TOXICOLOGICAL PROFILE FOR 

BENZENE 

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES 

Public Health Service 

Agency for Toxic Substances and Disease Registry
 

August 2007 
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DISCLAIMER 

The use of company or product name(s) is for identification only and does not imply endorsement by the 

Agency for Toxic Substances and Disease Registry. 
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UPDATE STATEMENT 

A Toxicological Profile for Benzene, Draft for Public Comment was released in August 2005. This 

edition supersedes any previously released draft or final profile. 

Toxicological profiles are revised and republished as necessary. For information regarding the update 

status of previously released profiles, contact ATSDR at: 

Agency for Toxic Substances and Disease Registry
 

Division of Toxicology and Environmental Medicine/Applied Toxicology Branch 

1600 Clifton Road NE 

Mailstop F-32 

Atlanta, Georgia 30333 
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v 

FOREWORD

This toxicological profile is prepared in accordance with guidelines developed by the Agency for 

Toxic Substances and Disease Registry (ATSDR) and the Environmental Protection Agency (EPA). The 

original guidelines were published in the Federal Register on April 17, 1987. Each profile will be revised 

and republished as necessary. 

The ATSDR toxicological profile succinctly characterizes the toxicologic and adverse health 

effects information for the hazardous substance described therein. Each peer-reviewed profile identifies 

and reviews the key literature that describes a hazardous substance's toxicologic properties. Other 

pertinent literature is also presented, but is described in less detail than the key studies. The profile is not 

intended to be an exhaustive document; however, more comprehensive sources of specialty information 

are referenced. 

The focus of the profiles is on health and toxicologic information; therefore, each toxicological 

profile begins with a public health statement that describes, in nontechnical language, a substance's 

relevant toxicological properties. Following the public health statement is information concerning levels 

of significant human exposure and, where known, significant health effects. The adequacy of information 

to determine a substance's health effects is described in a health effects summary. Data needs that are of 

significance to protection of public health are identified by ATSDR and EPA. 

Each profile includes the following: 

(A) The examination, summary, and interpretation of available toxicologic information and 

epidemiologic evaluations on a hazardous substance to ascertain the levels of significant 

human exposure for the substance and the associated acute, subacute, and chronic health 

effects; 

(B) A determination of whether adequate information on the health effects of each substance is 

available or in the process of development to determine levels of exposure that present a 

significant risk to human health of acute, subacute, and chronic health effects; and 

(C) Where appropriate, identification of toxicologic testing needed to identify the types or levels 

of exposure that may present significant risk of adverse health effects in humans. 

The principal audiences for the toxicological profiles are health professionals at the Federal, State, 

and local levels; interested private sector organizations and groups; and members of the public. 

This profile reflects ATSDR’s assessment of all relevant toxicologic testing and information that 

has been peer-reviewed. Staff of the Centers for Disease Control and Prevention and other Federal 

scientists have also reviewed the profile. In addition, this profile has been peer-reviewed by a 

nongovernmental panel and is being made available for public review. Final responsibility for the 

contents and views expressed in this toxicological profile resides with ATSDR. 
vi 

The toxicological profiles are developed in response to the Superfund Amendments and 

Reauthorization Act (SARA) of 1986 (Public Law 99-499) which amended the Comprehensive 

Environmental Response, Compensation, and Liability Act of 1980 (CERCLA or Superfund). This 

public law directed ATSDR to prepare toxicological profiles for hazardous substances most commonly 

found at facilities on the CERCLA National Priorities List and that pose the most significant potential 

threat to human health, as determined by ATSDR and the EPA. The availability of the revised priority 

list of 275 hazardous substances was announced in the Federal Register on December 7, 2005 (70 FR 

72840). For prior versions of the list of substances, see Federal Register notices dated April 17, 1987 

(52 FR 12866); October 20, 1988 (53 FR 41280); October 26, 1989 (54 FR 43619); October 17, 1990 (55 

FR 42067); October 17, 1991 (56 FR 52166); October 28, 1992 (57 FR 48801); February 28, 1994 (59 

FR 9486); April 29, 1996 (61 FR 18744); November 17, 1997 (62 FR 61332); October 21, 1999 (64 FR 

56792); October 25, 2001 (66 FR 54014); and November 7, 2003 (68 FR 63098). Section 104(i)(3) of 

CERCLA, as amended, directs the Administrator of ATSDR to prepare a toxicological profile for each 

substance on the list. 
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QUICK REFERENCE FOR HEALTH CARE PROVIDERS 

Toxicological Profiles are a unique compilation of toxicological information on a given hazardous 

substance. Each profile reflects a comprehensive and extensive evaluation, summary, and interpretation 

of available toxicologic and epidemiologic information on a substance. Health care providers treating 

patients potentially exposed to hazardous substances will find the following information helpful for fast 

answers to often-asked questions. 

Primary Chapters/Sections of Interest 

Chapter 1: Public Health Statement: The Public Health Statement can be a useful tool for educating 

patients about possible exposure to a hazardous substance. It explains a substance’s relevant 

toxicologic properties in a nontechnical, question-and-answer format, and it includes a review of 

the general health effects observed following exposure. 

Chapter 2: Relevance to Public Health: The Relevance to Public Health Section evaluates, interprets, 

and assesses the significance of toxicity data to human health. 

Chapter 3: Health Effects: Specific health effects of a given hazardous compound are reported by type 

of health effect (death, systemic, immunologic, reproductive), by route of exposure, and by length 

of exposure (acute, intermediate, and chronic). In addition, both human and animal studies are 

reported in this section. 

NOTE: Not all health effects reported in this section are necessarily observed in the clinical 

setting. Please refer to the Public Health Statement to identify general health effects observed 

following exposure. 

Pediatrics: Four new sections have been added to each Toxicological Profile to address child health 

issues: 

Section 1.6 How Can (Chemical X) Affect Children? 

Section 1.7 How Can Families Reduce the Risk of Exposure to (Chemical X)? 

Section 3.7 Children’s Susceptibility 

Section 6.6 Exposures of Children 

Other Sections of Interest: 

Section 3.8 Biomarkers of Exposure and Effect 

Section 3.11 Methods for Reducing Toxic Effects 

ATSDR Information Center 

Phone: 1-800-CDC-INFO (800-232-4636) or 1-888-232-6348 (TTY) Fax: (770) 488-4178 

E-mail: cdcinfo@cdc.gov Internet: http://www.atsdr.cdc.gov 

The following additional material can be ordered through the ATSDR Information Center: 

Case Studies in Environmental Medicine: Taking an Exposure History—The importance of taking an 

exposure history and how to conduct one are described, and an example of a thorough exposure 

history is provided. Other case studies of interest include Reproductive and Developmental 

Hazards; Skin Lesions and Environmental Exposures; Cholinesterase-Inhibiting Pesticide 

Toxicity; and numerous chemical-specific case studies. 
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Managing Hazardous Materials Incidents is a three-volume set of recommendations for on-scene 

(prehospital) and hospital medical management of patients exposed during a hazardous materials 

incident. Volumes I and II are planning guides to assist first responders and hospital emergency 

department personnel in planning for incidents that involve hazardous materials. Volume III— 

Medical Management Guidelines for Acute Chemical Exposures—is a guide for health care 

professionals treating patients exposed to hazardous materials. 

Fact Sheets (ToxFAQs) provide answers to frequently asked questions about toxic substances. 

Other Agencies and Organizations 

The National Center for Environmental Health (NCEH) focuses on preventing or controlling disease, 

injury, and disability related to the interactions between people and their environment outside the 

workplace. Contact: NCEH, Mailstop F-29, 4770 Buford Highway, NE, Atlanta, 

GA 30341-3724 • Phone: 770-488-7000 • FAX: 770-488-7015. 

The National Institute for Occupational Safety and Health (NIOSH) conducts research on occupational 

diseases and injuries, responds to requests for assistance by investigating problems of health and 

safety in the workplace, recommends standards to the Occupational Safety and Health 

Administration (OSHA) and the Mine Safety and Health Administration (MSHA), and trains 

professionals in occupational safety and health. Contact: NIOSH, 200 Independence Avenue, 

SW, Washington, DC 20201 • Phone: 800-356-4674 or NIOSH Technical Information Branch, 

Robert A. Taft Laboratory, Mailstop C-19, 4676 Columbia Parkway, Cincinnati, OH 45226-1998 

• Phone: 800-35-NIOSH. 

The National Institute of Environmental Health Sciences (NIEHS) is the principal federal agency for 

biomedical research on the effects of chemical, physical, and biologic environmental agents on 

human health and well-being. Contact: NIEHS, PO Box 12233, 104 T.W. Alexander Drive, 

Research Triangle Park, NC 27709 • Phone: 919-541-3212. 

Referrals 

The Association of Occupational and Environmental Clinics (AOEC) has developed a network of clinics 

in the United States to provide expertise in occupational and environmental issues. Contact: 

AOEC, 1010 Vermont Avenue, NW, #513, Washington, DC 20005 • Phone: 202-347-4976 

• FAX: 202-347-4950 • e-mail: AOEC@AOEC.ORG • Web Page: http://www.aoec.org/. 

The American College of Occupational and Environmental Medicine (ACOEM) is an association of 

physicians and other health care providers specializing in the field of occupational and 

environmental medicine. Contact: ACOEM, 25 Northwest Point Boulevard, Suite 700, Elk 

Grove Village, IL 60007-1030 • Phone: 847-818-1800 • FAX: 847-818-9266. 
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CONTRIBUTORS 

CHEMICAL MANAGER(S)/AUTHOR(S): 

Sharon Wilbur, M.A. 

Sam Keith, M.S., C.H.P.
 

Obaid Faroon, Ph.D. 

ATSDR, Division of Toxicology and Environmental Medicine, Atlanta, GA 

David Wohlers, Ph.D. 

Julie Stickney, Ph.D. 

Sari Paikoff, Ph.D. 

Gary Diamond, Ph.D 

Antonio Quiñones-Rivera, Ph.D. 

Syracuse Research Corporation, North Syracuse, NY 

THE PROFILE HAS UNDERGONE THE FOLLOWING ATSDR INTERNAL REVIEWS: 

1. Health Effects Review. The Health Effects Review Committee examines the health effects 

chapter of each profile for consistency and accuracy in interpreting health effects and classifying 

end points. 

2.	 Minimal Risk Level Review. The Minimal Risk Level Workgroup considers issues relevant to 

substance-specific Minimal Risk Levels (MRLs), reviews the health effects database of each 

profile, and makes recommendations for derivation of MRLs. 

3. Data Needs Review. The Applied Toxicology Branch reviews data needs sections to assure 

consistency across profiles and adherence to instructions in the Guidance. 

4. Green Border Review. Green Border review assures the consistency with ATSDR policy. 
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PEER REVIEW 

A peer review panel was assembled for benzene. The panel consisted of the following members: 

1. Dr. Jeffrey Fisher, Professor and Department Head, Department of Environmental Health 

Science, University of Georgia, Athens, Georgia; 

2. Dr. Tee Guidotti, Chair and Professor, Department of Occupational and Environmental Health, 

School of Public Health and Sciences, The George Washington University Medical Center, 

Washington, DC; and 

3. Dr. Rogene Henderson, Senior Scientist, Lovelace Respiratory Research Institute, Albuquerque, 

New Mexico. 

These experts collectively have knowledge of benzene's physical and chemical properties, toxicokinetics, 

key health end points, mechanisms of action, human and animal exposure, and quantification of risk to 

humans. All reviewers were selected in conformity with the conditions for peer review specified in 

Section 104(I)(13) of the Comprehensive Environmental Response, Compensation, and Liability Act, as 

amended. 

Scientists from the Agency for Toxic Substances and Disease Registry (ATSDR) have reviewed the peer 

reviewers' comments and determined which comments will be included in the profile. A listing of the 

peer reviewers' comments not incorporated in the profile, with a brief explanation of the rationale for their 

exclusion, exists as part of the administrative record for this compound. 

The citation of the peer review panel should not be understood to imply its approval of the profile's final 

content. The responsibility for the content of this profile lies with the ATSDR. 
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CONTENTS 

DISCLAIMER ..............................................................................................................................................ii 

UPDATE STATEMENT .............................................................................................................................iii 

FOREWORD ................................................................................................................................................v 

QUICK REFERENCE FOR HEALTH CARE PROVIDERS....................................................................vii 

CONTRIBUTORS.......................................................................................................................................ix 

PEER REVIEW ...........................................................................................................................................xi 

CONTENTS...............................................................................................................................................xiii 

LIST OF FIGURES ..................................................................................................................................xvii 

LIST OF TABLES.....................................................................................................................................xix 

1. PUBLIC HEALTH STATEMENT..........................................................................................................1 

1.1 WHAT IS BENZENE?................................................................................................................1 

1.2 WHAT HAPPENS TO BENZENE WHEN IT ENTERS THE ENVIRONMENT? ..................2 

1.3 HOW MIGHT I BE EXPOSED TO BENZENE?.......................................................................3 

1.4 HOW CAN BENZENE ENTER AND LEAVE MY BODY? ....................................................4 

1.5 HOW CAN BENZENE AFFECT MY HEALTH? .....................................................................4 

1.6 HOW CAN BENZENE AFFECT CHILDREN? ........................................................................ 6 

1.7 HOW CAN FAMILIES REDUCE THE RISK OF EXPOSURE TO BENZENE? ....................7 

1.8 IS THERE A MEDICAL TEST TO DETERMINE WHETHER I HAVE BEEN
 

EXPOSED TO BENZENE?........................................................................................................7 

1.9 WHAT RECOMMENDATIONS HAS THE FEDERAL GOVERNMENT MADE TO 

PROTECT HUMAN HEALTH?.................................................................................................8 

1.10 WHERE CAN I GET MORE INFORMATION? .......................................................................9 

2. RELEVANCE TO PUBLIC HEALTH .................................................................................................11 

2.1 BACKGROUND AND ENVIRONMENTAL EXPOSURES TO BENZENE IN THE 

UNITED STATES.....................................................................................................................11 

2.2 SUMMARY OF HEALTH EFFECTS......................................................................................12 

2.3 MINIMAL RISK LEVELS (MRLs) .........................................................................................21 

3. HEALTH EFFECTS..............................................................................................................................29 

3.1 INTRODUCTION.....................................................................................................................29 

3.2 DISCUSSION OF HEALTH EFFECTS BY ROUTE OF EXPOSURE ..................................29 

3.2.1 Inhalation Exposure ..............................................................................................................30 

3.2.2 Oral Exposure......................................................................................................................104 

3.2.3 Dermal Exposure.................................................................................................................137 

3.3 GENOTOXICITY ...................................................................................................................141 

3.4 TOXICOKINETICS................................................................................................................154 

3.4.1 Absorption...........................................................................................................................155 

3.4.2 Distribution .........................................................................................................................163 

3.4.3 Metabolism..........................................................................................................................166 

3.4.4 Elimination and Excretion...................................................................................................175 

3.4.5 Physiologically Based Pharmacokinetic (PBPK)/Pharmacodynamic (PD) Models ...........181 

3.5 MECHANISMS OF ACTION ................................................................................................ 200 

3.5.1 Pharmacokinetic Mechanisms............................................................................................. 200 

3.5.2 Mechanisms of Toxicity......................................................................................................201 

3.5.3 Animal-to-Human Extrapolations.......................................................................................204 

3.6 TOXICITIES MEDIATED THROUGH THE NEUROENDOCRINE AXIS ........................205 

3.7 CHILDREN’S SUSCEPTIBILITY.........................................................................................206 
 

 

 

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3.8 BIOMARKERS OF EXPOSURE AND EFFECT ..................................................................208 

3.8.1 Biomarkers Used to Identify or Quantify Exposure to Benzene.........................................209 

3.8.2 Biomarkers Used to Characterize Effects Caused by Benzene ...........................................211 

3.9 INTERACTIONS WITH OTHER CHEMICALS ..................................................................212 

3.10 POPULATIONS THAT ARE UNUSUALLY SUSCEPTIBLE.............................................214 

3.11 METHODS FOR REDUCING TOXIC EFFECTS.................................................................216 

3.11.1 Reducing Peak Absorption Following Exposure ............................................................216 

3.11.2 Reducing Body Burden...................................................................................................217 

3.11.3 Interfering with the Mechanism of Action for Toxic Effects .........................................217 

3.12 ADEQUACY OF THE DATABASE......................................................................................218 

3.12.1 Existing Information on Health Effects of Benzene .......................................................219 

3.12.2 Identification of Data Needs ...........................................................................................221 

3.12.3 Ongoing Studies..............................................................................................................236 

4. CHEMICAL AND PHYSICAL INFORMATION..............................................................................239 

4.1 CHEMICAL IDENTITY.........................................................................................................239 

4.2 PHYSICAL AND CHEMICAL PROPERTIES......................................................................239 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL..........................................................243 

5.1 PRODUCTION .......................................................................................................................243 

5.2 IMPORT/EXPORT .................................................................................................................244 

5.3 USE..........................................................................................................................................248 

5.4 DISPOSAL..............................................................................................................................249 

6. POTENTIAL FOR HUMAN EXPOSURE .........................................................................................251 

6.1 OVERVIEW............................................................................................................................251 

6.2 RELEASES TO THE ENVIRONMENT................................................................................253 

6.2.1 Air .......................................................................................................................................253 

6.2.2 Water...................................................................................................................................257 

6.2.3 Soil ......................................................................................................................................257 

6.3 ENVIRONMENTAL FATE....................................................................................................258
 

6.3.1 Transport and Partitioning...................................................................................................258 

6.3.2 Transformation and Degradation ........................................................................................259 

6.4 LEVELS MONITORED OR ESTIMATED IN THE ENVIRONMENT...............................265 

6.4.1 Air .......................................................................................................................................265 

6.4.2 Water...................................................................................................................................269 

6.4.3 Sediment and Soil ...............................................................................................................271 

6.4.4 Other Environmental Media................................................................................................271 

6.5 GENERAL POPULATION AND OCCUPATIONAL EXPOSURE .....................................273 

6.6 EXPOSURES OF CHILDREN...............................................................................................281 

6.7 POPULATIONS WITH POTENTIALLY HIGH EXPOSURES ...........................................283 

6.8 ADEQUACY OF THE DATABASE......................................................................................284 

6.8.1 Identification of Data Needs ...............................................................................................284 

6.8.2 Ongoing Studies ..................................................................................................................288 

7. ANALYTICAL METHODS ...............................................................................................................291 

7.1 BIOLOGICAL MATERIALS.................................................................................................291 

7.2 ENVIRONMENTAL SAMPLES............................................................................................295 

7.3 ADEQUACY OF THE DATABASE......................................................................................301 

7.3.1 Identification of Data Needs ...............................................................................................302 

7.3.2 Ongoing Studies..................................................................................................................304 
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8. REGULATIONS AND ADVISORIES ...............................................................................................307 

9. REFERENCES ....................................................................................................................................313 

10. GLOSSARY ......................................................................................................................................377 

APPENDICES 

A. ATSDR MINIMAL RISK LEVELS AND WORKSHEETS .............................................................A-1 

B. USER’S GUIDE.................................................................................................................................. B-1 

C. ACRONYMS, ABBREVIATIONS, AND SYMBOLS...................................................................... C-1 

D. INDEX ................................................................................................................................................D-1 
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LIST OF FIGURES 

3-1. Levels of Significant Exposure to Benzene – Inhalation.................................................................... 66 

3-2. Levels of Significant Exposure to Benzene – Oral........................................................................... 122 

3-3. Metabolic Pathways for Benzene .....................................................................................................167 

3-4. Conceptual Representation of a Physiologically Based Pharmacokinetic (PBPK) Model for a 

Hypothetical Chemical Substance....................................................................................................183 

3-5. General Structure of Physiologically Based Pharmacokinetic Models of Benzene .........................184 

3-6. Existing Information on Health Effects of Benzene.........................................................................220 

6-1. Frequency of NPL Sites with Benzene Contamination ....................................................................252 

6-2. Environmental Transformation Products of Benzene in Various Media..........................................260 

6-3. Benzene Emissions and Exposures...................................................................................................275 
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LIST OF TABLES 

3-1. Levels of Significant Exposure to Benzene – Inhalation.................................................................... 33 

3-2. Levels of Significant Exposure to Benzene – Oral........................................................................... 106 

3-3. Levels of Significant Exposure to Benzene – Dermal...................................................................... 139 

3-4. Genotoxicity of Benzene In Vivo......................................................................................................142 

3-5. Genotoxicity of Benzene In Vitro.....................................................................................................146 

3-6. Summary Comparison of Physiologically Based Pharmacokinetic Models for Benzene ................185 

3-7. Ongoing Studies on the Health Effects of Benzene..........................................................................237 

4-1. Chemical Identity of Benzene ..........................................................................................................240 

4-2. Physical and Chemical Properties of Benzene .................................................................................241 

5-1. Facilities that Produce, Process, or Use Benzene .............................................................................245 

5-2. Current U.S. Manufacturers of Benzene...........................................................................................247 

6-1. Releases to the Environment from Facilities that Produce, Process, or Use Benzene......................254 

6-2. Benzene Levels in Air Samples........................................................................................................266 

6-3. Benzene in Food ...............................................................................................................................272 

6-4. Percentage of Employees Exposed to Benzene by Exposure Level and Industry Division .............280 

6-5. Ongoing Studies on the Potential for Human Exposure to Benzene ................................................289 

7-1. Analytical Methods for Determining Benzene in Biological Samples.............................................292 

7-2. Analytical Methods for Determining Metabolites of Benzene in Urine...........................................294 

7-3. Analytical Methods for Determining Benzene in Environmental Samples......................................296 

7-4. Ongoing Studies on Benzene, Analytical Methods..........................................................................305 

8-1. Regulations and Guidelines Applicable to Benzene.........................................................................308 
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BENZENE 1 

1. PUBLIC HEALTH STATEMENT 

This public health statement tells you about benzene and the effects of exposure to it. 

The Environmental Protection Agency (EPA) identifies the most serious hazardous waste sites in 

the nation. These sites are then placed on the National Priorities List (NPL) and are targeted for 

long-term federal clean-up activities. Benzene has been found in at least 1,000 of the 

1,684 current or former NPL sites. Although the total number of NPL sites evaluated for this 

substance is not known, the possibility exists that the number of sites at which benzene is found 

may increase in the future as more sites are evaluated. This information is important because 

these sites may be sources of exposure and exposure to this substance may harm you. 

When a substance is released either from a large area, such as an industrial plant, or from a 

container, such as a drum or bottle, it enters the environment. Such a release does not always 

lead to exposure. You can be exposed to a substance only when you come in contact with it. 

You may be exposed by breathing, eating, or drinking the substance, or by skin contact. 

If you are exposed to benzene, many factors will determine whether you will be harmed. These 

factors include the dose (how much), the duration (how long), and how you come in contact with 

it. You must also consider any other chemicals you are exposed to and your age, sex, diet, 

family traits, lifestyle, and state of health. 

1.1 WHAT IS BENZENE? 

Benzene, also known as benzol, is a colorless liquid with a sweet odor. Benzene evaporates into 

air very quickly and dissolves slightly in water. Benzene is highly flammable. Most people can 

begin to smell benzene in air at approximately 60 parts of benzene per million parts of air (ppm) 

and recognize it as benzene at 100 ppm. Most people can begin to taste benzene in water at 0.5– 

4.5 ppm. One part per million is approximately equal to one drop in 40 gallons. Benzene is 

found in air, water, and soil. Benzene comes from both industrial and natural sources. 
BENZENE 2 

1. PUBLIC HEALTH STATEMENT 

Industrial Sources and Uses. Benzene was first discovered and isolated from coal tar in the 

1800s. Today, benzene is made mostly from petroleum. Because of its wide use, benzene ranks 

in the top 20 in production volume for chemicals produced in the United States. Various 

industries use benzene to make other chemicals, such as styrene (for Styrofoam® and other 

plastics), cumene (for various resins), and cyclohexane (for nylon and synthetic fibers). Benzene 

is also used in the manufacturing of some types of rubbers, lubricants, dyes, detergents, drugs, 

and pesticides. 

Natural Sources. Natural sources of benzene, which include gas emissions from volcanoes and 

forest fires, also contribute to the presence of benzene in the environment. Benzene is also 

present in crude oil and gasoline and cigarette smoke. For more information on characteristics 

and uses of benzene, see Chapters 4 and 5. 

1.2 WHAT HAPPENS TO BENZENE WHEN IT ENTERS THE ENVIRONMENT? 

Benzene is commonly found in the environment. Industrial processes are the main sources of 

benzene in the environment. Benzene levels in the air can be elevated by emissions from 

burning coal and oil, benzene waste and storage operations, motor vehicle exhaust, and 

evaporation from gasoline service stations. Tobacco smoke is another source of benzene in air, 

particularly indoors. Industrial discharge, disposal of products containing benzene, and gasoline 

leaks from underground storage tanks release benzene into water and soil. 

Benzene can pass into air from water and soil surfaces. Once in the air, benzene reacts with 

other chemicals and breaks down within a few days. Benzene in the air can also be deposited on 

the ground by rain or snow. 

Benzene in water and soil breaks down more slowly. Benzene is slightly soluble in water and 

can pass through the soil into underground water. Benzene in the environment does not build up 

in plants or animals. For more information on what happens to benzene after it gets into the 

environment, see Chapters 5 and 6. 
BENZENE 3 

1. PUBLIC HEALTH STATEMENT 

1.3 HOW MIGHT I BE EXPOSED TO BENZENE? 

Everyone is exposed to a small amount of benzene every day. You are exposed to benzene in the 

outdoor environment, in the workplace, and in the home. Exposure of the general population to 

benzene mainly occurs through breathing air that contains benzene. The major sources of 

benzene exposure are tobacco smoke, automobile service stations, exhaust from motor vehicles, 

and industrial emissions. Vapors (or gases) from products that contain benzene, such as glues, 

paints, furniture wax, and detergents, can also be a source of exposure. Auto exhaust and 

industrial emissions account for about 20% of the total national exposure to benzene. About half 

of the exposure to benzene in the United States results from smoking tobacco or from exposure 

to tobacco smoke. The average smoker (32 cigarettes per day) takes in about 1.8 milligrams 

(mg) of benzene per day. This amount is about 10 times the average daily intake of benzene by 

nonsmokers. 

Measured levels of benzene in outdoor air have ranged from 0.02 to 34 parts of benzene per 

billion parts of air (ppb) (1 ppb is 1,000 times less than 1 ppm). People living in cities or 

industrial areas are generally exposed to higher levels of benzene in air than those living in rural 

areas. Benzene levels in the home are usually higher than outdoor levels. People may be 

exposed to higher levels of benzene in air by living near hazardous waste sites, petroleum 

refining operations, petrochemical manufacturing sites, or gas stations. 

For most people, the level of exposure to benzene through food, beverages, or drinking water is 

not as high as through air. Drinking water typically contains less than 0.1 ppb benzene. Benzene 

has been detected in some bottled water, liquor, and food. Leakage from underground gasoline 

storage tanks or from landfills and hazardous waste sites that contain benzene can result in 

benzene contamination of well water. People with benzene-contaminated tap water can be 

exposed from drinking the water or eating foods prepared with the water. In addition, exposure 

can result from breathing in benzene while showering, bathing, or cooking with contaminated 

water. 
BENZENE 4 

1. PUBLIC HEALTH STATEMENT 

Individuals employed in industries that make or use benzene may be exposed to the highest 

levels of benzene. As many as 238,000 people may be occupationally exposed to benzene in the 

United States. These industries include benzene production (petrochemicals, petroleum refining, 

and coke and coal chemical manufacturing), rubber tire manufacturing, and storage or transport 

of benzene and petroleum products containing benzene. Other workers who may be exposed to 

benzene include coke oven workers in the steel industry, printers, rubber workers, shoe makers, 

laboratory technicians, firefighters, and gas station employees. For more information on how 

you might be exposed to benzene, see Chapter 6. 

1.4 HOW CAN BENZENE ENTER AND LEAVE MY BODY? 

Benzene can enter your body through your lungs, gastrointestinal tract, and across your skin. 

When you are exposed to high levels of benzene in air, about half of the benzene you breathe in 

passes through the lining of your lungs and enters your bloodstream. When you are exposed to 

benzene in food or drink, most of the benzene you take in by mouth passes through the lining of 

your gastrointestinal tract and enters your bloodstream. A small amount will enter your body by 

passing through your skin and into your bloodstream during skin contact with benzene or 

benzene-containing products. Once in the bloodstream, benzene travels throughout your body 

and can be temporarily stored in the bone marrow and fat. Benzene is converted to products, 

called metabolites, in the liver and bone marrow. Some of the harmful effects of benzene 

exposure are caused by these metabolites. Most of the metabolites of benzene leave the body in 

the urine within 48 hours after exposure. For more information on how benzene can enter and 

leave your body, see Chapter 3. 

1.5 HOW CAN BENZENE AFFECT MY HEALTH? 

Scientists use many tests to protect the public from harmful effects of toxic chemicals and to find 

ways for treating persons who have been harmed. 

One way to learn whether a chemical will harm people is to determine how the body absorbs, 

uses, and releases the chemical. For some chemicals, animal testing may be necessary. Animal 

testing may also help identify health effects such as cancer or birth defects. Without laboratory 
BENZENE 5 

1. PUBLIC HEALTH STATEMENT 

animals, scientists would lose a basic method for getting information needed to make wise 

decisions that protect public health. Scientists have the responsibility to treat research animals 

with care and compassion. Scientists must comply with strict animal care guidelines because 

laws today protect the welfare of research animals. 

After exposure to benzene, several factors determine whether harmful health effects will occur, 

as well as the type and severity of such health effects. These factors include the amount of 

benzene to which you are exposed and the length of time of the exposure. Most information on 

effects of long-term exposure to benzene are from studies of workers employed in industries that 

make or use benzene. These workers were exposed to levels of benzene in air far greater than 

the levels normally encountered by the general population. Current levels of benzene in 

workplace air are much lower than in the past. Because of this reduction and the availability of 

protective equipment such as respirators, fewer workers have symptoms of benzene poisoning. 

Brief exposure (5–10 minutes) to very high levels of benzene in air (10,000–20,000 ppm) can 

result in death. Lower levels (700–3,000 ppm) can cause drowsiness, dizziness, rapid heart rate, 

headaches, tremors, confusion, and unconsciousness. In most cases, people will stop feeling 

these effects when they are no longer exposed and begin to breathe fresh air. 

Eating foods or drinking liquids containing high levels of benzene can cause vomiting, irritation 

of the stomach, dizziness, sleepiness, convulsions, rapid heart rate, coma, and death. The health 

effects that may result from eating foods or drinking liquids containing lower levels of benzene 

are not known. If you spill benzene on your skin, it may cause redness and sores. Benzene in 

your eyes may cause general irritation and damage to your cornea. 

Benzene causes problems in the blood. People who breathe benzene for long periods may 

experience harmful effects in the tissues that form blood cells, especially the bone marrow. 

These effects can disrupt normal blood production and cause a decrease in important blood 

components. A decrease in red blood cells can lead to anemia. Reduction in other components 

in the blood can cause excessive bleeding. Blood production may return to normal after 

exposure to benzene stops. Excessive exposure to benzene can be harmful to the immune 
BENZENE 6 

1. PUBLIC HEALTH STATEMENT 

system, increasing the chance for infection and perhaps lowering the body's defense against 

cancer. 

Long-term exposure to benzene can cause cancer of the blood-forming organs. This condition is 

called leukemia. Exposure to benzene has been associated with development of a particular type 

of leukemia called acute myeloid leukemia (AML). The Department of Health and Human 

Services has determined that benzene is a known carcinogen (can cause cancer). Both the 

International Agency for Cancer Research and the EPA have determined that benzene is 

carcinogenic to humans. 

Exposure to benzene may be harmful to the reproductive organs. Some women workers who 

breathed high levels of benzene for many months had irregular menstrual periods. When 

examined, these women showed a decrease in the size of their ovaries. However, exact exposure 

levels were unknown, and the studies of these women did not prove that benzene caused these 

effects. It is not known what effects exposure to benzene might have on the developing fetus in 

pregnant women or on fertility in men. Studies with pregnant animals show that breathing 

benzene has harmful effects on the developing fetus. These effects include low birth weight, 

delayed bone formation, and bone marrow damage. 

We do not know what human health effects might occur after long-term exposure to food and 

water contaminated with benzene. In animals, exposure to food or water contaminated with 

benzene can damage the blood and the immune system and can cause cancer. See Chapters 2 

and 3 for more information on the health effects resulting from benzene exposure. 

1.6 HOW CAN BENZENE AFFECT CHILDREN? 

This section discusses potential health effects in humans from exposures during the period from 

conception to maturity at 18 years of age. 
BENZENE 7 

1. PUBLIC HEALTH STATEMENT 

Children can be affected by benzene exposure in the same ways as adults. Benzene can pass 

from the mother’s blood to a fetus. It is not known if children are more susceptible to benzene 

poisoning than adults. 

1.7 HOW CAN FAMILIES REDUCE THE RISK OF EXPOSURE TO BENZENE? 

If your doctor finds that you have been exposed to substantial amounts of benzene, ask whether 

your children might also have been exposed. Your doctor might need to ask your state health 

department to investigate. 

Gasoline and cigarette smoke are two main sources of human exposure to benzene. Benzene 

exposure can be reduced by limiting contact with these sources. People are exposed to benzene 

from both active and passive second-hand smoke. Average smokers take in about 10 times more 

benzene than nonsmokers each day. Families are encouraged not to smoke in their house, in 

enclosed environments, or near their children. 

Benzene is a major component of gasoline and used in many manufacturing processes. 

Increased levels of benzene can be found at fueling stations, and in air emissions from 

manufacturing plants and hazardous waste sites. Living near gasoline fueling stations or 

hazardous waste sites may increase exposure to benzene. People are advised not to have their 

families play near fueling stations, manufacturing plants, or hazardous waste sites. 

1.8 IS THERE A MEDICAL TEST TO DETERMINE WHETHER I HAVE BEEN 

EXPOSED TO BENZENE? 

Several tests can show whether you have been exposed to benzene. Some of these tests may be 

available at your doctor's office. All of these tests are limited in what they can tell you. The test 

for measuring benzene in your breath must be done shortly after exposure. This test is not very 

helpful for detecting very low levels of benzene in your body. Benzene can be measured in your 

blood. However, because benzene rapidly disappears in the blood, measurements may be useful 

only for recent exposures. 
BENZENE 8 

1. PUBLIC HEALTH STATEMENT 

In the body, benzene is converted to products called metabolites. Certain metabolites of 

benzene, such as phenol, muconic acid, and S-phenylmercapturic acid can be measured in the 

urine. The amount of phenol in urine has been used to check for benzene exposure in workers. 

The test is useful only when you are exposed to benzene in air at levels of 10 ppm or greater. 

However, this test must also be done shortly after exposure, and it is not a reliable indicator of 

how much benzene you have been exposed to, because phenol is present in the urine from other 

sources (diet, environment). Measurements of muconic acid or S-phenylmercapturic acid in the 

urine are more sensitive and reliable indicators of benzene exposure. The measurement of 

benzene in blood or of metabolites in urine cannot be used for making predictions about whether 

you will experience any harmful health effects. Blood counts of all components of the blood and 

examination of bone marrow are used to determine benzene exposure and its health effects. 

For people exposed to relatively high levels of benzene, complete blood analyses can be used to 

monitor possible changes related to exposure. However, blood analyses are not useful when 

exposure levels are low. For more information on tests for benzene exposure, see Chapters 3 

and 7. 

1.9 WHAT RECOMMENDATIONS HAS THE FEDERAL GOVERNMENT MADE TO 

PROTECT HUMAN HEALTH? 

The federal government develops regulations and recommendations to protect public health. 

Regulations can be enforced by law. The EPA, the Occupational Safety and Health 

Administration (OSHA), and the Food and Drug Administration (FDA) are some federal 

agencies that develop regulations for toxic substances. Recommendations provide valuable 

guidelines to protect public health, but cannot be enforced by law. The Agency for Toxic 

Substances and Disease Registry (ATSDR) and the National Institute for Occupational Safety 

and Health (NIOSH) are two federal organizations that develop recommendations for toxic 

substances. 

Regulations and recommendations can be expressed as “not-to-exceed” levels, that is, levels of a 

toxic substance in air, water, soil, or food that do not exceed a critical value that is usually based 

on levels that affect animals; they are then adjusted to levels that will help protect humans. 
BENZENE 9 

1. PUBLIC HEALTH STATEMENT 

Sometimes these not-to-exceed levels differ among federal organizations because they used 

different exposure times (an 8-hour workday or a 24-hour day), different animal studies, or other 

factors. 

Recommendations and regulations are also updated periodically as more information becomes 

available. For the most current information, check with the federal agency or organization that 

provides it. Some regulations and recommendations for benzene include the following: 

EPA has set 5 ppb as the maximum permissible level of benzene in drinking water. EPA has set 

a goal of 0 ppb for benzene in drinking water and in water such as rivers and lakes because 

benzene can cause leukemia. EPA estimates that 10 ppb benzene in drinking water that is 

consumed regularly or exposure to 0.4 ppb in air over a lifetime could cause a risk of one 

additional cancer case for every 100,000 exposed persons. EPA recommends 200 ppb as the 

maximum permissible level of benzene in water for short-term exposures (10 days) for children. 

EPA requires that the National Response Center be notified following a discharge or spill into 

the environment of 10 pounds or more of benzene. 

OSHA regulates levels of benzene in the workplace. The maximum allowable amount of 

benzene in workroom air during an 8-hour workday, 40-hour workweek is 1 ppm. Because 

benzene can cause cancer, NIOSH recommends that all workers wear special breathing 

equipment when they are likely to be exposed to benzene at levels exceeding the recommended 

(8-hour) exposure limit of 0.1 ppm. For more information on federal regulations, see Chapter 8. 

1.10 WHERE CAN I GET MORE INFORMATION? 

If you have any more questions or concerns, please contact your community or state health or 

environmental quality department, or contact ATSDR at the address and phone number below. 
BENZENE 10 

1. PUBLIC HEALTH STATEMENT 

ATSDR can also tell you the location of occupational and environmental health clinics. These 

clinics specialize in recognizing, evaluating, and treating illnesses that result from exposure to 

hazardous substances. 

Toxicological profiles are also available on-line at www.atsdr.cdc.gov and on CD-ROM. You 

may request a copy of the ATSDR ToxProfilesTM CD-ROM by calling the toll-free information 

and technical assistance number at 1-800-CDCINFO (1-800-232-4636), by e-mail at 

cdcinfo@cdc.gov, or by writing to: 

Agency for Toxic Substances and Disease Registry 

 Division of Toxicology and Environmental Medicine 

1600 Clifton Road NE 

 Mailstop F-32 

 Atlanta, GA 30333 

 Fax: 1-770-488-4178 

Organizations for-profit may request copies of final Toxicological Profiles from the following: 

National Technical Information Service (NTIS) 

5285 Port Royal Road 

 Springfield, VA 22161 

 Phone: 1-800-553-6847 or 1-703-605-6000 

 Web site: http://www.ntis.gov/ 
BENZENE 11 

2. RELEVANCE TO PUBLIC HEALTH 

2.1 BACKGROUND AND ENVIRONMENTAL EXPOSURES TO BENZENE IN THE UNITED 

STATES 

Benzene is widely distributed in the environment. The exposure scenario of most concern to the general 

public is low-level inhalation over long periods. This is because the general population is exposed to 

benzene mainly through inhalation of contaminated air, particularly in areas of heavy traffic and around 

gas stations, and through inhalation of tobacco smoke from both active and passive smoking. Smoking 

has been identified as the single most important source of benzene exposure for the estimated 40 million 

U.S. smokers. Smoking accounts for approximately half of the total benzene exposure of the general 

population. Individuals employed in industries that make or use benzene, or products containing benzene, 

are probably exposed to the highest concentrations of atmospheric benzene. In addition, benzene is a 

common combustion product, providing high inhalation exposure potential for firefighters. Of the 

general population, those residing around certain chemical manufacturing sites or living near waste sites 

containing benzene or near leaking gasoline tanks may be exposed to concentrations of benzene that are 

higher than background air concentrations. In private homes, benzene levels in the air have been shown 

to be higher in homes with attached garages, or where the inhabitants smoke inside the house. 

Although low levels of benzene have been detected in certain foods, beverages, and tap water, these do 

not constitute major sources of exposure for most people. However, leakage from underground gasoline 

storage tanks and seepage from landfills and hazardous waste sites have resulted in significant benzene 

contamination of well water. People with contaminated tap water can be exposed from drinking the water 

or eating foods prepared with it. In addition, exposure can also occur via inhalation during showering, 

bathing, or cooking with contaminated tap water. Showering and bathing with benzene-contaminated 

water can also contribute significantly to dermal exposure. 

The Benzene Subregistry Baseline Technical Report of the National Exposure Registry contains 

information on 1,143 persons who had documented exposure to benzene in their drinking water and were 

exposed for at least 30 days. No causal relationship has been proposed for health conditions identified in 

the base subregistry or continued follow-up of the population. 
BENZENE 12 

2. RELEVANCE TO PUBLIC HEALTH 

2.2 SUMMARY OF HEALTH EFFECTS 

The carcinogenicity of benzene is well documented in exposed workers. Epidemiological studies and 

case reports provide clear evidence of a causal relationship between occupational exposure to benzene 

and benzene-containing solvents and the occurrence of acute myelogenous leukemia (AML). The 

epidemiological studies are generally limited by confounding chemical exposures and methodological 

problems, including inadequate or lack of exposure monitoring and low statistical power, but a consistent 

excess risk of leukemia across studies indicates that benzene is the causal factor. 

In vivo and in vitro data from both humans and animals indicate that benzene and/or its metabolites are 

genotoxic. Chromosomal aberrations (hypo- and hyperdiploidy, deletions, breaks, and gaps) in peripheral 

lymphocytes and bone marrow cells are the predominant effects seen in humans. 

Damage to both the humoral and cellular components of the immune system has been known to occur in 

humans following inhalation exposure. This is manifested by decreased levels of antibodies and 

decreased levels of leukocytes in workers. Animal data support these findings. 

The most characteristic systemic effect resulting from intermediate and chronic benzene exposure is 

arrested development of blood cells. Early biomarkers of exposure to relatively low levels of benzene 

include depressed numbers of one or more of the circulating blood cell types. A common clinical finding 

in benzene hematotoxicity is cytopenia, which is a decrease in various cellular elements of the circulating 

blood manifested as anemia, leukopenia, or thrombocytopenia in humans and in animals. Benzeneassociated cytopenias vary and may involve a reduction in one (unicellular cytopenias) to all three 

(pancytopenia) cellular elements of the blood. 

Benzene also causes a life-threatening disorder called aplastic anemia in humans and animals. This 

disorder is characterized by reduction of all cellular elements in the peripheral blood and in bone marrow, 

leading to fibrosis, an irreversible replacement of bone marrow. Benzene has also been associated with 

acute non-lymphocytic leukemia in humans, and aplastic anemia may be an early indicator of developing 

acute non-lymphocytic leukemia in some cases. 

Limited information is available on other systemic effects reported in humans and is associated with 

high–level benzene exposure. Respiratory effects have been noted after acute exposure of humans to 

benzene vapors. Cardiovascular effects, particularly ventricular fibrillation, have been suggested as the 
BENZENE 13 

2. RELEVANCE TO PUBLIC HEALTH 

cause of death in fatal exposures to benzene vapor. Gastrointestinal effects have been noted in humans 

after fatal inhalation exposure (congestive gastritis), and ingestion (toxic gastritis and pyloric stenosis), of 

benzene. Myelofibrosis (a form of aplastic anemia) was reported by a gasoline station attendant who had 

been exposed to benzene by inhalation, and probably also through dermal contact. Myalgia was also 

reported in steel plant workers exposed to benzene vapors. Reports of renal effects in humans after 

benzene exposure consist of kidney congestion after fatal inhalation exposure. Dermal and ocular effects 

including skin irritation and burns, and eye irritation have been reported after exposure to benzene vapors. 

Swelling and edema have been reported to occur in a human who swallowed benzene. Studies in animals 

show systemic effects after inhalation exposure, including cardiovascular effects. Oral administration of 

benzene to animals has yielded information concerning hepatic effects. A study conducted in rabbits 

lends support to the finding that benzene is irritating and damaging to the skin and also shows that it is 

irritating and damaging to the eyes following dermal or ocular application. 

Neurological effects have been commonly reported in humans following high-level exposure to benzene. 

Fatal inhalation exposure has been associated with vascular congestion in the brain. Chronic inhalation 

exposure has been associated with distal neuropathy, difficulty in sleeping, and memory loss. Oral 

exposure results in symptoms similar to inhalation exposure. Studies in animals suggest that inhalation 

exposure to benzene results in depressed electrical activity in the brain, loss of involuntary reflexes and 

narcosis, decrease in hind-limb grip strength and tremors, and narcosis, among other symptoms. Oral 

exposure to benzene has not been shown to cause significant changes in behavior. No neurological 

effects have been reported after dermal exposure to liquid benzene in either humans or animals. 

Acute inhalation and oral exposures of humans to high concentrations of benzene have caused death. 

These exposures are also associated with central nervous system depression. Chronic low-level exposures 

have been associated with peripheral nervous system effects. Abnormalities in motor conduction velocity 

were noted in four of six pancytopenic individuals occupationally exposed to adhesives containing 

benzene. 

Evidence of an effect of benzene exposure on human reproduction is not sufficient to demonstrate a 

causal association. Some animal studies provide limited evidence that benzene affects reproductive 

organs following inhalation exposure. Results from studies of benzene administered orally to rats and 

mice indicate no adverse effect on male or female reproductive organs at 17 weeks, but at 2 years, 

endometrial polyps were observed in female rats, preputial gland lesions were observed in male mice, and 

ovarian lesions were observed in female mice. Results are conflicting or inconclusive as to whether 
BENZENE 14 

2. RELEVANCE TO PUBLIC HEALTH 

inhalation of benzene vapors reduces the number of live fetuses and/or the incidences of pregnancy. 

Other studies are negative for effects on reproductive competence. 

Epidemiological studies implicating benzene as a developmental toxicant have many limitations, and 

thus, it is not possible to assess the effect of benzene on the human fetus. Results of inhalation studies 

conducted in animals are fairly consistent across species and demonstrate that, at levels >47 ppm, benzene 

is fetotoxic as evidenced by decreased fetal weight and/or minor skeletal variants. Benzene has also been 

shown to reduce pup body weight in mice. A persistent decrease in the number of erythroid precursors 

was found in mice exposed in utero. Benzene has not been shown to be teratogenic, but has been shown 

to be fetotoxic in animals at high concentrations that are maternally toxic. 

Cancer. The strongest evidence for the leukemogenic potential of benzene comes from series of cohort 

mortality studies on workers exposed to benzene in Ohio (the Pliofilm study) and China (the NCI/CAPM 

study). The Pliofilm study investigated workers exposed to benzene in three rubber hydrochloride 

(‘Pliofilm’) manufacturing plants. Mortality from all leukemias was increased but declined after 

additional years of follow-up, suggesting that the excess risk diminished with time since exposure. 

Exposures in the most recent 10 years were most strongly associated with leukemia risk, and there was no 

significant relation between leukemia death and benzene exposures received more than 20 years 

previously. AML accounted for most of the increased leukemia, and the risk of AML increased with 

increasing cumulative exposure above 200 ppm-years. 

The NCI/CAPM study, a collaboration between the National Cancer Institute and the Chinese Academy 

of Preventive Medicine, evaluated lymphohematopoietic malignancies and other hematologic disorders in 

74,828 benzene-exposed workers employed in 672 factories in 12 cities in China. Findings included 

increased risks for all leukemias, acute nonlymphocytic leukemia (ANLL), and combined ANLL and 

precursor myelodysplastic syndromes. These risks were increased at average exposure levels of 10– 

24 ppm and cumulative exposure levels of 40–99 ppm-years, and tended to increase with increasing 

average and cumulative levels of exposure. 

The results of the Pliofilm and NCI/CAPM studies are consistent with epidemiologic studies and case 

reports showing increased incidences of leukemia in shoe factory and rotogravure plant workers exposed 

to high benzene levels during its use as a solvent. No significant increases in leukemia or other 

lymphohematopoietic malignancies were found in chemical industry workers or petroleum industry 

workers exposed to lower levels of benzene. 
BENZENE 15 

2. RELEVANCE TO PUBLIC HEALTH 

Possible associations between occupational exposure to benzene and non-Hodgkin’s lymphoma (NHL) 

and multiple myeloma have been suggested. The risk for mortality from NHL increased with increasing 

level and duration of benzene exposure the NCI/CAPM study. The significance of this finding is unclear 

because NHL mortality was not significantly elevated in the cohort overall, concerns regarding the 

adequacy of the data have been raised, and increases in NHL were not found in other cohort mortality 

studies or in case-control studies of benzene-exposed workers. 

The risk of mortality from multiple myeloma was increased in one of the early assessments of the 

Pliofilm cohort. The implication of this finding is unclear because the risk declined to non-significant 

levels in subsequent follow-up studies, and was not supported by the findings of other cohort mortality 

studies. Additionally, population-based and hospital-based case-control studies indicate that benzene 

exposure is not likely to be causally related to the risk of multiple myeloma. A meta-analysis of casecontrol studies found no significant association between occupational exposure to benzene and benzenecontaining products and risk of multiple myeloma from sources categorized as benzene and/or organic 

solvents, petroleum, or petroleum products. 

Animal studies provide supporting evidence for the carcinogenicity of benzene. Benzene has been shown 

to be a multiple site carcinogen in rats and mice following inhalation and oral exposure. Tumors that 

were increased in rats that were exposed to 200 or 300 ppm benzene by inhalation for 4–7 hours/day, 

5 days/week for up to 104 weeks included carcinomas of the Zymbal gland and oral cavity. Mice that 

were exposed to 100 or 300 ppm benzene for 6 hours/day, 5 days/week for 16 weeks and observed for 

18 months or life developed a variety of tumors, including thymic lymphomas, myelogenous leukemias, 

and Zymbal gland, ovarian, and lung tumors. 

In oral bioassays conducted by the National Toxicology Program, benzene was administered to rats and 

mice by gavage at dose levels of 25–200 mg/kg/day on 5 days/week for 103 weeks. Tumors that were 

induced in the rats included Zymbal gland carcinomas and squamous cell papillomas and carcinomas of 

the oral cavity and skin. In the mice, benzene caused tumors that included malignant lymphomas, 

Zymbal gland carcinomas, lung alveolar/bronchiolar adenomas and carcinomas, Harderian gland 

adenomas, preputial gland squamous cell carcinomas, and mammary gland carcinomas. Similar effects 

occurred in rats exposed to 50–500 mg/kg/day benzene by gavage on 4–5 days/week for up to 104 weeks 

and observed for life; induced tumors included carcinomas of the Zymbal gland, oral cavity, forestomach, 
BENZENE 16 

2. RELEVANCE TO PUBLIC HEALTH 

nasal cavity, and skin. Mice that were similarly exposed to 500 mg/kg/day for 52 or 78 weeks developed 

Zymbal gland carcinomas, mammary carcinomas, and lung adenomas. 

Application of benzene to the skin of animals has not produced evidence of carcinogenicity, although 

most of the dermal studies were inadequate for cancer evaluation. Many dermal carcinogenicity studies 

of other chemicals used benzene as a vehicle and treated large numbers of control animals (mice) with 

benzene alone. None of these studies indicated that benzene induced skin tumors; however, all possible 

tumor sites usually were not examined. 

EPA, IARC, and the Department of Health and Human Services have concluded that benzene is a human 

carcinogen. The Department of Health and Human Services determined that benzene is a known 

carcinogen based on human evidence showing a causal relationship between exposure to benzene and 

cancer. Two studies classified benzene in Group 1 (carcinogenic to humans) based on sufficient evidence 

in both humans and animals. EPA classified benzene in Category A (known human carcinogen) based on 

convincing evidence in humans supported by evidence from animal studies. Under EPA’s most recent 

guidelines for carcinogen risk assessment, benzene is characterized as a known human carcinogen for all 

routes of exposure based on convincing human evidence as well as supporting evidence from animal 

studies. Based on human leukemia data, EPA derived a range of inhalation unit risk values of 2.2x10-6– 

7.8x10-6 (μg/m3

)

-1 for benzene. For risks ranging from 1x10-4 to 1x10-7, the corresponding air 

concentrations range from 13.0–45.0 μg/m3

 (4–14 ppb) to 0.013–0.045 μg/m3 (0.004–0.014 ppb), 

respectively. 

The consensus conclusion that benzene is a human carcinogen is based on sufficient inhalation data in 

humans supported by animal evidence, including the oral studies in animals. The human cancer induced 

by inhalation exposure to benzene is predominantly acute nonlymphocytic (myelocytic) leukemia, 

whereas benzene is a multiple site carcinogen in animals by both the inhalation and oral routes. Due to 

the lack of oral carcinogenicity data in humans, as well as the lack of a well-demonstrated and 

reproducible animal model for leukemia from benzene exposure, EPA extrapolated an oral slope factor 

from the inhalation unit risk range. The oral slope factor ranges from 1.5x10-2 to 5.5x10-2 (mg/kg/day)-1, 

and for cancer risks from 1x10-4 to 1x10-7, the corresponding dose levels are 6.7x10-3–1.8x10-3 to 

6.7x10-6–1.8x10-6 mg/kg/day, respectively. 

Hematological Effects. Both human and animal studies have shown that benzene exerts toxic effects 

on various parts of the hematological system. All of the major types of blood cells are susceptible 
BENZENE 17 

2. RELEVANCE TO PUBLIC HEALTH 

(erythrocytes, leukocytes, and platelets). In the less severe cases of toxicity, specific deficiencies occur in 

individual types of blood elements. A more severe effect occurs when there is hypoplasia of the bone 

marrow, or hypercellular marrow exhibiting ineffective hematopoiesis so that all types of blood cells are 

found in reduced numbers. This is known as pancytopenia. A biphasic response (i.e., a hyperplastic 

effect in addition to destruction of the bone marrow cells) has been observed. Severe damage to the bone 

marrow involving cellular aplasia is known as aplastic anemia and can occur with prolonged exposure to 

benzene. This condition can lead to leukemia. 

Numerous earlier studies of benzene-exposed workers demonstrated that chronic exposure to benzene air 

concentrations of 10 ppm or more resulted in adverse hematological effects, which increased in severity 

with increasing benzene exposure levels. Animal studies support the findings in humans. Significantly 

reduced counts for all three blood factors (white blood cells [WBCs], red blood cells [RBCs], and 

platelets); and other evidence of adverse effects on blood-forming units (reduced bone marrow cellularity, 

bone marrow hyperplasia and hypoplasia, granulocytic hyperplasia, decreased numbers of colony-forming 

granulopoietic stem cells and erythroid progenitor cells, damaged erythrocytes and erythroblast-forming 

cells) have been observed in animals at benzene concentrations in the range of 10–300 ppm and above. 

Several more recent epidemiological studies have demonstrated hematological effects (including 

significant reductions in WBC, RBC, and platelet counts) in workers chronically exposed to benzene 

levels below 10 ppm, and even as low as 1 ppm or less. Results of one of these studies served as the basis 

for a chronic-duration inhalation MRL for benzene. Other reports demonstrated the lack of clinical signs 

of hematotoxicity following long-term, low-level occupational exposure to benzene levels below 

approximately 0.5 ppm (8-hour time-weighted average [TWA]). These investigators utilized a defined 

range of clinically normal hematological values and compared the prevalence of abnormal results 

between benzene-exposed workers and unexposed controls. The normal range for certain hematological 

parameters is necessarily broad due to large interindividual differences in clinical status. Restricting the 

comparison of benzene-exposed and nonexposed populations to only those values considered clinically 

abnormal or adverse may reduce the sensitivity of a particular study to detect meaningful changes at the 

population level. 

Only one study was found that described hematological effects in humans after oral exposure to benzene. 

No reports describing hematological effects in humans following direct dermal exposure to benzene were 

found. However, intermediate- and chronic-duration animal studies show that loss of blood elements 

occurs in animals exposed to benzene in drinking water or by gavage at doses as low as 8–25 mg/kg/day. 
BENZENE 18 

2. RELEVANCE TO PUBLIC HEALTH 

Based on information found in the literature, it is reasonable to expect that adverse hematological effects 

might occur in humans after inhalation, oral, or dermal exposure, since absorption of benzene through any 

route of exposure would increase the risk of damage to blood elements. Studies show that the 

hematological system is susceptible to chronic exposure at low levels, so people living in and around 

hazardous waste sites that may be exposed to contaminated air, drinking water, soil, or food may be at an 

increased risk for adverse hematological effects. Deficiencies in various types of blood cells lead to other 

disorders, such as hemorrhagic conditions from a lack of platelets, susceptibility to infection from the lack 

of leukocytes, and increased cardiac output from the lack of erythrocytes. 

Immunological and Lymphoreticular Effects. Benzene has been shown to have adverse 

immunological effects in humans following inhalation exposure for intermediate and chronic durations. 

Adverse immunological effects in animals occur following both inhalation and oral exposure for acute, 

intermediate, and chronic durations. The effects include damage to both humoral (antibody) and cellular 

(leukocyte) responses. Human studies of intermediate and chronic duration have shown that benzene 

causes decreases in the levels of circulating leukocytes in workers at low levels (30 ppm) of exposure and 

decreases in levels of circulating antibodies in workers exposed to benzene at 3–7 ppm. Other studies 

have shown decreases in human lymphocytes and other blood elements after exposure; these effects have 

been seen at occupational exposure levels as low as 1 ppm or less. Animal data support these findings. 

Both humans and rats have shown increases in leukocyte alkaline phosphatase activity. No studies 

regarding effects from oral or dermal exposure in humans were located. However, exposure to benzene 

through ingestion or dermal contact could cause immunological effects similar to those seen after 

inhalation exposure in humans and inhalation and oral exposure in animals. 

Animal studies have also shown that benzene decreases circulating leukocytes and decreases the ability of 

lymphoid tissue to produce the mature lymphocytes necessary to form antibodies. This has been 

demonstrated in animals exposed for acute, intermediate, or chronic periods via the inhalation route. This 

decrease in lymphocyte numbers is reflected in impaired cell-mediated immune functions in mice 

following intermediate inhalation exposure to 100 ppm of benzene. The impaired cellular immunity after 

benzene treatment was observed both in vivo and in vitro. Mice exposed to 100 ppm for a total of 

100 days were challenged with 104

 polyoma virus-induced tumor cells (PYB6). Nine of 10 mice had 

reduced tumor resistance resulting in the development of lethal tumors. In the same study, lymphocytes 

were obtained from spleens of benzene-treated mice and tested for their immune capacity in vitro. The 

results showed that two other immune functions, alloantigen response (capacity to respond to foreign 
BENZENE 19 

2. RELEVANCE TO PUBLIC HEALTH 

antigens) and cytotoxicity, were also impaired. Similar effects were noted in mice exposed to benzene via 

the oral route for intermediate time periods, and in rats and mice exposed for chronic time periods. A 

decrease in spleen weight was observed in mice after acute-duration exposure to benzene at 25 ppm, the 

same dose levels at which a decrease in circulating leukocytes was observed. Similar effects on spleen 

weight and circulating leukocytes were observed in mice exposed to 12 ppm benzene 2 hours/day for 

30 days. The acute-duration inhalation MRL was based on a study showing decreased mitogen-induced 

blastogenesis of B-lymphocytes following exposure of mice to benzene vapors at a concentration of 

10 ppm, 6 hours/day for 6 days. The intermediate-duration inhalation MRL was based on a study 

showing delayed splenic lymphocyte reaction to foreign antigens evaluated by in vitro mixed lymphocyte 

culture following exposure of mice to benzene vapors at a concentration of 10 ppm, 6 hours/day, 

5 days/week for a total of 20 exposures. 

Based on information found in the literature, it is reasonable to expect that adverse immunological effects 

might occur in humans after inhalation, oral, or dermal exposure, since absorption of benzene through any 

route of exposure would increase the risk of damage to the immunological system. Studies show that the 

immunological system is susceptible to chronic exposure at low levels, so people living in and around 

hazardous waste sites who may be exposed to contaminated air, drinking water, soil, or food may be at an 

increased risk for adverse immunological effects. 

Neurological Effects. In humans, results of occupational studies indicate that there is a cause-andeffect relationship between acute inhalation of very high concentrations of benzene and symptoms 

indicative of central nervous system toxicity. These symptoms, observed following both acute nonlethal 

and lethal exposures, include drowsiness, dizziness, headache, vertigo, tremor, delirium, and loss of 

consciousness. These symptoms are reversible when symptomatic workers are transferred from the 

problem area. Comparable toxicity in humans has been reported following ingestion of benzene at doses 

of 125 mg/kg and above. Occupational exposure to benzene has also been reported to produce 

neurological abnormalities in humans. Electromyographical and motor conduction velocity examinations 

were conducted on six patients with aplastic anemia, all of whom worked in environments where 

adhesives containing benzene were used (in one case, air concentrations bracketed around 210 ppm). 

Abnormalities in motor conduction velocity were noted in four of the six pancytopenic individuals and 

were thought to result from a direct effect of benzene on the peripheral nerves and/or spinal cord. 

In its acute stages, benzene toxicity appears to be due primarily to the direct effects of benzene on the 

central nervous system, whereas the peripheral nervous system appears to be the target following chronic 
BENZENE 20 

2. RELEVANCE TO PUBLIC HEALTH 

low-level exposures. In addition, because benzene may induce an increase in brain catecholamines, it 

may also have a secondary effect on the immune system via the hypothalamus-pituitary-adrenal axis. 

Increased metabolism of catecholamines can result in increased adrenal corticosteroid levels, which are 

immunosuppressive. 

Animal studies provide additional support that benzene affects the nervous system following acute 

inhalation and oral exposures, albeit at extremely high acute exposure levels. Effects reported include 

narcosis, nervous system depression, tremors, and convulsions. Acute and intermediate inhalation 

exposures have also been reported to produce adverse neurological effects in animals including a 

reduction in hind-limb grip strength and evoked electrical activity in the brain, and behavioral 

disturbances. Effects of benzene on learning were investigated in male hooded rats of the SpragueDawley strain given 550 mg/kg of benzene in corn oil or corn oil without benzene, intraperitoneally, 

on days 9, 11, and 13 postpartum. The rats exposed to benzene exhibited a significantly impaired 

learning ability when tested on problems of the closed-field, maze-learning task. This sign of 

neurotoxicity was not observed in control animals. In another study, 47-day-old juvenile cotton rats were 

maintained on one of two isocaloric diets containing either 4 or 16% crude protein for a 26-day 

experimental period. Animals were treated intraperitoneally with either 0 (corn oil), 100, 500, or 

1,000 mg/kg benzene in corn oil for 3 consecutive days. The first dose was administered on days 15– 

17 of the experimental period. Animals were terminated on day 27. During the experimental period, 

severe loss of coordination was observed in some rats on the low protein diet immediately after exposure 

to benzene, but this subsided. 

Intermediate oral exposures resulted in changes in the levels of monoamine transmitters in the brain 

without treatment-related behavioral changes. Mice exposed to 3 ppm for 2 hours/day for 30 days 

exhibited increased levels of acetylcholinesterase in the brain. In vitro studies suggest that benzene may 

have a direct effect on brain cells. Primary astrocyte cultures prepared from neonatal rat cerebella were 

treated with 3, 6, or 9 mmol/L benzene for 1 hour. ATPase and Mg2+-ATPase activity were inhibited in a 

dose-related manner, and were detected at 78–92% of control values for ATPase, and 60–74% of control 

values for Mg2+-ATPase. 

These data suggest that humans exposed to benzene in the occupational setting for acute, intermediate, or 

chronic durations via the inhalation and oral routes are at risk of developing neurological effects. 

However, benzene levels in ambient air, drinking water, and at hazardous waste sites are lower and not 

likely to be of concern. 
BENZENE 21 

2. RELEVANCE TO PUBLIC HEALTH 

2.3 MINIMAL RISK LEVELS (MRLs) 

Estimates of exposure levels posing minimal risk to humans (MRLs) have been made for benzene. An 

MRL is defined as an estimate of daily human exposure to a substance that is likely to be without an 

appreciable risk of adverse effects (noncarcinogenic) over a specified duration of exposure. MRLs are 

derived when reliable and sufficient data exist to identify the target organ(s) of effect or the most sensitive 

health effect(s) for a specific duration within a given route of exposure. MRLs are based on 

noncancerous health effects only and do not consider carcinogenic effects. MRLs can be derived for 

acute, intermediate, and chronic duration exposures for inhalation and oral routes. Appropriate 

methodology does not exist to develop MRLs for dermal exposure. 

Although methods have been established to derive these levels (Barnes and Dourson 1988; EPA 1990), 

uncertainties are associated with these techniques. Furthermore, ATSDR acknowledges additional 

uncertainties inherent in the application of the procedures to derive less than lifetime MRLs. As an 

example, acute inhalation MRLs may not be protective for health effects that are delayed in development 

or are acquired following repeated acute insults, such as hypersensitivity reactions, asthma, or chronic 

bronchitis. As these kinds of health effects data become available and methods to assess levels of 

significant human exposure improve, these MRLs will be revised. 

Inhalation MRLs 

•	 An MRL of 0.009 ppm has been derived for acute-duration inhalation exposure (14 days or less) 

to benzene. 

The acute-duration inhalation MRL of 0.009 ppm was derived from a lowest-observed-adverse-effect 

level (LOAEL) value of 10.2 ppm for reduced lymphocyte proliferation following mitogen stimulation in 

mice (Rozen et al. 1984). The concentration was adjusted for intermittent exposure by multiplying the 

LOAEL (10.2 ppm) by 6 hours/24 hours to correct for less than a full day of exposure. The resulting 

adjusted LOAEL, 2.55 ppm, was then converted to a human equivalent concentration (HEC) according to 

EPA (1994b) methodology for calculating a HEC for extrarespiratory effects of a category 3 gas (such as 

benzene) as follows: 

LOAELHEC = LOAELADJ x ([Hb/g]A/[Hb/g]H) 
BENZENE 22 

2. RELEVANCE TO PUBLIC HEALTH 

where: 

LOAELHEC = the LOAEL dosimetrically adjusted to a human equivalent concentration 

LOAELADJ = the LOAEL adjusted from intermittent to continuous exposure 

[Hb/g]A/[Hb/g]H = the ratio of the blood:gas partition coefficient of the chemical for the laboratory animal 

species to the human value 

If the animal blood:gas partition coefficient is greater than the human blood:gas partition coefficient, a 

default value of 1 is used for the ratio. According to Wiester et al. (2002), blood:gas partition coefficients 

for benzene in mice and humans are 17.44 and 8.12, respectively. Therefore, the default value of 1 is 

applied, in which case, the LOAEL[HEC] is equivalent to the LOAEL[ADJ]. 

Therefore: 

LOAELHEC = LOAELADJ = 2.55 ppm 

The resulting LOAEL(HEC) of 2.55 ppm was then divided by an uncertainty factor of 300 (10 for the use of 

LOAEL, 3 for extrapolation from animals to humans using dosimetric conversion, and 10 for human 

variability) to yield the MRL value of 0.009 ppm (see Appendix A). An increased number of 

micronucleated polychromatic erythrocytes (MN-PCEs), decreased numbers of granulopoietic stem cells 

(Toft et al. 1982), lymphopenia (Cronkite et al. 1985), lymphocyte depression, and increased 

susceptibility to bacterial infection (Rosenthal and Snyder 1985) are among the adverse hematological 

and immunological effects observed in several other acute-duration inhalation studies. The study by 

Rozen et al. (1984) shows benzene immunotoxicity (reduced mitogen-induced lymphocyte proliferation) 

at a slightly lower exposure level than these other studies. C57BL/6J mice were exposed to 0, 10.2, 31, 

100, and 301 ppm benzene for 6 hours/day for 6 days. Control mice were exposed to filtered, conditioned 

air only. Lymphocyte counts were depressed at all exposure levels; erythrocyte counts were elevated at 

10.2 ppm, equal to controls at 31 ppm, and depressed at 100 and 301 ppm. Femoral B-lymphocyte and 

splenic B-lymphocyte numbers were reduced at 100 ppm. Levels of circulating lymphocytes and 

mitogen-induced blastogenesis of femoral B-lymphocytes were depressed after exposure to 10.2 ppm 

benzene for 6 days. Mitogen-induced blastogeneses of splenic T-lymphocytes were depressed after 

exposure to 31 ppm of benzene for 6 days. In another study, mice exhibited a 50% decrease in the 
BENZENE 23 

2. RELEVANCE TO PUBLIC HEALTH 

population of colony-forming granulopoietic stem cells (CFU-E) after exposure to 10 ppm benzene for 

6 hours/day for 5 days (Dempster and Snyder 1991). In a study by Wells and Nerland (1991), groups of 

4–5 male Swiss-Webster mice were exposed to 3, 25, 55, 105, 199, 303, 527, 1,150, or 2,290 ppm 

benzene for 6 hours/day for 5 days. The number of leukocytes in peripheral blood and spleen weights 

were significantly decreased compared with untreated controls at all concentrations >25 ppm. Therefore, 

3 ppm was the no-observed-adverse-effect level (NOAEL) and 25 ppm was the LOAEL for these effects. 

These data support the choice of Rozen et al. (1984) as the study from which to derive the MRL. 

•	 An MRL of 0.006 ppm has been derived for intermediate-duration inhalation exposure (15– 

364 days) to benzene. 

The intermediate-duration inhalation MRL of 0.006 ppm was derived from a LOAEL value of 10 ppm for 

significantly delayed splenic lymphocyte reaction to foreign antigens evaluated in in vitro mixed 

lymphocyte reaction following the exposure of male C57Bl/6 mice to benzene vapors 6 hours/day, 

5 days/week for 20 exposure days (Rosenthal and Snyder 1987). The concentration was adjusted for 

intermittent exposure by multiplying the LOAEL (10 ppm) by 6 hours/24 hours to correct for less than a 

full day of exposure and by 5 days/7days to correct for less than a full week of exposure. The resulting 

adjusted LOAEL, 1.8 ppm, was then converted to a HEC according to EPA (1994b) methodology for 

calculating a HEC for extrarespiratory effects of a category 3 gas (such as benzene) as follows: 

LOAELHEC = LOAELADJ x ([Hb/g]A/[Hb/g]H) 

where: 

LOAELHEC = the LOAEL dosimetrically adjusted to a human equivalent concentration 

LOAELADJ = the LOAEL adjusted from intermittent to continuous exposure 

[Hb/g]A/[Hb/g]H = the ratio of the blood:gas partition coefficient of the chemical for the laboratory animal 

species to the human value 

If the animal blood:gas partition coefficient is greater than the human blood:gas partition coefficient, a 

default value of 1 is used for the ratio. According to Wiester et al. (2002), blood:gas partition coefficients 

for benzene in mice and humans are 17.44 and 8.12, respectively. Therefore, the default value of 1 is 

applied, in which case, the LOAEL[HEC] is equivalent to the LOAEL[ADJ]. 
BENZENE 24 

2. RELEVANCE TO PUBLIC HEALTH 

Therefore: 

LOAELHEC = LOAELADJ = 1.8 ppm 

The resulting LOAEL(HEC) of 1.8 ppm was then divided by an uncertainty factor of 300 (10 for the use of 

LOAEL, 3 for extrapolation from animals to humans using dosimetric conversion, and 10 for human 

variability) to yield the MRL value of 0.006 ppm (see Appendix A). 

Results of several studies support the choice of Rosenthal and Snyder (1987) as the basis for the 

intermediate-duration inhalation MRL for benzene (Baarson et al. 1984; Green et al. 1981a, 1981b), 

although the supporting studies employed a single exposure level, which precluded consideration as 

critical studies for MRL derivation. Exposure of C57BL mice to 10 ppm benzene for 6 hours/day, 

5 days/week caused significant depressions in numbers of lymphocytes (ca. 30% lower than controls) as 

early as exposure day 32; this effect was also noted at the other scheduled periods of testing (exposure 

days 66 and 178) (Baarson et al. 1984). Splenic RBCs were significantly reduced (ca. 15% lower than 

controls) at exposure days 66 and 178. The failure of the erythrons of benzene-exposed mice to support 

normal red cell mass was illustrated by the significant reduction in peripheral red cell numbers in these 

animals at 66 and 178 days of benzene exposure. Green et al. (1981a, 1981b) exposed male CD-1 mice to 

benzene vapors at concentrations of 0 or 9.6 ppm for 6 hours/day, 5 days/week for 50 days and assessed 

the effects of exposure on cellularity in the spleen, bone marrow, and peripheral blood. Exposure-related 

effects included a 90% increase in numbers of multipotential hematopoietic stem cells (CFU-S) (Green et 

al. 1981a), approximately 25% increase in spleen weight and total splenic nucleated cellularity (Green et 

al. 1981b), and 80% increase in nucleated RBCs (Green et al. 1981b). 

One intermediate-duration inhalation study reported increased rapid response to an electrical shock in 

mice at 0.78 ppm (Li et al. 1992). However, this study was not selected as the critical study for deriving 

an intermediate-duration inhalation MRL for benzene due to apparent discrepancies between reported and 

actual benzene exposure levels. Other animal studies identified neurological effects only at much higher 

exposure levels (Carpenter et al. 1944; Dempster et al. 1984; Evans et al. 1981; Frantik et al. 1994; Green 

et al. 1978). 

•	 An MRL of 0.003 ppm has been derived for chronic-duration inhalation exposure (365 days or 

more) to benzene. 
BENZENE 25 

2. RELEVANCE TO PUBLIC HEALTH 

This MRL is based on statistically significantly decreased counts of B-lymphocytes in workers of shoe 

manufacturing industries in Tianjin, China (Lan et al. 2004a, 2004b), using a benchmark dose (BMD) 

analysis. The 250 benzene-exposed workers had been employed for an average of 6.1±2.9 years. 

Controls consisted of 140 age- and gender-matched workers in clothing manufacturing facilities in which 

measurable benzene concentrations were not found (detection limit 0.04 ppm). Benzene exposure was 

monitored by individual organic vapor monitors (full shift) 5 or more times during 16 months prior to 

phlebotomy. Benzene-exposed workers were categorized into four groups (140 controls, 109 at <1 ppm, 

110 at 1–<10 ppm, and 31 at ≥10 ppm) according to mean benzene exposure levels measured during 

1 month prior to phlebotomy. Complete blood count (CBC) and differential were analyzed mechanically. 

Coefficients of variation for all cell counts were <10%. 

Mean 1-month benzene exposure levels in the four groups (controls, <1 ppm, 1–<10 ppm, and ≥10 ppm) 

were <0.04, 0.57±0.24, 2.85±2.11, and 28.73±20.74 ppm, respectively. Hematological values were 

adjusted to account for potential confounding factors (i.e., age, gender, cigarette smoking, alcohol 

consumption, recent infection, and body mass index). All types of WBCs and platelets were significantly 

decreased in the lowest exposure group (<1 ppm), ranging in magnitude from approximately 8 to 15% 

lower than controls. Although similar statistical analyses for the mid- and high-exposure groups were not 

included in the study report, decreases in all types of WBCs and platelets were noted at these exposure 

levels as well; the decreases in the highest exposure group ranged in magnitude from 15 to 36%. 

Lymphocyte subset analysis revealed significantly decreased CD4+-T cells, CD4+/CD8+ ratio, and 

B cells. Hemoglobin concentrations were significantly decreased only within the highest (≥10 ppm) 

exposure group. Tests for a linear trend using benzene air level as a continuous variable were significant 

for platelets and all WBC measures except monocytes and CD8+-T cells. Upon restricting the linear 

trend analyses to workers exposed to <10 ppm benzene, excluding controls, inverse associations remained 

for total WBCs, granulocytes, lymphocytes, B cells, and platelets. In order to evaluate the effect of past 

benzene exposures on the hematological effects observed in this study, the authors compared findings for 

a group of workers who had been exposed to <1 ppm benzene over the previous year (n=60) and a subset 

who also had <40 ppm-years lifetime cumulative benzene exposure (n=50). The authors stated that the 

same cell types were significantly reduced in these groups, but did not provide further information of the 

magnitude (i.e., percent change) of the hematological effects observed. These data suggest that the 

1-month benzene exposure results could be used as an indicator of longer-term, low-level benzene 

hematotoxicity. To demonstrate that the observed effects were attributable to benzene, significantly 

decreased levels of WBCs, granulocytes, lymphocytes, and B cells were noted in a subgroup of the 

<1-ppm group for which exposure to other solvents was negligible. 
BENZENE 26 

2. RELEVANCE TO PUBLIC HEALTH 

As shown in Table A-1 of Appendix A, exposure-response relationships were noted for several blood 

factors. Benzene-induced decreased B cell count was selected as the critical effect for BMD modeling 

because it represented the highest magnitude of effect (i.e., B cell count in the highest exposure group was 

approximately 36% lower than that of controls). A BMD modeling approach was selected to identify the 

point of departure because the critical study (Lan et al. 2004a, 2004b) identified a LOAEL in the absence 

of a NOAEL. 

As discussed in detail in Appendix A, the BMD analysis selected a point of departure (BMCL0.25sd) of 

0.1 ppm, which was adjusted from the 8-hour TWA to a continuous exposure concentration 

(BMCL0.25sdADJ) using the default occupational minute volume (EPA 1994b). The resulting BMCL0.25sdADJ 

of 0.03 ppm was divided by an uncertainty factor of 10 (for human variability) to yield the chronicduration inhalation MRL value of 0.003 ppm. 

Several recent epidemiological studies provide supporting information to the Lan et al. (2004a, 2004b) 

findings of hematotoxicity in workers chronically exposed to relatively low levels of benzene (Qu et al. 

2002, 2003a, 2003b; Rothman et al. 1996a, 1996b; Ward et al. 1996). Qu et al. (2002, 2003a, 2003b) 

compared hematologic values among 105 healthy workers (51 men, 54 women) in industries with a 

history of benzene usage (Tianjin, China) and 26 age-and gender-matched workers in industries that did 

not use benzene. A LOAEL of 2.26 ppm was identified for significantly reduced total WBCs, 

neutrophils, and RBCs; there was also an indication of benzene-induced changes in some hematological 

values at exposure levels lower than the current industry 8-hour TWA of 1 ppm. Rothman et al. (1996a, 

1996b) identified an 8-hour TWA LOAEL of 7.6 ppm for a group of 11 benzene-exposed workers in a 

cross-sectional study of 44 healthy workers in Chinese (Shanghai) industries with a history of benzene 

usage and age- and gender-matched workers in industries that did not use benzene. In a nested casecontrol study of a cohort of workers in the Pliofilm production departments of a rubber products 

manufacturer in Ohio (Ward et al. 1996), a strong exposure-response relationship was correlated with low 

WBC count. A weak positive exposure-response relationship was observed for RBCs, which was 

significant for cumulative exposure up until the blood test date. The study authors noted that there was no 

evidence for a threshold for hematologic effects and suggested that exposure to benzene levels <5 ppm 

may result in hematologic suppression. 
BENZENE 27 

2. RELEVANCE TO PUBLIC HEALTH 

Oral MRLs 

No acute-duration oral MRL was derived due to a lack of appropriate data on the effects of acute oral 

exposure to benzene. 

In a study by Thienes and Haley (1972), central nervous system symptoms of giddiness, vertigo, muscular 

incoordination, unconsciousness, and death of humans have been reported when doses of ≥125 mg 

kg/body weight benzene were ingested. The only lower doses in the acute oral database are 

50 mg/kg/day, at which pregnant rats experienced alopecia of hindlimbs and trunk (Exxon 1986), an end 

point not suitable for MRL derivation for benzene because the toxicological significance is not clear; and 

88 mg/kg/day, a concentration resulting in slight non-dose-related central nervous system depression in 

rats (Cornish and Ryan 1965). 

No intermediate-duration oral MRL was derived. Examination of the literature indicated that studies by 

Hsieh et al. (1988b, 1991) and Wolf et al. (1956) should be considered for derivation of an intermediateduration oral MRL. Adult male CD-1 mice were exposed to 0, 8, 40, or 180 mg/kg/day benzene in the 

drinking water for 4 weeks (Hsieh et al. 1988b, 1991). Serious hematological and immunological effects 

(leukopenia, erythrocytopenia, lymphopenia, enhanced splenic lymphocyte proliferation) occurred at the 

lowest dose, precluding its use for MRL derivation. In the study by Wolf et al. (1956), a NOAEL of 

1 mg/kg/day and a LOAEL of 50 mg/kg/day for leukopenia were identified in rats given benzene in oil by 

gavage for 6 months. However, closer examination of the study by Wolf et al. (1956) revealed that the 

results were not adequately supported by the data and analysis presented in the paper. Therefore, Wolf et 

al. (1956) was not selected for derivation of the MRL. 

•	 An MRL of 0.0005 mg/kg/day has been derived for chronic-duration oral exposure (365 days or 

more) to benzene. 

No human data are available to evaluate hematological effects following oral exposure to benzene. No 

adequate oral data were located from which a chronic-duration oral MRL for benzene could be derived, 

although a 2-year carcinogenesis bioassay of orally-exposed rats and mice is available (NTP 1986). Male 

rats were given 50, 100, or 200 mg/kg/day and female rats and mice of both sexes were given 0, 25, 50, 

and 100 mg/kg/day benzene in corn oil for 2 years. The dose of 25 mg/kg/day was a LOAEL for 

hematotoxicity and immunotoxicity in rats and mice, and is higher than the serious LOAEL of 

8 mg/kg/day in the intermediate-duration database. Therefore, the threshold for hematological and 

immunological effects of benzene was not identified. 
BENZENE 28 

2. RELEVANCE TO PUBLIC HEALTH 

However, results of toxicokinetic studies of inhaled benzene in humans (Nomiyama and Nomiyama 

1974a; Pekari et al. 1992; Srbova et al. 1950) and inhaled and orally-administered benzene in rats and 

mice (Sabourin et al. 1987) indicate that absorption of benzene at relatively low levels of exposure is 

approximately 50% of an inhaled dose and essentially 100% of an oral dose. Based on these assumptions, 

inhalation data can be used to estimate equivalent oral doses that would be expected to similarly affect the 

critical targets of benzene toxicity. Therefore, the point of departure for the chronic-duration inhalation 

MRL for benzene, namely the BMCL0.25sdADJ of 0.03 ppm for decreased B cell counts in benzene-exposed 

workers (Lan et al. 2004a, 2004b), serves as the point of departure for deriving the chronic-duration oral 

MRL as well. 

The point of departure (in ppm) was converted to mg/m3

 using the molecular weight of 78.11 for benzene 

and assuming 25 °C and 760 mm Hg: 

BMCL0.25sdADJ of 0.03 ppm x 78.11/24.45 = 0.096 mg/m3 

The BMCL0.25sdADJ of 0.096 mg/m3

 for inhaled benzene was converted to an equivalent BMDL0.25sdADJ for 

ingested benzene using EPA (1988b) human reference values for inhalation rate (20 m3

/day) and body 

weight (70 kg) and a factor of 0.5 to adjust for differences in absorption of benzene following inhalation 

versus oral exposure (50 versus 100%, respectively) as follows: 

BMDL0.25sdADJ = BMCL0.25sdADJ of 0.096 mg/m3

 x 20 m3/day x 0.5 ÷ 70 kg = 0.014 mg/kg/day 

A total uncertainty factor of 30 (10 for human variability and 3 for uncertainty in route-to-route 

extrapolation) was applied to the BMDL0.25sdADJ of 0.014 mg/kg/day; the resulting chronic-duration oral 

MRL is 0.0005 mg/kg/day. 
BENZENE 29 

3. HEALTH EFFECTS 

3.1 INTRODUCTION 

The primary purpose of this chapter is to provide public health officials, physicians, toxicologists, and 

other interested individuals and groups with an overall perspective on the toxicology of benzene. It 

contains descriptions and evaluations of toxicological studies and epidemiological investigations and 

provides conclusions, where possible, on the relevance of toxicity and toxicokinetic data to public health. 

A glossary and list of acronyms, abbreviations, and symbols can be found at the end of this profile. 

3.2 DISCUSSION OF HEALTH EFFECTS BY ROUTE OF EXPOSURE 

To help public health professionals and others address the needs of persons living or working near 

hazardous waste sites, the information in this section is organized first by route of exposure (inhalation, 

oral, and dermal) and then by health effect (death, systemic, immunological, neurological, reproductive, 

developmental, genotoxic, and carcinogenic effects). These data are discussed in terms of three exposure 

periods: acute (14 days or less), intermediate (15–364 days), and chronic (365 days or more). 

Levels of significant exposure for each route and duration are presented in tables and illustrated in 

figures. The points in the figures showing no-observed-adverse-effect levels (NOAELs) or lowestobserved-adverse-effect levels (LOAELs) reflect the actual doses (levels of exposure) used in the studies. 

LOAELs have been classified into "less serious" or "serious" effects. "Serious" effects are those that 

evoke failure in a biological system and can lead to morbidity or mortality (e.g., acute respiratory distress 

or death). "Less serious" effects are those that are not expected to cause significant dysfunction or death, 

or those whose significance to the organism is not entirely clear. ATSDR acknowledges that a 

considerable amount of judgment may be required in establishing whether an end point should be 

classified as a NOAEL, "less serious" LOAEL, or "serious" LOAEL, and that in some cases, there will be 

insufficient data to decide whether the effect is indicative of significant dysfunction. However, the 

Agency has established guidelines and policies that are used to classify these end points. ATSDR 

believes that there is sufficient merit in this approach to warrant an attempt at distinguishing between 

"less serious" and "serious" effects. The distinction between "less serious" effects and "serious" effects is 

considered to be important because it helps the users of the profiles to identify levels of exposure at which 

major health effects start to appear. LOAELs or NOAELs should also help in determining whether or not 
BENZENE 30 

3. HEALTH EFFECTS 

the effects vary with dose and/or duration, and place into perspective the possible significance of these 

effects to human health. 

The significance of the exposure levels shown in the Levels of Significant Exposure (LSE) tables and 

figures may differ depending on the user's perspective. Public health officials and others concerned with 

appropriate actions to take at hazardous waste sites may want information on levels of exposure 

associated with more subtle effects in humans or animals (LOAELs) or exposure levels below which no 

adverse effects (NOAELs) have been observed. Estimates of levels posing minimal risk to humans 

(Minimal Risk Levels or MRLs) may be of interest to health professionals and citizens alike. 

Levels of exposure associated with carcinogenic effects (Cancer Effect Levels, CELs) of benzene are 

indicated in Tables 3-1 and 3-2 and Figures 3-1 and 3-2. Because cancer effects could occur at lower 

exposure levels, Figures 3-1 and 3-2 also shows a range for the upper bound of estimated excess risks, 

ranging from a risk of 1 in 10,000 to 1 in 10,000,000 (10-4 to 10-7), as developed by EPA. 

A User's Guide has been provided at the end of this profile (see Appendix B). This guide should aid in 

the interpretation of the tables and figures for Levels of Significant Exposure and the MRLs. 

3.2.1 Inhalation Exposure 

Although occupational or environmental exposure to benzene or benzene-containing materials may 

include inhalation, oral, and dermal exposure routes, the inhalation and dermal routes are usually of 

primary concern in such scenarios. Data regarding occupational or environmental exposure in which 

inhalation is considered to have been the primary exposure route are summarized in this section 

(Section 3.2.1). Information regarding adverse health effects following oral or dermal exposure to 

benzene or benzene-containing materials is summarized in Sections 3.2.2 and 3.2.3, respectively. 

3.2.1.1 Death 

Case reports of fatalities due to acute benzene exposures have appeared in the literature since the early 

1900s (Cronin 1924; Greenburg 1926; Hamilton 1922). Deaths occurred suddenly or within several hours 

after exposure (Avis and Hutton 1993; Cronin 1924; Greenburg 1926; Hamilton 1922; Winek et al. 1967). 

The benzene concentrations encountered by the victims were not often known. However, it has been 

estimated that 5–10 minutes of exposure to 20,000 ppm benzene in air is usually fatal (Flury 1928). 

Lethality in humans has been attributed to asphyxiation, respiratory arrest, central nervous system 
BENZENE 31 

3. HEALTH EFFECTS 

depression, or suspected cardiac collapse (Avis and Hutton 1993; Hamilton 1922; Winek and Collom 

1971; Winek et al. 1967). Cyanosis, hemolysis, and congestion or hemorrhage of organs were reported in 

the cases for which there were autopsy reports (Avis and Hutton 1993; Greenburg 1926; Hamilton 1922; 

Winek et al. 1967). No studies were located regarding noncancer-related mortality in humans following 

long-term inhalation exposure to benzene. Cancer-related mortality data for chronic-duration human 

occupational exposure to benzene are presented in Section 3.2.1.7. 

In animals, acute inhalation exposure to high concentrations of benzene has caused death. An inhalation 

LC50 value for rats was calculated as 13,700 ppm for a 4-hour exposure (Drew and Fouts 1974). 

Additionally, 4 of 6 rats died following a 4-hour exposure to 16,000 ppm benzene (Smyth et al. 1962). 

However, in a study by Green et al. (1981b), male CD-1 mice exposed by inhalation to doses of benzene 

up to 4,862 ppm, 6 hours/day for 5 days showed no lethality. Lower doses (up to 400 ppm) for longer 

periods of time (2 weeks) did not cause death in mice (Cronkite et al. 1985). Lethality in monkeys and 

cats exposed to unspecified concentrations has been ascribed to ventricular fibrillation due to increased 

release of adrenaline (Nahum and Hoff 1934). Exposure of rabbits to 45,000 ppm of benzene for 

approximately 30 minutes caused narcosis that was followed by the death of all exposed animals 

(Carpenter et al. 1944). Furthermore, early deaths of rats and mice have occurred from intermediate and 

chronic exposure to air concentrations of 200 or 300 ppm of benzene in cancer studies (Cronkite et al. 

1989; Farris et al. 1993; Maltoni et al. 1982a, 1983). Intermediate exposures (6 hours/day, 5 days/week 

for 50 days) of male CD-1 mice to benzene at doses of 9.6 ppm caused no increase in mortality, although 

mice exposed to 302 ppm benzene under the same regimen for a total of 26 weeks showed mortality 

approaching 50% (Green et al. 1981b). Mortality was observed in 97% of the CBA/Ca mice exposed to 

300 ppm benzene for 16 weeks, as compared to 20% mortality in sham-exposed mice (Cronkite 1986). In 

Sprague-Dawley rats that received 300 ppm benzene vapor for 6 hours/day, 5 days/week for 691 days, the 

calculated median survival time was shown to be 51 weeks as compared to 65 weeks for controls (Snyder 

et al. 1978a). However, Snyder et al. (1984) reported a median survival time of 546 days for male 

Sprague-Dawley rats exposed to 100 ppm benzene for 5 days/week, 6 hours/day for life, compared to 

560 days for air-exposed controls. It is not clear whether the difference in survival was significant or due 

to benzene exposure since both controls and exposed rats experienced early mortality from respiratory 

infections. Companion studies were also conducted with AKR and C57BL mice exposed to 300 ppm 

benzene (Snyder et al. 1978a, 1980). The calculated median survival time for AKR and C57BL mice 

exposed to 300 ppm benzene was shown to be 11 and 41 weeks compared to 39 and 75 weeks, 

respectively, for controls. For AKR mice exposed to 100 ppm, the calculated median survival time was 

39 weeks (number of deaths not shown) as compared to 41 weeks for controls (Snyder et al. 1980). 
BENZENE 32 

3. HEALTH EFFECTS 

The LC50 value and all reliable LOAEL values for each species and duration category are recorded in 

Table 3-1 and plotted in Figure 3-1. 

3.2.1.2 Systemic Effects 

No studies were located regarding endocrine, metabolic, or body weight effects in humans or 

gastrointestinal, musculoskeletal, endocrine, metabolic, or dermal effects in animals following inhalation 

exposure to benzene. Available data pertaining to systemic effects are presented below. 

The highest NOAEL values and all reliable LOAEL values for systemic effects in each species and 

duration category are recorded in Table 3-1 and plotted in Figure 3-1. 

Respiratory Effects. Respiratory effects have been reported in humans after acute exposure to 

benzene vapors (Avis and Hutton 1993; Midzenski et al. 1992; Winek and Collom 1971; Winek et al. 

1967; Yin et al. 1987b). Fifteen male workers employed in removing residual fuel from shipyard tanks 

were evaluated for benzene exposure (Midzenski et al. 1992). Mucous membrane irritation was noted in 

80% and dyspnea was noted in 67% of the workers at occupational exposures of >60 ppm for up to 

3 weeks. Nasal irritation and sore throat were reported by male and female workers exposed to 33 and 

59 ppm benzene, respectively, for more than 1 year (Yin et al. 1987b). After a fatal occupational 

exposure to benzene vapors on a chemical cargo ship for only minutes, autopsy reports on three victims 

revealed hemorrhagic, edematous lungs (Avis and Hutton 1993). Acute granular tracheitis, laryngitis, 

bronchitis, and massive hemorrhages of the lungs were observed at autopsy of an 18-year-old male who 

died of benzene poisoning after intentional inhalation of benzene (Winek and Collom 1971). Similarly, 

acute pulmonary edema was found during the autopsy of a 16-year-old who died after sniffing glue 

containing benzene (Winek et al. 1967). 

Snyder et al. (1978a, 1984) reported no treatment-related effects on lung tissue in male Sprague-Dawley 

rats exposed to 0, 100, or 300 ppm benzene 5 days/week, 6 hours/day for life. In addition, no adverse 

histopathological effects on lung tissue were observed in AKR/J or C57BL/65 mice exposed to 300 ppm 

benzene for life (Snyder et al. 1978a, 1980). 
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation 

Exposure/ 

Duration/ 

a Frequency Key to Species (Route) Figure (Strain) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

ACUTE EXPOSURE Death 

1 Human 1 d 

5-10 min 20000 (death) 20000 Flury 1928 

12

2 Rat 4 hr 

(Sprague- 13700 (LC50) 13700 Drew and Fouts 1974 

Dawley) 

14

3 Rat 4 hr 

(NS) 16000 (4/6 died) 16000 Smyth et al. 1962 

13

4 Rabbit 3.7-36.2 min 

(NS) 45000 (death in 36.2 min) 45000 Carpenter et al. 1944 

226

Systemic 

5 Human 1-21 d 

2.5-8 hr/d Resp 60 M (mucous membrane irritation, dyspnea) Midzenski et al. 1992 

60

419

Hemato 60 M (leukopenia, anemia, 

thrombocytopenia, MCV elevation) 

60

Dermal 60 M (skin irritation) 

6 Rat Gd 6-15 

6 hr/d (SpragueDawley) 

60

Bd Wt 300 F 2200 F (decreased maternal 

300 body weight) 

Green et al. 1978 

2200

815

7 Rat Gd 6-15 

7 hr/d (SpragueDawley) Bd Wt 10 F 50 F (decreased maternal 10 body weight and weight gain) Kuna and Kapp 1981 

50

818
a 

Key to 

Figure 

8 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Species 

(Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

Rat (Wistar) 

910

7 d 8 hr/d Hemato 50 F 100 F (leukopenia) 50 100 Li et al. 1986 

9 Rat 

(Wistar) 

423

15 min Cardio 3526 M (ventricular arrhythmia) 

3526

Magos et al. 1990 

10 Rat 

(CFY) 

696

Gd 7-14 24 hr/d Hepatic 125 F 125 Tatrai et al. 1980a 

Bd Wt 125 F (decreased maternal 

weight gain of <22.08% of controls) 

125

11 Rat 

(CFY) 

Gd 7-14 24 hr/d Hepatic 47 F 141 F (increased relative liver 47 weight) Tatrai et al. 1980b 

824

141

Bd Wt 47 F (decreased maternal 

weight gain) 

47

12 Rat 

(SpragueDawley) 

2 wk 5 d/wk 6 hr/d 

Hemato 30 300 (decrease in leukocytes, 

30 males; decrease in 

lymphocytes) 

Ward et al. 1985 

951

300

13 Mouse 

(BALB/c) 

670

7 d 6 hr/d Hemato 47 M 211 M (depressed WBC count) 47 211 Aoyama 1986 

Bd Wt 47 M 211 M (16% decrease in body 

47 weight) 

211
a 

Key to 

Figure 

14 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Exposure/ 

Duration/ 

Frequency Species (Route) (Strain) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

Mouse 14 d 

6 hr/d (BALB/c) 

671

Hemato 48 M (depressed WBC count) 

48

Aoyama 1986

Bd Wt 48 M 208 M (18% decrease in body 

48 weight) 

208

15 Mouse 2 wk 

6 hr/d (DBA/2) 5 d/wk Hemato 300 M (hematocrit decreased by 26%, leukocytes decreased by 80%, bone Chertkov et al. 1992

marrow cellularity decreased by 93%) 

877

300

Bd Wt 300 M (15% decrease) 

16 Mouse 11 d 

5 d/wk (Hale- Stoner) 6 hr/d 

300

Hemato 400 M (decreased erythrocytes 

and leukocytes) 

Cronkite et al. 1982

381

400

17 Mouse 2 wk 

5 d/wk (C57B1/6 

BNL) 6 hr/d 

Hemato 25 100 (decreased hematocrit, 

25 hemolytic anemia) 

Cronkite et al. 1985

330

100

18 Mouse 2 d 

5 d/wk (Hale- Stoner) 6 hr/d 

821

Hemato 400 M (decreased CFU-E cells) 

400

Cronkite et al. 1989

19 Mouse 8 d 

6 hr/d (C57BL/6BNL) Hemato 3000 F (decreased marrow cellularity) Cronkite et al. 1989

828

3000
a 

Key to 

Figure 

20 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Species 

(Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

Mouse (DBA/2J) 

5 d 6 hr/d Hemato 10 M (50% decrease in CFU-E numbers) Dempster and Snyder 1

885

10

21 Mouse 

(C57B1/6) 

2-8 d 24 hr/d Hemato 100 M (leukopenia; decrease in marrow cellularity) Gill et al. 1980 

911

100

22 Mouse 

(CD-1) 

5 d 6 hr/d Hemato 9.9 M 103 M (decreased marrow 9.9 cellularity; Green et al. 1981b 

granulocytopenia, lymphocytopenia; decreased polymorphonucleucytes) 

396

103

Bd Wt 4862 M 

4862

23 Mouse 

(Swiss Webster, C57B1/6J) 

2 wk 4 d/wk 6 h/d 

Hemato 300 M (reduced bone marrow 

cellularity and CFU-E development) 

Neun et al. 1992 

807

300

24 Mouse 

(Hybrid) 

945

5 d 5 d/wk 6 hr/d 

Hemato 300 F 900 F (CFU-E depression) 

300 900

Plappert et al. 1994a 
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route)

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

25 Mouse 

(C57BI/6J) 

6 d 

6 hr/d Hemato 10.2 M (depressed lymphocyte counts; elevated RBCs) Rozen et al. 1984

10.2

845

26 Mouse 

(NMRI) 

1-10 d 24 hr/d Hemato 21 M (reduced bone marrow cellularity; increased Toft et al. 1982

polychromatic erythrocytes; decreased granulopoietic stem cells) 

21

25

27 Mouse 

(NMRI) 

1 wk Hemato 14 M 

14

Toft et al. 1982 

302

28 Mouse 

(NMRI) 

2 wk 5 d/wk 

8 hr/d 

Hemato 10.5 M 21 M (increased 

10.5 micronucleated 

polychromatic 

Toft et al. 1982 

erythrocytes; decreased granulopoietic stem cells) 

21

327

29 Mouse 

(CD-1) 

2 wk 5 d/wk 6 hr/d 

Hemato 30 300 (anemia, decreased 

30 hemoglobin,erythrocytes 

and hematocrit; 

Ward et al. 1985 

hypoplasia of bone marrow; leukopenia) 

300

346

300

Bd Wt 300 
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Exposure/ 

Duration/ 

a Frequency Key to Species (Route) Figure (Strain) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

30 Mouse 5 d 

6 hr/d (Swiss- Hemato 3 M 25 M (decrease in WBC count) 3 25 Wells and Nerland 1991 

Webster) 

894

31 Rabbit Gd 7-20 

24 hr/d Bd Wt 156.5 F 313 F (reduced maternal weight 156.5 gain) Ungvary and Tatrai 198

313

826

Immuno/ Lymphoret 

32 Rat 7 d 

8 hr/d (Wistar) 50 F 100 F (leukopenia; increased 50 leukocyte alkaline Li et al. 1986 

phosphatase) 

100

15

33 Rat 2 wk 

5 d/wk (SpragueDawley) 6 hr/d 200 M 400 M (29% reduction in total 200 splenic cells, 28% lower thymus weight) Robinson et al. 1997 

400

1012

34 Mouse 7 d 

6 hr/d (BALB/c) 47 M (depressed T- and B-lymphocytes; Aoyama 1986 

decreased spleen weight and WBC count) 

47

255

35 Mouse 14 d 

6 hr/d (BALB/c) 48 M (depressed T- and B-lymphocytes; Aoyama 1986 

decreased spleen and thymus weights and WBC count) 

48

672
a 

Key to 

Figure 

36 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Exposure/ 

Duration/ 

Frequency Species (Route) (Strain) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

Mouse 2 wk 

6 hr/d (DBA/2) 5 d/wk 300 M (leukocytes decreased by 80%, bone marrow cellularity decreased by Chertkov et al. 1992 

93%) 

906

300

37 Mouse 2 wk 

5 d/wk (CBA/Ca) 6 hr/d 

311

10 25 (lymphopenia) 

10 25

Cronkite 1986 

38 Mouse 11 d 

5 d/wk (Hale- Stoner) 6 hr/d 400 M (decreased bone marrow cellularity) Cronkite et al. 1982 

382

400

39 Mouse 2 wk 

5 d/wk (C57B1/6 

BNL) 6 hr/d 

400

10 25 (lymphopenia) 

10 25

Cronkite et al. 1985 

40 Mouse 2 d 

5 d/wk (CBA/Ca 

BNL) 6 hr/d 

3000 M (decreased lymphocytes, 

CFU-S content in marrow) 

Cronkite et al. 1989 

825

3000

41 Mouse 2-8 d 

24 hr/d (C57B1/6) 100 M (leukopenia; decrease in marrow cellularity) Gill et al. 1980 

998

100
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

42 Mouse 

(CD-1) 

5 d 6 hr/d 9.9 M 103 M (decreased femoral 9.9 marrow and splenic Green et al. 1981a 

cellularities; reduced splenic granulocytes) 

392

103

43 Mouse 

(Swiss Webster, 

2 wk 4 d/wk 6 h/d 

300 M (reduced bone marrow 

cellularity) 

Neun et al. 1992 

C57B1/6J) 

907

300

44 Mouse 

(Hybrid) 

5 d 5 d/wk 6 hr/d 

100 F 300 F (increased helper 

100 lymphocytes) 

Plappert et al. 1994a 

946

300

45 Mouse 

(C57BL/6) 

1-12 d 6 hr/d 10 M 30 M (Listeria infection, T and 10 B lymphocyte Rosenthal and Snyder 1

depression) 

37

30

46 Mouse 

(C57BI/6J) 

6 d 6 hr/d 

b

10.2 M (decreased circulating 

lymphocytes and 

Rozen et al. 1984 

mitogen-induced blastogenesis of femoral T and B-lymphocytes) 

847

10.2

47 Mouse 

(NMRI) 

2 wk 5 d/wk 8 hr/d 

10.5 M 21 M (decreased 

10.5 granulopoietic stem cells) 

Toft et al. 1982 

908

21
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

48 Mouse 

(NMRI) 

1-10 d 24 hr/d 21 M (decreased granulopoietic stem cells) Toft et al. 1982 

21

909

49 Mouse 

(CD-1) 

2 wk 5 d/wk 6 hr/d 

30 300 (leukopenia; 

30 lymphopenia; bone 

marrow hypoplasia; 

Ward et al. 1985 

histopathological lesions in spleen, thymus, selected lymph nodes) 

300

347

50 Mouse 

(SwissWebster) 

5 d 6 hr/d 3 M 25 M (decrease in spleen 3 weight and WBC count) Wells and Nerland 1991 

25

439

Neurological 

51 Human 30 min 300 (drowsiness, dizziness, Flury 1928 

headaches) 

300

359

52 Human 1-21 d 

2.5-8 hr/d 60 M (dizzyness, nausea, headache, peculiar or Midzenski et al. 1992 

strong odor, chemical taste, fatigue) 

60

420

53 Rat 

(SpragueGd 6-15 6 hr/d 300 F 2200 F (lethargy) 300 2200 Green et al. 1978 

Dawley) 

837
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

54 Mouse 

(C57BL) 

1-14 d 5 d/wk 6 hr/d 

100 M (increased milk licking: 3000 M (tremors; decreased grip 

behavioral index) strength) 

Dempster et al. 1984 

100 3000

360

55 Mouse 

(CD1, 

5 d 6 hr/d 300 (hyperactivity) 900 (narcosis) 300 900 Evans et al. 1981 

C57BL/6J) 

201

56 Rabbit 

(NS) 

3.7-36.2 min 45000 (narcosis, tremors, 

excitement, chewing, 

Carpenter et al. 1944 

loss of pupillary and blink reflex; pupillary contraction and involuntary blinking) 

45000

43

Reproductive 

57 Rat 

(SpragueGd 6-15 6 hr/d 100 F 100 Coate et al. 1984 

Dawley) 

904

58 Rat 

(SpragueGd 6-15 6 hr/d 2200 F 2200 Green et al. 1978 

Dawley) 

288

59 Rat 

(CFY) 

Gd 7-14 24 hr/d 125 F 125 Tatrai et al. 1980a 

209

60 Mouse 

(CF-1) 

Gd 6-15 7 hr/d 500 F 500 Murray et al. 1979 

289
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

61 Rabbit 

(New 

Gd 6-18 7 hr/d 500 F 500 Murray et al. 1979 

Zealand) 

291

62 Rabbit Gd 7-20 

24 hr/d 156.5 F 313 F (increased abortions and 156.5 resorptions) Ungvary and Tatrai 198

313

372

Developmental 

63 Rat 

(SpragueGd 6-15 6 hr/d 40 F 100 F (decreased fetal weight) 40 100 Coate et al. 1984 

Dawley) 

319

64 Rat 

(SpragueDawley) 

Gd 6-15 6 hr/d 100 F (increased incidence of missing sternebrae) Green et al. 1978 

100

132

65 Rat 

(SpragueGd 6-15 7 hr/d 10 F 50 F (decreased fetal weight) 10 50 Kuna and Kapp 1981 

Dawley) 

136

66 Rat 

(CFY) 

Gd 7-14 24 hr/d 125 F (decreased mean fetal weight; increased fetal Tatrai et al. 1980a 

weight retardation; skeletal retardation; 17% decrease in mean placental weight) 

125

210
a 

Key to 

Figure 

67 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Species 

(Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

Rat (CFY) 

Gd 7-14 24 hr/d 47 F (decreased fetal weight; 141 F (significant increase in signs of skeletal fetal mortality) Tatrai et al. 1980b 

retardation) 

320

141

47

68 Mouse 

(SwissWebster) 

Gd 6-15 6 hr/d 10 F 20 F (decreased circulating 10 erythroid precursors, 

elevation of granulocytic 

Keller and Snyder 1988 

precourser cells in neonates and 6-week old offspring) 

103

20

69 Mouse 

(CF-1) 

Gd 6-15 7 hr/d 500 F (decreased fetal body weight; delayed skeletal Murray et al. 1979 

ossifications) 

57

500

70 Mouse 

(CFLP) 

Gd 6-15 12 hr/d 156.5 F (decreased fetal body weight and skeletal Ungvary and Tatrai 198

retardation) 

61

156.5

71 Rabbit 

(New Zealand) 

Gd 6-18 7 hr/d 500 F (increased minor skeletal variants) Murray et al. 1979 

55

500

72 Rabbit 

(New Zealand) 

Gd 7-20 24 hr/d 156.5 F 313 F (decreased fetal weight; 156.5 increased minor 

anomalies) 

Ungvary and Tatrai 198

335

313
Table 3-1 Levels of Significant Exposure to Benzene -

Inhalation (continued) 

Exposure/ 

Duration/ 

a Frequency Key to Species NOAEL Less Serious (Route) Figure (Strain) System (ppm) (ppm) 

LOAEL 

Reference Chemical Form 

Serious 

(ppm) 

INTERMEDIATE EXPOSURE Death 

73 Rat 15 wk 

4-5 d/wk (SpragueDawley) 4-7 hr/d 200 (death) 200 Maltoni et al. 1983, 198

211

74 Mouse 16 wk 

5 d/wk (CBA/Ca) 6 hr/d 300 M (97% mortality) 300 Cronkite 1986 

664

75 Mouse 16 wk 

5 d/wk (CBA/Ca 

BNL) 6 hr/d 

300 (deaths in males during 

exposure; deaths in females shortly after 

Cronkite et al. 1989 

exposure) 

300

832

76 Mouse 16 wk 

5 d/wk (CBA/Ca) 6 hr/d 300 M (11/125 died during first 9 months after initiation of exposure) Farris et al. 1993 

300

447

77 Mouse 26 wk 

5 d/wk (CD-1) 6 hr/d 302 M (50% mortality) 302 Green et al. 1981b 

328

Systemic 

78 Human 4 mo- 1 yr Hemato (occup) 150 (pancytopenia) 150 Aksoy and Erdem 1978 

344

79 Human 4 mo -1 yr Hemato (occup) 210 (pancytopenia, hypocellular to Aksoy et al. 1972 

hypercellular bone marrow) 

210

324
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

80 Human 1 yr 

(occup) Hemato 40 (decrease in WBC counts in first 4 months) Cody et al. 1993 

880

40

81 Human 

8

3.5 mo- 19 yr (occup) Hemato 29 (aplastic anemia) 29 Yin et al. 1987c 

82 Rat 

(SpragueDawley) 

3 wk 5 d/wk 6 hr/d 

Hemato 500 (decreased WBC and 

lymphocytes; increased RBC and hemoglobin) 

Dow 1992 

213

500

83 Rat 

(SpragueDawley) 

401

10 wk Gd 0-20 Ld 5-20 5 d/wk 6 hr/d 

Bd Wt 300 F 

300

Kuna et al. 1992 

84 Rat 

(SpragueDawley) 

13 wk 5 d/wk 6 hr/d 

Hemato 30 300 (decrease in leukocytes, 

30 slight decrease in 

marrow cellularity) 

Ward et al. 1985 

16

300

Bd Wt 300 

300

85 Rat 

(Wistar) 

377

204 d 5 d/wk 7 hr/d 

Hemato 88 M (leukopenia) 

88

Wolf et al. 1956 
a 

Key to 

Figure 

86 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Exposure/ 

Duration/ 

Frequency Species (Route) (Strain) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

Mouse 24 wk 

5 d/wk (C57BL) 6 hr/d Hemato 10 M (depressed peripheral red blood cells, CFU-E) Baarson et al. 1984 

10 

287

87 Mouse 24 wk 

5 d/wk (C57BL) 6 hr/d Hemato 10 M (depressed splenic red cells) Baarson et al. 1984 

10 

314

88 Mouse 9.5 wk 

5 d/wk (Hale- Stoner) 6 hr/d Hemato 400 M (decreased erythrocytes and leukocytes, decreased bone marrow Cronkite et al. 1982 

cellularity) 

131

400

89 Mouse 4-16 wk 

5 d/wk (C57B1/ 6 hr/d 6BNL) Hemato 300 (stem cell depression in bone marrow, reversible after 2-4 weeks) Cronkite et al. 1985 

386

300

90 Mouse 16 wk 

5 d/wk (CBA/Ca) 6 hr/d Hemato 25 M 316 M (decreased stem cells in 25 bone marrow) Cronkite et al. 1989 

331

316

91 Mouse 16 wk 

5 d/wk (CBA/Ca) 6 hr/d Hemato 300 M (granulocytic hyperplasia in bone marrow) Farris et al. 1993 

806

300
a 

Key to 

Figure 

92 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Species 

(Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

Mouse (B6C3F1) 

up to 8 wk 5 d/wk 6 hr/d 

Hemato 10 M 100 M (decreased numbers of 

10 differentiating and 

maturing hematopoietic 

Farris et al. 1997a 

bone marrow cells) 

1015

100

93 Mouse 

(CD-1) 

26 wk 5 d/wk 6 hr/d 

Hemato 302 M (decreased WBC, RBC; 

altered RBC morphology) 

Green et al. 1981b 

332

302

Bd Wt 302 M 

302

94 Mouse 

(CD-1) 

399

50 d 5 d/wk 6 hr/d 

Hemato 9.6 M 

9.6

Green et al. 1981b 

Bd Wt 9.6 M 

9.6

95 Mouse 

(Kunming) 

30 d 6 d/wk 2 hr/d 

Hepatic 12.52 M 

12.52

Li et al. 1992 

412

Renal 12.52 M 

12.52

Bd Wt 12.52 M 

12.52

96 Mouse 

(DBA/2, B6C3F1, C57B1/6) 

13 wk 3 or 5 d/wk 6 hr/d 

Hemato 300 M (depressed rate of 

erythropoiesis, increased frequency of MN-PCE and MN-NCE) 

Luke et al. 1988b 

98

300
a 

Key to 

Figure 

97 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Species 

(Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

Mouse (Hybrid) 

8 wks 5 d/wk 6 hr/d 

Hemato 100 F 300 F (slight anemia; BFU-E 

100 and CFU-E depression in 

bone marrow) 

Plappert et al. 1994a 

715

300

98 Mouse 

(Hybrid) 

8 wk 5 d/wk 6 hr/d 

Hemato 300 F (decreased Hgb, Hct, 

erythrocyte counts) 

Plappert et al. 1994b 

723

300

99 Mouse 

(BDF1) 

8 wk 5 d/wk 6 hr/d 

Hemato 100 F (BFU-E and CFU-E 

depression) 

Seidel et al. 1989b 

96

100

100 Mouse 

(NMRI) 

605

8 wk Hemato 14 M 

14

Toft et al. 1982 

101 Mouse 

(Hybrid) 

6 or 7 wk 5 d/wk 6 h/d 

Hemato 300 F (decreased CFU-C, 

BFU-E and CFU-E) 

Vacha et al. 1990 

808

300

102 Mouse 

(CD-1) 

13 wk 5 d/wk 6 hr/d 

Hemato 30 300 (pancytopenia, bone 

30 marrow hypoplasia) 

Ward et al. 1985 

17

300

Bd Wt 300 

300

103 Gn Pig 

(NS) 

379

32 or 269 d 5 d/wk 7 hr/d 

Hemato 88 (leukopenia) 

88

Wolf et al. 1956 
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Exposure/ 

Duration/ 

a Frequency Key to Species (Route) Figure (Strain) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

104 Rabbit 243 d 

5 d/wk (NS) 7 hr/d Hemato 80 (leukopenia) 80 Wolf et al. 1956

378

105 Pig 3 wk 

5 d/wk (Duroc- 6 hr/d Jersey) Hemato 20 100 (decreased peripheral 20 WBC, and increased erythroid cells) Dow 1992

100

215

Immuno/ Lymphoret 

106 Human 4 mo- 1 yr 

(occup) 210 (pancytopenia, hypoplastic to Aksoy et al. 1972

hyperplastic bone marrow, enlarged spleen) 

210

997

107 Human 1 yr 

(occup) 40 (decreased lymphocytes) 40 Cody et al. 1993

294

108 Rat 3 wk 

5 d/wk (SpragueDawley) 6 hr/d 500 (decreased myeloid and lymphoid cells) Dow 1992

500

216

109 Rat 4 wk 

5 d/wk (SpragueDawley) 6 hr/d 200 M 400 M (29% reduction in total 200 splenic cells, 28% lower thymus weight) Robinson et al. 1997

400

1013

110 Rat 13 wk 

5 d/wk (SpragueDawley) 6 hr/d 30 300 (leukopenia and 30 lymphopenia) Ward et al. 1985

300

353
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

111 Rat 

(Wistar) 

204 d 5 d/wk 7 hr/d 

88 (leukopenia, increased 

spleen weight) 

Wolf et al. 1956 

18

88

112 Rat 

(NS) 

20 wk 6 d/wk 4 hr/d 

4570 (increased leukocyte 

alkaline phosphatase, decreased white blood 

Yin et al. 1982 

cell count) 

384

4570

113 Mouse 

(C57B1) 

24 wk 5 d/wk 6 hr/d 

10 M (decreased number of 

splenic lymphocytes) 

Baarson et al. 1984 

10 

603

114 Mouse 

(C57B1/ 6BNL) 

4-16 wk 5 d/wk 6 hr/d 

300 (reduced bone marrow 

cellularity; stem cell depression, reversible 

Cronkite et al. 1985 

after 2-4 weeks) 

803

300

115 Mouse 

(CBA/Ca BNL) 

20 d 5 d/wk 6 hr/d 

316 M (decreased lymphocytes, 

CFU-S content in marrow) 

Cronkite et al. 1989 

827

316
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

116 Mouse 

(CBA/Ca) 

16 wk 5 d/wk 6 hr/d 

300 M (granulocytic 

hyperplasia) 

Farris et al. 1993 

805

300

117 Mouse 

(B6C3F1) 

up to 8 wk 5 d/wk 6 hr/d 

10 M 100 M (reduced numbers of 

10 total bone marrow cells, 

progenitor cells, 

Farris et al. 1997a 

differentiating hematopoietic cells, peripheral blood leukocytes and RBCs) 

1014

100

118 Mouse 

(B6C3F1) 

8 wk 5 d/wk 6 hr/d 

10 M 100 M (reduced lymphocyte and 

10 total nucleated cell 

counts) 

Farris et al. 1997b 

1010

100

119 Mouse 

(C57B1/6) 

6 wk 5 d/wk 6 hr/d 

1000 (leukopenia, 

granulocytopenia, lymphocytopenia) 

Gill et al. 1980 

24

1000

120 Mouse 

(CD-1) 

50 d 6 hr/d 5 d/wk 

9.6 M (increased splenic 

CFU-S) 

Green et al. 1981a 

9.6 

393
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

121 Mouse 

(CD-1) 

26 wk 6 hr/d 5 d/wk 

302 M (reduced marrow and 

spleen cellularity; decreased spleen 

Green et al. 1981a 

weight) 

302

394

122 Mouse 

(CD-1) 

50 d 5 d/wk 6 hr/d 

9.6 M (increased spleen weight, 

total splenic nucleated cellularity and NRBC) 

Green et al. 1981b 

9.6 

316

123 Mouse 

(CD-1) 

26 wk 5 d/wk 6 hr/d 

302 M (lymphocytopenia, 

anemia, decreased spleen weight, decreased 

Green et al. 1981b

spleen and marrow cellularities) 

302

336

124 Mouse 

(Kunming) 

30 d 6 d/wk 2 hr/d 

3.13 M 12.52 M (26% decrease in relative 

3.13 spleen weight; decrease 

in myelocytes, premyelocytes, 

Li et al. 1992 

myeloblasts, and metamyelocytes in the bone marrow) 

12.52

414
a 

Key to 

Figure 

125 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Exposure/ 

Duration/ 

Frequency Species (Route) (Strain) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

Mouse 8 wks 

5 d/wk (Hybrid) 6 hr/d 

727

100 F 300 F (increased T4/T8 ratio) 

100 300

Plappert et al. 1994a 

126 Mouse 20 d 

5 d/wk (C57B1/6) 6 hr/d 

c

10 M (delayed splenic 

lymphocyte reaction to foreign antigens 

Rosenthal and Snyder 1

evaluated in in vitro mixed lymphocyte reaction) 

600

10

127 Mouse 100 d 

5 d/wk (C57B1/6) 6 hr/d 100 M (death in 9/10 mice due to depressed cell-mediated immunity) Rosenthal and Snyder 1

601

100

128 Mouse 4-5 wk 

5 d/wk (Hale- Stoner) 6 hr/d 50 F 200 F (suppressed antibody 50 response to fluid tetanus toxoid) Stoner et al. 1981 

34

200

129 Mouse 13 wk 

5 d/wk (CD-1) 6 hr/d 30 300 (leukocyte & lymphocyte 30 depression; bone marrow hypoplasia; Ward et al. 1985 

histopathologic lesions in spleen and selected lymph nodes) 

352

300

130 Gn Pig 32 or 269 d 

5 d/wk (NS) 7 hr/d 88 (leukopenia, increased spleen weight) Wolf et al. 1956 

20

88
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

131 Rabbit 

(NS) 

243 d 5 d/wk 7 hr/d 

80 (leukopenia) 

80

Wolf et al. 1956 

19

132 Pig 

(DurocJersey) 

3 wk 5 d/wk 6 hr/d 

20 F 100 F (T-cell depression; 

20 decreased peripheral 

WBC; decreased total 

Dow 1992 

lymphocytes) 

100

217

Neurological 

133 Rat 

(Wistar) 

3 wk 3-4 x 4 hr 

929 M (calculated 30% 

depression of evoked electrical activity) 

Frantik et al. 1994 

929

730

134 Mouse 

(H) 

3 wk 3-4 x 2 hr 

856 F (calculated 30% 

depression of evoked electrical activity) 

Frantik et al. 1994 

856

731

135 Mouse 

(Kunming) 

30 d 6 d/wk 2 hr/d 

0.78 M (increased rapid 

response) 

Li et al. 1992 

0.78 

413

Reproductive 

136 Rat 

(SpragueDawley) 

10 wk Gd 0-20 Ld 5-20 5 d/wk 6 hr/d 

300 F 

300

Kuna et al. 1992 

899
a 

Key to 

Figure 

137 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Species 

(Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

Rat (Wistar) 

93 d 5 d/wk 7-8 hr/d 

6600 M (testicular weight 

increase) 

Wolf et al. 1956 

73

6600

138 Mouse 

(CD-1) 

13 wk 5 d/wk 6 hr/d 

30 300 (bilateral cyst in ovaries; 

30 atrophy/degeneration of 

testes; decrease in 

Ward et al. 1985 

spermatozoa; increase in abnormal sperm) 

71

300

139 Gn Pig 

(NS) 

32 or 269 d 5 d/wk 7-8 hr/d 

88 M (testicular weight 

increase) 

Wolf et al. 1956 

75

88

140 Rabbit 

(NS) 

243 d 5 d/wk 7-8 hr/d 

80 M (degeneration of 

germinal epithelium in testes) 

Wolf et al. 1956 

119

80

Cancer 

141 Human 3.5 mo- 19 yr 

(occup) 29 (CEL: humanlymphocytic leukemia) Yin et al. 1987c 

293

29

142 Rat 

(SpragueDawley) 

219

15 wk 4-5 d/wk 4-7 hr/d 

200 (CEL: hepatomas) 

200

Maltoni et al. 1982a, 191985 
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) NOAEL Less Serious System (ppm) (ppm) 

LOAEL 

Reference Chemical Form 

Serious 

(ppm) 

143 Rat 

(SpragueDawley) 

15 wk 5 d/wk 4-7 hr/d 

200 (CEL: hepatomas; onset 

of Zymbal gland carcinoma) 

Maltoni et al. 1982b, 191985 

221

200

144 Mouse 

(CBA/Ca) 

309

16 wk 5 d/wk 6 hr/d 

100 M (CEL: leukemia) 

100

Cronkite 1986 

145 Mouse 

(C57BL/ 6BNL) 

4-16 wk 5 d/wk 6 hr/d 

300 (CEL: thymic and 

non-thymic lymphoma) 

Cronkite et al. 1984, 19

373

300

146 Mouse 

(CBA/Ca BNL) 

16 wk 5 d/wk 6 hr/d 

300 (Harderian and Zymbal 

gland, squamous cell and mammary carcinoma, papillary adenocarcinoma of the lung) 

Cronkite et al. 1989

833

300

147 Mouse 

(CBA/Ca BNL) 

16 wk 5 d/wk 6 hr/d 

100 M (CEL: hepatomata, 

lymphomatous and myelogenous neoplasms) 

Cronkite et al. 1989 

835

100

148 Mouse 

(CBA/Ca) 

16 wk 5 d/wk 6 hr/d 

300 M (CEL: lymphoma in 

12%) 

Farris et al. 1993 

896

300
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Exposure/ 

Duration/ 

a Frequency Key to Species (Route) Figure (Strain) NOAEL Less Serious System (ppm) (ppm) 

LOAEL 

Reference Chemical Form 

Serious 

(ppm) 

149 Mouse 10 wk 

5 d/wk (C57BL, 

CD-1) 6 hr/d 

1200 M (CEL: 46% lung 

adenoma on CD-1 mice) 

Snyder et al. 1988

1200

170

CHRONIC EXPOSURE Death 

150 Rat 104 wk 

5 d/wk (SpragueDawley) 4-7 hr/d 200 (61% died, versus 46% in controls) Maltoni et al. 1982a

200

383

151 Rat 691 d 

5 d/wk (SpragueDawley) 6 hr/d 300 M (median lifespan 51 weeks versus 65 weeks in controls) Snyder et al. 1978a

300

222

152 Mouse lifetime 

5 d/wk (AKR/J, 

C57Bl) 6 hr/d 

300 M (median lifespan 11-41 

weeks versus 39-75 weeks in controls) 

Snyder et al. 1978a, 19

300

1030

153 Mouse 222 d 

5 d/wk (CD-1) 6 hr/d 300 M (decreased survival) 300 Snyder et al. 1982

1007

Systemic 

154 Human 4 mo- 15 yr 

(occup) Hemato 150 (pancytopenia) 150 Aksoy and Erdem 1978

28
a 

Key to 

Figure 

155 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Species 

(Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) NOAEL Less Serious System (ppm) (ppm) 

LOAEL 

Reference Chemical Form 

Serious 

(ppm) 

Human 14 yr 

(Occup) Hemato 0.55 0.55 Collins et al. 1997 

999

156 Human 1-3 yr 

(occup) Hemato 3 (anemia, lymphocytosis, thrombocytopenia, Doskin 1971 

leukopenia, leukocytosis) 

3

307

157 Human NS 

(occup) Hemato 24 (pancytopenia, hypoplastic bone Erf and Rhoads 1939 

marrow) 

24

4

158 Human 3-29 yr 

(occup) Hemato 25 M (increased mean corpuscular volume) Fishbeck et al. 1978 

25

354

159 Human 0.5-5 yr 

(occup) Hemato 11 (anemia, macrocytosis, thrombocytopenia) Goldwater 1941, Green

al. 1939 

11

303

160 Human 1-25 yr 

(occup) Hemato 20 20 75 (anemia and leukopenia) 75 Kipen et al. 1989 

91
a 

Key to 

Figure 

161 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Species 

(Strain) 

Exposure/ LOAEL 

Reference Chemical Form 

Duration/ 

Frequency NOAEL Less Serious Serious (Route) System (ppm) (ppm) (ppm) 

Human 6.1 yr (avg) d Hemato 0.57 (reduced WBC and (Occup) platelet counts, Lan et al. 2004a 

approximately 7-18% lower than control values) 

0.57

1003

162 Human 4.5-9.7 yr 2.26 (reduced neutrophils and (mean duration) Hemato RBC counts, Qu et al. 2002, 2003 

approximately 12% lower than controls) 

2.26

1002

163 Human 6.3 yr (avg) Hemato 7.6 (reduced absolute (Occup) lymphocyte count, Rothman et al. 1996a, 1

approximately 16% lower than controls) 

7.6

1001

164 Human 1-21 yr Hemato 0.53 M (occup) 0.53 Tsai et al. 1983 

357

165 Human >1 yr Hemato 0.69 (leukopenia) (occup) 0.69 Xia et al. 1995 

912
a 

Key to 

Figure 

166 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Species 

(Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

Human >1 yr 

(occup) 

e

Resp 33 M (sore throat; nasal 

irritation) 

Yin et al. 1987b 

33

59 F (sore throat; nasal 

irritation) 

59

849

e

Hemato 33 M 

33

59 F 

59

e

Renal 33 M 

33

59 F 

59

e

Ocular 33 M (eye irritation) 

33

59 F (eye irritation) 

167 Rat 

(SpragueDawley) 

lifetime 5 d/wk 6 hr/d 

59

Resp 300 M 

300

Snyder et al. 1978a, 19

1033

Hemato 100 M (anemia, leukopenia) 

100

Hepatic 300 M 

300

Renal 300 M 

300

Bd Wt 100 M 300 M (unspecified decreased 

100 body weight gain relative 

to controls) 

300
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Exposure/ 

Duration/ 

a Frequency Key to Species (Route) Figure (Strain) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

168 Mouse lifetime 

5 d/wk (AKR/J, 

C57BL/6J) 6 hr/d 

Resp 300 M 

300

Snyder et al. 1978a, 19

145

Hemato 100 M (anemia, increased 

neutrophil levels; pancytopenia; bone marrow hypoplasia) 

100

Hepatic 300 M 

300

Renal 300 M 

300

Bd Wt 300 M (59% decrease in weight 

gain) 

300

169 Mouse 222 d 

5 d/wk (CD-1) 6 hr/d Hemato 300 M (decreased RBCs and lymphocytes) Snyder et al. 1982 

300

1008

Bd Wt 300 M (reduced body weight 

gain) 

300

Immuno/ Lymphoret 

170 Human 3-5 yr 

(occup) 11 (macrocytosis, thrombocytopenia) Goldwater 1941, Green

al. 1939 

11

500

171 Human 1-25 yr 

(occup) 20 75 (leukopenia) 20 75 Kipen et al. 1989 

296
a 

Key to 

Figure 

172 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Species 

(Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (ppm) (ppm) (ppm) 

Human 

744

>1 yr (occup) 0.69 (leukopenia) 0.69 Xia et al. 1995 

173 Rat 

(SpragueDawley) 

lifetime 5 d/wk 6 hr/d 

100 M (decreased lymphocyte 

counts, splenic hyperplasia) 

Snyder et al. 1978a, 19

1034

100

174 Mouse 

(AKR/J, C57Bl) 

lifetime 5 d/wk 6 hr/d 

100 M (lymphocytopenia, bone 

marrow hypoplasia) 

Snyder et al. 1978a, 19

1031

100

175 Mouse 

(C57BL, CD-1) 

169

lifetime 6 hr/d 5 d/wk 

300 M (lymphopenia) 

300

Snyder et al. 1988 

Cancer 

176 Human 

78

4-15 yr (occup) 150 (CEL: leukemia) 150 Aksoy and Erdem 1978 

177 Human 28 mo- 40 yr 1 (CEL: leukemia, Aksoy et al. 1987 

lymphoma) 

362

1

178 Human 

257

1-10 yr (occup) (occup) 

10 M (CEL: leukemia) 

10

Infante 1978; Infante 19

179 Human 

87

18 mo (occup) 0.3 M (CEL: leukemia) 0.3 Ott et al. 1978 
a 

Key to 

Figure 

180 

Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Species 

(Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) NOAEL Less Serious System (ppm) (ppm) 

LOAEL 

Reference Chemical Form 

Serious 

(ppm) 

Human 1-14 yr 

(occup) 16 M (CEL: leukemia) 16 Rinsky et al. 1981; Infan

al. 1977a 

253

181 Human 

349

1-30 yr (occup) 200 (CEL: leukemia) 200 Vigliani and Forni 1976 

182 Human >1 yr 

(occup) 2 (CEL: chronic erythroid leukemia) Yin et al. 1989 

341

2

183 Rat 

(SpragueDawley) 

326

104 wk 5 d/wk 4-7 hr/d 

200 (CEL: hepatomas) 

200

Maltoni et al. 1982a 

184 Rat 

(SpragueDawley) 

220

104 wk 5 d/wk 4-7 hr/d 

200 (CEL: hepatomas) 

200

Maltoni et al. 1983, 198

185 Rat 

(SpragueDawley) 

lifetime 5 d/wk 6 hr/d 

100 M (CEL: Zymbal gland 

carcinoma, myelogenous leukemia, liver tumors) 

Snyder et al. 1984 

704

100

186 Mouse 

(AKR/J, C57BL6J) 

lifetime 5 d/wk 6 hr/d 

300 M (CEL: hematopoietic 

neoplasms [8/40], including 6 thymic 

Snyder et al. 1978a, 19

lymphomas) 

144

300
Table 3-1 Levels of Significant Exposure to Benzene - Inhalation (continued) 

Exposure/ LOAEL 

Duration/ 

a Frequency Reference Key to Species NOAEL Less Serious Serious (Route) Figure (Strain) System (ppm) (ppm) (ppm) Chemical Form 

187 Mouse lifetime 300 M (CEL: 35% increase of Snyder et al. 1988 every (C57BL, Zymbal gland CD-1) 3rd wk carcinomas in C57BL 7 d/wk mice) 

300

262

a The number corresponds to entries in Figure 3-1. b Used to derive an acute-duration inhalation minimal risk level (MRL) of 0.009 ppm for benzene. Concentration was adjusted for intermittent exposure by multiplyihours and converted to a human equivalent concentration, which was divided by an uncertainty factor of 300 (10 for use of a LOAEL, 3 for extrapolation from animausing dosimetric adjustment, and 10 for human variability) (see Appendix A). c Used to derive an intermediate-duration inhalation minimal risk level (MRL) of 0.006 ppm for benzene. Concentration was adjusted for intermittent exposure by mhours/24 hours and 5 days/7 days and converted to a human equivalent concentration, which was divided by an uncertainty factor of 300 (10 for use of a LOAEL, 3 from animals to humans using dosimetric adjustment, and 10 for human variability) (see Appendix A). d Study results used to derive a chronic-duration inhalation minimal risk level (MRL) of 0.003 ppm for benzene, as described in detail in Appendix A. Benchmark doanalysis was performed on B-lymphocyte counts to select a point of departure, which was adjusted for intermittent exposure and divided by an uncertainty factor of variability. Study results also used to derive a chronic-duration oral minimal risk level (MRL) of 0.0005 mg/kg/day based on route-to-route extrapolation, as describeChapter 2 and Appendix A. Benchmark dose (BMD) analysis was performed on B-lymphocyte counts in benzene-exposed workers to select a point of departure, wfor intermittent exposure. An equivalent oral dose was estimated based on route-to-route extrapolation to determine a point of departure for deriving a chronic-durabenzene, which was divided by an uncertainty factor of 30 (10 for human variability and 3 for uncertainty in route-to-route extrapolation). e Differences in levels of health effects and cancer effects between male and females are not indicated in Figure 3-1. Where such differences exist, only the levels most sensitive gender are presented. AChE = acetylcholinesterase; Bd Wt = body weight; BFU-E = burst-forming units - erythroid; Cardio = cardiovascular; CEL = cancer effect level; CFU-E = colony-fo

erythroid progenitor cells; CFU-G = colony-forming units - granulopoietic stem cells; CFU-GM = colony-forming units - macrophages; CFU-S = colony-forming units central nervous system; d = day(s); F = female; Gd = gestational day; Hct = hematocrit; Hemato = hematological; Hgb = hemoglobin; hr = hour(s); LC50 = lethal cokill; Ld = lactational day(s); LOAEL = lowest-observed-adverse-effect level; M = male; min = minute(s); MN-NCE = micronucleated normochromatic erythrocytes; Mmicronucleated polychromatic erythrocytes; mo = month(s); Musc/skel = musculoskeletal; NOAEL = no-observed-adverse-effect level; NRBC = nucleated red blood specified; occup = occupational exposure; RBC = red blood cell; WBC = white blood cell; wk = week(s); yr = year(s) 
Death 

Respiratory Cardiovascular Hematological Hepatic Dermal Body Weight 

Figure 3-1 Levels of Significant Exposure to Benzene - Inhalation
	

Acute (≤14 days)
	

Systemic 

ppm 

100000 

4h 

1 3r 2r

10000 

22m

9r 19m 6r 

1000 24m 

16m 18m 15m 23m 24m 29m 12r 15m 29m 6r 

13m 13m 14m 

11r 10r 10r 100 17m 21m 22m 8r 

5 5 5 13m 14m 8r 11r 13m 14m 7r

29m 12r 17m 26m 28m 30m

27m 

10 20m 22m 25m 28m 7r

30m 

1 

0.1 

0.01 

0.001 

c-Cat 

d-Dog

r-Rat 

p-Pig

q-Cow

 -Humans 

k-Monkey

m-Mouse 

h-Rabbit 

a-Sheep 

f-Ferret 

j-Pigeon

e-Gerbil 

s-Hamster 

g-Guinea Pig 

n-Mink 

o-Other

 Cancer Effect Level-Animals

 LOAEL, More Serious-Animals

LOAEL, Less Serious-Animals

NOAEL - Animals

 Cancer Effect Level-Humans

 LOAEL, More Serious-Humans

LOAEL, Less Serious-Humans

NOAEL - Humans

 LD50/LC50

Minimal Risk Level

 for effects

 other than

 Cancer 
Body Weight 

Immuno/Lymphor Neurological Reproductive Developmental 

100000 

Figure 3-1 Levels of Significant Exposure to Benzene - Inhalation (Continued)
 

Acute (≤14 days)
	

Systemic
	

ppm 

56h 

10000 

40m 54m 53r 58r 

1000 55m 

60m 61h 69m 71h 38m 33r 31h 36m 43m 44m 49m 51 55m 53r 62h 72h

33r 31h 62h 70m 67r 72h 59r 66r 100 41m 44m 42m 32r 54m 57r 63r 64r 

52 11r 34m 35m 32r 65r 67r 63r 45m 49m 37m 39m 47m 48m 50m 68m 

10 37m 39m 42m 45m 46m 47m 68m 65r

50m 

1 

0.1 

0.01 

0.001 

c-Cat 

d-Dog

r-Rat 

p-Pig

q-Cow

 -Humans 

k-Monkey

m-Mouse 

h-Rabbit 

a-Sheep 

f-Ferret 

j-Pigeon

e-Gerbil 

s-Hamster 

g-Guinea Pig 

n-Mink 

o-Other

 Cancer Effect Level-Animals

 LOAEL, More Serious-Animals

LOAEL, Less Serious-Animals

NOAEL - Animals

 Cancer Effect Level-Humans

 LOAEL, More Serious-Humans

LOAEL, Less Serious-Humans

NOAEL - Humans

 LD50/LC50

Minimal Risk Level

 for effects

 other than

 Cancer 
Death 

Hematological Hepatic Renal Body Weight 

Figure 3-1 Levels of Significant Exposure to Benzene - Inhalation (Continued)
 

Intermediate (15-364 days)
	

Systemic 

ppm 

10000 

1000 

82r 88m

74m 75m 76m 77m 89m 90m 91m 93m 96m 97m 98m 101m 102m 84r 93m 

73r 79
	 78
	

100
	 92m 97m 99m 105p 103g 85r 104h 

80
	 81
	 102m 84r 90m 105p

100m 95m 95m 95m 10 86m 87m 92m 94m 94m

1 

0.1 

0.01 

0.001 

c-Cat 

d-Dog

r-Rat 

p-Pig

q-Cow

 -Humans 

k-Monkey

m-Mouse 

h-Rabbit 

a-Sheep 

f-Ferret 

j-Pigeon

e-Gerbil 

s-Hamster 

g-Guinea Pig 

n-Mink 

o-Other

 Cancer Effect Level-Animals

 LOAEL, More Serious-Animals

LOAEL, Less Serious-Animals

NOAEL - Animals

 Cancer Effect Level-Humans

 LOAEL, More Serious-Humans

LOAEL, Less Serious-Humans

NOAEL - Humans

 LD50/LC50

Minimal Risk Level

 for effects

 other than

 Cancer 
Body Weight 

Immuno/Lymphor Neurological Reproductive Cancer * 

10000 

Figure 3-1 Levels of Significant Exposure to Benzene - Inhalation (Continued)
 

Intermediate (15-364 days)
	

Systemic
 

ppm 

137r 

112r 

1000 119m 134m 133r

108r 109r

102m 83r 84r 114m 115m 116m 121m 123m 125m 129m 110r 138m 136r 145m 146m 

106 128m 109r 

100 130g 117m 118m 125m 127m 111r 132p 139g 144m 147m 131h 140h 

128m 107 

129m 110r 138m 141 

132p 

124m 10 113m 117m 118m 120m 122m 126m

124m 

1 135m 

0.1 

0.01 

*Doses represent the lowest dose tested per study that produced a tumorigenic 

response and do not imply the existence of a threshold for the cancer endpoint. 0.001 

c-Cat 

d-Dog

r-Rat 

p-Pig

q-Cow

 -Humans 

k-Monkey

m-Mouse 

h-Rabbit 

a-Sheep 

f-Ferret 

j-Pigeon

e-Gerbil 

s-Hamster 

g-Guinea Pig 

n-Mink 

o-Other

 Cancer Effect Level-Animals

 LOAEL, More Serious-Animals

LOAEL, Less Serious-Animals

NOAEL - Animals

 Cancer Effect Level-Humans

 LOAEL, More Serious-Humans

LOAEL, Less Serious-Humans

NOAEL - Humans

 LD50/LC50

Minimal Risk Level

 for effects

 other than

 Cancer 
Cancer * 

Figure 3-1 Levels of Significant Exposure to Benzene - Inhalation (Continued)
 

Intermediate (15-364 days)

ppm 

10000 

149m 1000 

148m
	

142r
	 143r 

100 

10 

1 

0.1 

0.01 

*Doses represent the lowest dose tested per study that produced a tumorigenic 

response and do not imply the existence of a threshold for the cancer endpoint. 

0.001 

c-Cat 

d-Dog

r-Rat 

p-Pig

q-Cow

 -Humans 

k-Monkey

m-Mouse 

h-Rabbit 

a-Sheep 

f-Ferret 

j-Pigeon

e-Gerbil 

s-Hamster 

g-Guinea Pig 

n-Mink 

o-Other

 Cancer Effect Level-Animals

 LOAEL, More Serious-Animals

LOAEL, Less Serious-Animals

NOAEL - Animals

 Cancer Effect Level-Humans

 LOAEL, More Serious-Humans

LOAEL, Less Serious-Humans

NOAEL - Humans

 LD50/LC50

 for effects

 other than

 Cancer 

Minimal Risk Level 
Death 

Respiratory Hematological Hepatic Renal Ocular Body Weight Immuno/Lymphor 

1000 

Figure 3-1 Levels of Significant Exposure to Benzene - Inhalation (Continued)
 

Chronic (≥365 days)
	

Systemic 

ppm 

152m 153m 151r 168m 167r 169m 168m 167r 168m 167r 168m 169m 167r 

150r 154
	 100
	 168m 167r 167r 160
	 171
	

166
	 166
	 166
	 166 157
	 158 160
	 171
	

10
	 159
	 170

163
	

156
	 162
	

1
	 165
	 172 155
	 161 164
	

0.1 

0.01 

0.001 

0.0001 

1E-5 

1E-6 

c-Cat 

d-Dog

r-Rat 

p-Pig

q-Cow

 -Humans 

k-Monkey

m-Mouse 

h-Rabbit 

a-Sheep 

f-Ferret 

j-Pigeon

e-Gerbil 

s-Hamster 

g-Guinea Pig 

n-Mink 

o-Other

 Cancer Effect Level-Animals

 LOAEL, More Serious-Animals

LOAEL, Less Serious-Animals

NOAEL - Animals

 Cancer Effect Level-Humans

 LOAEL, More Serious-Humans

LOAEL, Less Serious-Humans

NOAEL - Humans

 LD50/LC50

 for effects

 other than

 Cancer

 Minimal Risk Level 
Immuno/Lymphor 

Cancer * 

Figure 3-1 Levels of Significant Exposure to Benzene - Inhalation (Continued)

Chronic (≥365 days)

ppm 

1000
	

175m 187m 188m
	 181
	 183r 184r 185r 176
	 100
	 174m 173r 186r 

180
	 10
	 178
	

182
	

1
	 177
	

179
	

0.1 

0.01
	

-4 10
	

Estimated
	

0.001 Upper-Bound 

10-5 Human Cancer 
	

Risk Levels
	

0.0001
	

-6 10
	

1E-5
	

-7 10
	

*Doses represent the lowest dose tested per study that produced a tumorigenic
	

response and do not imply the existence of a threshold for the cancer endpoint.
	 1E-6 

c-Cat 

d-Dog

r-Rat 

p-Pig

q-Cow

 -Humans 

k-Monkey

m-Mouse 

h-Rabbit 

a-Sheep 

f-Ferret 

j-Pigeon

e-Gerbil 

s-Hamster 

g-Guinea Pig 

n-Mink 

o-Other

 Cancer Effect Level-Animals

 LOAEL, More Serious-Animals

LOAEL, Less Serious-Animals

NOAEL - Animals

 Cancer Effect Level-Humans

 LOAEL, More Serious-Humans

LOAEL, Less Serious-Humans

NOAEL - Humans

 LD50/LC50

 for effects

 other than

 Cancer 

Minimal Risk Level 
BENZENE 73 

3. HEALTH EFFECTS 

Cardiovascular Effects. No studies were located regarding cardiovascular effects in humans after 

inhalation exposure to benzene, although ventricular fibrillation has been proposed as the cause of death 

in some human poisonings (Avis and Hutton 1993; Winek and Collom 1971). 

One animal study was found that investigated the effects of acute inhalation exposure to high concentrations of benzene vapor on the heart muscle of cats and monkeys (Nahum and Hoff 1934). Information 

from the electrocardiograms indicated that exposure to benzene vapor caused extra systoles and 

ventricular tachycardia of the prefibrillation type. Animals that had their adrenals and stellate ganglias 

removed did not exhibit extra systoles or ventricular tachycardia. These findings suggest that the 

arrhythmias were caused by catecholamine release and sympathetic discharge. This study is limited in 

that exact levels of exposure are not available. An additional study investigated the influence of benzene 

inhalation on ventricular arrhythmia in the rat (Magos et al. 1990). Rats exposed to 3,526–8,224 ppm of 

benzene in a closed chamber for 15 minutes exhibited an increased number of ectopic ventricular beats. 

Gastrointestinal Effects. Very few data are available describing gastrointestinal effects in humans 

after inhalation exposure to benzene. In a case study involving the death of an 18-year-old boy who 

intentionally inhaled benzene, the autopsy revealed congestive gastritis (Winek and Collom 1971). No 

other details or data were given. 

Hematological Effects. Data regarding effects on the human hematological system following acute 

inhalation exposure to benzene are scant, but indicate leukopenia, anemia, and thrombocytopenia after 

more than 2 days of occupational exposure to more than 60 ppm benzene (Midzenski et al. 1992). 

Epidemiological studies on persons exposed to various levels of benzene in the workplace for 

intermediate and chronic periods of time also indicate hematological effects. Deficiencies in most of 

these studies include uncertainty in estimates of historical exposure levels, concomitant exposure to other 

chemicals, and lack of appropriate control groups. However, sufficient data are available to show that the 

hematopoietic system is a critical target for benzene toxicity. Studies that were conducted well and that 

show effects linked to specific exposure levels are presented in Table 3-1 and Figure 3-1. Effects on 

leukocytes, lymphocytes, and bone marrow are also discussed in Section 3.2.1.3. 

Inhalation exposure to benzene levels in excess of regulated workplace limits (8-hour TWA of 1 ppm) for 

several months to several years can result in deficits in the relative numbers of circulating blood cells, 

which may be severe enough to be considered clinical pancytopenia. Continued exposure to benzene can 
BENZENE 74 

3. HEALTH EFFECTS 

also result in aplastic anemia or leukemia (Aksoy et al. 1974; EPA 1995; Hayes et al. 1997; IARC 1982, 

1987; IRIS 2007; Rinsky et al. 1987, 2002; Yin et al. 1987c, 1996a, 1996b). 

Pancytopenia is the reduction in the number of all three major types of blood cells: erythrocytes (red 

blood cells), thrombocytes (platelets), and leukocytes (white blood cells). In adults, all three major types 

of blood cells are produced in the red bone marrow of the vertebrae, sternum, ribs, and pelvis. The red 

bone marrow contains immature cells, known as multipotent myeloid stem cells, that later differentiate 

into the various mature blood cells. Pancytopenia results from a reduction in the ability of the red bone 

marrow to produce adequate numbers of these mature blood cells. 

Aplastic anemia is a more severe effect of benzene and occurs when the bone marrow ceases to function 

and the stem cells never reach maturity. Depression in bone marrow function occurs in two stages— 

hyperplasia (increased synthesis of blood cell elements), followed by hypoplasia (decreased synthesis). 

As the disease progresses, bone marrow function decreases and the bone marrow becomes necrotic and 

filled with fatty tissue. This myeloblastic dysplasia without acute leukemia has been seen in persons 

exposed to benzene (Erf and Rhoads 1939). Aplastic anemia can progress to a type of leukemia known as 

acute myelogenous leukemia (Aksoy 1980), which is discussed in Section 3.2.1.7. 

Early biomarkers of exposure to relatively low levels of benzene include depressed numbers of one or 

more of the circulating blood cell types. For example, statistically significantly decreased total red blood 

cells (RBCs), white blood cells (WBCs), absolute lymphocyte count, platelets, and hematocrit were 

reported for a group of 44 healthy subjects exposed to benzene in the workplace (median 8-hour timeweighted average [TWA] of 31 ppm; minimal exposure to other solvents) for an average of 6.3 years in 

China (Rothman et al. 1996a, 1996b). Age- and gender-matched workers with no history of occupational 

exposure to benzene served as controls. Among the 22 workers whose mean 5-day benzene exposure 

levels did not exceed 31 ppm (median 8-hour TWA of 13.6 ppm), significantly depressed absolute 

lymphocyte count, RBCs, and platelets were noted. Only absolute lymphocyte count was significantly 

decreased in a subgroup of 11 workers with no 8-hour TWA exceeding 31 ppm (median 8-hour TWA of 

7.6 ppm). 

Qu et al. (2002, 2003a, 2003b) compared hematology values in a group of 130 chronically exposed 

workers in China with those obtained from 51 age- and gender-matched subjects without occupational 

exposure to benzene. Statistically significant trends for depressed RBCs, WBCs, and neutrophils were 

observed in the benzene-exposed workers (average measured 4-week benzene exposure levels ranged 
BENZENE 75 

3. HEALTH EFFECTS 

from 0.08 to 54.5 ppm just prior to blood testing). A subgroup of 73 of these workers, whose average 

4-week benzene exposure level was 2.26 ppm, exhibited significantly depressed RBCs and neutrophils. 

This study provided exposure-response data, but apparent discrepancies in reported low-concentration 

results render the study of limited value for MRL derivation. 

One recent cross-sectional study (Lan et al. 2004a, 2004b), performed on 250 workers exposed to benzene 

in shoe manufacturing industries in Tianjin, China, and 140 age- and gender-matched workers in clothing 

manufacturing facilities that did not use benzene, was of sufficient quality to serve as the basis for 

deriving a chronic-duration inhalation MRL for benzene (see footnote to Table 3-1 and Appendix A). 

The benzene-exposed workers had been employed for an average of 6.1±2.9 years. Controls consisted of 

140 age-and gender-matched workers in clothing manufacturing facilities in which measurable benzene 

concentrations were not found (detection limit 0.04 ppm). Benzene exposure was monitored by 

individual organic vapor monitors (full shift) 5 or more times during 16 months prior to phlebotomy. 

Benzene-exposed workers were categorized into four groups (controls, <1, 1–<10, and ≥10 ppm) 

according to mean benzene exposure levels measured during 1 month prior to phlebotomy. Complete 

blood count (CBC) and differential were analyzed mechanically. Coefficients of variation for all cell 

counts were <10%. 

Mean 1-month benzene exposure levels in the four groups (controls, <1, 1–<10, and ≥10 ppm) were 

<0.04, 0.57±0.24, 2.85±2.11, and 28.73±20.74 ppm, respectively. Hematological values were adjusted to 

account for potential confounding factors (i.e., age, gender, cigarette smoking, alcohol consumption, 

recent infection, and body mass index). All types of WBCs and platelets were significantly decreased in 

the lowest exposure group (<1 ppm), ranging in magnitude from approximately 8 to 15% lower than 

controls. Although similar statistical analyses for the mid- and high-exposure groups were not included 

in the study report, decreases in all types of WBCs and platelets were noted at these exposure levels as 

well; the decreases in the highest exposure group ranged in magnitude from 15 to 36%. Lymphocyte 

subset analysis revealed significantly decreased CD4+-T cells, CD4+/CD8+ ratio, and B cells. 

Hemoglobin concentrations were significantly decreased only within the highest (≥10 ppm) exposure 

group. Tests for a linear trend using benzene air level as a continuous variable were significant for 

platelets and all WBC measures except monocytes and CD8+-T cells. Upon restricting the linear trend 

analyses to workers exposed to <10 ppm benzene, excluding controls, inverse associations remained for 

total WBCs, granulocytes, lymphocytes, B cells, and platelets. In order to evaluate the effect of past 

benzene exposures on the hematological effects observed in this study, the authors compared findings for 

a group of workers who had been exposed to <1 ppm benzene over the previous year (n=60) and a subset 
BENZENE 76 

3. HEALTH EFFECTS 

who also had <40 ppm-years lifetime cumulative benzene exposure (n=50). The authors stated that the 

same cell types were significantly reduced in these groups, but did not provide further information of the 

magnitude (i.e., percent change) of the hematological effects observed. These data suggest that the 

1-month benzene exposure results could be used as an indicator of longer-term low-level benzene 

hematotoxicity. To demonstrate that the observed effects were attributable to benzene, significantly 

decreased levels of WBCs, granulocytes, lymphocytes, and B cells were noted in a subgroup (n=30; mean 

1-month exposure level of 0.29±0.15 ppm) of the <1 ppm group for which exposure to other solvents was 

negligible. Lan et al. (2004a, 2004b) also presented information on the effect of benzene on colony 

forming progenitor cells (data were only presented for the mid- and high-exposure groups). Benzene 

exposure was associated with a concentration-dependent decrease in colony formation and progenitor 

cells were suggested to be more sensitive than circulating cells. 

As described above, several epidemiology studies compared hematological variables for benzene-exposed 

workers to gender- and age-matched controls and noted hematological effects at relatively low 

concentrations (well below 1 ppm in some exposure groups) (Lan et al. 2004a, 2004b; Qu et al. 2002, 

2003a, 2003b; Rothman et al. 1996a, 1996b). An alternative approach was used by Collins et al. (1991, 

1997) and Tsai et al. (1983, 2004). This approach utilized a defined range of clinically normal 

hematological values and compared the prevalence of abnormal results between benzene-exposed 

workers and unexposed controls. Collins et al. (1991) found no significant correlations between benzene 

exposure and the prevalence of abnormal hematological values among 200 workers exposed to benzene at 

estimated concentrations ranging from 0.01 to 1.4 ppm, relative to the prevalence of abnormal 

hematological values obtained from 268 unexposed workers in the same plant. In a more recent 

evaluation, Collins et al. (1997) found no significant correlation between exposure to benzene at an 

8-hour TWA of 0.55 ppm and prevalence of clinically-defined lymphopenia (or other measures of 

hematotoxicity including mean corpuscular volume [MCV], and counts of WBCs, RBCs, hemoglobin, 

and platelets) among a group of 387 workers exposed for ≥5 years. Tsai et al. (1983) measured benzene 

levels in a Texas refinery at one time point (mean benzene concentration of 0.53 ppm) and found that 

hemoglobin, hematocrit, RBCs, WBCs, and thrombocytes of workers exposed for up to 21 years were 

within the range of normal values. Tsai et al. (2004) found no clinically adverse hematotoxic effects 

among a large group of 1,200 petrochemical employees with mean 8-hour TWA benzene exposure levels 

of 0.6 ppm from 1977 to 1988 and 0.14 ppm from 1988 to 2002. The normal range for certain 

hematological parameters is necessarily broad due to large interindividual differences in clinical status. 

Restricting the comparison of benzene-exposed and nonexposed populations to only those values 
BENZENE 77 

3. HEALTH EFFECTS 

considered clinically abnormal or adverse may reduce the sensitivity of the study to detect meaningful 

changes at the population level. 

Many reports of hematological effects in benzene-exposed workers involve estimated exposure levels 

well in excess of 1 ppm. A series of studies conducted on Turkish workers exposed to benzenecontaining adhesives in various occupations showed increased severity of effects with increased levels or 

duration of exposure. The initial hematological study examined 217 male workers who were exposed for 

between 4 months and 17 years to benzene-containing solvents (Aksoy et al. 1971). The concentration of 

benzene in the work area ranged from 15 to 30 ppm outside work hours and reached a maximum of 

210 ppm when benzene-containing adhesives were being used. Fifty-one of the workers showed clinical 

hematological abnormalities such as leukopenia, thrombocytopenia, eosinophilia, and pancytopenia. An 

additional cohort was identified that included 32 shoe manufacturers who had worked with benzene for 

4 months to 15 years at concentrations of 15–30 ppm outside work hours and 210–640 ppm during the 

use of benzene, and who showed pancytopenia (Aksoy et al. 1972). Examination of these people revealed 

disruptions in bone marrow function, including cases of hypoplastic, acellular, hyperplastic, or 

normoblastic bone marrow. The continuing assessment further identified that ineffective erythropoiesis 

or increased hemolysis may have been responsible for the reticulocytosis, hyperbilirubinemia, 

erythroblastemia, increase in quantitative osmotic fragility, and elevated serum lactate dehydrogenase 

levels observed in some patients. 

Leukopenia, lymphocytosis, a biphasic leukocyte response, and bone marrow hypercellularity were 

reported for workers in a Russian industry who were exposed to a complex of hydrocarbons that included 

benzene, cyclohexane, and 1,3-butadiene (Doskin 1971). Benzene levels were estimated at 3.2–12.8 ppm, 

which are 2–8 times the Russian standard. Leukopenia was observed by Xia et al. (1995) in Chinese 

workers exposed to 0.69–140 ppm (mean=6 ppm) benzene for more than 1 year. Enlarged RBCs, 

transient anemia, reduced hemoglobin concentrations, and clinically increased MCV were reported in 

workers in a chemical factory who were exposed to over 25 ppm of benzene in the workplace for an 

average of 9 years (Fishbeck et al. 1978). 

Dosemeci et al. (1996) assessed the relative risk of abnormal hematological values with increasing 

benzene levels in a study of workers employed in 672 rubber and rubber glue application facilities in 

China between 1949 and 1987. Compared to workers with estimated exposures <5 ppm, relative risks of 

abnormal hematological values (indicating benzene poisoning) by estimated exposure intensity at 

1.5 years prior to clinically-diagnosed benzene poisoning were 2.2 (95% confidence interval [CI] 1.7– 
BENZENE 78 

3. HEALTH EFFECTS 

2.9), 4.7 (95% CI 3.4–6.5), and 7.2 (95% CI 5.3–9.8) for exposures in the ranges of 5–19, 20–39, and 

>40 ppm, respectively. 

Examination of individual hematological records, demographic data, and chronological work histories of 

459 workers employed at a rubber products manufacturing plant in Ohio between 1940 and 1975 revealed 

significant decreases in WBC and RBC counts and hemoglobin for the period of 1940–1948 when the 

exposure was 75 ppm (Kipen et al. 1989). The trend was not apparent during later years (1949–1975) 

when the exposure was decreased to 15–20 ppm. A more focused study of many of the same workers 

during their first year of employment revealed significantly lower WBC and RBC counts in employees 

exposed to benzene levels greater than the median (estimated at 40–54 ppm), compared to those with 

lower estimated exposure levels (Cody et al. 1993). Using a nested case-control approach at the same 

rubber products manufacturing plant to assess a possible exposure-response relationship between benzene 

and the risk of developing a low WBC or RBC count in workers for whom hematologic screening data 

were available, Ward et al. (1996) reported a strong correlation between low WBC counts and benzene 

exposure and a weak correlation between RBC count and benzene exposure. 

More severe effects, including preleukemia or acute leukemia, were observed in 26 out of 28,500 benzene 

workers exposed to 210–650 ppm for 1–15 years (Aksoy et al. 1974). Clinical features of the 

preleukemia included one or more of the following: anemia, leukopenia, pancytopenia, bone marrow 

hyperplasia, pseudo-Pelger-Huet anomaly, and splenomegaly. A study was conducted 2–17 years 

following the last exposure of 44 pancytopenic patients exposed to benzene (150–650 ppm) in adhesives 

for 4 months to 15 years (Aksoy and Erdem 1978). Of these patients, complete remission was seen in 23, 

death due to complications of pancytopenia in 14, death due to myeloid metaplasia in 1, and leukemia in 

6. When benzene concentrations in factories decreased in later years, less severe effects were seen. At 

40 tire manufacturing plants, 231 workers were exposed for 28 months to 40 years (mean 8.8 years) to 

benzene-containing solvents and thinners (Aksoy et al. 1987). The decrease in benzene content of 

materials used in these workshops and the corresponding reduction in air concentration (most samples 

<1 ppm) paralleled the decrease in the number of hematological abnormalities reported for benzeneexposed workers. 

Another study revealed effects, ranging from mild to severe, of benzene exposure in factory workers in 

China (Yin et al. 1987c). Of the 528,729 workers, 95% were exposed to mixtures of benzene, toluene, 

and xylene, while 5% (26,319 workers) were exposed to benzene alone at 0.02–264 ppm in air in 95% of 

the work stations. Over half of the work stations had levels of benzene in the air of less than 13 ppm; 
BENZENE 79 

3. HEALTH EFFECTS 

about 1% had levels of 13–264 ppm. Benzene toxicity, as indicated by leukopenia, aplastic anemia, and 

leukemia, was seen in 0.94% of the workers exposed to benzene and 0.44% of the workers exposed to the 

mixtures. Similar toxicity was found in employees of 28 of the 141 shoe factories studied (124 cases in 

2,740 employees) (Yin et al. 1987c). A positive correlation was observed for prevalence of adverse 

benzene effects and benzene concentration in data from these 28 shoe factories. The authors determined 

that the affected people were exposed to benzene concentrations >29 ppm. In one workshop, there were 

4 cases of aplastic anemia in 211 workers. These workers were exposed to benzene at a mean 

concentration of 324 ppm during an 8-month period of employment. The prevalence of aplastic anemia 

in the shoe-making industry was about 5.8 times that in the general population. The main limitation of 

this study is the lack of information on the duration of exposure. 

People who were exposed to high levels of benzene vapors in the printing industry also showed severe 

hematological effects. One study evaluated 332 workers who were exposed to 11–1,060 ppm of benzene 

for 6 months to 5 years. Detailed blood studies performed on 102 of these workers revealed benzene 

poisoning in 22 workers characterized by pancytopenia or other clinical signs (Goldwater 1941; 

Greenburg et al. 1939). Another study reported 6 cases of pancytopenia and bone marrow dysplasia in 

printers exposed to 24–1,060 ppm of benzene (Erf and Rhoads 1939). 

Animal studies have been designed to characterize exposure-response relationships of benzene 

hematotoxicity; the results provide support to the human data. Animal responses to benzene exposure are 

variable and may depend on factors such as species, strain, duration of exposure, and whether exposure is 

intermittent or continuous. Wide variations have also been observed in normal hematological parameters, 

complicating statistical evaluation. However, the studies show that benzene exerts toxic effects at all 

phases of the hematological system, from stem cell depression in the bone marrow, to pancytopenia, to 

histopathological changes in the bone marrow. Effects on leukocytes, lymphocytes, and bone marrow are 

also discussed in Section 3.2.1.3. 

Repeated acute-, intermediate-, and chronic-duration inhalation exposure of laboratory animals (mainly 

mice, but also rats, rabbits, and guinea pigs) to benzene vapor concentrations ranging from 10 to 

>300 ppm demonstrate significant decreases in blood values that include RBCs, total WBCs, 

lymphocytes, granulocytes, hematocrit, and hemoglobin (Aoyama 1986; Baarson et al. 1984; Chertkov et 

al. 1992; Cronkite et al. 1982; Farris et al. 1997a, 1997b; Gill et al. 1980; Green et al. 1981a, 1981b; Li et 

al. 1986; Rozen and Snyder 1985; Rozen et al. 1984; Snyder et al. 1982; Ward et al. 1985; Wells and 

Nerland 1991; Wolf et al. 1956). Pancytopenia was noted in the study of Ward et al. (1985). 
BENZENE 80 

3. HEALTH EFFECTS 

Short-duration exposure of mice to benzene has been shown to cause both concentration- and durationrelated reductions in bone marrow cellularity (number of nucleated cells) and the number of colonyforming granulopoietic stem cells (CFU-C), and an increased frequency of micronucleated polychromatic 

erythrocytes (MN-PCE) (Toft et al. 1982). Mice that were exposed continuously at benzene 

concentrations ≥21 ppm in air for 4–10 days showed significant changes in all three parameters. 

Intermittent exposure (8 hours/day, 5 days/week for 2 weeks) to 21 ppm significantly reduced the number 

of CFU-C and elevated the frequency of MN-PCE, but did not affect bone marrow cellularity. 

Intermittent exposure at concentrations ≥50 ppm caused significant changes in all three parameters and 

decreased the ability of the spleen to form mature cells (as measured by numbers of colony-forming units 

of stem cells). When mice were intermittently exposed for 2 weeks, decreased cellularity and CFU-C per 

tibia were observed at 95 ppm after 6–8 hours/day exposure, whereas at 201 ppm benzene, decreased 

cellularity (but no effect on CFU-C) was noted after 2 hours/day exposure. A decrease in cellularity and 

CFU-C was observed after 4–8 hours/day exposure to 201 ppm benzene. 

Results of numerous additional studies of laboratory animals (mainly mice, which are particularly 

sensitive to benzene hematotoxicity, but also rats and pigs) support findings of benzene-induced effects 

on bone marrow cellularity (hyper- and/or hypocellularity) and colony-forming stem cells, as well as 

granulocytic hyperplasia, following repeated acute-, intermediate-, or chronic-duration inhalation 

exposure to benzene vapors at concentrations ranging from 10 to 500 ppm (Baarson and Snyder 1991; 

Baarson et al. 1984; Chertkov et al. 1992; Corti and Snyder 1996; Cronkite et al. 1982, 1985, 1989; 

Dempster and Snyder 1991; Dow 1992; Farris et al. 1993, 1997b; Neun et al. 1992, 1994; Plappert et al. 

1994a, 1994b; Snyder et al. 1978a, 1980, 1982; Vacha et al. 1990). For example, Dempster and Snyder 

(1991) observed a 50% reduction in CFU-E (erythroid progenitor cells) in bone marrow of DBA/2 mice 

exposed to 10 ppm benzene for 6 hours/day for 5 days. Farris et al. (1997b) reported benzene-induced 

decreased numbers of total bone marrow forming cells, progenitor cells, and differentiating hematopoietic 

cells in mice exposed to benzene vapor concentrations ≥200 ppm, 6 hours/day, 5 days/week for up to 

8 weeks. Replication of primitive progenitor cells in the bone marrow was increased during the exposure 

period, presumably as compensation for cytotoxicity. Granulocytic hyperplasia was detected in the bone 

marrow of mice exposed to 300 ppm benzene in air for 6 hours/day, 5 days/week for 16 weeks, and held 

18 months after the last exposure (Farris et al. 1993). Prolonged exposure to lower levels had a greater 

hematotoxic effect than exposure to higher levels for a shorter period of time. Recovery from 

hematotoxicity has been demonstrated following the cessation of exposure (Cronkite et al. 1982, 1985, 

1989) and may be most closely associated with rate of exposure, since longer-term exposure of mice to 
BENZENE 81 

3. HEALTH EFFECTS 

316 ppm of benzene caused irreparable hematologic injury, while the hematotoxic effects caused by 

shorter-term exposure to 10 times higher benzene concentrations were reversible (Cronkite et al. 1989). 

One study revealed damaged erythrocytes in the peripheral blood of mice (Luke et al. 1988b). Cytotoxic 

damage in the bone marrow was dependent on strain and exposure duration. Peripheral blood smears 

were analyzed weekly from three strains of mice (DBA/2, B6C3F1, and C57BL/6) exposed to 300 ppm 

benzene for 13 weeks (6 hours/day) for either 5 days/week (Regimen 1) or 3 days/week (Regimen 2). In 

all three strains, an initial severe depression in rate of erythropoiesis was observed. The return to normal 

was dependent on strain (Luke et al. 1988b) and regimen (Cronkite et al. 1989; Luke et al. 1988b). An 

increase in frequency of micronucleated normochromatic erythrocytes (MN-NCE) was observed to be 

dependent on strain (C57BL/6=B6C3F1>DBA/2) and regimen (Regimen 1 > Regimen 2), whereas the 

increase in frequency of MN-PCE was dependent on strain (DBA/2>C57BL/6=B6C3F1) but, for the most 

part, was not dependent on exposure regimen. 

Damaged erythroblast-forming cells were also noted in bone marrow (Seidel et al. 1989). A substantial 

decrease in erythroid colony-forming units and smaller decreases in erythroid burst-forming units 

occurred in BDF1 mice intermittently exposed to 100 ppm of benzene for 8 weeks (6 hours/day, 

5 days/week). Although the effects in the 100 ppm group were not apparent at 8 weeks (the end of 

experiment), they did occur, and it took over 3 weeks for the erythroid burst-forming units and erythroid 

colony-forming units to return to their initial values. This reduction in the number of erythroid precursors 

was reflected by a slight reduction in the number of erythrocytes. 

Benzene-induced hematotoxicity was also demonstrated in the spleen of rats and mice following 

intermediate- or chronic-duration repeated inhalation exposure (Snyder et al. 1978a, 1984; Ward et al. 

1985). Snyder et al. (1978a, 1984) reported benzene-induced increased extramedullary hematopoiesis in 

the spleen. Ward et al. (1985) noted that the finding of hemosiderin in the spleen of benzene-exposed rats 

could be due to erythrocyte hemolysis. 

Musculoskeletal Effects. A case of myelofibrosis was diagnosed in a 46-year-old man in October 

1992 (Tondel et al. 1995). The patient worked from 1962 to 1979 as a gasoline station attendant. The 

patient was referred to the Department of Hematology, University Hospital in Linkoping, Sweden, where 

a bone marrow biopsy was performed. The patient described symptoms of increasing muscle pain for 

1 year, fatigue for 3 weeks, night sweats, and weight loss. A bone marrow biopsy showed myelofibrosis. 

The TWA concentration for gasoline station attendants was estimated to be <0.2 ppm. The occupational 
BENZENE 82 

3. HEALTH EFFECTS 

standard for benzene in Sweden was 0.5 ppm (TWA) and the Swedish short-term exposure limit was 

3 ppm. Ruiz et al. (1994) reported musculoskeletal effects in employees from a steel plant of Cubatao, 

S. Paulo, Brazil, who presented with neutropenia due to benzene exposure. Patients either were employed 

at the steel plant (mean time of 7 years and 4 months), or were employees of a building construction 

company working at repairs in the steel plant (mean time of 5 years and 5 months). Sixty percent of the 

workers had nonspecific clinical complaints such as myalgia. 

Hepatic Effects. No specific reports of adverse hepatic effects of inhalation exposure to benzene in 

humans were found, although Aksoy et al. (1972) reported enlarged livers in workers chronically exposed 

to benzene at airborne concentrations ranging from 150 to 650 ppm. 

CFY rats (20/group) were exposed to pure air, benzene (125 ppm) or benzene (400 ppm) and toluene 

(265 ppm) for 24 hours/day from gestational day (Gd) 7 through 14 (Tatrai et al. 1980a). The rats were 

then sacrificed on day 21 of pregnancy. Exposure to 125 ppm benzene caused a slight increase in relative 

liver weight of 4.67% compared to 4.25% in controls, which was not considered adverse. In a companion 

study, CFY rats were exposed to continuous benzene inhalation 24 hours/day from day 7 to day 14 of 

gestation at 0, 47, 141, 470, or 939 ppm atmospheric concentrations (Tatrai et al. 1980b). At 141 ppm 

benzene, there was a significant increase in relative maternal liver weight. 

No treatment-related non-neoplastic histopathological effects on hepatic tissue were found in male 

Sprague-Dawley rats exposed to 0, 100, or 300 ppm benzene 5 days/week, 6 hours/day for life (Snyder et 

al. 1978a, 1984) or in AKR/J or C57B1/6J mice similarly exposed to 300 ppm for life (Snyder et al. 

1978a, 1980). 

Renal Effects. Very little data are available describing renal effects in humans after inhalation 

exposure to benzene. In a case study involving the death of an 18-year-old boy who intentionally inhaled 

benzene, the autopsy revealed acute kidney congestion (Winek and Collom 1971). No other details or 

data were given. 

No treatment-related histopathological effects on kidney tissue were found in male Sprague-Dawley rats 

were exposed to 0, 100, or 300 ppm benzene 5 days/week, 6 hours/day for life (Snyder et al. 1978a, 1984) 

or in AKR/J or C57B1/6J mice similarly exposed to 300 ppm (Snyder et al. 1978a, 1980). 
BENZENE 83 

3. HEALTH EFFECTS 

Dermal Effects. Dermal effects in humans have been reported after acute exposure to benzene vapors 

(Avis and Hutton 1993). After a fatal occupational exposure to benzene vapors on a chemical cargo ship 

for only minutes, autopsy reports on three victims revealed hemorrhagic respiratory tissues, and second 

degree burns on the face, trunk, and limbs (Avis and Hutton 1993). Skin irritation has been noted at 

occupational exposures of >60 ppm for up to 3 weeks (Midzenski et al. 1992). These effects are due to 

direct contact of the skin with the vapor, and other dermal effects resulting from direct contact of the skin 

are discussed in Section 3.2.3.2. 

Ocular Effects. Three hundred solvent workers who had inhalation exposures for >1 year to benzene 

at 33 and 59 ppm for men and women, respectively, complained of eye irritation (Yin et al. 1987b). 

Male Charles River CD rats exposed to 0, 1, 10, 30, or 300 ppm benzene 6 hours/day, 5 days/week for 

10 weeks exhibited lacrimation at concentrations >10 ppm during the first 3 weeks of treatment (Shell 

1980). 

These effects are due to direct contact of the eyes with the vapor, and other ocular effects resulting from 

direct contact of the eyes are discussed in Section 3.2.3.2. 

Body Weight Effects. Relatively few studies in animals report changes in body weight after 

inhalation exposure to benzene. No change in body weight was noted in Sprague-Dawley rats or 

CD-1 mice exposed to 300 ppm benzene for 13 weeks (Ward et al. 1985). No significant decrease in 

body weight was observed in CD-1 mice exposed to doses up to 4,862 ppm for 6 hours/day, for 5 days, or 

at lower doses of 9.6 ppm for 50 days (Green et al. 1981b). Decreases in body weight (15%) were seen in 

DBA/2 mice after exposure to 300 ppm benzene in air for 6 hours/day, 5 days/week for 2 weeks 

(Chertkov et al. 1992). Decreased body weight (16–18%) has also been noted in mice exposed to doses 

of approximately 200 ppm of benzene for 6 hours/day for 7 or 14 days (Aoyama 1986). C57BL mice 

exhibited a 59% decrease in body weight gain after exposure to 300 ppm benzene, 5 hours/day, 

6 days/week over their lifetime (Snyder et al. 1980). Decreased maternal body weight and weight gain 

have been observed in Sprague-Dawley rats exposed to 50 ppm benzene during gestation days (Gds) 

6-15 (Kuna and Kapp 1981), but not in rats exposed to doses up to 300 ppm during premating, mating, 

gestation, and lactation (Kuna et al. 1992). CFY rats (20/group) were exposed to pure air, benzene 

(125 ppm), or benzene (400 ppm) plus toluene (265 ppm) for 24 hours/day from Gd 7 through 14 (Tatrai 

et al. 1980a). The rats were then sacrificed on day 21 of pregnancy. Exposure to 125 ppm benzene 

caused decreased maternal weight gain (46.74% of starting weight as opposed to 68.82% of starting 
BENZENE 84 

3. HEALTH EFFECTS 

weight in controls). Decreased maternal weight gain was also observed in a companion experiment in 

which the rats were exposed to doses as low as 47 ppm using the same study design (Tatrai et al. 1980b). 

Decreased maternal body weight was also observed in rats exposed to 2,200 ppm benzene during 

gestation (Green et al. 1978). Rabbits exposed to 313 ppm benzene on Gd 7–20 exhibited decreased 

maternal weight gain (Ungvary and Tatrai 1985). Kunming mice exposed to 12.52 ppm benzene for 

2 hours/day, 6 days/week for 30 days exhibited no adverse effect on body weight (Li et al. 1992). 

Sprague-Dawley rats received 100 or 300 ppm benzene vapor for 6 hours/day, 5 days/week for life 

(Snyder et al. 1978a, 1984). Decreased weight gain, which continued throughout the study, was observed 

at 30 weeks at 300 ppm, but not at 100 ppm. AKR mice exposed to 100 or 300 ppm and C57BL mice 

exposed to 300 ppm benzene vapor for 6 hours/day, 5 days/week for life had decreased weight gain at 

300 ppm (Snyder et al. 1978a, 1980). 

3.2.1.3 Immunological and Lymphoreticular Effects 

Immunological effects have been reported in humans with occupational exposure to benzene. There are 

two types of acquired immunity, humoral and cellular, and benzene damages both. First, benzene has 

been shown to alter humoral immunity (i.e., to produce changes in levels of antibodies in the blood). 

Painters who were exposed to benzene (3–7 ppm), toluene, and xylene in the workplace for 1–21 years 

showed increased serum immunoglobulin values for IgM and decreased values for IgG and IgA (Lange et 

al. 1973b). The decreased levels of immunoglobulins may represent suppression of immunoglobulinproducing cells by benzene. Other adverse reactions, characterized by a reaction between leukocytes and 

agglutinins, occurred in 10 of 35 of these workers (Lange et al. 1973a). These results suggest the 

occurrence of allergic blood dyscrasia in some persons exposed to benzene. However, since the workers 

were exposed to multiple solvents, the specific role of benzene is uncertain. 

The second type of immunity, cellular immunity, is affected by changes in circulating leukocytes and a 

subcategory of leukocytes, called lymphocytes. Leukopenia was found in a series of studies of workers 

exposed to benzene at levels ranging from 15 to 210 ppm in various manufacturing processes in Turkey 

(Aksoy et al. 1971, 1987). Another study also noted signs of preleukemia that included loss of leukocytes 

and other blood elements, bone marrow histopathology, and enlarged spleens (Aksoy et al. 1972, 1974). 

Other studies in chronically exposed workers also showed losses of lymphocytes and other blood 

elements (Cody et al. 1993; Goldwater 1941; Greenburg et al. 1939; Kipen et al. 1989; Ruiz et al. 1994; 

Yin et al. 1987c). In these studies, benzene levels in workplace air ranged from 1 to 1,060 ppm. 

Hematological effects reported in these studies are described in Section 3.2.1.2. In one study, routine 
BENZENE 85 

3. HEALTH EFFECTS 

leukocyte counts conducted every 3 months on employees of a small-scale industry in China revealed 

leukopenia in workers exposed to as little as 0.69–140 ppm (mean=6 ppm) for an average period of 5– 

6 years (Xia et al. 1995). Leukocyte alkaline phosphatase (LAP) activity was increased in benzene 

workers exposed to about 31 ppm for a chronic time period (Songnian et al. 1982). Increased LAP 

activity is an indicator of myelofibrosis and is associated with both decreased white blood cell counts and 

with changes in bone marrow activity. The change in LAP activity could be used in the diagnosis of 

benzene poisoning since it was more sensitive than the change in the leukocyte count, although it is not a 

biomarker that is specific for benzene exposure. A study conducted by Li et al. (1994) during 1972–1987 

examined 74,828 benzene-exposed workers employed in 672 factories and 35,805 unexposed workers 

from 109 factories located in 12 cities in China. Estimates of gender-specific rate ratios and a comparison 

of the rate ratios for females to the rate ratios for males were calculated for the incidence of hematopoietic 

and lymphoproliferative (HLP) disorders, comparing all exposed workers in each of the occupational 

groups to unexposed workers. Small increases in relative risks for all HLP disorders for both genders 

were observed among chemical and rubber manufacturing workers, painters, and paint manufacturers. In 

another study, an increase in leukocyte count and alkaline phosphatase score was observed in a pipe-fitter 

who was chronically exposed to 0.9 ppm benzene in addition to other solvents (Froom et al. 1994). 

Animal studies support the observations made in humans and show that benzene affects humoral and 

cellular immunity. A decrease in spleen weight was observed in mice after exposure to benzene at a 

concentration of 25 ppm, 6 hours/day for 5 days, the same exposure concentration at which a decrease in 

circulating leukocytes was observed (Wells and Nerland 1991). Benzene decreases the formation of the 

B-lymphocytes that produce the serum immunoglobulins or antibodies. Exposure to benzene at 10 ppm 

and above for 6 days decreased the ability of bone marrow cells to produce mature B-lymphocytes in 

C57BL/6 mice (Rozen et al. 1984). The spleen was also inhibited from forming mature T-lymphocytes at 

exposure levels of 31 ppm and above. Mitogen-induced blastogenesis of B- and T-lymphocytes was 

depressed at 10 ppm and above. Peripheral lymphocyte counts were depressed at all levels, whereas 

erythrocyte counts were depressed only at 100 and 300 ppm. This study is the basis for the acute-duration 

inhalation MRL of 0.009 ppm (see footnote to Table 3-1 and Appendix A). 

A continuation of this line of studies for 6 days to 23 weeks at 300 ppm showed continued decreases in 

numbers of mature B- and T-lymphocytes produced in the bone marrow, spleen, and thymus (Rozen and 

Snyder 1985). Abnormalities of humoral and cell-mediated immune responses following benzene 

exposure are presumably caused by a defect in the lymphoid stem cell precursors of both T- and 

B-lymphocytes. Bone marrow cellularity increased 3-fold, and the number of thymic T-cells increased 
BENZENE 86 

3. HEALTH EFFECTS 

15-fold in benzene-exposed mice between the 6th and the 30th exposure. No corresponding increase in 

splenic cells was noted. The marked increase in the numbers of cells in bone marrow and thymus was 

interpreted by the authors to indicate a compensatory proliferation in these cell lines in response to 

benzene exposure, which may play a role in the carcinogenic response of C57BL mice to inhaled 

benzene. The lack of response in the spleen suggests a lack of lymphoid restorative capacity in that 

organ. 

Other studies have also shown similar effects on immune functions following inhalation exposure to 

benzene. These include decreased numbers of circulating leukocytes and decreases in bone marrow 

cellularity in mice exposed to 100 ppm and higher, 24 hours/day for up to 8 days (Gill et al. 1980); 

decreased leukocytes and increased leukocyte alkaline phosphatase in rats exposed to 100 ppm for 7 days 

(Li et al. 1986); decreased leukocytes and bone marrow cellularity in DBA/2 mice exposed to 300 ppm 

benzene for 2 weeks (Chertkov et al. 1992); decreased leukocytes in mice exposed to 300 ppm for 

10 days (Ward et al. 1985); bone marrow hyperplasia, lymphocytopenia, and anemia in mice repeatedly 

exposed to 100 ppm or higher for a lifetime (Snyder et al. 1978a, 1980); and leukopenia and lymphopenia 

in rats repeatedly exposed to 100 ppm for life (Snyder et al. 1978a, 1984). Aoyama (1986) noted 

decreased spleen and thymus weights and decreased levels of B- and T-lymphocytes in blood and spleen 

in mice exposed 6 hours/day to approximately 48 ppm of benzene for 14 days. Exposures of mice to 

benzene vapor concentrations ≥100 ppm, 6 hours/day, 5 days/week resulted in significantly reduced 

numbers of lymphocytes (ranging from 25 to 43% lower than controls) in the spleen, thymus, and femur 

as early as 1 week following the initial exposure, and persisting throughout an 8-week exposure period 

(Farris et al. 1997a). In rats, exposure to benzene vapors at a concentration of 400 ppm (but not 200 ppm) 

for 6 hours/day, 5 days/week for 4 weeks resulted in reduced thymus weight and decreased splenic 

lymphocytes (Robinson et al. 1997). 

Reduced bone marrow cellularity was observed in Swiss Webster and C57BL/6J mice after exposure to 

300 ppm benzene for 2 weeks (Neun et al. 1992). Decreased granulopoietic stem cells were observed at 

21 ppm in NMRI mice after exposures of 10 days to 2 weeks (Toft et al. 1982). Green and colleagues 

examined the effect of benzene inhalation on peripheral blood and bone marrow and spleen cells (Green 

et al. 1981a, 1981b) of CD-1 mice following exposures to a number of regimens. In the acute studies, 

individual test groups were exposed by inhalation to mean benzene concentrations of 0, 1.1, 9.9, 103, 306, 

603, 1,276, 2,416, or 4,862 ppm, 6 hours/day for 5 days. Intermediate studies consisted of two different 

exposure regimens and concentrations: 6 hours/day, 5 days/week for 10 weeks (50 days) to a mean 

concentration of 0 or 9.6 ppm benzene; or 6 hours/day, 5 days/week for 26 weeks to a mean concentration 
BENZENE 87 

3. HEALTH EFFECTS 

of 0 or 302 ppm benzene. In the acute studies, marrow and splenic cellularities were reduced at 103 ppm. 

Splenic granulocytes were reduced at all exposure levels except at 9.9 ppm (Green et al. 1981b). The 

decrease in spleen cellularity correlated with a reduction in spleen weight at all concentrations ≥103 ppm. 

Mean spleen weights were significantly depressed at 1.1 ppm and at doses above 9.9 ppm, but not at 

9.9 ppm. In Green et al. (1981a), marrow concentration of GM-CFU-C was equivalent to or greater than 

control values at all levels; however, splenic GM-CFU-C concentration was decreased at 103 ppm. 

Femoral and splenic CFU-S and GM-CFU-C per organ were depressed at 103 ppm. Absolute numbers of 

GM-CFU-C/femur or spleen were significantly reduced at all higher concentrations. There was no 

change in the colony/cluster ratio observed in the 5-day experiment. In the 50-day experiments, increased 

spleen weight, splenic nucleated cellularity, splenic nucleated erythrocytes, and CFU-S were seen at 

9.6 ppm (Green et al. 1981a, 1981b). Exposure to 302 ppm for 26 weeks resulted in reduced marrow and 

spleen cellularity and decreased spleen weight (Green et al. 1981a, 1981b). 

Sprague-Dawley SD/Tex rats were exposed to benzene vapor at 0 or 500 ppm for 5 days/week, 

6 hours/day for 3 weeks (Dow 1992). In the bone marrow differential counts, rats showed a relative 

decrease in lymphoid cells at the 500 ppm dose level. There were also decreases in myeloid cells of 

animals exposed to 500 ppm benzene. In a companion study, purebred Duroc-Jersey pigs were exposed 

to 0, 20, 100, and 500 ppm benzene vapors 6 hours/day, 5 days/week for 3 weeks (Dow 1992). Exposure 

to 500 ppm resulted in significant decreases in total white blood cells, T-cells, peripheral blood 

lymphocytes, and proportion of myeloid cells in bone marrow counts. 

Another series of experiments revealed that exposures as low as 25 ppm for 2 weeks caused decreases in 

the numbers of circulating lymphocytes but did not affect the bone marrow cellularity in C57BL/6 or 

CBA/Ca mice (Cronkite 1986; Cronkite et al. 1985). Increasing the duration (up to 16 weeks) or 

concentration (up to 300–400 ppm) produced the same effect of reduced peripheral blood lymphocytes as 

well as decreased bone marrow cellularity that persisted for up to 8 weeks after exposure (Cronkite et al. 

1982, 1985). Similar observations were made in two groups of male CBA/Ca mice exposed to a total of 

6,000 ppm of benzene by inhalation using two different regimens (Cronkite et al. 1989). One group was 

exposed to 316 ppm for a total of 19 times, while the second group was exposed to 3,000 ppm twice. 

Although both groups had significantly decreased lymphocyte counts, the lymphocyte numbers were 

much more depressed in the group of mice exposed to 316 ppm of benzene over 19 days. This suggests 

that, at relatively high exposure levels, repeated or prolonged exposure is more potent than the magnitude 

of exposure in causing lymphopenia. In both groups, the lymphocyte numbers had not returned to normal 
BENZENE 88 

3. HEALTH EFFECTS 

values by 214 days post exposure. The femoral bone marrow cellularity returned to normal levels 32 days 

after exposure in both groups of mice. 

In female BDF1 mice (C57BL/6xDBA/2F1 hybrids) exposed to 0, 300, or 900 ppm benzene 6 hours/day 

for 5 days, there was a relative increase in helper lymphocytes (CD4+) at days 3–5, leading to an increase 

of the T4/T8 (CD4+–CD8+) ratio from 2 in controls to higher values at 8 weeks (Plappert et al. 1994a). 

No concentration dependency was observed. Granulocytic hyperplasia of the spleen was observed in 

CBA/Ca mice exposed to 300 ppm benzene for 16 weeks (Farris et al. 1993). 

Leukopenia, granulocytopenia, and lymphocytopenia were observed in C57BL/6 mice exposed to 

1,000 ppm benzene for 6 hours/day, 5 days/week for up to 6 weeks (Gill et al. 1980). Decreased 

leukocyte counts were also noted in Sprague-Dawley rats and CD-1 mice exposed to 300 ppm benzene 

6 hours/day, 5 days/week for up to 13 weeks (Ward et al. 1985). In mice, the most common compoundrelated histopathological findings were splenic periarteriolar lymphoid sheath depletion, lymphoid 

depletion in the mesenteric lymph node, and plasma cell infiltration of the mandibular lymph node. 

Exposure to the highest concentration caused a decrease in leukocyte and lymphocyte counts in SpragueDawley rats. Hematological changes at 300 ppm were accounted for by decreased leukocyte counts in 

males on day 14 and in females on day 91. Decreases in percentage of lymphocytes in males and females 

started on day 14 and lasted through day 91. Rats exhibited decreased femoral marrow cellularity as the 

only histological change. Treatment-related changes were not observed at lower concentrations. Rabbits, 

rats, and guinea pigs exposed to 80–88 ppm for 32–269 days also had decreased leukocyte counts (Wolf 

et al. 1956). Leukocyte alkaline phosphatase values were increased and leukocyte counts were decreased 

in rats exposed to 4,570 ppm for 20 weeks (Songnian et al. 1982). Exposure of C57BL mice to 10 ppm 

benzene for 6 hours/day, 5 days/week, for 24 weeks caused depressions in the numbers of splenic 

nucleated red cells and lymphocytes (Baarson et al. 1984). 

Li et al. (1992) observed a 26% decrease in spleen weight in male Kunming mice exposed to 12.52 ppm 

benzene 2 hours/day, 6 days/week for 30 days. Examination of the bone marrow showed decreases in 

myelocytes, premyelocytes, myeloblasts, and metamyeloblasts at the same dose level. 

Benzene also affects functional immune responses, as indicated by decreased resistance to infectious 

agents. Pre-exposure to benzene at 30 ppm for 5–12 days increased the bacterial counts in mice on 

day 4 of infection with Listeria monocytogenes (Rosenthal and Snyder 1985). Recovery of the immune 

system was noted on day 7. The effects did not occur at 10 ppm. In addition, a concentration-dependent 
BENZENE 89 

3. HEALTH EFFECTS 

statistically significant depression was noted in T- and B-lymphocyte populations from day 1 through 

day 7 at 30 ppm and above. B-cells were more sensitive to benzene than were T-cells on a percentage-ofcontrol basis. These results indicate a benzene-induced delay in immune response to L. monocytogenes. 

Concentrations of 200 or 400 ppm for 4–5 weeks (5 days/week) suppressed the primary antibody 

response to tetanus toxin in mice, but there was no effect at 50 ppm (Stoner et al. 1981). In another 

intermediate-duration exposure study, no changes were noted in the numbers of splenic B-cells, T-cells, 

or T-cell subsets in C57BL/6 mice exposed to 100 ppm of benzene 5 days/week for 20 days (Rosenthal 

and Snyder 1987). However, when splenic T-cells from mice treated with 10 ppm and 100 ppm were 

tested in vitro for their capacity to respond to foreign antigens (alloantigens) in the mixed lymphocyte 

reaction (MLR), the MLR response was delayed. Further analysis showing that this delayed MLR 

response was not due to the presence of benzene-induced suppressor cells and indicated that benzene 

impaired the functional abilities of alloreactive T-cells. This study is the basis for the intermediateduration inhalation MRL of 0.006 ppm (see footnote to Table 3-1 and Appendix A). A similar in vitro 

observation was made using T-cells from mice exposed to 100 ppm of benzene 5 days/week for 3 weeks. 

These cells had a reduced tumor cytolytic activity, suggesting a benzene-induced impairment of cellmediated immunity. The impaired cell-mediated immune function was also apparent in vivo. Mice 

exposed to 100 ppm for a total of 100 days were challenged with 10,000 polyoma virus-induced tumor 

cells (PYB6), and 9 of 10 mice had reduced tumor resistance and developed tumors that were lethal 

(Rosenthal and Snyder 1987). 

The highest NOAEL values and all reliable LOAEL values for immunological effects in each species and 

duration category are recorded in Table 3-1 and plotted in Figure 3-1. 

3.2.1.4 Neurological Effects 

Following acute inhalation of benzene, humans exhibit symptoms indicative of central nervous system 

effects (Cronin 1924; Flury 1928; Greenburg 1926; Midzenski et al. 1992). These symptoms, reported to 

occur at levels ranging from 300 to 3,000 ppm, include drowsiness, dizziness, headache, vertigo, tremor, 

delirium, and loss of consciousness. Acute exposure (5–10 minutes) to higher concentrations of benzene 

(approximately 20,000 ppm) can result in death, which has been associated with vascular congestion in 

the brain (Avis and Hutton 1993; Flury et al. 1928). Lethal exposures are also associated with 

nonspecific neurological symptoms similar to those reported for nonlethal exposures. These symptoms 

are similar to the consequences of exposure to multiple organic solvents and are reversible when 

symptomatic workers are removed from the problem area (Kraut et al. 1988; Yin et al. 1987b). In reports 
BENZENE 90 

3. HEALTH EFFECTS 

of cases of benzene poisoning, subjects exhibited headaches, nausea, tremor, convulsions, and 

unconsciousness, among other neurological effects (Cronin 1924; Greenburg 1926; Midzenski et al. 1992; 

Tauber 1970). 

Chronic exposure to benzene has been reported to produce neurological abnormalities in humans. Of 

eight patients (six with aplastic anemia and two with preleukemia) with previous occupational exposure to 

adhesives and solutions containing 9–88% benzene, four of the six patients with aplastic anemia showed 

neurological abnormalities (global atrophy of lower extremities and distal neuropathy of upper 

extremities) (Baslo and Aksoy 1982). Air concentrations in the workplace were reported to have reached 

levels of ≥210 ppm. These findings suggest that benzene may induce toxic effects on the nervous system 

involving peripheral nerves and/or spinal cord. The limitations of this study are that benzene exposure 

levels were not monitored and that there was a possibility of an additional exposure to toluene (6.37– 

9.25%). 

Chronic exposure to benzene and toluene was studied in 121 workers exposed to benzene for 2–9 years 

(Kahn and Muzyka 1973). The air concentration of benzene between 1962 and 1965 was 6–15.6 ppm 

(20–50 mg/m3

), while the toluene vapors did not exceed the 5 mg/m3 level. Subsequently (the authors do 

not specify when), the air levels of both benzene and toluene have not exceeded the 5 mg/m3

 level. 

Seventy-four of the examined workers complained of frequent headaches (usually at the end of the work 

day), became tired easily, had difficulties sleeping, and complained of memory loss. The limitations of 

this study are that workers were exposed to both benzene and toluene and that the precise dose and 

duration of exposure are not known. 

Tondel et al. (1995) reported the case of a gasoline station attendant who had worked from 1962 to 1979. 

The patient described symptoms of fatigue for 3 weeks and night sweats, among other symptoms. 

The neurotoxicity of benzene has not been studied extensively in animals. Female Sprague-Dawley rats 

exhibited lethargy after exposure to 2,200 ppm benzene, but not 300 ppm, on Gd 6–15 (Green et al. 

1978). Male albino SPF rats from a Wistar-derived strain exposed to benzene for 4 hours in glass 

chambers (dose not specified) exhibited depression of evoked electrical activity in the brain; the authors 

calculated the 30% effect level (depressed activity) as 929 ppm (Frantik et al. 1994). When female 

H strain mice were exposed to benzene for 2 hours, the 30% effect level for depression of evoked 

electrical activity in the brain was 856 ppm (Frantik et al. 1994). In rabbits, symptoms that occurred 

3.7 minutes following acute exposure to benzene at 45,000 ppm were relaxation and light narcosis 
BENZENE 91 

3. HEALTH EFFECTS 

(Carpenter et al. 1944). As the time after exposure progressed, so did the symptoms to include excitation, 

chewing, and tremors (after 5 minutes), loss of pupillary reflex to strong light (after 6.5 minutes), loss of 

blinking reflex (after 11.4 minutes), pupillary contraction (after 12 minutes), and involuntary blinking 

(after 15.6 minutes). Behavioral tests of C57BL/6 mice showed significant increase in licking of 

sweetened milk after 1 week of exposure to 300 ppm; a 90% decrease in hind limb grip strength after one 

exposure to 1,000 or 3,000 ppm (data for 100 ppm were not reported); and tremors after one exposure to 

3,000 ppm that subsided 30 minutes after the exposure (Dempster et al. 1984). In another study, designed 

to reflect occupational exposure, male CD-1 and C57BL/6 mice were exposed to 300 or 900 ppm of 

benzene 6 hours/day for 5 days followed by 2 weeks of no exposure after which the exposure regimen 

was repeated for an unspecified amount of time (Evans et al. 1981). The following seven categories of 

behavioral activities were monitored in exposed and control animals: stereotypic behavior, sleeping, 

resting, grooming, eating, locomotion, and fighting. Only minimal and insignificant differences were 

observed between the two strains of mice. Increased behavioral activity was observed after exposure to 

benzene in both strains of mice. Mice exposed to 300 ppm of benzene had a greater increase than those 

exposed to 900 ppm, probably because of narcosis-like effects induced at the higher exposure level 

(Evans et al. 1981). It is not known if benzene induces behavioral changes by directly acting upon the 

central nervous system. It is also not known whether these changes occur before or after hematological 

changes. 

Li et al. (1992) exposed male Kunming mice to 0, 0.78, 3.13, or 12.52 ppm benzene for 2 hours/day, 

6 days/week for 30 days, and then monitored brain and blood acetylcholinesterase, forelimb grip strength, 

locomotor activity, and rapid response. Significantly increased grip strength was observed at 0.78 ppm, 

whereas at the higher doses, grip strength decreased significantly. Rapid response showed a significant 

increase at the low dose; the two higher doses showed a significant depressed rapid response. Locomotor 

activity increased at the low dose, was similar to control values at the middle dose, and decreased at the 

high dose. However, these changes were not significantly different from the control values. A significant 

decrease in acetylcholinesterase activity was noted in the brain, but it was not large enough to be 

considered adverse; no change in acetylcholinesterase levels in the blood was observed. However, the 

exposure levels used by Li et al. (1992) were more than 10-fold lower than those eliciting signs of 

neurotoxicity in other animal studies, an indication that the actual exposure levels in the study of Li et al. 

(1992) may have been higher than indicated. Uncertainty regarding actual exposure levels renders this 

study of little value for purposes of risk assessment. 
BENZENE 92 

3. HEALTH EFFECTS 

All reliable LOAEL values for neurological effects in each species and duration category are recorded in 

Table 3-1 and plotted in Figure 3-1. 

3.2.1.5 Reproductive Effects 

Data on the reproductive effects of occupational exposure to benzene suggest that benzene may impair 

fertility in women (Mukhametova and Vozovaya 1972; Vara and Kinnunen 1946). However, the findings 

are inconclusive due to uncertainties in exposure assessment and the limited data collected. In one study, 

30 women with symptoms of benzene toxicity were examined (Vara and Kinnunen 1946). The levels of 

benzene in air were not specified, but are assumed to have been much greater than the 1 ppm permitted in 

today's working environment. Twelve of these women had menstrual disorders (profuse or scanty blood 

flow and dysmenorrhea). Ten of the 12 women provided information on fertility. Of these 10 women, 

2 had spontaneous abortions, and no births occurred during their employment, even though no 

contraceptive measures had been taken. This led the investigators to suggest that benzene has a 

detrimental effect on fertility at high levels of exposure. However, the study failed to provide verification 

that the absence of birth was due to infertility. Gynecological examinations revealed that the scanty 

menstruations of five of the patients were due to ovarian atrophy. This study is limited in that an 

appropriate comparison population was not identified. Additionally, little follow-up was conducted on 

the 30 women with regard to their continued work history and possible symptoms of benzene toxicity. 

Disturbances of the menstrual cycle were found in women workers exposed to aromatic hydrocarbons 

(benzene, toluene, xylene) (Michon 1965). The exposure levels of benzene and toluene were below 

0.25 ppm. The observed group consisted of 500 women, 20–40 years old. One hundred controls were 

included in the study. The results showed that 21% of exposed women whose work involved sitting or 

standing had irregular menstrual cycles compared to 12% in the control group. Brief (up to 2 days), long 

(6–9 days), and prolonged (over 9 days) menstrual cycles were present in 26% of women who performed 

lifting during their work as compared to 13% in the control group. Irregular amounts of menstrual flow 

and pain were also observed in female workers exposed to aromatic hydrocarbons. The major limitations 

of this study are that the exposure occurred from a mixture of chemicals, levels of exposure were not well 

defined, duration of exposure was not stated, and activities of the controls were not provided. 

Another study examined the reproductive function and incidence of gynecological effects in 360 women 

exposed to petroleum (a major source of benzene) and chlorinated hydrocarbons both dermally and by 

inhalation (Mukhametova and Vozovaya 1972). However, dermal exposure was considered to be 
BENZENE 93 

3. HEALTH EFFECTS 

negligible. The concentrations of benzene in the air were not well documented. When compared to 

female workers with no chemical exposure, female gluers had developed functional disturbances of the 

menstrual cycle. Additionally, as chemical exposure time increased, there were increases in the number 

of premature interruptions of pregnancy, the percentage of cases in which the membranes ruptured late, 

and the number of cases of intrauterine asphyxia of the fetus. The study limitations (including lack of 

exposure history, simultaneous exposure to other substances, and lack of follow-up) make it difficult to 

assess the effects of benzene on reproduction. 

Reproductive competence of male workers in two organic chemical factories in France was evaluated by 

Stucker et al. (1994). Analysis of 1,739 pregnancies that ended in spontaneous abortion or birth was 

presented. Paternal exposure to benzene for each pregnancy was described as exposure in the 3 months 

immediately before conception, and as previous job exposures. Benzene exposure was graded at two 

levels: <5 ppm (low) and ≥5 ppm (moderate). Of the 1,739 pregnancies described, 171 ended in a 

spontaneous abortion (rate=9.8%). According to exposure categories, 1,277 pregnancies were defined as 

non-exposed (mate not exposed) and 270 whose mates were exposed at some time before conception. For 

the 270 pregnant women, 145 of their mates were exposed during the 3 months immediately preceding 

conception. The frequency of spontaneous abortion was not significantly higher for the paternal group 

exposed at any time before conception than in the non-exposed group; nor was it higher for the group 

exposed during the 3 months immediately before conception. 

In CFY rats exposed to either pure air or benzene (125 ppm) for 24 hours/day from Gd 7 through 14, no 

effect on implantation number was observed (Tatrai et al. 1980a). No changes in maternal body weight 

were observed in Sprague-Dawley rats exposed to 100 ppm benzene for 6 hours/day on Gd 6–15 (Coate 

et al. 1984). Pregnant rabbits exposed 12 hours/day to 156.5 or 313 ppm benzene on Gd 7–20 showed an 

increase in the number of abortions and resorptions at 312 ppm (Ungvary and Tatrai 1985). However, in 

other developmental toxicity studies, no effect on the number of resorptions was seen in rats at doses as 

high as 2,200 ppm (Green et al. 1978), in mice at 500 ppm (Murray et al. 1979), or in rabbits at doses of 

500 ppm (Murray et al. 1979). 

Reproductive effects have been noted in experimental animals exposed for intermediate durations, but the 

levels of exposure were higher (80–6,600 ppm) than those to which humans are exposed in the modern 

industrial environment (Ward et al. 1985; Wolf et al. 1956). In an intermediate-duration inhalation study, 

groups of male and female CD-1 mice were exposed to benzene vapor concentrations of 0, 1, 10, 30, or 

300 ppm, 5 days/week, 6 hours/day to benzene vapor for 13 weeks (Ward et al. 1985). Histopathological 
BENZENE 94 

3. HEALTH EFFECTS 

changes were observed in ovaries (bilateral cysts) and testes (atrophy/degeneration, decrease in 

spermatozoa, moderate increase in abnormal sperm forms) of mice exposed to 300 ppm benzene; the 

severity of gonadal lesions was greater in the males. An inhalation study was conducted exposing rats 

(6,600 ppm), rabbits (80 ppm), and guinea pigs (88 ppm) to benzene for 7–8 hours/day, 5 days/week for 

93, 243, and 32 or 269 days, respectively (Wolf et al. 1956). Male rats showed an increase in testicular 

weight after 93 days at the 6,600 ppm level. The guinea pigs showed a slight increase in average 

testicular weight at the 88 ppm level. Rabbits showed slight histopathological testicular changes 

(degeneration of the germinal epithelium) when exposed to 80 ppm. Since only one or two rabbits were 

used in this study, it was not possible to draw any conclusions regarding benzene's ability to induce 

testicular damage in the rabbits. Continuous exposure of female rats to 210 ppm benzene for 10–15 days 

before cohabitation with males and 3 weeks after cohabitation resulted in a complete absence of litters 

(Gofmekler 1968). It is not known whether this was due to failure to mate, infertility, or early 

preimplantation losses of fertilized ova. In a fertility study, female rats exposed up to 300 ppm benzene 

for 10 weeks during premating, mating, gestation, and lactation showed no effect on indices of fertility, 

reproduction, and lactation (Kuna et al. 1992). 

The highest NOAEL values and all reliable LOAEL values for reproductive effects in each species and 

duration category are recorded in Table 3-1 and plotted in Figure 3-1. 

3.2.1.6 Developmental Effects 

The available human data on the developmental effects of benzene after inhalation exposure are 

inconclusive. The studies designed specifically to investigate developmental effects are limited, primarily 

because of concomitant exposure to other chemicals, inadequate sample size, and lack of quantification of 

exposure levels (Budnick et al. 1984; Goldman et al. 1985; Heath 1983; Olsen 1983). Benzene crosses 

the human placenta and is present in the cord blood in amounts equal to or greater than those in maternal 

blood (Dowty et al. 1976). In a study of subjects with known benzene poisoning in Italy, Forni et al. 

(1971a) reported the case of one pregnant worker exposed to benzene in the air during her entire 

pregnancy. Although she had severe pancytopenia and increased chromosomal aberrations, she delivered 

a healthy son with no evidence of chromosomal alterations. The following year, she delivered a healthy 

daughter. However, increased frequency of chromatid and isochromatid breaks and sister chromatid 

exchange was found in lymphocytes from 14 children of female workers exposed by inhalation to 

benzene (dose not specified) and other organic solvents during pregnancy (Funes-Cravioto et al. 1977). 
BENZENE 95 

3. HEALTH EFFECTS 

No mention was made of whether the mothers showed signs of toxicity or whether physical abnormalities 

occurred among their offspring. 

There are numerous inhalation studies in which animals have been exposed to benzene during pregnancy 

(Coate et al. 1984; Green et al. 1978; Kuna and Kapp 1981; Murray et al. 1979; Tatrai et al. 1980a, 

1980b; Ungvary and Tatrai 1985). None of these studies demonstrated that benzene was teratogenic even 

at levels that induced maternal and fetal toxicity. Fetotoxicity was evidenced by decreased body weight 

and by increased skeletal variants such as missing sternebrae and extra ribs, which were not considered to 

be malformations. Alterations in hematopoiesis have also been observed in the fetuses and offspring of 

pregnant mice exposed to low levels of benzene (Keller and Snyder 1986, 1988). These studies are 

discussed below. 

Mice exposed to 500 ppm benzene for 7 hours/day on days 6–15 of pregnancy exhibited fetal growth 

retardation (i.e., decreased fetal body weight) and increased minor skeletal variants (i.e., delayed 

ossification) (Murray et al. 1979). There were no fetal malformations and no significant effect on the 

incidence of pregnancy, average number of live fetuses, resorptions per litter, or maternal weight gain. 

No malformations in fetuses and no significant effects on incidence of pregnancy, average number of live 

fetuses, or resorptions per litter were observed when rabbits were exposed to benzene 7 hours/day at 

500 ppm during gestation for 9 days, although an increase in minor skeletal variations was observed 

(Murray et al. 1979). 

Pregnant mice exposed 12 hours/day to 156.5 or 313 ppm benzene on Gd 6–15 had pups with significant 

weight retardation and retardation of skeletal development, but no malformations (Ungvary and Tatrai 

1985). A parallel study in rabbits showed that inhalation of benzene at 313 ppm caused fetal weight 

reduction, and an increase in minor fetal anomalies (Ungvary and Tatrai 1985). 

As was the case with mice and rabbits, the fetotoxicity of benzene in rats is also demonstrated by retarded 

fetal weight and/or minor skeletal variants (Coate et al. 1984; Green et al. 1978; Kuna and Kapp 1981; 

Tatrai et al. 1980b). In an experiment conducted by Green et al. (1978), pregnant Sprague-Dawley rats 

were exposed to 100, 300, or 2,200 ppm benzene for 6 hours/day on Gd 6–15. Exposure to high levels of 

benzene (2,200 ppm) during gestation resulted in a significant decrease in fetal weight, whereas dams 

breathing air containing lower levels of benzene (100 or 300 ppm) during gestation bore young that were 

similar in weight and crown–rump length to control pups. Statistically significant numbers of fetuses 

with delayed ossification were found in groups exposed to concentrations of 300 and 2,200 ppm. The 
BENZENE 96 

3. HEALTH EFFECTS 

litter incidence of missing sternebrae was significantly increased in the 100 and 2,200 ppm exposure 

groups. Maternal toxicity, as indicated by a decrease in maternal weight gain, was evident only at the 

2,200 ppm level. The female offspring appeared to be affected to a greater extent than males with respect 

to delayed ossification and missing sternebrae. 

Kuna and Kapp (1981) found decreased fetal weight after exposure to 50 ppm. From a group of 151 pups 

examined after in utero exposure to 500 ppm benzene, a single pup exhibited exencephaly. In the same 

study, of 98 pups examined for skeletal effects after in utero exposures of 500 ppm, a single pup had 

angulated ribs and 2 others had nonsequential ossification of the forefeet. These anomalies were not 

statistically significant and may have resulted from maternal nutritional stress. 

No significant skeletal malformations occurred in pups of rats exposed during gestation to 47 ppm for 

8 days, 24 hours/day (Tatrai et al. 1980b) or 100 ppm for 10 days, 6 hours/day (Coate et al. 1984). 

Decreased fetal weights were seen at 47 ppm (Tatrai et al. 1980b) and 100 ppm (Coate et al. 1984), and 

increased fetal mortality was observed at 141 ppm (Tatrai et al. 1980b). In CFY rats exposed to pure air 

or 125 ppm benzene on Gd 7–14, there was a 17% decrease in placental weight, a decrease in mean fetal 

weight, and evidence of skeletal retardation (Tatrai et al. 1980a). Continuous exposure of female rats to 

6 concentrations of benzene ranging from 0.3 to 210 ppm for 10–15 days before cohabitation with males 

and 3 weeks after did not affect newborn weight or induce malformations, but there were differences in 

the weights of individual organs of the dams at all exposure levels (Gofmekler 1968). There was a slight 

tendency toward decreased litter sizes at 20 ppm of benzene. A complete absence of litters resulted from 

exposure to 210 ppm. It is not known whether this was due to failure to mate, infertility, or early 

preimplantation losses of fertilized ova. 

Alterations in hematopoiesis have also been observed in the fetuses and offspring of pregnant mice 

exposed to benzene (Keller and Snyder 1986). Administration of 20 ppm benzene to pregnant Swiss 

Webster mice for 6 hours/day on Gd 6–15 caused reductions in the levels of the CFU-E of the fetuses, 

whereas 5 and 10 ppm benzene caused enhancement of these colony-forming cells. In 2-day-old 

neonates, CFU-E numbers in the 5 ppm group returned to control values, but the 10 ppm neonates showed 

a bimodal response by litter. Granulocytic colony-forming cells were enhanced in neonates exposed in 

utero to 20 ppm benzene. Some of the mice exposed to 10 ppm prenatally were re-exposed to 10 ppm as 

adults. Their hematopoietic progenitor cell numbers were depressed compared with controls exposed for 

the first time as adults. No tests were conducted on the dams after benzene exposure. 
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In a follow-up study, pregnant Swiss Webster mice were exposed 6 hours/day on Gd 6–15 to 5, 10, or 

20 ppm benzene (Keller and Snyder 1988). The results indicated that 16-day fetuses, when checked for 

erythrocyte and leukocyte counts, hemoglobin analysis, and the proliferating pool of differentiating 

hematopoietic cells, had no noteworthy change at any of the exposure levels. In contrast, 2-day neonates 

exposed in utero to all concentrations of benzene exhibited a reduced number of circulating erythroid 

precursor cells and, at 20 ppm, had increased numbers of hepatic hematopoietic blast cells and 

granulopoietic precursor cells accompanied by decreased numbers of erythropoietic precursor cells. 

Six-week-old adult mice exposed in utero to 20 ppm of benzene had a similar pattern of enhanced 

granulopoiesis. However, this effect was not clearly evident in 6-week-old adult mice exposed in utero to 

5 or 10 ppm. 

The results of inhalation studies conducted in experimental animals have been fairly consistent across 

species. It has been suggested that benzene fetotoxicity in animals is a function of maternal toxicity 

because the joint occurrence of a decrease in fetal weight and an increase in skeletal variants usually 

occurs when there is a decrease in maternal weight (Tatrai et al. 1980b). However, the mechanism 

underlying developmental toxicity has not been fully elucidated, and there are few data on the effect of 

benzene on maternal food consumption and on blood levels of benzene and its metabolites in the dams 

and their fetuses. There are apparently none of the usual fetotoxic findings after exposure in utero to low 

concentrations of benzene (10 ppm) (Coate et al. 1984; Kuna and Kapp 1981). As stated above, there is 

evidence for persistent hematopoietic anomalies in animals exposed in utero to benzene at 20 ppm (Keller 

and Snyder 1988). They may also exist at lower concentrations, but adequate testing has not been 

performed. 

The highest NOAEL value and all reliable LOAEL values for developmental effects in each species 

following acute exposure are recorded in Table 3-1 and plotted in Figure 3-1. 

3.2.1.7 Cancer 

Epidemiological studies and case reports provide clear evidence of a causal relationship between 

occupational exposure to benzene and benzene-containing solvents and the occurrence of acute 

nonlymphocytic leukemia (ANLL), particularly the myeloid cell type (acute myelogenous leukemia, 

AML) (EPA 1985b, 1998; Hayes et al. 1997; IARC 1982, 1987; IRIS 2007; Rinsky et al. 1987, 2002; Yin 

et al. 1996a, 1996b). Some of the studies also provide suggestive evidence of associations between 

benzene exposure and non-Hodgkin’s lymphoma (NHL) and multiple myeloma (Hayes et al. 1997; 
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3. HEALTH EFFECTS 

Rinsky et al. 1987). The epidemiological studies are generally limited by confounding chemical 

exposures and methodological problems, including inadequate or lack of exposure monitoring and low 

statistical power (due to small numbers of cases), but a consistent excess risk of leukemia across studies 

indicates that benzene is the causal factor. Many of the earlier studies are additionally limited by a lack of 

information on leukemia cell types other than AML, because leukemia used to be considered a single 

diagnostic category for epidemiological purposes, due in part to historical nomenclature, small numbers 

of deaths by cell type, and unavailability of cell-type-specific rates for comparison. 

Two series of studies on workers exposed to benzene in Ohio (the Pliofilm study) (e.g., Rinsky et al. 

1981, 1987, 2002) and China (the NCI/CAPM study) (e.g., Hayes et al. 1997; Yin et al. 1996a, 1996b) 

have yielded particularly strong data on the leukemogenic potential of benzene and are summarized 

below. The studies of the Pliofilm cohort provide the best set of data for evaluating human cancer risks 

from benzene exposure because, in comparison to other published studies, the Pliofilm workers had the 

fewest reported co-exposures to other potentially carcinogenic substances and experienced a greater range 

of estimated exposures to benzene (EPA 1998). The NCI/CAPM Chinese study is one of the largest of its 

type ever undertaken and evaluated many thousands of benzene-exposed workers, enabling detection of 

significantly elevated risks at unusually low levels of exposure. 

A number of studies were performed on a cohort of workers exposed to benzene in three rubber 

hydrochloride (‘Pliofilm’) manufacturing plants in Ohio (Collins et al. 1997; Finkelstein 2000; Infante 

1978; Infante et al. 1977; Ireland et al. 1997; Paxton et al. 1994a, 1994b; Rinsky et al. 1981, 1987, 2002; 

Schnatter et al. 1996a; Wong 1995). Analyses using various expansions and updates of the cohort, 

statistical methods, and exposure estimates have consistently shown a significant relationship between 

exposure to benzene and excess mortality from leukemia. In the report by Rinsky et al. (1987), a cohort 

of 1,165 white males employed between 1940 and 1965 and followed through 1981 experienced 

increased mortality from all leukemias (9 observed versus 2.7 expected; standardized mortality ratio 

[SMR]=3.37, 95% CI 1.54–6.41) and multiple myeloma (4 observed versus 1 expected; SMR=4.09, 95% 

CI 1.10–10.47). Death rates for age-matched U.S. white males during the same calendar period were 

used for comparison with death rates observed in the cohort. Assessment after an additional 15 years of 

follow-up showed declines in the SMRs for both leukemias (2.56, 95% CI 1.43–4.22) and multiple 

myeloma (2.12, 95% CI 0.69–4.96), suggesting that the excess risks diminished with time since exposure 

(Rinsky et al. 2002). Exposures in the most recent 10 years were most strongly associated with leukemia 

risk, and there was no significant relation between leukemia death and benzene exposures received more 

than 20 years previously (Finkelstein 2000). AML accounted for most of the increased leukemia 
BENZENE 99 

3. HEALTH EFFECTS 

(SMR=5.03, 95% CI 1.84–10.97) (Schnatter et al. 1996a; Wong 1995). The risk of all leukemias and 

AML, but not multiple myeloma, significantly increased with increasing cumulative exposure above 

200 ppm-years (Rinsky et al. 1987, 2002; Wong 1995). Analysis of 4,417 workers from one of the 

Pliofilm plants showed no clear evidence that the risks of all leukemias, ANLL, multiple myeloma, or 

other lymphohematopoietic cancers increased with increasing exposure at lower levels of cumulative 

exposure (1–72 ppm-years), and the number of peak exposures over 100 ppm for ≥40 days was a better 

predictor of risk for all leukemias and multiple myeloma (Collins et al. 2003; Ireland et al. 1997). No 

clear associations between exposure to benzene and mortality from NHL or Hodgkin’s disease were 

reported (Collins et al. 2003; Ireland et al. 1997; Rinsky et al. 2002). 

A collaborative study between the National Cancer Institute and the Chinese Academy of Preventive 

Medicine (NCI/CAPM) evaluated incidence rates (occurrence of disease and cause of death) for 

lymphohematopoietic malignancies and other hematologic disorders in a cohort of 74,828 benzeneexposed and 35,805 nonexposed workers employed in 672 factories in 12 cities in China (Hayes et al. 

1996, 1997, 2001; Linet et al. 1996). The joint NCI/CAPM study is an expansion of earlier studies 

performed by CAPM alone (Yin et al. 1987a, 1987b, 1987c, 1989). The workers were employed from 

1972–1987, followed for an average of nearly 12 years, and worked in various job categories using 

benzene as a solvent for paints, varnishes, glues, coatings, and other products. The derivation of the 

cohort from many different factories and job types suggests that the members were concurrently exposed 

to many other chemicals. Findings in the exposed workers included significantly increased relative risk 

(RR) for all hematologic neoplasms (RR=2.6, 95% CI 1.4–4.7), all leukemias (RR=2.5, 95% CI 1.2–5.1), 

ANLL (RR=3.0, 95% CI 1.0–8.9), and combined ANLL and precursor myelodysplastic syndromes 

(ANLL/MDS; RR=4.1, 95% CI 1.4–11.6) (Hayes et al. 1997). Increases were also observed for other 

(unspecified) leukemias (RR=2.0, 95% CI=0.7–5.4), but the results were not statistically significant. 

Analysis by level of average benzene exposure (<10, 10–24, and ≥25 ppm) and cumulative exposure 

(<40, 40–99, and ≥100 ppm-years) indicated that the risk for all hematologic neoplasms was significantly 

increased at <10 ppm (RR=2.2, 95% CI 1.1–4.2) and <40 ppm-years (RR=2.2, 95% CI 1.1–4.5), 

respectively. The risks for all leukemias, ANLL, and ANLL/MDS were increased at exposure levels of 

10–24 ppm (average) and 40–99 ppm-years (cumulative). The elevated risks showed weak tendencies to 

increase with increasing average and cumulative levels of exposure. Analysis by duration of exposure 

(<5, 5–9, or ≥10 years) did not show increased risk with increasing exposure duration. Analysis by 

occupational group (coatings, rubber, chemical, shoe, other/mixed) showed that the increased risks for 

ANLL and ANLL/MDS were consistent across the spectrum of industries studied, implying that the 

associations were due to the common exposure to benzene rather than other industry-specific exposures. 
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3. HEALTH EFFECTS 

The results of a study of shoe factory workers in Italy are similar to those of the Pliofilm and NCI/CAPM 

studies in showing that the risk of leukemia increases with increasing exposure to benzene (Costantini et 

al. 2003; Paci et al. 1989). The cohort was followed from 1950 to 1999 and consisted of 891 men and 

796 women who were exposed to estimated benzene concentrations ranging from 0 to 92 ppm, and had 

mean cumulative exposures of 71.8 and 43.4 ppm-years, respectively (exposure durations not reported). 

Leukemia risk was significantly increased in both sexes in the highest of four exposure categories and 

most apparent in the men. For cumulative exposures of <40, 40–99, 100–199, and >200 ppm-years, the 

leukemia SMR values for the men were 1.4 (95% CI 0.2–5.0), 3.7 (95% CI 0.1–20.6), 3.0 (95% CI 0.4– 

10.9), and 7.0 (95% CI 1.9–18.0), respectively. Leukemia subtypes were not evaluated. The findings are 

consistent with earlier epidemiologic studies and case reports showing increased incidences of leukemia 

in shoe factory and rotogravure plant workers exposed to high benzene levels during its use as a solvent 

(Aksoy et al. 1974, 1987; IARC 1982; Vigliani and Forni 1976). 

No significant increases in leukemia were found in chemical industry workers (Bloemen et al. 2004; Bond 

et al. 1986; Ott et al. 1978) or petroleum industry workers (Lewis et al. 1997; Raabe and Wong 1996; 

Rushton and Romaniuk 1997; Schnatter et al. 1993, 1996b, 1996c; Tsai et al. 1983) exposed to lower 

levels of benzene. Cause-specific mortality was determined in a prospective study of 2,266 chemical 

workers who were exposed to benzene in various Dow Chemical Company manufacturing processes 

between 1938 and 1970 (Bloemen et al. 2004). The workers were followed from 1940–1996 and had an 

average duration of exposure, intensity of exposure, and cumulative exposure of 4.8 years, 9.6 ppm, and 

39.7 ppm-years, respectively. There were no significant increases in risk for any lymphohematopoietic 

malignancies, including all leukemias, ANLL, chronic lymphatic leukemia (CLL), NHL, and multiple 

myeloma. A previous investigation of this cohort found a significantly increased risk of ‘myelogenous’ 

leukemias (SMR=4.44, 95% CI not reported) (Bond et al. 1986; Ott et al. 1978), but this result is less 

reliable than those of the current study because it is based on a much smaller number of deaths. 

A meta-analysis was conducted on 19 cohorts of petroleum workers in the United States and the United 

Kingdom that were pooled into a single database for cell-type-specific leukemia analysis (Raabe and 

Wong 1996). The combined cohort consisted of 208,741 workers, mainly refinery employees who 

contributed more than 4.6 million person-years of observation. Benzene exposures were mainly from 

handling gasoline and the estimated mean and cumulative exposures for the most exposed jobs were 

<1 ppm and <45 ppm-years, respectively. No increased risks were found for mortality from AML, 

chronic myeloid leukemia (CML), acute lymphocytic leukemia (ALL), or CLL. Analyses limited to 
BENZENE 101 

3. HEALTH EFFECTS 

studies of refinery workers or studies with at least 15 years of follow-up yielded similar results. The 

negative results of this meta-analysis are consistent with those of case-control studies of the common 

leukemia cell types in petroleum workers (Rushton and Romaniuk 1997; Schnatter et al. 1993, 1996b, 

1996c). A case-control study of hairy cell leukemia, a rare B-lymphoid chronic leukemia, found no 

association between reported exposure to benzene and the risk of this cancer in workers from various 

occupations with generally low levels of exposure (Clavel et al. 1996). 

A possible association between occupational exposure to benzene and NHL is suggested by results of the 

most recent analysis (Hayes et al. 1997) of the cohort from the NCI/CAPM Chinese study summarized 

above. The relative risk for mortality from NHL in the whole cohort was 3.0 (95% CI 0.9–10.5), an 

increase that was not statistically significant. However, the risk for NHL did significantly increase at the 

highest level and duration of benzene exposure. For exposure to average concentrations of <10, 10–24, 

and ≥25 ppm, the RR values were 2.7 (95% CI 0.7–10.6), 1.7 (95% CI 0.3–10.2), and 4.7 (95% CI 1.2– 

18.1), respectively (P for trend=0.04). For cumulative exposures of <40, 40–99, and ≥100 ppm-years, the 

RR values were 3.3 (95% CI 0.8–13.1), 1.1 (95% CI 0.1–11.1), and 3.5 (95% CI 0.9–13.2), respectively 

(P for trend=0.02). Additionally, the risk for NHL significantly increased with increasing duration of 

exposure among workers exposed for <5 years (RR=0.7, 95% CI 0.1–7.2), 5–9 years (RR=3.3, 95% CI 

0.7–14.7), and >10 years (RR=4.2, 95% CI 1.1–15.9) (P for trend=0.01). 

Although the results of the NCI/CAPM Chinese study (Hayes et al. 1997) indicated a possible association 

between exposure to benzene and NHL, other cohort mortality studies found no significant increases in 

NHL mortality. These studies include the entire Pliofilm cohort (Rinsky et al. 2002); a total of 

4,417 workers from one of the three Pliofilm plants assessed for cumulative exposures of 0, <1, 1–6, and 

>6 ppm-years (Collins et al. 2003); a cohort of 2,266 chemical manufacturing workers exposed to a 

cumulative benzene level of 39.7 ppm-years (Bloemen et al. 2004); and a meta-analysis of 26 cohorts of 

petroleum workers from the United States and five other countries with expected low exposures to 

benzene (Wong and Raabe 2000). Furthermore, case-control studies provide no indications of an 

association between benzene exposure and risk of NHL (Schnatter et al. 1996b; Wong and Raabe 2000), 

and the adequacy of the data from the NCI/CAPM Chinese study (Hayes et al. 1997) has been questioned 

(Dosemeci et al. 1994; Wong 1999; Wong and Raabe 2000). 

Other evidence suggests a possible association between occupational exposure to benzene and increased 

risk of multiple myeloma. Mortality from multiple myeloma was initially elevated in the Pliofilm cohort, 

but appeared to diminish with increasing duration of observation. Based on an evaluation of 1,165 white 
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3. HEALTH EFFECTS 

male workers followed through 1981, Rinsky et al. (1987) found an SMR for multiple myeloma of 

4.09 (95% CI 1.10–10.47), an increase that was statistically significant. Assessment of the cohort after an 

additional 6 and 15 years of follow-up yielded lower SMRs of 2.91 (95% CI 0.79–7.45) (Wong 1995) and 

2.12 (95% CI 0.69–4.96) (Rinsky et al. 2002), respectively, risks that were no longer statistically 

significant. The risk of multiple myeloma did not increase with increasing level of cumulative exposure 

to benzene or with duration of employment, but this finding is particularly limited by the small number of 

deaths. No increases in mortality from multiple myeloma were found in other cohort mortality studies, 

including the NCI/CAPM study of Chinese workers (no observed deaths) (Yin et al. 1996a, 1996b), the 

Bloemen et al. (2004) study of chemical workers (SMR=0.72, 95% CI 0.15–2.10), and the Paci et al. 

(1989) study of shoe factory workers (no observed deaths in males; SMR=1.11 in females, 95% CI not 

calculated) summarized above. Additionally, mortality from multiple myeloma was not increased in a 

meta-analysis of 22 other cohorts of petroleum workers with potential exposure to benzene or benzenecontaining petroleum products (Wong and Raabe 1997). The combined cohort consisted of 

250,816 workers (from the United States, Canada, the United Kingdom, and Australia) who were 

observed over a period of 55 years from 1937 to 1991. The overall SMR for multiple myeloma was 

0.93 (95% CI=0.81–1.07), and analyses by type of facility/industrial process (refinery, distribution, and 

crude oil production and pipeline workers) and duration of observation showed no differences in risk. 

The individual cohorts used in the meta-analysis also had no increased risk of multiple myeloma. 

As summarized above, one of the early assessments of the Pliofilm cohort found an increased risk of 

mortality from multiple myeloma (Rinsky et al. 1987). The implications of this finding are unclear 

because the risk declined to non-significant levels in subsequent follow-ups (Rinsky et al. 2002; Wong 

1995), and was not supported by the findings of other cohort mortality studies (Bloemen et al. 2004; Paci 

et al. 1989; Wong and Raabe 2000; Yin et al. 1996a, 1996b). Additionally, population-based and 

hospital-based case-control studies indicate that benzene exposure is not likely to be causally related to 

the risk of mulitple myeloma (Bezabeh et al. 1996; Heineman et al. 1992; Linet et al. 1987; Schnatter et 

al. 1996b; Sonoda et al. 2001; Wong and Raabe 1997). A meta-analysis of case-control studies found no 

significant association between occupational exposure to benzene and benzene-containing products and 

risk of multiple myeloma from sources categorized as benzene and/or organic solvents (summary odds 

ratio [OR]=0.74, 95% CI 0.60–0.90), petroleum (summary OR=1.11, 95% CI 0.96–1.28), or petroleum 

products (summary OR=1.08, 95% CI 0.89–1.33) (Sonoda et al. 2001). 
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Studies in animals provide supporting evidence for the carcinogenicity of inhaled benzene. As 

summarized below, inhalation exposure to benzene induced tumors at multiple sites in rats and mice, with 

a tendency towards induction of lymphomas in mice. 

Carcinomas of the Zymbal gland and oral cavity were clearly increased in Sprague-Dawley rats that were 

exposed to 200–300 ppm benzene for 4–7 hours/day, 5 days/week for up to 104 weeks (Maltoni et 

al. 1982a, 1982b, 1983, 1985, 1989). The Zymbal gland is a specialized sebaceous gland and a site for 

benzene-induced tumors. Marginal increases in nasal cavity carcinomas, mammary tumors, and 

hepatomas were also observed. The significance of these findings is unclear because statistical analyses 

were not performed and there were no clearly increased incidences of Zymbal gland carcinoma or tumors 

at other sites in Sprague-Dawley rats that were similarly exposed to 100 or 300 ppm benzene for 

6 hours/day, 5 days/week for life (Snyder et al. 1978b, 1984). 

Benzene was carcinogenic in mice exposed to 100 or 300 ppm benzene for 6 hours/day, 5 days/week for 

16 weeks and observed for 18 months or life (Cronkite 1986; Cronkite et al. 1984, 1985, 1989; Farris et 

al. 1993). These exposures consistently induced a variety of tumors, including leukemia (mainly thymic 

lymphoma) and Zymbal gland and ovarian tumors in C57BL/6 mice, and myelogenous leukemias, 

malignant lymphomas, and Zymbal gland and lung tumors in CBA/Ca mice. Incidences of lymphocytic 

lymphoma with thymic involvement were significantly increased in C57BL/6 mice similarly exposed to 

300 ppm benzene for life (Snyder et al. 1980). Intermittent lifetime exposure to 300 ppm benzene 

(6 hours/day, 5 days/week on every third week) was more tumorigenic (in the Zymbal gland and lungs) to 

CD-1 and C57BL/6 mice than short-term exposure to 1,200 ppm (6 hours/day, 5 days/week for 10 weeks) 

followed by lifetime observation, although neither of these exposures induced leukemia (Snyder et al. 

1988). 

The cancer effect levels (CELs) for each species and duration category are recorded in Table 3-1 and 

plotted in Figure 3-1. 

EPA, IARC, and the Department of Health and Human Services have concluded that benzene is a human 

carcinogen. The Department of Health and Human Services (NTP 2005) determined that benzene is a 

known carcinogen based on human evidence showing a causal relationship between exposure to benzene 

and cancer. IARC (1987, 2004, 2007) classified benzene in Group 1 (carcinogenic to humans) based on 

sufficient evidence in both humans and animals. EPA (IRIS 2007) classified benzene in Category A 

(known human carcinogen) based on convincing evidence in humans supported by evidence from animal 
BENZENE 104 

3. HEALTH EFFECTS 

studies. Under EPA’s most recent guidelines for carcinogen risk assessment, benzene is characterized as 

a known human carcinogen for all routes of exposure based on convincing human evidence as well as 

supporting evidence from animal studies (IRIS 2007). Based on the Rinsky et al. (1981, 1987) human 

leukemia data, EPA derived a range of inhalation unit risk values of 2.2x10-6–7.8x10-6 (μg/m3

)

-1 for 

benzene (IRIS 2007). For risks ranging from 1x10-4 to 1x10-7, the corresponding air concentrations for 

lifetime exposure range from 13.0–45.0 μg/m3

 (4–14 ppb) to 0.013–0.045 μg/m3 (0.004–0.014 ppb), 

respectively. These risk levels are presented in Figure 3-1. 

3.2.2 Oral Exposure 

3.2.2.1 Death 

Individual case reports of death from acute oral exposure to benzene have appeared in the literature since 

the early 1900s. The benzene concentrations encountered by the victims were often not known. 

However, lethal oral doses for humans have been estimated at 10 mL (8.8 g or 125 mg/kg for a 70-kg 

person) (Thienes and Haley 1972). Lethality in humans has been attributed to respiratory arrest, central 

nervous system depression, or cardiac collapse (Greenburg 1926). Accidental ingestion and/or attempted 

suicide with lethal oral doses of benzene have produced the following signs and symptoms: staggering 

gait; vomiting; shallow and rapid pulse; somnolence; and loss of consciousness, followed by delirium, 

pneumonitis, collapse, and then central nervous system depression, coma, and death (Thienes and Haley 

1972). Ingestion of lethal doses may also result in visual disturbances and/or feelings of excitement and 

euphoria, which may quite suddenly change to weariness, fatigue, sleepiness, convulsion, coma, and 

death (Von Oettingen 1940). 

Animal lethality data indicate that benzene is of low toxicity following acute oral exposure (O'Bryan and 

Ross 1988). Oral LD50 values for rats ranged from 930 to 5,600 mg/kg; the values varied with age and 

strain of the animals (Cornish and Ryan 1965; Wolf et al. 1956). Male Sprague-Dawley rats were given 

various doses of benzene to determine the LD50 (Cornish and Ryan 1965). The LD50 for nonfasted rats 

was found to be 930 mg/kg. In 24-hour fasted rats, the LD50 was 810 mg/kg. No increase in mortality 

was reported in Fischer 344 rats or B6C3F1 mice treated with 600 mg/kg/day for up to 17 weeks (Huff et 

al. 1989; NTP 1986). 

Sprague-Dawley rats (30–35 males, 30–35 females) were exposed to benzene by ingestion (stomach 

tube), in olive oil, at 0, 50, or 250 mg/kg/day for 4–5 days weekly for 52 weeks and then kept under 

supervision until spontaneous death (Maltoni et al. 1983). Exposure to 50 mg/kg/day benzene after 
BENZENE 105 

3. HEALTH EFFECTS 

52 weeks resulted in deaths in 9 of 30 male (same as controls) and 2 of 30 female rats. At 250 mg/kg/day 

exposure, 13 of 35 males and 9 of 35 females died. For rats receiving only olive oil, 9 of 30 males and 

0 of 30 females died. In a companion study, Sprague-Dawley rats were exposed to 500 mg/kg/day 

benzene by ingestion (stomach tube), in olive oil, 4–5 days/week for 92 weeks, and then kept under 

observation until spontaneous death (Maltoni et al. 1983). Mortality rates were the same as the controls. 

In a chronic-duration oral study conducted by the NTP (1986), increased mortality was observed in male 

Fischer 344 rats exposed to 200 mg/kg/day benzene in corn oil, and in female Fischer 344 rats exposed to 

50 mg/kg/day benzene. B6C3F1 mice given 100 mg/kg/day also had increased mortality compared to 

control mice. 

The LD50 values and all reliable LOAEL values for death in each species following acute and chronic 

exposure are recorded in Table 3-2 and plotted in Figure 3-2. 

3.2.2.2 Systemic Effects 

No studies were located regarding respiratory, cardiovascular, musculoskeletal, hepatic, renal, endocrine, 

ocular, metabolic, or body weight effects in humans. Human and animal data pertaining to other systemic 

effects are presented below. 

The highest NOAEL value and all reliable LOAEL values for systemic effects in each species and 

duration category are recorded in Table 3-2 and plotted in Figure 3-2. 

Respiratory Effects. Male and female Fischer 344 rats and B6C3F1 mice were given oral doses of 0, 

25, 50, 100, 200, 400, or 600 mg/kg/day benzene in corn oil for 120 days (NTP 1986). No histopathological lesions were observed in lungs, trachea, or mainstream bronchi. After chronic-duration exposure 

to 50, 100, or 200 mg/kg/day (male rats) or 25, 50, or 100 mg/kg/day (female rats, male and female mice), 

no histopathological lesions were observed in trachea, lungs, or mainstream bronchi in rats (NTP 1986). 

In mice, a significantly increased incidence of alveolar hyperplasia was observed at 50 and 

100 mg/kg/day in females and at 100 mg/kg/day in males. 

Cardiovascular Effects. No histopathological lesions were observed in cardiac tissue from male and 

female Fischer 344 rats or B6C3F1 mice given oral doses of 0, 25, 50, 100, 200, 400, or 600 mg/kg/day 

benzene in corn oil for 120 days (NTP 1986). After chronic-duration exposure to ≤200 mg/kg/day (male 
Table 3-2 Levels of Significant Exposure to Benzene - Oral 

Exposure/ 

Duration/ 

a Frequency Key to Species (Route) Figure (Strain) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (mg/kg/day) (mg/kg/day) (mg/kg/day) 

ACUTE EXPOSURE Death 

1 Human once 126 (death) Thienes and Haley 1972 

338

126

2 Rat once 

(Sprague- (G) 930 M (LD50) 930 Cornish and Ryan 1965 

Dawley) 

121

3 Rat once 

(Wistar) (GO) 5600 M (LD50) 5600 Wolf et al. 1956 

162

Systemic 

4 Rat Gd 6-15 

daily (Sprague- Renal 1000 F 1000 Exxon 1986 

Dawley) (G) 

709

Dermal 50 F (alopecia of hindlimbs 

and trunk) 

50

Bd Wt 500 F 1000 F (body weight decreased 

500 11%) 

1000

Other 50 F 250 F (decreased food 

50 consumption) 

250

5 Rat 1-3 d 

(CF) Hepatic 1402 M (increased liver weight, biochemical changes) Pawar and Mungikar 19

1402

138

Neurological 

6 Human once 126 (muscular incoordination, Thienes and Haley 1972 

unconsciousness) 

126

350
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

Exposure/ 

Duration/ 

a Frequency Key to Species (Route) Figure (Strain) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (mg/kg/day) (mg/kg/day) (mg/kg/day) 

7 Rat 1 d 

(Sprague- 88 M (slight CNS depression) 1870 M (tremors) 88 1870 Cornish and Ryan 1965 

Dawley) 

122

8 Rat once 

(Sprague- (G) Dawley) 

950 M (altered neurotransmitter 

concentrations) 

Kanada et al. 1994 

950

752

Reproductive 

9 Rat Gd 6-15 

daily (Sprague- 1000 F 1000 Exxon 1986 

Dawley) (G) 

229

Developmental 

10 Rat Gd 6-15 

daily (Sprague- 1000 F 1000 Exxon 1986 

Dawley) (G) 

230

11 Mouse Gd 8-12 

(ICR/SIM) (GO) 1300 F (decreased pup weight on neonatal days 1-3) Seidenberg et al. 1986 

1300

266

INTERMEDIATE EXPOSURE Death 

12 Rat 52 wk 

4-5 d/wk (SpragueDawley) 1 x/d 250 F (9/35 died) 250 Maltoni et al. 1983, 198

(GO) 

234
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (mg/kg/day) (mg/kg/day) (mg/kg/day) 

Systemic 

13 Rat 

(SpragueDawley) 

660

52 wk 4-5 d/wk 1 x/d (GO) 

Bd Wt 50 250 (body weight decreased 

50 19%) 

250

Maltoni et al. 1983, 198

14 Rat 

(F-344/N) 

<1 yr 5 d/wk (GO) 

Hemato 50 M (lymphocytopenia and 

leukocytopenia) 

50

b

25 F (lymphocytopenia and 

leukocytopenia) 

NTP 1986 

450

25

Bd Wt 100 200 M (body weight decreased 

100 11% or more in 25 

weeks) 

200
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

Exposure/ 

Duration/ 

a Frequency Key to Species (Route) Figure (Strain) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (mg/kg/day) (mg/kg/day) (mg/kg/day) 

15 Rat 60-120 d 

5 d/wk (F-344/N) Resp 600 600 NTP 1986 

(GO) 

702

Cardio 600 

600

Gastro 600 

600

Hemato 200 (dose-related leukopenia 

at 60 days) 

200

b

25 (dose-related leukopenia 

at 120 days) 

25

Musc/skel 600 

600

Hepatic 600 

600

Renal 600 

600

Endocr 600 

600

Bd Wt 200 (body weight decreased 

14% in males and 16% in females) 

200

16 Rat 6 wk 

(Fischer- 344) 5 d/wk Hepatic 400 M 400 Taningher et al. 1995 

(GO) 

736

Bd Wt 400 M 

400

17 Rat 6 mo 

5 d/wk (Wistar) (GO) 

Hemato 1 F 50 F (leukopenia, 

1 erythrocytopenia) 

Wolf et al. 1956 

50

163
714

1000

166

8

431

31.5

31.531.531.5

451

25

100

Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

LOAEL 

NOAEL Less Serious Serious 

System (mg/kg/day) (mg/kg/day) (mg/kg/day) 

a 

Key to Species 

Figure (Strain) 

18 19 

Mouse (C57BL/6) Mouse (CD-1) 

20 Mouse 

(CD-1) 

21 Mouse 

(B6C3F1) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

28 d (W) 4 wk ad lib (W) 4 wk (W) <1 yr 5 d/wk (GO) 

Bd Wt 1000 M Hemato 8 M Hemato 31.5 M Hepatic Renal Bd Wt Hemato Bd Wt 

31.5 M 

31.5 M 31.5 M 

100 

25 (lymphocytopenia) 

(erythrocytopenia, 

increased mean corpuscular volume; leukopenia) 

(decreased erythrocyte, 

leukocyte counts) 

Reference Chemical Form 

Fan 1992 Hsieh et al. 1988b Hsieh et al. 1990 NTP 1986 
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (mg/kg/day) (mg/kg/day) (mg/kg/day) 

22 Mouse 

(B6C3F1) 

707

60-120 d 5 d/wk (GO) 

Resp 600 

600

Cardio 600 

600

Gastro 600 

600

b

Hemato 200 F 50 M (dose-related leukopenia) 

200 50

25 400 F (dose-related leukopenia) 

25 400

Musc/skel 600 

600

Hepatic 600 

600

Renal 600 

600

Endocr 600 

600

Bd Wt 600 

600

NTP 1986 

23 Mouse 

(B6C3F1) 

30 d ad lib (W) 

Hemato 12 F 195 F (decreased leukocytes) 350 F (decreased hemoglobin, 

12 195 hematocrit, leukocytes, 

MCV, and MCH) 

Shell 1992 

231

350

Hepatic 350 F 

350

Renal 350 F 

350

Bd Wt 350 F 

350

Other 12 F (decreased fluid intake) 

12
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

Exposure/ 

Duration/ 

a Frequency Key to Species (Route) Figure (Strain) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (mg/kg/day) (mg/kg/day) (mg/kg/day) 

Immuno/ Lymphoret 

24 Rat 60-120 d 

5 d/wk (F-344/N) (GO) 

200 (lymphopenia at 60 days, 

lymphoid depletion in B-cell of the spleen) 

Huff et al. 1989; NTP 19

200

25 F dose-related 

lymphopenia at 120 days) 

25

703

25 Rat 6 mo 

5 d/wk (Wistar) 1 F 50 F (leukopenia) 1 50 Wolf et al. 1956 

(GO) 

161

26 Mouse 28 d 

(C57BL/6) (W) 27 M (decreased number of splenocytes and IL-2 Fan 1992 

production) 

27

502

27 Mouse 4 wk 

ad lib (CD-1) (W) 

8 M (leukopenia and 

lymphopenia; enhanced splenic lymphocyte 

Hsieh et al. 1988b 

proliferation) 

8

340

28 Mouse 4 wk 

(CD-1) (W) 31.5 M (reduction in thymus mass; suppresion of both Hsieh et al. 1990 

B- and T-cell mitogeneses; suppressed IL-2 secretions; leukopenia, lymphopenia) 

31.5

432
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) NOAEL Less Serious System (mg/kg/day) (mg/kg/day) 

LOAEL 

Reference Chemical Form 

Serious 

(mg/kg/day) 

29 Mouse 

(CD-1) 

4 wk ad lib (W) 

40 M (elevated corticosterone 

levels; T-lymphocyte suppression) 

Hsieh et al. 1991 

40

898

30 Mouse 

(B6C3F1) 

60-120 d 5 d/wk (GO) 

b b

25 M 50 M (dose-related 

25 lymphopenia) 

200 F 

Huff et al. 1989; NTP 19

200

50

400 F 

400

708

31 Mouse 

(B6C3F1) 

30 d ad lib (W) 

12 F (decreased leukocytes) 

12

Shell 1992 

232

Neurological 

32 Rat 

(F-344/N) 

60-120 d 5 d/wk (GO) 

600 

600

NTP 1986 

704

33 Mouse 

(CD-1) 

4 wk ad lib (W) 

8 M (fluctuation of 

neurotransmitter levels) 

Hsieh et al. 1988a 

8

165

34 Mouse 

(CD-1) 

4 wk (W) 31.5 M (decreased NE, DA, 5-HT) Hsieh et al. 1990 

31.5

886

35 Mouse 

(CD-1) 

4 wk ad lib (W) 

8 M 40 M (increased hypothalmic 

8 NE and VMA) 

Hsieh et al. 1991 

40

443
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (mg/kg/day) (mg/kg/day) (mg/kg/day) 

36 Mouse 

(B6C3F1) 

60-120 d 5 d/wk (GO) 

200 400 (intermittent tremors) 

200 400

NTP 1986

709

37 Mouse 

(B6C3F1) 

30 d ad lib (W) 

195 F 350 F (decreased brain weight) 

195 350

Shell 1992

233

Reproductive 

38 Rat 

(F344/N) 

17 wk 5 d/wk (GO) 

600 

600

NTP 1986

705

39 Mouse 

(B6C3F1) 

17 wk 5 d/wk (GO) 

600 

600

NTP 1986

710

Cancer 

40 Rat 

(SpragueDawley) 

52 wk 4-5 d/wk 1 x/d (GO) 

50 F (CEL: Zymbal gland 

carcinoma in 2/30; oral cavity carcinoma) 

Maltoni et al. 1983, 198

50

237

41 Rat 

(SpragueDawley) 

52 wk 4-5 d/wk 1 x/d 

50 (CEL: Zymbal gland 

carcinoma) 

Maltoni et al. 1989

(GO) 50

250 (CEL: nasal cavity 

carcinoma, fore-stomach liver angiosarcoma; Zymbal gland carcinoma) 

250

146
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

Exposure/ 

Duration/ 

a Frequency Key to Species (Route) Figure (Strain) NOAEL Less Serious System (mg/kg/day) (mg/kg/day) 

LOAEL 

Reference Chemical Form 

Serious 

(mg/kg/day) 

42 Mouse 52 wk 

4-5 d/wk (RF/J) (GO) 

500 (CEL: mammary 

pulmonary leukemias) 

Maltoni et al. 1989

500

141

CHRONIC EXPOSURE Death 

43 Rat 2 yr 

5 d/wk (F-344/N) (GO) 

200 M (30/50 died) 

200

b

50 F (14/50 died) 

NTP 1986

711

50

44 Mouse 2 yr 

5 d/wk (B6C3F1) (GO) 

100 (41/50 males died, 35/50 

females died) 

NTP 1986

100

717

Systemic 

45 Human 6.1 yr (avg) 

(Occup) 

c

Hemato 0.29 (reduced WBC and 

platelet counts, approximately 7-18% lower than control values) 

Lan et al. 2004a 

0.29 

1035
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) NOAEL Less Serious System (mg/kg/day) (mg/kg/day) 

LOAEL 

Reference Chemical Form 

Serious 

(mg/kg/day) 

46 Rat 

(F-344/N) 

2 yr 5 d/wk (GO) 

Resp 200 M 

200

b

100 F 

Huff et al. 1989; NTP 19

100

712

Cardio 200 M 

200

b

100 F 

100

Gastro 100 200 M (hyperkeratosis and 

100 acanthosis in 

nonglandular forestomach) 

200

Hemato 50 M (lymphocytopenia and 

leukocytopenia) 

50

b

25 F (lymphocytopenia and 

leukocytopenia) 

25

Musc/skel 200 M 

200

b

100 F 

100

Hepatic 200 M 

200

b

100 F 

100

Renal 200 M 

200

b

100 F 

100

Endocr 200 M 

200

b

100 F 

100

Dermal 200 M 

200

b

100 F 

100

Ocular 200 M 

200

b

100 F 

100
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (mg/kg/day) (mg/kg/day) (mg/kg/day) 

47 Rat 

(F-344/N) 

712

2 yr 

5 d/wk(GO) 

Bd Wt 100 M 200 M (body weights decreased 

100 23% in 103 weeks)

200

Huff et al. 1989; NTP 19

48 Rat 

(SpragueDawley) 

236

92 wk 4-5 d/wk 1 x/d (GO) 

Hemato 500 (decreased RBCs and 

WBCs after 84 weeks) 

500

Bd Wt 500 (decreased body weights 

in 92 weeks) 

Maltoni et al. 1983, 198

500
a 

Key to 

Figure 

49 

Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

Species 

(Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (mg/kg/day) (mg/kg/day) (mg/kg/day) 

Mouse (B6C3F1) 

2 yr 5 d/wk (GO) 

Resp 50 M 100 M (alveolar hyperplasia) 

50 100

b b

25 F 50 F 

25 50

NTP 1986 

718

Cardio 100 

100

Gastro 25 (epithelial hyperplasia 

and hyperkeratosis of forestomach) 

25

Hemato 25 (lymphocytopenia; 

increased frequency of micronucleated normochromatic peripheral erythrocytes) 

25

Musc/skel 100 

100

Hepatic 100 

100

Renal 100 

100

Endocr 25 (hyperplasia of adrenal 

gland and harderian gland) 

25

Dermal 100 

100

Bd Wt 50 100 (mean body weight 

50 decreased 10% in 47 

weeks to 19% in 103 weeks in males; decreased 14-15% in week 99-103 in females) 

100
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

Exposure/ 

Duration/ 

a Frequency Key to Species (Route) Figure (Strain) NOAEL Less Serious System (mg/kg/day) (mg/kg/day) 

LOAEL 

Reference Chemical Form 

Serious 

(mg/kg/day) 

Immuno/ Lymphoret 

50 Rat 2 yr 

5 d/wk (F-344/N) (GO) 

50 M (lymphoid depletion of 

spleen and thymus) 

Huff et al. 1989; NTP 19

50

b

25 F (lymphoid depletion of 

spleen and thymus) 

25

713

51 Rat 92 wk 

4-5 d/wk (SpragueDawley) 1 x/d (GO) 

500 (decreased WBCs after 

84 weeks) 

500

Maltoni et al. 1983, 198

905

52 Mouse 2 yr 

5 d/wk (B6C3F1) (GO) 

25 (lymphopenia, 

hematopoietic hyperplasia in the bone 

Huff et al. 1989; NTP 19

marrow, splenic hematopoiesis) 

25

719

Neurological 

53 Rat 2 yr 

5 d/wk (F-344/N) (GO) 

200 M 

200

b

100 F 

NTP 1986 

100

714

54 Mouse 2 yr 

5 d/wk (B6C3F1) (GO) 

100 

100

NTP 1986 

720
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

a 

Key to Species 

Figure (Strain) 

Exposure/ 

Duration/ 

Frequency 

(Route) 

LOAEL 

Reference Chemical Form 

NOAEL Less Serious Serious 

System (mg/kg/day) (mg/kg/day) (mg/kg/day) 

Reproductive 

55 Rat 

(F-344/N) 

2 yr 5 d/wk (GO) 

200 M 100 F (endometrial polyps) 

200 100

b

50 F 

NTP 1986 

50

715

56 Mouse 

(B6C3F1) 

2 yr 5 d/wk (GO) 

25 (preputial gland 

hyperplasia in males; ovarian hyperplasia and 

NTP 1986 

senile atrophy in females) 

25

721

Cancer 

57 Rat 

(F-344/N) 

2 yr 5 d/wk (GO) 

50 M (CEL: squamous cell 

papillomas and carcinomas of the oral 

Huff et al. 1989; NTP 19

cavity) 

50

b

25 F (CEL: Zymbal gland 

carcinomas) 

25

955

58 Rat 

(SpragueDawley) 

92 wk 4-5 d/wk 1 x/d (GO) 

500 (CEL: Zymbal gland 

carcinoma; oral and nasal cavity carcinoma; angiosarcoma of the 

Maltoni et al. 1983, 198

liver) 

500

238
Table 3-2 Levels of Significant Exposure to Benzene - Oral (continued) 

Exposure/ LOAEL 

Duration/ 

a Frequency Reference Key to Species NOAEL Less Serious Serious (Route) Figure (Strain) System (mg/kg/day) (mg/kg/day) (mg/kg/day) Chemical Form 

59 Rat 104 wk 500 (CEL: Zymbal gland and Maltoni et al. 1989 4-5 d/wk (Wistar) oral and nasal cavity 1 x/d carcinoma; (GO) angiosarcoma of the 

liver) 

500

140

60 Rat 104 wk 50 M (CEL: Zymbal gland Maltoni et al. 1989 5 d/wk (Sprague- carcinoma) Dawley) (GO) 

50

167

61 Mouse 2 yr 25 (CEL: harderian gland Huff et al. 1989; NTP 19

5 d/wk (B6C3F1) adenoma, lymphoma in (GO) males; CEL: lymphoma 

in females) 

25

956

62 Mouse 78 wk 500 (CEL: mammary tumors, Maltoni et al. 1989 4-5 d/wk (Swiss) lung tumors, Zymbal 1 x/d gland) (GO) 

500

143

a The number corresponds to entries in Figure 3-2. b Differences in levels of health effects and cancer effects between male and females are not indicated in Figure 3-2. Where such differences exist, only the levels most sensitive gender are presented. c Study results used to derive a chronic-duration oral minimal risk level (MRL) of 0.0005 mg/kg/day based on route-to-route extrapolation, as described in detail in CAppendix A. Benchmark dose (BMD) analysis was performed on B-lymphocyte counts to select a point of departure, which was adjusted for intermittent exposure. 

dose was estimated based on route-to-route extrapolation to determine a point of departure for deriving a chronic-duration oral MRL for benzene, which was divided factor of 30 (10 for human variability and 3 for uncertainty in route-to-route extrapolation). ad lib = ad libitum; Bd Wt = body weight; CEL = cancer effect level; CNS = central nervous system; d = day(s); DA = dopamine; F = female; (F) = feed; (G) = gavagday; (GO) = gavage in oil; Hemato = hematological; 5-HT = 5-hydroxystryptamine; LD50 = lethal dose, 50% kill; LOAEL = lowest-observed-adverse-effect level; M = mean corpuscular hemoglobin; MCHC = mean corpuscular hemoglobin concentration; MCV = mean corpuscular volume; mo = month(s); NE = norepinephrine; NOno-observed-adverse-effect level; NS = not specified; RBC = red blood cells; VMA = vanillin mandelic acid; (W) = water; WBC = white blood cells; wk = week(s); yr times 
Death 

Hepatic Renal Dermal Body Weight Other Neurological Reproductive Developmental 

Figure 3-2 Levels of Significant Exposure to Benzene - Oral 

Acute (≤14 days) 

Systemic 

mg/kg/day 

10000 

3r 

7r 

5r 11m 

1000 4r 4r 9r 10r 2r 8r

4r 

4r 

1 6 

100 

7r 

4r 4r 

10 

c-Cat 

d-Dog

r-Rat 

p-Pig

q-Cow

 -Humans 

k-Monkey

m-Mouse 

h-Rabbit 

a-Sheep 

f-Ferret 

j-Pigeon

e-Gerbil 

s-Hamster 

g-Guinea Pig 

n-Mink 

o-Other

 Cancer Effect Level-Animals

 LOAEL, More Serious-Animals

LOAEL, Less Serious-Animals

NOAEL - Animals

 Cancer Effect Level-Humans

 LOAEL, More Serious-Humans

LOAEL, Less Serious-Humans

NOAEL - Humans

 LD50/LC50

Minimal Risk Level

 for effects

 other than

 Cancer 
Death 

Respiratory Cardiovascular Gastrointestinal Hematological Musculoskeletal Hepatic Renal Endocrine Body Weight 

Figure 3-2 Levels of Significant Exposure to Benzene - Oral (Continued)

Intermediate (15-364 days) 

Systemic 

mg/kg/day 

10000 

1000 18m 

22m 15r 22m 15r 22m 15r 22m 15r 22m 15r 22m 15r 22m 15r 22m 

16r 23m 23m 23m 23m 

12r 13r 

22m 23m 

100 21m 

22m 17r 13r 

20m 20m 20m 20m 

21m 22m 14r 15r 

23m 10 

19m 

1 17r 

0.1 

c-Cat 

d-Dog

r-Rat 

p-Pig

q-Cow

 -Humans 

k-Monkey

m-Mouse 

h-Rabbit 

a-Sheep 

f-Ferret 

j-Pigeon

e-Gerbil 

s-Hamster 

g-Guinea Pig 

n-Mink 

o-Other

 Cancer Effect Level-Animals

 LOAEL, More Serious-Animals

LOAEL, Less Serious-Animals

NOAEL - Animals

 Cancer Effect Level-Humans

 LOAEL, More Serious-Humans

LOAEL, Less Serious-Humans

NOAEL - Humans

 LD50/LC50

Minimal Risk Level

 for effects

 other than

 Cancer 
Body Weight 

Other Immuno/Lymphor 

Neurological 

Reproductive Cancer * 

Figure 3-2 Levels of Significant Exposure to Benzene - Oral (Continued)
 

Intermediate (15-364 days)
	

Systemic
	

mg/kg/day 

10000 

1000 

14r 

100 14r 

10 

1 

0.1 

32r 39m 38r 42m 

16r 36m 37m 

41r 

15r 24r 36m 37m 

30m 25r 40r 41r 

29m 35m 

28m 34m 26m 30m 24r 

23m 31m 

27m 33m 35m 

25r 

*Doses represent the lowest dose tested per study that produced a tumorigenic 

response and do not imply the existence of a threshold for the cancer endpoint. 

c-Cat 

d-Dog

r-Rat 

p-Pig

q-Cow

 -Humans 

k-Monkey

m-Mouse 

h-Rabbit 

a-Sheep 

f-Ferret 

j-Pigeon

e-Gerbil 

s-Hamster 

g-Guinea Pig 

n-Mink 

o-Other

 LOAEL, Less Serious-Animals

NOAEL - Animals

 LOAEL, Less Serious-Humans for effects

 Cancer Effect Level-Animals

 LOAEL, More Serious-Animals LOAEL, More Serious-Humans Minimal Risk Level 

Cancer Effect Level-Humans

 NOAEL - Humans

 LD50/LC50

 other than

 Cancer 
Death 

Respiratory Cardiovascular Gastrointestinal Hematological Musculoskeletal Hepatic Renal Endocrine Dermal Ocular Body Weight Immuno/Lymphor 

Figure 3-2 Levels of Significant Exposure to Benzene - Oral (Continued)
 

Chronic (≥365 days)
	

Systemic
	

mg/kg/day 

1000 

48r 48r 

46r 47r 

100 44m 46r 49m 46r 46r 49m 46r 49m 46r 49m 46r 46r 49m 46r 46r 49m 47r 

43r 49m 49m 

49m 49m 49m 46r 49m 52m 

10 

1 

45 

0.1 

0.01 

0.001 

0.0001 

1E-5 

1E-6 

c-Cat 

d-Dog

r-Rat 

p-Pig

q-Cow

 -Humans 

k-Monkey

m-Mouse 

h-Rabbit 

a-Sheep 

f-Ferret 

j-Pigeon

e-Gerbil 

s-Hamster 

g-Guinea Pig 

n-Mink 

o-Other

 Cancer Effect Level-Animals

 LOAEL, More Serious-Animals

LOAEL, Less Serious-Animals

NOAEL - Animals

 Cancer Effect Level-Humans

 LOAEL, More Serious-Humans

LOAEL, Less Serious-Humans

NOAEL - Humans

 LD50/LC50

 for effects

 other than

 Cancer 

Minimal Risk Level 
Immuno/Lymphor

Neurological Reproductive Cancer * 

Figure 3-2 Levels of Significant Exposure to Benzene - Oral (Continued)
 

Chronic (≥365 days)
	

mg/kg/day 

1000 

51r 62m 58r 59r 

100 54m 53r 55r 

57r 60r 

50r 56m 61m 57r
	

10
	

1 

0.1 

0.01 

10-4 

0.001 Estimated 

Upper-Bound 

10-5 Human Cancer 

0.0001 Risk Levels 

10-6 

1E-5 

*Doses represent the lowest dose tested per study that produced a tumorigenic 

10-7 response and do not imply the existence of a threshold for the cancer endpoint. 1E-6 

c-Cat 

d-Dog

r-Rat 

p-Pig

q-Cow

 -Humans 

k-Monkey

m-Mouse 

h-Rabbit 

a-Sheep 

f-Ferret 

j-Pigeon

e-Gerbil 

s-Hamster 

g-Guinea Pig 

n-Mink 

o-Other

 Cancer Effect Level-Animals

 LOAEL, More Serious-Animals

LOAEL, Less Serious-Animals

NOAEL - Animals

 Cancer Effect Level-Humans

 LOAEL, More Serious-Humans

LOAEL, Less Serious-Humans

NOAEL - Humans

 LD50/LC50

 for effects

 other than

 Cancer 

Minimal Risk Level 
BENZENE 127 

3. HEALTH EFFECTS 

rats) or ≤100 mg/kg/day (female rats, male and female mice) no histopathological lesions were observed 

in the heart (NTP 1986). 

Gastrointestinal Effects. A man swallowed an unspecified amount of benzene and survived, but 

developed an intense toxic gastritis and later pyloric stenosis (Greenburg 1926). 

No histopathological lesions were observed in esophageal and stomach tissue or in the small intestine and 

colon from male and female Fischer 344 rats or B6C3F1 mice given oral doses of 0, 25, 50, 100, 200, 400, 

or 600 mg/kg/day benzene in corn oil for 120 days (NTP 1986). After chronic-duration exposure to 50– 

200 mg/kg/day (male rats) or 25–100 mg/kg/day (female rats, male and female mice), male rats exhibited 

hyperkeratosis and acanthosis in the nonglandular forestomach at 200 mg/kg/day, and mice exhibited 

epithelial hyperplasia and hyperkeratosis in the forestomach at 25 mg/kg/day (NTP 1986). 

Hematological Effects. Prior to 1913, benzene was used as a treatment for leukemia. Benzene was 

given in gelatin capsules starting with 43 mg/kg/day and increasing to 71 mg/kg/day for unspecified 

durations (Selling 1916). Leukemia patients showed a great reduction in leukocyte count and multiple 

hemorrhages with advanced anemia. However, it is difficult to determine which effects were due to the 

leukemia and which were due to the benzene treatment. 

Intermediate-duration studies in animals have revealed decreases in numbers of erythrocytes and 

leukocytes following exposure to benzene. Male and female Fischer 344 rats and B6C3F1 mice were 

given oral doses of 0, 25, 50, 100, 200, 400, and 600 mg/kg/day benzene in corn oil for 120 days. Doserelated leukopenia and lymphopenia were observed at 200 and 600 mg/kg/day for both male and female 

rats killed on day 60, and at all doses in female rats killed on day 120. Dose-related leukopenia and 

lymphopenia were observed in male mice at 50 mg/kg/day and female mice at 400 mg/kg/day for 

120 days, but not for 60 days. Mice exposed to 8 mg/kg/day in the drinking water for 4 weeks had 

decreased numbers of erythrocytes, increased mean corpuscular volumes, and decreased numbers of 

lymphocytes (Hsieh et al. 1988b, 1990). Female B6C3F1 mice were exposed to 0, 12, 195, or 

350 mg/kg/day benzene in drinking water for 30 days (Shell 1992). Decreased hemoglobin, hematocrit, 

leukocytes, MCV, and MCH were observed at 350 mg/kg/day. Decreased leukocytes also occurred at 

195 mg/kg/day. Decreased leukocytes also occurred at 195 mg/kg/day. Rats gavaged with 50 mg/kg/day 

of benzene 5 days/week for 6 months also had decreased numbers of erythrocytes and leukocytes (Wolf et 

al. 1956). One chronic-duration study showed that gavage doses of 25 mg/kg/day resulted in leukopenia 

and/or lymphocytopenia in both rats and mice, both at the interim sacrifices at 3–18 months, and at the 
BENZENE 128 

3. HEALTH EFFECTS 

end of 2 years (Huff et al. 1989; NTP 1986). Increased frequency of micronucleated normochromatic 

peripheral erythrocytes was observed in mice at 25 mg/kg/day after 2 years. Sprague-Dawley rats were 

exposed to 500 mg/kg benzene by ingestion (stomach tube), in olive oil, once daily, 4–5 days/week for 

92 weeks, and then kept under observation until spontaneous death (Maltoni et al. 1983). Decreased 

erythrocytes and leukocytes were observed after 84 weeks. 

Musculoskeletal Effects. No histopathological lesions were observed in femoral tissue from male 

and female Fischer 344 rats or B6C3F1 mice given oral doses of 0, 25, 50, 100, 200, 400, or 

600 mg/kg/day benzene in corn oil for 120 days, or in the sternebrae, femur, or vertebrae from 

Fischer 344 rats and mice exposed to 50–200 mg/kg/day (male rats) or 25–100 mg/kg/day (female rats, 

male and female mice) for 2 years (NTP 1986). 

Hepatic Effects. Acute oral administration of 1,402 mg/kg/day benzene for 3 days induced hepatic 

changes in rats evidenced by increased liver weight, decreased protein in the postmitochondrial 

supernatant fractions (9,000 times specific gravity), and changes in hepatic drug metabolism and lipid 

peroxidation (Pawar and Mungikar 1975). The initiation-promotion-progression (IPP) model for the 

induction of malignant neoplasms in the liver was evaluated for benzene in male and female SpragueDawley rats (Dragan et al. 1993). Initiation was begun in 5-day-old rats with administration of a single 

intraperitoneal injection of diethylnitrosamine during the time when the liver is undergoing rapid growth. 

Promotion began at 6 months of age with phenobarbital in the feed, and continued into young adulthood. 

Partial hepatectomy was performed, and at the height of the regenerative proliferation phase following the 

hepatectomy, benzene (1 g/kg) was administered by gavage; phenobarbital treatment was maintained after 

the administration of benzene. A slight increase in the incidence of altered hepatic foci was observed 

after initiation and promotion. Few hepatic foci were observed in the livers of male Fischer 344 rats 

treated by gavage with 400 mg/kg/day benzene in corn oil for 5 days/week for 6 weeks (Taningher et al. 

1995). No histopathological non-neoplastic lesions were observed in hepatic tissue from male and female 

Fischer 344 rats given oral doses of 0, 25, 50, 100, 200, 400, or 600 mg/kg/day benzene in corn oil for 

120 days or in male rats exposed to 50–200 mg/kg/day and female rats exposed to 25–100 mg/kg/day for 

2 years (NTP 1986). 

Female B6C3F1 mice were exposed to 0, 12, 195, or 350 mg/kg/day benzene in drinking water for 

30 days (Shell 1992). No adverse liver effects, as evidenced by gross necropsy, liver weight 

determination, and serum levels of hepatic enzymes, were observed. Oral administration of 

31.5 mg/kg/day benzene continuously in drinking water for 4 weeks did not affect liver weight in CD-1 
BENZENE 129 

3. HEALTH EFFECTS 

mice (Hsieh et al. 1990). No histopathological non-neoplastic lesions effects were observed in hepatic 

tissue from male and female B6C3F1 mice given oral doses of 0, 25, 50, 100, 200, 400, or 600 mg/kg/day 

benzene in corn oil for 120 days, or in male and female mice exposed to 25–100 mg/kg/day for 2 years 

(NTP 1986). 

Renal Effects. Female Sprague-Dawley rats were dosed by gavage with 0, 50, 250, 500, or 

1,000 mg/kg/day benzene on Gd 6–15 and killed on Gd 20 (Exxon 1986). No adverse effects were noted 

in the kidneys based on gross necropsy. No adverse effects based on histological examination were 

observed on renal tissue or the urinary bladder from male and female Fischer 344 rats given oral doses of 

0, 25, 50, 100, 200, 400, or 600 mg/kg/day benzene in corn oil for 120 days or in male rats exposed to 

50–200 mg/kg/day and female rats exposed to 25–100 mg/kg/day for 2 years (NTP 1986). Female 

B6C3F1 mice were exposed to 0, 12, 195, or 350 mg/kg benzene in drinking water for 30 days (Shell 

1992). No adverse effects were observed in the kidneys, based on kidney weights, gross examination, and 

blood urea nitrogen and creatinine determinations. Oral administration of 31.5 mg/kg/day benzene 

continuously in drinking water for 4 weeks did not affect kidney weight in CD-1 mice (Hsieh et al. 1990). 

No adverse effect based on histological examination was observed on renal tissue or the urinary bladder 

from male and female B6C3F1 mice given oral doses of 0, 25, 50, 100, 200, 400, or 600 mg/kg/day 

benzene in corn oil for 120 days, or in male and female mice exposed to 25–100 mg/kg/day for 2 years 

(NTP 1986). 

Endocrine Effects. No histopathological lesions were observed in salivary, thyroid, parathyroid, 

pancreas, adrenal, or pituitary glands from male and female Fischer 344 rats or B6C3F1 mice given oral 

doses of 0, 25, 50, 100, 200, 400, or 600 mg/kg/day benzene in corn oil for 120 days (NTP 1986). In the 

companion chronic-duration oral study, male Fischer 344 rats were exposed to 25, 50, 100, or 

200 mg/kg/day benzene, while female rats received 25, 50, or 100 mg/kg/day benzene. Hyperplasia of 

the Zymbal gland was increased in low-dose males and in mid-dose females. In the adrenal gland, 

hyperplasia was observed in both sexes (males: 27 and 4% at 50 and 200 mg/kg, respectively; females: 

34% at 25 mg/kg). In the thyroid gland, incidences of C-cell hyperplasia were 14, 26, 15, and 15% in 

males treated with 0, 50, 100, and 200 mg/kg, respectively. Analysis of the pituitary gland showed 

incidence of hyperplasia in males treated with 0, 50, 100, and 200 mg/kg at 6, 16, 20, and 10%, 

respectively; in females treated with 0, 25, 50, and 100 mg/kg at 11, 20, 10, and 14%, respectively. None 

of the increased incidences of hyperplasia in these glands were considered to be treatment-related by 

NTP. The non-dose-related increase of hyperplasia of the Zymbal gland could represent a progression to 

the neoplasms (see Section 3.2.2.7). In mice, Zymbal gland lesions showed epithelial hyperplasia in 
BENZENE 130 

3. HEALTH EFFECTS 

males (0, 9, 30, and 26%) and in females (2, 3, 5, and 19%) exposed to 0, 25, 50, or 100 mg/kg, 

respectively. Hyperplasia of the adrenal cortex occurred at incidences of 4, 67, 29, and 9% in males and 

10, 43, 68, and 13% in females, respectively. Hyperplasia of the harderian gland occurred at incidences 

of 0, 11, 22, and 15% in males and 13, 23, 22, and 21% in females, respectively (NTP 1986). 

Dermal Effects. A case of accidental poisoning in which the patient survived but developed an odd 

skin condition consisting of swelling and edema has been reported (Greenburg 1926). 

Female Sprague-Dawley rats were dosed by gavage with 0, 50, 250, 500, or 1,000 mg/kg/day benzene 

daily on Gd 6–15 and killed on Gd 20 (Exxon 1986). Alopecia of the hind limbs and trunk was noted in 

all dose groups. 

No histopathological lesions were observed in the skin of male and female Fischer 344 rats and B6C3F1 

mice after chronic oral exposure to 50–200 mg/kg/day (male rats) or 25–100 mg/kg/day (female rats and 

male and female mice) (NTP 1986). 

Ocular Effects. No histopathological lesions were noted in the eyes of male and female Fischer 344 

rats and B6C3F1 mice after chronic-duration oral exposure to 50–200 mg/kg/day (male rats) or 25– 

100 mg/kg/day (female rats and male and female mice) (NTP 1986). 

Body Weight Effects. No significant change in body weight was observed in male Fischer 344 rats 

treated by gavage with 400 mg/kg/day benzene in corn oil for 5 days/week for 6 weeks (Taningher et al. 

1995). Body weight was unaffected in male and female Fischer 344 rats given oral doses of 0, 25, 50, or 

100 mg/kg/day benzene in corn oil for 120 days (NTP 1986). However, animals receiving 200, 400, or 

600 mg/kg/day benzene exhibited a 14–22% decrease in body weight after 120 days. Female SpragueDawley rats were dosed by gavage with 0, 50, 250, 500, or 1,000 mg/kg benzene daily on Gd 6–15 and 

killed on Gd 20 (Exxon 1986). Maternal body weight decreased 11% at the high dose. C57BL/6 male 

mice were given benzene at concentration levels of 200 and 1,000 mg/L (assumed benzene intake of 

27 and 154 mg/kg/day) in drinking water for 28 days (Fan 1992). Control groups were given untreated 

tap water. Groups of mice were killed on day 7, 14, 21, and 28 of administration, and on day 7, 14, 21, 

and 28 after the last administration of benzene at 200 mg/L. There was no effect of treatment on body 

weight. Female B6C3F1 mice were exposed to 0, 12, 195, or 350 mg/kg/day benzene in drinking water 

for 30 days (Shell 1992). There was no significant effect on body weight at the highest treatment level. 

Oral administration of 31.5 mg/kg/day benzene continuously in drinking water for 4 weeks did not affect 
BENZENE 131 

3. HEALTH EFFECTS 

body weight in CD-1 mice (Hsieh et al. 1990). No adverse effect was observed on body weight of male 

and female B6C3F1 mice given oral doses of 0, 25, 50, 100, 200, 400, or 600 mg/kg/day benzene in corn 

oil for 120 days (NTP 1986). 

Sprague-Dawley rats were exposed to benzene by gavage in olive oil, at 0, 50, or 250 mg/kg/day body 

weight for 4–5 days/week for 52 weeks, and then kept under supervision until spontaneous death (Maltoni 

et al. 1983, 1985). A 19% decrease in body weight was reported in animals exposed to 250 mg/kg 

benzene for 52 weeks. In a companion study, Sprague-Dawley rats were exposed to 500 mg/kg benzene 

by ingestion (stomach tube), in olive oil, once daily, 4–5 days/week for 92 weeks, and then kept under 

observation until spontaneous death (Maltoni et al. 1983). Decreased body weight was observed after 

92 weeks. In a chronic-duration oral study, male Fischer 344 rats exhibited a decrease in body weight of 

11% or more after 25 weeks exposure to doses of 200 mg/kg/day benzene in corn oil (NTP 1986). 

Females rats and male and female B6C3F1 mice in the same study exposed to doses up to 100 mg/kg/day 

benzene did not show any change in body weight after 12 months of exposure, or after 2 years exposure 

(female rats). Male and female mice exhibited body weight effects after chronic exposure. In male mice 

given 100 mg/kg, mean body weights decreased from 10% after 47 weeks to 19% in 103 weeks of 

exposure relative to controls. In female mice given 100 mg/kg, mean body weights decreased 14–15% in 

weeks 99–103 of exposure (NTP 1986). 

Other Systemic Effects. C57BL/6 male mice were given benzene at concentration levels of 200 and 

1,000 mg/L (assumed benzene intake of 27 and 154 mg/kg/day) in drinking water for 28 days (Fan 1992). 

Control groups were given untreated tap water. Groups of mice were killed on day 7, 14, 21, and 28 of 

administration, and on day 7, 14, 21, and 28 after the last administration of benzene at 200 mg/L. There 

was no effect of treatment on food or water consumption. Female Sprague-Dawley rats were dosed by 

gavage with 0, 50, 250, 500, or 1,000 mg/kg/day on Gd 6–15 and killed on Gd 20 (Exxon 1986). 

Decreased feed consumption was noted at doses of 250 mg/kg and above, and body weight decreased 

11% at the high dose. Female B6C3F1 mice were exposed to 0, 12, 195, or 350 mg/kg/day benzene in 

drinking water for 30 days (Shell 1992). Decreased fluid consumption was observed at >12 mg/kg. 

3.2.2.3 Immunological and Lymphoreticular Effects 

No studies were located regarding immunological effects in humans after oral exposure to benzene. 
BENZENE 132 

3. HEALTH EFFECTS 

Oral administration of benzene to CD-1 mice produced an immunotoxic effect on both the humoral and 

cellular immune responses (Hsieh et al. 1988b). Exposure to benzene at 8, 40, or 180 mg/kg/day for 

4 weeks caused a significant dose-response reduction of total peripheral blood leukocytes and 

erythrocytes. Lymphocytes, but not neutrophils or other leukocytes, were decreased in number. Splenic 

lymphocyte proliferative response to B- and T-cell mitogens was biphasic—enhanced in the 8 mg/kg/day 

dosage group and depressed in the 40 and 180 mg/kg/day dosage groups. Cell-mediated immunity 

evaluated in mixed-lymphocyte reaction and in the 51Cr-release assay showed a similar biphasic response. 

Antibody production was significantly suppressed in mice dosed at 40 and 180 mg/kg/day. The results 

indicate that administration of 40 mg/kg/day benzene has an immunosuppressive effect evident in 

decreased immune functions evaluated in in vitro assays for cell-mediated immunity and antibody 

production. A dose-related decrease in spleen weight was observed, which was significant only in the 

180 mg/kg/day group. 

C57BL/6 male mice were given benzene at concentration levels of 200 and 1,000 mg/L (assumed benzene 

intake of 27 and 154 mg/kg/day) in drinking water for 28 days (Fan 1992). Control groups were given 

untreated tap water. Mice were sacrificed on days 7, 14, 21, and 28 of administration. Selected mice of 

the 27 mg/kg/day dose group were sacrificed on postexposure days 7, 14, and 21 in order to assess the 

postexposure time course of the benzene toxicity. At 27 mg/kg/day, a decreased number of splenocytes 

was observed on day 21 and 28 of exposure. At 154 mg/kg/day, spleen cell numbers decreased 

significantly as a function of time in mice treated for 14, 21, and 28 days. Benzene treatment for 3 weeks 

raised natural killer (NK) cell activity significantly at both doses. However, after another week of 

benzene treatment, NK cell activity resumed to control levels. Significant depression of interleukin-2- 

(IL-2) production was detected in both levels for 28 days. The NK cell activity showed normal levels on 

day 7, 14, and 21 after the last administration of benzene exposure for 28 days at 27 mg/kg/day. 

IL-2 production decreased significantly on day 7 and 14 after cessation of benzene administration, but 

recovered with time (43, 71, and 79% of control on day 7, 14, and 21, respectively). Spleen cell number 

decreased significantly on the 7th day, but recovered on day 14 and 21. Female B6C3F1 mice were 

exposed to 0, 12, 195, or 350 mg/kg/day benzene in drinking water for 30 days (Shell 1992). Decreased 

leukocytes were observed at 12 mg/kg/day, and decreased spleen cell number was observed at 

195 mg/kg/day. 

In the NTP-sponsored intermediate-duration oral study using Fischer 344 rats and B6C3F1 mice, doserelated leukopenia and lymphopenia were observed for both male and female Fischer 344 rats at 200 and 

600 mg/kg/day killed on day 60, and at all doses in female rats killed on day 120 (NTP 1986). Decreased 
BENZENE 133 

3. HEALTH EFFECTS 

leukocytes were observed in male and female rats exposed for 60 days to 200 and 600 mg/kg/day 

benzene. Lymphoid depletion in the B-cells of the spleen was observed in animals exposed to 

200 mg/kg/day (3 of 5 males, 4 of 5 females) and 600 mg/kg/day (5 of 5 males, 5 of 5 females) benzene 

for 60 days and in animals that received 600 mg/kg/day (10 of 10 males, 10 of 10 females) benzene for 

120 days. At 600 mg/kg/day benzene exposure, increased extramedullary hematopoiesis was observed in 

the spleen of 4 of 5 male and 3 of 5 female rats. Dose-related leukopenia and lymphopenia were observed 

for both male and female mice exposed for 120 days, but not for 60 days. Leukocytes and lymphocytes 

were significantly decreased in male mice exposed for 120 days to 50, 100, 200, 400, and 600 mg/kg/day 

benzene. At 120 days of exposure, leukocytes were significantly decreased in female mice at 

600 mg/kg/day and lymphocytes at 400 and 600 mg/kg/day. Histological examination revealed no 

adverse effects in mandibular lymph node or the thymus for either rats or mice (Huff et al. 1989; NTP 

1986). Rats exposed to benzene at 50 and 100 mg/kg/day for 6 months had significant leukopenia (Wolf 

et al. 1956). 

Leukopenia and lymphopenia were observed in mice at 31.5 mg/kg/day after 4 weeks of oral exposure 

(Hsieh et al. 1990). Reduction in thymus mass, suppression of B- and T-cell mitogenesis, and suppressed 

IL-2 release were also noted. Similar results were noted at 40 mg/kg/day (Hsieh et al. 1991). Oral 

administration of benzene to B6C3F1 mice and Fischer 344 rats at doses of 50–200 mg/kg/day (male rats) 

or 25–100 mg/kg/day (female rats and male and female mice), 5 days/week for 103 weeks resulted in 

significant leukocytopenia and lymphocytopenia in both species (Huff et al. 1989; NTP 1986). In the 

thymus, lymphoid depletion was observed at 0, 10, 20, or 28% in male rats treated with 0, 50, 100, or 

200 mg/kg/day, respectively. Increased incidences of lymphoid depletion of the spleen were observed in 

male rats treated with 0 (0%), 50 (40%), 100 (17%), and 200 (49%) mg/kg/day and in female rats treated 

with 0 (0%), 25 (22%), 50 (16%), and 100 (20%) mg/kg/day. In mice, an increased incidence of 

hematopoietic hyperplasia was observed in the bone marrow of dosed animals in both sexes. Splenic 

hematopoiesis was increased in dosed animals of both sexes of mice (Huff et al. 1989; NTP 1986). 

Maltoni et al. (1983, 1985) observed decreased leukocytes in Sprague-Dawley rats dosed with 

500 mg/kg/day benzene for 84 weeks or more. 

The highest NOAEL values and all reliable LOAEL values for immunologic effects in each species and 

duration category are recorded in Table 3-2 and plotted in Figure 3-2. 
BENZENE 134 

3. HEALTH EFFECTS 

3.2.2.4 Neurological Effects 

In humans, symptoms of central nervous system toxicity (including euphoria, vertigo, muscular 

incoordination, and unconsciousness) have been reported following one-time ingestion of benzene at 

125 mg/kg (Thienes and Haley 1972). 

Neurochemical profiles were conducted on rats after oral exposure to benzene (Kanada et al. 1994). 

Sprague-Dawley rats received a single dose of 950 mg/kg benzene by gavage and were sacrificed 2 hours 

after treatment. The control group received nothing. Brains were dissected into small-brain areas and 

stored until analysis. Acetylcholine, 3,4-dihydroxyphenylalanine (DOPA), dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), norepinephrine, 3-methoxy-4-hydroxyphenylglycol (MHPG), serotonin, and 5-hydroxyindoleacetic acid (5HIAA) contents in the small-brain 

regions were measured. Results showed that benzene decreased acetylcholine content of rat 

hippocampus. DOPA and norepinephrine content decreased in the rat midbrain. Dopamine, serotonin 

and 5HIAA content increased in the rat midbrain. Dopamine, DOPAC, norepinephrine, and 5HIAA 

content increased and serotonin content decreased in the rat hypothalmus after oral administration of 

benzene. Increased dopamine, HVA, MHPG, and serotonin content of rat medulla oblongata was 

observed. Decreased norepinephrine and 5HIAA content of rat medulla oblongata by benzene treatment 

was observed. 

Oral exposure to benzene induced both synthesis and catabolism of monoamine neurotransmitters in 

CD-1 mice (Hsieh et al. 1988a). Mice given 8, 40, or 180 mg/kg/day of benzene for 4 weeks in drinking 

water exhibited changes in the levels of norepinephrine, dopamine, serotonin, and catecholamine 

metabolites in several brain regions but no treatment-related behavioral changes. Similar results were 

seen at 31.5–40 mg/kg/day (Hsieh et al. 1990, 1991). Because of the lack of association with behavioral 

changes, the effects on the neurotransmitters cannot be adequately assessed. Female B6C3F1 mice were 

exposed to 0, 12, 195, or 350 mg/kg/day benzene in drinking water for 30 days (Shell 1992). Decreased 

brain weight was observed at 350 mg/kg/day. Sprague-Dawley rats given one oral dose of 88 mg/kg 

benzene exhibited slight central nervous system depression, whereas at 1,870 mg/kg/day, tremors were 

observed (Cornish and Ryan 1965). Histological examination of the brain revealed no treatment-related 

lesions after gavage treatment of male and female Fischer 344 rats and B6C3F1 mice with doses up to 

600 mg/kg/day for 120 days (NTP 1986). In the same experiment, B6C3F1 mice exhibited tremors 

intermittently at doses of 400 mg/kg/day, which were more pronounced in males during the last 3 weeks 

of the study. No adverse effects based on histological examination of brain or spinal cord were observed 
BENZENE 135 

3. HEALTH EFFECTS 

in male and female Fischer 344 rats and B6C3F1 mice after chronic oral exposure to 50–200 mg/kg/day 

(male rats) or 25–100 mg/kg/day (female rats and male and female mice) (NTP 1986). 

The highest NOAEL values and all reliable LOAEL values for neurological effects in each species and 

duration category are recorded in Table 3-2 and plotted in Figure 3-2. 

3.2.2.5 Reproductive Effects 

No studies were located regarding reproductive effects in humans after oral exposure to benzene. 

Female Sprague-Dawley rats were dosed by gavage with 0, 50, 250, 500, or 1,000 mg/kg benzene daily 

on Gd 6–15 and killed on Gd 20 (Exxon 1986). No adverse effects were noted on reproductive 

competency. No histological changes were reported in the prostate, testes, ovaries, mammary gland, or 

uterus of male and female Fischer 344 rats and B6C3F1 mice dosed by gavage with up to 600 mg/kg/day 

benzene for 17 weeks (NTP 1986). In male and female Fischer 344 rats and B6C3F1 mice after chronic 

oral exposure to 50–200 mg/kg/day (male rats) or 25–100 mg/kg/day (female rats and male and female 

mice), endometrial stromal polyps occurred with a significant positive trend in female rats (NTP 1986). 

The incidence in high dose group (14/50) was significantly greater than that in the control (7/50). In 

mice, analysis of preputial gland lesions in male mice dosed at 0, 25, 50, or 100 mg/kg showed increased 

incidences of focal, diffuse or epithelial hyperplasia (5, 65, 31, and 3%, respectively). The lower 

incidences of hyperplasia in the higher dose groups were probably due to the progression of the preputial 

gland lesions to neoplasias (see Section 3.2.2.7). Various non-neoplastic and neoplastic ovarian lesions 

were observed in dosed female mice, including epithelial hyperplasia and senile atrophy (NTP 1986). 

The NOAEL and LOAEL values for reproductive effects in rats and mice are recorded in Table 3-2 and 

plotted in Figure 3-2. 

3.2.2.6 Developmental Effects 

No studies were located regarding developmental effects in humans after oral exposure to benzene. 

Benzene was embryotoxic as evidenced by reduced pup body weights when mice were administered 

1,300 mg/kg/day of benzene by gavage on Gd 8–12 (Seidenberg et al. 1986). No maternal toxicity was 

observed. Female Sprague-Dawley rats were dosed by gavage with 0, 50, 250, 500, or 1,000 mg/kg 
BENZENE 136 

3. HEALTH EFFECTS 

benzene daily on Gd 6–15 and killed on Gd 20 (Exxon 1986). No adverse effects were noted on 

morphological development. 

The NOAEL value for rats and the LOAEL value for mice for developmental effects following acute oral 

exposure are recorded in Table 3-2 and plotted in Figure 3-2. 

3.2.2.7 Cancer 

Essentially no information was located regarding the oral carcinogenicity of benzene in humans. 

Lymphatic and hematopoietic cancers were increased in vehicle maintenance workers who occasionally 

siphoned gasoline by mouth (Hunting et al. 1995), but the skin and lungs were the main routes of 

exposure (see Section 3.2.3.7). 

Benzene has been shown to be a multiple site carcinogen by the oral route in animals (Huff et al. 1989; 

Maltoni et al. 1983, 1985, 1989; NTP 1986). In bioassays conducted by the NTP, benzene in corn oil was 

administered groups of 50 F344/N rats and 50 B6C3F1 of each sex by gavage on 5 days/week for 

103 weeks (Huff et al. 1989; NTP 1986). The male rats were exposed to dose levels of 0, 50, 100, or 

200 mg/kg/day, and female rats and mice of both sexes were exposed to 0, 25, 50, or 100 mg/kg/day. In 

the rats, benzene caused significantly increased incidences of Zymbal gland carcinomas in males at 

≥100 mg/kg/day and females at ≥25 mg/kg/day, oral cavity squamous cell papillomas and carcinomas in 

males at ≥50 mg/kg/day and females at ≥25 mg/kg/day, and skin squamous cell papillomas and 

carcinomas in males at 200 mg/kg/day. In the mice, benzene mainly caused significantly increased 

incidences of malignant lymphomas in both sexes at ≥25 mg/kg/day, Zymbal gland carcinomas in males 

at ≥50 mg/kg/day and females at 100 mg/kg/day, lung alveolar/bronchiolar adenomas and carcinomas in 

males at ≥100 mg/kg/day and females at ≥50 mg/kg/day, Harderian gland adenomas in males at 

≥25 mg/kg/day, preputial gland squamous cell carcinomas in males at ≥50 mg/kg/day, and mammary 

gland carcinomas in females at ≥50 mg/kg/day. NTP (1986) concluded that there was clear evidence of 

carcinogenicity of benzene in male and female F344/N rats and B6C3F1 mice under the conditions of 

these studies. 

Maltoni et al. (1983, 1985, 1989) assessed carcinogenicity in groups of 30–50 rats and mice of each sex 

that were exposed to benzene in olive oil by gavage on 4–5 days/week for up to 104 weeks and observed 

for life. Sprague-Dawley rats were exposed to 0, 50, or 250 mg/kg/day for 52 weeks or 0 or 

500 mg/kg/day for 104 weeks; effects included increased incidences of Zymbal gland carcinomas in 
BENZENE 137 

3. HEALTH EFFECTS 

females at ≥50 mg/kg/day, oral cavity carcinomas in females at 250 mg/kg/day and both sexes at 

500 mg/kg/day, forestomach carcinomas in females at 500 mg/kg/day, nasal cavity and skin carcinomas 

in males at 500 mg/kg/day, and liver angiosarcomas in both sexes at 500 mg/kg/day. Wistar rats were 

exposed to 0 or 500 mg/kg/day for 104 weeks; effects included increased incidences of Zymbal gland, 

nasal cavity, and oral cavity carcinomas. Swiss mice were exposed to 0 or 500 mg/kg/day for 78 weeks; 

effects included increased incidences Zymbal gland carcinomas in males, mammary carcinomas in 

females, and lung adenomas in both sexes. RF/J mice were exposed to 0 or 500 mg/kg/day for 52 weeks; 

effects included increased incidences of mammary carcinomas in females and lung adenomas in both 

sexes. 

As discussed in Section 3.2.1.7, there is a consensus that benzene is a human carcinogen (IARC 1987, 

2004, 2007; IRIS 2007; NTP 2005). This conclusion is based on sufficient inhalation data in humans 

supported by animal evidence, including the oral studies summarized above. The human cancer induced 

by inhalation exposure to benzene is predominantly acute nonlymphocytic leukemia, whereas benzene is 

a multiple site carcinogen in animals by both the inhalation and oral routes. Due to the lack of oral 

carcinogenicity data in humans, as well as the lack of a well-demonstrated and reproducible animal model 

for leukemia from benzene exposure, EPA extrapolated an oral slope factor from the inhalation unit risk 

range (IRIS 2007). The oral slope factor ranges from 1.5x10-2 to 5.5x10-2 (mg/kg/day)-1, and for cancer 

risks of 1x10-4–1x10-7, the corresponding dose levels range from 6.7x10-3–1.8x10-3 to 6.7x10-6–1.8x10-6 

mg/kg/day, respectively. These risk levels are presented in Figure 3-1. 

3.2.3 Dermal Exposure 

3.2.3.1 Death 

No studies were located regarding deaths in animals after dermal exposure to benzene. 

A cohort of 338 men was investigated as to causes of death among employees of the fleet maintenance 

division of Washington DC's Department of Public Works (Hunting et al. 1995). This mortality study 

was undertaken because of three cases of leukemia among car and mobile equipments mechanics. 

Preliminary evaluation showed that the garage mechanics regularly used gasoline to clean parts and wash 

their hands; these workers also experienced dermal and inhalation exposure to gasoline during 

maintenance of vehicles. The men were employed for at least 1 year between January 1, 1977, and 

December 31, 1989. Cause-specific SMRs were calculated. Increased risk of death was found in some 

categories. 
BENZENE 138 

3. HEALTH EFFECTS 

3.2.3.2 Systemic Effects 

No studies were located regarding respiratory, cardiovascular, gastrointestinal, musculoskeletal, hepatic, 

renal, endocrine, or body weight effects in humans or animals after dermal exposure to benzene. 

Available data pertaining to hematological, dermal, and ocular effects are presented below. All reliable 

LOAEL values for systemic effects in humans and rabbits for acute- and chronic-duration dermal 

exposure are recorded in Table 3-3. 

Hematological Effects. Tondel et al. (1995) reported a case of myelofibrosis that was diagnosed in a 

46-year-old man who had worked from 1962 to 1979 as a gasoline station attendant. Although the 

exposure was primarily by inhalation, it is likely that dermal exposure also occurred. 

Dermal Effects. In humans, benzene is a skin irritant. By defatting the keratin layer, it may cause 

erythema, vesiculation, and dry and scaly dermatitis (Sandmeyer 1981). Acute fatal exposure to benzene 

vapors caused second degree burns on the face, trunk, and limbs of the victims (Avis and Hutton 1993). 

Fifteen male workers were exposed to benzene vapors (>60 ppm) over several days during the removal of 

residual fuel from shipyard fuel tanks (Midzenski et al. 1992). Exposures to benzene range from 1 day to 

3 weeks (mean of 5 days), 2.5–8 hours/day (mean of 5.5 hours). Workers with more than 2 days 

(16 hours) exposure reported mucous membrane irritation (80%), and skin irritation (13%) after exposure 

to the vapor. 

Benzene was slightly irritating to the skin of rabbits (Wolf et al. 1956). The skin showed moderate 

erythema, edema, and moderate necrosis following application 1 time/day for 4 weeks. 

Ocular Effects. Solvent workers who were exposed to 33 (men) or 59 (women) ppm benzene 

exhibited eye irritation while being exposed to the vapors (Yin et al. 1987b). 

A transient increase in lacrimation was observed in male rats exposed to 10–300 ppm benzene for 

6 hours/day, 5 days/week (Shell 1980). Moderate conjunctival irritation and transient corneal damage 

were observed in rabbits subsequent to placement of 2 drops of benzene onto the eyeball (Wolf et al. 

1956). 
Table 3-3 Levels of Significant Exposure to Benzene - Dermal 

Exposure/ 

Duration/ 

Frequency Species (Route) (Strain) 

LOAEL 

System NOAEL Less Serious Serious 

Reference Chemical Form 

ACUTE EXPOSURE 

Systemic 

Human 1-21 d 

2.5-8 hr/d 

921

Rabbit once (NS) 

164

INTERMEDIATE EXPOSU

Systemic 

Rat 10 wk (CD) 5 d/wk 6 hr/d 

930

CHRONIC EXPOSURE 

Systemic 

Human >1 yr 

(occup) 

952

d = day(s); F = female; hr = houryr = year 

Dermal 60 M (mucous membrane and 

ppm skin irritation)

ppm

Ocular 2 (moderate conjunctival 

Unknown irritations; light corneal 

injury) 

Unknown

RE 

Ocular 1M 10 M (lacrimation) 

ppm ppm 

ppm ppm

Ocular 33 M (eye irritation) 

ppm 

ppm

59 F (eye irritation) 

ppm 

ppm

(s); LOAEL = lowest-observed-adverse-effect level; M = male; NOAEL = no-observed-adverse-effect level; NS = not specified; occ

Midzenski et al. 1992 

Wolf et al 1956 

Shell 1980 Yin et al. 1987b 
BENZENE 140 

3. HEALTH EFFECTS 

3.2.3.3 Immunological and Lymphoreticular Effects 

No studies were located regarding immunological and lymphoreticular effects in humans or animals after 

dermal exposure to benzene. 

3.2.3.4 Neurological Effects 

Tondel et al. (1995) reported the case of a gasoline station attendant who had worked from 1962 to 1979 

and who developed myelofibrosis. The patient described symptoms of fatigue for 3 weeks and night 

sweats, among other symptoms. Although the exposure was primarily by inhalation, it is probable that 

dermal exposure also occurred. 

No studies were located regarding neurological effects in animals after dermal exposure to benzene. 

3.2.3.5 Reproductive Effects 

No studies were located regarding reproductive effects in humans or animals after dermal exposure to 

benzene. 

3.2.3.6 Developmental Effects 

No studies were located regarding developmental effects in humans or animals after dermal exposure to 

benzene. 

3.2.3.7 Cancer 

A cohort mortality study was conducted to estimate the relative risk of hematological cancer among 

335 male vehicle maintenance workers who were employed for at least 1 year between 1977 and 1989 

and followed through 1991 (Hunting et al. 1995). The workers were car and mobile equipment 

mechanics who regularly used gasoline to clean parts and wash their hands. Exposure to gasoline also 

occurred via inhalation, and some of the workers occasionally siphoned gasoline by mouth. Since most of 

the cohort consisted of non-white (race data were missing for 9.5% of the cohort) vehicle maintenance 

workers who lived within the District of Columbia, death rates for age- and calendar-matched non-white 

males in the District of Columbia were used to calculate expected deaths for the cohort. Three deaths due 

to lymphatic and hematopoietic cancer were observed, yielding SMRs of 3.63 (95% CI 0.75–10.63) in the 

whole cohort and 4.22 (95% CI 0.87–12.34) in a subgroup of 297 workers with the highest potential for 
BENZENE 141 

3. HEALTH EFFECTS 

exposure. Neither of these SMRs were significant, although analysis by cancer subtype showed a 

significantly elevated risk for leukemia and aleukemia in the high exposure subgroup (two cases, 

SMR=9.26, 95% CI 1.12–33.43). Aleukemia refers to leukemias in which the blood has normal or nearnormal white blood cell counts, but few numbers of young leukocytes. Two additional cases of leukemia 

were identified among workers who were not included in the cohort (one died after the end of the followup period and another was still alive). Mortalities from causes other than lymphatic/hematopoietic cancer 

were not significantly increased. 

Application of benzene to the skin of animals has not produced evidence of carcinogenicity, although 

most studies were inadequate for evaluation. As summarized by IARC (1982, 1987), many dermal 

carcinogenicity studies of chemicals other than benzene used benzene as a vehicle, and treated large 

numbers of control animals (mice) with benzene alone. None of these studies indicated that benzene 

induced skin tumors; however, all possible tumor sites usually were not examined. 

3.3 GENOTOXICITY 

The genotoxic effects of benzene have been studied extensively. The in vivo and in vitro data are 

summarized in Tables 3-4 and 3-5, respectively. In chronically-exposed humans, benzene and/or its 

metabolites primarily cause chromosomal aberrations (Andreoli et al. 1997; Bogadi-Šare et al. 1997; Ding 

et al. 1983; Forni and Moreo 1967, 1969; Forni et al. 1971a; Hartwich et al. 1969; Hedli et al. 1991; 

Karacic et al. 1995; Kašuba et al. 2000; Major et al. 1992, 1994; Picciano 1979; Popp et al. 1992; 

Rothman et al. 1995; Sardas et al. 1994; Sasiadek et al. 1989; Sellyei and Kelemen 1971; Smith et al. 

1998; Sul et al. 2002; Tompa et al. 1994; Tough and Court Brown 1965; Tough et al. 1970; Türkel and 

Egeli 1994; Van den Berghe et al. 1979; Yardley-Jones et al. 1990; Zhang et al. 1998b, 1999). 

Chromosomal aberrations in humans are frequently demonstrated in peripheral blood lymphocytes and 

bone marrow. Although inhalation, oral, and dermal routes are all potential pathways of exposure 

relevant to humans, available in vivo human data are usually drawn from occupational settings in which 

inhalation and dermal exposure routes are most prevalent. In most of these studies, chromosome 

abnormalities were detected in workers exposed to high concentrations of benzene, sufficient to produce 

blood dyscrasias. However, Qu et al. (2003a, 2003b) noted a concentration-related increase in 

chromosomal aberrations across a wide range of exposure concentrations, including workers with 

relatively low-level benzene exposure. Limitations of many of the occupational studies include lack of 

accurate exposure data, possible coexposure to other chemicals, and lack of appropriate control groups. 
BENZENE	 142 

3. HEALTH EFFECTS 

Table 3-4. Genotoxicity of Benzene In Vivo 

Species (test system) End point Result Reference 

Prokaryotic cells: 

Escherichia coli (host mediated DNA synthesis – Hellmér and Bolcsfoldi 1992a 

DNA repair) 

Invertebrate animal cells: 

Drosophila melanogaster Sex-linked recessive lethal – Kale and Baum 1983 

 Spermatocytes Recombination – 

 Spermatogonia Recombination + 

 Spermatocytes Heritable translocation – 

Mammalian cells: 

Mouse (bone marrow) Chromosomal aberrations + Giver et al. 2001; Shelby and 

Witt 1995; Siou et al. 1981 

+a Mouse (bone marrow) Chromosomal aberrations Meyne and Legator 1980 

Mouse (bone marrow) Chromosomal aberrations (+) Tice et al. 1980; 1982 

Mouse (spleen lymphocytes) Chromosomal aberrations + Au et al. 1991; Rithidech et al. 

1987 

Mouse (lymphoid cells, myeloid Chromosomal aberrations + Giver et al. 2001 

cells) 

Rat (bone marrow) Chromosomal aberrations + Fujie et al. 1990; Hoechst 

1977; Philip and Jensen 1970; 

Styles and Richardson 1984 

Rat (bone marrow) Chromosomal aberrations +b Anderson and Richardson 

1981b 

Rat (bone marrow) Chromosomal aberrations – Hoechst 1977 

Chinese hamster (bone marrow) Chromosomal aberrations + Siou et al. 1981 

Rabbit (bone marrow) Chromosomal aberrations + Kissling and Speck 1972; 1973

 Human (occupational Chromosomal aberrations + Bogadi-Šare et al. 1997; Ding 

exposure/lymphocytes) et al. 1983; Forni et al. 1971a; 

Kašuba et al. 2000; Picciano 

1979; Sasiadek 1992; Sasiadek 

and Jagielski 1990; Sasiadek et 

al. 1989; Smith et al. 1998; 

Tompa et al. 1994; Tough and 

Court Brown 1965; Tough et al. 

1970; Zhang et al. 1998a, 1999

 Human (occupational Chromosomal aberrations (+) Yardley-Jones et al. 1990 

exposure/lymphocytes) 

 Human (occupational Chromosomal aberrations – Bogadi-Šare et al. 1997; 

exposure/lymphocytes) Jablonická et al. 1987 

 Mouse (bone marrow) Micronuclei + Shelby and Witt 1995; Shelby 

et al. 1993 

+c Mouse (bone marrow PCEs) Micronuclei Ciranni et al. 1988 

Moused

 (bone marrow PCEs) Micronuclei +e Suzuki et al. 1989 
BENZENE	 143 

3. HEALTH EFFECTS 

Table 3-4. Genotoxicity of Benzene In Vivo 

Species (test system) End point Result Reference 

Mouse (bone marrow PCEs) Micronuclei + Au et al. 1990; Barale et al. 

1985; Chen et al. 1994; Diaz et 

al. 1980; Erexson et al. 1986; 

Farris et al. 1996; Harper et al. 

1984; Hite et al. 1980; Siou et 

al. 1981; Toft et al. 1982 

+a Mouse (bone marrow PCEs) Micronuclei Meyne and Legator 1980 

+f Mouse (bone marrow PCEs) Micronuclei Eastmond et al. 2001 

Mouse (bone marrow NCEs) Micronuclei + Farris et al. 1996 

+f Mouse (bone marrow NCEs) Micronuclei Eastmond et al. 2001 

Mouse (pregnant/bone marrow Micronuclei (+) Ciranni et al. 1988 

PCEs) 

Mouse (peripheral blood) Micronuclei + Hayashi et al. 1992; Healy et 

al. 2001 

Mouse (peripheral blood PCEs) Micronuclei + Farris et al. 1996 

+c,g Mouse (peripheral blood PCEs) Micronuclei Luke et al. 1988a 

Mouse (peripheral blood NCEs) Micronuclei + Barale et al. 1985; Choy et al. 

1985; Farris et al. 1996; 

Rithidech et al. 1988 

+c,f Mouse (peripheral blood NCEs) Micronuclei Luke et al. 1988a 

Mouse (lung fibroblasts) Micronuclei + Ranaldi et al. 1998 

Mouse (fetus/liver cells) Micronuclei + Ciranni et al. 1988 

Rat (lymphocytes) Micronuclei + Erexson et al. 1986 

Chinese hamster (bone marrow) Micronuclei + Siou et al. 1981 

Human (lymphocytes) Micronuclei + Robertson et al. 1991 

 Human (occupational Micronuclei + Liu et al. 1996 

exposure/lymphocytes) 

 Human (occupational Micronuclei – Pitarque et al. 1996; Surrallés 

exposure/lymphocytes) et al. 1997 

Mouse (bone marrow) Sister chromatid exchange + Tice et al. 1980; 1982 

Mouse (pregnant/bone marrow) Sister chromatid exchange + Sharma et al. 1985 

Mouse (lymphocytes) Sister chromatid exchange + Erexson et al. 1986 

Mouse (fetus/liver cells) Sister chromatid exchange + Sharma et al. 1985 

Rat (lymphocytes) Sister chromatid exchange + Erexson et al. 1986 

 Human (occupational Sister chromatid exchange + Popp et al. 1992 

exposure/lymphocytes) 

 Human (occupational Sister chromatid exchange – Kašuba et al. 2000; Pitarque et 

exposure/lymphocytes) al. 1997; Seiji et al. 1990; 

Yardley-Jones et al. 1988 

Mouse (spleen lymphocytes) Mutations + Ward et al. 1992 

Mouse (lung tissue) Mutations + Mullin et al. 1998 

Mouse embryo (premelanocytes) Mutations (deletions) + Schiestl et al. 1997 

Human (bone marrow) Mutations (gene- + Rothman et al. 1995 

duplicating) 
BENZENE	 144 

3. HEALTH EFFECTS 

Table 3-4. Genotoxicity of Benzene In Vivo 

Species (test system) End point Result Reference 

Human (bone marrow) Mutations (gene- – Rothman et al. 1995 

inactivating) 

Mouse (bone marrow) DNA adducts + Arfellini et al. 1985; Creek et al. 

1997; Lévay et al. 1996; Pathak 

et al. 1995; Turteltaub and 

Mani 2003 

Mouse (white blood cells) DNA adducts + Lévay et al. 1996 

Mouse (liver) DNA adducts + Arfellini et al. 1985; Creek et al. 

1997; Mani et al. 1999; 

Turteltaub and Mani 2003 

Rat (bone marrow) DNA adducts + Arfellini et al. 1985; Creek et al. 

1997; Lévay et al. 1996; Pathak 

et al. 1995; Turteltaub and 

Mani 2003 

Rat (liver) DNA adducts + Arfellini et al. 1985; Creek et al. 

1997; Lutz and Schlatter 1977; 

Mani et al. 1999; Mazzullo et al. 

1989; Turteltaub and Mani 

2003 

Mouse (peripheral blood DNA strand breaks + Tuo et al. 1996 

lymphocytes) 

Rat (lymphocytes, bone marrow, DNA strand breaks + Lee et al. 2005 

spleen, liver) 

 Human (occupational DNA strand breaks + Andreoli et al. 1997; Nilsson et 

exposure/lymphocytes) al. 1996; Sul et al. 2002 

Mouse (peripheral blood DNA damage + Chang et al. 2005 

lymphocytes, bone marrow) 

 Human (occupational DNA repair efficiency _ Hallberg et al. 1996 

exposure/lymphocytes) 

Rat (bone marrow) DNA oxidative damage + Kolachana et al. 1993 

 Human (occupational DNA oxidative damage + Liu et al. 1996 

exposure/lymphocytes) 

 Mouse (bone marrow) DNA synthesis inhibition + Lee et al. 1988 

Rabbit (bone marrow) DNA synthesis inhibition + Kissling and Speck 1972 

 Mouse (bone marrow) RNA synthesis inhibition + Kissling and Speck 1972 

Rat (liver mitochondria) RNA synthesis inhibition + Kalf et al. 1982 
BENZENE 145 

3. HEALTH EFFECTS 

Table 3-4. Genotoxicity of Benzene In Vivo 

Species (test system) End point Result Reference 

Mouse (spermatogonia) Sperm head abnormality + Topham 1980 

a

This result was observed following both oral and intraperitoneal exposure.

b

This result was observed following both inhalation and intraperitoneal exposure. 

c

Males affected to a significantly greater degree than females. d

Two strains of mouse were tested; Ms/Ae and CD-1. The result applies to both strains. e

This result was observed following both oral and intraperitoneal exposure; however, oral exposure produced the 

greater effect.

f

Increase in micronuclei was exposure duration-dependent.

g

Increase in micronuclei was exposure duration-independent. 

+ = Positive result; – = negative result; (+) = weakly positive result; DNA = deoxyribonucleic acid; 

NCEs = normochromatic erythrocytes; PCEs = polychromatic erythrocytes; RNA = ribonucleic acid 
BENZENE 146 

3. HEALTH EFFECTS 

Table 3-5. Genotoxicity of Benzene In Vitro 

Result 

With Without 

Species (test system) End point activation activation Reference 

Prokaryotic organisms: 

Salmonella typhimurium Gene mutation – – De Flora et al. 1984 

(Ames test) 

S. typhimurium (histidine Gene mutation + – Glatt et al. 1989 

reversion) 

S. typhimurium (azaquanine Gene mutation + No data Kaden et al. 1979; 

reversion) Seixas et al. 1982 

Bacillus subtilis (histidine Gene mutation – – Tannoka 1977 

reversion) 

Escherichia coli (DNA DNA synthesis No data – Lee et al. 1988 

polymerase 1/cell-free DNA inhibition 

synthetic system) 

E. coli (host meditated DNA DNA synthesis No data No data Hellmér and Bolcsfoldi 

repair) 1992b 

Plasmid DNA ΦX-174 RF I DNA degradation No data + Li et al. 1995 

Eukaryotic organisms: 

Fungi: 

Aspergillus nidulans Gene mutation No data – Crebelli et al. 1986 

(methionine supressors) 

 Mammalian cells: 

 Mouse (L5178Y cells/TK Gene mutation – – Oberly et al. 1984 

test) 

 Chinese hamster (ovary Chromosomal – – Gulati et al. 1989 

cell culture) aberrations 

 Human (lymphocyte cell Chromosomal No data + Eastmond et al. 1994; 

culture) aberrations Morimoto 1976 

 Human (lymphocyte cell Chromosomal No data – Gerner-Smidt and 

culture) aberrations Friedrich 1978 

 Human (lymphoblastoid Intrachromosomal No data + Aubrecht et al. 1995 

culture) recombination 

 Chinese hamster (ovary Micronuclei – – Douglas et al. 1985 

cell culture) 

Human (whole blood cells) Micronuclei _ _ Zarani et al. 1999 

 Chinese hamster (ovary Sister chromatid – – Douglas et al. 1985; 

cell culture) exchange Gulati et al. 1989 

 Human (lymphocyte cell Sister chromatid + No data Morimoto 1983 

culture) exchange 

 Human (lymphocyte cell Sister chromatid No data – Gerner-Smidt and 

culture) exchange Friedrich 1978 

 Rabbit (bone marrow DNA adducts No data + Rushmore et al. 1984 

mitoplasts) 
BENZENE 147 

3. HEALTH EFFECTS 

Table 3-5. Genotoxicity of Benzene In Vitro 

Result 

With Without 

Species (test system) End point activation activation Reference 

Rat (liver mitoplasts) 

Calf thymus DNA 

Human (bone marrow) 

Human (leukemia cells) 

Rat (hepatocytes) 

 Chinese hamster (ovary 

cell culture) 

 Chinese hamster (ovary 

cell culture) 

Chinese hamster (V79 cell 

culture) 

 Mouse (L5178Y cell 

culture) 

Human (leukemia cells) 

 Human (lymphocyte cell 

culture) 

Rat liver epithelial cells 

 Rat (hepatocyte culture)

 Rat (hepatocyte culture)
 

Human (HeLa S3 cells) 

 Mouse (bone marrow cell 

culture) 

 Mouse (bone marrow cell 

culture) 

 Calf (thymus DNA 

polymerase α/cell-free 

DNA synthetic system) 

Human (HeLa cells) 

 Mouse (spleen
 

lymphocytes) 

DNA adducts 

DNA adducts 

DNA adducts 

DNA adducts 

DNA breaks 

DNA breaks 

DNA breaks 

DNA breaks 

DNA breaks 

DNA oxidative 

damage 

DNA repair 

DNA hyperphosphorylatioin 

Unscheduled DNA 

synthesis 

Unscheduled DNA 

synthesis 

Unscheduled DNA 

synthesis 

DNA synthesis 

inhibition 

DNA synthesis 

inhibition 

DNA synthesis 

inhibition 

DNA synthesis 

inhibition 

RNA synthesis 

inhibition 

No data 

No data 

No data 

No data 

No data 

+ 

+ 

– 

No data 

No data 

No data 

No data 

No data 

No data 

– 

No data 

+ 

No data 

– 

No data 

+ 

+ 

+ 

+ 

– 

+ 

+a 

– 

– 

+ 

_ 

+ 

(+) 

– 

– 

+ 

(+) 

+ 

– 

+ 

Rushmore et al. 1984 

Chenna et al. 1995 

Bodell et al. 1993; 

Lévay and Bodell 1992 

Bodell et al. 1993; 

Lévay and Bodell 1992 

Bradley 1985 

Douglas et al. 1985 

 Lakhanisky and 

Hendricks 1985 

Swenberg et al. 1976 

Pellack-Walker and 

Blumer 1986 

Kolachana et al. 1993 

Hallberg et al. 1996 

Dees and Travis 1994 

Glauert et al. 1985 

Probst and Hill 1985; 

Williams et al. 1985 

Barrett 1985 

Lee et al. 1988 

Lee et al. 1989 

Lee et al. 1988 

Painter and Howard 

1982 

Post et al. 1985 
BENZENE 148 

3. HEALTH EFFECTS 

Table 3-5. Genotoxicity of Benzene In Vitro 

Result 

With Without 

Species (test system) End point activation activation Reference 

Rat (liver mitoplasts) RNA synthesis No data + Kalf et al. 1982 

inhibition 

Cat, rabbit (bone marrow RNA synthesis No data + Kalf et al. 1982 

mitoplasts) inhibition 

a

Benzene's effect on DNA breaks was reduced when metabolic activators were used. 

– = negative results; + = positive results; (+) = weakly positive result; DNA = deoxyribonucleic acid; 

RNA = ribonucleic acid 
BENZENE 149 

3. HEALTH EFFECTS 

Chromosomal aberrations observed in workers chronically exposed to benzene include hypo- and 

hyperdiploidy, deletions, breaks, and gaps. For example, analysis of peripheral lymphocytes of workers 

exposed to benzene vapors at a mean concentration of 30 ppm revealed significant increases in 

monosomy of chromosomes 5, 7, and 8 (but not 1), and tri- and/or tetrasomy of chromosomes 1, 5, 7, and 

8 (Zhang et al. 1998b, 1999). In another series of epidemiological studies in workers chronically exposed 

to benzene, nonrandom effects were apparent in chromosomes 1, 2, 4, and 9; nonrandom breaks in 

chromosomes 2, 4, and 9 were twice as prevalent in benzene-exposed workers versus controls; and 

chromosomes 1 and 2 were nearly twice as prone to gaps (Sasiadek and Jagielski 1990; Sasiadek et al. 

1989). Twenty-one people with hematological signs of chronic benzene poisoning exhibited significantly 

more chromosomal abnormalities than controls (Ding et al. 1983). A significant increase in dicentric 

chromosomes and unstable aberrations was noted in 36 female workers exposed to benzene in a shoe 

factory for up to 32 years (Kašuba et al. 2000). Significant increases in hyperploidy of chromosomes 8 

and 21 and translocations between chromosomes 8 and 21 were observed in workers exposed to benzene 

vapors at a mean TWA of 31 ppm (Smith et al. 1998). 

DNA repair efficiency was evaluated in blood lymphocytes collected from exposed or unexposed workers 

in a petrochemical plant (Hallberg et al. 1996). Plasmids (pCMVCAT) were irradiated with UV light 

(254 nm) to induce thymidine dimers and were then transfected into blood lymphocytes from workers. 

Transfected plasmids that were repaired in the lymphocytes would express the chloramphenicol 

actetyltransferase reporter gene (CAT) product whereas unrepaired plasmids would not. Lymphocytes 

from exposed or unexposed workers did not show significant differences in their ability to repair lightdamaged DNA; however, the authors suggest that the sample population was too small to detect any 

differences given the large individual variations in repair capacity (Hallberg et al. 1996). 

Oxidative DNA damage was assessed in workers (n=87) exposed to benzene by conducting 

measurements of 8-hydroxy-2-deoxyguanosine (8-OHdG) in peripheral blood lymphocytes (Liu et al. 

1996). 8-OHdG is formed by hydroxy radical (OH·) addition at the C-8 position of deoxyguanosine 

(Kasai and Nishimura 1986) and appears to be a biomarker of oxidative DNA damage (Liu et al. 1996). 

The exposure to benzene was classified as low, medium, or high (mean benzene levels of 2.46, 103, or 

424 mg/m3

, respectively [corresponding to 0.78, 32.2, or 133 ppm]). Levels of 8-OHdG in exposed 

workers rose in a concentration-related manner although the increases were significant only in the 

medium and high exposure groups. Toluene, also detected in the workplace air, did not alter levels of 

8-OHdG. Formation of micronuclei, a measure of DNA damage, increased in a concentration-related 

manner to levels that were significantly higher than those in controls. The levels of urinary trans,trans­
BENZENE 150 

3. HEALTH EFFECTS 

muconic acid, a biomarker of benzene exposure, were well correlated with levels of 8-OHdG which, in 

turn, were well correlated with levels of lymphocyte micronuclei. The findings of Liu et al. (1996) are 

supported by the results of a study by Nilsson et al. (1996) in which concentration-related increased 

urinary levels of 8-OHdG were reported in male workers (n=30) at a gasoline station. Breathing zone 

benzene concentrations ranged from 0.003 to 0.6 ppm (mean 0.13 ppm). Significant concentrationrelated increases in DNA single-strand breaks were also noted in these workers. Collectively, the results 

of Liu et al. (1996) and Nilsson et al. (1996) provide suggestive evidence that benzene metabolites may 

induce reactive oxygen species, which could result in oxidative DNA damage and formation of 

hydroxylated bases such as 8-OHdG (Liu et al. 1996). 

Rothman et al. (1995) used the glycophorin A (GPA) gene loss mutation assay to assess the nature of 

DNA damage in workers heavily exposed to benzene. The GPA assay measures the frequency of variant 

cells that have lost the expression of the M form of the GPA gene in the peripheral blood of heterozygous 

(MN) subjects. The variant cells possess the NN phenotype (double-copy expression of the N allele and 

no expression of M) or the NØ phenotype (single-copy expression of the N allele and no expression of 

M). The NN variants are thought to arise from mitotic recombination, chromosome loss, and 

reduplication, or gene conversion. The NØ variants appear to be associated with point mutations, 

deletions, or gene inactivation. Rothman et al. (1995) demonstrated a significant increase in the 

frequency of NN cells in benzene-exposed workers (compared with unexposed control subjects) in the 

absence of a significant effect on the frequency of NØ cells. These results suggest that benzene induces 

gene-duplicating, but not gene-inactivating, mutations at the GPA locus in benzene-exposed humans. 

Sister chromatid exchange was not found to be a significant effect of benzene exposure in humans 

(Kašuba et al. 2000; Seiji et al. 1990; Yardley-Jones et al. 1988); however, the selection of control 

subjects in the studies by Seiji et al. (1990) and Yardley-Jones et al. (1988) was poor. Refer to Table 3-4 

for a further summary of these results. 

In vivo animal studies provide convincing evidence of benzene's genotoxicity (Table 3-4). Furthermore, 

the finding that male mice are more sensitive than females to benzene-induced chromosomal damage is 

consistent among reports (Armstrong and Galloway 1993; Barale et al. 1985; Choy et al. 1985; Ciranni et 

al. 1988; Hatakeyama et al. 1992; Meyne and Legator 1980; Siou et al. 1981). Consistently positive 

findings for chromosomal aberrations in bone marrow and lymphocytes in animals support the human 

case reports and epidemiological studies in which chromosomal damage was linked to benzene exposure. 

Micronucleus assays are popular methods for crudely analyzing DNA damage in animals. Positive results 
BENZENE 151 

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were observed in all studies testing for increased micronuclei frequencies. One of these micronucleus 

assays (Luke et al. 1988a) investigated the effects of different inhalation exposure durations on 

polychromatic erythrocytes (PCEs) and normochromatic erythrocytes (NCEs) in peripheral blood. PCEs 

are newly formed erythrocytes that contain mRNA and as a result, exhibit staining when RNA staining 

reagents are used. NCEs are mature erythrocytes that lack mRNA and are not stained under the same 

conditions. The researchers found that PCEs are good indicators of recent and acute exposure, while 

NCEs are good indicators of accumulated long-term exposure (Luke et al. 1988a). Although no human 

studies were located that reported increased sister chromatid exchange in exposed individuals, increases 

in sister chromatid exchange were reported in mice and rats (Erexson et al. 1986; Sharma et al. 1985; Tice 

et al. 1980, 1982). In vivo, trans,trans-muconaldehyde (a metabolite of benzene) has also been shown to 

induce highly significant increases in sister chromatid exchanges in mice (Witz et al. 1990a). In addition 

to oral and inhalation routes, many researchers tested subcutaneous and intraperitoneal routes as well; the 

results for these alternate routes of exposure were largely positive for chromosomal aberrations in bone 

marrow (Anderson and Richardson 1981; Kissling and Speck 1972, 1973; Kolachana et al. 1993; Meyne 

and Legator 1980; Philip and Jensen 1970), micronuclei in bone marrow (Diaz et al. 1980), and sister 

chromatid exchange in mouse fetus liver cells (Sharma et al. 1985). Binding of benzene and/or its 

metabolites to DNA, RNA, and proteins has been consistently observed in rats and mice (Arfellini et al. 

1985; Creek et al. 1997; Lévay et al. 1996; Mani et al. 1999; Mazullo et al. 1989; Turteltaub and Mani 

2003). Arfellini et al. (1985) noted that binding to RNA and proteins was more prevalent than binding to 

DNA. Lévay et al. (1996) observed dose- and time-dependent formation of two DNA adducts in white 

blood cells and bone marrow cells of mice administered benzene for 7 days via intraperitoneal injection. 

Turteltaub and Mani (2003) consistently observed DNA and protein binding in mice at intraperitoneal 

doses as low as 5 μg/kg body weight. One study using intraperitoneal injections reported a dosedependent increase in sperm head abnormalities in mice exposed to 0.5 or 0.6 mL benzene/kg/day 

(Topham 1980). The author views sperm head abnormality as a possible indication of heritable 

mutations. However, from this study alone, it cannot be determined if benzene causes such transmissible 

genetic mutations. For these and other results from animal in vivo studies, see Table 3-4. 

Molecular mechanisms of benzene-induced genotoxicity have been studied to some extent in laboratory 

animals. Schiestl et al. (1997) noted benzene-induced reversion of the mouse pink-eye unstable mutation 

in premelanocytes of mouse embryos following intraperitoneal injection of the chemical into pregnant 

dams; the reversions resulted from genomic DNA deletions. Results of Chen et al. (1994) and Eastmond 

et al. (2001) indicate that benzene-induced micronuclei in mouse bone marrow erythrocytes are formed 

predominantly from chromosome breakage, but also from aneuploidy. 
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In vitro studies strongly imply that benzene's genotoxicity is derived primarily from its metabolites. 

Positive results were obtained for gene mutation in Salmonella typhimurium (Glatt et al. 1989; Kaden et 

al. 1979; Seixas et al. 1982) and sister chromatid exchange in human lymphocyte cell culture (Morimoto 

1983) only when exogenous metabolic activators of benzene were used. Similarly, endogenous metabolic 

activation was required for effects to be seen on DNA synthesis in rat hepatocyte culture (Glauert et al. 

1985), DNA adduct formation in rat liver mitoplasts (Rushmore et al. 1984), and RNA synthesis in rat 

liver mitoplasts and in rabbit and cat bone marrow mitoplasts (Kalf et al. 1982). Endogenous activation 

occurs naturally by enzymes already within the cells. Exogenous activation requires the addition of 

enzymes to cellular preparations. In one study using benzene and 13 possible metabolites, Glatt et al. 

(1989) found that trans-1,2-dihydrodiol (with metabolic activators) and the diol epoxides (with or without 

metabolic activators) produced histidine reversion in S. typhimurium. The same researchers also 

investigated the genotoxicity of these 13 proposed metabolites in V79 Chinese hamster cells; anti-diol 

epoxide, syn-diol epoxide, 1,2,3-trihydroxybenzene, 1,2,4-trihydroxybenzene, quinone, hydroquinone, 

catechol, phenol, and 1,2-dihydrodiol were found to produce genotoxic effects ranging from sister 

chromatid exchange and micronuclei increases to gene mutations (Glatt et al. 1989). Similar studies with 

trans,trans-muconaldehyde showed that this metabolite is strongly mutagenic in V79 cells and weakly 

mutagenic in bacteria (Glatt and Witz 1990). Chang et al. (1994) showed that muconaldehyde and its 

aldehyde metabolites 6-hydroxy-trans,trans-2,4-hexadienal and 6-oxo-trans,trans-hexadienoic acid were 

mutagenic in V79 cells. 

Several studies revealed a possible connection between certain benzene metabolites and DNA damage via 

the formation of oxygen radicals. Lewis et al. (1988) found that both 1,2,4-benzenetriol and 

hydroquinone produce DNA strand breaks, 1,2,4-benzenetriol to a greater degree than hydroquinone. 

Consistent with these findings was the observation that 1,2,4-benzenetriol generates a greater 

concentration of oxygen radicals than hydroquinone. The study concluded that 1,2,4-benzenetriol 

damages DNA by producing oxygen radicals, while hydroquinone probably exerts its genotoxic effects by 

some other mechanism (Lewis et al. 1988). However, results of Li et al. (1995b) indicate that much of 

the benzene-mediated DNA damage may result from the generation of reactive oxygen species via a 

copper-redox cycling mechanism involved in the oxidation of the benzene metabolite, hydroquinone. 

Increased recombination, which can lead to adverse genetic changes, was observed in Chinese hamster 

ovary cells exposed to phenol, catechol, or benzoquinone (Winn 2003). Benzene had no effect on 

recombination. The observed increases in recombination were abolished by the addition of catalase to the 

cells, suggesting that the effect of the benzene metabolites was elicited by oxidative stress (Winn 2003). 
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Oxidative damage (manifested as DNA strand breaks) was observed in HL60 cells treated with 

1,4-benzoquinone or 1,4-hydroquinone (Hiraku and Kawanishi 1996). 1,4-Benzoquinone and 

hydroquinone induced DNA strand breaks in Chinese hamster ovary cells while phenol, catechol, 

1,2,4-benzenetriol, trans,trans-muconic acid, and S-phenyl mercapturic acid did not (Sze et al. 1996). No 

synergism between hydroquinone and the other metabolites was observed. 

Phenol, catechol, hydroquinone, and benzene induced morphological transformation and gene mutations 

in Syrian hamster embryo cells (Tsutsui et al. 1997). Chromosomal aberrations, sister chromatid 

exchange, and unscheduled DNA synthesis were increased by the benzene metabolites while aneuploidy 

was observed in cells treated with benzene or catechol (Tsutsui et al. 1997). There was no significant 

increase (p>0.005) in DNA fragmentation in human respiratory epithelial cells exposed to an atmosphere 

of 5 mg benzene/m3

 for 8 hours, although there was evidence of an inflammatory response (Gosepath et 

al. 2003). Chromosomal breaks and hyperdiploidy were observed in human lymphocytes after exposure 

to hydroquinone in vitro (Eastmond et al. 1994). Aneusomy of chromosomes 7 and 8 were observed in 

human umbilical cord blood cells treated with hydroquinone (2, 10, or 50 μM) for 72 hours (Smith et al. 

2000). Monosomy 7 and trisomy 8 are two common clonal aberrations observed in myeloid leukemias 

(Smith et al. 2000). Hydroquinone (26–49 μM) also induced monosomy of chromosomes 5, 7, and 8 in a 

human lymphoblast cell line (Stillman et al. 1997). Hydroquinone (10–100 μM) and 1,2,4-benzenetriol 

(10–50 μM) significantly increased monosomy 5 and 7 in human lymphocytes and long-arm deletions in 

chromosomes 5 and 7 (Zhang et al. 1998b). Benzene metabolites have been shown to form DNA adducts 

in human bone marrow and HL-60 cells (Bodell et al. 1993; Lévay and Bodell 1992). Zhang et al. (1993) 

showed that 1,2,4-benzenetriol increased the frequency of micronuclei formation in human lymphocytes 

in culture, and in HL60 cells in a dose-related manner. An increase in the level of oxidative damage to 

DNA was also noted in HL60 cells in culture. Extracts from human cells have been shown to have repair 

activity toward benzoquinone-DNA adducts in vitro (Chenna et al. 1995). Benzene (1–5 mM) did not 

elicit micronuclei formation in whole blood cells treated for 48 hours with or without metabolic activation 

with 10% S9 rat liver fraction for 2 hours (Zarani et al. 1999). The assay was conducted with blood 

collected from four subjects. Chen and Eastmond (1995) showed that benzene metabolites can adversely 

affect human topoisomerases, enzymes involved in DNA replication and repair. No effect of any 

metabolite was seen on human topoisomerase I or for topoisomerase II for hydroquinone, phenol, 

2,2'-biphenol, 4,4'-biphenol, and catechol at concentrations as high as 500 μM. 1,4-Benzoquinone and 

1,2,4-benzenetriol inhibited human topoisomerase II in vitro, at 500 and 250 μM without bioactivation. 

However, following bioactivation, phenol and 2,2'-biphenol showed inhibitory effects at doses as low as 

50 μM, whereas 4,4'-biphenol inhibited topoisomerase II at concentrations of 10 μM. More recently, 
BENZENE 154 

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Eastmond et al. (2001) demonstrated decreased activity of topoisomerase II activity in nucleated bone 

marrow cells of mice administered benzene by oral gavage for subchronic durations. 

Available in vitro data suggest that benzene itself is genotoxic. Two studies reported that benzene 

produced DNA breaks in Chinese hamster ovary cells independent of metabolic activators (Douglas et al. 

1985; Lakhanisky and Hendrickx 1985). In a study by Aubrecht et al. (1995), benzene was shown to 

induce intrachromosomal recombination in human lymphoblastoid cell culture. Therefore, benzene 

appears to have some genotoxic capabilities of its own, but its metabolites seem to be the primary 

genotoxins in systems in which normal metabolism is occurring. Refer to Table 3-5 for the results of 

these and other in vitro studies. 

In summary, chromosome aberrations have been found consistently in bone marrow cells of persons 

occupationally exposed to benzene. The conclusion, based on human epidemiological studies, that 

benzene is a human clastogen is well supported by in vivo animal studies and in vitro cell cultures and 

subcellular studies. Virtually all studies that looked for effects at the chromosomal level were positive 

when the ability to metabolize benzene was present. These experimental results are consistent with the 

chromosomal damage seen in exposed humans. The leukemia observed in some benzene-exposed 

persons may result from the appearance of a clone of chromosomally abnormal cells in the bone marrow. 

With respect to genetic effects, no safe human exposure level can be determined from available 

epidemiological data. Significant increases in sister chromatid exchanges were produced in bone marrow 

cells and lymphocytes of animals. The significance of sister chromatid exchanges is unknown, but their 

production by a chemical is generally considered to indicate a genotoxic potential. Exposures generally 

occur via inhalation, and based on animal studies, effects following oral exposure may be greater than 

effects following inhalation exposure to comparable levels of benzene. Data presented in this section and 

elsewhere in this profile (Section 3.4) show that benzene metabolites are the genotoxic entities. It is 

possible that each metabolite causes a different genotoxic effect. Differences in metabolic capability are 

probably responsible for some of the variations in response to benzene seen in different test systems. 

3.4 TOXICOKINETICS 

The toxicokinetics of benzene has been extensively studied. Inhalation exposure is probably the major 

route of human exposure to benzene, although oral and dermal exposure are also important. Benzene is 

readily absorbed following inhalation or oral exposure. Although benzene is also readily absorbed from 

the skin, a significant amount of a dermal application evaporates from the skin surface. Absorbed 
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benzene is rapidly distributed throughout the body and tends to accumulate in fatty tissues. The liver 

serves an important function in benzene metabolism, which results in the production of several reactive 

metabolites. Although it is widely accepted that benzene toxicity is dependent upon metabolism, no 

single benzene metabolite has been found to be the major source of benzene hematopoietic and 

leukemogenic effects. At low exposure levels, benzene is rapidly metabolized and excreted 

predominantly as conjugated urinary metabolites. At higher exposure levels, metabolic pathways appear 

to become saturated and a large portion of an absorbed dose of benzene is excreted as parent compound in 

exhaled air. Benzene metabolism appears to be qualitatively similar among humans and various 

laboratory animal species. However, there are quantitative differences in the relative amounts of benzene 

metabolites. 

3.4.1 Absorption 

3.4.1.1 Inhalation Exposure 

Inhalation exposure is probably the major route of human exposure to benzene, and numerous studies of 

absorption of benzene after inhalation exposure in different settings have been conducted (Ashley et al. 

1994; Avis and Hutton 1993; Boogaard and van Sittert 1995; Brunnemann et al. 1989; Byrd et al. 1990; 

Etzel and Ashley 1994; Fustinoni et al. 1995; Ghittori et al. 1995; Gordian and Guay 1995; Hajimiragha 

et al. 1989; Hanzlick 1995; Karacic et al. 1995; Kok and Ong 1994; Lagorio et al. 1994a; Laitinen et al. 

1994; Lauwerys et al. 1994; Lindstrom et al. 1994; Mannino et al. 1995; Nomiyama and Nomiyama 

1974a; Ong et al. 1994, 1995; Pekari et al. 1992; Popp et al. 1994; Rauscher et al. 1994; Rothman et al. 

1995; Ruppert et al. 1995; Scherer et al. 1995; Shamy et al. 1994; Srbova et al. 1950; Yu and Weisel 

1996). Existing evidence indicates that benzene is rapidly absorbed by humans following inhalation 

exposure. Results from a study of 23 subjects who inhaled 47–110 ppm benzene for 2–3 hours showed 

that absorption was highest in the first few minutes of exposure, but decreased rapidly thereafter (Srbova 

et al. 1950). In the first 5 minutes of exposure, absorption was 70–80%, but by 1 hour, it was reduced to 

approximately 50% (range, 20–60%). Respiratory uptake (the amount of benzene absorbed from the 

lungs following inhalation of the vapors) in six volunteers including males and females exposed to 52– 

62 ppm benzene for 4 hours was determined to be approximately 47%(Nomiyama and Nomiyama 1974a). 

In a similar study, three healthy nonsmoking volunteers were exposed to benzene at levels of 1.6 or 

9.4 ppm for 4 hours (Pekari et al. 1992). The amount of benzene absorbed was estimated from the 

difference between the concentration inhaled and the concentration exhaled. Estimates were 48% for the 

high dose and 52% for the low dose, supporting the evidence of Nomiyama and Nomiyama (1974a). Yu 

and Weisel (1996) measured the uptake of benzene by three female subjects exposed to benzene in smoke 
BENZENE 156 

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generated by burning cigarettes, which resulted in airborne benzene concentrations in the range of 32–69 

ppm. Average absorption for exposure periods of 30 or 120 minutes was 64% and did not appear to be 

influenced by exposure duration. 

Studies of occupational exposure to benzene suggest that absorption occurs both by inhalation and 

dermally in many workplace settings. In a study conducted in 1992 in Finland, car mechanics’ exposure 

to benzene was evaluated (Laitinen et al. 1994). Different work phases were measured at five Finnish 

garages. Blood samples from car mechanics (eight nonsmokers) were taken 3–9 hours after exposure to 

benzene. The results were approximated to the time point of 16 hours after exposure. Fourteen air 

samples were taken from the breathing zone and five stationary samples were collected from the middle 

of the garage for background concentration levels. The average background concentration (stationary 

samples) of gasoline vapors was 6±7 cm3

/m3 (2±2 ppm) and the concentration of benzene was under the 

detection limit of 0.2 cm3

/m3 (0.1 ppm). The concentrations of benzene in the breathing zone varied from 

the detection limit of 0.2 cm3

/m3 to 1.3 cm3/m3 (0.1–0.4 ppm) for unleaded gasoline and from the 

detection limit to 3.7 cm3

/m3 (1.2 ppm) for leaded gasoline. The highest benzene exposure level (2.4– 

3.7 cm3

/m3 or 0.8–1.2 ppm) was measured when changing the filter to the fuel pump. The mechanics 

worked without protective gloves, and the risk of contamination and penetration through the skin was 

significant. During carburetor renewal and gathering, benzene concentrations were 0.5–1.1 cm3

/m3 (0.2– 

0.3 ppm). During changing of the fuel filter to electronic fuel-injection system, benzene concentration 

ranged from 0.9 to 3.4 cm3

/m3 (0.3–1.1 ppm). The approximated benzene concentrations in blood 

corresponding to the time point of 16 hours after the exposure showed much higher levels of exposure 

than could be expected according to the corresponding air measurements (8-hour TWA). The comparison 

of expected benzene concentrations in blood, if no dermal exposure were present, to the levels at the time 

point of 16 hours after the exposure showed that the dermal route must be the source of about 68% of 

exposure (range 1.1–88.2%). Two of eight workers had minimal exposure through the skin (0–1.1%). 

The other six workers showed high dermal exposure (79.4%). 

Exposure to benzene-contaminated water can also provide an opportunity for both inhalation and dermal 

absorption. In a series of experiments conducted in a single-family residence from June 11 to 13, 1991, 

exposure to benzene through contaminated residential water was monitored (Lindstrom et al. 1994). The 

residential water was contaminated with benzene and other hydrocarbons in 1986. Periodic testing 

conducted from 1986 to 1991 showed benzene concentrations ranging from 33 to 673 μg/L (ppb). The 

experiment involved an individual taking a 20-minute shower with the bathroom door closed, followed by 

5 minutes for drying and dressing; then the bathroom door was opened and this individual was allowed to 
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3. HEALTH EFFECTS 

leave the house. Integrated 60- and 240-minute whole-air samples were collected from the bathroom, an 

adjacent bedroom, living room, and in ambient air. Glass, gas-tight syringe grab samples were 

simultaneously collected from the shower, bathroom, bedroom, and living room at 0, 10, 18, 20, 25, 25.5, 

and 30 minutes. Two members of the monitoring team were measured for 6 hours using personal Tenax 

gas GC monitors. For the first 30 minutes of each experiment, one member was based in the bathroom 

and the other in the living room. Benzene concentrations in the shower head ranged from 185 to 

367 μg/L (ppb), while drain level samples ranged from below the detectable limit (0.6 μg/L or ppb) to 

198 μg/L (ppb). Analysis of the syringe samples suggested a pulse of benzene moving from the shower 

stall to the rest of the house over approximately 60 minutes. Peak levels of benzene measured 758– 

1,670 μg/m3

 (235–518 ppb) in the shower stall at 18–20 minutes, 366–498 μg/m3 (113–154 ppb) in the 

bathroom at 10–25 minutes, 81–146 μg/m3

 (25–45 ppb) in the bedroom at 25.5–30 minutes, and 40– 

62 μg/m3

 (12–19 ppb) in the living room at 36–70 minutes. The individual who took the 20-minute 

shower had estimated inhalation doses of 79.6, 105, and 103 μg (mean=95.9 μg) for the 3 consecutive 

sampling days. These doses were estimated by taking the products of the concentration of benzene in 

water, the minute ventilation rate, the duration of exposure, and a 70% benzene absorption factor. This 

was 2.1–4.9 times higher than corresponding 20-minute bathroom exposures. Adding the average dose 

absorbed in the bathroom during the 5.5 minutes following the shower (using the overall 20–25 minutes 

mean syringe level of 318 μg/m3 [99 ppb]) gave a total average shower-related inhalation dose of 113 μg. 

An average dermal dose of 168 μg was estimated for the 20-minute shower by multiplying the average 

concentration of benzene in water by the surface area of the male volunteer, an exposure factor of 75% 

body surface area exposed, a dermal permeability constant for benzene of 0.11 cm/hour, an exposure 

duration of 0.33 hours, and a unit conversion factor of 11/1,000 cm2

. The total benzene dose resulting 

from the shower was estimated to be approximately 281 μg (40% via inhalation and 60% via dermal), 

suggesting a higher potential exposure to benzene via dermal contact from the water than through 

vaporization and inhalation. This exposure was 2–3.5 times higher than the mean 6-hour inhalation dose 

received by the sampling members. The estimated inhalation and dermal doses reported by Lindstrom et 

al. (1994) have not been validated by others and are therefore of questionable value for quantitative 

analysis. 

Additional evidence of benzene absorption following inhalation exposure comes from data on cigarette 

smokers. Benzene levels were significantly higher in the venous blood of 14 smokers (median level of 

493 ng/L) than in a control group of 13 nonsmokers (median level of 190 ng/L) (Hajimiragha et al. 1989). 

Cigarette smoke is known to contain benzene (Brunnemann et al. 1989; Byrd et al. 1990), and the subjects 

had no known exposure to other sources of benzene (Hajimiragha et al. 1989). Kok and Ong (1994) 
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3. HEALTH EFFECTS 

report blood and urine levels of benzene as 110.9 and 116.4 ng/L, respectively for nonsmokers, and 

328.8 and 405.4 ng/L, respectively for smokers. The National Association of Medical Examiners 

Pediatric Toxicology (PedTox) Registry reported blood benzene concentrations ranging from 0.2 to 

4.9 mg/L in eight children who died in fires and were dead at the scene, indicating absorption of benzene 

from burning materials (Hanzlick 1995). Blood benzene levels taken from U.S. engineers (Group I) and 

firefighters (Group II) working at burning oil wells in Kuwait were compared to blood benzene levels 

from non-exposed U.S. citizens (Etzel and Ashley 1994). The median concentrations of benzene in whole 

blood from Groups I, II, and U.S. reference group were 0.035 μg/L (range ND–0.055 μg/L), 0.18 μg/L 

(range 0.063–1.1 μg/L), and 0.066 μg/L (range ND–0.54 μg/L), respectively. The median concentration 

in group II was generally higher than the median concentrations in group I or the reference group. 

Statistically significant higher concentrations of benzene (p<0.0001) were found in group II smokers than 

in Group II nonsmokers. 

Animal data confirm that benzene is rapidly absorbed through the lungs. Inhalation studies with 

laboratory dogs indicate that distribution of benzene throughout the animal's body is rapid, with tissue 

values dependent on blood supply. A linear relationship existed between the concentration of benzene in 

air (200–1,300 ppm) and the equilibrium concentration in blood (Schrenk et al. 1941). At these 

exposures, the concentrations of benzene in the blood of dogs exposed to benzene reached a steady state 

within 30 minutes. 

In rodents, the extent of uptake increased linearly with concentration for exposures up to 200 ppm. At 

concentrations of >200 ppm, zero-order kinetics were observed (i.e., uptake became nonlinear, indicating 

saturation of the metabolic capacity). The percentage of inhaled benzene that was absorbed and retained 

during a 6-hour exposure period decreased from 33 to 15% in rats and from 50 to 10% in mice as the 

exposure concentration was increased from about 10 to 1,000 ppm (Sabourin et al. 1987). When rats and 

mice were exposed to approximately 300 ppm, mice had greater uptake than rats. Mice and rats had 

different absorption characteristics; the cumulative inhaled dose in mice was greater than that in rats 

(Eutermoser et al. 1986; Sabourin et al. 1987). Purebred Duroc-Jersey pigs were exposed to 0, 20, 100, 

and 500 ppm benzene vapors 6 hours/day, 5 days/week for 3 weeks (Dow 1992). The average 

concentration of phenol in the urine increased linearly with dose. 
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3. HEALTH EFFECTS 

3.4.1.2 Oral Exposure 

Although definitive scientific data are not available on oral absorption of benzene in humans, case studies 

of accidental or intentional poisoning indicate that benzene is absorbed by the oral route (Thienes and 

Haley 1972). 

Benzene appears to be efficiently absorbed following oral dosing in animals. Oral absorption of benzene 

was first demonstrated by Parke and Williams (1953a). After radiolabeled (14C) benzene was 

administered orally to rabbits (340–500 mg/kg), the total radioactivity eliminated in exhaled air and urine 

accounted for approximately 90% of the administered dose, indicating that at least this much of the 

administered dose was absorbed. Studies in rats and mice showed that gastrointestinal absorption was 

greater than 97% in both species when the animals were administered benzene by gavage (in corn oil) at 

doses of 0.5–150 mg/kg/day (Sabourin et al. 1987). In many animal studies, benzene is administered 

orally in oil to insure predictable solubility and dose concentration control. This is unlike the predicted 

human oral exposure, which is likely to be in drinking water. There are a number of studies in which 

benzene has been administered to animals in the drinking water, which more closely resembles predicted 

human oral exposure (Lindstrom et al. 1994). Although no information was located regarding the extent 

of oral absorption of benzene in aqueous solutions, it is reasonable to assume that oral absorption from 

water solutions would be nearly 100%. 

The bioavailability of pure as opposed to soil-adsorbed benzene was conducted in adult male rats (Turkall 

et al. 1988). Animals were gavaged with an aqueous suspension of benzene alone, or adsorbed to clay or 

sandy soil. Plasma concentration, half-life, tissue distribution, respiratory excretion, and urinary 

excretion were monitored. Peak plasma concentration of radioactivity was increased in the presence of 

either soil as opposed to benzene alone, while sandy soil also decreased the time to peak plasma 

concentration as opposed to benzene alone. Soil increased the area under the plasma radioactivity-time 

curve as opposed to benzene alone, a difference that was significant with clay soil. The half-life in 

plasma was not affected by soil. 

3.4.1.3 Dermal Exposure 

Studies conducted in vivo in humans and in vitro using human skin indicates that benzene can be 

absorbed dermally. The movement of a substance through the skin to the blood occurs by passive 

diffusion and has been described mathematically by Fick's law. However, this is an oversimplification of 
BENZENE 160 

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the process of skin absorption; various factors (e.g., interaction of benzene with molecules within the 

skin) affect the transport of the solvent through the skin (Lodén 1986). 

In vivo experiments on four volunteers, to whom 0.0026 mg/cm2

 of 14C-benzene was applied to forearm 

skin, indicated that approximately 0.05% of the applied dose was absorbed (Franz 1984). Absorption was 

rapid, with more than 80% of the total excretion of the absorbed dose occurring in the first 8 hours after 

application. Calculations were based on urinary excretion data and no correction was made for the 

amount of benzene that evaporated from the applied site before absorption occurred. In addition, the 

percentage of absorbed dose excreted in urine that was used in the calculation was based only on data 

from rhesus monkeys and may not be accurate for humans. In another study, 35–43 cm2

 of the forearm 

was exposed to approximately 0.06 g/cm2

 of liquid benzene for 1.25–2 hours (Hanke et al. 1961). The 

absorption was estimated from the amount of phenol eliminated in the urine. The absorption rate of liquid 

benzene by the skin (under the conditions of complete saturation) was calculated to be low, approximately 

0.4 mg/cm2

/hour. The absorption due to vapors in the same experiment was negligible. The results 

indicate that dermal absorption of liquid benzene is of concern, while dermal absorption from vapor 

exposure may not be of concern because of the low concentration of benzene in vapor form at the point of 

contact with the skin. No signs of acute intoxication due to liquid benzene dermally absorbed were noted. 

These results confirm that benzene can be absorbed through skin. However, non-benzene-derived phenol 

in the urine was not accounted for. 

Studies of occupational exposure to benzene suggest that absorption occurs both by inhalation and 

dermally in many workplace settings. In a study conducted in 1992 in Finland, car mechanics’ exposure 

to benzene was evaluated (Laitinen et al. 1994). Different work phases were measured at five Finnish 

garages. Blood samples from car mechanics (eight nonsmokers) were taken 3–9 hours after exposure to 

benzene. The results were approximated to the time point of 16 hours after exposure. Fourteen air 

samples were taken from the breathing zone and five stationary samples were collected from the middle 

of the garage for background concentration levels. The mechanics worked without protective gloves, and 

the risk of contamination and penetration through the skin was significant. The approximated benzene 

concentrations in blood corresponding to the time point of 16 hours after the exposure showed much 

higher levels of exposure than could be expected according to the corresponding air measurements 

(8 hour TWA). The comparison of expected benzene concentrations in blood, if no dermal exposure was 

present, to the levels at the time point of 16 hours after the exposure showed that the dermal route must be 

the source of about 68% of exposure (range 1.1–88.2%). Two of eight workers had minimal exposure 

through the skin (0–1.1%). The other six workers showed high dermal exposure (79.4%). 
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Exposure to benzene-contaminated water can also provide an opportunity for both inhalation and dermal 

absorption. In a series of experiments conducted in a single-family residence from June 11 to 13, 1991, 

exposure to benzene through contaminated residential water was monitored (Lindstrom et al. 1994). The 

residential water was contaminated with benzene and other hydrocarbons in 1986. Exposure was 

monitored for a person taking a 20-minute shower and for people in other parts of the house during and 

after the shower. An average dermal dose of 168 μg was estimated for a 20-minute shower using this 

water. The total benzene dose resulting from the shower was estimated to be approximately 281 μg (40% 

via inhalation and 60% via dermal), suggesting a higher potential exposure to benzene via dermal contact 

from the water than through vaporization and inhalation (see Section 3.4.1.1 for a more detailed 

discussion). This exposure was 2–3.5 times higher than the mean 6-hour inhalation dose received by the 

sampling team members in other parts of the house. The estimated inhalation and dermal doses reported 

by Lindstrom et al. (1994) have not been validated by others and are therefore of questionable value for 

quantitative analysis. 

In vitro experiments using human skin support the fact that benzene can be absorbed dermally. An 

experiment on the permeability of excised human skin with regard to benzene (specific activity 

99.8 mCi/mmol; total volume of applied benzene not reported) resulted in the absorption of 0.17 mg/cm2 

after 0.5 hours and 1.92 mg/cm2

 after 13.5 hours (Lodén 1986). Following application of 5, 120, 270, and 

520 μL/cm2

 of benzene to human skin, total absorption was found to be 0.01, 0.24, 0.56, and 0.9 μL/cm2, 

respectively. Thus, the total amount absorbed appears to increase linearly with dose. The study author 

indicated that evaporation of benzene did not exceed 5%. When exposure time (i.e., the time to complete 

evaporation) at each dose was measured and plotted as the ordinate of absorption, total absorption was 

found to increase linearly with exposure time. The percentage of the applied dose absorbed at each 

concentration was constant at about 0.2% (Franz 1984). 

Using results from an in vitro study, it was estimated that an adult working in ambient air containing 

10 ppm benzene would absorb 7.5 μL/hour from inhalation and 1.5 μL/hour from whole-body (2 m2

) 

dermal exposure (Blank and McAuliffe 1985). It was also estimated that 100 cm2

 of smooth and bare 

skin in contact with gasoline containing 5% benzene would absorb 7.0 μL/hour. Diffusion through the 

stratum corneum was considered the most likely rate-limiting step for dermal absorption because of 

benzene's low water solubility (Blank and McAuliffe 1985). 
BENZENE 162 

3. HEALTH EFFECTS 

Based on an observational study of workers in a tire factory, it was estimated that a worker exposed to 

benzene as a result of direct skin contact with petroleum naphtha containing 0.5% benzene could absorb 

4–8 mg of benzene per day through intact skin (Susten et al. 1985). This amount absorbed was compared 

with an estimated 14 mg of benzene absorbed as a result of inhalation of 1 ppm for an 8-hour day. The 

estimate for dermal absorption is theoretical since in many facilities the concentration of benzene in 

rubber solvents such as petroleum naphtha is less than 0.5% and may be as low as 0.09%. 

Benzene is also absorbed dermally by animals. In Rhesus monkeys, minipigs, and hairless mice, dermal 

absorption was <1% following a single direct (unoccluded) application of liquid benzene (Franz 1984; 

Maibach and Anjo 1981; Susten et al. 1985). As with humans, absorption appeared to be rapid, with the 

highest urinary excretion of the absorbed dose observed in the first 8 hours following exposure (Franz 

1984). Multiple applications, as well as application to stripped skin, resulted in greater skin penetration 

(Maibach and Anjo 1981). The percentage of absorption of the applied dose of benzene in each of these 

animals was approximately 2–3-fold higher than that of humans. 

Data indicate that soil adsorption decreases the dermal bioavailability of benzene. A study in which male 

rats were treated dermally with 0.004 mg/cm2 14C-benzene, with or without 1 g of clay or sandy soil, 

reported benzene absorption half-lives of 3.1, 3.6, and 4.4 hours for pure benzene, sandy soil, and clay 

soil, respectively (Skowronski et al. 1988). 

Benzene in air was rapidly absorbed through the skin of hairless mice that were attached to respirators to 

avoid pulmonary uptake of the benzene vapors (Tsuruta 1989). The rate of absorption of benzene through 

the skin increased linearly with dose. The skin absorption rate for 200 ppm was 4.11 nmol/cm2

/hour 

(0.31 μg/cm2

/hour); at 1,000 ppm, the rate was 24.2 nmol/cm2/hour (1.89 μg/cm2/hour), and at 3,000 ppm, 

the rate was 75.5 nmol/cm2

/hour (5.90 μg/cm2/hour). The skin absorption coefficient was 0.619 cm/hour. 

McDougal et al. (1990) estimated permeability constants of 0.15 and 0.08 cm/hour for rat and human 

skin, respectively, based on the appearance of benzene in the blood of rats dermally exposed to benzene 

vapors at a concentration of 40,000 ppm for 4 hours. A physiologically based pharmacodynamic (PBPK) 

model was used to estimate the permeability of the vapor in rat and human skin. These results indicate 

that dermal absorption of benzene may be greater in rats than humans. Therefore, results in rats may 

provide a conservative estimate of dermal absorption of benzene in humans. 
BENZENE 163 

3. HEALTH EFFECTS 

In an in vitro experiment using Fischer 344 rat skin, the partition coefficient for skin:air was determined 

for benzene at 203 ppm (Mattie et al. 1994). The partition coefficient of a chemical in skin is an indicator 

of the capacity of the skin to retain the chemical, and may reflect the rate at which a chemical is absorbed 

through the skin and enters the circulation. Results indicated a partition coefficient of 35, with an 

equilibration time of 4 hours. The skin:air partition coefficient is necessary for developing the dermal 

compartment of a PBPK model. 

Based on data for skin absorption of benzene vapors in mice and occupational exposure data, Tsuruta 

(1989) estimated the ratio of skin absorption rate to pulmonary uptake for humans exposed to benzene to 

be 0.037. Dermal absorption could account for a relatively higher percentage of total benzene uptake in 

occupational settings where personnel, using respirators but not protective clothing, are exposed to high 

concentrations of benzene vapor. 

3.4.2 Distribution 

3.4.2.1 Inhalation Exposure 

Information on the distribution of benzene in humans comes primarily from case studies. The data 

suggest that benzene is distributed throughout the body following absorption into blood. Since benzene is 

lipophilic, a high distribution to fatty tissue might be expected. Following inhalation exposure to 

benzene, the chemical has been detected in the biological fluids and tissues of the subjects (Pekari et al. 

1992; Tauber 1970; Winek and Collom 1971; Winek et al. 1967). Fluid and tissue levels of benzene have 

been reported in cases of both accidental and intentional lethal exposures. Levels of 0.38 mg% in blood 

(mg% = mg per 100 mL of blood or mg per 100 g of tissue), 1.38 mg% in the brain, and 0.26 mg% in the 

liver were reported in a worker who died from exposure to very high air concentrations of the chemical 

(Tauber 1970). An autopsy (time after death not indicated) performed on a youth who died while sniffing 

reagent-grade benzene revealed benzene concentrations of 2.0 mg% in blood, 3.9 mg% in brain, 1.6 mg% 

in liver, 1.9 mg% in kidney, 1 mg% in stomach, 1.1 mg% in bile, 2.23 mg% in abdominal fat, and 

0.06 mg% in urine (Winek and Collom 1971). Benzene crosses the human placenta and is present in the 

cord blood in amounts equal to or greater than those in maternal blood (Dowty et al. 1976). Benzene is 

expected to readily bind to plasma proteins (Travis and Bowers 1989). Furthermore, benzene metabolites 

have been found to form covalent adducts with proteins from blood in humans (Bechtold et al. 1992b; 

Rappaport et al. 2002a, 2002b; Yeowell-O’Connell et al. 1998) and mice (McDonald et al. 1994). The 

relatively widespread distribution of benzene and its metabolites to other tissues and organs indicates that 
BENZENE 164 

3. HEALTH EFFECTS 

protein adduct formation in the blood does not adversely affect distribution, although no confirming 

studies were located. 

Results from animal studies indicate that absorbed benzene is distributed among several compartments. 

The parent compound is preferentially stored in the fat, although the relative uptake in tissues also 

appears to be dependent on the perfusion rate of tissues by blood. 

Following a 10-minute inhalation exposure of pregnant mice to 2,000 ppm benzene, parent compound and 

its metabolites were found to be present in lipid-rich tissues, such as brain and fat, and in well-perfused 

tissues, such as liver and kidney. Benzene was also found in the placenta and fetuses immediately 

following inhalation of benzene (Ghantous and Danielsson 1986). During inhalation exposure of rats to 

500 ppm, benzene levels reached a steady-state concentration within 4 hours in blood (11.5 μg/mL), 

6 hours in fat (164.4 μg/g), and less than 2 hours in bone marrow (37.0 μg/g) (Rickert et al. 1979). 

Benzene was also distributed to the kidney, lung, liver, brain, and spleen. The benzene metabolites 

phenol, catechol, and hydroquinone were detected in blood and bone marrow following 6 hours of 

exposure to benzene, with levels in bone marrow exceeding the respective levels in blood. The levels of 

phenol in blood and bone marrow decreased much more rapidly after exposure ceased than did those of 

catechol or hydroquinone, suggesting the possibility of accumulation of the latter two compounds. 

Benzene was rapidly distributed throughout the bodies of dogs exposed via inhalation to concentrations of 

800 ppm for up to 8 hours/day for 8–22 days (Schrenk et al. 1941). Fat, bone marrow, and urine 

contained about 20 times the concentration of benzene in blood; benzene levels in muscles and organs 

were 1–3 times that in blood; and erythrocytes contained about twice the amount of benzene found in 

plasma. During inhalation exposure of rats to 1,000 ppm (2 hours/day, for 12 weeks), benzene was stored 

longer (and eliminated more slowly) in female and male rats with higher body fat content than in leaner 

animals (Sato et al. 1975). 

Benzene was detected in the liver, lung, and blood of rats and mice examined immediately following a 

6-hour exposure to benzene vapors at a concentration of 50 ppm (Sabourin et al. 1988). Sabourin and 

coworkers (Sabourin et al. 1987, 1988) also examined effects exposure concentration, exposure rate, and 

route of administration on the comparative metabolism of benzene in rats and mice. Results of these 

studies are summarized in Section 3.4.3 (Metabolism). 
BENZENE 165 

3. HEALTH EFFECTS 

3.4.2.2 Oral Exposure 

No studies were located regarding distribution in humans after oral exposure to benzene. 

In Sprague-Dawley rats administered a single dose of 0.15, 1.5, 15, 150, or 500 mg/kg of 14C-benzene by 

gavage, benzene was rapidly absorbed and distributed to various organs and tissues within 1 hour of 

administration (Low et al. 1989). One hour after rats were dosed with 0.15 or 1.5 mg/kg of benzene, 

tissue distribution of benzene was highest in liver and kidney, intermediate in blood, and lowest in the 

Zymbal gland, nasal cavity tissue, and mammary gland. At higher doses, beginning with 15 mg/kg, 

benzene disproportionately increased in the mammary glands and bone marrow. Bone marrow and 

adipose tissue proved to be depots of benzene at the higher dose levels. The highest tissue concentrations 

of benzene’s metabolite hydroquinone 1 hour after administration of 15 mg/kg of benzene were in the 

liver, kidney, and blood, while the highest concentrations of the metabolite phenol were in the oral cavity, 

nasal cavity, and kidney. The major tissue sites of benzene’s conjugated metabolites were blood, bone 

marrow, oral cavity, kidney, and liver for phenyl sulfate and hydroquinone glucuronide; muconic acid 

was also found in these sites. Additionally, the Zymbal gland and nasal cavity were depots for phenyl 

glucuronide, another conjugated metabolite of benzene. The Zymbal gland is a specialized sebaceous 

gland and a site for benzene-induced tumors. Therefore, it is reasonable to expect that lipophilic 

chemicals like benzene would partition readily into this gland. However, benzene did not accumulate in 

the Zymbal gland; within 24 hours after administration, radiolabel derived from 14C-benzene in the 

Zymbal gland constituted less than 0.0001% of the administered dose. 

The bioavailability of pure as opposed to soil-adsorbed benzene was conducted in adult male rats (Turkall 

et al. 1988). Animals were gavaged with an aqueous suspension of 14C-benzene alone, or adsorbed to 

clay or sandy soil. Two hours after exposure, stomach tissue contained the highest amount of 

radioactivity, followed by fat in all treatment groups. No differences in tissue distribution patterns were 

detected for the three treatments. 

3.4.2.3 Dermal Exposure 

No studies were located regarding distribution in humans after dermal exposure to benzene. 

A study of male rats treated dermally with 0.004 mg/cm2

 of 14C-benzene, with and without 1 g of clay or 

sandy soil, revealed soil-related differences in tissue distribution following treatment. The 14C activity 

(expressed as a percentage of initial dose per g of tissue) 48 hours after treatment with soil-adsorbed 
BENZENE 166 

3. HEALTH EFFECTS 

benzene was greatest in the treated skin (0.059–0.119%), followed by the kidney (0.024%) and liver 

(0.013–0.015%), in both soil groups. In the pure benzene group, the kidney contained the largest amount 

of radioactivity (0.026%), followed by the liver (0.013%) and treated skin (0.11%) (Skowronski et al. 

1988). In all three groups, <0.01% of the radioactivity was found in the following tissues: duodenum, 

fat, bone marrow, esophagus, pancreas, lung, heart, spleen, blood, brain, thymus, thyroid, adrenal, testes, 

untreated skin, and remaining carcass. 

3.4.3 Metabolism 

Although the metabolism of benzene has been studied extensively, the steps leading to benzene toxicity 

are not yet fully understood. It is generally understood that both cancer and noncancer effects are caused 

by one or more reactive metabolites of benzene. Available data indicate that metabolites produced in the 

liver are carried to the bone marrow where benzene toxicity is expressed. Benzene metabolism may 

occur, at least in part, in the bone marrow. Benzene metabolism has been demonstrated in isolated 

perfused rabbit lung preparations (Powley and Carlson 2002). As discussed in detail in Section 3.5, 

available evidence suggests that multiple benzene metabolites may collectively be responsible for the 

expression of benzene toxicity. 

Data regarding metabolism of benzene in humans are derived primarily from studies using inhalation 

exposures. Benzene is excreted both unchanged via the lungs and as metabolites (but also as parent 

compound in small amounts) in the urine. The rate and percentage of excretion via the lungs are 

dependent on exposure dose and route. Qualitatively, the metabolism and elimination of benzene appear 

to be similar in humans and laboratory animals, but no directly comparable studies are available 

(Henderson et al. 1989; Sabourin et al. 1988). 

The metabolic scheme shown in Figure 3-3 is based on results of numerous mechanistic studies of 

benzene metabolism (see Henderson et al. 1989; Huff et al. 1989; and Ross 1996, 2000 for 

comprehensive reviews of benzene metabolism). The first step is the cytochrome P-450 2E1 (CYP2E1) 

catalyzed oxidation of benzene to form benzene oxide (Lindstrom et al. 1997), which is in equilibrium 

with its oxepin (Vogel and Günther 1967). Several pathways are involved in the metabolism of benzene 

oxide. The predominant pathway involves nonenzymatic rearrangement to form phenol (Jerina et al. 

1968), the major initial product of benzene metabolism (Parke and Williams 1953a). Phenol is oxidized 

in the presence of CYP2E1 to catechol or hydroquinone, which are oxidized via myeloperoxidase (MPO) 

to the reactive metabolites 1,2- and 1,4-benzoquinone, respectively (Nebert et al. 2002). The reverse 
BENZENE 167

3. HEALTH EFFECTS 

Figure 3-3. Metabolic Pathways for Benzene 

Benzene 

dihydrodiol Catechol 

Benzene OH OH DHDD 

O 

EH 

CYP2E1 

OH OH

H3C NH 

OH CYP2E1 

O 

S ADH OH

FE/OH· ALDH 

O O O

O O 

GSH 

O OH trans,trans-Muconaldehyde 

S-Phenylmercapturic Benzene Benzene trans,trans-Muconic acid 

acid oxide oxepin 

Rearrangement 

Hydroquinone
 Phenol Catechol OH OH OH CYP2E1 CYP2E1 

HO 

CYP2E1 

OH 

CYP2E1 

NQO1 MPO NQO1 MPO 

O 1,2,4-Benzenetriol 

OH 

O 

HO 

O 

OH 1,4-Benzoquinone O 

1,2-Benzoquinone 

ADH = alcohol dehydrogenase; ALDH = aldehyde dehydrogenase; CYP2E1 = cytochrome P-450 2E1; 

DHDD = dihydrodiol dehydrogenase; EH = epoxide hydrolase; GSH = glutathione; MPO = myeloperoxidase; 

NQ01 = NAD(P)H:quinone oxidoreductase 

Source: adapted from Nebert et al. 2002; Ross 2000 
BENZENE 168 

3. HEALTH EFFECTS 

reaction (reduction of 1,2- and 1,4-benzoquinone to catechol and hydroquinone, respectively) is catalyzed 

by NAD(P)H:quinone oxidoreductase (NQ01) (Nebert et al. 2002). Both catechol and hydroquinone may 

be converted to the reactive metabolite 1,2,4-benzenetriol via CYP2E1 catalysis. Alternatively, benzene 

oxide may undergo epoxide hydrolase-catalyzed conversion to benzene dihydrodiol and subsequent 

dihydrodiol dehydrogenase-catalyzed conversion to catechol (Nebert et al. 2002; Snyder et al. 1993a, 

1993b). Each of the phenolic metabolites of benzene (phenol, catechol, hydroquinone, and 

1,2,4-benzenetriol) can undergo sulfonic or glucuronic conjugation (Nebert et al. 2002; Schrenk and Bock 

1990); the conjugates of phenol and hydroquinone are major urinary metabolites of benzene (Sabourin et 

al. 1989a; Wells and Nerland 1991). Other pathways of benzene oxide metabolism include: (1) reaction 

with glutathione (GSH) to form S-phenylmercapturic acid (Nebert et al. 2002; Sabourin et al. 1988; 

Schafer et al. 1993; Schlosser et al. 1993; Schrenk et al. 1992; van Sittert et al. 1993), and (2) ironcatalyzed ring-opening conversion to trans,trans-muconic acid, presumably via the reactive trans,transmuconaldehyde intermediate (Bleasdale et al. 1996; Nebert et al. 2002; Ross 2000; Witz et al. 1990b, 

1990c, 1996). 

Results of several studies provide strong evidence for the involvement of CYP2E1 in the oxidation of 

benzene. For example, no signs of benzene-induced toxicity were observed in transgenic CYP2E1 

knockout mice (that do not express hepatic CYP2E1 activity) following exposure to benzene vapors 

(200 ppm, 6 hours/day for 5 days) that caused severe genotoxicity and cytotoxicity in wild-type mice 

(Valentine et al. 1996a, 1996b). Pretreatment of mice with CYP inhibitors (toluene, propylene glycol, 

β-diethyl amino ethyl diphenyl propyl acetate hydrogen chloride [SKF-525A]) has been demonstrated to 

reduce both benzene metabolite formation (Andrews et al. 1977; Gill et al. 1979; Ikeda et al. 1972; Tuo et 

al. 1996) and resulting genotoxicity (DNA damage as assessed by the alkaline comet assay) in mice (Tuo 

et al. 1996). Pretreatment with CYP inducers (3-methylcholanthrene and β-naphthoflavone) increased 

both benzene metabolism and benzene clastogenicity (Gad-El-Karim et al. 1986). Immunoinhibition 

studies in rat and rabbit hepatic microsomes provide additional support to the major role of CYP2E in 

benzene metabolism (Johansson and Ingelman-Sundberg 1988; Koop and Laethem 1992). 

Occupationally exposed workers with a phenotype corresponding to rapid CYP2E1 metabolism were 

more susceptible to benzene hematotoxicity than workers not expressing this phenotype (Rothman et al. 

1997). In vitro studies using human liver microsomes demonstrate a positive correlation between 

benzene metabolism and CYP2E1 activity (Nedelcheva et al. 1999; Seaton et al. 1994). Although 

CYP2E1 appears to be the major catalyzing agent in initial benzene metabolism, other CYPs, such as 

CYP2B1 and CYP2F2, may also be involved (Gut et al. 1996a, 1996b; Powley and Carlson 2000, 2001; 

Sheets and Carlson 2004; Sheets et al. 2004; Snyder et al. 1993a, 1993b). 
BENZENE 169 

3. HEALTH EFFECTS 

CYPs involved in benzene metabolism are found in all tissues. However, the predominant repository is 

the liver, which is considered to be the primary site of benzene metabolism. By demonstrating that partial 

hepatectomy diminished both the rate of metabolism of benzene and its toxicity in rats exposed to 

benzene via subcutaneous injection, Sammett et al. (1979) provided suggestive evidence that one or more 

benzene metabolites formed in the liver are necessary for toxicity. In vitro studies have demonstrated that 

pulmonary microsomes of humans and laboratory animals are capable of metabolizing benzene, which 

appears to be catalyzed by both CYP2E1 and CYP2F2 (Powley and Carlson 1999, 2000; Sheets et al. 

2004). There is some indication that CYP2E1-catalyzed benzene metabolism may also occur in bone 

marrow, a major target tissue of benzene toxicity. Andrews et al. (1979) demonstrated that rabbit bone 

marrow is capable of metabolizing benzene. Schnier et al. (1989) subsequently found that rabbit bone 

marrow contains CYP2E1. Irons et al. (1980) demonstrated that benzene metabolism by rat bone marrow 

(in situ) was complete and independent of metabolism by the liver, with concentrations of phenol greater 

than catechol and hydroquinone. Although the total metabolism by bone marrow was limited (total 

metabolites present were 25% of those in blood), the concentration of metabolites in the bone marrow 

exceeded that in the blood. Similar studies have been conducted in mice (Ganousis et al. 1992). 

Fibroblasts had elevated levels of glutathione-S-transferase activity relative to macrophages, whereas 

macrophages had higher levels of UDP-glucuronyltransferase and peroxidase activity. These data suggest 

that cell-specific metabolism of benzene in the marrow may contribute to the toxicity of benzene in this 

tissue compartment. In addition, comparison of the detoxifying activities of rat and mouse bone marrow 

stromal cells indicates that rats have higher levels of glutathione and quinone reductase, which are known 

to play critical roles in modulating hydroquinone-induced toxicity; this suggests a metabolic basis for the 

observed increased susceptibility of mice to benzene-induced hematotoxicity (Zhu et al. 1995). Bernauer 

et al. (1999, 2000) recently noted the presence of CYP2E1 in bone marrow samples of mice (several 

strains), rats, rabbits, and humans. However, although Irons et al. (1980) demonstrated that the isolated 

perfused rat femur was capable of metabolizing a very small amount of benzene (approximately 0.0002% 

of 14C-benzene was recovered as metabolites), neither benzene oxide nor phenol were detected in a test of 

benzene metabolism using microsomal preparations of bone marrow from rats (Lindstrom et al. 1999), 

indicating that bone marrow is not a likely source of initial metabolic oxidation for benzene. No studies 

were located the potential for human bone marrow tissue to metabolize benzene. 

Mouse liver microsomes and cytosol have been shown to catalyze ring opening in the presence of 

nicotinamide adenine dinucleotide phosphate (NADPH) in vitro, producing trans,trans-muconaldehyde, a 

six-carbon diene dialdehyde also referred to as muconic dialdehyde (Goon et al. 1993; Latriano et al. 
BENZENE 170 

3. HEALTH EFFECTS 

1986), a known hematotoxin (Witz et al. 1985) and toxic metabolite of benzene (Henderson et al. 1989). 

Metabolism of benzene and trans,trans-muconaldehyde in the isolated perfused rat liver indicated that 

benzene was metabolized to muconic acid, a ring-opened metabolite of benzene (Grotz et al. 1994). 

Trans,trans-muconaldehyde was metabolized to muconic acid and three other metabolites. These studies 

indicate that ring-opening of benzene occurs in the liver. Other recent literature identifies the following 

metabolites after incubation of benzene with mouse liver microsomes: phenol, hydroquinone, 

trans,trans-muconaldehyde, 6-oxo-trans,trans-2,4-hexadienoic acid, 6-hydroxy-trans,trans-2,4-hexadienal, and 6-hydroxy-trans,trans-2,4-hexadienoic acid (Zhang et al. 1995a). β-Hydroxymuconaldehyde, 

a new metabolite, was also identified. Additional work by Zhang et al. (1995b) suggests that cis,cismuconaldehyde is formed first, followed by cis,trans-muconaldehyde, and finally converted to 

trans,trans-muconaldehyde. Muconic dialdehyde has been shown to be metabolized in vivo in mice to 

muconic acid (Witz et al. 1990c). These data suggest that muconic dialdehyde is the precursor of 

muconic acid in animals exposed to benzene. Small amounts of muconic acid were found in the urine of 

rabbits and mice that received oral doses of 14C-benzene (Gad-El-Karim et al. 1985; Parke and Williams 

1953a). The percentage of this metabolite formed varied with the administered benzene dose and was 

quite high at low doses (17.6% of 0.5 mg/kg benzene administered to C57BL/6 mice) (Witz et al. 1990c). 

Other studies in animals support these results (Brondeau et al. 1992; Ducos et al. 1990; McMahon and 

Birnbaum 1991; Sabourin et al. 1989a; Schad et al. 1992). This pathway also appears to be active in 

humans (Bechtold and Henderson 1993; Ducos et al. 1990, 1992; Lee et al. 1993; Melikian et al. 1993, 

1994). For instance, urine samples from male and female smokers and nonsmokers were obtained from 

subjects who applied for life insurance (Melikian et al. 1994). Samples from pregnant women were 

obtained during 7–35 weeks of pregnancy. Questionnaires were filled out on smoking history and 

occupation. The levels of muconic acid and cotinine (a biomarker for cigarette smoking) in the urine for 

the groups of pregnant and nonpregnant smokers and nonsmokers were compared with previously 

reported data in male smokers. Results showed the mean levels of muconic acid in the groups of male, 

female-nonpregnant, and female-pregnant smokers were 3.6-, 4.8-, and 4.5-fold higher than the mean 

concentration of this acid in the nonsmoking groups. The differences in the mean muconic acid 

concentrations between smoking and nonsmoking groups were significant in male, and nonpregnant and 

pregnant female smokers. Mean concentrations of muconic acid levels in nonpregnant female smokers 

are similar to that of male smokers. Mean concentrations of muconic acid in groups of 42 male smokers 

and 53 female smokers were 0.22±0.03 and 0.24±0.02 mg/g creatinine, or 0.13±0.06 and 

0.13±0.07 mg/mg cotinine, respectively. Mean concentrations of muconic acid in groups of 63 pregnant 

and 53 nonpregnant female smokers were 0.27±0.04 and 0.24±0.02 mg/g creatinine, or 0.24±0.06 and 

0.13±0.07 mg/mg cotinine, respectively. Because of its relative importance in benzene toxicity, 
BENZENE 171 

3. HEALTH EFFECTS 

additional modeling studies, including molecular orbital studies, have been conducted to further describe 

how trans,trans-muconaldehyde is transformed to muconic acid (Bock et al. 1994). 

Kenyon et al. (1995) compared their urinary profile of metabolites in B6C3F1 mice after oral dosing with 

phenol with the results of Sabourin et al. (1989a) who administered a comparable oral dose of benzene to 

B6C3F1 mice. The analysis of Kenyon et al. (1995) indicated that phenol administration resulted in lower 

urinary levels of hydroquinone glucuronide, and higher levels of phenol sulfate and phenol glucuronide 

compared to benzene administration. Kenyon et al. (1995) hypothesized that the differences in the 

urinary metabolite profiles between phenol and benzene after oral dosing were due to zonal differences in 

the distribution of metabolizing enzymes within the liver. Conjugating enzymes are more concentrated in 

the periportal area of the liver, the first region to absorb the compound, whereas oxidizing enzymes are 

more concentrated in the pericentral region of the liver. Based on this hypothesis, during an initial pass 

through the liver after oral administration, phenol would have a greater opportunity to be conjugated as it 

was absorbed from the gastrointestinal tract into the periportal region of the liver, thus resulting in less 

free phenol being delivered into the pericentral region of the liver to be oxidized. With less free phenol 

available for oxidation, less hydroquinone would be produced, relative to conjugated phenol metabolites. 

In contrast, benzene must be oxidized before it can be conjugated. Therefore, metabolism of benzene 

would be minimal in the periportal region of the liver, with most of the benzene reaching the pericentral 

region to be oxidized to hydroquinone. Based on this scheme, the authors suggest that benzene 

administration would result in more free phenol being delivered to oxidizing enzymes in the pericentral 

region of the liver than administration of phenol itself (Kenyon et al. 1995). 

Benzene has been found to stimulate its own metabolism, thereby increasing the rate of toxic metabolite 

formation. Pretreatment of mice, rats, and rabbits subcutaneously with benzene increased benzene 

metabolism in vitro without increasing CYP2E1 concentrations (Arinc et al. 1991; Gonasun et al. 1973; 

Saito et al. 1973). In contrast, there was no significant effect on the metabolism of benzene when 

Fischer 344 rats and B6C3F1 mice, pretreated with repeated inhalation exposure to 600 ppm of benzene, 

were again exposed to 600 ppm benzene (Sabourin et al. 1990). The rate of benzene metabolism can be 

altered by pretreatment with various compounds. Benzene is a preferential substrate of CYP2E1, which 

also metabolizes alcohol and aniline. CYP2E1 can be induced by these substrates and is associated with 

the generation of hydroxyl radicals, probably via futile cycling of the cytochrome (Chepiga et al. 1991; 

Parke 1989; Snyder et al. 1993a, 1993b). It is possible that hydroxy radical formation by CYP2E1 may 

play a role in the benzene ring-opening pathway, leading to the formation of trans,trans-muconaldehyde. 

Phenol, hydroquinone, benzoquinone, and catechol have also been shown to induce CYPs in human 
BENZENE 172 

3. HEALTH EFFECTS 

hematopoietic stem cells (Henschler and Glatt 1995). Therefore, exposure to chemicals that stimulate the 

activity of this enzyme system prior to exposure to benzene could increase the rate of benzene 

metabolism. 

Both NADPH-linked and ascorbate-induced lipid peroxidation activities induced in vitro were lowered 

5.5 and 26%, respectively, in rats following oral administration of 1,400 mg/kg/day of benzene for 3 days, 

followed by intraperitoneal injection of phenobarbital. These results suggest that benzene alters hepatic 

drug metabolism and lipid peroxidation. The decrease in lipid peroxidation could be due to the 

antioxidant property of the metabolites (Pawar and Mungikar 1975). 

The ultimate disposition and metabolic fate of benzene depends on animal species, dose, and route of 

exposure. The dose of benzene affects both the total metabolism and the concentrations of individual 

metabolites formed. In mice, the percentage of hydroquinone glucuronide decreased as the dose 

increased. In both rats and mice, the percentage of muconic acid decreased as the dose increased. The 

shift in metabolism may affect the dose-response relationship for toxicity, and has been observed in all 

animal species studies thus far (Sabourin et al. 1989a, 1992; Witz et al. 1990b, 1990c). The effect of 

species differences in metabolism of inhaled benzene was evidenced by the fact that mice have a higher 

minute volume per kg body weight than rats (1.5 times higher). This caused the blood concentration of 

benzene to reach equilibrium more quickly in mice than in rats, but the steady-state level in blood was not 

influenced (Sabourin et al. 1987). Species differences in benzene metabolism following oral exposure 

were elucidated in rats and mice administered benzene by gavage at doses of 0.5–150 mg/kg/day 

(Sabourin et al. 1987). At doses below 15 mg/kg, >90% of the benzene was metabolized, while at doses 

above 15 mg/kg, an increasing percentage of orally administered benzene was exhaled unmetabolized. 

Total metabolites per unit body weight were equal in rats and mice at doses up to 50 mg/kg/day. 

However, total metabolites in mice did not increase at higher doses, suggesting saturation of metabolic 

pathways (Sabourin et al. 1987). 

The integrated dose to a tissue over a 14-hour period (6-hour exposure, 8 hours following exposure) was 

calculated for benzene metabolites in rats and mice that were exposed to 50 ppm of radiolabeled (3

H) 

benzene (Sabourin et al. 1988). The major metabolic products in rats were detoxification products that 

were marked by phenyl conjugates. In contrast, mice had substantial quantities of the markers for 

toxification pathways (muconic acid, hydroquinone glucuronide, and hydroquinone sulfate) in their 

tissues. Muconic acid and hydroquinone glucuronide were also detected in mouse bone marrow. These 

results may explain why mice are more susceptible to benzene-induced toxicity than rats. 
BENZENE 173 

3. HEALTH EFFECTS 

In a study by Orzechowski et al. (1995), hepatocytes from adult male Wistar rats and NMRI mice were 

incubated for 1 hour with 0.5 mM 14C-benzene, and the supernatant analyzed for metabolites. Formation 

of sulfate conjugates of benzene, hydroquinone, and 1,2,4-benzenetriol was also studied in a separate 

experiment. Mouse hepatocytes produced two metabolites (1,2,4-trihydroxybenzene sulfate and hydroquinone sulfate) that were not found in rat hepatocyte incubations. These sulfate metabolites were found 

in incubations including benzene, or the metabolites themselves, hydroquinone and 1,2,4-benzenetriol. 

Mouse hepatocytes were almost three times more effective in metabolizing benzene, compared to rat 

hepatocytes. This difference was accounted for in the formation of hydroquinone, hydroquinone sulfate, 

and 1,2,4-trihydroxybenzene sulfate. These in vitro experiments indicate there are both quantitative and 

qualitative differences in rodent metabolism of benzene. 

Data produced in vitro by mouse and rat liver microsomes also indicate species differences in benzene 

metabolism (Schlosser et al. 1993). Quantitation of metabolites from the microsomal metabolism of 

benzene indicated that after 45 minutes, mouse liver microsomes from male B6C3F1 mice had converted 

20% of the benzene to phenol, 31% to hydroquinone, and 2% to catechol. In contrast, rat liver 

microsomes from male Fischer 344 rats converted 23% to phenol, 8% to hydroquinone, and 0.5% to 

catechol. Mouse liver microsomes continued to produce hydroquinone and catechol for 90 minutes, 

whereas rat liver microsomes had ceased production of these metabolites by 90 minutes. Muconic acid 

production by mouse liver microsomes was <0.04 and <0.2% from phenol and benzene, respectively, 

after 90 minutes. 

There are quantitative differences in the benzene metabolites produced by different species (Sabourin et 

al. 1988). Fischer 344 rats exposed to 50 ppm benzene had undetectable amounts of phenol, catechol, and 

hydroquinone in the liver, lungs, and blood. The major water-soluble metabolites were muconic acid, 

phenyl sulfate, prephenyl mercapturic acid, and an unknown. The unknown was present in amounts equal 

to the amounts of phenyl sulfate in the liver; phenyl sulfate and the unknown were the major metabolites 

in the liver. B6C3F1 mice exposed to 50 ppm benzene had detectable levels of phenol and hydroquinone 

in the liver, lungs, and blood; catechol was detectable only in the liver and not in the lungs or blood. As 

in the rat, the unknown was present in amounts equal to the amounts of phenyl sulfate in the liver. Mice 

had more muconic acid in the liver, which indicates a greater risk for them from trans,trans-muconaldehyde (Sabourin et al. 1988). 
BENZENE 174 

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The effect of dose rate on benzene metabolism was studied in Fischer 344 rats and B6C3F1 mice that had 

either long inhalation exposures to low concentrations or short exposures to high concentrations of 

benzene (Sabourin et al. 1989a, 1989b). Inhalation occurred at 1 of 3 exposure regimens, all having the 

same integral amount of benzene: 600 ppm benzene for 0.5 hour, 150 ppm for 2 hours, or 50 ppm for 

6 hours. Results indicated no dose-rate effect in rats. In mice, however, the fast exposure rate 

(0.5 hour times 600 ppm) produced less muconic acid in the blood, liver, and lungs. In the blood and 

lungs, less hydroquinone glucuronide and more prephenyl mercapturic acid were produced at the higher 

exposure rates. At the highest benzene exposure concentrations or fastest benzene exposure rate in mice, 

there was a reduction in the ratios of muconic acid and hydroquinone glucuronide to the metabolite 

phenylsulfate. Furthermore, with increased dose rate or increased exposure concentration, mice tended to 

shift a greater portion of their benzene metabolism toward detoxification pathways. Likewise, the 

detoxification pathways for benzene appear to be low-affinity, high-capacity pathways, whereas pathways 

leading to the putative toxic metabolites appear to be high-affinity, low-capacity systems (Henderson et 

al. 1989). Accordingly, if the exposure dose regimen, via inhalation, extends beyond the range of linear 

metabolism rates of benzene (200 ppm by inhalation) (Sabourin et al. 1989b), then the fraction of toxic 

metabolites formed relative to the amount administered will be reduced. Bois and Paxman (1992) used a 

PBPK model to assess effects of dose rate on the disposition of benzene metabolites. Simulations were 

performed for rats exposed either for 15 minutes to 32 ppm or for 8 hours to 1 ppm (equivalent 8-hour 

TWAs). The amount of metabolites (hydroquinone, catechol, and muconaldehyde) formed was 20% 

higher after the 15-minute exposure at the higher level than after the 8-hour exposure at the lower level. 

Differences between the model predictions (Bois and Paxman 1992) and the empirical data of Sabourin et 

al. (1989a, 1989b) may be related, at least in part, to the higher benzene exposure levels (50, 150, and 

600 ppm) used by Sabourin and coworkers. 

A number of investigators have suggested that covalent binding of benzene metabolites to cellular 

macromolecules is related to benzene's mechanism of toxicity, although the relationship between adduct 

formation and toxicity is not clear. Benzene metabolites have been found to form covalent adducts with 

proteins from blood in humans (Bechtold et al. 1992b). Benzene metabolites form covalent adducts with 

nucleic acids and proteins in rats and mice (Norpoth et al. 1988; Rappaport et al. 1996); covalently bind to 

proteins in mouse or rat liver, bone marrow, kidney, spleen, blood, and muscle in vivo (Bechtold and 

Henderson 1993; Bechtold et al. 1992a, 1992b; Creek et al. 1997; Longacre et al. 1981a, 1981b; Sun et 

al. 1990); bind to proteins in perfused bone marrow preparations (Irons et al. 1980) and in rat and mouse 

liver DNA in vivo (Creek et al. 1997; Lutz and Schlatter 1977); and bind to DNA in rabbit and rat bone 

marrow mitochondria in vitro (Rushmore et al. 1984). Exposure-related increases in blood levels of 
BENZENE 175 

3. HEALTH EFFECTS 

albumin adducts of benzene oxide and 1,4-benzoquinone were noted among workers occupationally 

exposed to benzene air concentrations ranging from 0.07 to 46.6 ppm (Rappaport et al. 2002a, 2002b). 

Several reactive metabolites of benzene have been proposed as agents of benzene hematotoxic and 

leukemogenic effects. These metabolites include benzene oxide, reactive products of the phenol pathway 

(catechol, hydroquinone, and 1,4-benzoquinone), and trans,trans-muconaldehyde. See Section 3.5.2 for a 

discussion of mechanisms of benzene toxicity. 

3.4.4 Elimination and Excretion 

3.4.4.1 Inhalation Exposure 

Available human data indicate that following inhalation exposure to benzene, the major route for 

elimination of unmetabolized benzene is via exhalation. Absorbed benzene is also excreted in humans via 

metabolism to phenol and muconic acid followed by urinary excretion of conjugated derivatives (sulfates 

and glucuronides). In six male and female volunteers exposed to 52–62 ppm benzene for 4 hours, 

respiratory excretion (the amount of absorbed benzene excreted via the lungs) was approximately 17%; 

no gender-related differences were observed (Nomiyama and Nomiyama 1974a, 1974b). Results from a 

study of 23 subjects who inhaled 47–110 ppm benzene for 2–3 hours showed that 16.4–41.6% of the 

retained benzene was excreted by the lungs within 5–7 hours (Srbova et al. 1950). The rate of excretion 

of benzene was the greatest during the first hour. The study also showed that only 0.07–0.2% of the 

retained benzene was excreted in the urine. Other studies suggest that benzene in the urine may be a 

useful biomarker of occupational exposure (Ghittori et al. 1993). Results of a study involving a single 

human experimental subject exposed to concentrations of benzene of 6.4 and 99 ppm for 8 hours and 

1 hour, respectively, suggested that excretion of benzene in breath has three phases and could possibly 

have four phases. The initial phase is rapid and is followed by two (or three) slower phases (Sherwood 

1988). The initial phase with a high exposure concentration (99 ppm) and a short-term exposure duration 

(1 hour) had a more rapid excretion rate (half-life=42 minutes) and a greater percentage of the total dose 

excreted (17%) than did the initial phase with a low exposure concentration (6.4 ppm) and longer 

exposure duration (8 hours) (half-life=1.2 hours, percentage of total dose excreted=9.3%). Subsequent 

phases showed an increase in the half-lives. These results also showed that urinary excretion of phenol 

conjugate was biphasic, with an initial rapid excretion phase, followed by a slower excretion phase. A 

greater proportion of the total dose was excreted in urine than in breath (Sherwood 1988). The urinary 

excretion of phenol in workers was measured following a 7-hour workshift exposure to 1–200 ppm 

benzene. A correlation of 0.881 between exposure level and urinary phenol excretion was found (Inoue et 

al. 1986). Urine samples were collected from randomly chosen subjects not exposed to known sources of 
BENZENE 176 

3. HEALTH EFFECTS 

benzene, from subjects exposed to sidestream cigarette smoke, or from supermarket workers presumed 

exposed to benzene from polyvinyl chloride (PVC) meat packing wrap (Bartczak et al. 1994). Samples 

were analyzed for identification of muconic acid. Muconic acid concentrations of 8–550 ng/mL were 

found in all urine samples. Kok and Ong (1994) report blood and urine levels of benzene as 110.9 and 

116.4 ng/L, respectively, in nonsmokers, and 328.8 and 405.4 ng/L, respectively in smokers. A 

significant correlation was found between benzene levels in blood and benzene levels in urine. Similar 

results were found for filling station attendants in Italy (Lagorio et al. 1994b). 

Popp et al. (1994) reported a mean blood benzene level in car mechanics of 3.3 μg/L. Urinary muconic 

acid and S-phenylmercapturic acid levels increased during the work shift, and were well correlated with 

the blood levels and the benzene air levels, which reached a maximum of 13 mg/m3

. 

As discussed in Section 3.4.3, the mean urinary levels of muconic in groups of male, female-nonpregnant, 

and female-pregnant smokers were 3.6-, 4.8-, and 4.5-fold higher than the mean concentration of this acid 

in the nonsmoking groups (Melikian et al. 1994). The differences in the mean muconic acid 

concentrations between smoking and nonsmoking groups were significant in male (p=0.001), and 

nonpregnant (p=0.001) and pregnant female smokers (p=0.002). Mean concentrations of muconic acid 

levels in nonpregnant female smokers are similar to that of male smokers. Mean concentrations of 

muconic acid in groups of 42 male smokers and 53 female smokers were 0.22±0.03 and 0.24±0.02 mg/g 

creatinine, or 0.13±0.06 and 0.13±0.07 mg/mg cotinine, respectively. Mean concentrations of muconic 

acid in groups of 63 pregnant and 53 nonpregnant female smokers were 0.27±0.04 and 0.24±0.02 mg/g 

creatinine, or 0.24±0.06 and 0.13±0.07 mg/mg cotinine, respectively. Mean concentrations of urinary 

cotinine in pregnant smokers were significantly lower than in the group of nonpregnant female smokers 

(1.13±0.12 mg/g creatinine compared to 1.82±0.14 mg/g creatinine). Benzene levels ranging from 0.01 to 

0.18 μg/kg have been detected in samples of human breast milk (Fabietti et al. 2004). 

Animal data show that exhalation is the main route for excretion of unmetabolized benzene and that 

metabolized benzene is excreted primarily in urine. Only a small amount of an absorbed dose is 

eliminated in feces. A biphasic pattern of excretion of unmetabolized benzene in expired air was 

observed in rats exposed to 500 ppm for 6 hours, with half-times for expiration of 0.7 hour for the rapid 

phase and 13.1 hours for the slow phase (Rickert et al. 1979). The half-life for the slow phase of benzene 

elimination suggests the accumulation of benzene. The major route of excretion following a 6-hour noseonly inhalation exposure of rats and mice to various concentrations of 14C-benzene appeared to be 

dependent on the inhaled concentration (Sabourin et al. 1987). At similar exposures to vapor 
BENZENE 177 

3. HEALTH EFFECTS 

concentrations of 10–1,000 ppm, the mice received 150–200% of the equivalent dose in rats on a per kg 

body weight basis. At all concentrations, fecal excretion accounted for <3.5% of the radioactivity for rats 

and <9% for mice. At lower exposure concentrations (i.e., 13–130 ppm in rats and 11–130 ppm in mice), 

<6% of the radioactivity was excreted in expired air. At the highest exposure concentrations (rats, 

870 ppm; mice, 990 ppm), both rats and mice exhaled a significant amount of unmetabolized benzene 

(48 and 14%, respectively) following termination of the exposure. The majority of the benzeneassociated radioactivity that was not exhaled was found in the urine and in the carcass 56 hours after the 

end of exposure to these high concentrations. The radioactivity in the carcass was associated with the pelt 

of the animals. The authors assumed that this was due to contamination of the pelt with urine, since the 

inhalation exposure had been nose-only. Further investigation confirmed that the radioactivity was 

associated with the fur of the animals. Accordingly, the percentage of the total radioactivity excreted by 

these animals (urine and urine-contaminated pelt) that was not exhaled or associated with feces was 47– 

92% for rats and 80–94% for mice. At exposures of 260 ppm in rats, 85–92% of the radioactivity was 

excreted as urinary metabolites, while at exposures of 130 ppm in mice, 88–94% of the radioactivity was 

excreted as urinary metabolites. The total urinary metabolite formation was 5–37% higher in mice than in 

rats at all doses. This may be explained by the greater amount of benzene inhaled by mice per kg of body 

weight (Sabourin et al. 1987). Purebred Duroc-Jersey pigs were exposed to 0, 20, 100, and 500 ppm 

benzene vapors 6 hours/day, 5 days/week for 3 weeks (Dow 1992). The average concentration of phenol 

in the urine increased linearly with dose. 

3.4.4.2 Oral Exposure 

No studies were located regarding excretion in humans after oral exposure to benzene. Data on excretion 

of benzene or its metabolites in human breast milk after oral exposure were not found. 

Radiolabeled benzene (340 mg/kg) was administered by oral intubation to rabbits; 43% of the label was 

recovered as exhaled unmetabolized benzene and 1.5% was recovered as carbon dioxide (Parke and 

Williams 1953a). Urinary excretion accounted for about 33% of the dose. The isolated urinary 

metabolites were mainly in the form of conjugated phenols. Phenol was the major metabolite accounting 

for about 23% of the dose or about 70% of the benzene metabolized and excreted in the urine. The other 

phenols excreted (percentage of dose) were hydroquinone (4.8%), catechol (2.2%), and trihydroxybenzene (0.3%). L-Phenyl-N-acetyl cysteine accounted for 0.5% of the dose. Muconic acid accounted 

for 1.3%; the rest of the radioactivity (5–10%) remained in the tissues or was excreted in the feces (Parke 

and Williams 1953a). 
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3. HEALTH EFFECTS 

Mice received a single oral dose of either 10 or 200 mg/kg radiolabeled benzene (McMahon and 

Birnbaum 1991). Radioactivity was monitored in urine, feces, and breath. At the low dose, urinary 

excretion was the major route of elimination. Hydroquinone glucuronide, phenylsulfate, and muconic 

acid were the major metabolites at this dose, accounting for 40, 28, and 15% of the dose, respectively. At 

200 mg/kg, urinary excretion decreased to account for 42–47% of the administered dose, while 

respiratory excretion of volatile components increased to 46–56% of the administered dose. Fecal 

elimination was minor and relatively constant over both doses, accounting for 0.5–3% of the dose. 

The effect of dose on the excretion of radioactivity, including benzene and metabolites, following oral 

administration of 14C-benzene (0.5–300 mg/kg) has been studied in rats and mice (Sabourin et al. 1987). 

At doses of <15 mg/kg for 1 day, 90% of the administered dose was excreted in the urine of both species. 

There was a linear relationship for the excretion of urinary metabolites up to 15 mg/kg; above that level, 

there was an increased amount of 14C eliminated in the expired air. Mice and rats excreted equal amounts 

up to 50 mg/kg; above this level, metabolism apparently became saturated in mice. In rats, 50% of the 

150 mg/kg dose of 14C was eliminated in the expired air; in mice, 69% of the 150 mg/kg dose of 14C was 

eliminated in expired air (Sabourin et al. 1987). The label recovered during exhalation was largely in the 

form of unmetabolized benzene, suggesting that saturation of the metabolic pathways had occurred. Dose 

also affected the metabolite profile in the urine. At low doses, a greater fraction of the benzene was 

converted to putative toxic metabolites than at high doses, as reflected in urinary metabolites. 

Mathews et al. (1998) reported similar results following oral (gavage) administration of 14C-benzene to 

rats, mice, and hamsters in single doses from as low as 0.2 mg/kg and up to 100 mg/kg. For example, 

>95% of a 0.5 mg/kg dose was recovered in the urine of rats; a small amount (3%) was recovered in 

expired air. At benzene doses of 10 and 100 mg/kg, elimination in the breath rose to 9 and 50%, 

respectively, indicating the likely saturation of benzene metabolism. Excretion in the feces was minimal 

at all dose levels. Similar results were noted for mice and hamsters. Both dose and species differences 

were noted in the composition of urinary metabolites. Phenyl sulfate was the major metabolite in rat 

urine at all dose levels, accounting for 64–73% of urinary radioactivity. Phenyl sulfate (24–32%) and 

hydroquinone glucuronide (27–29%) were the predominant urinary metabolites in mice. At a dose of 

0.1 mg/kg, mice produced a considerably higher proportion of muconic acid than rats (15 versus 7%). In 

hamsters, hydroquinone glucuronide (24–29%) and muconic acid (19–31%) were the primary urinary 

metabolites. Two additional metabolites (1,2,4-trihydroxybenzene and catechol sulfate) were recovered 

from the urine of hamsters, but not rats or mice. 
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3. HEALTH EFFECTS 

3.4.4.3 Dermal Exposure 

Limited data on excretion of benzene after dermal exposure in humans were found. Four human male 

subjects were given a dermal application of 0.0024 mg/cm2 14C benzene (Franz 1984). A mean of 

0.023% (range, 0.006–0.054%) of the applied radiolabel was recovered in the urine over a 36-hour period. 

Urinary excretion of the radiolabel was greatest in the first two hours following skin application. More 

than 80% of the total excretion occurred in the first 8 hours. In another study, 35–43 cm2

 of the forearm 

were exposed to approximately 0.06 g/cm2

 of liquid benzene for 1.25–2 hours (Hanke et al. 1961). The 

absorption was estimated from the amount of phenol eliminated in the urine. The absorption rate of liquid 

benzene by the skin (under the conditions of complete saturation) was calculated to be low, approximately 

0.4 mg/cm2

/hour. The absorption due to vapors in the same experiment was negligible. Although there 

was a large variability in the physiological values, the amount of excreted phenol was 8.0–14.7 mg during 

the 24-hour period after exposure. It is estimated that approximately 30% of dermally absorbed benzene 

is eliminated in the form of phenol in the urine. 

Data on excretion of benzene or its metabolites in human breast milk after dermal exposure were not 

found. 

Monkeys and minipigs were exposed dermally to 0.0026–0.0036 mg/cm2

 of 14C-benzene (Franz 1984). 

After application, the urine samples were collected over the next 2–4 days at 5-hour intervals. The rate of 

excretion was highest in the first two collection periods. The total urinary excretion of radioactivity was 

found to be higher in monkeys than in minipigs with the same exposure. Mean excretion in monkeys was 

0.065% (range, 0.033–0.135%) of the applied dose compared to 0.042% (range, 0.030–0.054%) in 

minipigs. 

Results of a study in which male rats were dermally treated with 0.004 mg/cm2

 of 14C-benzene, with or 

without 1g of clay or sandy soil, showed that for all treatment groups, the major routes of excretion were 

the urine and, to a lesser extent, the expired air (Skowronski et al. 1988). The highest amount of 

radioactivity in urine appeared in the first 12–24 hours after treatment (58.8, 31.3, and 25.1% of the 

absorbed dose, respectively, for pure benzene, sandy soil–adsorbed benzene, and clay soil-adsorbed 

benzene). In the group treated with pure benzene, 86.2% of the absorbed dose was excreted in the urine. 

Sandy soil and clay soil significantly decreased urinary excretion to 64.0 and 45.4%, respectively, of the 

absorbed dose during the same time period. Rats receiving pure benzene excreted 12.8% of the absorbed 
BENZENE 180 

3. HEALTH EFFECTS 

dose in expired air within 48 hours. Only 5.9% of the radioactivity was collected in expired air 48 hours 

after treatment with sandy soil–adsorbed benzene, while experiments with clay soil–adsorbed benzene 

revealed that 10.1% of the radioactivity was located in expired air. Less than 1% of the absorbed dose 

was expired as 14CO2 in all groups. The 14C activity in the feces was small (<0.5% of the applied 

radioactivity) in all groups 48 hours after treatment. Phenol was the major urinary metabolite detected in 

the 0–12-hour urine samples of all treatment groups. The percentage of total urinary radioactivity 

associated with phenol was 37.7% for benzene alone, 44.2% for benzene adsorbed to sandy soil, and 

45.5% for benzene adsorbed to clay soil. Smaller quantities of hydroquinone, catechol, and benzenetriol 

were also detected (Skowronski et al. 1988). 

3.4.4.4 Other Routes of Exposure 

The metabolic fate of benzene can be altered in fasted animals. In nonfasted rats that received an 

intraperitoneal injection of 88 mg of benzene, the major metabolites present in urine were total conjugated 

phenols (14–19% of dose), glucuronides (3–4% of dose), and free phenol (2–3% of dose). However, in 

rats fasted for 24 hours preceding the same exposure, glucuronide conjugation increased markedly (18– 

21% of dose) (Cornish and Ryan 1965). Free phenol excretion (8–10% of dose) was also increased in 

fasted, benzene-treated rats. There was no apparent increase in total conjugated phenol excretion in 

fasted rats given benzene. 

When 14C-benzene (0.5 and 150 mg/kg) was injected intraperitoneally into rats and mice, most of the 

14C-benzene and 14C-metabolites were excreted in the urine and in the expired air. A smaller amount of 

14C-benzene was found in the feces due to biliary excretion (Sabourin et al. 1987). Monkeys were dosed 

intraperitoneally with 5–500 mg/kg radiolabeled benzene, and urinary metabolites were examined 

(Sabourin et al. 1992). The proportion of radioactivity excreted in the urine decreased with increasing 

dose, whereas as the dose increased, more benzene was exhaled unchanged. This indicated saturation of 

benzene metabolism at higher doses. Phenyl sulfate was the major urinary metabolite. Hydroquinone 

conjugates and muconic acid in the urine decreased as the dose increased. When C57BL/6 mice and 

DBA/2 mice were given benzene subcutaneously in single doses (440, 880, or 2,200 mg/kg) for 1 day, or 

multiple doses (880 mg/kg) 2 times daily for 3 days, no strain differences were observed in the total 

amount of urinary ring-hydroxylated metabolites (Longacre et al. 1981a). Although each strain excreted 

phenol, catechol, and hydroquinone, differences in the relative amounts of these metabolites were noted. 

The more sensitive DBA/2 mice excreted more phenol but less hydroquinone than the more resistant 

C57BL/6 mice, while both strains excreted similar amounts of catechol. DBA/2 mice excreted more 
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3. HEALTH EFFECTS 

phenyl glucuronide but less sulfate conjugate. Both strains excreted similar amounts of phenyl 

mercapturic acid (Longacre et al. 1981a). 

3.4.5 Physiologically Based Pharmacokinetic (PBPK)/Pharmacodynamic (PD) Models 

Physiologically based pharmacokinetic (PBPK) models use mathematical descriptions of the uptake and 

disposition of chemical substances to quantitatively describe the relationships among critical biological 

processes (Krishnan et al. 1994). PBPK models are also called biologically based tissue dosimetry 

models. PBPK models are increasingly used in risk assessments, primarily to predict the concentration of 

potentially toxic moieties of a chemical that will be delivered to any given target tissue following various 

combinations of route, dose level, and test species (Clewell and Andersen 1985). Physiologically based 

pharmacodynamic (PBPD) models use mathematical descriptions of the dose-response function to 

quantitatively describe the relationship between target tissue dose and toxic end points. 

PBPK/PD models refine our understanding of complex quantitative dose behaviors by helping to 

delineate and characterize the relationships between: (1) the external/exposure concentration and target 

tissue dose of the toxic moiety, and (2) the target tissue dose and observed responses (Andersen and 

Krishnan 1994; Andersen et al. 1987). These models are biologically and mechanistically based and can 

be used to extrapolate the pharmacokinetic behavior of chemical substances from high to low dose, from 

route to route, between species, and between subpopulations within a species. The biological basis of 

PBPK models results in more meaningful extrapolations than those generated with the more conventional 

use of uncertainty factors. 

The PBPK model for a chemical substance is developed in four interconnected steps: (1) model 

representation, (2) model parameterization, (3) model simulation, and (4) model validation (Krishnan and 

Andersen 1994). In the early 1990s, validated PBPK models were developed for a number of 

toxicologically important chemical substances, both volatile and nonvolatile (Krishnan and Andersen 

1994; Leung 1993). PBPK models for a particular substance require estimates of the chemical substancespecific physicochemical parameters, and species-specific physiological and biological parameters. The 

numerical estimates of these model parameters are incorporated within a set of differential and algebraic 

equations that describe the pharmacokinetic processes. Solving these differential and algebraic equations 

provides the predictions of tissue dose. Computers then provide process simulations based on these 

solutions. 
BENZENE 182 

3. HEALTH EFFECTS 

The structure and mathematical expressions used in PBPK models significantly simplify the true 

complexities of biological systems. If the uptake and disposition of the chemical substance(s) are 

adequately described, however, this simplification is desirable because data are often unavailable for 

many biological processes. A simplified scheme reduces the magnitude of cumulative uncertainty. The 

adequacy of the model is, therefore, of great importance, and model validation is essential to the use of 

PBPK models in risk assessment. 

PBPK models improve the pharmacokinetic extrapolations used in risk assessments that identify the 

maximal (i.e., the safe) levels for human exposure to chemical substances (Andersen and Krishnan 1994). 

PBPK models provide a scientifically sound means to predict the target tissue dose of chemicals in 

humans who are exposed to environmental levels (for example, levels that might occur at hazardous waste 

sites) based on the results of studies where doses were higher or were administered in different species. 

Figure 3-4 shows a conceptualized representation of a PBPK model. 

If PBPK models for benzene exist, the overall results and individual models are discussed in this section 

in terms of their use in risk assessment, tissue dosimetry, and dose, route, and species extrapolations. 

Several PBPK models have been developed that simulate the disposition of benzene in humans (Bois et 

al. 1996; Brown et al. 1998; Fisher et al. 1997; Medinsky et al. 1989c; Sinclair et al. 1999; Travis et al. 

1990), mice (Cole et al. 2001; Medinsky et al. 1989a, 1989b; Sun et al. 1990; Travis et al. 1990), and rats 

(Bois et al. 1991a; Medinsky et al. 1989a, 1989b; Sun et al. 1990; Travis et al. 1990). A comparative 

summary of the models is provided in Table 3-6. All of the models have the same general structure 

(Figure 3-5). Most of the models simulate inhalation and oral exposures; one model provides a 

simulation of dermal absorption (Sinclair et al. 1999). Physiological parameters and partition coefficients 

for simulating benzene biokinetics of human females were reported for the Brown et al. (1998) and Fisher 

et al. (1997) models. Flow-limited exchange of benzene between blood and tissues is assumed in all 

models, with excretion of benzene in exhaled air and, in one case, to breast milk (Fisher et al. 1997). All 

models include simulations of blood, fat, liver, lung, and lumped compartments representing other 

slowly-perfused tissues (e.g., skeletal muscle) and rapidly-perfused tissues (e.g., kidneys, other viscera). 

Simulation of bone marrow, the primary target for benzene toxicity, is included in the models reported by 

Bois et al. (1991a, 1996), Sinclair et al. (1999), and Travis et al. (1990). 

Simulations of metabolism in the various models vary in complexity. In the simplest representation, 

metabolic elimination of benzene is simulated as a single capacity-limited process, represented with 
Inhaled chem ic al E x haled chem ic al 

Lungs 

Liver 

F a t 

S lo w ly 

perfu sed 

tissues 

R ic h ly 

perfu sed 

tissues 

K idney 

S k in 

V 

E 

N 

O 

U 

S 

B 

L 

O 

O 

D 

A 

R 

T 

E 

R 

I 

A 

L 

B 

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O 

O 

D 

V m a x 

K m 

Ingestion 

GI 

T ra c t 

F e c e s 

U rin e

 C hem ic als 

contacting skin 

Note: This is a conceptual representation of a physiologically based pharmacokinetic (PBPK) model for a 

hypothetical chemical substance. The chemical substance is shown to be absorbed via the skin, by inhalation, or by 

ingestion, metabolized in the liver, and excreted in the urine or by exhalation. 

Source: adapted from Krishnan and Andersen 1994 

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Figure 3-4. Conceptual Representation of a Physiologically Based 

Pharmacokinetic (PBPK) Model for a 

Hypothetical Chemical Substance 
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Figure 3-5. General Structure of Physiologically Based Pharmacokinetic Models 

of Benzene* 

Bone Marrow 

Rapidly-perfused 

Slowly-perfused 

Metabolites 

Fat 

Mammary 

Urine 

Metabolites 

Milk 

Gastrointestinal 

Liver 

Alveolar Space 

Lung Blood 

Air Air 

Tract 

*Tissues shown with dashed lines are not simulated in all models. Flow-limited exchange of benzene between blood 

and tissues is assumed. Metabolism is simulated to varying degrees of complexity (see Table 3-6 for model 

comparison). 
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Table 3-6. Summary Comparison of Physiologically Based Pharmacokinetic 

Models for Benzene 

Simulated areas 

Refer- Absorption Excretion 

ence Speciesa pathwaysb

 Tissuesc Metabolic pathwaysd pathwayse Comment 

Bois et al. R IH, OR BL, BM, BM, LI: BZ→BO(c) EH: BZ Simulates metabolic 

1991a FA, LI, pathways in bone BO→BG(c) UR: PH LU, RP, marrow, and phenol 

SP BO→PH(f) conjugation in lung 

BO→GSH(c) and gastrointestinal 

tract BG→DI(c) 

PH→HQ(c) 

PH→CA(c) 

BM, PH→PHCO(c)
 

LU, 

LU, GI:
 

Bois et al. H IH BL, BM, BM, LI: BZ→Mtot(c) EH: BZ Simulates metabolic 

1996 FA, LU, pathways in bone 

LI, RP, marrow, and endoSP genous production LI: PHXend→PH(z) UR: Mtot of phenolic metaPH bolites 

Brown et H (m,f) IH BL, FA, LI: BZ→Mtot(f) EH: BZ Simulates males or 

al. 1998 LI, LU, females 

RP, SP 

Cole et M IH, OR BL, FA, LI: BZ→BO(c) EH: BZ All metabolism is 

al. 2001 LI, LU, assigned to the liver BO→PH(f) UR: CA 

RP, SP BO→PMA(f) MA 

BO→MA(f) PHCO 

PH→HQ(c) PMA 

PH→PHCO(c) HQCO 

PH→CA(c) THB 

CA→THB(c) 

HQ→HQCO(c) 

Fisher et H IH FA, LU, LI: BZ→Mtot(c) EH: BZ Simulates transfer of 

al. 1997 LI, RP, MI: BZ benzene to breast 

SP, MI milk 

Medinsky H, M, R IH, OR FA, LI, LI: BZ→BO(c) EH: BZ All metabolism is 

et al. LU, RP, assigned to the liver BO→PHCO(c) 1989a, SP 

BO→PMA(c) 1989b, 

1989c BO→HQCO(c) 

BO→MA(c) 
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Table 3-6. Summary Comparison of Physiologically Based Pharmacokinetic 

Models for Benzene 

Simulated areas 

Refer- Absorption Excretion 

ence Speciesa pathwaysb Tissuesc Metabolic pathwaysd pathwayse Comment 

Sinclair et H IH, OR, DE BL, BM, BM, LI: BZ→Mtot(c) EH: BZ Simulates dermal 

al. 1999 LI, LU, UR: M exposure and tot MU, RP absorption PH 

Sun et al. M, R IH, OR BL, FA, LI: BZ→BO(c) EH: BZ Simulates formation 

1990 LI, LU, of hemoglobin BO→PHCO(c) RBC, RP, adducts in red blood 

BO→PMA(c) SP cells derived from 

BO→HQCO(c) benzene oxide 

BO→MA(c) 

RBC: BO→HBA(c,f) 

Travis et H, M, R IH, OR BL, BM, BM, LI: BZ→Mtot(c) EH: BZ Total metabolism of 

al. 1990 FA, LI, benzene in the bone 

LU, MU, marrow and liver 

RP 

a

Species simulated: H=human; M=mouse; R=rat; m=male; f=female b

Absorption pathways simulated: IH=inhalation; OR=oral; DE=dermal 

c

Tissues simulated: BL=blood; BM=bone marrow; FA=fat; LI=liver; LU=lung; MU=muscle; RBC=red blood cells; 

RP=other rapidly-perfused tissues; SP=other slowly-perfused tissues 

d

Metabolic pathways simulated: BZ=benzene; BD=benzene diols; BO=benzene oxide; BG=Benzene glycol; 

CA=catechol; HBA=hemoglobin adduct; HQ=hydroquinone; HQCO=ydroqinione conjugates; MA=muconic acid; 

Mtot=total metabolites; PH=phenol; PHCO=phenol conjugates; PMA=phenylmercapturic acid; 

THB=trihydroxybenzene; PHXend=endogenous phenolic metabolites; (c)=capacity-limited; (f)=first-order; (z)= zeroorder 

e

Excretion pathways simulated: EH=exhalation; MI=breast milk; UR=urine 
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Michaelis-Menten function of benzene concentration in tissue (Bois et al. 1996; Brown et al. 1998; Fisher 

et al. 1997; Sinclair et al. 1999; Travis et al. 1990). In the more complex representations, the major 

pathways of metabolism of benzene, including conjugation reactions, are simulated as capacity-limited or 

first-order processes (Bois et al. 1991a; Cole et al. 2001; Medinsky et al. 1989a, 1989b, 1989c; Sun et al. 

1990). In most of the models, all metabolic pathways are attributed to the liver; however, four of the 

models include simulations of metabolism in bone marrow (Bois et al. 1991a, 1996; Sinclair et al. 1999; 

Travis et al. 1990), and one model includes simulations of the formation of sulfate and glucuronide 

conjugates of phenol in the gastrointestinal and respiratory tracts (Bois et al. 1991a). The Sun et al. 

(1990) model includes a simulation of the formation of hemoglobin adducts derived from benzene oxide. 

In models that simulate the disposition of the metabolites, metabolites are assumed to be excreted in urine 

either at a rate equal to their formation (Cole et al. 2001), or in accordance with a first-order excretion rate 

constant (Bois et al. 1991a, 1996; Sinclair et al. 1999); the difference being, in the latter, the mass balance 

for formation and excretion of metabolites is simulated, allowing predictions of metabolite levels in 

tissues. All of the models use typical parameters and values for species-specific blood flows and tissue 

volumes. 

Brief summaries of the models presented in Table 3-6 are provided below, with emphasis on unique 

features that are applicable to risk assessment. 

Medinsky et al. 1989a, 1989b, 1989c 

Description of the Model. The Medinsky et al. (1989a, 1989b, 1989c) model simulates absorption 

and disposition of benzene in the human, mouse, and rat. Tissues simulated include the blood, bone 

marrow, fat, liver, lung, other slowly-perfused tissues, and other rapidly-perfused tissues. Gastrointestinal 

absorption of benzene is simulated as a first-order process; absorption and excretion of benzene in the 

lung are assumed to be flow-limited. Exchange of benzene between blood and tissues is assumed to be 

flow-limited. The model simulates capacity-limited (i.e., Michaelis-Menten) metabolism of benzene to 

benzene oxide as a function of the concentration of benzene in liver. Conversion of benzene oxide to 

phenol conjugates, phenylmercapturic acid, hydroquinone conjugates, and muconic acid are simulated as 

parallel, capacity-limited reactions in liver. The model simulates rates of formation of metabolites, but 

not the disposition (e.g., excretion) of metabolites. Metabolism parameter values (Vmax, Km) for the 

mouse and rat models were estimated by optimization of the model to observations of total metabolites 

formed in mice and rats exposed by inhalation or oral routes to benzene (Medinsky et al. 1989b; Sabourin 
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3. HEALTH EFFECTS 

et al. 1987). Human metabolism parameter values were derived from allometric scaling of the values for 

mice (Medinsky et al. 1989c). 

Risk Assessment. The model has been used to predict the amounts of benzene metabolites formed in 

rats and mice after inhalation or oral exposures (Medinsky et al. 1989a, 1989b). For inhalation 

concentrations up to 1,000 ppm, mice were predicted to metabolize at least 2–3 times more benzene than 

rats. For oral doses >50 mg/kg, rats were predicted to metabolize more benzene on a kg-body weight 

basis than mice. The model also predicts different metabolite profiles in the two species: mice were 

predicted to produce primarily hydroquinone glucuronide and muconic acid, metabolites linked to toxic 

effects, whereas rats were predicted to produce primarily phenyl sulfate, a detoxification product. These 

predictions agree with experimental data and provide a framework for understanding the greater 

sensitivity of the mouse to benzene toxicity. 

Validation of the Model. The model was calibrated with data from Sabourin et al. (1987). Bois et al. 

(1991b) compared predictions made to observations of benzene exhaled by rats following exposures to 

490 ppm benzene, reported by Rickert et al. (1979), as well as the data from which the model was 

calibrated (Sabourin et al. 1987). In general, the model tended to overestimate observations to which it 

was not specifically fitted. 

Target Tissues. The model simulates amounts and concentrations of benzene in blood, liver, fat, and 

lumped compartments for other rapidly-perfused and slowly-perfused tissues as well as amounts of 

metabolites formed. It does not simulate concentrations of metabolites in these tissues. It does not 

simulate bone marrow, a target of benzene metabolites. 

Species Extrapolation. The model has been applied to simulations of mice, rats, and humans 

(Medinsky et al. 1989a, 1989b, 1989c). 

High-low Dose Extrapolation. The model has been evaluated for simulating inhalation exposures in 

rodents ranging from 1 to 1,000 ppm and oral gavage doses ranging from 0.1 to 300 mg/kg (Bois et al. 

1991b; Medinsky et al. 1989a, 1989b, 1989c). 

Interroute Extrapolation. The model simulates inhalation and oral exposures and has been applied 

to predicting internal dose metrics (e.g., amounts of metabolites formed) resulting from exposures by 

these routes (Medinsky et al. 1989a, 1989b, 1989c). 
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Strengths and Limitations. Strengths of the model are that it simulates disposition of inhaled and 

ingested (single dose) benzene, including rates and amounts of major metabolites formed in mice, rats, 

and humans. Limitations include: (1) the model has not been evaluated for multiple exposures; (2) the 

model attributes all metabolism to the liver; (3) the model does not simulate the fate of metabolites 

formed and, therefore, cannot be used to predict concentrations of metabolites (e.g., muconaldehyde) in 

tissues; and (4) the model does not simulate bone marrow, a major target tissue for benzene metabolites. 

Sun et al. 1990 

Description of the Model. The Sun et al. (1990) model is an extension of the mouse and rat models 

developed by Medinsky et al. (1989a, 1989b, 1989c). The Sun et al. (1990) model includes a simulation 

of the formation of hemoglobin adducts derived from benzene oxide. Adduct formation is represented as 

the sum of capacity-limited and first-order functions of the concentration of benzene oxide in the liver. 

Parameter values were estimated by optimization to measurements of hemoglobin adduct formations in 

rats and mice exposed to single oral gavage doses of benzene (Sun et al. 1990). 

Risk Assessment. The model has been applied to predicting the levels of hemoglobin adducts in 

mice and rats following inhalation or oral exposures to benzene. This approach could be potentially 

useful for predicting exposure levels that correspond to measured hemoglobin adduct levels, for use of 

adducts as an exposure biomarker. 

Validation of the Model. The model was calibrated against measurements of hemoglobin adduct 

formation in mice and rats that received single oral gavage doses of benzene ranging from 0.008 to 

800 mg/kg (Sun et al. 1990). The model was evaluated by comparing predictions to observations of 

amounts of hemoglobin adducts formed in mice and rats exposed to benzene vapor concentrations of 5, 

50, or 600 ppm for 6 hours (Sabourin et al. 1989a). 

Target Tissues. The model predicts hemoglobin adduct formation after oral and inhalation exposure 

(Sun et al. 1990). 

Species Extrapolation. The model has been applied to simulations for mice and rats. 
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3. HEALTH EFFECTS 

High-low Dose Extrapolation. The model was calibrated with observations made in mice and rats 

exposed to single gavage doses ranging from 0.1 to 10,000 μmol/kg (0.008–800 mg/kg), and evaluated 

for predicting observations in mice and rats exposed by inhalation to 600 ppm benzene. 

Interroute Extrapolation. The Sun model examined two routes of exposure, oral and inhalation. 

The model was found to be useful in predicting the concentrations of hemoglobin adducts in blood in 

rodents after oral and inhalation exposure. 

Strengths and Limitations. Strengths of the model are that it extends the Medinsky et al. (1989a, 

1989b, 1989c) models to simulate hemoglobin adduct formation secondary to formation of benzene 

oxide. A limitation of the adduct model is that it simulates production of adducts as a function of benzene 

oxide concentration in liver and does not consider other potential pathways of adduct formation through 

hydroquinone, phenol, or muconaldehyde. 

Travis et al. 1990 

Description of the Model. The Travis et al. (1990) model simulates the absorption and disposition 

of benzene in the human, mouse, and rat. Tissues simulated include the blood, bone marrow, fat, liver, 

lung, other slowly-perfused tissues, and rapidly-perfused tissues. Gastrointestinal absorption of benzene 

is simulated as a first-order process. Absorption and excretion of benzene in the lung are assumed to be 

flow-limited, as are exchanges of benzene between blood and tissues. The model simulates capacitylimited (i.e., Michaelis-Menten) metabolic elimination of benzene as a function of the concentration of 

benzene in bone marrow and liver. The model simulates rates of metabolic elimination of benzene, but 

not the rates of formation of specific metabolites or their disposition (e.g., excretion). For the purpose of 

comparing model predictions to observations, 80% of the total metabolite formed in 24 hours (and 

excreted in urine) was assumed to be phenol. Metabolism parameter values (Vmax, Km) were estimated by 

optimization of the model to observations of total metabolites formed (i.e., excreted in urine) in humans, 

mice, and rats exposed to benzene by inhalation or oral routes to benzene. The Vmax for metabolism in 

bone marrow in humans was assumed to be 4% of that of liver, consistent with optimized values for 

rodents. 

Risk Assessment. This model has been used to predict the amounts of benzene in expired air, 

concentrations of benzene in blood, and total amount of benzene metabolized following inhalation 

exposures to humans and inhalation, intraperitoneal, oral gavage, or subcutaneous exposures in mice or 
BENZENE 191 

3. HEALTH EFFECTS 

rats (Travis et al. 1990). Cox (1996) applied the model to derive internal dose-response relationships for 

benzene in humans. 

Validation of the Model. The model was evaluated by comparing predictions with observations 

made in mouse and rat inhalation studies (Rickert et al. 1979; Sabourin et al. 1987; Sato et al. 1975; 

Snyder et al. 1981); mouse oral gavage studies (Sabourin et al. 1987); mouse subcutaneous injection 

studies (Andrews et al. 1977); and rat intraperitoneal injection studies (Sato and Nakajima 1979). 

Predictions of benzene in expired air and/or blood concentrations were also compared to observations 

made in humans who inhaled concentrations ranging from 5 to 100 ppm benzene (5 ppm: Berlin et al. 

1980; Sherwood 1972; 25–57 ppm: Sato et al. 1975; Sherwood 1972; Nomiyama and Nomiyama 1974a, 

1974b; 99–100 ppm: Sherwood 1972; Teisinger and Fiserova-Bergerova 1955). Further evaluations of 

predictions of benzene in workers are reported in Sinclair et al. (1999) and Sherwood and Sinclair (1999), 

who compared model predictions with observations of benzene in exhaled breath and urinary excretion of 

phenol in workers who were exposed to benzene at concentrations ranging from 1 to 1,100 ppm. 

Target Tissues. The model simulates amounts and concentrations of benzene in blood, bone marrow 

(a target tissue), liver, fat, and lumped compartments for other rapidly-perfused and slowly-perfused 

tissues; and amounts of metabolites formed in liver and bone marrow. It does not simulate concentrations 

of metabolites in these tissues. 

Species Extrapolation. The model has been applied to simulations for mice, rats, and humans 

(Sherwood and Sinclair 1999; Sinclair et al. 1999; Travis et al. 1990). 

High-low Dose Extrapolation. The model has been evaluated for simulating inhalation exposures in 

humans ranging from 1 to 1,110 ppm (Sherwood and Sinclair 1999; Sinclair et al. 1999; Travis et al. 

1990). Evaluations of predictions in rodents included observations made during inhalation exposures that 

ranged from 11 to 1,000 ppm and oral gavage doses that ranged from 0.5 to 300 mg/kg. 

Interroute Extrapolation. The model simulates inhalation and oral exposures and has been applied 

to predicting internal dose metrics (e.g., benzene concentration in blood, amount benzene metabolized) 

resulting from exposures by these routes (Travis et al. 1990). 

Strengths and Limitations. Strengths of the model are that it simulates (1) disposition of inhaled 

and ingested (single dose) benzene in mice, rats, and humans; and (2) concentrations of benzene, and 
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3. HEALTH EFFECTS 

rates and amount of benzene metabolized in bone marrow, a target tissue for benzene metabolites. 

Limitations of the model include: (1) the model simulates metabolic elimination of benzene, but not the 

rates of formation of major metabolites; and (2) the model does not simulate fate of metabolites formed 

and, therefore, cannot be used to predict concentrations of metabolites in tissues. 

Fisher et al. 1997 

Description of the Model. The Fisher et al. (1997) model extends the model reported by Travis et al. 

(1990) to include a simulation of lactational transfer of benzene to breast milk in humans. Other tissues 

simulated include blood, fat, liver, lung, other slowly-perfused tissues, and rapidly-perfused tissues. 

Absorption and excretion of benzene in the lung and exchange of benzene between blood and tissues are 

assumed to be flow-limited, as is excretion of benzene in breast milk. The lactational transfer model 

includes simulations of breast milk production and loss from nursing; the latter is represented as a firstorder process. Estimates of blood:air and blood:milk partition coefficients during lactation (from which 

the milk:blood partition coefficient could be calculated) were measured in nine lactating subjects (Fisher 

et al. 1997). The model simulates capacity-limited (i.e., Michaelis-Menten) metabolism of metabolic 

elimination of benzene as a function of the concentration of benzene in liver. Rates of formation of 

specific metabolites and their disposition (e.g., excretion) are not simulated. Metabolism parameter 

values (Km,Vmax) and tissue:blood partition coefficients were derived from Travis et al. (1990). 

Risk Assessment. This model has been used to predict benzene concentrations in breast milk and 

lactational transfers to breast feeding infants (Fisher et al. 1997). Exposures to the threshold limit value 

(TLV) (10 ppm, 8 hours/day, 5 days/week) were predicted to yield 0.053 mg of benzene in breast milk 

per 24 hours. This approach has potential applicability to assessing lactational exposures to infants 

resulting from maternal exposures. 

Validation of the Model. The lactation model was evaluated (Fisher et al. 1997) by comparing 

predictions for perchloroethylene (not benzene) with those predicted by a perchloroethylene model 

developed by Schreiber (1993). Other components of the biokinetics model were derived from the Travis 

et al. (1990) model, which has undergone evaluations against data obtained from studies in humans. 

Target Tissues. The model simulates concentrations of benzene in blood, breast milk, liver, fat, and 

lumped compartments for other rapidly-perfused and slowly-perfused tissues as well as rates of metabolic 
BENZENE 193 

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elimination of benzene. It does not simulate concentrations of metabolites in these tissues and does not 

simulate metabolism in bone marrow, a major target of benzene metabolites. 

Species Extrapolation. The model has been applied to simulations for humans (Fisher et al. 1997). 

High-low Dose Extrapolation. The lactational model has not been evaluated for simulating 

inhalation exposures to benzene in humans; therefore, applicability to high-low dose extrapolations 

cannot be assessed. 

Interroute Extrapolation. The model was developed to simulate inhalation exposures. Extrapolation 

to other routes (e.g., oral, dermal) would require the extension of the model to include simulations of 

absorption from these routes. 

Strengths and Limitations. Strengths of the model are that it simulates the disposition of inhaled 

benzene in females during lactation, including transfers of benzene to breast milk and nursing infants; 

concentrations of benzene in blood and tissues; and rates of metabolic elimination of benzene 

metabolized. Limitations of the model include that the model does not simulate rates of formation of 

major metabolites and that the model does not simulate kinetics of uptake or metabolism of benzene in 

bone marrow, a major target of benzene toxicity. 

Sinclair et al. 1999 

Description of the Model. The Sinclair et al. (1999) model is an extension of the human model 

developed by Travis et al. (1990) to include a simulation of first-order urinary excretion of total 

metabolites and phenol, and dermal absorption of benzene. 

Risk Assessment. The model has been applied to predicting the levels of benzene in exhaled air and 

phenol in urine in workers exposed to benzene (Sherwood and Sinclair 1999; Sinclair et al. 1999). 

Validation of the Model. The model was evaluated against measurements of benzene in exhaled 

breath and urinary excretion of phenol in workers who were exposed to benzene at concentrations ranging 

from 1 to 1,100 ppm (Sherwood and Sinclair 1999; Sinclair et al. 1999). 
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Target Tissues. The model simulates amounts and concentrations of benzene in blood, bone marrow 

(a target tissue), liver, fat, and lumped compartments for other rapidly-perfused and slowly-perfused 

tissues; rates of metabolic elimination of benzene in liver and bone marrow; and excretion of total 

metabolites formed and phenol. 

Species Extrapolation. The model has been applied to simulations for humans (Sinclair et al. 1999). 

High-low Dose Extrapolation. The model was evaluated against observations of benzene in exhaled 

breath and urinary excretion of phenol in workers who were exposed to benzene at concentrations ranging 

from 1 to 1,100 ppm (Sherwood and Sinclair 1999; Sinclair et al. 1999). 

Interroute Extrapolation. The model simulates inhalation, oral, and dermal exposures. 

Strengths and Limitations. Strengths of the model are that it extends the Travis et al. (1990) model 

to include simulation of dermal absorption of benzene. 

Bois et al. 1991a 

Description of the Model. The Bois et al. (1991a) model simulates absorption and disposition of 

benzene and the benzene metabolite, phenol, in the rat. Tissues simulated include the blood, bone 

marrow, fat, liver, lung, other slowly-perfused tissues, and other rapidly-perfused tissues. Gastrointestinal 

absorption of benzene and phenol are simulated as a first-order function for dose. Absorption and 

excretion of benzene in the lung are assumed to be flow-limited as are exchanges of benzene and phenol 

between blood and tissues. Excretion of phenol is simulated as a first-order transfer to urine. The model 

simulates capacity-limited (i.e., Michaelis-Menten) and first-order metabolism of benzene and 

metabolites in bone marrow, liver, gastrointestinal tract, and respiratory tract (see Table 3-6). All 

pathways are assumed to be capacity-limited reactions, except for the spontaneous hydrolysis of benzene 

oxide to form phenol, which is simulated as a first-order process. The model simulates rates of formation 

of metabolites and first-order excretion of phenol; however, disposition (e.g., excretion) of other 

metabolites is not simulated. Parameter values, including metabolism parameter values, were optimized 

to a reference set of observations of metabolites formed in rats exposed by inhalation or to single gavage 

doses of benzene (see below). 
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Risk Assessment. This model has been used to predict amounts of benzene and phenol metabolites 

formed in rats during oral gavage exposures to benzene equivalent to those administered in NTP (1986) 

and to inhalation exposures equivalent to the OSHA PEL (Bois and Paxman 1992; Bois et al. 1991a). 

Model simulations indicate that dose rate may be an important factor in benzene toxicity. For example, 

when the model was applied to simulations for rats exposed either for 15 minutes to a benzene vapor 

concentration of 32 ppm or for 8 hours to 1 ppm (equivalent 8-hour TWAs), the amount of metabolites 

(hydroquinone, catechol, and muconaldehyde) formed was 20% higher after the 15-minute exposure at 

the higher level than after the 8-hour exposure at the lower level (Bois and Paxman 1992). These 

metabolites have been identified as being important in the genesis of bone marrow toxicity after benzene 

exposure (Eastmond et al. 1987). These types of analyses, if extended to humans, would be applicable to 

evaluations of the adequacy of short-term exposure limits. 

Validation of the Model. The model was calibrated (Bois and Paxman 1992; Bois et al. 1991a) with 

observations made in rats exposed to single oral gavage doses of benzene, or to inhalation exposures of 

13–870 ppm (Sabourin et al. 1987, 1989b), in rats administered single parenteral doses of phenol 

(Cassidy and Houston 1984), and in in vitro metabolism studies (Sawahata and Neal 1983). Further 

evaluations against data not used in the calibration were not reported. 

Target Tissues. The model simulates amounts and concentrations of benzene and phenol in bone 

marrow, a target tissue for benzene metabolites, as well as in blood, liver, fat, and lumped compartments 

for other rapidly-perfused and slowly-perfused tissues. The model also simulates amounts of specific 

metabolites formed and urinary excretion of the major urinary metabolite, phenol. It does not simulate 

concentrations of metabolites, other than phenol, in these tissues. 

Species Extrapolation. The model has been applied to simulations for rats. A human model has 

been developed that implements a scaled-down version of the rat metabolism model (see Bois et al. 

1996). 

High-low Dose Extrapolation. The model has been evaluated for simulating inhalation exposures in 

rats ranging from 13 to 870 ppm and oral gavage doses ranging from 15 to 300 mg/kg. 

Interroute Extrapolation. The model simulates inhalation and oral exposures and has been applied 

to predicting internal dose metrics (e.g., amounts of metabolites formed) resulting from exposures by 

these routes. 
BENZENE 196 

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Strengths and Limitations. Strengths of the model are that it simulates disposition of inhaled and 

ingested benzene (and phenol), including rates and amounts of most of the major metabolites formed in 

rats. Limitations include: (1) the model has not been evaluated for multiple exposures; (2) although the 

model simulates the fate of benzene and phenol, it does not simulate the fate of other metabolites formed 

and, therefore, cannot be used to predict concentrations of these metabolites in tissues; and (3) the model, 

as configured in Bois et al. (1991a), does not simulate benzene disposition in humans. 

Bois et al. 1996 

Description of the Model. The Bois et al. (1996) model simulates inhalation absorption and 

disposition of benzene in humans. Tissues simulated include the blood, bone marrow, fat, liver, lung, 

other slowly-perfused tissues, and other rapidly-perfused tissues. Absorption and excretion of benzene in 

the lung are assumed to be flow-limited as are exchanges of benzene between blood and tissues. The 

model simulates metabolic elimination of benzene as a single capacity-limited (i.e., Michaelis-Menten) 

reaction, occurring in bone marrow and liver. Endogenous formation of phenolic metabolites is also 

simulated as a zero-order process occurring in liver. The model simulates first-order excretion of total 

metabolites and the phenol fraction (approximately 80% of total). Parameter values (physiological and 

chemical) were estimated by Bayesian optimization techniques (Markov Chain Monte Carlo analysis) 

using reference observations of benzene concentration in blood and urinary excretion of phenol in human 

subjects who were exposed to benzene in air (Pekari et al. 1992). 

Risk Assessment. The model has been used to predict rates and amounts of benzene metabolized in 

human populations (Bois et al. 1996). The population model (population geometric means and standard 

deviations of parameter values) was derived using Markov Chain Monte Carlo analysis with observations 

from three human subjects serving as the reference data for inter-individual variability (from Pekari et al. 

1992). The population model predicts probability distributions of model outputs (for example, rates or 

amounts of benzene metabolized for a given exposure). This approach could be used to evaluate 

uncertainty factors in risk assessments intended to account for uncertainties in our understanding of 

benzene pharmacokinetics variability. 

Validation of the Model. The model was calibrated with observations of benzene concentrations in 

blood and urinary phenol levels, made in three human subjects who were exposed to 1.7 or 10 ppm 
BENZENE 197 

3. HEALTH EFFECTS 

benzene for 4 hours (Pekari et al. 1992). Further evaluations against data not used in the calibration have 

not been reported. 

Target Tissues. The model simulates amounts and concentrations of benzene in bone marrow, a 

target of benzene toxicity, as well as blood, liver, fat, and lumped compartments for other rapidlyperfused and slowly-perfused tissues. Amounts of total metabolites formed and excreted are simulated; 

however, the model does not simulate concentrations of metabolites in these tissues. 

Species Extrapolation. The model has been applied to simulations for humans (Bois et al. 1996). 

High-low Dose Extrapolation. The model has been evaluated for simulating inhalation exposures in 

human subjects ranging from 1.7 to 10 ppm (Bois et al. 1996). 

Interroute Extrapolation. The model simulates inhalation exposures. Extrapolation to other routes 

(e.g., oral, dermal) would require the extension of the model to include simulations of absorption from 

these routes. 

Strengths and Limitations. Strengths of the model are that it simulates disposition of inhaled 

benzene and rates of total metabolism in humans. Limitations include that the model has not been 

evaluated for multiple exposures and that the model simulates total metabolism of benzene, and not the 

rates of formation of the major metabolites of benzene of toxicological interest. 

Brown et al. 1998 

Description of the Model. The Brown et al. (1998) model simulates inhalation absorption and 

disposition of benzene in humans. Tissues simulated include the blood, fat, liver, lung, other slowlyperfused tissues, and other rapidly-perfused tissues. Absorption and excretion of benzene in the lung and 

exchange of benzene between blood and tissues are assumed to be flow-limited. The model simulates 

capacity-limited (i.e., Michaelis-Menten) metabolic elimination of benzene as a function of the 

concentration of benzene in liver. Rates of formation of specific metabolites, or their disposition (e.g., 

excretion), are not simulated. For the purpose of comparing model predictions to observations, 80% of 

the total metabolites formed and excreted in urine (i.e., amount of benzene eliminated by metabolism) in 

24 hours was assumed to be phenol. The Km parameter for metabolism was derived from Travis et al. 

1990; the Vmax was estimated by optimization of the model to observations of blood concentrations of 
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benzene and benzene in exhaled breath of female and male subjects who were exposed to 25 ppm 

benzene for 2 hours (Sato et al. 1975). Partition coefficients for males and females were derived from 

vial equilibrium studies conducted on blood and/or tissues from males and females (Fisher et al. 1997; 

Paterson and Mackay 1989). 

Risk Assessment. This model has been used to predict the benzene concentrations in blood and 

amounts of benzene metabolized in females and males who experience the same inhalation exposure 

scenarios. Females were predicted to metabolize 23–26% more benzene than similarly-exposed males. 

This difference was attributed, in part, to a higher blood:air partition coefficient for benzene in females. 

Validation of the Model. The model was calibrated by comparing predictions of blood 

concentrations of benzene and benzene in exhaled breath of female and male subjects who were exposed 

to 25 ppm benzene for 2 hours (Brown et al. 1998; Sato et al. 1975). Further evaluations against data not 

used in the calibration were not been reported. 

Target Tissues. The model simulates concentrations of benzene in blood, liver, fat, and lumped 

compartments for other rapidly-perfused and slowly-perfused tissues as well as rates of metabolic 

elimination of benzene. It does not simulate concentrations of metabolites in these tissues and does not 

simulate metabolism in bone marrow, a major target of benzene metabolites. 

Species Extrapolation. The model has been applied to simulations for humans. 

High-low Dose Extrapolation. The model has been evaluated for simulating inhalation exposures in 

humans. Evaluations of predictions included observations made during inhalation exposures to 25 ppm 

(Brown et al. 1998; Sato et al. 1975). 

Interroute Extrapolation. The model simulates inhalation and has been applied to predicting internal 

dose metrics (e.g., benzene concentration in blood, amount benzene metabolized) resulting from 

exposures by this route (Brown et al. 1998). Extrapolation to other routes (e.g., oral, dermal) would 

require the extension of the model to include simulations of absorption from these routes. 

Strengths and Limitations. Strengths of the model are that it simulates disposition of inhaled 

benzene in female and male humans as well as the concentrations of benzene and rates of metabolic 

elimination of benzene metabolized. Limitations of the model include: (1) the model does not simulate 
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rates of formation of major benzene metabolites; (2) the model does not simulate fate of metabolites 

formed and, therefore, cannot be used to predict concentrations of metabolites in tissues; and (3) the 

model does not simulate kinetics of uptake or metabolism of benzene in bone marrow, a major target of 

benzene toxicity. 

Cole et al. 2001 

Description of the Model. The Cole et al. (2001) model simulates absorption and disposition of 

benzene in the mouse. Tissues simulated include the blood, fat, liver, lung, other slowly-perfused tissues, 

and other rapidly-perfused tissues. Gastrointestinal absorption of benzene is simulated as a first-order 

process. Absorption and excretion of benzene in the lung are assumed to be flow-limited as are 

exchanges of benzene between blood and tissues. The model simulates capacity-limited (i.e., MichaelisMenten) and first-order metabolism of benzene and metabolites in liver (see Table 3-6). Capacity-limited 

reactions in bone marrow and liver include benzene to benzene oxide, phenol to hydroquinone, phenol to 

catechol, catechol to trihydroxybenzene, and conjugation of phenol and hydroquinone. First-order 

reactions in liver include conversion of benzene oxide to phenol, muconic acid, and phenylmercapturic 

acid. The model simulates rates of formation of metabolites, tissue distribution of benzene oxide, phenol, 

and hydroquinone; and first-order excretion of metabolites in urine. Capacity-limited metabolism 

parameter values were estimated from in vitro studies of mouse liver (Lovern et al. 1999; Nedelcheva et 

al. 1999; Seaton et al. 1995); first-order parameters were estimated by optimization of model output to 

observations of metabolites formed in mice exposed by inhalation or to single gavage doses (Kenyon et 

al. 1995; Mathews et al. 1998; Sabourin et al. 1988). Blood:tissue partition coefficients for benzene and 

metabolites were derived from Medinsky et al. (1989a) or estimated based on the n-octanol-water 

partition coefficient (Poulin and Krishnan 1995). 

Risk Assessment. This model has been used to predict amounts of benzene exhaled and amounts of 

benzene metabolites produced in mice during inhalation exposures or following oral gavage exposures to 

benzene (Cole et al. 2001). 

Validation of the Model. The model was calibrated with observations made in mice exposed to 

single oral gavage doses of benzene, or to inhalation exposures (Cole et al. 2001; Kenyon et al. 1995; 

Mathews et al. 1998; Sabourin et al. 1988). Further evaluations against data not used in the calibration 

have not been reported. 
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Target Tissues. The model simulates amounts and concentrations of benzene in blood, liver, fat, and 

lumped compartments for other rapidly perfused and slowly perfused tissues; rates of formation of 

metabolites; tissue distribution of benzene oxide, phenol, and hydroquinone; and first-order excretion of 

metabolites in urine. It does not simulate concentrations of metabolites in bone marrow, a target tissue 

for benzene metabolites. 

Species Extrapolation. The model has been applied to simulations for mice (Cole et al. 2001). 

High-low Dose Extrapolation. The model has been evaluated for simulating inhalation exposures in 

mice (50 ppm) and oral gavage doses ranging from 0.1 to 100 mg/kg (Cole et al. 2001; Kenyon et al. 

1995; Mathews et al. 1998; Sabourin et al. 1988). 

Interroute Extrapolation. The model simulates inhalation and oral exposures and has been applied 

to predicting internal dose metrics (e.g., amounts of metabolites formed) resulting from exposures by 

these routes (Cole et al. 2001). 

Strengths and Limitations. Strengths of the model are that it simulates disposition of inhaled and 

ingested benzene, including rates and amounts of major metabolites. Most of the metabolism parameter 

values were derived empirically from in vitro studies, rather than by model optimization. Limitations 

include: (1) the model has not been evaluated for multiple exposures; (2) the model does not simulate the 

metabolism of benzene in bone marrow, a major target of benzene toxicity; (3) the model, as configured 

in Cole et al. (2001), does not simulate benzene disposition in humans. 

3.5 MECHANISMS OF ACTION 

3.5.1 Pharmacokinetic Mechanisms 

Benzene is readily absorbed via all natural routes of exposure (inhalation, oral, and dermal) and 

distributed throughout the body via the blood. Based on physical properties such as slight water 

solubility, high lipid solubility, and nonpolarity, benzene is expected to enter the blood via passive 

diffusion from gut, lungs, and skin. Benzene is expected to readily bind to plasma proteins (Travis and 

Bowers 1989). Being moderately lipophilic, benzene tends to accumulate in fatty tissues. However, 

benzene metabolism is relatively rapid and required for hematopoietic and leukemogenic effects to be 

expressed. Multiple reactive metabolites appear to be involved in benzene toxicity. As discussed in 

detail in Section 3.5.2, potential candidates include benzene oxide, phenolic metabolites (phenol, 
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catechol, hydroquinone, 1,2,4-benzenetriol, and 1,2- and 1,4-benzoquinone), and trans,trans-muconaldehyde. Both human and animal data demonstrate the importance of CYP2E1 in benzene metabolism (see 

Section 3.4.3 for a detailed discussion). Metabolism is assumed to take place primarily in the liver, with 

some secondary metabolism in the bone marrow, the site of characteristic benzene toxicity. Processes 

involved in transport of hepatic metabolites of benzene to the critical toxicity target (bone marrow) are 

not known, although some degree of covalent binding of reactive benzene metabolites to blood proteins is 

expected. At relatively low exposure levels, urinary excretion of conjugated benzene derivatives 

represents the major excretory pathway for benzene. Biliary excretion represents a minor excretory 

pathway. 

3.5.2 Mechanisms of Toxicity 

Numerous mechanistic studies have been conducted in an effort to elucidate mechanisms of benzeneinduced hematotoxic and leukemogenic effects, widely recognized as the most critical effects of benzene 

exposure. Conversely, benzene-induced effects on reproduction, development, and the nervous system 

have not been studied in sufficient detail to assess mechanisms of toxicity for these end points. The 

database of information for benzene-induced hematotoxic and leukemogenic effects has been reviewed 

extensively (e.g., Bird et al. 2005; Irons 2000; Morgan and Alvares 2005; Ross 1996, 2000, 2005; 

Schnatter et al. 2005; Smith 1996a, 1996b; Snyder 2000a, 2000b, 2002; Snyder and Hedli 1996; Snyder 

and Kalf 1994). It is generally believed that reactive hepatic metabolites of benzene are transported to the 

major toxicity target (bone marrow). Additional metabolism likely occurs in bone marrow. Phenolic 

metabolites (phenol, hydroquinone, catechol, 1,2,4-benzenetriol, and 1,2- and 1,4-benzoquinone) appear 

to play a major role in benzene toxicity. Smith (1996a, 1999b) noted that the phenolic metabolites can be 

metabolized by bone marrow peroxidases, such as myeloperoxidase (MPO), to highly reactive 

semiquinone radicals and quinones that stimulate the production of reactive oxygen species. These steps 

lead to damage to tubulin, histone proteins, topoisomerase II, other DNA associated proteins, and DNA 

itself (clastogenic effects such as strand breakage, mitotic recombination, chromosome translocations, and 

aneuploidy). Damage to stem or early progenitor cells would be expressed as hematopoietic and 

leukemogenic effects. 

Results of several mechanistic studies demonstrate that benzene hematotoxicity is dependent upon 

metabolism (see Section 3.4.3 for a detailed discussion). Inhibition of benzene metabolism reduced its 

toxicity (Andrews et al. 1977). Partial hepatectomy decreased both benzene metabolism and toxicity 

(Sammett et al. 1979). Pretreatment with inducers of metabolism increased both benzene metabolism and 
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toxicity (Gad-El-Karim et al. 1985, 1986). Inhibition of CYPs (enzymes that catalyze oxidation pathways 

in benzene metabolism) reduced benzene-induced genotoxicity (Tuo et al. 1996). Mice lacking CYP2E1 

or microsomal epoxide hydrolase expression were not susceptible to benzene levels known to cause 

myelotoxicity and cytotoxicity in wild type mice (Bauer et al. 2003; Valentine et al. 1996a, 1996b). 

Occupationally-exposed workers with a phenotype corresponding to rapid CYP2E1 metabolism were 

more susceptible to benzene toxicity than those expressing slow CYP2E1 metabolism (Rothman et al. 

1997). The enzyme NAD(P)H:quinone oxidoreductase (NQ01), which maintains quinones in reduced 

form where they are more readily conjugated and excreted (Nebert et al. 2002), is another example of the 

importance of metabolism in benzene hematotoxicity. Between 22% (Caucasian) and 45% (Asian) of the 

population is homozygous for an NQ01 allele whereby NQ01 production is negligible. Rothman et al. 

(1997) found that workers homozygous for an NQ01 allele whereby NQ01 production is negligible (wild 

type) exhibited a 2.4-fold increased risk for benzene hematotoxicity than workers with the normal 

genotype. Greater than 7-fold increased risk of benzene hematotoxicity was noted in workers who 

expressed both rapid CYP2E1 metabolism and the NQ01 wild type (Rothman et al. 1997). 

Benzene metabolism involves the production of reactive metabolites that may act directly on cellular 

macromolecules (proteins and DNA). No single metabolite has been implicated; effects are probably due 

to many metabolites, which include benzene oxide, reactive products of the phenol pathway (catechol, 

hydroquinone, and 1,4-benzoquinone), and trans,trans-muconaldehyde. Evidence that benzene oxide 

may play a role in benzene toxicity includes findings that benzene oxide is a product of oxidative benzene 

metabolism in mouse, rat, and human liver microsomes (Lovern et al. 1997), benzene oxide can be 

released from the liver into the blood (Lindstrom et al. 1997), benzene oxide-protein adducts have been 

found in the blood and bone marrow of mice exposed to benzene (McDonald et al. 1994), and benzene 

oxide hemoglobin and albumin adducts have been detected in the blood of workers exposed to benzene 

(Rappaport et al. 2002a, 2002b; Yeowell-O’Connell et al. 1998). 

Urinary trans,trans-muconic acid has been detected in humans and animals following benzene exposure, 

although its purported reactive precursor (trans,trans-muconaldehyde; see Figure 3-3) has not been 

detected in vivo. The highly reactive trans,trans-muconaldehyde, which has been found in mouse hepatic 

microsomes (Latriano et al. 1986), can undergo reductive and oxidative metabolism (Goon et al. 1992) 

and has been shown to be hematotoxic (Witz et al. 1985). A small amount (<0.05%) of parenterallyadministered trans,trans-muconaldehyde to mice reached the bone marrow (Zhang et al. 1997). Rivedal 

and Witz (2005) found trans,trans-muconaldehyde to be a strong inhibitor of gap junction intercellular 

communication in rat liver epithelial cells. 
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Phenolics (phenol, catechol, hydroquinone; 1,2,4-benzenetriol; 1,2- and 1,4-benzoquinone) are major 

metabolites of benzene that have been shown to persist in bone marrow following inhalation exposure 

(Rickert et al. 1979; Sabourin et al. 1988). Hydroquinone induces chromosomal damage in lymphocytes 

in vitro in a manner similar to that observed in lymphocytes of benzene-exposed workers (Eastmond et al. 

1994; Stillman et al. 1997; Zhang et al. 1998b). Kolachana et al. (1993) demonstrated that both 

hydroquinone and 1,2,4-benzenetriol cause oxidative damage to DNA in mouse bone marrow (in vivo) 

and human myeloid cells (in vitro). Glutathione adducts of 1,4-benzoquinone have been shown to be 

hematotoxic in bone marrow of mice exposed to benzene (Bratton et al. 1997). Additional evidence that 

phenolic metabolites may play an important role in benzene toxicity includes the finding that MPO, an 

enzyme found in high concentration in bone marrow (Bainton et al. 1971), catalyzes the oxidation of 

polyphenols to reactive quinones, semiquinones, and oxygen radicals (Nebert et al. 2002). This can lead 

to strand breaks and inhibition of topoisomerases and microtubule assembly, which could result in 

chromosome damage (Chen and Eastmond 1995; Eastmond et al. 2001; Irons and Neptune 1980; Smith 

1996a, 1996b). Eastmond et al. (2005) demonstrated that hydroquinone can be readily activated to a 

potent topoisomerase II inhibitor in the presence of human MPO and H2O2 and that partial inhibition 

occurs at hydroquinone concentrations as low as 50 nM. Irons and Neptune (1980) suggested that 

benzene-derived hydroquinone may inhibit cell replication by covalently binding to tubulin, a protein 

essential for spindle formation in mitosis. 

Benzene-induced effects on DNA have been studied in some detail. Schwartz et al. (1985) demonstrated 

that benzene metabolites inhibit mitochondrial DNA polymerase. Hydroquinone has been shown to 

inhibit ribonucleotide reductase, a key step in DNA synthesis (Li et al. 1997, 1998). Rushmore et al. 

(1984) found that benzene metabolites covalently bind to mitochondrial DNA and inhibit RNA synthesis. 

Benzene metabolites bound to hepatic DNA have been observed in animals following inhalation exposure 

to radiolabeled benzene (Lutz and Schlatter 1977). However, the reported levels of DNA adduct 

formation appear to be low. For example, Creek et al. (1997) observed adducts to both protein and DNA 

in the range of nanograms per kilogram in mice given radiolabeled benzene. 

Benzene-induced DNA damage may result from the oxidation of DNA by reactive oxygen species that 

are produced during benzene metabolism. Both 1,4-benzoquinone and hydroquinone are known to 

increase superoxide, nitric oxide, and hydrogen peroxide in HL-60 cells (Rao and Snyder 1995). Chen et 

al. (2004) observed nitric oxide-derived benzene metabolites, namely nitrobenzene, nitrophenyl, and 

nitrophenol isomers in the bone marrow of mice 1 hour following intraperitoneal injection of a 400 mg/kg 
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dose of benzene. These nitro metabolites were either not detected in other tissues or were present in 

much smaller concentrations, indicating that they were most likely produced in bone marrow. Brunmark 

and Cadenas (1988) described a metabolic pathway from hydroquinone leading to the production of 

glutathionyl-benzenetriol, which can undergo autoxidation leading to superoxide formation. Rao (1996) 

suggested that benzene-induced DNA damage is mediated by the release of free iron in the bone marrow 

(probably by polyphenolic metabolites), followed by the chelation of iron by hydroquinone or 

benzenetriol to yield a reactive oxygen-generating species such as superoxide, which causes oxidative 

damage to DNA. 

The expression of benzene toxicity may involve multiple benzene metabolites. All of the known 

unconjugated metabolites of benzene, with the exception of phenol and 1,2,4-benzenetriol, have been 

shown to decrease erythropoiesis (Snyder and Hedli 1996). In mice, the combination of phenol and 

hydroquinone resulted in exacerbated loss of bone marrow cellularity (Eastmond et al. 1987), increased 

peroxidatic activation of hydroquinone (Subrahmanyam et al. 1989, 1990), and increased DNA damage 

(Lévay and Bodell 1992; Marrazzini et al. 1994). Combinations of either phenol and hydroquinone, or 

phenol and catechol, were more hematotoxic than any of the metabolites given alone (Guy et al. 1991). 

The combination of hydroquinone and muconaldehyde was the most potent in inhibiting erythropoiesis 

(Snyder et al. 1989). Catechol was found to stimulate the peroxidase-mediated activation of 

hydroquinone and produced a synergistic genotoxic effect in lymphocytes (Robertson et al. 1991). 

3.5.3 Animal-to-Human Extrapolations 

Pathways of benzene metabolism are generally similar among various rodent and nonhuman primate 

species. However, species differences exist regarding capacity to metabolize benzene and relative 

proportions of various benzene metabolites formed. 

Species differences exist in absorption and retention of benzene. For example, following 6-hour 

exposures to low concentrations (7–10 ppm) of benzene vapors, mice retained 20% of the inhaled 

benzene, whereas rats and monkeys retained only 3–4% (Sabourin et al. 1987, 1992). Mice exhibit a 

greater overall capacity to metabolize benzene, compared to rats. An Inhalation exposure to 925 ppm 

resulted in an internal dose of 152 mg/kg in mice, approximately 15% of which was excreted as parent 

compound, and an internal dose of 116 mg/kg in rats, approximately 50% of which was excreted 

unchanged (Henderson et al. 1992; Sabourin et al. 1987). 
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The proportions of benzene metabolites produced depend on both species and exposure concentration. 

Hydroquinones and muconic acid (potential sources of benzene toxicity) were detected in much higher 

concentrations in the blood, liver, lung, and bone marrow of mice than rats, following a 6-hour inhalation 

exposure to benzene at a concentration of 50 ppm (Sabourin et al. 1988). It is generally understood that 

metabolic profiles of benzene in mice and humans are more similar than those of humans and rats. 

Sabourin et al. (1989a) noted increased production of detoxification metabolites (phenylglucuronide and 

prephenylmercapturic acid) and decreased production of potentially toxic metabolites (hydroquinones and 

muconic acid) in both mice and rats exposed to benzene at much higher concentrations (600 ppm in air or 

200 mg/kg orally), which indicates that extrapolation of toxicological results from studies using high 

exposure concentrations to low exposure scenarios may result in an underestimation of risk. 

Recent PBPK models have tried to address benzene metabolism in an effort to derive animal-to-human 

extrapolations (Bois et al. 1991a, 1996; Cole et al. 2001; Medinsky 1995; Medinsky et al. 1989a, 1989b, 

1989c; Travis et al. 1990). Each model described a multicompartmental model that attempted to relate 

the generation of metabolites to end points of benzene toxicity. The generation of hydroquinone and 

muconaldehyde in the liver, with further metabolism in the bone marrow, has been addressed as well as 

the available data allow. However, the models are not sufficiently refined to allow them to accurately 

predict human metabolism. 

3.6 TOXICITIES MEDIATED THROUGH THE NEUROENDOCRINE AXIS 

Recently, attention has focused on the potential hazardous effects of certain chemicals on the endocrine 

system because of the ability of these chemicals to mimic or block endogenous hormones. Chemicals 

with this type of activity are most commonly referred to as endocrine disruptors. However, appropriate 

terminology to describe such effects remains controversial. The terminology endocrine disruptors, 

initially used by Thomas and Colborn (1992), was also used in 1996 when Congress mandated the EPA to 

develop a screening program for “...certain substances [which] may have an effect produced by a 

naturally occurring estrogen, or other such endocrine effect[s]...”. To meet this mandate, EPA convened a 

panel called the Endocrine Disruptors Screening and Testing Advisory Committee (EDSTAC), and in 

1998, the EDSTAC completed its deliberations and made recommendations to EPA concerning endocrine 

disruptors. In 1999, the National Academy of Sciences released a report that referred to these same types 

of chemicals as hormonally active agents. The terminology endocrine modulators has also been used to 

convey the fact that effects caused by such chemicals may not necessarily be adverse. Many scientists 

agree that chemicals with the ability to disrupt or modulate the endocrine system are a potential threat to 
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the health of humans, aquatic animals, and wildlife. However, others think that endocrine-active 

chemicals do not pose a significant health risk, particularly in view of the fact that hormone mimics exist 

in the natural environment. Examples of natural hormone mimics are the isoflavinoid phytoestrogens 

(Adlercreutz 1995; Livingston 1978; Mayr et al. 1992). These chemicals are derived from plants and are 

similar in structure and action to endogenous estrogen. Although the public health significance and 

descriptive terminology of substances capable of affecting the endocrine system remains controversial, 

scientists agree that these chemicals may affect the synthesis, secretion, transport, binding, action, or 

elimination of natural hormones in the body responsible for maintaining homeostasis, reproduction, 

development, and/or behavior (EPA 1997). Stated differently, such compounds may cause toxicities that 

are mediated through the neuroendocrine axis. As a result, these chemicals may play a role in altering, 

for example, metabolic, sexual, immune, and neurobehavioral function. Such chemicals are also thought 

to be involved in inducing breast, testicular, and prostate cancers, as well as endometriosis (Berger 1994; 

Giwercman et al. 1993; Hoel et al. 1992). 

No information was located to indicate that benzene may adversely affect the endocrine system. 

3.7 CHILDREN’S SUSCEPTIBILITY 

This section discusses potential health effects from exposures during the period from conception to 

maturity at 18 years of age in humans, when all biological systems will have fully developed. Potential 

effects on offspring resulting from exposures of parental germ cells are considered, as well as any indirect 

effects on the fetus and neonate resulting from maternal exposure during gestation and lactation. 

Relevant animal and in vitro models are also discussed. 

Children are not small adults. They differ from adults in their exposures and may differ in their 

susceptibility to hazardous chemicals. Children’s unique physiology and behavior can influence the 

extent of their exposure. Exposures of children are discussed in Section 6.6, Exposures of Children. 

Children sometimes differ from adults in their susceptibility to hazardous chemicals, but whether there is 

a difference depends on the chemical (Guzelian et al. 1992; NRC 1993). Children may be more or less 

susceptible than adults to health effects, and the relationship may change with developmental age 

(Guzelian et al. 1992; NRC 1993). Vulnerability often depends on developmental stage. There are 

critical periods of structural and functional development during both prenatal and postnatal life, and a 

particular structure or function will be most sensitive to disruption during its critical period(s). Damage 
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may not be evident until a later stage of development. There are often differences in pharmacokinetics 

and metabolism between children and adults. For example, absorption may be different in neonates 

because of the immaturity of their gastrointestinal tract and their larger skin surface area in proportion to 

body weight (Morselli et al. 1980; NRC 1993); the gastrointestinal absorption of lead is greatest in infants 

and young children (Ziegler et al. 1978). Distribution of xenobiotics may be different; for example, 

infants have a larger proportion of their bodies as extracellular water, and their brains and livers are 

proportionately larger (Altman and Dittmer 1974; Fomon 1966; Fomon et al. 1982; Owen and Brozek 

1966; Widdowson and Dickerson 1964). The infant also has an immature blood-brain barrier (Adinolfi 

1985; Johanson 1980) and probably an immature blood-testis barrier (Setchell and Waites 1975). Many 

xenobiotic metabolizing enzymes have distinctive developmental patterns. At various stages of growth 

and development, levels of particular enzymes may be higher or lower than those of adults, and 

sometimes unique enzymes may exist at particular developmental stages (Komori et al. 1990; Leeder and 

Kearns 1997; NRC 1993; Vieira et al. 1996). Whether differences in xenobiotic metabolism make the 

child more or less susceptible also depends on whether the relevant enzymes are involved in activation of 

the parent compound to its toxic form or in detoxification. There may also be differences in excretion, 

particularly in newborns who all have a low glomerular filtration rate and have not developed efficient 

tubular secretion and resorption capacities (Altman and Dittmer 1974; NRC 1993; West et al. 1948). 

Children and adults may differ in their capacity to repair damage from chemical insults. Children also 

have a longer remaining lifetime in which to express damage from chemicals; this potential is particularly 

relevant to cancer. 

Certain characteristics of the developing human may increase exposure or susceptibility, whereas others 

may decrease susceptibility to the same chemical. For example, although infants breathe more air per 

kilogram of body weight than adults breathe, this difference might be somewhat counterbalanced by their 

alveoli being less developed, which results in a disproportionately smaller surface area for alveolar 

absorption (NRC 1993). 

No clear evidence of age-related differences in susceptibility to benzene toxicity was located. Benzene 

crosses the placenta and can be found in cord blood at concentrations that equal or exceed those of 

maternal blood (Dowty et al. 1976). Nursing infants can be exposed to benzene in the breast milk 

(Fabietti et al. 2004). Limited animal studies indicate that in utero exposure to benzene results in 

hematological changes similar to those observed in animals exposed only as adults (Corti and Snyder 

1996; Keller and Snyder 1986, 1988). There is some indication that parental occupational exposure to 

benzene may play a role in childhood leukemia (Buckley et al. 1989; McKinney et al. 1991; Shaw et al. 
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1984; Shu et al. 1988). However, none of these studies indicate whether children may be at greater risk 

than adults for benzene toxicity. Results of Infante-Rivard et al. (2005) indicate that maternal exposure to 

benzene during pregnancy or from 2 years before pregnancy up to birth does not result in increased risk of 

childhood acute lymphoblastic leukemia (ALL), a frequent form of childhood cancer (Infante-Rivard et 

al. 2005). However, no information was located regarding the risk of childhood AML, the form of 

leukemia most frequently associated with exposure to benzene. 

Children could potentially be at increased risk for significant benzene exposure via the inhalation route 

based on higher activity levels and ventilation rates than adults. However, no information was located to 

indicate that children are at increased risk for benzene toxicity. Age-related differences in benzene 

metabolism could potentially affect susceptibility. Results of one human study indicate that CYP2E1, a 

major enzyme involved in benzene metabolism, is not present in the fetus, but appears in rapidly 

increasing concentrations during early postnatal development (Vieira et al. 1996). This suggests that 

fetuses and neonates may be at decreased risk of benzene toxicity due to a reduced metabolic capacity. 

No information was located regarding potential age-related differences in pharmacodymanic processes 

such as benzene-target interactions in the hematopoietic system. 

3.8 BIOMARKERS OF EXPOSURE AND EFFECT 

Biomarkers are broadly defined as indicators signaling events in biologic systems or samples. They have 

been classified as markers of exposure, markers of effect, and markers of susceptibility (NAS/NRC 

1989). 

Due to a nascent understanding of the use and interpretation of biomarkers, implementation of biomarkers 

as tools of exposure in the general population is very limited. A biomarker of exposure is a xenobiotic 

substance or its metabolite(s) or the product of an interaction between a xenobiotic agent and some target 

molecule(s) or cell(s) that is measured within a compartment of an organism (NAS/NRC 1989). The 

preferred biomarkers of exposure are generally the substance itself, substance-specific metabolites in 

readily obtainable body fluid(s), or excreta. However, several factors can confound the use and 

interpretation of biomarkers of exposure. The body burden of a substance may be the result of exposures 

from more than one source. The substance being measured may be a metabolite of another xenobiotic 

substance (e.g., high urinary levels of phenol can result from exposure to several different aromatic 

compounds). Depending on the properties of the substance (e.g., biologic half-life) and environmental 

conditions (e.g., duration and route of exposure), the substance and all of its metabolites may have left the 
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body by the time samples can be taken. It may be difficult to identify individuals exposed to hazardous 

substances that are commonly found in body tissues and fluids (e.g., essential mineral nutrients such as 

copper, zinc, and selenium). Biomarkers of exposure to benzene are discussed in Section 3.8.1. 

Biomarkers of effect are defined as any measurable biochemical, physiologic, or other alteration within an 

organism that, depending on magnitude, can be recognized as an established or potential health 

impairment or disease (NAS/NRC 1989). This definition encompasses biochemical or cellular signals of 

tissue dysfunction (e.g., increased liver enzyme activity or pathologic changes in female genital epithelial 

cells), as well as physiologic signs of dysfunction such as increased blood pressure or decreased lung 

capacity. Note that these markers are not often substance specific. They also may not be directly 

adverse, but can indicate potential health impairment (e.g., DNA adducts). Biomarkers of effects caused 

by benzene are discussed in Section 3.8.2. 

A biomarker of susceptibility is an indicator of an inherent or acquired limitation of an organism's ability 

to respond to the challenge of exposure to a specific xenobiotic substance. It can be an intrinsic genetic or 

other characteristic or a preexisting disease that results in an increase in absorbed dose, a decrease in the 

biologically effective dose, or a target tissue response. If biomarkers of susceptibility exist, they are 

discussed in Section 3.10, Populations That Are Unusually Susceptible. 

3.8.1 Biomarkers Used to Identify or Quantify Exposure to Benzene 

Several biomarkers of exposure to benzene have been reported in the literature. Unmetabolized benzene 

can be detected in the expired air and urine of humans exposed to benzene vapors (Farmer et al. 2005; 

Fustinoni et al. 2005; Ghittori et al. 1993; Nomiyama and Nomiyama 1974a, 1974b; Sherwood 1988; 

Srbova et al. 1950; Waidyanatha et al. 2001). Urinary phenol measurements have routinely been used for 

monitoring occupational exposure to benzene (OSHA 1987), and urinary phenol levels appear to be 

correlated with exposure levels (Astier 1992; Inoue et al. 1986, 1988b; Karacic et al. 1987; Pagnotto et al. 

1961; Pekari et al. 1992). 

Urinary trans,trans-muconic acid has been widely studied as a biomarker of exposure to benzene 

(Boogaard and van Sittert 1995, 1996; Ducos et al. 1990, 1992; Inoue et al. 1989b, 2000; Lee et al. 1993; 

Melikian et al. 1993, 1994; Pezzagno et al. 1999; Popp et al. 1994; Qu et al. 2005; Rothman et al. 1998; 

Ruppert et al. 1997; Sanguinetti et al. 2001; van Sittert et al. 1993; Weaver et al. 2000). Urinary 

S-phenylmercapturic acid levels have also been correlated with occupational exposure to benzene 
BENZENE 210 

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(Boogaard and van Sittert 1995, 1996; Farmer et al. 2005; Inoue et al. 2000; Jongeneelen et al. 1987; 

Popp et al. 1994; Qu et al. 2005). Significant exposure-response trends for urinary trans,trans-muconic 

acid and S-phenylmercapturic acid levels have been demonstrated in occupationally-exposed subjects at 

exposure levels of ≤1 ppm (Qu et al. 2005). The American Conference of Governmental Industrial 

Hygienists (ACGIH) has established 25 μg S-phenylmercapturic acid/g creatinine in the urine and 500 μg 

trans,trans-muconic acid/g creatinine in the urine as Biological Exposure Indices (BEIs) for benzene 

exposure in the workplace (ACGIH 2006). The BEI is primarily an index of exposure and not a level at 

which health effects might occur from exposure to benzene. Positive correlations have been made 

between benzene workplace air levels and urinary catechol and hydroquinone in exposed workers (Inoue 

et al. 1988a, 1988b; Rothman et al. 1998). 

Hemoglobin and albumin adducts of the benzene metabolites, benzene oxide and 1,4-benzoquinone, have 

been used as biomarkers of exposure to benzene (Bechtold and Henderson 1993; Bechtold et al. 1992a, 

1992b; Smith and Rothman 2000; Yeowell-O’Connell et al. 1998, 2001). Furthermore, DNA adducts 

with benzene metabolites have been found after benzene exposure (Hedli et al. 1991; Lutz and Schlatter 

1977; Reddy et al. 1989). 

Ong et al. (1995) evaluated various biomarkers of benzene exposure for their relationship with 

environmental benzene levels. Muconic acid in the urine correlated best with environmental benzene 

concentrations. Urinary hydroquinone levels were the most accurate biomarker of exposure for the 

phenolic metabolites of benzene, followed by phenol and catechol. No correlation was found between 

environmental benzene levels and unmetabolized benzene in the urine, although other studies suggest that 

benzene in the urine may be a useful biomarker of occupational exposure (Ghittori et al. 1993). 

The biomarkers discussed in the preceding paragraphs appear to be adequate indicators of exposure to 

benzene at relatively high occupational exposure levels, and may serve as biomarkers in acute exposure 

scenarios involving relatively high levels of benzene. However, some of these biomarkers do not appear 

to be reliable indicators of environmental exposure to benzene (concentrations below the common 

industrial standard of 1 ppm TWA). For example, results from data collected on 152 chemical workers 

showed a linear relationship between the concentration of benzene in breathing zone air (when greater 

than 10 ppm) and urinary concentrations of catechol and hydroquinone (Inoue et al. 1988a). Workers 

who had an average work-site exposure of 10 ppm benzene showed no significant differences in the 

concentration of urinary catechol or hydroquinone when compared to a group of unexposed subjects. In a 

study of pharmacy workers exposed to benzene levels measured in the parts per billion (ppb) range, there 
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was no significant difference in urinary levels of trans,trans-muconic acid between subjects exposed to 

1.5 ppb and those exposed to 2.5 ppb (Sanguinetti et al. 2001). Recent reports indicate that urinary 

benzene may serve as the most sensitive biomarker of exposure to benzene concentrations well below 

1 ppm (Farmer et al. 2005; Fustinoni et al. 2005). 

Several additional factors must be taken into account when assessing the reliability of biomarkers of 

exposure to benzene. High and variable background levels of phenol and its metabolites result from 

ingestion of vegetables, exposure to other aromatic compounds, ingestion of ethanol, and inhalation of 

cigarette smoke (Nakajima et al. 1987). Relatively high urinary phenol levels (5–42 mg/L) have been 

found in persons with no known exposure to benzene (NIOSH 1974). Although muconic acid is used as a 

marker for benzene exposure, muconic acid in the urine can also result from ingestion of sorbic acid, a 

common food preservative (Ducos et al. 1990). Inoue et al. (1989b) suggested that individual urinary 

trans,trans-muconic acid content was not a useful index of benzene exposure due to large variations in 

measured individual background urinary trans,trans-muconic acid values. 

In summary, several benzene metabolites may serve as biomarkers of exposure to benzene. Urinary 

benzene appears to be the most sensitive biomarker for low-level exposure to benzene. Refer to 

Tables 7-1 and 7-2 for information regarding analytical methods for determining benzene in biological 

samples and benzene metabolites in urine. 

3.8.2 Biomarkers Used to Characterize Effects Caused by Benzene 

In addition to using levels of benzene and benzene metabolites for monitoring purposes, various 

biological indices might also be helpful in characterizing the effects of exposure to benzene. As with 

monitoring for benzene exposure, monitoring for effects may best be accomplished through the use of a 

series of biomarkers with correlation of the results. Decreases in erythrocyte and leukocyte counts have 

been used as an indicator of high occupational exposures. Monitoring of benzene workers has included 

monthly blood counts, with workers being removed from areas of high benzene exposure when leukocyte 

counts fell below 4,000/mm3

 or erythrocyte counts fell below 4,000,000/mm3 (ITII 1975; OSHA 1987). 

Hayes (1992) indicates that benzene-related leukopenia, commonly thought of as an intermediate end 

point in the process of developing benzene-related leukemia, is considered a biomarker of benzene 

poisoning in China, not necessarily related to leukemia. Leukocyte alkaline phosphatase (LAP) activity 

was increased in benzene workers exposed to about 31 ppm for a chronic time period (Songnian et al. 

1982). Increased LAP activity is an indicator of myelofibrosis and is associated with both decreased 
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white blood cell counts and changes in bone marrow activity. The change in LAP activity could be used 

in the diagnosis of benzene poisoning since it was more sensitive than the change in the leukocyte count, 

although it is not a biomarker that is specific for benzene. Additionally, it seems reasonable that 

chromosomal aberrations in bone marrow and peripheral blood lymphocytes and sister chromatid 

exchanges could be used to monitor for benzene effects (Eastmond et al. 1994; Van Sittert and de Jong 

1985). Benzene metabolites have also been found to form adducts with DNA (Chenna et al. 1995; Lutz 

and Schlatter 1977; Norpoth et al. 1988; Rushmore et al. 1984; Snyder et al. 1987). 

Exposure to benzene causes toxic effects in the bone marrow via its metabolites and possibly by benzene 

via solvent effects (Eastmond et al. 1987; Gad-El-Karim et al. 1985; Hedli et al. 1991; Irons et al. 1980). 

Therefore, it is possible that hematological tests could be used as markers of hematotoxicity. To date, 

surveillance and early diagnosis of benzene hematotoxicity rely primarily on the complete blood count, 

including hemoglobin, hematocrit, erythrocyte count, leukocyte count, and differential and platelet 

counts. In effect, complete blood counts and marrow exams should be good for early detection of 

preleukemic lesions. Additionally, cytogenetic tests of marrow cells are being used. Workers exposed to 

benzene in the air have shown elevated levels of delta-aminolevulinic acid in erythrocytes and elevated 

coproporphyrin in the urine (Kahn and Muzyka 1973). These may be biomarkers for disruption of 

porphyrin synthesis and may be early indicators of adverse hematological effects. These effects are not 

specific for benzene. Hedli et al. (1991) observed that in rats, benzene metabolite-DNA adducts were 

observed in the bone marrow at doses that did not affect bone marrow cellularity, and suggested that 

monitoring of the bone marrow DNA adducts might be a sensitive bioassay of genotoxic effects of 

benzene exposure. Work by other researchers also suggests that monitoring DNA adducts or products of 

DNA damage might be useful (Bodell et al. 1993; Chen et al. 1994; Lagorio et al. 1994a; Reddy et al. 

1989). 

3.9 INTERACTIONS WITH OTHER CHEMICALS 

Studies have been conducted on the interaction of benzene with other chemicals, both in vivo and in the 

environment. Benzene metabolism is complex, and various xenobiotics can induce or inhibit specific 

routes of detoxification and/or activation in addition to altering the rate of benzene metabolism and 

clearance from the body. Toluene, Aroclor 1254, phenobarbital, acetone, and ethanol are known to alter 

the metabolism and toxicity of benzene. Interactions reported in in vivo studies occurred at relatively 

high benzene exposure levels, which would not likely be encountered near hazardous waste sites. 
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Ethanol and benzene induce the formation of the hepatic cytochrome P-450 isoenzyme, CYP2E1, in 

rabbits and rats (Gut et al. 1993; Johansson and Ingelman-Sundberg 1988), although benzene derivatives, 

such as toluene and xylene, can inhibit the enzymatic activity of the isozyme (Koop and Laethem 1992). 

Ethanol enhances both the metabolism (in vitro) and the toxicity (in vivo) of benzene in animals (Baarson 

et al. 1982; Nakajima et al. 1985). Administration of ethanol (5 or 15% in drinking water, 4 days/week 

for 13 weeks) to mice exposed to benzene vapors at a concentration of 300 ppm, 6 hours/day, 

5 days/week for 13 weeks) resulted in greater severity of benzene-induced hematological effects (anemia, 

lymphocytopenia, bone marrow aplasia, transient increases in normoblasts and peripheral blood atypia) 

relative to benzene-exposed mice not given ethanol (Baarson et al. 1982). The modulating effects of 

benzene were dose dependent. The enhancement of the hematotoxic effects of benzene by ethanol may 

be of particular concern for benzene-exposed workers who consume alcohol (Nakajima et al. 1985), 

although the interactions demonstrated in the mice occurred at much higher benzene exposure 

concentrations than would likely be experienced in workplace air. Benzene can interfere with the 

disappearance of ethanol from the body. Accordingly, increased central nervous system disturbances 

(e.g., depression) may occur following concurrent exposure to high levels of benzene and ethanol. 

Other chemicals that induce specific isoenzymes of cytochrome P-450 can increase the rate of benzene 

metabolism and may alter metabolism pathways favoring one over another. Ikeda and Ohtsuji (1971) 

presented evidence that benzene hydroxylation was stimulated when rats were pretreated with 

phenobarbital and then exposed to 1,000 ppm of benzene vapor for 8 hours/day for 2 weeks. 

Additionally, phenobarbital pretreatment increased the rate of metabolism by 40% in rats and 70% in 

mice (Pawar and Mungikar 1975). In contrast, rats exposed to phenobarbital showed no effects on the 

metabolism of micromolar amounts (35–112.8 μmol) of benzene in vitro (Nakajima et al. 1985). 

Co-administration of toluene inhibited the biotransformation of benzene to phenol in rats (Ikeda et al. 

1972; Inoue et al. 1988b). This was due to competitive inhibition of the oxidation mechanisms involved 

in the metabolism of benzene. Phenobarbital pretreatment of the rats alleviated the suppressive effect of 

toluene on benzene hydroxylation by the induction of oxidative activities in the liver. This effect has 

been observed in other studies in rats (Purcell et al. 1990). 

Mathematical models of benzene and phenol metabolism suggest that the inhibition by benzene of phenol 

metabolism, and by phenol on benzene metabolism, occurs through competition for a common reaction 

site, which can also bind catechol and hydroquinone (Schlosser et al. 1993). Flavonoids have been shown 
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to inhibit phenol hydroxylase or increase phenol hydroxylase activity in a dose-dependent manner, 

dependent on the oxidation potential of the flavonoid (Hendrickson et al. 1994). 

SKF-525A and carbon monoxide are classic inhibitors of cytochrome P-450s. The binding between 

P-450 and carbon monoxide or SKF-525A is coordinate covalent. Carbon monoxide inhibits all 

cytochrome P-450 isoenzymes since it binds to the heme component of cytochrome P-450, whereas 

SKF-525A inhibits specific types. SKF-525A inhibited benzene metabolism in the rat (Ikeda et al. 1972). 

Injection of 80 mg/kg of SKF-525A in rats resulted in a depression of phenol excretion. It also prolonged 

phenol excretion and interfered in the conversion of benzene to glucuronides and free phenols. Carbon 

monoxide, aniline, aminopyrine, cytochrome C, and metyrapone inhibited benzene metabolism in vitro by 

mouse liver microsomes (Gonasun et al. 1973). 

3.10 POPULATIONS THAT ARE UNUSUALLY SUSCEPTIBLE 

A susceptible population will exhibit a different or enhanced response to benzene than will most persons 

exposed to the same level of benzene in the environment. Reasons may include genetic makeup, age, 

health and nutritional status, and exposure to other toxic substances (e.g., cigarette smoke). These 

parameters result in reduced detoxification or excretion of benzene, or compromised function of organs 

affected by benzene. Populations who are at greater risk due to their unusually high exposure to benzene 

are discussed in Section 6.7, Populations with Potentially High Exposures. 

Variability in human susceptibility to benzene toxicity may be related, at least in part, to genetic 

polymorphisms associated with metabolic processes. As discussed in Section 3.4.3, the flavoenzyme, 

NAD(P)H:quinone oxidoreductase (NQ01), catalyzes the reduction of 1,2- and 1,4-benzoquinone 

(reactive metabolites of benzene) to catechol and hydroquinone, respectively (Nebert et al. 2002), thus 

protecting cells from oxidative damage by preventing redox cycling. The NQ01*1 (wild type) allele 

codes for normal NQ01 enzyme and activity. An NQ01*2 allele encodes a nonsynonymous mutation that 

has negligible NQ01 activity. Approximately 5% of Caucasians and African Americans, 15% of 

Mexican-Americans, and 20% of Asians are homozygous for the NQ01*2 allele (Kelsey et al. 1997; 

Smith and Zhang 1998). Rothman et al. (1997) demonstrated that workers expressing negligible NQ01 

activity were at increased risk of benzene poisoning (Rothman et al. 1997). In the same group of workers, 

those expressing rapid CYP2E1 activity were also at increased risk of benzene poisoning. Those workers 

with polymorphisms for both negligible NQ01 activity and rapid CYP2E1 activity exhibited greater than 

7-fold increased risk of benzene poisoning than workers not expressing these polymorphisms. These 
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results indicate that individuals expressing rapid CYP2E1 activity may also be at increased risk for 

benzene toxicity. 

Polymorphisms have been described for many of the glutathione genes (Cotton et al. 2000; Hengstler et 

al. 1998; Strange and Fryer 1999), the myeloperoxidase gene (Williams 2001), and the epoxide hydrolase 

gene (Omiecinski et al. 2000), which are known to be involved in benzene metabolism. However, the 

potential involvement of such polymorphisms in benzene toxicity have not been demonstrated in 

benzene-exposed workers. 

Individuals with medical conditions that include reduced bone marrow function or decreased blood 

factors would be at increased risk for benzene toxicity. Treatments for certain medical conditions might 

result in decreases in particular blood factors, which could lead to increased susceptibility to benzene 

poisoning. 

Ethanol can increase the severity of benzene-induced anemia, lymphocytopenia, and reduction in bone 

marrow cellularity, and produce transient increases in normoblasts in the peripheral blood and atypical 

cellular morphology (Baarson et al. 1982). The enhancement of the hematotoxic effects of benzene by 

ethanol is of particular concern for benzene-exposed workers who consume alcohol (Nakajima et al. 

1985). Accordingly, increased central nervous system disturbances (e.g., depression) may be expected 

following concurrent exposure to benzene and ethanol. 

Gender-related differences in susceptibility to benzene toxicity have been observed in animals. For 

example, Kenyon et al. (1998) exposed male and female mice to benzene vapor concentrations of 100 or 

600 ppm and found increased benzene metabolism and associated genotoxicity in males, relative to 

females. Brown et al. (1998) used a PBPK modeling approach to assess potential gender-related 

differences in susceptibility to benzene. Their results suggest that women exhibit a higher blood/air 

partition coefficient and maximum velocity of benzene metabolism than men, and that women metabolize 

23-26% more benzene than men under similar exposure scenarios. However, gender-related differences 

in susceptibility among benzene-exposed workers were not located in available reports. 

At early stages of human development, metabolic pathways may not be fully functional, which might 

result in a lower level of susceptibility. Young children might experience increased susceptibility to 

benzene by inhalation due to increased breathing rates and potential for increased absorption, relative to 
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adults. However, no definitive human or animal data were located regarding age-related differences in 

susceptibility to benzene. 

3.11 METHODS FOR REDUCING TOXIC EFFECTS 

This section will describe clinical practice and research concerning methods for reducing toxic effects of 

exposure to benzene. However, because some of the treatments discussed may be experimental and 

unproven, this section should not be used as a guide for treatment of exposures to benzene. When 

specific exposures have occurred, poison control centers and medical toxicologists should be consulted 

for medical advice. The following texts provide specific information about treatment following exposures 

to benzene: 

Goldfrank LR, Flomenbaum NE, Lewin NA, et al., eds. 1998. Goldfrank’s toxicologic emergencies. 6th 

ed. Stamford: Appleton & Lange, 1384-1398. 

Haddad LM, Shannon MW, Winchester JF. 1998. Clinical management of poisoning and drug overdose. 

3rd ed. Philadelphia, PA: W.B Saunders Company, 940-941. 

3.11.1 Reducing Peak Absorption Following Exposure 

Human exposure to benzene can occur by inhalation, oral, or dermal routes. General recommendations 

for reducing absorption of benzene following acute high-level inhalation exposure have included moving 

the patient to fresh air and monitoring for respiratory distress (Bronstein and Currance 1988; Haddad et 

al. 1998; HSDB 2007; Kunisaki and Augenstein 1994; Stutz and Janusz 1988). The administration of 

100% humidified supplemental oxygen with assisted ventilation, when required, has also been suggested 

(Bronstein and Currance 1988; Haddad et al. 1998; Kunisaki and Augenstein 1994; Stutz and Janusz 

1988). In the case of eye exposure, irrigation with copious amounts of water or saline has been 

recommended (Bronstein and Currance 1988; Haddad et al. 1998; HSDB 2007; Stutz and Janusz 1988). 

The removal of contaminated clothing and a thorough washing of exposed areas with soap and water have 

also been recommended (Bronstein and Currance 1988; Ellenhorn and Barceloux 1988; Haddad et al. 

1998; Kunisaki and Augenstein 1994; Stutz and Janusz 1988). Some sources suggest the administration 

of water or milk to the victim after ingestion of benzene (Stutz and Janusz 1988). Emesis may be 

indicated following recent substantial ingestion of benzene (Ellenhorn and Barceloux 1988); however, 

other sources do not recommend the use of emetics because of the risk of aspiration pneumonitis, 

especially once the patient loses consciousness (Bronstein and Currance 1988; Haddad et al. 1998). Some 

sources recommend gastric lavage if indicated (Haddad et al. 1998; Kunisaki and Augenstein 1994). 
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While lavage may be useful, induction of emesis should be regarded with great caution because the rapid 

onset of central nervous system depression may lead to aspiration. Although of unproven value, 

administration of a charcoal slurry, aqueous or mixed with saline cathartic or sorbitol, has also been 

suggested as a way to stimulate fecal excretion of the chemical before it is completely absorbed by the 

body (HSDB 2007; Kunisaki and Augenstein 1994; Stutz and Janusz 1988). Diazepam and phenytoin 

may be helpful in controlling seizures (Bronstein and Currance 1988; HSDB 2007; Stutz and Janusz 

1988). Administration of epinephrine or other catecholamines has not been recommended because of the 

possibility of myocardial sensitization and subsequent arrhythmia (Haddad et al. 1998; HSDB 2007; 

Nahum and Hoff 1934). 

3.11.2 Reducing Body Burden 

Following absorption into the blood, benzene is rapidly distributed throughout the body. Since benzene is 

lipophilic, it is preferentially distributed to lipid-rich tissues. The initial stage of benzene metabolism is 

the formation of benzene oxide via P-450 mixed-function oxidases. Detoxification pathways generally 

involve the formation of glutathione conjugates of benzene oxide and glucuronide or sulfate conjugates of 

phenol or its subsequent metabolites, catechol, hydroquinone, and trihydroxybenzene. Other metabolites 

of benzene also have known toxic effects. Exhalation is the main route for excretion of unmetabolized 

benzene, while metabolized benzene is excreted primarily in the urine. Studies in humans and animals 

indicate that both exhalation and urinary excretion occur in several phases, with half-lives of minutes 

to hours. Hence, benzene and its metabolites have relatively short half-lives in the body, and while some 

of these metabolites are clearly toxic, accumulation of substantial body burdens are not expected. 

No methods are currently used for reducing the body burden of benzene. It is possible that methods could 

be developed to enhance the detoxification and elimination pathways, such as ensuring sufficient 

glutathione stores in the body by the administration of N-acetyl-L-cysteine. 

3.11.3 Interfering with the Mechanism of Action for Toxic Effects 

Administration of indomethacin, a nonsteroidal anti-inflammatory drug, has been shown to prevent 

benzene-induced myelotoxicity in mice and the accompanying increase in prostaglandin E in the bone 

marrow (Kalf et al. 1989; Renz and Kalf 1991). Co-administration of indomethacin also prevented an 

increase in the number of micronucleated polychromatic erythrocytes in peripheral blood. The authors 

suggest that these results suggest a role for prostaglandin synthetase in benzene-induced myelotoxic and 

genotoxicity, and a way to interfere with that process with substances such as indomethacin. 
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Prostaglandins have been shown to inhibit hematopoiesis (Kalf et al. 1989). Additionally, prostaglandin 

synthetase could be involved in the oxidation of phenol and/or hydroquinone to toxic metabolites (Kalf et 

al. 1989). 

The use of indomethacin to block benzene toxicity has led to data that indicate that myelotoxicity may 

involve the destruction of stromal macrophages that produce IL-1, a cytokine essential for hematopoiesis 

(Renz and Kalf 1991). External administration of recombinant IL-1 to mice prior to benzene 

administration prevents the myelotoxicity, presumably by providing a source of the cytokine (Renz and 

Kalf 1991). Further indication that IL-1 is affected by benzene exposure comes from the work of 

Carbonnelle et al. (1995), who showed that exposure of human monocytes to micromolar amounts of 

hydroquinone for 2 hours resulted in significantly decreased secretion of IL-1α and IL-1β at 

concentrations of 5 μM and above. RNA and protein synthesis were also inhibited. Additional research 

in this area indicates that tumor necrosis factor may provide protection against the inhibitory effects of 

hydroquinone on human hematopoietic progenitor cells (Colinas et al. 1995). The research of Shankar et 

al. (1993) determined that pretreatment with Protein A, a glycoprotein that acts as a multipotent 

immunostimulant, modulated the toxicity of benzene. Groups of six female albino rats (Swiss Wistar) 

were injected intraperitoneally with 1.0 mL/kg/body weight (879 mg/kg) benzene once daily for 

3 consecutive days. Another group (six per group) was administered intravenously with 60 μg/kg Protein 

A twice weekly for 2 weeks and then injected with 879 mg/kg benzene intraperitoneally once daily for 

3 consecutive days. Controls were injected with normal saline. All of the animals were killed 24 hours 

after receiving the last benzene injection. Blood was collected from the jugular vein for enumeration of 

total leukocyte counts. Routine autopsy was performed on all animals and the liver, thymus, spleen, and 

kidney organs were collected for organ weights. In benzene-only treated animals, there was a significant 

decrease in the total leukocyte counts in the peripheral blood as well as a significant decrease in the 

number of lymphocytes, a decrease in the gross organ weights of thymus and spleen, a significant 

increase in the iron content and lipid peroxidation of the liver and bone marrow, and an increase in low 

molecular weight iron in the bone marrow. Pretreatment with Protein A prevented these parameters from 

changing. 

3.12 ADEQUACY OF THE DATABASE 

Section 104(I)(5) of CERCLA, as amended, directs the Administrator of ATSDR (in consultation with the 

Administrator of EPA and agencies and programs of the Public Health Service) to assess whether 

adequate information on the health effects of benzene is available. Where adequate information is not 
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3. HEALTH EFFECTS 

available, ATSDR, in conjunction with the National Toxicology Program (NTP), is required to assure the 

initiation of a program of research designed to determine the health effects (and techniques for developing 

methods to determine such health effects) of benzene. 

The following categories of possible data needs have been identified by a joint team of scientists from 

ATSDR, NTP, and EPA. They are defined as substance-specific informational needs that if met would 

reduce the uncertainties of human health assessment. This definition should not be interpreted to mean 

that all data needs discussed in this section must be filled. In the future, the identified data needs will be 

evaluated and prioritized, and a substance-specific research agenda will be proposed. 

3.12.1 Existing Information on Health Effects of Benzene 

The existing data on health effects of inhalation, oral, and dermal exposure of humans and animals to 

benzene are summarized in Figure 3-6. The purpose of this figure is to illustrate the existing information 

concerning the health effects of benzene. Each dot in the figure indicates that one or more studies provide 

information associated with that particular effect. The dot does not necessarily imply anything about the 

quality of the study or studies, nor should missing information in this figure be interpreted as a “data 

need”. A data need, as defined in ATSDR’s Decision Guide for Identifying Substance-Specific Data 

Needs Related to Toxicological Profiles (Agency for Toxic Substances and Disease Registry 1989), is 

substance-specific information necessary to conduct comprehensive public health assessments. 

Generally, ATSDR defines a data gap more broadly as any substance-specific information missing from 

the scientific literature. 

Virtually all of the information regarding health effects in humans comes from studies of workers 

exposed to benzene-containing solvents and/or adhesives. Exposures to benzene occurred at rotogravure 

printing shops; at shoe, rubber, and raincoat manufacturing plants; and during chemical manufacturing 

processes. Case reports and cohort studies describe both acute and chronic health effects. The 

predominant route of exposure in these studies is inhalation. Dermal contact is also suspected as a 

possible route of exposure in these studies. 

As seen in Figure 3-6, inhalation information for humans is available regarding death; acute-, 

intermediate-, and chronic-duration systemic effects; immunologic, neurologic, reproductive, 

developmental, and genotoxic effects; and cancer. However, as mentioned above, human exposure to 
Inhalation 

Oral 

Dermal 

Death

Acute 

Intermediate 

Chronic 

Immunologic/Lymphoretic 

Neurologic 

Reproductive 

Developmental 

Genotoxic 

Cancer 

Systemic 

Human 

BENZENE 220 

3. HEALTH EFFECTS 

Figure 3-6. Existing Information on Health Effects of Benzene 

Inhalation 

Oral 

Dermal 

Existing Studies 

Death

Acute 

Intermediate 

Chronic 

Immunologic/Lymphoretic 

Neurologic 

Reproductive 

Developmental 

Genotoxic 

Cancer 

Systemic 

Animal 
BENZENE 221 

3. HEALTH EFFECTS 

benzene in specific work environments probably occurs not only by inhalation, but also by the dermal 

route. Limited information is available regarding direct skin contact with benzene by humans. 

Additionally, oral studies in humans are limited to isolated case reports of death and acute-duration 

systemic effects subsequent to accidental or intentional ingestion of benzene, although one study (Hunting 

et al. 1995) described effects in vehicle maintenance workers who siphoned gasoline by mouth. There is 

limited information on effects of dermal exposure of humans to benzene, including death, acute-duration 

effects, and cancer. 

Inhalation and oral studies in animals provide data on death; systemic effects after acute-, intermediate-, 

and chronic-duration exposure; and immunologic, neurologic, reproductive, developmental, genotoxic, 

and cancer effects. Furthermore, data exist regarding acute- and intermediate-duration systemic effects 

and cancer in animals after dermal exposure to benzene. 

3.12.2 Identification of Data Needs 

Acute-Duration Exposure. There are reports on the health effects resulting from acute exposure of 

humans and animals to benzene via the inhalation, oral, and dermal routes. The primary target 

organs for acute exposure are the hematopoietic system, nervous system, and immune system. Acute 

effects on the nervous system and immune system are discussed below under Neurotoxicity and 

Immunotoxicity. Information is also available for levels that cause death in humans (e.g., Cronin 1924; 

Flury 1928; Greenburg 1926; Tauber 1970; Thienes and Haley 1972) and in animals (e.g., Cornish and 

Ryan 1965; Drew and Fouts 1974; Smyth et al. 1962) following inhalation and oral exposures. 

No acute human or animal data on hematological effects from oral or dermal exposure are available. 

However, there are acute inhalation data that characterize the effects of benzene on the hematological 

system in humans and animals. Data regarding effects on the human hematological system following 

acute inhalation exposure to benzene are scant, but indicate leukopenia, anemia, and thrombocytopenia 

after more than 2 days of occupational exposure to more than 60 ppm benzene (Midzenski et al. 1992). 

Data for hematological effects in animals after acute-duration inhalation exposure are more extensive. 

Changes in peripheral erythrocytes (Chertkov et al. 1992; Cronkite et al. 1985; Rozen et al. 1984; Ward et 

al. 1985), in peripheral leukocytes (Aoyama et al. 1986; Chertkov et al. 1992; Gill et al. 1980; Green et al. 

1981b; Li et al. 1986; Ward et al. 1985; Wells and Nerland 1991), and in bone marrow cells (Chertkov et 

al. 1992; Corti and Snyder 1996; Cronkite et al. 1989; Dempster and Snyder 1991; Gill et al. 1980; Green 

et al. 1981b; Neun et al. 1992; Toft et al. 1982) were seen in rats and mice. An acute-duration inhalation 
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MRL of 0.009 ppm was determined based on the LOAEL for immunologic effects in the mouse (Rozen et 

al. 1984) (discussed below under Immunotoxicity). No acute-duration oral studies were suitable for 

deriving MRLs. Additional studies that include dose-response information on hematological effects 

following acute oral exposure could be designed to provide information that could be useful in deriving 

an acute-duration oral MRL for benzene. Such studies could be designed to serve as validation for 

existing PBPK models. Acute dermal exposure at levels that are likely to be found in the environment 

and at hazardous waste sites is not likely to cause adverse health effects. 

Intermediate-Duration Exposure. There is sufficient information in humans and animals to 

identify the hematopoietic, nervous, and immunological systems as targets for benzene toxicity. The 

effects on the nervous system and immune system are discussed in the sections below titled Neurotoxicity 

and Immunotoxicity. Data on adverse hematological effects in humans are available following 

intermediate-duration exposures to benzene in the workplace (Aksoy and Erdem 1978; Aksoy et al. 

1972). However, the exposure levels and durations were not well defined and, therefore, could not be 

used to calculate an MRL. Studies in rats and mice following inhalation exposure can be used to define 

NOAELs and LOAELs for hematological and immunological effects (e.g., Baarson et al. 1984; Cronkite 

et al. 1982, 1985, 1989; Dow 1992; Farris et al. 1993, 1997a, 1997b; Green et al. 1981a, 1981b; Luke et 

al. 1988b; Plappert et al. 1994a, 1994b; Rosenthal and Snyder 1987; Seidel et al. 1989; Snyder et al. 

1978a, 1980; Toft et al. 1982; Vacha et al. 1990; Ward et al. 1985; Wolf et al. 1956). An intermediateduration inhalation MRL of 0.006 ppm was derived, based on a LOAEL for immunologic effects in the 

mouse (Rosenthal and Snyder 1987) (discussed below under Immunotoxicity). Data on hematological 

effects from oral exposures in animals are also available (Hsieh et al. 1988b, 1990, 1991; NTP 1986; 

Shell 1992; Wolf et al. 1956), but no intermediate-duration oral MRL could be derived because the 

threshold for hematological (and immunological) effects could not be identified. Additional studies that 

include dose-response information on hematological effects following intermediate-duration oral 

exposure could be designed to provide information that could be useful in deriving an intermediateduration oral MRL for benzene. Such studies could be designed to serve as validation for existing PBPK 

models. Intermediate-duration dermal exposure at levels that are likely to be found in the environment 

and at hazardous waste sites is not likely to cause adverse health effects. 

Chronic-Duration Exposure and Cancer. The primary target for adverse systemic effects of 

benzene following chronic exposure is the hematological system. Hematotoxicity was reported in studies 

of humans chronically exposed to benzene in the workplace air (Aksoy and Erdem 1978; Aksoy et al. 

1971, 1972, 1974, 1987; Cody et al. 1993; Dosemeci et al. 1996; Doskin 1971; Erf and Rhoads 1939; 
BENZENE 223 

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Goldwater 1941; Greenburg et al. 1939; Kipen et al. 1989; Lan et al. 2004a, 2004b; Li et al. 1994; Qu et 

al. 2002, 2003a, 2003b; Rothman et al. 1996a, 1996b; Townsend et al. 1978; Ward et al. 1996; Yin et al. 

1987c). Several studies of occupational inhalation exposure to benzene did not find clinically-defined 

hematological effects (Collins et al. 1991, 1997; Tsai et al. 1983, 2004), but they had a high degree of 

uncertainty regarding estimations of benzene exposure levels. The study of workers of shoe 

manufacturing industries in Tianjin, China (Lan et al. 2004a, 2004b) identified the lowest LOAEL for 

hematotoxicity and was selected as the principal study for deriving a chronic-duration inhalation MRL of 

0.003 ppm for benzene. Chronic-duration animal studies are available for hematological effects 

following inhalation exposure and provide support to the human data (Snyder et al. 1978a, 1980, 1982, 

1984). No human data are available to evaluate hematological effects following oral exposure. Although 

chronic duration oral animal studies are available for hematological effects (Huff et al. 1989; Maltoni et 

al. 1983, 1985; NTP 1986), the most extensive study (NTP 1986) did not conclusively define a NOAEL 

or less serious LOAEL for end points that could be used to derive an MRL. However, a chronic-duration 

oral MRL of 0.0005 mg/kg/day was derived for benzene based on route-to-route extrapolation of the same 

results (Lan et al. 2004a, 2004b) that served as the basis for the chronic-duration inhalation MRL. 

Additional chronic-duration oral animal data could be designed to provide support to the chronic-duration 

oral MRL and to assist in defining threshold levels for populations living near hazardous waste sites. 

Dermal data for humans and animals were not available. However, chronic-duration dermal exposure at 

levels that are likely to be found in the environment and at hazardous waste sites is not likely to cause 

adverse health effects. 

EPA, IARC, and the Department of Health and Human Services have concluded that benzene is a human 

carcinogen based on sufficient data in humans supported by animal evidence (IARC 2004; IRIS 2007; 

NTP 2005). Epidemiological studies and case reports provide clear evidence of a causal relationship 

between occupational exposure to benzene and the occurrence of acute myelogenous leukemia (AML) 

(Hayes et al. 1997; IARC 1982, 1987; EPA 1995, 1998; IRIS 2007; Rinsky et al. 1987, 2002; Yin et al. 

1996a, 1996b), as well as is suggestive evidence of associations between benzene and non-Hodgkin’s 

lymphoma (NHL) and multiple myeloma (Hayes et al. 1997; Rinsky et al. 1987). Studies of workers in 

Ohio (the Pliofilm study) (e.g., Rinsky et al. 1981, 1987; 2002) and China (the NCI/CAPM study) (e.g., 

Hayes et al. 1997; Yin et al. 1996a, 1996b) provide the strongest data on the leukemogenic potential of 

benzene, including exposure-response information, and data from the Pliofilm study was used as the basis 

of inhalation and oral cancer risk values for benzene (EPA 1998; IRIS 2007). Additional studies on these 

and other cohorts could better characterize exposure level and exposure duration relationships for benzene 
BENZENE 224 

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and leukemia, particularly at low levels of exposure, and clarify the potential of benzene to induce NHL 

and multiple myeloma. 

Benzene is a multiple site carcinogen in rats and mice following inhalation exposure (Cronkite 1986; 

Cronkite et al. 1984, 1985, 1989; Farris et al. 1993; Maltoni et al. 1982a, 1982b, 1983, 1985, 1989; 

Snyder et al. 1980) and oral exposure (Huff et al. 1989; Maltoni et al. 1983, 1985, 1989; NTP 1986), 

inducing lymphomas and other neoplasms in numerous tissues not affected in humans. Although 

contributing to the weight of evidence for carcinogenicity, the animal studies do not identify a suitable 

model for leukemia in humans. An appropriate animal model would help to provide a better 

understanding of how benzene causes cancer, particularly the mechanism of benzene leukemogenesis. 

The exact mechanism of benzene carcinogenicity is not known, but it has been postulated that some 

benzene metabolites are capable of forming adducts with DNA and are responsible for reduced immune 

function which could potentially lead to cancer. The clastogenic properties of benzene may play a role in 

its carcinogenicity. DNA adduct formation could occur with both inhalation and oral exposures (Ding et 

al. 1983; Sasiadek et al. 1989). Questions that need to be answered with regard to the mechanism of 

benzene carcinogenesis include how benzene metabolites produce greater-than-additive effects, 

determination of the critical target genes, whether aplastic anemia is essential to the development of 

leukemia, and determination of the role of cytokines and growth factor pathways in benzene toxicity. 

Genotoxicity. Evidence for the genotoxicity of benzene in humans comes from studies of chronicallyexposed populations (Andreoli et al. 1997; Bogadi-Šare et al. 1997; Ding et al. 1983; Forni and Moreo 

1967, 1969; Forni et al. 1971a; Hallberg et al. 1996; Hartwich et al. 1969; Hedli et al. 1991; Karacic et al. 

1995; Kašuba et al. 2000; Liu et al. 1996; Major et al. 1992, 1994; Nilsson et al. 1996; Picciano 1979; 

Pitarque et al. 1996, 1997; Popp et al. 1992; Qu et al. 2003a, 2003b; Rothman et al. 1995; Sardas et al. 

1994; Sasiadek et al. 1989; Sellyei and Kelemen 1971; Smith et al. 1998; Sul et al. 2002; Tompa et al. 

1994; Tough and Court Brown 1965; Tough et al. 1970; Türkel and Egeli 1994; Van den Berghe et al. 

1979; Yardley-Jones et al. 1990; Zhang et al. 1998b, 1999). These exposures have occurred primarily via 

inhalation, although some dermal exposure cannot be ruled out. In spite of the lack of accurate exposure 

data, exposure to multiple chemicals, and often inappropriate control groups, the association between 

benzene exposure and the appearance of structural and numerical chromosome aberrations in human 

lymphocytes suggests that benzene can be considered a human clastogen. Benzene-induced cytogenetic 

effects, including chromosome and chromatid aberrations, sister chromatid exchanges, and micronuclei, 

have been consistently found in in vivo animal studies (Anderson and Richardson 1981; Au et al. 1991; 

Chen et al. 1994; Chang et al. 2005; Eastmond et al. 2001; Erexson et al. 1986; Farris et al. 1996; Fujie et 
BENZENE 225 

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al. 1992; Healy et al. 2001; Lee et al. 2005; Kolachana et al. 1993; Ranaldi et al. 1998; Siou et al. 1981; 

Toft et al. 1982; Ward et al. 1992). Binding of benzene and/or its metabolites to DNA, RNA, and 

proteins has been consistently observed in rats and mice (Arfellini et al. 1985; Creek et al. 1997; Lévay et 

al. 1996; Mani et al. 1999; Mazullo et al. 1989; Turteltaub and Mani 2003). Inhalation exposure of mice 

has yielded identifiable benzene-derived hemoglobin adducts (Sabourin et al. 1990). Ward et al. (1992) 

have shown that intermediate-duration exposure of mice to benzene by inhalation at levels below the 

current PEL may induce gene mutations in the lymphocytes. 

Standard short-term in vitro assays indicate that benzene's genotoxicity is derived primarily from its 

metabolites, although benzene has been shown to produce DNA breaks in Chinese hamster ovary cells 

independent of metabolic activators (Douglas et al. 1985; Eastmond et al. 1994; Lakhanisky and 

Hendrickx 1985; Zhang et al.1993). There is good evidence that benzene affects cell cycle progression, 

RNA and DNA synthesis, as well as DNA binding (Forni and Moreo 1967, 1969; Hartwich et al. 1969; 

Sellyei and Kelemen 1971; Van den Berghe et al. 1979). Chen and Eastmond (1995) showed that 

benzene metabolites can adversely affect human topoisomerases (enzymes involved in DNA replication 

and repair); however, evidence exists that some repair of DNA binding with benzene metabolites occurs 

in human cells (Chenna et al. 1995). 

The genotoxicity of benzene has been extensively studied and demonstrated in both humans and 

laboratory animals. Although some information is available regarding possible mechanisms of benzene 

genotoxicity and carcinogenicity, additional mechanistic studies would be helpful to more completely 

characterize mechanisms responsible for these effects. 

Reproductive Toxicity. Reproductive data are available on women occupationally exposed to 

benzene (Mukhametova and Vozovaya 1972; Vara and Kinnunen 1946). The data suggest spontaneous 

abortions, menstrual disturbances, and ovarian atrophy. These studies are limited by the difficulty in 

identifying appropriate controls, problems in controlling for concomitant exposures to other chemicals, 

and inadequate follow-up. Only one study was found that described the reproductive effects on men 

exposed to benzene (Stucker et al. 1994). There are some data available from inhalation studies on 

reproductive effects of benzene in animals. Although there are data on adverse gonadal effects (e.g., 

atrophy/degeneration, decrease in spermatozoa, moderate increases in abnormal sperm forms) (Ward et 

al. 1985; Wolf et al. 1956), data on reproductive outcomes are either negative (Coate et al. 1984; Green et 

al. 1978; Kuna et al. 1992; Murray et al. 1979; Tatrai et al. 1980a) or inconclusive or conflicting (i.e., 

number of live fetuses, incidences of pregnancies) (Murray et al. 1979; Ungvary and Tatrai 1985). 
BENZENE 226 

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Gofmekler (1968) showed infertility in female rats after intermediate-duration inhalation exposure to 

210 ppm, but the results are poorly reported. Thus, they provide only suggestive evidence that benzene 

may have an adverse effect on reproductive outcomes. No data were located on reproductive effects 

following oral exposure to benzene in humans. Negative effects on reproductive outcome have been 

reported in one oral study in animals (Exxon 1986). NTP (1986) reported neoplastic changes in the 

reproductive tissues of female rats and male and female mice after chronic oral exposure. Given the 

paucity of available data across all exposure routes, it would be useful to have additional 90-day studies 

conducted by the oral and inhalation routes that assess reproductive organs histologically or 

cytogenetically. Although dermal contact is not likely to be the most relevant route of exposure for 

humans at hazardous waste sites, data on dermal exposure would also be useful. If the results of the 

suggested inhalation or oral studies indicate reproductive toxicity, multigeneration or continuous breeding 

studies for oral and inhalation exposures would help clarify the potential for benzene to cause 

reproductive effects in humans. 

Developmental Toxicity. Based on epidemiological studies at hazardous waste sites, an increased 

susceptibility to benzene of pregnant women or their offspring has not been demonstrated (Budnick et al. 

1984; Goldman et al. 1985; Heath 1983; Olsen 1983). However, these studies have several limitations 

that make it impossible to assess the effect of benzene on the human fetus. For example, the few studies 

that do exist are limited by a lack of control incidences for end points, problems in identifying exposed 

populations, a lack of data on exposure levels, and/or exposure to multiple substances (Budnick et al. 

1984; Forni et al. 1971a; Funes-Cravioto et al. 1977; Goldman et al. 1985; Heath 1983; Olsen 1983). In 

the occupational setting, however, there may be stronger evidence of increased susceptibility to benzene 

of pregnant women and/or their offspring (Forni et al. 1971a; Funes-Cravioto et al. 1977). For example, 

severe pancytopenia and increased chromosomal aberrations occurred in a pregnant worker exposed to 

benzene (levels not reported) during the entire pregnancy. She gave birth to a healthy boy (Forni et al. 

1971b). On the other hand, increased frequency of chromatid and isochromatid breaks and sister 

chromatid exchanges was found in lymphocytes from 14 children of female workers exposed to benzene 

(levels not reported) and other organic solvents during pregnancy (Funes-Cravioto et al. 1977). 

Irrespective of the hematological effects reported in the pregnant worker or the 14 children of female 

workers, the lack of complete and detailed medical records and the lack of follow-up limit the 

significance of effects with regard to post exposure morbidity. 

A number of investigations have evaluated the developmental/maternal toxicity of benzene in animals 

following inhalation exposures. In these investigations, fetotoxicity was evidenced by reduced fetal 
BENZENE 227 

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weight and/or minor skeletal variations at concentrations 47 ppm (Coate et al. 1984; Green et al. 1978; 

Kimmel and Wilson 1973; Kuna and Kapp 1981; Murray et al. 1979; Tatrai et al. 1980a, 1980b; Ungvary 

and Tatrai 1985). However, persistently altered fetal hematopoiesis occurred in mice at 20 ppm (Keller 

and Snyder 1986, 1988). Additional information designed to assess the hematopoietic system of the 

developing fetus (human or animal) following low-level in utero exposures to benzene is needed. Oral 

data are limited to two animal studies in which benzene was shown to reduce pup body weight when mice 

were administered a high single oral dose (Seidenberg et al. 1986) or have no effect after gestational 

exposure (Exxon 1986). Additional oral studies designed to assess the developmental effects (human or 

animal) of low-level exposures to benzene would be useful. No data are available on the developmental 

toxicity of benzene following dermal exposure. Although these data would be useful, the most likely 

routes of exposure for humans at hazardous waste sites are the inhalation and oral routes. 

Immunotoxicity. The immune system is known to be a target for benzene toxicity. The effects do not 

appear to be route- or species-specific. The evidence for immunotoxicity in humans comes from workers 

exposed by inhalation of intermediate and chronic durations. A series of studies demonstrated decreases 

in numbers of circulating leukocytes (Aksoy et al. 1971, 1972, 1974), but levels of exposure were not 

well documented. In other studies, alterations in human serum immunoglobulins were observed, but there 

was concomitant exposure to xylene and toluene (Lange et al. 1973a, 1973b). There is a need to test for 

subtle alterations in the immune system and immune competence in workers with intermediate- and 

chronic-duration exposure to benzene. Animal studies support the findings of immune dysfunction and 

indicate that additional parameters of the immune system are affected by exposure to benzene in the air 

for acute (Aoyama 1986; Chertkov et al. 1992; Cronkite 1986; Cronkite et al. 1982, 1985, 1989; Gill et al. 

1980; Green et al. 1981a; Li et al. 1986; Neun et al. 1992; Rozen et al. 1984; Rosenthal and Snyder 1985; 

Toft et al. 1982; Ward et al. 1985; Wells and Nerland 1991), intermediate (Baarson et al. 1984; Cronkite 

et al. 1982, 1985, 1989; Dow 1992; Gill et al. 1980; Green et al. 1981a, 1981b; Li et al. 1992; Plappert et 

al. 1994b; Rosenthal and Snyder 1987; Rozen and Snyder 1985; Seidel et al. 1990; Snyder et al. 1980; 

Stoner et al. 1981; Ward et al. 1985; Wolf et al. 1956; Songnian et al. 1982), and chronic (Snyder et al. 

1980, 1984, 1988) periods. An acute-duration inhalation study found depressions of proliferative 

responses of bone-marrow-derived B-cells and splenic T-cells in mice at 10 ppm (Rozen et al. 1984), and 

an acute-duration inhalation MRL has been derived based on this study. An intermediate-duration 

inhalation study found delayed splenic lymphocyte reaction to foreign antigens evaluated by in vitro 

mixed lymphocyte culture following exposure of mice to benzene vapors at a concentration of 10 ppm 

(Rosenthal and Snyder 1987), and an intermediate-duration inhalation MRL has been derived based on 

this study. No human data on the oral route were available, but animal data showed immunological 
BENZENE 228 

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effects after intermediate (Fan 1992; Hsieh et al. 1988b, 1990, 1991; Shell 1992; Wolf et al. 1956) and 

chronic exposures (Huff et al. 1989; Maltoni et al. 1983, 1985; NTP 1986) via the oral route. Since the 

immune system is known to be a target organ, it is important to have information on subtle alterations in 

immune competence in people exposed to benzene in the air and in the drinking water. A decrease in 

cell-mediated immune functions, including alloantigen response and cytotoxicity, was reported in mice 

following intermediate inhalation exposure to 100 ppm of benzene (Rosenthal and Snyder 1987). This 

impaired cell-mediated immune function was also apparent in other in vivo studies (Stoner et al. 1981). 

Mice exposed to 100 ppm of benzene for a total of 100 days had reduced tumor resistance when 

challenged with syngeneic tumor cells and developed tumors that were lethal (Rosenthal and Snyder 

1987). Further animal studies would also be useful in defining more NOAEL and LOAEL values. No 

data are available that document immunotoxicity in humans or animals exposed by dermal application. 

Dermal sensitization tests may also provide useful data on the likelihood of an allergic response in 

humans, since skin contact may occur in the workplace and at hazardous waste sites. 

Neurotoxicity. In humans, the nervous system is a target of benzene toxicity following both inhalation 

and oral exposures. No data are available that demonstrate neurologic effects in humans or animals 

exposed dermally. There are sufficient data to suggest that it is the central nervous system which is 

affected following acute exposures. Neurological symptoms reported in humans following acute oral and 

inhalation exposures are similar and include drowsiness, dizziness, headache, vertigo, tremor, delirium, 

and loss of consciousness (Cronin 1924; Flury 1928; Greenburg 1926; Midzenski et al. 1992; Tauber 

1970; Thienes and Haley 1972). Acute- and intermediate-duration inhalation and oral animal studies 

provide supportive evidence that benzene effects the central nervous system (Carpenter et al. 1944; 

Cornish and Ryan 1965; Evans et al. 1981; Frantik et al. 1994). Effects observed include narcosis, 

hyperactivity, tremors, tonic-clonic convulsions, decreased evoked electrical activity in the brain, and 

slight nervous system depression. Behavioral changes were found in mice after 1 week of exposure to 

300 ppm, and decreased grip strength was found after three exposures to 1,000 ppm (Dempster et 

al. 1984). An intermediate-duration inhalation study found increased rapid response in mice at 0.78 ppm 

(Li et al. 1992), but this study was limited due to apparent discrepancies between reported and actual 

benzene exposure levels, since neurological effects reported in other animal studies occurred only at 

much higher exposure levels. Acute-duration oral exposures of 950 mg/kg benzene altered neurotransmitter levels in the brains of rats (Kanada et al. 1994). Intermediate-duration oral exposures of 

8 mg/kg/day resulted in changes in the levels of monoamine transmitters in the brain without treatmentrelated behavioral changes (Hsieh et al. 1988a). 
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One chronic-duration occupational study reported neurological abnormalities of the peripheral nervous 

system (global atrophy of lower extremities and distal neuropathy of upper extremities) in four of six 

benzene-exposed patients with aplastic anemia (Baslo and Aksoy 1982). Additional studies are needed to 

verify the peripheral nervous system effects that might occur following chronic exposures to low doses of 

benzene. 

Although there are sufficient data to indicate that the nervous system is a target of benzene toxicity, the 

neurotoxicity of benzene has not been extensively studied. Additional studies in animals are needed to 

identify the thresholds of neurotoxicity following acute-, intermediate-, and chronic-duration inhalation 

and oral exposures. 

Epidemiological and Human Dosimetry Studies. A large segment of the U.S. population is 

exposed to benzene. This exposure occurs primarily as a result of benzene emitted to the air from tobacco 

smoke, gasoline stations, and automobile exhaust. Benzene has been found in about a third of the NPL 

hazardous waste sites. The magnitude of exposure is greater for those occupationally exposed. 

The predominance of available epidemiological data comes from occupational studies and associated 

analysis (Aksoy and Erdem 1978; Aksoy et al. 1971, 1972, 1974; Baslo and Aksoy 1982; Ciccone et al. 

1993; Cody et al. 1993; Dosemeci et al. 1994, 1996; Doskin 1971; EPA 1986, 1995, 1998; Erf and 

Rhoads 1939; Goldwater 1941; Greenburg et al. 1939; Hayes et al. 1997; IARC 1982, 1987; Infante 1978; 

Infante et al. 1977; IRIS 2007; Kipen et al. 1989; Lan et al. 2004a, 2004b; Li et al. 1994; Ott et al. 1978; 

Paustenbach et al. 1992; Paxton et al. 1994a, 1994b; Qu et al. 2002, 2003a, 2003b; Rinsky et al. 1981, 

1987, 2002; Rothman et al. 1996a, 1996b; Townsend et al. 1978; Travis et al. 1994; Utterback and Rinsky 

1995; Ward et al. 1996; Yin et al. 1987c, 1989, 1994, 1996a, 1996b). While these studies provide data on 

hematological and neurotoxic effects and evidence of carcinogenicity, they only offer information on the 

effects of inhalation exposure. Retrospective and prospective studies of populations that have been 

identified as being exposed to contaminated groundwater or drinking water could provide information on 

long-term health effects from oral exposures. The Benzene Subregistry Baseline Technical Report of the 

National Exposure Registry contains information on 1,143 persons who had documented exposure to 

benzene in their drinking water and were exposed for at least 30 days (Agency for Toxic Substances and 

Disease Registry 2001). No causal relationship has been proposed for health conditions identified in the 

base subregistry (Agency for Toxic Substances and Disease Registry 1995) or continued follow-up of the 

population (Agency for Toxic Substances and Disease Registry 2001). 
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Few well-conducted epidemiological studies of exposed populations exist. An epidemiological study of a 

population with chronic exposure to two NPL hazardous waste sites illustrates the problems inherent in 

assessing adverse health effects from waste site exposure (Dayal et al. 1995). Benzene was only 1 of 

20 compounds or groups of compounds identified as being chemicals of concern in the waste site. The 

authors note that assessment of long-term exposure to a dumpsite is problematic because of a lack of 

history of materials deposited, the complexity of the waste components, the interaction of the components 

over time, and the emphasis on statistically significant versus biologically significant effects. The study 

had certain limitations, including lifestyle characteristics, the use of mailed-in responses, subjective 

symptom-reports without independent confirmation, and various problems with exposure estimation. A 

relationship was detected between exposed individuals and neurological symptoms, including learning 

difficulties, nervousness, numbness in fingers or toes, sleeping difficulties, unusual fatigue, and decreased 

sense of smell. Chest pains, irregular heartbeat, skin rashes, and detection of a peculiar odor or taste also 

occurred more often in the exposed population. However, no effort was made to relate the symptoms to 

particular chemicals in the waste site. Clearly, more studies of exposed populations surrounding waste 

sites would be warranted. Other studies include analyses of workers in China (Dosemeci et al. 1994, 

1996; Lan et al. 2004a, 2004b; Qu et al. 2002, 2003a, 2003b; Rothman et al. 1996a, 1996b; Yin et al. 

1987c, 1994), reanalysis of data from the Pliofilm cohort (Paxton et al. 1994a, 1994b; Utterback and 

Rinsky 1995; Ward et al. 1996), and assessment of clinical manifestations of benzene hematotoxicity in 

U.S. industries with potential for benzene exposure (Collins et al. 1991, 1997; Tsai et al. 1983, 2004). A 

publication by Lioy and Pellizzari (1995) outlines an appropriate framework for the National Human 

Exposure Assessment Survey, using benzene as a candidate compound. Other articles also address 

population-based exposure models and measurements (MacIntosh et al. 1995; Pellizzari et al. 1995). 

Analysis of benzene dosimetry in animal studies suggests that the metabolic pathways leading to the 

production of the putative toxic metabolites appear to be low-capacity, high-affinity pathways that are 

saturated at relatively low-exposure concentrations (Henderson et al. 1992). The authors suggest that this 

could also be true for humans. Additional dosimetry studies are necessary to further define the role of 

toxic metabolites in the development of benzene-related leukemia. 

Studies of DNA damage have primarily been performed on workers with preexisting hematological 

effects and cancer (Ding et al. 1983; Forni et al. 1971a; Picciano 1979; Yardley-Jones et al. 1988) or as 

biomonitoring of benzene exposure (Andreoli et al. 1997; Bogadi-Šare et al. 1997; Hallberg et al. 1996; 

Kašuba et al. 2000; Liu et al. 1996; Nilsson et al. 1996; Pitarque et al. 1996; Sul et al. 2002; Surrallés et 

al. 1997; Zhang et al. 1999). Additional data on benzene-induced DNA damage could be useful in 
BENZENE 231 

3. HEALTH EFFECTS 

monitoring possible adverse effects in individuals living near hazardous waste sites if more quantitative 

dose-response data were available for clastogenicity. 

Studies to determine whether benzene causes solid tumors and hematological malignancies other than 

acute nonlymphocytic leukemia in humans have been suggested as a data need (Goldstein and Warren 

1993). These data could be obtained by revisiting already established cohorts of exposed individuals, or 

by examining new cohorts. 

Biomarkers of Exposure and Effect. 

Exposure. There is no clinical disease state unique to benzene toxicity. However, the effects on the 

hematopoietic and immune systems are well recognized, and analytical methodologies exist for 

monitoring benzene levels in expired breath and blood (Brugnone et al. 1989, 1992; DeLeon and Antoine 

1985; Pellizzari et al. 1988). Several biomarkers of exposure to benzene exist, including unmetabolized 

benzene in expired air and urine (Farmer et al. 2005; Fustinoni et al. 2005; Ghittori et al. 1993; 

Nomiyama and Nomiyama 1974a, 1974b; Sherwood 1988; Srbova et al. 1950; Waidyanatha et al. 2001), 

urinary phenol levels (Astier 1992; Inoue et al. 1986, 1988b; Jongeneelen et al. 1987; Karacic et al. 1987; 

Pagnotto et al. 1961; Pekari et al. 1992), and urinary trans,trans-muconic acid and S-phenylmercapturic 

acid levels (Boogaard and van Sittert 1995, 1996; Ducos et al. 1990, 1992; Farmer et al. 2005; Inoue et al. 

1989b, 2000; Lee et al. 1993; Melikian et al. 1993, 1994; Pezzagno et al. 1999; Popp et al. 1994; Qu et al. 

2005; Rothman et al. 1998; Ruppert et al. 1997; Sanguinetti et al. 2001; van Sittert et al. 1993; Weaver et 

al. 2000). Estimates of benzene exposure can be made by comparing the ratio of inorganic to organic 

sulfates in the urine (Hammond and Hermann 1960). However, urinary sulfate levels are variable, and 

they have not been used to identify exposure levels of benzene associated with minimal toxic effect. 

Furthermore, benzene-derived adducts with hemoglobin and albumin may be useful as biomarkers 

(Bechtold et al. 1992a, 1992b; Bechtold and Henderson 1993; Farmer et al. 2005; Fustinoni et al. 2005; 

Ghittori et al. 1993; Hedli et al. 1991; Lutz and Schlatter 1977; Nomiyama and Nomiyama 1974a, 1974b; 

Pekari et al. 1992; Reddy et al. 1989; Schad et al. 1992; Sherwood 1988; Smith and Rothman 2000; 

Srbova et al. 1950; Waidyanatha et al. 2001; Yeowell-O’Connell et al. 1998, 2001). 

Phenol measurements have routinely been used for monitoring occupational exposures, and there is 

evidence that urinary phenol levels can be correlated with exposure levels (Astier 1992; Inoue et al. 1986; 

Pekari et al. 1992). Correlating urinary phenol with benzene exposure is complicated by high and 

variable background levels in nonexposed persons. The data suggest that variations in urinary phenol will 
BENZENE 232 

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obscure phenol formed from low levels of benzene (Ong and Lee 1994; Perbellini et al. 1988). For 

exposures to 1 ppm or less, the current workplace standard, urinary phenol is not an adequate assay to 

determine the extent of benzene exposure. In addition, urinary excretion of phenol is lowered by 

coexposure to toluene (Inoue et al. 1988b). In many exposure situations (e.g., occupational settings and 

hazardous waste sites), toluene and other chemicals are present and could interfere with the metabolism 

and elimination of benzene. Studies that attempt to identify another biomarker that is specific to benzene, 

such as S-phenylmercapturic acid, the proposed urinary BEI (ACGIH 1996b), may be helpful for medical 

surveillance. 

The metabolism of benzene to ring-opened compounds (e.g., muconic acid) may be pertinent to the 

development of a biomarker of benzene exposure and effect (Ducos et al. 1990, 1992; Inoue et al. 1989b; 

Lee et al. 1993; Melikian et al. 1993; Ong and Lee 1994; Witz et al. 1990b). However, the dose-response 

curve for metabolism of benzene to urinary muconic acid in humans needs to be determined. If urinary 

muconic acid increases in humans at low benzene exposures, as it does in mice, then it may be a valid 

biomarker of benzene exposure. Excretion of urinary muconic acid in humans has also been shown to be 

lowered by coexposure to toluene (Inoue et al. 1989b). The effect of coexposure to toluene and other 

chemicals on muconic acid formation needs to be assessed in order to determine the usefulness of 

muconic acid as a biomarker of benzene exposure. 

Breath levels of benzene have been used as a measure of exposure (Brugnone et al. 1989; Money and 

Gray 1989; Nomiyama and Nomiyama 1974a, 1974b; Ong and Lee 1994; Pekari et al. 1992). However, 

the amount of benzene lost in expired air will vary not only with the dose, duration, and time from 

exposure, but also with the extent of metabolism in the body. Benzene levels in blood have been 

measured. However, blood levels rapidly decrease after exposure (Brugnone et al. 1989; DeLeon and 

Antoine 1985; Schrenk et al. 1941). 

Benzene metabolites also form DNA adducts (Popp et al. 1992). The use of hemoglobin adducts formed 

from benzene metabolites as a biomarker of benzene exposure has been developed by Sun et al. (1990). 

Bechtold and colleagues have conducted further studies using this model (Bechtold et al. 1992a; Bechtold 

and Henderson 1993). Further refinement of this model would be useful to quantitate cumulative lowlevel exposures to benzene. Development of specific markers of exposure to use in epidemiological 

studies to clarify the shape of the dose-response curve in the low-dose region has been suggested as a data 

need (Goldstein and Warren 1993). Recent reports indicate that urinary benzene may serve as a sensitive 
BENZENE 233 

3. HEALTH EFFECTS 

biomarker of exposure to benzene concentrations well below 1 ppm (Farmer et al. 2005; Fustinoni et al. 

2005). 

Effect. Monitoring of benzene workers has included monthly blood counts. Toxic effects occur in the 

bone marrow and arise either from benzene via a solvent effect, or via its metabolites (Gad-El-Karim et 

al. 1985; Irons et al. 1980). Hematological tests could be used as markers of hematotoxicity, but medical 

laboratories lack tests specific to benzene hematotoxicity. The tests cannot be relied upon to find 

preclinical disease but can identify the subtle changes that are early indicators of effects. Furthermore, 

these markers of effect may not be useful for long periods following cessation of exposure, nor do they 

distinguish between acute and chronic exposures. As stated above, additional studies are needed to define 

the role of benzene-related leukopenia in the disease process initiated by benzene exposure, to determine 

if it is a biomarker of effect, or an intermediate end point in the development of leukemia (Hayes 1992). 

Absorption, Distribution, Metabolism, and Excretion. Data from both humans and animals 

consistently indicate that benzene is rapidly absorbed through the lungs (Eutermoser et al. 1986; 

Nomiyama and Nomiyama 1974a; Sabourin et al. 1987; Schrenk et al. 1941; Srbova et al. 1950; Yu and 

Weisel 1996). Although experimentally-acquired data are not available on oral absorption of benzene in 

humans, case reports of accidental or intentional poisoning suggest that benzene is rapidly absorbed from 

the gastrointestinal tract (Thienes and Haley 1972). The efficient absorption of oral doses in animals is 

well documented (Cornish and Ryan 1965; Parke and Williams 1953a; Sabourin et al. 1987). Benzene 

can be absorbed through the skin, but the rate of absorption is much lower than that for inhalation 

(Maibach and Anjo 1981; Susten et al. 1985; Tsuruta 1989). Following absorption into the body, benzene 

is widely distributed to tissues, with the relative uptake dependent on the perfusion of the tissue by blood, 

and the total potential uptake dependent on fat content and metabolism (Sato et al. 1975; Tauber 1970). 

There is no evidence to suggest that the route of administration has any substantial effect on the 

subsequent metabolism of benzene, either in humans or animals. Benzene is metabolized primarily in the 

liver and to a lesser extent, in the bone marrow. Benzene is a preferential substrate of CYP2E1, which 

also metabolizes alcohol. The induction of CYP2E1 by benzene (and some of its metabolites) with 

subsequent generation of reactive metabolites, oxygen radicals, circulating lipid peroxides, and hydroxyl 

radicals could be associated with hematopoietic toxicity and carcinogenicity of benzene (Irons 2000; 

Parke 1989; Ross 1996, 2000; Smith 1996a, 1996b; Snyder 2000a, 2000b, 2002; Snyder and Hedli 1996; 

Snyder and Kalf 1994). CYP2E1 is not confined to the liver, but has also been detected in bone marrow. 

Andrews et al. (1979) demonstrated that rabbit bone marrow is capable of metabolizing benzene. Schnier 
BENZENE 234 

3. HEALTH EFFECTS 

et al. (1989) subsequently found that rabbit bone marrow contains CYP2E1. Irons et al. (1980) 

demonstrated that benzene metabolism by rat bone marrow (in situ) was complete and independent of 

metabolism by the liver, with concentrations of phenol greater than catechol and hydroquinone. Although 

the total metabolism by bone marrow was limited (total metabolites present were 25% of those in blood), 

the concentration of metabolites in the bone marrow exceeded that in the blood. Similar studies have 

been conducted in mice (Ganousis et al. 1992). Benzene metabolism in bone marrow is not well 

understood; additional data regarding the initial oxidation step and the comparatively low levels of 

CYP2E1 activity in bone marrow would be useful in identifying the mechanisms of benzene's 

hematotoxicity. This aspect of metabolism may have implications for long-term exposures, which could 

be explored in chronic exposure studies. The intermediary metabolites of benzene are responsible for 

many of the toxic effects observed (Eastmond et al. 1987; Gad-El-Karim et al. 1985). Biotransformation 

is believed to be essential for benzene-induced bone marrow damage. 

However, there is disagreement as to whether benzene is activated in the marrow, activated elsewhere and 

transported to the marrow, or metabolized in the liver and the metabolites activated in the marrow. 

Further studies on the metabolism of benzene would help define its mechanism of action. Additionally, 

more information is needed on the pathways of metabolism in humans, the chemical nature of the toxic 

metabolites, and the mechanism of toxicity. Recently published data comparing urinary metabolite 

profiles of orally administered benzene and phenol in mice suggest that zonal differences in metabolism 

in the liver may be responsible for relative differences in the production of hydroquinone, thus explaining 

the higher toxicity observed after benzene administration compared with phenol administration (Kenyon 

et al. 1995). Additional work in this area would aid in further understanding the kinetic determinants of 

benzene toxicity. Ethanol and dietary factors such as food deprivation and carbohydrate restriction 

enhance the hematotoxic effects of benzene. Therefore, more information regarding differences in 

metabolic pattern according to sex, age, nutritional status, and species, and correlation to differences in 

health effects would be useful. 

Humans and animals both excrete inhaled benzene via expiration. Additionally, benzene metabolites are 

excreted primarily in the urine in both humans and animals. No studies in humans exist for excretion of 

oral doses of benzene. Studies in several animal species indicate that the route of excretion of benzene 

and/or its metabolites is a function of exposure level and the saturation of metabolic systems (Henderson 

et al. 1989). Data regarding excretion following dermal exposure in humans are limited. However, the 

major route of excretion in both humans and animals following dermal exposure is the urine. 
 

BENZENE 235 

3. HEALTH EFFECTS 

Comparative Toxicokinetics. Qualitatively, absorption, distribution, metabolism, and excretion 

appear to be similar in humans and laboratory animals. However, quantitative variations in the 

absorption, distribution, metabolism, and excretion of benzene have been observed with respect to 

exposure routes, sex, nutritional status, and species. Further studies that focus on these differences and 

their implications for human health would be useful. Additionally, in vitro studies using human tissue 

and further research into PBPK modeling in animals would contribute significantly to the understanding 

of the kinetics of benzene and would aid in the development of pharmacokinetic models of exposure in 

humans. These topics are being addressed in ongoing studies (see Section 3.12.3). 

Methods for Reducing Toxic Effects. Development of methods and practices that are specific for 

benzene is needed for reducing peak absorption, body burden and for interfering with the mechanism of 

action following benzene exposures. Since benzene metabolites are thought to play the major role in the 

toxicity and carcinogenicity, more information is needed about their covalent binding to nucleic acids and 

cellular macromolecules. This information would help the development of methods for possible 

prevention of benzene-induced toxicity. Related lines of investigation include the use of non-steroidal 

anti-inflammatory drugs to block prostaglandin and prostaglandin synthetase-mediated activity after 

benzene exposure, and the role of IL-1 cytokine activity in preventing depression of hematopoiesis. 

Children’s Susceptibility. Data needs relating to both prenatal and childhood exposures, and 

developmental effects expressed either prenatally or during childhood, are discussed in detail in the 

Developmental Toxicity subsection above. 

No clear evidence of age-related differences in susceptibility to benzene toxicity was located. Benzene 

crosses the placenta (Dowty et al. 1976). Nursing infants can be exposed to benzene in the breast milk 

(Fabietti et al. 2004). Limited animal studies indicate that in utero exposure to benzene results in 

hematological changes similar to those observed in animals exposed only as adults (Corti and Snyder 

1996; Keller and Snyder 1986, 1988). There is some indication that parental occupational exposure to 

benzene may play a role in childhood leukemia (Buckley et al. 1989; McKinney et al. 1991; Shaw et al. 

1984; Shu et al. 1988). However, none of these studies indicate whether children may be at greater risk 

than adults for benzene toxicity. Children could potentially be at increased risk for benzene toxicity via 

the inhalation exposure route based on higher activity levels and ventilation rates than adults. Age-related 

differences in benzene metabolism could potentially affect susceptibility. Well-designed animal studies 

should be performed to adequately assess the potential for age-related increased susceptibility to benzene. 
BENZENE 236 

3. HEALTH EFFECTS 

Child health data needs relating to exposure are discussed in Section 6.8.1, Identification of Data Needs: 

Exposures of Children. 

3.12.3 Ongoing Studies 

Ongoing studies pertaining to benzene have been identified and are shown in Table 3-7 (FEDRIP 2005). 
BENZENE 237 

3. HEALTH EFFECTS 

Table 3-7. Ongoing Studies on the Health Effects of Benzene 

Investigator Affiliation Research description Sponsor 

Conaway CC University of California 

Lawrence Livermore 

DNA binding of radiolabeled benzene National Center for 

Research Resources 

National Laboratory 

French JE National Institute of 

Environmental Health 

Sciences 

Carcinogen inactivation of tumor 

suppressor genes in p53 mice 

National Institute of 

Environmental Health 

Sciences 

French JE National Institute of 

Environmental Health 

Sciences 

Mechanisms of leukemogenesis in 

genetically-altered mice 

National Institute of 

Environmental Health 

Sciences 

Sabri MI Oregon Health and 

Science University 

Biomarkers of exposure and effect in 

neurotoxicants including neurotic 

benzene derivatives 

National Institute of 

Environmental Health 

Sciences 

Monks TJ University of Texas at 

Austin 

Benzene metabolites and 

hematotoxicity 

National Institute of 

Environmental Health 

Sciences 

Ross D Texas A&M University 

System 

Protective effect of NQ01 against 

benzene toxicity 

National Institute of 

Environmental Health 

Sciences 

Smith MT University of California 

Berkeley 

Biomarkers of benzene exposure and 

genotoxicity 

National Institute of 

Environmental Health 

Sciences 

Source: FEDRIP 2005 
BENZENE 238 

3. HEALTH EFFECTS 

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BENZENE 239 

4. CHEMICAL AND PHYSICAL INFORMATION 

4.1 CHEMICAL IDENTITY 

Information regarding the chemical identity of benzene is located in Table 4-1. Although the term benzol 

is found in older literature for the commercial product, benzene is the name presently approved by the 

International Union of Pure and Applied Chemistry (IUPAC), the Chemical Manufacturers Association 

(CMA), and the American Society for Testing and Materials (ASTM) for the pure product. 

4.2 PHYSICAL AND CHEMICAL PROPERTIES 

Information regarding the physical and chemical properties of benzene is located in Table 4-2. The major 

impurities found in commercial products are toluene, xylene, phenol, thiophene, carbon disulfide, 

acetylnitrile, and pyridine (NIOSH 1974). Commercial refined benzene-535 is free of hydrogen sulfide 

and sulfur dioxide, but contains a maximum of 1 ppm thiophene and a maximum of 0.15% nonaromatics. 

Refined nitration-grade benzene is free of hydrogen sulfide and sulfur dioxide. Benzene is also 

commercially available as thiophene-free, 99 mole%, 99.94 mole%, and nanograde quality (HSDB 2007). 
BENZENE 240 

4. CHEMICAL AND PHYSICAL INFORMATION 

Table 4-1. Chemical Identity of Benzene 

Characteristic Information Reference 

Chemical name Benzene HSDB 2007 

Synonym(s) Annulene, benzeen (Dutch), benzen HSDB 2007 

(Polish), benzol, benzole; benzolo (Italian), 

coal naphtha, cyclohexatriene, fenzen 

(Czech), phene, phenyl hydride, 

pyrobenzol, pyrobenzole 

Registered trade name(s) Polystream IARC 1982 

Chemical formula C6H6 Budavari et al. 2001 

Chemical structure Budavari et al. 2001 

Identification numbers: 

CAS registry 71-43-2 HSDB 2007 

NIOSH RTECS CY-1400000 HSDB 2007 

EPA hazardous waste NA 

OHM/TADS No Data 

DOT/UN/NA/IMCO shipping UN1114; IMO3.2 HSDB 2007 

HSDB 35 HSDB 2007 

NCI C55276 HSDB 2007 

Merck 1066 Budavari et al. 2001 

CAS = Chemical Abstracts Service; DOT/UN/NA/IMO=Department of Transportation/United Nations/North America/ 

Intergovernmental Maritime Dangerous Goods Code; EPA = Environmental Protection Agency; HSDB=Hazardous 

Substances Data Bank; NCI = National Cancer Institute; NIOSH= National Institute for Occupational Safety and 

Health; OHM/TADS = Oil and Hazardous Materials/Technical Assistance Data System; RTECS = Registry of Toxic 

Effects of Chemical Substances 
BENZENE 241 

4. CHEMICAL AND PHYSICAL INFORMATION 

Table 4-2. Physical and Chemical Properties of Benzene 

Property Information Reference 

Molecular weight 78.11 Budavari et al. 2001 

Color Clear, colorless liquid Budavari et al. 2001 

Physical state colorless to light yellow liquid HSDB 2007 

Melting point 5.5 °C Budavari et al. 2001 

Boiling point 80.1 °C Budavari et al. 2001 

Density at 15 °C, g/cm3 0.8787 Budavari et al. 2001 

Odor Aromatic NFPA 1994 

Odor threshold: 

Water 2.0 mg/L HSDB 2007 

Aira Detection range: 34–119 ppm AIHA 1989 

(geometric mean: 61 ppm) 

Recognition: 97 ppm 

Taste threshold: 0.5–4.5 mg/L HSDB 2007 

Solubility: 

Water at 25 °C w/w: 0.188% Budavari et al. 2001 

Organic solvents Alcohol, chloroform, ether, carbon Budavari et al. 2001 

disulfide, acetone, oils, carbon, 

tetrachloride, glacial acetic acid 

Partition coefficients: HSDB 2007; Karickhoff 1981; 

Kenaga 1980

 Log Kow 

b 2.13 

 Log Koc 

c 1.8–1.9 

Vapor pressure at 20 °C 75 mm Hg NFPA 1994 

Henry's law constant at 25 °C 5.5x10-3 atm-m3

/mol Mackay and Leinonen 1975 

Autoignition temperature 498 °C NFPA 1994 

Flashpoint -11 °C (closed cup) Budavari et al. 2001 

NFPA hazard classification: HSDB 2007 

Health 2.2 

Flammability 3.3 

 Reactivity 0.0 

Flammability limits in air 1.2% (lower limit; 7.8% (upper limit) NFPA 1994 

Conversion factors 1 ppm=3.26 mg/m3

 at 20 °C and 

1 atm pressure; 1 mg/m3

=0.31 ppm 

HSDB 2007 

Explosive limits 1.4% (lower limit); 8% (upper limit) HSDB 2007 

a

Odor threshold values considered by AIHA (1989) to be acceptable based on review of peer-reviewed reports of 

odor thresholds for benzene (range 0.78–100 ppm). b

Kow = octanol-water partitioning coefficient c

Koc = soil adsorption coefficient 

NFPA = National Fire Protection Association 
BENZENE 242 

4. CHEMICAL AND PHYSICAL INFORMATION 

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BENZENE 243 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 

5.1 PRODUCTION 

In 1825, Faraday isolated benzene from a liquid condensed by compressing oil gas. Benzene was first 

synthesized by Mitscherlich in 1833 by distilling benzoic acid with lime. Benzene was first commercially 

recovered from light oil derived from coal tar in 1849 and from petroleum in 1941 (IARC 1982a). 

Several years after the end of World War II, the rapidly expanding chemical industry created an increased 

demand for benzene that the coal carbonization industry could not fulfill. To meet this demand, benzene 

was produced by the petroleum and petrochemical industries by recovery from reformat and liquid 

byproducts of the ethylene manufacturing process (Fruscella 1992). 

Currently, benzene is commercially recovered from both coal and petroleum sources. More than 98% of 

the benzene produced in the United States is derived from the petrochemical and petroleum refining 

industries (OSHA 1987). These sources include refinery streams (catalytic reformats), pyrolysis gasoline, 

and toluene hydrodealkylation. Catalytic reformat is the major source of benzene (Greek 1990). Between 

1978 and 1981, catalytic reformats accounted for approximately 44–50% of the total U.S. benzene 

production (Fishbein 1988). During catalytic reforming, cycloparaffins (also known by the obsolescent 

term "naphthenes") such as cyclohexane, methyl cyclohexane, and dimethylcyclohexane are converted to 

benzene by isomerization, dehydrogenation, and dealkylation, and paraffins in naphtha (such as hexane) 

are converted to benzene by cyclodehydrogenation. The process conditions and the catalyst determine 

which reaction will predominate. The benzene is recovered by solvent extraction (e.g., with sulpholane or 

tetraethylene glycol). Pyrolysis gasoline is a liquid byproduct produced by the steam cracking of lower 

paraffins (gas oil) or heavier hydrocarbons (heavy naphtha). Pyrolysis gasoline contains unsaturated 

aliphatic hydrocarbons (such as ethylene and propylene) and aromatics. Several integrated pyrolysis 

gasoline treatment processes are available including partial hydrogenation and extractive distillation; 

hydrogenation, hydrodesulfurization, and solvent extraction; or partial hydrogenation, desulfurization, 

hydrocracking, hydrodealkylation, and distillation for the optimization of benzene yield and the recovery 

of benzene (Fruscella 1992; IARC 1982a). In the toluene hydrodealkylation process, toluene or 

toluene/xylene mixtures are reacted with hydrogen at temperatures ranging from 500 to 595 °C with usual 

pressures of 4–6 mPa (40–60 atm), and demethylated to produce benzene and methane. Another process 

whereby toluene is converted to benzene and xylenes by transalkylation or disproportionation is also used 

for the production of benzene (Fruscella 1992). Small quantities of benzene are also produced from 

destructive distillation of coal used for coke manufacture. Benzene is derived from the light oil fraction 

produced during the coking process (Greek 1990; Fruscella 1992). New coking, gasification, and 
BENZENE 244 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 

liquefaction processes for coal are all potential sources of benzene (IARC 1982a). Of the total U.S. 

production capacity of 3.109 billion gallons in 2004, catalytic reformats constituted 45%, toluene and 

xylene 30%, pyrolysis gasoline 23%, and coke oven <2% (SRI 2004). 

Benzene ranks in the top 20 most abundantly produced chemicals in the United States (C&EN 1994; 

Kirschner 1995; Reisch 1994). Production data from 1984 to 1994 indicate that the production of 

benzene increased by about 4% annually (C&EN 1995). 

According to the Toxics Release Inventory (TRI), 2,528 facilities in the United States produced or 

processed benzene in 2004 (TRI04 2006). Table 5-1 lists the facilities in each state that manufacture or 

process benzene, the intended use, and the range of maximum amounts of benzene that are stored on site. 

The TRI data listed in Table 5-1 should be used with caution since only certain types of facilities were 

required to report. Therefore, this is not an exhaustive list. 

SRI (2004) lists the companies producing benzene in the United States in 2004. This list is summarized 

in Table 5-2. Facilities listed as being responsible for producing the top 50% of regional capacity in the 

United States include: Exxon Mobil Chemical Company, 518 million gallons (16%); Equistar Chemicals 

LP, 330 million gallons (11%), Dow Chemical Company, 300 million gallons (10%); Flint Hills 

Resources LP, 250 million gallons (8%), and BP Oil Company 230 million gallons (7%) (SRI 2004). 

5.2 IMPORT/EXPORT 

Benzene is imported and exported to the United States as both the pure chemical and as a mixture of 

mineral fuels. The import of pure benzene into the United States is dependent on domestic production 

and demand. Imports of benzene (from mineral fuels and pure benzene) into the United States were 

approximately 5,252 million L (10,176 million pounds) in 2004, 6,024 million L (11,672 million pounds) 

in 2003, and 5,912 million L (11,455 million pounds) in 2002 (USITC 2005). The largest exporters of 

benzene to the United States during 2002-2004 were Canada, Iraq, Israel, Kuwait, Venezuela, and Saudi 

Arabia (USITC 2005). 

As in the case of import, the export of benzene from the United States to other countries is dependent on 

domestic and world production and demand. The 2002–2004 data indicate an increase in export volumes 

for benzene during this period. Exports of benzene (both pure benzene and benzene derived from mineral 

fuels) to other countries were approximately 11 million L (21 million pounds) in 2002, 150 million L 
BENZENE 245 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 

Table 5-1. Facilities that Produce, Process, or Use Benzene 

Minimum Maximum 

Number of amount on site amount on site 

Statea facilities in poundsb in poundsb Activities and usesc 

AK 26 0 99,999,999 1, 2, 3, 4, 5, 7, 8, 9, 11, 12 

AL 85 0 499,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 

AR 24 100 9,999,999 1, 2, 3, 4, 6, 8, 9, 11, 12, 13 

AS 1 100,000 999,999 9 

AZ 30 100 9,999,999 1, 2, 4, 5, 7, 9, 10, 11, 12, 13, 14 

CA 172 0 99,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14 

CO 23 0 9,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 13 

CT 8 1,000 9,999,999 1, 4, 6, 7, 9, 10, 12 

DE 13 1,000 49,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13 

FL 36 0 499,999,999 1, 2, 3, 4, 5, 7, 8, 9, 10, 12, 13, 14 

GA 33 0 49,999,999 1, 2, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 

GU 6 100 9,999,999 2, 3, 4, 7, 9, 12 

HI 15 100 49,999,999 1, 2, 3, 4, 5, 6, 7, 9, 12, 13, 14 

IA 21 0 10,000,000,000 1, 2, 3, 4, 5, 7, 9, 10, 11, 12, 13, 14 

ID 3 10,000 9,999,999 7, 9, 12 

IL 102 0 99,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 

IN 80 0 99,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 

KS 59 0 499,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14 

KY 66 0 99,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 

LA 212 0 10,000,000,000 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 

MA 15 100 99,999,999 2, 4, 7, 8, 9 

MD 20 100 49,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12 

ME 9 100 9,999,999 2, 3, 4, 7, 9, 12, 13 

MI 108 0 499,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 

MN 36 100 99,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 

MO 30 0 9,999,999 1, 5, 7, 8, 9, 11, 12 

MP 7 0 999,999 2, 3, 4, 7, 9 

MS 43 0 499,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14 

MT 27 10,000 499,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 13, 14 

NC 34 0 49,999,999 1, 2, 3, 4, 5, 6, 7, 9, 10, 11, 12, 13 

ND 9 0 49,999,999 1, 2, 3, 4, 5, 6, 9, 12, 13 

NE 11 0 9,999,999 1, 7, 10, 12, 13 

NH 9 0 999,999 1, 2, 7, 9, 12, 13 

NJ 61 0 99,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13 

NM 32 100 9,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 13, 14 

NV 5 100 99,999 1, 2, 4, 8, 9, 12, 13 

NY 48 0 49,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13 

OH 101 0 99,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 
BENZENE 246 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 

Table 5-1. Facilities that Produce, Process, or Use Benzene 

Minimum Maximum 

Number of amount on site amount on site 

Statea facilities in poundsb in poundsb Activities and usesc 

OK 48 0 499,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, 14 

OR 12 1,000 9,999,999 1, 2, 3, 4, 5, 7, 9, 12, 13 

PA 100 0 99,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 

PR 52 0 10,000,000,000 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13 

RI 9 10,000 9,999,999 1, 2, 4, 7, 8, 9 

SC 26 0 9,999,999 1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13 

SD 8 100 999,999 2, 3, 4, 7, 8, 11, 12, 13, 14 

TN 53 0 9,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 

TX 361 0 10,000,000,000 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 

UT 42 1,000 49,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13 

VA 36 0 49,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13 

VI 8 10,000 499,999,999 1, 2, 3, 4, 6, 7, 8, 9 

WA 57 0 499,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 13, 14 

WI 27 0 9,999,999 1, 4, 5, 7, 8, 9, 11, 12, 13, 14 

WV 32 0 99,999,999 1, 2, 4, 5, 6, 7, 8, 9, 12, 13 

WY 37 0 9,999,999 1, 2, 3, 4, 5, 6, 7, 9, 12, 13, 14 

a

Post office state abbreviations used 

b

Amounts on site reported by facilities in each state 

c

Activities/Uses: 

1. Produce 

2. Import 

3. Onsite use/processing 

4. Sale/Distribution 

5. Byproduct 

6. Impurity 

7. Reactant 

8. Formulation Component 

9. Article Component 

10. Repackaging 

11. Chemical Processing Aid 

12. Manufacturing Aid 

13. Ancillary/Other Uses 

14. Process Impurity 

Source: TRI04 2006 (Data are from 2004) 
BENZENE	 247 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 

Table 5-2. Current U.S. Manufacturers of Benzenea 

Company Location (annual capacity millions of gallons) 

ATOFINA Petrochemicals, Inc. Port Arthur, Texas (87)
 

BASF FINA Petrochemicals, Inc. Port Arthur, Texas (34) 

BP America, Inc. Texas City, Texas (230) 

Ceveron Phillips Chemical Company, LP Pascagoula, Mississippi (150) 

Olefins and Polyolefins Business Unit Port Arthur, Texas (35) 

CITGO Petroleum Corporation Corpus Christi, Texas (78) 

Lake Charles, Louisiana (55) 

Lemont, Illinois (19) 

Conoco Phillips Refining Marketing and Alliance Louisiana (65) 

Transportation Division Sweeny, Texas (39) 

Wood River Illinois (60) 

Dow Chemical USA Freeport, Texas (80) 

Plaquemine, Louisiana (317) 

Equistar Chemicals, LP Alvin, Texas (137) 

Channelveiw, Texas (90) 

Corpus Christi, Texas (103) 

ExxonMobil Chemical Company Baton Rouge Louisiana (100) 

Baytown, Texas (180) 

Beaumont, Texas (182) 

Chalmette, Louisiana (56) 

Flint Hills Resources, LP Corpus Christi, Texas (250) 

Frontier El Dorado Refining Company El Dorado, Kansas (15) 

HOVENSA, LLC St. Croix, Virgin Islands (60) 

Huntsman, LLC Port Arthur, Texas (90) 

Lyondell-Citgo Refining, LP Houston, Texas (50) 

Marathon Ashland Petroleum, LLC Catlettsburg, Kentucky (58) 

Texas City, Texas (3) 

Motiva Enterprises, LLC Delaware City, Delaware (15) 

NOVA Chemicals Corp. Bayport, Texas (15) 

The Premcor Refining Group Inc. Lima, Ohio (145) 

Shell Chemical Company Deer Park, Texas (180) 

Sunoco, Inc. Marcus Hook, Pennsylvania (60) 

Philadelphia, Pennsylvania (24) 

Toledo Ohio (20) 

Westville, New Jersey (18) 

Valero Energy Corporation Corpus Christi, Texas (82) 

Three Rivers, Texas (20) 

Eagle-Pitcher Industries, Inc. Lenexa, Kansas 

Eagle-Pitcher Technologies, LLC 

Chem Syn Laboratiories 

(

14C-benzene) 

a

Derived from Stanford Research Institute (SRI) 2004, receipt where otherwise noted. SRI reports production of 

chemicals produced in commercial quantities (defined as exceeding 5,000 pounds or $10,000 in value annually) by 

the companies listed. 
BENZENE 248 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 

(290 million pounds) in 2003, and 75 million L (145 million pounds) in 2004. These numbers are up 

from 15 million L (29 million pounds) in 2001 and 3.3 million L (6.4 million pounds) in 1993 (USITC 

2005). The largest importers of benzene from the United States are Canada, the Netherlands, Taiwan, 

Spain, and Korea (USITC 2005). 

5.3 USE 

Benzene has been used extensively as a solvent in the chemical and drug industries, as a starting material 

and intermediate in the synthesis of numerous chemicals, and as a gasoline additive (NTP 1994). 

Benzene recovered from petroleum and coal sources is used primarily as an intermediate in the 

manufacture of other chemicals and end products. The major uses of benzene are in the production of 

ethylbenzene, cumene, and cyclohexane. Ethylbenzene (55% of benzene production volume) is an 

intermediate in the synthesis of styrene, which is used to make plastics and elastomers. Cumene (24%) is 

used to produce phenol and acetone. Phenols are used in the manufacture of phenolic resins and nylon 

intermediates; acetone is used as a solvent and in the manufacture of pharmaceuticals. Cyclohexane 

(12%) is used to make nylon resins. Other industrial chemicals manufactured from benzene include 

nitrobenzene (5%), which is used in the production of aniline, urethanes, linear alkylbenzene sulfonates, 

chlorobenzene, and maleic anhydride (Eveleth 1990; Greek 1990; HSDB 2007). Benzene is also a 

component of gasoline since it occurs naturally in crude oil and since it is a byproduct of oil refining 

processes (Brief et al. 1980; Holmberg and Lundberg 1985). Benzene is especially important for 

unleaded gasoline because of its anti-knock characteristics. For this reason, the concentration of 

aromatics, such as benzene, in unleaded fuels has increased (Brief et al. 1980). The percentage by 

volume of benzene in unleaded gasoline is approximately 1–2% (NESCAUM 1989). 

The EPA has listed benzene as a hazardous air pollutant and a hazardous waste (EPA 1977, 1981). In 

addition, there is sufficient evidence to support classifying benzene as a human carcinogen (Group A) 

(IRIS 2007). One result of EPA's action is that the widespread use of benzene as a solvent has decreased 

in recent years. Many products that used benzene as solvents in the past have replaced it with other 

organic solvents; however, benzene may still occur as a trace impurity in these products. Less than 2% of 

the amount produced is used as a solvent in products such as trade and industrial paints, rubber cements, 

adhesives, paint removers, artificial leather, and rubber goods. Benzene has also been used in the shoe 

manufacturing and rotogravure printing industries (EPA 1978; OSHA 1977). In the past, certain 

consumer products (such as some paint strippers, carburetor cleaners, denatured alcohol, and rubber 
BENZENE 249 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 

cement used in tire patch kits and arts and crafts supplies) contained small amounts of benzene (Young et 

al. 1978). Other consumer products that contained benzene were certain types of carpet glue, textured 

carpet liquid detergent, and furniture wax (Wallace et al. 1987). 

The Consumer Products Safety Commission (CPSC) withdrew an earlier proposal to ban consumer 

products, except gasoline and laboratory reagents, that contained benzene as an intentional ingredient or 

as a contaminant at >0.1% by volume. The withdrawal of the rulemaking was based on CPSC findings 

that benzene was no longer used as an intentional ingredient and that the contaminant levels remaining in 

certain consumer products were unlikely to result in significant exposures (NTP 1994). Products 

containing >5% benzene, and paint solvents and thinners containing <10% of petroleum distillates such as 

benzene, are required to meet established labeling requirements. In a guidance document targeting school 

science laboratories, the CPSC recommended that benzene not be used or stored in schools. The 

document identified benzene as a carcinogen and ascertained that the hazards posed by its use in high 

school laboratories may be greater than its potential usefulness. 

The U.S. Food and Drug Administration (FDA) regulates benzene as an indirect food additive under the 

Food, Drug, and Cosmetics Act (FDCA). Under the FDCA, benzene is restricted to use only as a 

component of adhesives used on articles intended for packaging, transport, or holding foods (FDA 1977). 

5.4 DISPOSAL 

Benzene-containing wastes, such as commercial chemical products, manufacturing chemical 

intermediates, and spent solvents, are subject to federal and/or state hazardous waste regulations (HSDB 

2007). Regulations governing the treatment and disposal of benzene-containing wastes are presented in 

Chapter 8. Waste byproducts from benzene production processes include acid and alkali sludges, liquidsolid slurries, and solids (EPA 1982b; Saxton and Narkus-Kramer 1975). In the past, landfilling and 

lagooning have been the major methods of disposal of benzene-containing industrial wastes (EPA 1982b). 

In addition to biodegradation, a portion of the benzene is expected to be lost due to volatilization. 

Unfortunately benzene, along with other hazardous contaminants, also leaches into groundwater from the 

lagooned wastes. Currently, the recommended method of disposal is to incinerate solvent mixtures and 

sludges at a temperature that ensures complete combustion. The recommended methods for combustion 

are liquid injection incineration at a temperature range of 650–1,600 °C and a residence time of 0.1– 

2 seconds; rotary kiln incineration at a temperature range of 820–1,600 °C and residence times of seconds 

for liquids and gases, and hours for solids; and fluidized bed incineration at a temperature range of 450– 
BENZENE 250 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 

980 °C and residence times of seconds for liquids and gases and longer for solids (HSDB 2007). Since 

benzene burns with a very smoky flame, dilution with alcohol or acetone is suggested to minimize smoke. 

Small quantities of benzene waste can be destroyed by chemical reaction. For example, treating benzene 

with dichromate in strong sulfuric acid for 1–2 days is sufficient for total destruction (HSDB 2007). 

Underground injection also appears to be an important disposal method in some states. Approximately 

436,000 pounds of benzene (6% of the total environmental release) was disposed of by underground 

injection. This disposal via underground injection in 2006 was higher than the amount (356,000 pounds) 

released in 1992 (TRI90 1992), but lower than the release in 1990 (654,000 pounds) (TRI90 1992) and 

2002 (692,000 pounds) (TRI02 2005). In addition, 24,000 pounds of benzene (0.3% of the total 

environmental release) was disposed of via land disposal (TRI04 2006). The amount discharged to soil in 

2002 was less than one quarter of the amount (724,000 pounds) discharged in 1990 (TRI90 1992) and less 

than one third of the amount (340,000 pounds) discharged in 1992 (TRI92 1994). 

Several methods exist for the treatment of waste water that contains benzene: biological treatment 

(aeration or activated sludge process), solvent extraction, air and/or steam stripping, and activated carbon 

process (EPA 1994a; IRPTC 1985). Full-scale chemical treatability studies have demonstrated 95–100% 

reductions in benzene concentrations for industrial waste waters receiving biological treatments (HSDB 

2007). A combination of steam stripping and air stripping, and a vapor extraction system that removes 

the separated benzene vapor may be suitable for the treatment of contaminated groundwater and soil (Naft 

1992). An in situ bioremediation process has been used to decontaminate a site by delivering a controlled 

amount of nitrate (to accelerate biodegradation of benzene) to the site under hydraulic control (Kennedy 

and Hutchins 1992). 
BENZENE 251 

6. POTENTIAL FOR HUMAN EXPOSURE 

6.1 OVERVIEW 

Benzene has been identified in at least 1,000 of the 1,684 hazardous waste sites that have been proposed 

for inclusion on the EPA National Priorities List (NPL) (HazDat 2006). However, the number of sites 

evaluated for benzene is not known. The frequency of these sites can be seen in Figure 6-1. Of these 

sites, 995 are located within the United States, 3 are located in the Commonwealth of Puerto Rico (not 

shown), and 2 are located in the Virgin Islands (not shown). 

Benzene is released to the environment by both natural and industrial sources, although the anthropogenic 

emissions are undoubtedly the most important. Emissions of benzene to the atmosphere result from 

gasoline vapors, auto exhaust, and chemical production and user facilities. EPA's estimate of nationwide 

benzene atmospheric emissions from various sources was 34,000 metric tons/year (EPA 1989). 

According to the Toxics Release Inventory, releases to the air from manufacturing and processing 

facilities were about 6,7 million pounds (3,055 metric tons) in 2004 (TRI04 2006). Releases to air 

accounted for about 93% of the total industry-related releases to the environment (TRI04 2006). Benzene 

is released to water and soil from industrial discharges, landfill leachate, and gasoline leaks from 

underground storage tanks. 

Chemical degradation reactions, primarily reaction with hydroxyl radicals, limit the atmospheric 

residence time of benzene to only a few days, and possibly to only a few hours. Benzene released to soil 

or waterways is subject to volatilization, photooxidation, and biodegradation. Biodegradation, principally 

under aerobic conditions, is an important environmental fate process for water- and soil-associated 

benzene. 

Benzene is ubiquitous in the atmosphere. It has been identified in air samples of both rural and urban 

environments and in indoor air. Although a large volume of benzene is released to the environment, 

environmental levels are low because of efficient removal and degradation processes. Benzene partitions 

mainly into air (99.9%) and inhalation is the dominant pathway of human exposure accounting for >99% 

of the total daily intake of benzene (Hattemer-Frey et al. 1990; MacLeod and MacKay 1999). The 

general population is exposed to benzene primarily by tobacco smoke (both active and passive smoking) 

and by inhaling contaminated air (particularly in areas with heavy motor vehicle traffic and around filling 

stations). Air around manufacturing plants that produce or use benzene and air around landfills and 

hazardous waste sites that contain benzene are additional sources of exposure. Exposure to benzene can 

BENZENE 253 

6. POTENTIAL FOR HUMAN EXPOSURE 

also result from ingestion of contaminated food or water. Use of contaminated tap water for cooking, 

showering, etc., can also be a source of inhalation exposure since benzene can volatilize from water. 

Compared to inhalation, dermal exposure accounts for a minor portion of the total exposure of the general 

population. The magnitude of exposure is greatest for those individuals occupationally exposed to 

benzene; however, a far greater number of individuals are exposed as a result of benzene released from 

smoking tobacco products, from gasoline filling stations, and from auto exhaust. Smoking was found to 

be the largest anthropogenic source of direct human exposure to benzene (Duarte-Davidson et al. 2001; 

Hattemer-Frey et al. 1990). 

6.2 RELEASES TO THE ENVIRONMENT 

The Toxics Release Inventory (TRI) data should be used with caution because only certain types of 

facilities are required to report (EPA 2005g). This is not an exhaustive list. Manufacturing and 

processing facilities are required to report information to the TRI only if they employ 10 or more full-time 

employees; if their facility is included in Standard Industrial Classification (SIC) Codes 10 (except 1011, 

1081, and 1094), 12 (except 1241), 20–39, 4911 (limited to facilities that combust coal and/or oil for the 

purpose of generating electricity for distribution in commerce), 4931 (limited to facilities that combust 

coal and/or oil for the purpose of generating electricity for distribution in commerce), 4939 (limited to 

facilities that combust coal and/or oil for the purpose of generating electricity for distribution in 

commerce), 4953 (limited to facilities regulated under RCRA Subtitle C, 42 U.S.C. section 6921 et seq.), 

5169, 5171, and 7389 (limited S.C. section 6921 et seq.), 5169, 5171, and 7389 (limited to facilities 

primarily engaged in solvents recovery services on a contract or fee basis); and if their facility produces, 

imports, or processes ≥25,000 pounds of any TRI chemical or otherwise uses >10,000 pounds of a TRI 

chemical in a calendar year (EPA 2005g). 

6.2.1 Air 

Estimated releases of 6.7 million pounds (~ 3,055 metric tons) of benzene to the atmosphere from 

968 domestic manufacturing and processing facilities in 2004, accounted for about 93% of the estimated 

total environmental releases from facilities required to report to the TRI (TRI04 2006). These releases are 

summarized in Table 6-1. 

Benzene is released into the atmosphere from both natural and industrial sources. Natural sources include 

crude oil seeps, forest fires, and plant volatiles (Brief et al. 1980; Graedel 1978). Major anthropogenic 

sources of benzene include environmental tobacco smoke, automobile exhaust, automobile refueling 
BENZENE 254 

6. POTENTIAL FOR HUMAN EXPOSURE 

Table 6-1. Releases to the Environment from Facilities that Produce, Process, or 

Use Benzenea
 

Reported amounts released in pounds per yearb 

Total release 

Statec

 RFd Aire Waterf UIg Landh Otheri On-sitej

 Off-sitek 

On- and offsite 

AK 7 19,568 19 0 375 0 19,962 0 19,962 

AL 15 290,660 290 0 280 2 290,955 277 291,232 

AR 8 33,137 66 0 0 0 33,203 0 33,203 

AZ 13 4,910 0 0 9 0 4,915 4 4,919 

CA 65 57,611 129 228 1,292 598 57,970 1,888 59,858 

CO 11 19,630 0 0 10 36 19,630 46 19,676 

CT 9 2,552 2 0 0 2 2,554 2 2,556 

DE 5 3,425 6,006 0 26 0 9,431 26 9,457 

FL 26 396,187 41 0 0 157 396,228 157 396,385 

GA 15 36,893 128 0 9 0 37,026 4 37,030 

GU 3 7,106 1 0 0 401 7,107 401 7,508 

HI 9 11,171 30 0 12 0 11,201 12 11,213 

IA 14 42,671 1 0 0 0 42,672 0 42,672 

ID 2 1,732 No data 0 0 0 1,732 0 1,732 

IL 35 171,104 102 0 2,152 104 172,227 1,235 173,462 

IN 30 211,476 805 14,001 4,654 29 227,079 3,886 230,965 

KS 19 69,823 164 231 259 0 70,226 251 70,477 

KY 22 77,345 799 0 766 20 78,904 26 78,930 

LA 62 605,128 1,075 122,723 1,688 968 729,109 2,473 731,582 

MA 10 5,556 55 0 26 537 5,611 563 6,173 

MD 11 12,644 8 0 184 1 12,821 15 12,836 

ME 5 3,027 23 0 0 467 3,049 467 3,516 

MI 41 345,255 8 0 127 2,295 345,363 2,322 347,686 

MN 16 16,614 2,724 0 131 1,561 19,360 1,670 21,030 

MO 17 98,498 0 0 0 0 98,498 0 98,498 

MP 2 710 0 0 0 0 710 0 710 

MS 13 59,076 25 0 14 0 59,102 13 59,115 

MT 5 17,242 5 0 15 0 17,248 14 17,262 

NC 12 17,290 0 0 5 48 17,295 48 17,343 

ND 2 6,400 0 0 1 0 6,400 1 6,401 

NE 9 1,333 No data 0 0 6 1,333 6 1,339 

NJ 20 51,054 443 0 150 169 51,510 306 51,816 

NM 9 38,730 No data 7 2 0 38,739 0 38,739 

NV 3 1,005 No data 0 0 0 1,005 0 1,005 

NY 44 60,682 56 0 10 1,335 60,738 1,345 62,084 

OH 52 183,972 67 5 730 82 184,049 807 184,856 

OK 11 114,701 14 0 278 71 114,984 80 115,064 
BENZENE 255 

6. POTENTIAL FOR HUMAN EXPOSURE 

Table 6-1. Releases to the Environment from Facilities that Produce, Process, or 

Use Benzenea
 

Reported amounts released in pounds per yearb 

Total release 

Statec

 RFd Aire Waterf UIg Landh Otheri On-sitej

 Off-sitek 

On- and offsite 

OR 6 4,651 8 0 41 0 4,659 41 4,700 

PA 39 234,647 419 0 345 11,389 235,066 11,733 246,800 

PR 10 49,879 7 0 0 0 49,886 0 49,886 

RI 2 1,841 4 0 0 820 1,845 820 2,665 

SC 6 64,627 6 0 250 1,023 64,883 1,023 65,906 

SD 11 272 1 0 0 0 273 0 273 

TN 17 73,926 63 0 280 0 73,989 280 74,269 

TX 150 2,681,863 621 298,595 8,326 1,713 2,935,539 55,579 2,991,118 

UT 11 31,314 750 0 809 233 32,069 1,038 33,107 

VA 17 183,748 787 0 26 0 184,535 26 184,561 

VI 3 20,576 0 0 9 0 20,576 9 20,585 

WA 18 50,667 14 0 95 48 50,770 54 50,824 

WI 14 141,183 0 0 251 63 141,183 314 141,496 

WV 5 76,556 284 0 393 0 76,840 393 77,233 

WY 7 24,986 0 0 1 0 24,986 1 24,987 

Total 968 6,736,655 16,051 435,790 24,033 24,179 7,147,049 89,659 7,236,707 

a

The TRI data should be used with caution since only certain types of facilities are required to report. This is not an 

exhaustive list. Data are rounded to nearest whole number.
 b

Data in TRI are maximum amounts released by each facility.
 c

Post office state abbreviations are used. 

d

Number of reporting facilities.
 e

The sum of fugitive and point source releases are included in releases to air by a given facility. f

Surface water discharges, waste water treatment-(metals only), and publicly owned treatment works (POTWs)
 

(metal and metal compounds).

g

Class I wells, Class II-V wells, and underground injection. 

h

Resource Conservation and Recovery Act (RCRA) subtitle C landfills; other on-site landfills, land treatment, surface 

impoundments, other land disposal, other landfills. i

Storage only, solidification/stabilization (metals only), other off-site management, transfers to waste broker for 

disposal, unknown 

j

The sum of all releases of the chemical to air, land, water, and underground injection wells. k

Total amount of chemical transferred off-site, including to POTWs. 

RF = reporting facilities; UI = underground injection 

Source: TRI04 2006 (Data are from 2004) 
BENZENE 256 

6. POTENTIAL FOR HUMAN EXPOSURE 

operations, and industrial emissions. Using source exposure modeling, it was estimated that benzene 

emissions were highest from coke oven blast furnaces (Edgerton and Shah 1992). Other sources that 

contributed to emissions of benzene include automobiles, petrochemical industries, waste water treatment 

plants, and petroleum industries (Edgerton and Shah 1992). 

Industrial and automotive sources of benzene are well monitored. EPA (1989) estimates of nationwide 

benzene atmospheric emissions, in metric tons/year (kkg/year), from various industrial sources include: 

(1) coke byproduct recovery plants (17,000 kkg/year), (2) benzene waste operations (5,300 kkg/year), 

(3) gasoline marketing systems, including bulk gasoline terminals and plants, service stations, and 

delivery tank trucks (4,800 kkg/year), (4) transfer operations at chemical production facilities, bulk 

terminals, and coke byproduct recovery plants (4,600 kkg/year), (5) benzene storage vessels (620– 

1,290 kkg/year), (6) industrial solvent use (450 kkg/year), (7) chemical manufacturing process vents 

(340 kkg/year), and (8) ethylbenzene/styrene process vents (135 kkg/year). Benzene is present in 

passenger car tailpipe emissions at compositions ranging from 2.9 to 15% of the total tailpipe 

hydrocarbon composition (Black et al. 1980). The 2002 benzene industrial emission inventory for 

California totaled 266 metric tons/year (CARB 2005); these numbers did not include motor vehicle 

exhaust,which accounted for 71% of emissions in 1984 (Cal EPA 1987). The contribution of mobile 

source hazardous air pollutant emissions has been compared to that of stationary sources in the SeattleTacoma area. Mobile sources were estimated to contribute approximately 83% of the benzene from 

stationary areas and mobile sources combined, with major stationary point sources excluded (EPA 

1994d). Benzene is also released by off-gassing from particle board (Glass et al. 1986), vaporization 

from oil spills, and emissions from landfills (Bennett 1987; Wood and Porter 1987). While all of these 

sources release more benzene into the environment, a large percentage of the benzene inhaled by humans 

comes from cigarette smoke. Exhaled breath of smokers contains benzene (Wallace 1989a, 1989b; 

Wallace and Pellizzari 1986; Wester et al. 1986). 

According to the Texas Natural Resource Conservation Commission (TNRCC), of the 93 districts in 

Texas reporting benzene emissions from industrial sources, in 1980, 1985, and 1988, 10 districts reported 

0 tons/year, 79 districts reported 0.1–100 tons/year, 4 districts reported 100–400 tons/year, 2 districts 

reported 400–1,000 tons/year, and 1 district had emissions projected at above 1,000 tons/year (Pendleton 

1995). 

The natural sources most monitored for benzene released to air are fires (Austin et al. 2001; Lowry et al. 

1985). A study of nine municipal fires in Canada found a mean concentration of 3.45 ppm of benzene 
BENZENE 257 

6. POTENTIAL FOR HUMAN EXPOSURE 

and had very high relative concentration of other volatile organic compounds (VOCs) monitored (Austin 

et al. 2001). 

There is a potential for atmospheric release of benzene from hazardous waste sites. Benzene has been 

detected in air in 200 of the 1,684 current and former NPL sites where it has been detected in some 

medium (HazDat 2006). 

6.2.2 Water 

Estimated releases of 16,051 pounds (~7 metric tons) of benzene to surface water from 968 domestic 

manufacturing and processing facilities in 2004, accounted for about 0.2% of the estimated total 

environmental releases from facilities required to report to the TRI (TRI04 2006). These releases are 

summarized in Table 6-1. 

Benzene is released to water from the discharges of both treated and untreated industrial waste water, 

gasoline leaks from underground storage tanks, accidental spills during marine transportation of chemical 

products, and leachate from landfills and other contaminated soils, (CDC 1994; Crawford et al. 1995; 

EPA 1979; NESCAUM 1989; Staples et al. 1985). A fire in a tire dump site in western Frederick County, 

Virginia, produced a free-flowing oily tar containing benzene among other chemicals. The seepage from 

this site contaminated nearby surface water (EPA 1993). Between 1986 and 1991, 3,000 gallons of 

benzene were accidentally released into Newark Bay and its major tributaries. Another 3,000 gallons 

were released in 1991 (Crawford et al. 1995). 

There is a potential for release of benzene to water from hazardous waste sites. Benzene has been 

detected in groundwater samples collected at 832 of the 1,684 current and former NPL sites and in surface 

water samples collected at 208 of the 1,684 sites (HazDat 2006). 

6.2.3 Soil 

Estimated releases of 24,033 pounds (~11 metric tons) of benzene to soils from 968 domestic 

manufacturing and processing facilities in 2004, accounted for about 0.3% of the estimated total 

environmental releases from facilities required to report to the TRI (TRI04 2006). An additional 435,000 

pounds (~197 metric tons), constituting about 6% of the total environmental emissions, were released via 

underground injection (TRI04 2006). These releases are summarized in Table 6-1. 
BENZENE 258 

6. POTENTIAL FOR HUMAN EXPOSURE 

Benzene is released to soils through industrial discharges, land disposal of benzene-containing wastes, 

and gasoline leaks from underground storage tanks. In northern Virginia, approximately 200,000 gallons 

of liquid hydrocarbons were released from a fuel-storage terminal into the underlying soil (Mushrush et 

al. 1994). 

There is a potential for release of benzene to soil from hazardous waste sites. Benzene has been detected 

in soil samples collected at 436 of the 1,684 sites, and in sediment samples collected at 145 of the 

1,684 sites where benzene has been detected in some medium (HazDat 2006). 

6.3 ENVIRONMENTAL FATE 

6.3.1 Transport and Partitioning 

The high volatility of benzene is the controlling physical property in the environmental transport and 

partitioning of this chemical. Benzene is considered to be highly volatile with a vapor pressure of 

95.2 mm Hg at 25 °C. Benzene is moderately soluble in water, with a solubility of 1,780 mg/L at 25 °C, 

and the Henry's law constant for benzene (5.5x10-3 atm-m3

/mole at 25 °C) indicates that benzene 

partitions readily to the atmosphere from surface water (Mackay and Leinonen 1975). Since benzene is 

soluble in water, some minor removal from the atmosphere via wet deposition may occur. A substantial 

portion of any benzene in rainwater that is deposited to soil or water will be returned to the atmosphere 

via volatilization. 

Benzene released to soil surfaces partitions to the atmosphere through volatilization, to surface water 

through runoff, and to groundwater as a result of leaching. The soil organic carbon sorption coefficient 

(Koc) for benzene has been measured with a range of 60–83 (Karickhoff 1981; Kenaga 1980), indicating 

that benzene is highly mobile in soil and readily leaches into groundwater. Other parameters that 

influence leaching potential include the soil type (e.g., sand versus clay), amount of rainfall, depth of the 

groundwater, and extent of degradation. In a study of the sorptive characteristics of benzene to 

groundwater aquifer solids, benzene showed a tendency to adsorb to aquifer solids. Greater soil 

adsorption was observed with increasing organic matter content (Uchrin and Mangels 1987). An 

investigation of the mechanisms governing the rates of adsorption and desorption of benzene by dry soil 

grains revealed that periods of hours are required to achieve equilibrium and that adsorption is much 

faster than desorption (Lin et al. 1994). The rate of volatilization and leaching are the principal factors 

that determine overall persistence of benzene in sandy soils (Tucker et al. 1986). 
BENZENE 259 

6. POTENTIAL FOR HUMAN EXPOSURE 

Studies suggest that benzene does not bioaccumulate in marine organisms. The bioconcentration/ 

bioaccumulation potential of benzene in aquatic organisms of the open coastal ocean was investigated by 

sampling final effluent from the Los Angeles County waste water treatment plant quarterly from 

November 1980 to August 1981 (Gossett et al. 1983). Benzene has a relatively low octanol/water 

partition coefficient (log Kow=2.13 or 2.15) (Gossett et al. 1983; HSDB 2007). In the alga, Chlorella, a 

bioaccumulation factor of 30 was determined experimentally (Geyer et al. 1984). An experimental 

bioconcentration factor (BCF) of 4.27 was measured in goldfish reared in water containing 1 ppm of 

benzene (Ogata et al. 1984). Based on these measured values, bioconcentration/bioaccumulation of 

benzene in the aquatic food chains does not appear to be important. These results are consistent with the 

fact that benzene has a relatively low octanol/water partition coefficient (Gossett et al. 1983; HSDB 

2007), suggesting relatively low bioaccumulation. There is no evidence in the literature of 

biomagnification of benzene in aquatic food chain. 

Evidence exists for the uptake of benzene by cress and barley plants from soil (Scheunert et al. 1985; 

Topp et al. 1989). BCFs for barley plants after 12, 33, 71, and 125 days were 17, 2.3, 2.9, and 4.6, 

respectively. BCFs for cress plants after 12, 33, and 79 days were 10, 2.3, and 1.9, respectively. The 

relative decrease in the BCFs with time was attributed to growth dilution (Topp et al. 1989). Since 

benzene exists primarily in the vapor phase, air-to-leaf transfer is considered to be the major pathway of 

vegetative contamination (Hattemer-Frey et al. 1990). Based on an equation to estimate vegetative 

contamination, the total concentration of benzene on exposed food crops consumed by humans and used 

as forage by animals was estimated to be 587 ng/kg, 81% of which was from air-to-leaf transfer and 19% 

was from root uptake (Hattemer-Frey et al. 1990). 

Benzene also accumulates in leaves and fruits of plants. After 40 days, plants grown in benzene-rich 

environments showed bioaccumulation in the leaves and fruit that were greater than the air portioning 

coefficient of benzene in the atmosphere. Blackberries exposed to 0.313 ppm and apples exposed to 

2.75 ppm contained about 1,000 and 36 ng/g of benzene, respectively (Collins et al. 2000). 

6.3.2 Transformation and Degradation 

Benzene undergoes a number of different transformation and degradation reactions in the environment as 

discussed in the following sections. The resulting environmental transformation products within different 

media are shown in Figure 6-2. 
2

Air OH NO2 

O H 

photooxidation NO2 H Benzene­ ring-cleavage C 

OH + + O C

OH adduct C

H O H 

Intermediate Phenol Bandow et al. Nitrobenzene Glyoxal Formaldehyde 1985 

photooxidation 

+ 

O O O 

Maleic anhydride 

NO2 OH OH OH OH 

NO 

hr 

NO NO2 O2N NO2 Nojima et al. + 

, + 1975 

, 

NO2 

Nitrobenzene o-Nitrophenol p-Nitrophenol 2,4-Dinitrophenol 2,6-Dinitrophenol 

NO2 

Water OH OH

OH 

Harayama and Timmis Biodegradation or + 1992 

OH 

OH 

Phenol Catechol Hydroquinone 

Soil 

OH microbial Substrate Benzene-OH Hopper 1978 for ring adduct 

O fusion 2 OH 

Catechol Intermediate 

BENZENE 260 

6. POTENTIAL FOR HUMAN EXPOSURE 

Figure 6-2. Environmental Transformation Products of Benzene in Various Media 
BENZENE 261 

6. POTENTIAL FOR HUMAN EXPOSURE 

6.3.2.1 Air 

Benzene in the atmosphere exists predominantly in the vapor phase (Eisenreich et al. 1981). The most 

significant degradation process for benzene is its reaction with atmospheric hydroxyl radicals. The rate 

constant for the vapor phase reaction of benzene with photochemically produced hydroxyl radicals has 

been determined to be 1.3x10-12 cm3

/molecule-second, which corresponds to a residence time of 8 days at 

an atmospheric hydroxyl radical concentration of 1.1x106

 molecules/cm3 (Gaffney and Levine 1979; 

Lyman 1982). With a hydroxyl radical concentration of 1x108

 molecules/cm3, corresponding to a 

polluted atmosphere, the estimated residence time is shortened to 2.1 hours (Lyman 1982). Benzene may 

also react with other oxidants in the atmosphere such as nitrate radicals and ozone; however, the rate of 

degradation is considered insignificant compared to the rate of reaction with hydroxyl radicals. 

Residence times of 472 years for rural atmospheres and 152 years for urban atmospheres were calculated 

for the reaction of benzene with ozone (O3) using a rate constant for O3 of 7x10-23 cm3

/molecule-second 

(Pate et al. 1976) and atmospheric concentrations for O3 of 9.6x1011 molecules/cm3

 (rural) and 

3x1012 molecules/cm3

 (urban) (Lyman 1982). 

The reaction of benzene and nitric oxide in a smog chamber was investigated to determine the role of 

benzene in photochemical smog formation (Levy 1973). The results showed that benzene exhibited low 

photochemical smog reactivity in the four categories tested: rate of photooxidation of nitric oxide, 

maximum oxidant produced, eye-irritation response time, and formaldehyde formation. The authors 

concluded that benzene probably does not play a significant role in photochemical smog formation (Levy 

1973). In the presence of active species such as nitrogen oxides and sulfur dioxide, the rate of 

photodegradation of benzene in the gas phase was greater than that in air alone. Its half-life in the 

presence of such active species (100 ppm benzene in the presence of 10–110 ppm NOx or 10– 

100 ppm SO2) was 4–6 hours with 50% mineralization to carbon dioxide in approximately 2 days (Korte 

and Klein 1982). Some of the products of the reaction of benzene with nitrogen monoxide gas (e.g., 

nitrobenzene, o- and p-nitrophenol, and 2,4- and 2,6-dinitrophenol) may have potentially adverse effects 

on human health (Nojima et al. 1975); however, these species also have relatively short atmospheric 

lifetimes, which should limit the potential exposure to these compounds. Photooxidation of benzene in a 

nitrogen monoxide/nitrogen dioxide-air system formed formaldehyde, formic acid, maleic anhydride, 

phenol, nitrobenzene, and glyoxal (ethane-1,2-dione) (Bandow et al. 1985). 
BENZENE 262 

6. POTENTIAL FOR HUMAN EXPOSURE 

Direct photolysis of benzene in the atmosphere is not likely because the upper atmosphere effectively 

filters out wavelengths of light <290 nm, and benzene does not absorb wavelengths of light >260 nm 

(Bryce-Smith and Gilbert 1976). 

6.3.2.2 Water 

Benzene is subject to indirect photolysis in sunlit surface water, but does not undergo direct photolysis. 

For direct photolysis to occur, a substance must absorb photons of light >290 nm. During indirect 

photolysis, light energy is absorbed by other constituents (photosensitizers) of the media (water, soil) and 

the excited species can then transfer energy to benzene (indirectly promoting it to an excited electronic 

state), or lead to the formation of reactive species, such as singlet oxygen or hydroxy radicals, which react 

with benzene. Humic and fulvic acids are well-known photosensitizing agents and are practically 

ubiquitous in natural waters. A half-life of 16.9 days was reported for photolysis of benzene dissolved in 

oxygen-saturated deionized water and exposed to sunlight (Hustert et al. 1981). 

Benzene is readily degraded in water under aerobic conditions. Results of a biochemical oxygen demand 

(BOD) test determined that benzene was completely biodegradable after the second week of static 

incubation at 25 °C at benzene concentrations of 5 and 10 mg/L using domestic waste water as the 

microbial inoculum (Tabak et al. 1981). A study of the degradation of benzene by the microbial 

population of industrial waste water at 23 °C using a shaker flask system showed that after 6 hours, only 

8% (4 mg/L) of the initial 50 mg/L dose of benzene remained (Davis et al. 1981). Water from a 

petroleum production site was successfully biotreated for complete removal of benzene using a 

flocculated culture of Thiobacillus denitrificans strain F and mixed heterotrophs (Rajganesh et al. 1995). 

Microbial degradation of benzene in aquatic environments is influenced by many factors including 

microbial population, dissolved oxygen, nutrients, other sources of carbon, inhibitors, temperature, pH, 

and initial concentration of benzene. Vaishnav and Babeu (1987) reported biodegradation half-lives for 

benzene in surface water (river water) and groundwater of 16 and 28 days, respectively. Benzene was 

found to be resistant to biodegradation in surface water taken from a harbor and supplemented with either 

nutrients (nitrogen and phosphorus) or acclimated microbes, however, biodegradation did occur, with a 

half-life of 8 days, in surface water enriched with both nutrients and microbes (Vaishnav and Babeu 

1987). At very high levels, as may be the case of a petroleum spill, benzene (and other compounds 

contained in petroleum) is toxic to microorganisms and the rate of degradation is slow compared to low 

initial starting concentrations. In another study, Davis et al. (1994) observed rapid aerobic biodegradation 
BENZENE 263 

6. POTENTIAL FOR HUMAN EXPOSURE 

of benzene in aquifer groundwater samples and measured times for 50% disappearance ranging from 

4 days for an initial benzene concentration of 1 mg/kg to 14 days for an initial benzene concentration of 

10 mg/kg. Under acidic conditions (pH 5.3, 20 °C), benzene was completely microbially degraded in 

16 days in groundwater taken from a shallow well (Delfino and Miles 1985). 

The aerobic biodegradation of benzene is also influenced by the presence of other aromatic hydrocarbons. 

A bacterial culture grown with aromatic hydrocarbons plus nitrogen-, sulfur-, and oxygen-containing 

aromatic compounds was much less efficient in degrading benzene than the culture grown with aromatic 

hydrocarbons alone. Pyrrole strongly inhibited benzene degradation. Benzene degradation was high 

when toluene and xylene were present (Arvin et al. 1989). 

Laboratory studies on microbial degradation of benzene with mixed cultures of microorganisms in 

gasoline-contaminated groundwater revealed that both oxygen and nitrogen concentrations are major 

controlling factors in the biodegradation of benzene. Nitrogen enhanced the biodegradation rate of 

benzene 4.5-fold, over innoculum-enriched water alone. More than 95% of the benzene in groundwater 

was removed through microbial action within 73.5 hours (Karlson and Frankenberger 1989). 

Benzene biodegradation under anaerobic conditions does not readily occur. When dissolved oxygen is 

depleted, an alternative electron acceptor such as nitrate, carbonate, or iron(III) must be available, and 

microbes capable of using the alternative electron acceptor to degrade the benzene must be present 

(McAllister and Chiang 1994). Using aquifer material obtained from a landfill from Norman, Oklahoma, 

no significant benzene biodegradation was reported during the first 20 weeks of incubation under 

anaerobic conditions at 17 °C; however, after 40 weeks of incubation, benzene concentrations were 

reduced by 72 and 99% of the benzene was degraded after 120 weeks (Wilson et al. 1986). No 

degradation of benzene was observed in 96 days under anaerobic conditions (20 °C) using raw water 

intake from a water treatment plant (Delfino and Miles 1985). 

Use of water as an oxygen source in the anaerobic degradation of benzene has been demonstrated. 

Experiments indicated that incorporation of 18O from 18O-labeled water is the initial step in the anaerobic 

oxidation of benzene by acclimated methanogenic cultures. Phenol was the first major product (Vogel 

and Grbić-Galić 1986). 
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6. POTENTIAL FOR HUMAN EXPOSURE 

6.3.2.3 Sediment and Soil 

Benzene is biodegraded in soil under aerobic conditions. Microbial metabolism of benzene proceeds 

through the formation of cis-dihydrodiols and, with further metabolism, to catechols, which are the 

substrates for ring fission (Gibson 1980; Hopper 1978). Pseudomonas putida oxidized benzene through 

cis-1,2-dihydroxy-1,2-dihydrobenzene (Gibson 1977; Hopper 1978). After acclimation, Norcardia 

species and Pseudomonas species, effectively degraded benzene to carbon dioxide within 7 days (45– 

90%). A strain of Rhodococcus isolated from contaminated river sediment mineralized 71% of benzene 

at an initial concentration of 0.7 mg/L in 14 days (Malachowsky et al. 1994). The soil bacterium 

Nitrosomonas europaea catabolized benzene to phenol and hydroquinone (Keener and Arp 1994). 

Another new mixotrophic bacteria, a strain of Pseudomonas sp. isolated from contaminated soil, grew 

under both anaerobic and aerobic conditions and used benzene for its growth (Morikawa and Imanaka 

1993). The biodegradation of 2 mg of radiolabeled benzene in 100 g of soil with a mixed microbial 

population transformed 47% of the added radioactivity to carbon dioxide after 10 weeks (Haider et al. 

1981). The authors concluded that specific organisms that mineralize benzene were present in the soil in 

only small numbers. 

ULTRA, a fate and transport model used to predict the environmental fate of benzene following leakage 

of gasoline from an underground storage tank into shallow sandy soil, indicated that only about 1% of the 

benzene in the gasoline would be degraded over a 17-month period, and 3% would remain in the soil 

(Tucker et al. 1986). According to this model, most of the benzene that would be leaked in soil would 

either volatilize (67%) or move into groundwater (29%). 

Salanitro (1993) summarized the aerobic degradation rates for BTEX in laboratory subsoil-groundwater 

slurries and aquifers. The data indicate that decay rates for benzene are highest (19–52% per day) for 

benzene concentrations <1 ppm when initial dissolved oxygen levels are about 8 ppm. Rates are 

significantly reduced (0–1.1% per day) when benzene levels are 1–2 ppm, and no degradation was 

observed when benzene levels were >2 ppm. This is particularly relevant in the case of petroleum spills 

as high concentrations of petroleum compounds are toxic to organisms and decrease the rate of 

biodegradation. 

Benzene has been shown to be anaerobically transformed by mixed methanogenic cultures derived from 

ferulic acid-degrading sewage sludge enrichments. In most of the experiments, benzene was the only 

semicontinuously supplied energy sources in the defined mineral medium (Grbić-Galić and Vogel 1987). 
BENZENE 265 

6. POTENTIAL FOR HUMAN EXPOSURE 

After an initial acclimation time of 11 days, at least 50% of the substrate was converted to CO2 and 

methane. The intermediates were consistent with benzene degradation via initial oxidation by ring 

hydroxylation. 

It has been demonstrated that when mixtures of benzene, toluene, xylenes, and ethylbenzene are present 

in an anaerobic environment, there is a sequential utilization of the substrate hydrocarbons, with toluene 

usually being the first to be degraded, followed by the isomers of xylene in varying order. Benzene and 

ethylbenzene tend to be degraded last, if they are degraded at all (Edwards and Grbić-Galić 1992). 

Similar studies using only benzene at initial concentrations ranging from 40 to 200 μM showed 

degradation rates ranging from 0.36 to 3.7 μM/day depending upon substrate concentration and the 

presence of other carbon sources (Edwards and Grbić-Galić 1992). 

6.3.2.4 Other Media 

Twenty-day-old spinach leaves placed in a hermetic chamber containing vapors of 14C-labeled benzene 

were shown to assimilate benzene, which was subsequently metabolized to various nonvolatile organic 

acids (Ugrekhelidze et al. 1997). 

6.4 LEVELS MONITORED OR ESTIMATED IN THE ENVIRONMENT 

Reliable evaluation of the potential for human exposure to benzene depends in part on the reliability of 

supporting analytical data from environmental samples and biological specimens. Concentrations of 

benzene in unpolluted atmospheres and in pristine surface waters are often so low as to be near the limits 

of current analytical methods. In reviewing data on benzene levels monitored or estimated in the 

environment, it should also be noted that the amount of chemical identified analytically is not necessarily 

equivalent to the amount that is bioavailable. The analytical methods available for monitoring benzene in 

a variety of environmental media are detailed in Chapter 7. 

6.4.1 Air 

Benzene is ubiquitous in the atmosphere. It has been identified in outdoor air samples of both rural and 

urban environments and in indoor air. Table 6-2 lists benzene levels in outdoor air from various cities in 

the United States. 
BENZENE 266 

6. POTENTIAL FOR HUMAN EXPOSURE 

Table 6-2. Benzene Levels in Air Samples 

Location Concentration (ppb) Comments References 

Outdoor air 

San Francisco, California 0.8–5.2 (range); 2.6±1.3a 

Stinson Beach, California 0.38±0.39a 

Houston, Texas 1.2–18.7 (range); 37.5b 

Houston, Texas 0.55–6.3; 1.73b 

St. Louis, Missouri 0.63–12.1 (range); 1.85b 

Denver, Colorado 3.0–6.6 (range); 4.1b 

Philadelphia, Pennsylvania 0.32–3.0 (range); 1b 

New York (Manhattan), 0.88–5.3 (range); 1.75b 

New York 

Chicago, Illinois 0.63–5.05 (range); 3.45b 

Boston, Massachusetts 0.69–3.1; 1.06b 

New York (Staten Island), 0.1–34 (range); 4.4±6.6a 

New York 

73 km northwest of Denver, 0.02–0.85 (range) 

Colorado 

Elizabeth and Bayonne, Night 2.7d

, 28.5 (max); 

New Jersey day, 3.0d

, 13.8 (max) 

Personal air 

Elizabeth and Bayonne, Night 9.7d

, 159.6 (max); 

New Jersey day, 8.5d

, 84.5 (max) 

Elizabeth and Bayonne, Winter 5.2, 

New Jersey summer 2.7, 

fall 4.8 

Los Angles, California Winter 4.5, spring 2.2, 

fall 2.8 

Data from six different urban Wester et al. 1986 

locations in 1984; n=25 

Data from remote coastal Wester et al. 1986 

area in 1984; n=21 

EPA survey, urban area; 

n=14; summer 1986 

EPA 1987ac 

Semirural area; n=22; 

1990–1991 

Kelly et al. 1993 

EPA survey, urban area; 

n=18; summer 1985 

EPA 1987ac 

EPA survey, urban area; 

n=13; summer 1986 

EPA 1987ac 

EPA survey, urban area; 

n=14; summer 1985 

EPA 1987ac 

EPA survey, urban area; 

n=12; summer 1986 

EPA 1987ac 

EPA survey, urban area; 

n=14; summer 1986 

EPA 1987ac 

Urban area; n=22; 1990– 

1991 

Kelly et al. 1993 

Urban area; spring 1984 Singh et al. 1985 

Rural area; May 1981– Roberts et al. 1985 

December 1982 

Night, n=81–86; day, n=86– Wallace et al. 1985 

90; fall 1981 

Night, n=346–348; day, Wallace et al. 1985 

n=339–341; fall 1981 

 Rappaport and 

Kupper 2004 

 Rappaport and 

Kupper 2004 

a

Average±standard deviation 

b

Median 

c

Concentrations reported as ppb carbon; values presented in table represent conversion equivalents as ppb 

benzene. 

d

Weighted average 

EPA = Environmental Protection Agency; max = maximum; N = number of samples 
BENZENE 267 

6. POTENTIAL FOR HUMAN EXPOSURE 

Volatile organic compounds, including benzene, were measured at 11 monitoring sites in Anchorage, 

Alaska, in a year-long study ending in April 1994 (Taylor and Morris 1995). Average annual benzene 

concentrations ranged from a minimum of 1.15 parts per billion by volume (ppbv) in a low density 

residential area to 5.44 ppbv near a major midtown intersection. In a neighborhood where residents were 

complaining of petroleum odors, the highest benzene concentrations (annual average 3.74 ppbv) were 

measured on a cliff over the petroleum tank farm, within 50 meters of a petroleum storage tank. 

Ambient air samples from 44 sites in 39 U.S. urban areas were collected from 6 to 9 a.m. during June 

through September of 1984, 1985, and 1986. Benzene was present in every sample. The median benzene 

site concentrations ranged from 0.8 to 6 ppb, with the overall median being 2.1 ppb (estimated detection 

limit ~0.04 ppb carbon; conversion equivalent ≈0.007 ppb benzene). The data indicated that mobile 

sources were the major source of benzene in the vast majority of samples (EPA 1987a). Benzene 

concentrations were in the range of >1–<5 ppbv for all of the 13 sites tested in a 1996 study. The sites 

included: Baton Rouge, Louisiana; Brownsville, Texas; Brattleboro, Vermont; Burlington, Vermont; 

Camden, New Jersey; El Paso, Texas; Garyville, Louisiana; Galveston, Texas; Hanville, Louisiana; Port 

Neches, Texas; Rutland, Vermont; Underhill, Vermont; and Winooski, Vermont (Mohamed et al. 2002). 

The following daily median benzene air concentrations were reported in the Volatile Organic Compound 

National Ambient Database (1975–1985): remote (0.16 ppb), rural (0.47 ppb), suburban (1.8 ppb), urban 

(1.8 ppb), indoor air (1.8 ppb), and workplace air (2.1 ppb). The outdoor air data represent 300 cities in 

42 states while the indoor air data represent 30 cities in 16 states (Shah and Singh 1988). In a NHEXAS 

study of six states in the Great Lakes region, benzene was found in 99.7% of the 386 personal air samples 

taken with an average concentration of 2.35 ppb. Benzene was also found in 100% of both indoor and 

outdoor samples with concentrations averaging 2.25 and 1.13 ppb, respectively (Clayton et al. 1999). 

An EPA study of the concentration of various pollutants in 1990 showed an average concentration of 

benzene of 0.948 ppb. The source of benzene included manufacturing facilities and manufacturers that 

emit benzene to the environment (17%), mobile (60%), and background (23%) sources (Morello-Frosch 

et al. 2000). 

In California, motor vehicle exhaust accounted for over 70% of the nonsmoking population’s exposure to 

ambient benzene (Cal EPA 1987). The 1984 population-weighted average benzene concentration in 

California was estimated to be 3.3 ppb (Cal EPA 1987). Benzene emissions in a Los Angeles roadway 

tunnel were measured at a concentration of 382 mg/L (118,420 ppm) (Fraser et al. 1998). 
BENZENE 268 

6. POTENTIAL FOR HUMAN EXPOSURE 

Industrial sites that produce benzene are monitored often. Analysis of ambient air samples collected in 

industrial areas showed benzene levels ranging from 0.13 to 5 ppb in Iberville Parish, Louisiana, an area 

that included many organic chemical and petroleum producer, user, and storage facilities located along 

the Mississippi River (Pellizzari 1982). Indoor and outdoor air measurements were made in August 1987 

in the Kanawha Valley region of West Virginia, which is the center of a heavily industrialized area known 

for its chemical manufacturing. The mean, maximum, and median indoor concentrations of benzene were 

2.1, 14.9, and 0.64 ppb, respectively. The median outdoor ambient air concentration of benzene was 

0.78 ppb (Cohen et al. 1989). 

An analysis of gas from 20 Class II (municipal) landfills revealed a maximum concentration of 32 ppm 

for benzene (Wood and Porter 1987). Benzene was measured in the vicinity of the BKK landfill, a 

hazardous waste landfill in California, at a maximum concentration of 1.2 ppb (Bennett 1987). Maximum 

estimated levels of benzene in air near uncontrolled (Superfund) hazardous waste sites were 59.5 ppb at 

the Kin-Buc Landfill (Edison, New Jersey) and 162.8 ppb in Love Canal basements (Niagara Falls, New 

York) (Bennett 1987; Pellizzari 1982). 

According to the National Cancer Institute, the average concentration of benzene in 1998 was 0.58 ppb in 

metropolitan areas (NCI 2003). A study of New York City air found mean values of 0.80, 1.87, and 

1.47 ppb for outdoor (home), indoor (home), and personal air, respectively (Kinney et al. 2002). 

According to the New York Department of Health, the air from about 50% of oil fuel heated homes 

between the years 1997 and 2003 contained benzene concentrations ≥0.69 ppb inside the home, and 

0.47 ppb in the area outside their homes. New York City was not included in this study (NYSDOH 

2005). Concentrations of VOCs were measured in 12 northern California office buildings with three 

different types of ventilation. Benzene concentrations ranged from <0.1 to 2.7 ppb, with a geometric 

mean of 0.98 ppb (Daisey et al. 1994). 

Population-weighted personal exposures to benzene exceeded the outdoor air concentrations in data from 

EPA's Total Exposure Assessment Methodology (TEAM) study. The overall mean personal exposure 

was about 4.7 ppb, compared to an overall mean outdoor concentration of only 1.9 ppb (Wallace 1989a). 

The study also reported that the median level of benzene in 185 homes without smokers was 2.2 ppb, and 

the median level of benzene in 343 homes with one or more smokers was 3.3 ppb (Wallace 1989a). This 

finding points to the possible significance of passive smoking as a source of benzene exposure. Indoor air 

samples taken from a smoke-filled bar contained 8.08–11.3 ppb of benzene (Brunnemann et al. 1989). A 
BENZENE 269 

6. POTENTIAL FOR HUMAN EXPOSURE 

study conducted by R.J. Reynolds Tobacco Company in smoking and nonsmoking homes revealed that 

benzene levels were elevated in smoking homes. In 24 nonsmoking homes, the mean benzene 

concentration was 1.21 ppb with a maximum of 5.93 ppb. In 25 smoking homes, the mean benzene 

concentration was 1.73 ppb, with a maximum of 8.43 ppb. However, benzene was not significantly 

correlated or associated with 3-ethenylpyridine, a proposed vapor phase environmental tobacco smoke 

marker (Heavner et al. 1995). 

A British study measured benzene concentrations from both rural and urban areas in 1995. The highest 

concentrations of benzene were in Southampton urban center, London roadside, and Liverpool, which 

contained measured benzene levels of 2.53, 1.69,, and 1.60 ppb, respectively. The lowest measured 

benzene concentrations were in rural Hartwell and urban Edinborough, which contained 0.4 and 0.69 ppb 

of benzene, respectively (Duarte-Davidson 2001). 

6.4.2 Water 

Data from EPA's STOrage and RETrieval (STORET) database (2003–2005) showed that benzene was 

positively detected in 38% of the surface water samples collected at 571 observation stations ranging 

from concentrations too low to quantify to 100 μg/L (100 ppb) in one Utah test site. The sampling sites in 

the STORET database include both ambient and pipe sites. Ambient sites include streams, lakes, ponds, 

wells, reservoirs, canals, estuaries, and oceans and are intended to be indicative of general U.S. waterway 

conditions. Pipe sites refer to municipal or industrial influents or effluents (EPA 2005f). 

Benzene was found in 904 of the 8,200 samples tested by the U.S. Geological Survey across the United 

States and Canada (USGS 2005). The EPA found benzene in 13,919 and 1,119 groundwater and surface 

water samples, respectively (EPA 2001). 

Benzene is found in both polluted and unpolluted waters and rainwater. Measured benzene levels in open 

ocean from the Gulf of Mexico in 1977 in relatively unpolluted and polluted waters were 5–15 ng/kg (5– 

15 parts per trillion [ppt]), and 5–40 ng/kg (5–40 ppt), respectively (Sauer 1981). Benzene levels 

measured in coastal surface waters of the Gulf of Mexico were 6 ng/kg (6 ppt) in relatively unpolluted 

waters and 50–175 ng/kg (50–175 ppt) in polluted coastal waters (Sauer 1981). Benzene has been 

detected in rainwater in the United Kingdom at a concentration of 87.2 ppb (Colenutt and Thorburn 

1980). 
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6. POTENTIAL FOR HUMAN EXPOSURE 

Benzene levels in water in the vicinity of five industrial facilities using or producing benzene ranged from 

<1 ppb to a high of 179 ppb found in plant effluent. In general, benzene in plant effluents quickly 

dispersed in rivers or streams to levels of 1–2 ppb or less (EPA 1979). The maximum benzene levels 

observed in monitoring wells in plumes from fuel spills at gasoline service stations ranged from 1,200 to 

19,000 ppb (Salanitro 1993). A monitoring well in the vicinity of a bulk storage facility had a maximum 

benzene level of 45,000 ppb (Salanitro 1993). In northern Virginia, approximately 200,000 gallons of 

liquid hydrocarbons were released from a fuel-storage terminal into the underlying soil. A dichloromethane extract of groundwater from a monitoring well in the same area gave a benzene concentration of 

52.1 ppm (Mushrush et al. 1994). Benzene has been detected at concentrations ranging from 16 to 

110 ppb in landfill leachate from a landfill that accepted both municipal and industrial wastes (Cline and 

Viste 1985). 

Composite data from the Comprehensive Emergency Response, Compensation, and Liability Act 

(CERCLA) monitoring program indicate that benzene was detected at a frequency of 11.2% in 

groundwater in the vicinity of 178 inactive hazardous waste disposal sites (Plumb 1987). Data from a 

1980 national survey by the Council on Environmental Quality on groundwater and surface water 

contamination showed benzene concentrations in contaminated drinking water wells in New York, New 

Jersey, and Connecticut ranged from 30 to 330 ppb, with the highest concentration in drinking water from 

surface water sources reported to be 4.4 ppb (Burmaster 1982). Benzene has also been identified but not 

quantified as one of the major organic constituents in commercially bottled artisan water in the United 

States (Dowty et al. 1975). At detection limits ranging from 0.2 to 1.0 ppb, benzene was detected at a 

concentration of 2 ppb in only one sample of 182 samples of bottled drinking water (Page et al. 1993). In 

six states of the Great Lakes region, unspecified amounts of benzene were found in 5.7% of the 

247 drinking water sites tested (Clayton et al. 1999). 

Because the New Jersey Department of Environmental Protection identified serious water issues in a 

930 km2

 area within metropolitan Philadelphia, tests have been done to find other water sources such as 

the Kirkwood-Cohansey aquifer system. A study of 57 water samples taken from the shallow ground 

water supply at Kirkwood Cohansey aquifer in New Jersey showed that the majority of samples (50) 

contained between 0.2 and 1 ppbv of benzene, three samples contained <0.1 ppbv, and four samples 

contained >1 ppbv (Baehr et al. 1999). 
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6. POTENTIAL FOR HUMAN EXPOSURE 

6.4.3 Sediment and Soil 

A Canadian study estimated that benzene is absorbed by the soil at a rate of 100 tons/year (MacLeod and 

MacKay 1999). Data from EPA's STORET database (2003–2005) showed that benzene had been 

positively detected in sediment samples taken at 9% of 355 observation stations with a median level of 

<5 ppb (EPA 2005f). The concentration of benzene in soil near factories where benzene was produced or 

used ranged from 2 to 191 ppb (IARC 1982). 

Benzene levels ranging from <2 to 191 ppb were recorded in the vicinity of five industrial facilities using 

or producing benzene (EPA 1979). Sediment levels ranging from 8 to 21 ppb were detected in Lake 

Pontchartrain in Louisiana (Ferrario et al. 1985). In northern Virginia, approximately 200,000 gallons of 

liquid hydrocarbons were released from a fuel-storage terminal into the underlying soil. Soil about 

1,000 feet from that storage terminal contained benzene gas at a concentration of 1,500 ppm at a depth of 

10 feet (Mushrush et al. 1994). 

6.4.4 Other Environmental Media 

Benzene has been detected in a variety of food. Benzene has been reported to occur in fruits, fish, 

vegetables, nuts, dairy products, beverages, and eggs (EPA 1982a). Although benzene has been detected 

in dairy products, there is no evidence of the presence of benzene in either cow's milk or human breast 

milk (Hattemer-Frey et al. 1990). Eggs had the highest concentrations (2,100 ppb [uncooked] and 500– 

1,900 ppb [hard-boiled]), followed by haddock (100–200 ppb), Jamaican rum (120 ppb), irradiated beef 

(19 ppb), heat-treated canned beef (2 ppb), and butter (0.5 ppb). Lamb, mutton, veal, and chicken all had 

<10 ppb benzene (when the meats were cooked) (EPA 1980b, 1982a). A survey of more than 50 foods 

collected and analyzed from 1991 to early 1992 (McNeal et al. 1993) revealed that foods (including eggs) 

without added benzoates contained benzene at levels ≤2 ng/g. The level of benzene in foods containing 

added benzoates in addition to ascorbates (e.g., imitation strawberry preserves, taco sauce, and duck 

sauce) ranged from <1 to 38 ng/g. In many foods, the presence of benzene is likely to be due to uptake 

from the air (Grob et al. 1990). This conclusion was supported by the fact that the uptake decreased with 

a decrease in exposed surface area of foods and contact time with air (Grob et al. 1990). 

The U.S. Food and Drug Administration sponsored a 5-year study to determine the amount of volatile 

organics in food from 1996 to 2000. The results are shown in Table 6-3 (Fleming-Jones and Smith 2003). 

Benzene was found in cheddar cheese, cream cheese, margarine, butter, sour cream, ground beef, 

bologna, hamburger, cheeseburger, pork, beef frankfurters, tuna canned in oil, chicken nuggets, chocolate 
BENZENE 272 

6. POTENTIAL FOR HUMAN EXPOSURE 

Table 6-3. Benzene in Fooda 

Number of cases found Concentration minimumFood (n=14) maximum in ppb 

Cheddar cheese 2 20–47 

Mixed nuts 3 1–6 

Ground beef 12 9–190 

Pork bacon 6 2–17 

Banana, raw 13 11–132 

Frankfurters, beef 4 2–11 

Cream cheese 3 1–17 

Chocolate cake icing 2 2–23 

Tuna canned in oil 7 4–13 

Fruit flavored cereal 5 2–21 

Eggs scrambled 4 2–40 

Peanut butter 5 2–25 

Avocado, raw 10 3–30 

Popcorn, popped in oil 3 4–22 

Blueberry muffin 3 3–8 

Strawberries, raw 1 1 

Cola carbonated 3 1–138 

Orange, raw 2 11–15 

Coleslaw with dressing 14 11–102 

Sweet roll danish 1 3–3 

Potato chips 2 2–7 

Fruit flavored sherbet 3 3–61 

Quarter pound hamburger cooked 11 4–47 

Margarine 1 7 

Sandwich cookies 3 1–39 

Butter 6 4-22 

Chocolate chip cookies 2 1–8 

Sour cream 2 3–15 

Apple pie fresh/frozen 4 2–11 

Chicken nuggets fast food 4 2–5 

Graham crackers 2 1–9 

French fries fast food 3 2–56 

Cheeseburger quarter pound 8 5–44 

Cheese pizza 2 1–2 

Bologna 4 2–44 

Cheese and pepperoni pizza 4 8–30 

Olive/safflower oil 6 1–46 

Sugar cookies 3 8–30 

Cake doughnuts with icing 2 3 

Popscicle 4 1–10 

a

Derived from Fleming-Jones and Smith 2003 
BENZENE 273 

6. POTENTIAL FOR HUMAN EXPOSURE 

cake icing, sandwich cookie, chocolate chip cookies, graham crackers, sugar cookies, cake doughnuts 

with icing, french fries, apple pie, cola carbonated beverages, sweet roll Danish, potato chips, cheese 

pizza, cheese and pepperoni pizza, mixed nuts, fruit-flavored cereal, fruit flavored sorbet, popsicles, 

olive/safflower oil, scrambled eggs, peanut butter, popcorn popped in oil, blueberry muffins, coleslaw 

with dressing, raw banana, avocado, oranges, and strawberries. American cheese was the only food tested 

that did not contain benzene. Foods with the greatest maximum concentration of benzene included 

ground beef (maximum 190 ppb), raw bananas (maximum 132 ppb), carbonated cola (maximum 

138 ppb), and coleslaw with dressing (maximum 102 ppb). 

As part of a program to identify possible exposures that may be important in the high incidence of lung 

cancer among women in Shanghai, China, Pellizzari et al. (1995) qualitatively identified the volatile 

components emitted during heating of cooking oils to 265 °C. This study found that, on a relative basis, 

the intensity of the benzene peak in the total ion current chromatogram of vapors from Chinese rapeseed 

oil (commonly used in wok cooking) was 14-, 6.6-, and 1.7-fold greater than in vapors from peanut, 

soybean, and other canola (rapeseed) oils, respectively. 

Cigarette smoke remains an important source of human exposure to benzene. The amount of benzene 

measured in mainstream smoke ranged from 5.9 to 73 μg/cigarette (Brunnemann et al. 1990). Larger 

amounts of benzene were found in sidestream smoke, ranging from 345 to 653 μg/cigarette (Brunnemann 

et al. 1990). Benzene has been found in vapor from cigarette smoke in concentrations ranging from 3.2 to 

61.2 μg/cigarette depending on the brand of cigarette. The amount of tar in the cigarette was not directly 

related to the benzene concentration (Darrall et al. 1998). 

6.5 GENERAL POPULATION AND OCCUPATIONAL EXPOSURE 

Exposure to low levels of environmental benzene is unavoidable due to the ubiquitous presence of 

benzene in the environment from a variety of anthropogenic sources. Benzene can be detected in blood 

samples. In a study designed to establish reference blood concentrations of benzene and other selected 

volatile organic compounds in the general population of the United States, Ashley et al. (1994) measured 

blood benzene levels in 883 people (not occupationally exposed) in the United States who had 

participated in the Third National Health and Nutrition Examination Survey (NHANES III). Within this 

group of nonoccupationally exposed subjects, the mean and median blood benzene levels were 0.13 and 

0.061 ppb, respectively. Seven nonsmoking subjects from this study group were assessed for blood 

benzene levels just prior to entering examination vans with limited potential for benzene exposure and at 
BENZENE 274 

6. POTENTIAL FOR HUMAN EXPOSURE 

the end of a 3-hour period in this environment. Mean blood benzene levels prior to van entry and after 

3 hours in the van measured 0.046 ppb (range from not detected to 0.061 ppb) and 0.033 ppb (range from 

not detected to 0.055 ppb), respectively. Lemire et al. (2004) reported a median blood benzene level of 

0.047 ppb in a subgroup of 546 nonsmoking and nonoccupationally exposed subjects from the NHANES 

III study group. Elevated blood benzene levels may be expected among subjects with potential for 

elevated exposure to benzene, such as smokers, commuters, gas station attendants, and people who use 

products that emit benzene. 

The Total Exposure Assessment Monitoring (TEAM) studies, carried out by the EPA between 1980 and 

1990, suggested that for many chemicals, including benzene, the most important sources of pollution are 

small and close to the person, and that exposures are not clearly correlated with emissions. For example, 

the TEAM study findings indicated that nearly 85% of atmospheric benzene in outdoor air is produced by 

cars burning petroleum products and the remaining 15% is produced by industry. Despite the fact that 

petroleum products contribute to the majority of benzene in the atmosphere, half of the total national 

personal exposure to benzene comes from cigarette smoke (Wallace 1995). In fact, breath measurements 

of benzene provided by the TEAM study between 1979 and 1988 identified smoking as the single most 

important source of benzene exposure for about 40 million U.S. smokers (Wallace 1989b). Even passive 

exposure to cigarette smoke is responsible for more benzene exposure (about 5% of the total) than the 

emissions from the entire industrial capacity of the United States (about 3% of the total) (Wallace 1995). 

A breakdown of the emissions and exposure sources for benzene that was derived from the Los Angeles 

TEAM study data (Wallace et al. 1991) is given in Figure 6-3. The reason that a relatively small source 

of emissions can have such a large effect on exposure is the efficiency of delivery. Wallace (1995) 

reports that one cigarette delivers an average of 55 μg of benzene with nearly 100% efficiency to the 

smoker. Benzene from industrial sources is dissipated into the atmosphere. 

Smokers (n=200) in the TEAM study had a mean breath concentration of 15 μg/m3

 (4.7 ppb), almost 

10 times the level of 1.5–2 μg/m3

 (0.47–0.63 ppb) observed in more than 300 nonsmokers (Wallace 

1989b). Smokers also had about 6–10 times as much benzene in their blood as non-smokers (Wallace 

1995). In another study, benzene concentrations were compared in the breath of smokers and nonsmokers 

and in ambient air in both an urban area of San Francisco and in a more remote area of Stinson Beach, 

California (Wester et al. 1986). In the urban area, benzene in smokers' breath (6.8±3.0 ppb) was greater 

than in nonsmokers' breath (2.5±0.8 ppb) and smokers' ambient air (3.3±0.8 ppb). In the remote area, the 

same pattern was observed. This suggests that smoking represented an additional source of benzene 

above that of outdoor ambient air (Wester et al. 1986). In 10 of 11 homes inhabited by tobacco smokers, 
Activitie s, 

Hom e 

Sources , 16% 

Cigare ttes, 

BENZENE 275 

6. POTENTIAL FOR HUMAN EXPOSURE 

Figure 6-3. Benzene Emissions and Exposures 

Benzene Emissions 

Personal and 

Home, 3% 

Cigarettes, 

0.1% 

Benzene Exposures 

40% Car Exhaust, 

18% 

Indus try 3%, Personal 

ETS, 5% 

18% 

Note: A comparison of benzene emission sources (home sources include paints and petroleum products; personal 

activities include driving and use of consumer products that contain benzene). Data taken from Wallace et al. (1991). 

Industry, 14% 

Cars, 82% 
BENZENE 276 

6. POTENTIAL FOR HUMAN EXPOSURE 

mean indoor and personal benzene concentrations were 2–5 times higher than outdoor levels (Thomas et 

al. 1993). 

Assuming that the average sales-weighted tar and nicotine cigarette yields 57 μg benzene in mainstream 

smoke, Wallace (1989a) estimated that the average smoker (32 cigarettes per day) takes in about 1.8 mg 

benzene per day from smoking. This is nearly 10 times the average daily intake of nonsmokers (Wallace 

1989a). On the assumption that intake of benzene from each cigarette is 30 μg, Fishbein (1992) has 

calculated that a smoker who consumes two packs of cigarettes per day will have an additional daily 

intake of 1,200 μg. 

A British study of rural and urban environments suggests that benzene exposure is greatly affected by 

proximity to smokers (Duarte-Davidson et al. 2001). Air concentrations of benzene at an urban center in 

South Hampton averaged about 2.5 ppb, while in a rural location in Hartwell, the average amount of 

benzene in the air was 0.41 ppb. Air at a smoky pub was found to contain 22 ppb of benzene. Comparing 

the daily doses of rural nonsmokers, urban nonsmokers, urban passive smokers, and urban smokers, very 

little difference between the rural nonsmokers’ 24 ppb daily dose and the urban nonsmokers’ 30 ppb daily 

dose was found. Passive urban smokers, on average, have a daily benzene exposure dose of 38 ppb of 

benzene while smokers have a daily exposure dose of benzene of 163 ppb. On average, nonsmokers in 

urban and rural environments have estimated benzene intakes of 1.15 and 1.5 μg/kg body weight/day. 

Women tend to intake more of benzene per kg body weight than men. Passive smokers’ estimated daily 

intake averages are 2.10 and 1.74 μg/kg body weight/day for women and men, respectively. Urban 

women and men smokers’ estimated intakes are estimated at 9.00 and 7.46 μg/kg body weight/day, 

respectively; this is equivalent to an atmospheric concentration of 8.2 ppb (Duarte-Davidson et al. 2001). 

In 1990, a study in Germany analyzed factors that predicted people’s exposures to VOCs and found that 

while smoking was the most significant determinant of benzene exposure, automobile-related activities, 

such as refueling and driving, were also significant (Hoffmann et al. 2000). Virtually all (99.9%) of the 

benzene released into the environment finally distributes itself into the air. The general population may 

be exposed to benzene through inhalation of contaminated air, particularly in areas of heavy motor 

vehicle traffic and around gas stations. Compared to inhalation, dermal exposure probably constitutes a 

minor portion of benzene exposure for the general population. Personal sources account for 18% of the 

total exposure of the general population to benzene. The main personal sources (other than smoking 
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6. POTENTIAL FOR HUMAN EXPOSURE 

cigarettes) are driving or riding in automobiles and using products that emit benzene (paints, adhesives, 

marking pens, rubber products, and tapes) (Wallace 1989a). 

Benzene constitutes 1–2% of most blends of gasoline and is released as a vapor from vehicular emissions. 

Since benzene is a constituent of auto exhaust and fuel evaporation, people who spend more time in cars 

or in areas of heavy traffic have increased personal exposure to benzene. Assuming an average benzene 

concentration of 40 μg/m3

 (12.5 ppb) for a moving automobile and an exposure duration of 1 hour/day, 

the calculated intake for driving or riding in an automobile is 40 μg/day (Wallace 1989a). In an 

investigation of exposure to methyl tertiary-butyl ether (MTBE) in oxygenated gasoline in Stamford, 

Connecticut, venous blood samples were collected from 14 commuters and from 30 other persons who 

worked in the vicinity of traffic or automobiles. In addition to MTBE, the samples were analyzed for five 

chemicals, including benzene. Levels of benzene in the blood of 11 nonsmoking men and women 

commuters ranged from 0.10 to 0.20 μg/L (median=0.12 μg/L). Blood benzene levels of 0.29, 0.14, and 

0.58 μg/L were measured in one female and two male smoking commuters, respectively. In 

12 nonsmoking male car repair workers, blood benzene levels ranged from 0.11 to 0.98 μg/L 

(median=0.19 μg/L); in 8 smoking male car repair workers, levels ranged from 0.17 to 0.67 μg/L 

(median=0.42 μg/L). Three nonsmoking male gasoline attendants had blood benzene levels ranging from 

0.32 to 0.47 μg/L (median=0.36 μg/L) (White et al. 1993). 

Pumping gasoline can also be a significant source of exposure to benzene. A study conducted between 

July 1998 and March 1999 that comprised of 39 customers of gasoline self service stations from North 

Carolina, measured the benzene level in the air around the station as well as the levels of benzene in 

customers’ breath prior to and immediately after fueling (Egeghy et al. 2000). Benzene levels in the air 

around the station ranged from <0.02 to 11.16 ppm, with a mean (±1 standard deviation) of 

0.91 (±1.8) ppm. The range of benzene levels in the breath of customers prior to fueling was <0.001– 

0.022 ppm with a mean (±1 standard deviation) of 0.0027 (±0.0034) ppm while the range of benzene 

levels in the breath of customers after re-fueling was <0.001–0.434 ppm with a mean (±1 standard 

deviation) of 0.05 (±0.081) ppm (Egeghy et al. 2000). Another study reported a benzene concentration of 

1 ppm at the breathing-level of a person pumping gas (EPA 1986). Using this concentration and an 

estimated 70 minutes/year of time spent pumping gasoline, a benzene intake of 10 μg/day has been 

calculated (Wallace 1989a). In a group of 26 subjects who were not occupationally exposed to benzene, 

but were exposed to benzene during refueling in Fairbanks, Alaska, median blood benzene levels prior to 

and immediately following refueling were 0.19 ppb (range 0.08–0.65 ppb) and 0.54 ppb (range 0.13– 

1.70 ppb), respectively (Backer et al. 1997). 
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6. POTENTIAL FOR HUMAN EXPOSURE 

Gasoline vapors vented into the home from attached garages can also increase indoor air exposure to 

benzene (Wallace 1989a, 1989b). Depending on airflow from garage to living areas, mean indoor 

benzene concentrations in houses with a garage were 2–5 times higher than outdoor levels in most homes 

(Thomas et al. 1993). Benzene levels in four garages during different times in a day ranged from 0.94 to 

61.3 ppb. The higher concentrations of benzene in these garages were not only from vehicular activity, 

but also in varying proportions from stored gasoline, paints, and benzene-containing consumer products 

(Thomas et al. 1993). Inhalation exposure to off-gassing from benzene-containing products and to 

evaporative emissions from automobiles in attached garages has been estimated to be 150 μg/day 

(Wallace 1989a). 

A study of human exposure to benzene in the California South Coast Air Basin showed that benzene 

exposure in that area has decreased greatly since 1989. Adult smokers saw a 28% decrease in benzene 

exposure from 1989 to 1997, while adult nonsmokers and adolescents saw a decrease in benzene exposure 

of 67 and 55%, respectively. Where people were being exposed to benzene also changed during that 

time. Ambient air was the source of 49% of the benzene that nonsmokers were exposed to in 1997, which 

is less than the 59% from 1987. For smokers in the region, 85% of their benzene exposure was from 

smoking and 4% was from the ambient air in 1997. This shows an increase in benzene exposure from 

smoking since 1987 (78%) and a decrease in exposure from the ambient air (12%) (Fruin et al. 2001). 

The main sources credited with this decrease in exposure are reformulated gasoline and stricter air 

emission laws as well as smoking control measures. 

Another source of exposure to benzene for the general population is the use of domestic wood stoves. It 

has been estimated that approximately 10% of the space heating in urban areas of the northern United 

States is from wood burning, with up to 50% in smaller, rural towns (Larson and Koenig 1994). Benzene 

has been found to be a major component of the emissions from wood burning, especially from efficient 

flame combustion, and constituted roughly 10–20% by weight of total non-methane hydrocarbons 

(Barrefors and Petersson 1995). It should be noted, however, that chimney emissions result in much 

lower human exposure than equally large emissions at the ground level. 

Other sources of inhalation exposure to benzene include air around hazardous waste sites, industrial 

facilities, off-gassing from particle board, and off-gassing from contaminated water during showering and 

cooking. Based on the TEAM study findings, it appears that the following are not important sources of 
BENZENE 279 

6. POTENTIAL FOR HUMAN EXPOSURE 

exposure to benzene on a nationwide basis: chemical manufacturing facilities, petroleum refining 

operations, oil storage tanks, drinking water, food, and beverages (Wallace 1989a). 

Average water intake of benzene (assuming a typical drinking water concentration of 0.1 ppb and a 

consumption of 2 L/day) is 0.2 μg/day (HSDB 2007). According to another estimate, the daily intakes of 

benzene for a nonsmoking individual (not exposed to secondary smoke) are 1–550 μg (Fishbein 1992). 

While most human exposure to benzene is believed to be through inhalation, studies show that benzene 

can permeate skin with a permeability factor of about 0.14–0.18 cm/hour at 25 °C. The permeability 

factor was not affected by moisturizer, baby oil, or insect repellent; however, it was affected by 

temperature (50 °C) and sunscreen with the permeability factors increasing to 0.26 and 0.24 cm/hour, 

respectively (Nakai et al. 1997). 

Individuals employed in industries that use or make benzene or products containing benzene may be 

exposed to the highest concentrations of benzene. The National Occupational Exposure Survey (NOES), 

conducted by NIOSH from 1981 to 1983, estimated that approximately 272,300 workers employed in 

various professions were potentially exposed to benzene in the United States. Approximately half of 

these workers were employed in general medical and surgical hospitals, and their occupations included 

nurses and aides, physicians, technicians, technologists, therapists, dieticians, pharmacists, and janitors 

(NIOSH 1990). The NOES database does not contain information on the frequency, concentration, or 

duration of exposure; the survey provides only estimates of workers potentially exposed to chemicals in 

the workplace. The current OSHA permissible limit for an 8-hour TWA exposure to benzene is 1 ppm 

and a short-term exposure limit in any 15-minute period is 5 ppm (OSHA 2003). The NIOSH 

recommended exposure limit is 0.1 ppm for an 8-hour TWA and 1 ppm for short-term exposure (NIOSH 

1992b). In 1987, OSHA estimated that approximately 238,000 workers were exposed to benzene in seven 

major industry sectors, including petrochemical plants, petroleum refineries, coke and coal chemicals, tire 

manufacturers, bulk terminals, bulk plants, and transportation via tank trucks (Table 6-4) (OSHA 1987). 

Approximately 10,000 workers were estimated to be exposed to TWA concentrations in excess of the 

1 ppm standard. This estimate did not include firms covered by the exclusions, firms under jurisdiction of 

other agencies, or firms involved in the use of products containing small quantities of benzene. The 

uptake of benzene by workers in a municipal waste incinerator in Germany was assessed by measuring 

benzene levels in blood (Angerer et al. 1991). No significant difference (p<0.05) in blood benzene levels 

between workers and controls were detected (mean 0.22 μg/L for nonsmoking workers versus 0.25 μg/L 
BENZENE 280 

6. POTENTIAL FOR HUMAN EXPOSURE 

Table 6-4. Percentage of Employees Exposed to Benzene by Exposure Level and 

Industry Divisiona
 

Percentage of observations in each exposure category 

according to range of 8-hour TWA benzene Total 

concentrations (ppm) number of 

Industry sector 0.0–0.1 0.11–0.5 0.51–1.0 1.1–5.0 5.1–10 10+ employees 

Petrochemical plantsb 74.6 23.0 2.4 0.0 4,300 

Petroleum refineriesc,d 64.6 26.1 4.6 3.8 0.5 0.4 47,547 

Coke and coal chemicalse 0.0 39.3 27.6 27.5 4.4 1.3 947f 

Tire manufacturersc

 53.4 37.5 6.3 2.8 0.0 0.0 65,000 

Bulk terminalsc

 57.8 32.8 5.3 3.7 0.3 0.1 27,095 

Bulk plantsc

 57.8 32.8 5.3 3.7 0.3 0.1 45,323 

Transportation via tank truckc

 68.4 23.1 5.3 2.9 0.1 0.2 47,600 

Total 237,812 

a

Derived from OSHA 1987 

b

Percentages represent the portion of workers whose average exposures are in each category. c

Percentages represent the portion of sampling results in each category. d

Data do not reflect respirator use and sampling biases. 

e

Percentages represent the portion of workers whose average exposures are in each category. f

Excludes workers employed at the coke ovens. 

TWA = time-weighted average 
BENZENE 281 

6. POTENTIAL FOR HUMAN EXPOSURE 

for nonsmoking controls). OSHA requires the use of engineering controls and/or respiratory protection in 

situations where compliance with the TWA is not feasible (OSHA 1987). 

Certain jobs, such as gasoline station workers, firefighters, and dry cleaners, are believed to put people at 

a higher risk of benzene exposure. In an analysis of literature, it was estimated that workers in the area of 

crude petroleum and natural gas are exposed to 0.04 ppm benzene, while workers in petroleum refining, 

gas stations, and crude petroleum pipelines are exposed to 0.22, 0.12 and 0.25 ppm benzene, respectively. 

This study also showed that fire fighters are exposed to an average of 0.38 ppm benzene (van 

Wijngaarden and Stewart 2003). Workers from four different dry cleaning facilities in Korea had mean 

benzene air concentrations ranging from 2.7 to 3.2 ppb. Their exposure to benzene was dependent upon 

the type of solvent used for cleaning (Jo and Kim 2001). Benzene concentrations of 25.46 and 

1,331.29 ppb were found near the kiln and at the rotary line, respectively, inside a hazardous waste 

incinerator in Turkey (Bakoglu et al. 2004). 

A study comparing workers who were exposed to benzene regularly at work to people who were not 

exposed to benzene at work showed that while the general population in Italy had average blood benzene 

concentration of 165 ng/L, the people who were exposed to high benzene levels at work had an average 

benzene blood concentration of 186 ng/L. Immediately following their shift, the average benzene blood 

level samples from of benzene-exposed workers was 420 ng/L. The average blood benzene levels for 

smoking and nonsmoking occupationally exposed workers were 264 and 123 ng/L respectively 

(Brugnone et al. 1998). 

6.6 EXPOSURES OF CHILDREN 

This section focuses on exposures from conception to maturity at 18 years in humans. Differences from 

adults in susceptibility to hazardous substances are discussed in Section 3.7, Children’s Susceptibility. 

Children are not small adults. A child’s exposure may differ from an adult’s exposure in many ways. 

Children drink more fluids, eat more food, breathe more air per kilogram of body weight, and have a 

larger skin surface in proportion to their body volume. A child’s diet often differs from that of adults. 

The developing human’s source of nutrition changes with age: from placental nourishment to breast milk 

or formula to the diet of older children who eat more of certain types of foods than adults. A child’s 

behavior and lifestyle also influence exposure. Children crawl on the floor, put things in their mouths, 
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6. POTENTIAL FOR HUMAN EXPOSURE 

sometimes eat inappropriate things (such as dirt or paint chips), and spend more time outdoors. Children 

also are closer to the ground, and they do not use the judgment of adults to avoid hazards (NRC 1993). 

Children can be subject to increased benzene exposure by inhalation of second-hand smoke. In a study of 

nonsmoking rural families, urban families, and urban smoking families, infant exposure to benzene was 

estimated at doses of 15.3, 19.7, and 25.9 μg/day, respectively, with daily intakes of 1.68, 2.16, and 

2.55 μg/kg body weight/day, respectively. For children of the same classification, benzene exposure was 

measured at doses of 29.3, 37.6, and 49.3 μg/day, respectively, with daily intakes of 0.71, 0.91, and 

1.20 μg/kg bodyweight/day, respectively. For all infants and children, benzene exposure predominantly 

comes from the indoors (Duarte-Davidson et al. 2001). 

Depending on a child’s living environment, they may have higher exposure to benzene than adults. In a 

study of two lower-income areas of Minneapolis, children were found to have average personal benzene 

exposures of 0.66 and 0.53 ppb in the winter and spring, respectively. The highest concentration of 

benzene in their environment came from the home with winter and spring concentrations of 0.69 and 

0.66 ppb respectively, while the outdoor and school benzene concentrations were 0.41 and 0.19 ppb, 

respectively (Adgate et al. 2004). In Italy, concentrations of the benzene metabolite, trans,trans-muconic 

acid (MA), was measured in the urine of children from both urban areas in Naples and rural areas in 

Pollica. The mean urinary concentrations of MA detected for rural and urban children were 48.4 and 

98.7 μg/L (Amodio-Cocchieri et al. 2001). These studies also found no strong link between passive 

smoking and MA levels. The only factor that affected levels of MA in urine samples was how close the 

family lived to the road. A study in Rouen, France, compared benzene exposure and concentrations in 

nonsmoking parents and their children. Despite the fact that the children were exposed to slightly less 

benzene 3.47 ppb (11.1 μg/m3

) than their parents 4.51 ppb (14.4 μg/m3), there was no significant 

correlation between exposure means and urinary metabolite levels (Kouniali et al. 2003). 

Benzene concentrations in women and their tissue as well as breast milk have been a major concern. In a 

study of South Korean housewives living near service stations, indoor, outdoor and breath benzene 

concentrations averaged 5.73, 3.63, and 3.29 ppm, respectively (Jo and Moon 1999). Benzene has been 

found in mother’s milk (Pellizzari 1982). Benzene was found at mean concentrations of 0.06 μg/kg in 

23 samples of breast milk taken from a children’s hospital in Rome (Fabietti et al. 2004). While this may 

provide a mechanism by which infants are exposed to benzene, these concentrations are lower than in 

other foods. 
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6. POTENTIAL FOR HUMAN EXPOSURE 

6.7 POPULATIONS WITH POTENTIALLY HIGH EXPOSURES 

Individuals who live near hazardous waste sites or near leaking underground fuel storage tanks might be 

exposed to potentially high concentrations of benzene in their drinking water if they obtain tap water from 

wells located near these sources. In a series of experiments conducted in a single-family residence from 

June 11 to 13, 1991, exposure to benzene through contaminated residential water was monitored 

(Lindstrom et al. 1994). The residential water was contaminated with benzene and other hydrocarbons in 

1986. Periodic testing conducted from 1986 to 1991 showed benzene concentrations ranging from 33 to 

673 μg/L (ppb). The experiment involved an individual taking a 20-minute shower with the bathroom 

door closed, followed by 5 minutes for drying and dressing; then the bathroom door was opened and this 

individual was allowed to leave the house. Integrated 60- and 240-minute whole-air samples were 

collected from the bathroom, an adjacent bedroom, living room, and in ambient air. Glass, gas-tight 

syringe grab samples were simultaneously collected from the shower, bathroom, bedroom, and living 

room at 0, 10, 18, 20, 25, 25.5, and 30 minutes. Two members of the monitoring team were measured for 

6 hours using personal Tenax gas GC monitors. For the first 30 minutes of each experiment, one member 

was based in the bathroom and the other in the living room. Benzene concentrations in the shower head 

ranged from 185 to 367 μg/L (ppb), while drain level samples ranged from below the detectable limit 

(0.6 μg/L or ppb) to 198 μg/L (ppb). Analysis of the syringe samples suggested a pulse of benzene 

moving from the shower stall to the rest of the house over approximately 60 minutes. Peak benzene 

levels were measured in the shower stall at 18–20 minutes (758–1,670 μg/m3

), in the bathroom at 10– 

25 minutes (366–498 μg/m3

), in the bedroom at 25.5–30 minutes (81–146 μg/m3), and in the living room 

at 36–70 minutes (40–62 μg/m3

). The total benzene dose resulting from the shower was estimated to be 

approximately 281 μg, with 40% via inhalation and 60% via the dermal pathway (Lindstrom et al. 1994). 

The major source of exposure to benzene is cigarette smoke. A smoker of 32 cigarettes per day (the U.S. 

average per smoker) would have a benzene intake of approximately 1.8 mg/day (at least 10 times the 

average nonsmoker's intake) (Wallace 1989a). Median benzene concentrations in 343 homes with 

smokers averaged 3.3 ppb compared to 2.2 ppb in 185 homes without smokers. This represents a 50% 

increase in benzene exposure for nonsmokers exposed to passive smoke compared to nonsmokers not 

exposed to passive smoke (Wallace 1989a). In a study in Germany, the mean benzene concentrations for 

frequent smokers and nonsmokers were 6.1 and 2.4 ppb, respectively (Hoffmann et al. 2000). 
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6. POTENTIAL FOR HUMAN EXPOSURE 

6.8 ADEQUACY OF THE DATABASE 

Section 104(i)(5) of CERCLA, as amended, directs the Administrator of ATSDR (in consultation with the 

Administrator of EPA and agencies and programs of the Public Health Service) to assess whether 

adequate information on the health effects of benzene is available. Where adequate information is not 

available, ATSDR, in conjunction with NTP, is required to assure the initiation of a program of research 

designed to determine the health effects (and techniques for developing methods to determine such health 

effects) of benzene. 

The following categories of possible data needs have been identified by a joint team of scientists from 

ATSDR, NTP, and EPA. They are defined as substance-specific informational needs that if met would 

reduce the uncertainties of human health assessment. This definition should not be interpreted to mean 

that all data needs discussed in this section must be filled. In the future, the identified data needs will be 

evaluated and prioritized, and a substance-specific research agenda will be proposed. 

6.8.1 Identification of Data Needs 

Physical and Chemical Properties. The physical and chemical properties of benzene are well 

characterized and allow prediction of the transport and transformation of the compound in the 

environment. 

Production, Import/Export, Use, Release, and Disposal. According to the Emergency 

Planning and Community Right-to-Know Act of 1986, 42 U.S.C. Section 11023, industries are required 

to submit substance release and off-site transfer information to the EPA. The TRI, which contains this 

information for 2004, became available in May of 2006. This database is updated yearly and should 

provide a list of industrial production facilities and emissions. 

Benzene is one of the top 20 highest volume chemicals produced in the United States. In 1994, the U.S. 

production volume of benzene was 14.7 billion pounds (C&EN 1995). The production volume during the 

1984–1994 period has increased by 4% annually (C&EN 1995). The United States currently has a 

benzene production capacity of 11.8 billion liters (SRI 2004). Imports of benzene into the United States 

have generally ranged from 10,176 to 11,672 million pounds during 2002–2004 (USITC 2005). Exports 

increased from 21 million pounds in 2002 to 290 million pounds in 2003, but decreased to 145 million 

pounds in 2004 (USITC 2005). The major use of benzene is in the production of other chemicals 

(primarily ethylbenzene, cumene, and cyclohexane), accounting for approximately 91% of benzene 
BENZENE 285 

6. POTENTIAL FOR HUMAN EXPOSURE 

production volume. Benzene is also used in chemical laboratories as a solvent and a reactant (OSHA 

1977, 1987), and as an anti-knock agent in unleaded gasoline (Brief et al. 1980; EPA 1985a). The 

widespread use of benzene as a solvent has decreased in recent years. Many products that used benzene 

as a solvent in the past have replaced it with other organic solvents; however, benzene may still occur as a 

trace impurity in these products. Less than 2% of the amount of benzene produced is used as a solvent in 

such products as trade and industrial paints, rubber cements, adhesives, paint removers, artificial leather, 

and rubber goods. Benzene has also been used in the shoe manufacturing and rotogravure printing 

industries (EPA 1978; OSHA 1977). In the past, certain consumer products (such as some paint strippers, 

carburetor cleaners, denatured alcohol, and rubber cement used in tire patch kits and arts and crafts 

supplies) contained small amounts of benzene (Young et al. 1978). Other consumer products that 

contained benzene were certain types of carpet glue, textured carpet liquid detergent, and furniture wax 

(Wallace et al. 1987). The use of benzene in certain pesticides has been canceled. Benzene-containing 

wastes, such as commercial chemical products, manufacturing chemical intermediates, and spent solvents, 

are subject to federal and/or state hazardous waste regulations (HSDB 2007). Currently, the 

recommended method of disposal is to incinerate solvent mixtures and sludges at a temperature that 

ensures complete combustion. No additional information on the production, import/export, use, release, 

or disposal of benzene is needed at this time. 

Environmental Fate. Benzene released to the environment partitions mainly to the atmosphere 

(Mackay and Leinonen 1975). However, the compound can also be found in surface water and 

groundwater. Benzene is mobile in soil (Karickhoff 1981; Kenaga 1980); however, there is a need for 

more information on the leachability potential of benzene to groundwater in different soil types. Benzene 

is transformed in the atmosphere by photooxidation. Biodegradation, principally aerobic, is the most 

important fate process of benzene in water (Delfino and Miles 1985; McAllister and Chiang 1994; 

Salanitro 1993) and soil (Gibson 1980; Hopper 1978; Salanitro 1993). Benzene can persist in 

groundwater. No additional information on the environmental fate of benzene is needed at this time. 

Bioavailability from Environmental Media. Benzene can be absorbed following oral exposure 

(Thienes and Haley 1972), dermal exposure (Blank and McAuliffe 1985; Franz 1984; Laitinen et al. 

1994; Lindstrom et al. 1994; Lodén 1986; Susten et al. 1985), and inhalation exposure (Ashley et al. 

1994; Avis and Hutton 1993; Boogaard and van Sittert 1995; Brunnemann et al. 1989; Byrd et al. 1990; 

Etzel and Ashley 1994; Fustinoni et al. 1995; Ghittori et al. 1995; Gordian and Guay 1995; Hajimiragha 

et al. 1989; Hanzlick 1995; HazDat 2006; Karacic et al. 1995; Kok and Ong 1994; Lagorio et al. 1994a; 

Laitinen et al. 1994; Lauwerys et al. 1994; Lindstrom et al. 1994; Mannino et al. 1995; Nomiyama and 
BENZENE 286 

6. POTENTIAL FOR HUMAN EXPOSURE 

Nomiyama 1974a; Ong and Lee 1994; Ong et al. 1995; Pekari et al. 1992; Popp et al. 1994; Rauscher et 

al. 1994; Rothman et al. 1995; Ruppert et al. 1995; Scherer et al. 1995; Shamy et al. 1994; Srbova et al. 

1950). These routes of exposure may be of concern to humans because of the potential for benzene to 

contaminate the air (Bennett 1987; Black et al. 1980; Brief et al. 1980; Cal EPA 1987; Edgerton and Shah 

1992; EPA 1989, 1994d; Glass et al. 1986; Graedel 1978; Mayer et al. 1994; TRI02 2005; Wallace 1989a, 

1989b; Wallace and Pellizzari 1986; Wester et al. 1986; Wood and Porter 1987), drinking water (CDC 

1994; EPA 1979), and soil (HazDat 2006; Mushrush et al. 1994; TRI02 2005). Information on inhalation 

exposure and on the absorption of benzene following ingestion of plants grown in contaminated 

environments near hazardous waste sites would be helpful in determining bioavailability of the compound 

in these media. 

Food Chain Bioaccumulation. Benzene has an estimated low-to-moderate bioconcentration 

potential in aquatic organisms (Miller et al. 1985; Ogata et al. 1984) and some plants (Geyer et al. 1984). 

Most of the benzene accumulation on vegetation results from air-to-leaf transfer. Root uptake is not 

believed to be important (Hattemer-Frey et al. 1990). Biomagnification in aquatic food chains does not 

appear to be important (Ogata et al. 1984). No further information appears to be needed. 

Exposure Levels in Environmental Media. Reliable monitoring data for the levels of benzene in 

contaminated media at hazardous waste sites are needed so that the information obtained on levels of 

benzene in the environment can be used in combination with the known body burden of benzene to assess 

the potential risk of adverse health effects in populations living in the vicinity of hazardous waste sites. 

Benzene is widely distributed in the environment and has been detected in air (Clayton et al. 1999; EPA 

1987a; Mohamed et al. 2002; Morello-Frosch et al. 2000), water (EPA 2001, 2005f; Sauer 1981, USGS 

2005), soil (EPA 1979; Ferrario et al. 1985; MacLeod and MacKay 1999; Staples et al. 1985), sediment, 

and some foods (EPA 1980b, 1982a; Fleming-Jones and Smith 2003). The levels of benzene in air and 

water are well documented, but there is a need for more current information. Benzene is not expected to 

be a significant contaminant in aquatic foods (Geyer et al. 1984; Gossett et al. 1983; Miller et al. 1985; 

Ogata et al. 1984); however, some contamination of food crops consumed by humans may occur, 

primarily from air-to-leaf transfer (Hattemer-Frey et al. 1990). The total concentration of benzene on 

exposed food crops consumed by humans was estimated to be 587 ng/kg (Hattemer-Frey et al. 1990). 

Humans are at risk of exposure to benzene because of its widespread distribution in the environment, 

particularly in the atmosphere. Releases to the air from gasoline, smoking, and automobile exhaust 

constitute the major risk of potential exposure for the general population (Wallace 1995). Additional data 
BENZENE 287 

6. POTENTIAL FOR HUMAN EXPOSURE 

characterizing the concentration of benzene in drinking water, air, and soil surrounding hazardous waste 

sites would be helpful in assessing human exposure for populations living near these waste sites. In 

addition, more current data on levels of benzene in foods would be helpful in estimating intake of benzene 

from food. 

Exposure Levels in Humans. Benzene has been detected in human body fluids and tissues such as 

blood, urine, and fat (Brugnone et al. 1989; Chao et al. 1993; Karacic et al. 1987). Most of the 

monitoring data have come from occupational studies of specific worker populations exposed to benzene 

(Inoue et al. 1989b; Karacic et al. 1987; OSHA 1987; van Wijngaarden and Stewart 2003). Biological 

monitoring studies exist for the general population (Melikian et al. 1994). There is information for 

background levels in breath of smokers and nonsmokers (Wallace 1989b), baseline blood levels (Karacic 

et al. 1987), and levels of urinary metabolites in unexposed people (Inoue et al. 1989b). Information on 

exposure levels for populations living in the vicinity of hazardous waste sites would be helpful in 

estimating exposure in these groups. 

This information is necessary for assessing the need to conduct health studies on these populations. 

Exposures of Children. Benzene levels have been monitored in children and the environments in 

which they live. This information gives levels found for infants and children in rural and urban area as 

well as the levels found for children in homes of parents who smoke (Duarte-Davidson et al. 2001). 

There have been many studies relating oil and petroleum exposure to childhood leukemia and other 

diseases; however, the majority of these studies have not recorded benzene levels. More information 

about the exposures of children, particularly those subject to high exposures such as smoking, busy roads, 

and gasoline stations, are needed. 

Child health data needs relating to susceptibility are discussed in Section 3.12.2, Identification of Data 

Needs: Children’s Susceptibility. 

Exposure Registries. In 2001, 1,143 people were included in the benzene subset of the Volatile 

Organics Compounds subregistry of the National Exposure Registry. These people were exposed to 

benzene at a site in Texas. Demographic and health information was obtained on all the exposed persons; 

the information will be updated longitudinally. For those who were identified as exposed and were 

deceased, a death certificate will be obtained to ascertain cause of death. This activity was carried out by 

the Exposure and Disease Registry Branch (EDRB), Division of Health Studies (DHS), ATSDR. The 
BENZENE 288 

6. POTENTIAL FOR HUMAN EXPOSURE 

data became part of public-user data files maintained by ATSDR. The statistical analyses of the baseline 

data was completed and published (Agency for Toxic Substances and Disease Registry 1995). The 

information that was amassed in the National Exposure Registry will be used to facilitate the 

epidemiological research needs to assess adverse health outcomes that may be related to the exposure to 

this substance. 

6.8.2 Ongoing Studies 

The Federal Research in Progress (FEDRIP 2005) database provides additional information obtainable 

from a few ongoing studies that may fill in some of the data needs identified in Section 6.8.1. These 

studies are summarized in Table 6-5. 
BENZENE 289 

6. POTENTIAL FOR HUMAN EXPOSURE 

Table 6-5. Ongoing Studies on the Potential for Human Exposure to Benzenea 

Investigator Affiliation Research description Sponsor 

Cho, CY CHA Corporation; Laramie, 

Wyoming 

Study of the use of microwave 

technology for superfund site 

remediation 

NIEHS 

Fischer, LJ Understanding potential health 

hazards from groundwater and 

soil contamination of chemicals 

USDA 

commonly found at hazardous 

waste sites 

Greenberg, A Study of critical steps in the 

ring-opening metabolism of the 

human carcinogen benzene to 

muconaldehyde 

USDA 

Gurian, PL University of El Paso Texas; 

El Paso, Texas 

Modeling of organic 

compounds in drinking water 

National Institute of 

General Medical 

Sciences 

Nylander-French, LA University of North Carolina; 

Chapel Hill, North Carolina 

Assessment of dermal 

exposure to benzene and 

naphthalene 

NIEHS 

Rothman, N Study of occupational and 

environmental exposures 

Division of cancer 

epidemiology and 

genetics 

Scow, KM University of California; 

Davis, California 

Molecular characterization of 

aquifer microbial communities 

NIEHS 

Stenzel, PS National Institute of Health Study of the link between 

occupational exposure of 

carcinogens and cancer 

Division of cancer 

epidemiology and 

genetics 

Thrall, KD Oregon Health and Science 

University; Portland, Oregon 

Assessment of human volatile 

organic compounds exposure 

near Superfund sites 

NIEHS 

Weisel, CP Study of the modulation of 

benzene metabolism by 

exposure to environmental 

mixtures 

USDA 

a

FEDRIP 2005 

NIEHS = National Institute of Environmental Health Sciences; USDA = U.S. Department of Agriculture 
BENZENE 290 

6. POTENTIAL FOR HUMAN EXPOSURE 

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BENZENE 291 

7. ANALYTICAL METHODS 

The purpose of this chapter is to describe the analytical methods that are available for detecting, 

measuring, and/or monitoring benzene, its metabolites, and other biomarkers of exposure and effect to 

benzene. The intent is not to provide an exhaustive list of analytical methods. Rather, the intention is to 

identify well-established methods that are used as the standard methods of analysis. Many of the 

analytical methods used for environmental samples are the methods approved by federal agencies and 

organizations such as EPA and the National Institute for Occupational Safety and Health (NIOSH). Other 

methods presented in this chapter are those that are approved by groups such as the Association of 

Official Analytical Chemists (AOAC) and the American Public Health Association (APHA). 

Additionally, analytical methods are included that modify previously used methods to obtain lower 

detection limits and/or to improve accuracy and precision. 

7.1 BIOLOGICAL MATERIALS 

Analytical methods have been developed to measure benzene levels in exhaled breath, blood, and various 

body tissues. The primary method of analyzing for benzene in exhaled breath, body fluids, and tissues is 

gas chromatography (GC) coupled with either flame ionization detection (FID), photoionization detection 

(PID), or mass spectrometry (MS). Rigorous sample collection and preparation methods must be 

followed when analyzing for benzene to prevent contamination of the sample. A summary of commonly 

used methods of measuring benzene in biological samples is presented in Table 7-1. 

Breath samples are collected on a solid sorbent (EPA 1987b; Gruenke et al. 1986; Pellizzari et al. 1988; 

Wallace et al. 1985), in canisters (Thomas et al. 1991), or in a breath sampling tube and analyzed directly 

(Sherwood and Carter 1970). Samples collected on Tenax sorbent are subjected to a thermal 

desorption/cryofocussing step prior to analysis by capillary GC/MS (EPA 1987b; Pellizzari et al. 1988; 

Wallace et al. 1985). Techniques involving headspace analysis of benzene adsorbed on silica gel have 

also been used (Gruenke et al. 1986). MS detection generally provides the most sensitivity, from the low 

to sub-ppb. The selectivity of the methods is improved if capillary GC columns are used (Pellizzari et al. 

1988). Extraction of benzene from blood is frequently accomplished by either purge-and-trap or 

headspace analysis. In purge-and-trap analysis, an inert gas such as helium or nitrogen is passed through 

the sample, and purged volatiles are trapped on an appropriate solid sorbent (Antoine et al. 1986; Ashley 

et al. 1992, 1994; Michael et al. 1980). Recent improvements in the method have resulted in excellent 

sensitivity (300 ppt) and acceptable precision and accuracy (Ashley et al. 1992, 1994). The purge-andtrap method has also been used to analyze breast milk for other volatile organic compounds and could be 
BENZENE 292 

7. ANALYTICAL METHODS 

Table 7-1. Analytical Methods for Determining Benzene in Biological Samples 

Sample Analytical Sample Percent 

matrix Preparation method method detection limit recovery Reference 

Breath Collection on Tenax GC; HRGC/MS 3 ppt 70–130 Pellizzari et al. 

thermal desorption (IARC estimated 1988 

Method 5) 

Breath Collection in sampling tube; GC/FID 100 ppb 100 Sherwood and 

direct injection of sample Carter 1970 

Breath Collection on Tenax GC; HRGC/MS 1.6 ppb 86–90 Wallace et al. 

thermal desorption to on­ (5-L sample) 1986, 1985 

column cryogenic trap 

Breath Collection in bags, adsorption GC/MS-SIM 0.1 ppb NR Gruenke et al. 

on silica gel; desorption to 1986 

headspace vial; analysis of 

headspace gases 

Blood Purge and trap HRGC/MS 30 ppt 112–128 Ashley et al. 

1992; 1994 

Blood Heparinization; transfer to HRGC/PID 0.4 μg/L NR Pekari et al. 

isotonic saline in headspace 1989 

vial; equilibration with heat 

Blood Collection and transfer to GC/MS-SIM 2 μg/L NR Gruenke et al. 

headspace vial; analysis of 1986 

headspace gases 

Blood Purging with nitrogen; GC/MS 0.5 μg/L NR Antoine et al. 

collection on Tenax GC-silica 1986 

gel 

Blood Extraction with toluene + HCl; GC/FID 100 μg/L 98–100 Jirka and 

centrifugation; analysis of Bourne 1982 

toluene layer 

Blood Extraction device coupled to ITMS 90 ppt NR St-Germain et 

MS al. 1995 

Blood Centrifugation; dialysis; HPLC 20 pmol/g NR Hanway et al. 

precipitation; acidic globin 2000 

hydrolysis; Sep Pak cartridge 

column purification 

Urine Incubation; analysis of GC/PID 0.51 nmol/L >90 Kok and Ong 

headspace gases 1994 

Tissues Homogenization with internal GC/MS-SIM NR NR Rickert et al. 

(bone standard; centrifugation; 1979 

marrow, fat) analysis of supernatant 

Tissues Homogenization in buffer; RID-preparative 20 pg/g NR Bechtold et al. 

(lung, liver) centrifugation; analysis of HPLC/UV 1988 

supernatant 

FID = flame ionization detection; GC = gas chromatography; HCl = hydrochloric acid; HPLC = high-performance 

liquid chromatography; HRGC = high resolution gas chromatography; IARC = International Agency for Research on 

Cancer; ITMS = ion trap mass spectrometry; MS = mass spectrometry; NR = not reported; PID = photoionization 

detection; RID = reverse isotope dilution; SIM = selected ion monitoring; UV = ultraviolet detection 
BENZENE 293 

7. ANALYTICAL METHODS 

used for analyzing benzene (Michael et al. 1980; Pellizzari 1982). For headspace analysis, the samples 

are placed in a special vial, and the gas generated above the liquid sample under equilibrium conditions is 

analyzed (Gruenke et al. 1986; Pekari et al. 1989). Sensitivity is in the sub- to low-ppb range. A third 

method of sample preparation involves extraction of the blood sample with an organic solvent (Jirka and 

Bourne 1982) and analysis of the organic fraction. These methods are generally less sensitive, with 

reported detection limits usually in the low- to mid-ppb range. Selectivity is improved with use of high 

resolution gas chromatography (HRGC). Accuracy and precision could not be adequately compared 

given the limited data available. 

Screening methods are available for analysis of benzene in feces and urine (Ghoos et al. 1994) and body 

fluids (Schuberth 1994). Both employ analysis by capillary GC with an ion trap detector (ITD). Benzene 

in urine has been determined by trapping benzene stripped from the urine on a Carbotrap tube, followed 

by thermal desorption GC/FID. Care must be taken when preparing benzene metabolite samples from 

urine and bodily fluids in order to protect against enzymatic and oxidative degradation. These samples 

are often treated to denature enzymes and avoid oxidation by hydroquinone. The detection limit is 

50 ng/L and the average recovery is approximately 82% (Ghittori et al. 1993). Benzene in urine has also 

been determined using headspace analysis with capillary GC/PID. The detection limit is 40 ng/L (Kok 

and Ong 1994). 

Methods are also available for determining metabolites of benzene in urine. A summary of available 

methods is shown in Table 7-2. Both GC/FID or GC/MS and high-performance liquid chromatography 

(HPLC) with ultraviolet detection (UV) have been used to measure urinary metabolites. 

The primary metabolite of benzene is phenol. Phenol is excreted as glucuronide and sulphate conjugates 

in urine. Total phenolic metabolites (phenol, phenyl sulfate, and phenyl glucuronide) have been 

determined by hydrolyzing urine samples either enzymatically or by acid, then extracting the phenol with 

solvent. Phenol is then measured by GC or HPLC techniques. Enzymatic hydrolysis coupled with 

GC/FID has been reported; the detection limit is 1 mg/L and recovery is excellent (92–98%) (IARC 

1988). Sulfate and glucuronide conjugates have been determined directly by HPLC/UV (Ogata and 

Taguchi 1987). The normal baseline levels of urinary phenolic metabolites from humans are usually 2– 

18 mg/L (Ong and Lee 1994). The available methods are sensitive enough to measure these relatively 

high amounts accurately. 
BENZENE 294 

7. ANALYTICAL METHODS 

Table 7-2. Analytical Methods for Determining Metabolites of Benzene in Urine 

Analytical Sample Percent 

Sample matrix Preparation method method detection limit recovery Reference 

Urine (phenol, Centrifugation HPLC/UV 4 mg/L (PS); 100.5 (PS); Ogata and 

phenyl sulfate, 5 mg/L (PG) 101.8 (PG) Taguchi 1987 

phenyl 

glucuronide) 

Urine (phenol, Digestion (enzymatic and GC/FID 1 mg/L 92–98 IARC 1988 

phenyl sulfate, with acid); extraction with (IARC 

phenyl diethyl ether Method 6) 

glucuronide) 

Urine (phenols Hydrolysis with perchloric GC/FID NR NR NIOSH 1974 

and cresols) acid; extraction with 

diisopropyl ether 

Urine (phenols Hydrolysis with perchloric GC/FID 2 mg/L NR Roush and Ott 

and cresols) acid; saturation with NaCl; 1977 

extraction with diisopropyl 

ether 

Urine (phenol) Enzymatic reaction HPLC/fluoro­ 50 ppb 97.7 Jen and Tsai 

metric 1994 

detection 

Urine Mixing with methanol; HPLC/UV 0.1 mg/L NR Inoue et al. 

(trans,trans­ centrifugation 1989 

muconic acid) 

Urine (muconic Mixing with formic acid; GC/MS 10 μg/L NR Bechtold et al. 

acid, phenol) extraction (twice) with ethyl 1991 

ether; evaporation of 

combined extracts 

Urine Cleanup on anion GC/MS 0.01 mg/L 93–106 Ruppert et al. 

(trans,trans­ exchange resin; 1995 

muconic acid) derivitization 

Urine (S-phenyl- Solid-phase extraction; HPLC 1 μg/L NR Einig and 

mercapturic acid hydrolysis; derivatization Dehnen 1995 

Urine Cleanup on anion HPLC/UV 5 ppb NR Weaver et al. 

(trans,trans­ exchange resin 2000 

muconic acid) 

FID = flame ionization detection; GC = gas chromatography; HPLC = high performance liquid chromatography; 

HRGC = high resolution gas chromatography; IARC = International Agency for Research on Cancer; MS = mass 

spectrometry; NaCl = sodium chloride; NR = not reported; PG = phenyl glucuronide; PS = phenyl sulfate; 

UV = ultraviolet detection 
BENZENE 295 

7. ANALYTICAL METHODS 

Analysis of urinary trans,trans-muconic acid seems to be a better indicator than phenol for assessing 

exposure to low levels of benzene (Ducos et al. 1990). However, muconic acid is a minor metabolic route 

and background levels of muconic acid in urine are much lower than levels of phenolic metabolites and 

are frequently below the limit of detection of the method used to determine them (Inoue et al. 1989b). 

The detection of low levels of trans,trans-muconic acid in urine was difficult by earlier methods because 

of low recovery of trans,trans-muconic acid (37% with ether) by the commonly used solvent extraction 

method (Gad-El-Karim et al. 1985). An improved method for the determination of urinary trans,transmuconic acid utilizes solid phase extraction with SAX sorbent in combination with the HPLC/UV for 

quantitation. The detection limit is 0.06–0.1 mg/L, and recovery is very good (90%) (Boogaard and van 

Sittert 1995; Ducos et al. 1990). Weaver et al. (2000) used an anion exchange column to extract the urine 

sample, which was then analyzed using HPLC-UV; detection limits were around 5 ppb. The relative 

standard deviation of the method was 5% in the concentration range 1–20 ng/L. trans,trans-muconic acid 

has been determined directly by HPLC/UV with similar sensitivity (detection limit=0.1 mg/L) (Inoue et 

al. 1989b). The detection limit and specificity for the determination of urinary trans,trans-muconic acid 

may be improved by using HPLC with diode array detector, GC/FID of the methylated product, or 

GC/MS of trimethylsilyated product (Bartczak et al. 1994). Both GC/FID and HPLC/diode array 

detection are capable of detecting urinary trans,trans-muconic acid at concentrations above 40 μg/L, but 

GC/MS is capable of detecting the metabolite at concentrations below 40 μg/L (Bartczak et al. 1994). 

The metabolite, S-phenyl-mercaptic acid, may be an indicator of exposure to benzene. It can be detected 

at low levels (1 μg/L) in urine using solid phase extraction and determination by HPLC (Einig and 

Dehnen 1995). After purification by reverse phase cartridge chromatography, S-phenyl cysteine was 

detected with sensitivity of about 20 pmol/g globin using HPLC (Hanway et al. 2000). 

7.2 ENVIRONMENTAL SAMPLES 

Methods exist for determining benzene in air (ambient, occupational, and industrial), water, sediment, 

soil, foods, cigarette smoke, gasoline, and jet fuel. Most involve separation by GC with detection by FID, 

PID, or MS. HPLC/UV and spectrophotometry have also been used. Table 7-3 summarizes several of 

the methods that have been used to analyze for benzene in environmental samples. 

Numerous methods exist for detecting and measuring benzene in ambient air. Air samples for benzene 

analysis may be preconcentrated by passing the sample through a trap containing a solid adsorbent (Bayer 

et al. 1988; EPA 1979, 1980a; Fung and Wright 1986; Gruenke et al. 1986; Harkov et al. 1985; Reineke 
BENZENE	 296 

7. ANALYTICAL METHODS 

Table 7-3. Analytical Methods for Determining Benzene in Environmental 

Samples 

Sample Analytical Sample Percent 

matrix Preparation method method detection limit recovery Reference 

Air Sample trapped on silica gel; 

thermal desorption 

Air Cryogenically trap; thermal 

desorption 

Air Direct on-line analysis 

Air (ambient) Direct injection of ambient air 

Air (ambient) Direct analysis of ambient air 

Air (ambient) Sample collection in Tedlar 

bag; Cryogenically trap; 

thermally desorb 

Air (ambient) Sample collection in stainless 

steel canisters or sorbent 

tubes; trap cryogenically; 

thermal desorption 

Air (ambient) Collect on charcoal (vapor 

badge or tube); desorb with 

carbon disulfide 

Air (ambient) Cryogenically trap; thermal 

desorption 

Air (ambient) Collection in canisters; 

preconcentration using 

2-stage trap 

Air (ambient) Direct analysis 

Air (ambient) Sample collection onto oncolumn cryogenic sample loop 

or into stainless steel 

containers 

Air (ambient) Sample trapped on Tenax 

GC; thermal desorption to oncolumn cryogenic trap 

Air (ambient) Sample collected on Tenax 

GC; thermal desorion to oncolumn cryogenic trap 

Air (ambient) Sample trapped on Tenax 

GC; thermal desorption to oncolumn cryogenic trap 

GC/MS 

GC/FID 

GC/FID 

GC/PID 

Electrochemical 

GC/PID 

HRGC/PID or 

HRGC/FID 

GC/FID 

GC/PID/FID 

HRGC/ITD 

ALMS 

GC/PID/FID 

HRGC 

GC/FID 

HRGC/FID; 

conf. by 

HRGC/MS 

0.1 ppb 

NR 

NR 

0.25 ppb 

NR 

0.5 ppb 

5 ppt (PID) 

24 ppt (FID) 

0.3 ppb 

(estimated) 

1 ppt 

sub-ppb level 

250 ppb 

1 ppt 

<0.1 ppb 

0.03 ppb 

3 ppt 

88–105 Gruenke et al. 

1986 

85–115 Singh et al. 

1985 

NR Bayer et al. 

1988 

NR Clark et al. 

1984 

NR Stetter et al. 

1986 

NR Kowalski et 

al. 1985 

97–104 Reineke and 

(PID) 96– Bächmann 

104 (FID) 1985 

NR Fung and 

Wright 1986 

70–130 Nutmagul and 

Cronn 1985 

8–13% Kelly et al. 

bias 1993 

NR EPA 1985d 

70–130 Nutmagul and 

Cronn 1985 

69–126 EPA 1979 

56–144 EPA 1980a 

NR Roberts et al. 

1984 

Air (consumer Sample trapped on solid GC/MS NR NR Bayer et al. 

products) sorbent; thermal desorption 1988 
BENZENE	 297 

7. ANALYTICAL METHODS 

Table 7-3. Analytical Methods for Determining Benzene in Environmental 

Samples 

Sample Analytical Sample Percent 

matrix Preparation method method detection limit recovery Reference 

Air (at waste 

sites and 

landfills) 

Air 

(occupational) 

Air 

(occupational) 

Air 

(occupational) 

Air 

(occupational) 

Air 

(occupational; 

jet fuel fumes) 

Soil air 

Drinking 

water 

Drinking 

water 

Water 

Water 

Water 

Water 

Water 

Water 

Water 

Sample trapped on Tenax 

GC; thermal desorption 

Sample trapped on charcoal; 

desorption with carbon 

disulfide 

Sample collection on silica 

gel; desorption with ethanol 

Sample collection in Tedlar 

bag; direct injection 

Sample collection on charcoal 

disk in miniature passive 

dosimeter; thermal desorption 

Sample collection on 

charcoal; desorption with 

methyl chloride-ethyl acetate 

Sample collection on activated 

charcoal; desorption with 

carbon disulfide 

Purge and trap 

Purge and trap 

Purge and trap onto Tenax 

GC; thermal desorption to oncolumn cryogenic trap 

Purge and trap on Tenax GC; 

thermal desorption to oncolumn cryogenic trap 

Purge and trap onto Tenax 

GC; thermal desorption 

Solvent extraction with 

dichloromethane; 

concentration 

Purge and trap on adsorbent 

column; thermal desorption 

Purge and trap on activated 

carbon; desorption with 

carbon disulfide 

Purge and trap on Tenax; 

thermal desorption 

HRGC/FID/ECD; 0.05 ppb NR Harkov et al. 

conf. by 1985 

HRGC/MS 

GC/FID (NIOSH 10–100 ppb NR NIOSH 1984 

Methods 1500 

and 1501) 

GC/FID 100 ppb 90 Sherwood 

and Carter 

1970 

GC/PID (NIOSH 10 ppb NR NIOSH 1994 

Method 3700) 

GC/PID 60 ppb 85–115 Gonzalez and 

Levine 1986 

HPLC/UV 0.08 ppm 94–112 Dibben et al. 

1989 

GC/FID NR 97–100 Colenutt and 

Davies 1980 

GC/MS 0.2 μg/L NR Brass et al. 

1977 

HRGC/MS (EPA 0.03–0.04 μg/L 97–99 EPA 1992a 

Method 524.2) 

HRGC/MS 0.1–10 μg/L 74–78 Michael et al. 

1988 

HRGC <1 μg/L 69–126 EPA 1979 

GC/MS NR 85–125 Harland et al. 

1985 

GC/MS 15 μg/L NR Sporstøl et al. 

1985 

GC/PID (EPA 0.2 μg/L 106 NEMI 2005a 

Method 602) 

GC/FID; conf. by NR 96–99 Colenutt and 

GC/MS Thorburn 

1980 

GC/FID 0.001 μg/L 94–111 Hammers and 

Bosman 1986 
BENZENE	 298 

7. ANALYTICAL METHODS 

Table 7-3. Analytical Methods for Determining Benzene in Environmental 

Samples 

Sample Analytical Sample Percent 

matrix Preparation method method detection limit recovery Reference 

Water Permeation of benzene 

through a silicone 

polycarbonate membrane into 

an inert gas stream 

Waste water Purge and trap onto 

adsorbent column; thermal 

desorption 

Waste water Addition of isotopically labeled 

benzene analog; purge and 

trap onto adsorbent column; 

thermal desorption 

Water, Purge and trap on Tenax; 

industrial thermal desorption 

effluents 

Landfill Purge sample and trap on 

leachate Tenax-silica gel; thermally 

desorb 

Landfill Extract sample with pentane 

leachate 

Solid wastes Purge and trap, direct 

injection, vacuum distillation, 

or headspace 

Soil Sample mixed with NaOH 

solution; equilibration; 

analysis of headspace gases 

Soil Purge and trap on Tenax; 

thermal desorption to oncolumn cryogenic trap 

Soil Purge and trap on Tenax; 

thermal desorption 

Soil Supercritical fluid extraction 

Sediment and Purge and trap on Tenax GCbiota silica gel; thermal desorption 

Sediment Purge and trap on Tenax; 

thermal desorption 

Fruits and Mix with water and methanol; 

vegetables	 filter; distill azeotrope 

Shellfish Tissue homogenized; purge 

with inert gas and trap on 

Tenax GC-silica gel; thermally 

desorb 

GC/FID 

GC/MS (EPA 

Method 624) 

GC/IDMS (EPA 

Method 1624) 

GC/MS 

GC/FID/FID 

GC/MS 

PID (EPA 

Method 8021B) 

HRGC/FID; 

conf. by 

HRGC/MS 

HRGC 

GC/FID 

HRGC/FID 

HRGC/MS 

GC/MS 

GC/FID 

GC/MS 

7.2 μg/L 

4.4 μg/L 

10 μg/L 

<5 μg/L 

1 μg/L 

1,000– 

10,000 μg/L 

0.009 μg/L 

0.02 ng/mL 

<0.1 ppb 

1 ppt 

low ppb 

NR 

NR 

NR 

NR 

NR Blanchard 

and Hardy 

1986 

113 NEMI 2005b 

65–141 NEMI 2005c 

95–106 Pereira and 

Hughes 1980 

NR EPA 1984b 

NR Schultz and 

Kjeldsen 

1986 

99 EPA 1994c 

75–98 Kiang and 

Grob 1986 

69–126 EPA 1979 

52 Hammers and 

Bosman 1986 

77–81 Burford et al. 

1994 

NR Ferrario et al. 

1985 

64 Harland et al. 

1985 

84–96 Kozioski 1985 

NR Ferrario et al. 

1985 
BENZENE	 299 

7. ANALYTICAL METHODS 

Table 7-3. Analytical Methods for Determining Benzene in Environmental 

Samples 

Sample Analytical Sample Percent 

matrix Preparation method method detection limit recovery Reference 

Mainstream Collection on filters and HRGC/IDMS- 0.05 μg/ 75–85 Byrd et al. 

cigarette impingers; [2H6]-benzene SIM cigarette (trapping 1990 

smoke added to impinger efficiency) 

Cigarette Mainstream smoke filtered HRGC/MS-SIM 0.1 μg/ NR Brunnemann 

smoke and analyzed directly; side- cigarette et al. 1989, 

stream smoke and smoke- 1990 

polluted air filtered and 

collected in cryogenic 

methanol-filled impingers 

Gasoline Dilute sample with hexane GC/FID NR NR Poole et al. 

1988 

Gasoline Dilute sample with methanol; HPLC/UV NR NR Ludwig and 

elute benzene to analytical Eksteen 1988 

column with 50% methanol; 

back-flush guard column with 

100% methanol 

ECD = electron capture detection; EPA = Environmental Protection Agency; FID = flame ionization detection; 

GC = gas chromatography; HPLC = high-performance liquid chromatography; HRGC = high resolution gas 

chromatography; IARC = International Agency for Research on Cancer; IDMS = isotope dilution mass spectrometry; 

ITD = ion trap mass spectrometry; LRS = laser Raman spectroscopy; MS = mass spectrometry; NaOH = sodium 

hydroxide; NIOSH = National Institute of Occupational Safety and Health; NR = not reported; PID = photoionization 

detection; SIM = selected ion monitoring; UV = ultraviolet detection 
BENZENE 300 

7. ANALYTICAL METHODS 

and Bächmann 1985; Roberts et al. 1984). Commonly used adsorbents are Tenax resins (e.g., Tenax TA, 

Tenax GC), silica gel, activated carbon, and carbonaceous polymeric compounds. Benzene in ambient air 

can be collected in stainless steel canisters (EPA 1988a; Kelly et al. 1993) or Tedlar bags (Kowalski et al. 

1985) and can be analyzed with or without preconcentration. Preconcentration of benzene can be 

accomplished by direct on-column cryogenic trapping (EPA 1985c; Kowalski et al. 1985; Nutmagul and 

Cronn 1985; Reineke and Bächmann 1985; Singh et al. 1985), or samples may be analyzed directly 

without preconcentration (Bayer et al. 1988; Clark et al. 1984). 

The most common methods of analysis for benzene in air are GC/PID, GC/FID, and GC/MS. The limit 

of detection for GC/FID and GC/PID ranges from low ppb to low ppt. GC/MS is generally considered to 

be more reliable than GC/FID or GC/PID in identifying benzene in samples, particularly those containing 

multiple components having similar GC elution characteristics. Benzene has been quantified in ambient 

air samples at sub-ppb levels by GC/MS (Gruenke et al. 1986) and ion trap mass spectrometry (Kelly et 

al. 1993). The ion trap detector has the advantage of remaining largely unaffected by water vapor in the 

sample. A continuous monitoring instrument using Fourier transform infrared (FTIR) spectroscopy is 

being developed for measuring the levels of benzene or other toxic chemicals in exhaust emissions from 

hazardous waste incinerators (DOE 1992). 

Several analytical methods are available for determining atmospheric levels of benzene in the workplace. 

The OSHA recommended procedure involves the collection of the sample vapors on charcoal adsorption 

tubes, and then desorption followed by GC/MS analysis (OSHA 1985). Samples desorbed from charcoal 

are also analyzed by GC/FID (NIOSH 1984) or HPLC/UV (Dibben et al. 1989). Detection limits are in 

the ppb range (Dibben et al. 1989; NIOSH 1984). Passive dosimeters are also utilized, with GC/PID 

quantitation; detection limits are in the ppb range (Gonzalez and Levine 1986). Other acceptable methods 

include portable direct reading instruments and real-time continuous monitoring systems; these methods 

generally have a sensitivity in the ppm range. 

The most frequently used analytical methods for water samples containing benzene are GC/MS, GC/FID, 

and GC/PID (Blanchard and Hardy 1986; Colenutt and Thorburn 1980; DOI 1984; EPA 1984, 1992; 

Hammers and Bosman 1986; Harland et al. 1985; Lysyj et al. 1980; Michael et al. 1988; Pereira and 

Hughes 1980; Sporstol et al. 1985). Benzene is usually isolated from aqueous media by the purge-andtrap method (Brass et al. 1977; Colenutt and Thorburn 1980; DOI 1984; EPA 1979, 1984, 1992; 

Hammers and Bosman 1986; Harland et al. 1985; Michael et al. 1988). An inert gas such as nitrogen is 

used to purge the sample. The purged benzene is trapped on an adsorbent substance, such as Tenax GC 
BENZENE 301 

7. ANALYTICAL METHODS 

or activated charcoal, and thermally desorbed. Recovery, where reported, ranges from acceptable (≈70%) 

(EPA 1979, 1984; Michael et al. 1988) to very good (≥90%) (Colenutt and Thorburn 1980; EPA 1984, 

1992; Hammers and Bosman 1986). Detection limits in the sub-ppb to ppt range may be attained with 

HRGC/MS techniques (EPA 1992a); Michael et al. 1988). Liquid-liquid extraction procedures (Harrison 

et al. 1994; Schultz and Kjeldsen 1986; Sporstol et al. 1985) are less commonly used, having been 

replaced by more sensitive purge-and-trap methods. Interference from contamination can occur with all 

methods if extreme care is not used in the handling of samples and cleaning of all equipment. 

Solid samples, such as soil, sediment, and foods, are most frequently prepared for analysis using the 

purge-and-trap method (EPA 1979, 1994c; Ferrario et al. 1985; Hammers and Bosman 1986; Harland et 

al. 1985), although supercritical fluid extraction has recently been utilized (Burford et al. 1994). 

Detection and quantitation of benzene may be GC/FID, GC/PID, or GC/MS. Detection limits as low as 

1 ppt have been reported, but recoveries and precision have frequently been low. Improvements in the 

method, including analysis by HRGC/PID, have resulted in low detection limits (9 ppt) and excellent 

recovery (99%) for benzene (EPA 1994c). Screening methods are available for benzene; some may be 

used at field sites. Immunoassay may be used as a screening and semiquantitative tool (Van Emon and 

Gerlach 1995). 

Methods exist for detection of benzene in other environmental media such as cigarette smoke, gasoline, 

and jet fuel and its fumes (Brunnemann et al. 1989; Byrd et al. 1990; Ludwig and Eksteen 1988; Poole et 

al. 1988). HPLC/UV, GC/FID, and GC/MS separation and detection techniques have been used for these 

analyses. Sensitivity and reliability of these methods cannot be compared because of the lack of data. 

Few methods have been reported for measurement of benzene in foods; performance data are generally 

lacking. 

7.3 ADEQUACY OF THE DATABASE 

Section 104(i)(5) of CERCLA, as amended, directs the Administrator of ATSDR (in consultation with the 

Administrator of EPA and agencies and programs of the Public Health Service) to assess whether 

adequate information on the health effects of benzene is available. Where adequate information is not 

available, ATSDR, in conjunction with NTP, is required to assure the initiation of a program of research 

designed to determine the health effects (and techniques for developing methods to determine such health 

effects) of benzene. 
BENZENE 302 

7. ANALYTICAL METHODS 

The following categories of possible data needs have been identified by a joint team of scientists from 

ATSDR, NTP, and EPA. They are defined as substance-specific informational needs that if met would 

reduce the uncertainties of human health assessment. This definition should not be interpreted to mean 

that all data needs discussed in this section must be filled. In the future, the identified data needs will be 

evaluated and prioritized, and a substance-specific research agenda will be proposed. 

7.3.1 Identification of Data Needs 

Methods for Determining Biomarkers of Exposure and Effect. 

Exposure. Methods exist for measuring benzene in breath (Gruenke et al. 1986; Pellizzari et al. 1988; 

Sherwood and Carter 1970; Wallace et al. 1986), blood (Antoine et al. 1986; Ashley et al. 1992, 1994; 

Gruenke et al. 1986; Jirka and Bourne 1982; Pekari et al. 1989), and tissues (Bechtold et al. 1988; Rickert 

et al. 1979). The methods for breath are sensitive and accurate for determining exposure levels of 

benzene at which health effects have been observed to occur, as well as for background levels in the 

general population. The methods are relatively precise and selective. Methods for determining benzene 

in blood are sensitive; based on the limited recovery data available, they appear to be accurate. More 

information on the performance obtained with different methods would be helpful. The application of 

GC/MS techniques to the analysis of blood specimens has resulted in a rapid, cost-effective, clinical 

screening test for common volatile organic compounds, including benzene (DeLeon and Antoine 1985). 

This test, the VOST (Volatile Organics Screening Test), has demonstrated the presence (down to 0.1 ppb) 

of a variety of toxic volatile organics in the blood of environmentally sensitive patients and has provided 

preliminary baseline concentration levels for the test population (DeLeon and Antoine 1985). The data on 

determination of benzene in urine and tissue samples are very limited. In general, the available methods 

have limits of detection that are too high to be useful in other than acute exposure situations. Methods 

that could be used to measure low levels in human tissues would be useful for determining the 

relationship between chronic low-level exposure and the effects observed in specific tissues. 

Methods are available for measuring phenolic benzene metabolites in urine (Bechtold et al. 1991; IARC 

1988; Jen and Tsai 1994; Jongeneelen et al. 1987; NIOSH 1974; Ogata and Taguchi 1987). Available 

methods for determining most benzene metabolites in urine are sufficiently sensitive and reliable to allow 

measurement of background concentrations in nonoccupationally exposed individuals. However, the 

phenolic metabolites are not unique to benzene. Improved methods to detect phenolic metabolites are not 

needed. Sensitive assays have been developed for detection of urinary trans,trans-muconic acid 
BENZENE 303 

7. ANALYTICAL METHODS 

(detection limit 10 μg/L) (Bechtold et al. 1991; Ruppert et al. 1995). Since urinary trans,trans-muconic 

acid concentration can be correlated with benzene exposure, this may provide a useful biomarker of 

exposure on an individual basis (Bechtold et al. 1991; EPA 1992b; Weaver et al. 2000). In addition, 

information is needed to assess the effect of co-exposure to other chemicals (e.g., toluene) on urinary 

muconic acid levels. Also needed are specific biomarkers of cumulative exposure to benzene, based on 

albumin or hemoglobin adducts, and lymphocyte DNA adducts of N-7-phenylguanine. It would also be 

useful to develop specific biomarkers of acute- and chronic-duration exposure to benzene based on 

adducts of muconaldehyde. The levels of such biomarkers formed in vivo would be useful later for 

correlation with toxic effects of acute- or chronic-duration exposure to benzene. S-phenyl-mercapturic 

acid has also been useful in biological monitoring of benzene exposure in humans and animals. S-phenylmercapturic acid can be measured with a sensitivity of about 20 pmol/g globin using HPLC (Hanway et 

al. 2000). S-phenyl-mercapturic acid levels can also be correlated to environmental benzene exposure 

(Popp et al. 1994), which may indicate its utility as a biomarker. 

Effect. Methods for determining benzene in breath, blood, and tissues and for determining its metabolites 

in urine could also be used as biomarkers of effect. However, efforts to correlate these measures with 

observed toxic effects of benzene exposure have been unsuccessful. Other biomarkers of effect (e.g., 

complete blood cell counts, red and white blood cell counts, chromosomal aberrations, sister chromatid 

exchanges, and examination of bone marrow) have been suggested for benzene, but they are not specific 

for benzene exposure. Further development of methods for determining reliable unique biomarkers of 

effect for benzene would be useful. 

Methods for Determining Parent Compounds and Degradation Products in Environmental 

Media. Methods for determining benzene in air (Clark et al. 1984; Gruenke et al. 1986) and water 

(Brass et al. 1977; EPA 1979, 1984; Hammers and Bosman 1986; Pereira and Hughes 1980), the media of 

most concern for human exposure, are sensitive enough to measure background levels in the environment 

and levels at which health effects might occur. Their reliability is limited primarily by the ubiquitous 

presence of benzene in the environment, which makes contamination a constant problem. The accuracy 

and precision of some methods for water analyses (e.g., GC/MS) need to be improved to produce more 

reliable results. Methods for soil and other solid media appear to have the same problems as those for air 

and water. In addition, there is a lack of information on methods for determining benzene in media such 

as shellfish, fish, foods, and plants. Although exposure to benzene via ingestion of food is believed to be 

minimal, standardized methods for these media are needed to better assess the extent of benzene 

contamination in the environment and the resulting risk of exposure. 
BENZENE 304 

7. ANALYTICAL METHODS 

7.3.2 Ongoing Studies 

The information in Table 7-4 was found as a result of a search of the Federal Research in Progress 

database (FEDRIP 2005). 

The Environmental Health Laboratory Sciences Division of the National Center for Environmental 

Health, Centers for Disease Control and Prevention, is developing methods for the analysis of benzene 

and other volatile organic compounds in blood. These methods use purge and trap methodology, highresolution gas chromatography, and magnetic sector mass spectrometry, which give detection limits in the 

low parts per trillion (ppt) range. 
BENZENE 305 

7. ANALYTICAL METHODS 

Table 7-4. Ongoing Studies on Benzene, Analytical Methodsa 

Investigator Affiliation Research description Sponsor 

Fales, HM Analysis of proteins, peptides, USDA 

and metabolites for carcinogen 

exposure using mass 

spectrometry 

Kalman, DA University of Washington Study of the use of human NIEHS 

Seattle, Washington dosimetry for assessment of 

exposure to volatile compounds 

Kavanaugh, TJ Study of glutathione biosynthesis NSF 

as a biomarker of toxic exposure 

O’Brien, RJ Assessment Environmental NIEHS 

Health Breath using PF-GC 

Pavel, K VOC Technologies Inc.; Assessment Environmental NIEHS 

Portland, Oregon Health Breath using PF-GC 

Rappaport, SM University of North Carolina; Development and application of NIEHS 

Chapel Hill, North Carolina biomarkers in the study of 

exposure to carcinogens 

Sabri, MI Oregon Health Science Study of biomarkers of NIH 

University; Portland, Oregon neurotoxicant exposure and 

neurodegeneration 

Smejtek, PK VOC Technologies Inc.; A PF-GC for Environmental NIEHS 

Portland, Oregon Health Breath Assessment 

Smith, MT University of California at Study of biomarkers of benzene NIEHS 

Berkeley exposure and genotoxicity 

Spencer, PS Oregon Health Sciences Study of neurotoxic chemicals NIH 

University; Portland, Oregon and biomarkers at superfund 

sites 

Stewart, P Use of biomarkers to study Division of cancer 

occupational exposure of epidemiology and 

carcinogens in order to enhance genetics 

exposure assessment 

Turtletaub, KW University of California at Study and development of NIEHS 

Berkeley biomarkers for exposure using 

accelerator mass spectrometry 

Weaver, VM Use of biomarkers to study USDA 

benzene exposure in inner city 

residents 

Wiencke, JK Use of molecular biomarkers of USDA 

occupational benzene exposure 

a

FEDRIP 2005 

NIEHS = National Institute of Environmental Health Sciences; NIH = National Institutes of Health; NSF = National 

Science Foundation; USDA = U.S. Department of Agriculture 
BENZENE 306 

7. ANALYTICAL METHODS 

This page is intentionally blank. 
BENZENE 307 

8. REGULATIONS AND ADVISORIES 

The international and national regulations and guidelines regarding benzene in air, water, and other media 

are summarized in Table 8-1. 

ATSDR has derived an acute-duration inhalation MRL of 0.009 ppm for benzene based on a LOAEL of 

10.2 ppm for immunological effects in mice exposed for 6 hours/day for 6 consecutive days (Rozen et al. 

1984). The LOAEL of 10.2 ppm was adjusted from intermittent to continuous exposure 

(LOAELADJ=2.55 ppm) and converted to a human equivalent concentration (LOAELHEC=2.55 ppm) using 

EPA (1994b) methodology for a category 3 gas; an uncertainty factor of 300 (10 for use of a LOAEL, 

3 for extrapolation from animals to humans using dosimetric conversion, and 10 to protect sensitive 

individuals) was applied. 

ATSDR has derived an intermediate-duration inhalation MRL of 0.006 ppm for benzene based on a 

LOAEL of 10 ppm for significantly delayed splenic lymphocyte reaction to foreign antigens evaluated in 

in vitro mixed lymphocyte reaction following the exposure of male C57Bl/6 mice to benzene vapors for 

6 hours/day, 5 days/week for 20 exposure days (Rosenthal and Snyder 1987). The concentration was 

adjusted from intermittent to continuous exposure (LOAELADJ=1.8 ppm) and converted to a human 

equivalent concentration (LOAELHEC=1.8 ppm) using EPA (1994b) methodology for a category 3 gas; an 

uncertainty factor of 300 (10 for the use of LOAEL, 3 for extrapolation from animals to humans using 

dosimetric conversion, and 10 for human variability) was applied. 

ATSDR has derived a chronic-duration inhalation MRL of 0.003 ppm for benzene based on the results of 

benchmark dose (BMD) modeling of B cell counts in workers of shoe manufacturing industries in 

Tianjin, China (Lan et al. 2004a). The resulting BMCL0.25sd of 0.10 ppm was adjusted from intermittent 

to continuous exposure (BMCL0.25sdADJ=0.03 ppm) using EPA (1994b) methodology; an uncertainty factor 

of 10 (to protect sensitive individuals) was applied. 

ATSDR has derived a chronic-duration oral MRL of 0.0005 mg/kg/day for benzene based on estimation 

of equivalent chronic-duration oral dosing that would result in effects similar to those observed in the 

occupationally-exposed workers assessed by Lan et al. (2004a, 2004b). The BMCL0.25sdADJ of 0.03 ppm 

(0.096 mg/m3

) was converted to an equivalent BMDL0.25sdADJ of 0.014 mg/kg/day for ingested benzene 

using EPA (1988b) human reference values for inhalation rate (20 m3

/day) and body weight (70 kg) and a 

factor of 0.5 to adjust for differences in absorption of benzene following inhalation versus oral exposure; 
BENZENE 308 

8. REGULATIONS AND ADVISORIES 

Table 8-1. Regulations and Guidelines Applicable to Benzene 

Agency Description Information Reference 

INTERNATIONAL 

Guidelines: 

IARC Carcinogenicity classification Group 1a IARC 1987, 2004, 

2007 

WHO Air quality guidelines 

Drinking water quality guidelines 

6x10-6 unit risk 

0.01 mg/Lb 

WHO 2000 

 WHO 2004 

NATIONAL 

Regulations and Guidelines: 

a. Air 

ACGIH TLV (TWA) 

STEL 

0.5 ppmc 

2.5 ppmc 

ACGIH 2006 

EPA Hazardous air pollutant Yes EPA 2004b 

42 USC 7412

 NAS/NRC AEGL-1d EPA 2005a 

10 minutes 130 ppm 

30 minutes 73 ppm 

60 minutes 52 ppm 

4 hours 18 ppm 

8 hours 

AEGL-2d 

9.0 ppm 

10 minutes 2,000 ppme 

30 minutes 1,100 ppm 

60 minutes 800 ppm 

4 hours 400 ppm 

8 hours 

AEGL-3d 

200 ppm 

10 minutes 

30 minutes 

60 minutes 

4 hours 

9,700 ppmf 

5,600 ppme 

4,000 ppme 

2,000 ppme 

NIOSH 

8 hours 

REL (10-hour TWA) 

STEL 

IDLH 

990 ppm 

0.1 ppmg 

1.0 ppmg 

500 ppmg 

NIOSH 2005 

OSHA PEL (8-hour TWA) for general industry 1 ppm OSHA 2005b, 2005e 

29 CFR 1910.1000 

29 CFR 1910.1028 

PEL (8-hour TWA) for construction 

industry 

1 ppm OSHA 2005d, 2005f 

29 CFR 1926.55, 

Appendix A 

29 CFR 1926.1128 
BENZENE 

8. REGULATIONS AND ADVISORIES 

309 

Agency 

Table 8-1. Regulations and Guidelines Applicable to Benzene 

Description Information Reference 

NATIONAL (cont.) 

OSHA PEL (8-hour TWA) for shipyard industry 1 ppm OSHA 2005a,2005b 

29 CFR 1915.1000 

29 CFR 1915.1028 

b. Water 

EPA Designated as hazardous substances 

in accordance with Section 311(b)(2)(A) 

of the Clean Water Act 

Yes EPA 2005b 

40 CFR 116.4 

Drinking water standards and health 

advisories 

EPA 2004a 

1-day health advisory for a 10-kg 

child 

0.2 mg/L 

10-day health advisory for a 10-kg 

child 

0.2 mg/L 

DWEL 

10-4 Cancer risk 

0.1 mg/L 

0.1 mg/L 

National primary drinking water 

standards 

EPA 2002a 

MCLG Zero 

MCL 0.005 mg/L 

Reportable quantities of hazardous 

substances designated pursuant to 

Section 311 of the Clean Water Act 

10 pounds EPA 2005c 

40 CFR 117.3 

Water quality criteria for human health 

consumption of: 

Water + organism 

Organism only 

2.2 μg/Lh 

51 μg/Lh 

EPA 2002b 

c. Food 

FDA Bottled drinking water 0.005 mg/L FDA 2004 

21 CFR 165.110 

d. Other 

 ACGIH Carcinogenicity classification A1i ACGIH 2006 

Biological exposure indices (end of 

shift) 

S-phenylmercapturic acid in urine 25 μg/g creatinine 

EPA 

t,t-Muconic acid in urine 

Carcinogenicity classification 

Oral slope factor 

Inhalation unit risk 

RfC 

RfD 

500 μg/g creatinine 

Group Aj 

1.5x10-2–5.5x10-2 

per (mg/kg)/day 

2.2x10-6–7.8x10-6 

per μg/m3 

0.03 mg/m3 

4x10-3 mg/kg/day 

IRIS 2007 
BENZENE 310 

8. REGULATIONS AND ADVISORIES 

Table 8-1. Regulations and Guidelines Applicable to Benzene 

Agency Description Information Reference 

NATIONAL (cont.) 

EPA Superfund, emergency planning, and 

community right-to-know 

Designated CERCLA hazardous EPA 2005d 

substance 40 CFR 302.4 

Reportable quantity 10 poundsk 

RCRA hazardous waste number U019 

Effective date of toxic chemical 01/01/87 EPA 2005e 

release reporting 40 CFR 372.65 

NTP Carcinogenicity classification Known human NTP 2005 

carcinogen 

a

Group 1: carcinogenic to humans 

b

For substances that are considered to be carcinogenic, the guideline value is the concentration in drinking water 

associated with an upper-bound excess lifetime cancer risk of 10-5 (one additional cancer per 100,000 of the 

population ingesting drinking water containing the substance at the guideline value for 70 years). Concentrations 

associated with upper-bound estimated excess lifetime cancer risks of 10-4 and 10-6 can be calculated by multiplying 

and dividing, respectively, the guideline value by 10. 

c

Skin notation: refers to the potential significant contribution to the overall exposure by the cutaneous route, 

including mucous membranes and the eyes, either by contact with vapors or, of probable greater significance, by 

direct skin contact with the substance. 

d

AEGL-1 is the airborne concentration of a substance above which it is predicted that the general population, 

including susceptible individuals, could experience notable discomfort, irritation, or certain asymptomatic nonsensory 

effects. AEGL-2 is the airborne concentration of a substance above which it is predicted that the general population, 

including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects or 

an impaired ability to escape. AEGL-3 is the airborne concentration of a substance above which it is predicted that 

the general population, including susceptible individuals, could experience life-threatening health effects or death. e

Values denoted as having safety considerations against the hazard of explosion, whereas the Lower Explosive Limit 

(LEL) =14,000 ppm and each value should be ≥10% LEL. f

Value denoted as having extreme safety considerations against the hazard of explosion must be taken into account, 

whereas the LEL =14,000 ppm and each value should be ≥50% LEL. 

g

NIOSH potential occupational carcinogen 

h

This criterion is based on carcinogenicity of 10-6 risk. i

A1: confirmed human carcinogen 

j

Group A: known human carcinogen 

k

Designated CERCLA hazardous substance pursuant to Section 311(b)(2) and 307(a) of the Clean Water Act, 

Section 112 of the Clean Air Act, and Section 3001 of RCRA. 

ACGIH = American Conference of Governmental Industrial Hygienists; AEGL = Acute Emergency Exposure 

Guideline Levels; CERCLA = Comprehensive Environmetnal Response, Compensation, and Liability Act; 

CFR = Code of Federal Regulations; DWEL = drinking water equivalent level; EPA = Environmental Protection 

Agency; FDA = Food and Drug Administration; IARC = International Agency for Research on Cancer; 

IDLH = immediately dangerous to life or health; IRIS = Integrated Risk Information System; MCL = maximum 

contaminant level; MCLG = maximum contaminant level goal; NAS/NRC = National Academy of Sciences/National 

Research Council; NIOSH = National Institute for Occupational Safety and Health; NTP = National Toxicology 

Program; OSHA = Occupational Safety and Health Administration; PEL = permissible exposure limit; 

RCRA = Resource Conservation and Recovery Act; REL = recommended exposure limit; RfC = inhalation reference 

concentration; RfD = oral reference dose; STEL = short-term expsoure limit; TLV = threshold limit values; 

TWA = time-weighted average; USC = United States Code; WHO = World Health Organization 
BENZENE 311 

8. REGULATIONS AND ADVISORIES 

an uncertainty factor of 30 (10 to protect sensitive individuals and 3 for uncertainty in route-to-route 

extrapolation) was applied. 

EPA (IRIS 2007) derived an inhalation reference concentration (RfC) for benzene of 0.03 mg/m3 

(0.009 ppm) based on the results of BMD modeling of absolute lymphocyte (ALC) data from the 

occupational epidemiologic study of Rothman et al. (1996a), in which workers were exposed to benzene 

by inhalation. The resulting BMCL of 7.2 ppm for decreased lymphocyte count was converted to 

23.0 mg/m3

 and adjusted from intermittent to continuous exposure (BMCLADJ=8.2 mg/m3); a total 

uncertainty factor of 300 (3 for effect-level extrapolation, 10 to protect sensitive individuals, 3 for 

subchronic-to-chronic extrapolation, and 3 for database deficiencies) was applied. 

EPA (IRIS 2007) derived an oral reference dose (RfD) for benzene of 0.004 mg/kg/day, based on the 

results of BMD modeling of ALC data from the occupational epidemiologic study of Rothman et al. 

(1996a), in which workers were exposed to benzene by inhalation. The resulting BMCL of 7.2 ppm for 

decreased lymphocyte count was converted to 23.0 mg/m3

 and adjusted from intermittent to continuous 

exposure (BMCLADJ=8.2 mg/m3

). Route-to-route extrapolation methodology was applied to convert from 

inhalation to equivalent oral exposure, resulting in an equivalent oral dose rate of 1.2 mg/kg/day. This 

value was divided by a total uncertainty factor of 300 (3 for effect-level extrapolation, 10 to protect 

sensitive individuals, 3 for subchronic-to-chronic extrapolation, and 3 for database deficiencies). 

The International Agency for Research on Cancer (IARC) classifies benzene as a Group 1 carcinogen 

(carcinogenic to humans) (IARC 2004). EPA classified benzene in Category A (known human 

carcinogen) based on convincing evidence in humans supported by evidence from animal studies. Under 

EPA’s most recent guidelines for carcinogen risk assessment, benzene is characterized as a known human 

carcinogen for all routes of exposure based on convincing human evidence as well as supporting evidence 

from animal studies (IRIS 2007). The National Toxicology Programs lists benzene as a "substance 

known to be carcinogenic," that is, a substance for which the evidence from human studies indicates that 

there is a causal relationship between exposure to the substance and human cancer (NTP 2005). 

The EPA has a current maximum contaminant level (MCL) of 0.005 mg/L for benzene in drinking water 

(EPA 2002a). The World Health Organization (WHO) has established a guideline value of 0.01 mg/L for 

benzene in drinking water (WHO 2004). 
BENZENE 312 

8. REGULATIONS AND ADVISORIES 

Benzene is on the list of chemicals in "The Emergency Planning and Community Right-to-Know Act of 

1986" (EPA 2005d). Section 313 of Title III of the Superfund Amendments and Reauthorization Act 

(SARA) requires owners and operators of certain facilities that manufacture, import, process, or otherwise 

use the chemicals on this list to report annually any release of those chemicals to any environmental 

media over a specified threshold level (U.S. Congress 1986). 

OSHA requires employers of workers who are occupationally exposed to benzene to institute engineering 

controls and work practices to reduce and maintain employee exposure at or below permissible exposure 

limits (PEL). If an employer can document that benzene is used in the workplace <30 days/year, the 

employer can use any combination of engineering controls, work practice controls, or respirators to 

reduce employee exposure to or below the PEL of 1 ppm (8-hour TWA). Respirators must be provided 

and used during the time period necessary to install or implement feasible engineering and work practice 

controls, or where controls are not yet sufficient. Respirators are also required when the employer 

determines that compliance with the PEL is not feasible with engineering or work practice controls, such 

as maintenance and repair activities, vessel cleaning, or other operations where exposures are intermittent 

and limited in duration, and in emergencies (OSHA 2005c). 

ACGIH limits exposure to benzene to 0.5 ppm (8-hour TWA) (ACGIH 2006). The National Institute for 

Occupational Safety and Health (NIOSH 2005) has established a recommended exposure level (REL) of 

0.1 ppm (15-minute ceiling limit). 
_______________________ 

BENZENE 313 

9. REFERENCES 

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Environmental Control. Atlanta, GA: Agency for Toxic Substances and Disease Registry. Division of 

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BENZENE 377 

10. GLOSSARY 

Absorption—The taking up of liquids by solids, or of gases by solids or liquids. 

Acute Exposure—Exposure to a chemical for a duration of 14 days or less, as specified in the 

Toxicological Profiles. 

Adsorption—The adhesion in an extremely thin layer of molecules (as of gases, solutes, or liquids) to the 

surfaces of solid bodies or liquids with which they are in contact. 

Adsorption Coefficient (Koc)—The ratio of the amount of a chemical adsorbed per unit weight of 

organic carbon in the soil or sediment to the concentration of the chemical in solution at equilibrium. 

Adsorption Ratio (Kd)—The amount of a chemical adsorbed by sediment or soil (i.e., the solid phase) 

divided by the amount of chemical in the solution phase, which is in equilibrium with the solid phase, at a 

fixed solid/solution ratio. It is generally expressed in micrograms of chemical sorbed per gram of soil or 

sediment. 

Benchmark Dose (BMD)—Usually defined as the lower confidence limit on the dose that produces a 

specified magnitude of changes in a specified adverse response. For example, a BMD10 would be the 

dose at the 95% lower confidence limit on a 10% response, and the benchmark response (BMR) would be 

10%. The BMD is determined by modeling the dose response curve in the region of the dose response 

relationship where biologically observable data are feasible. 

Benchmark Dose Model—A statistical dose-response model applied to either experimental toxicological 

or epidemiological data to calculate a BMD. 

Bioconcentration Factor (BCF)—The quotient of the concentration of a chemical in aquatic organisms 

at a specific time or during a discrete time period of exposure divided by the concentration in the 

surrounding water at the same time or during the same period. 

Biomarkers—Broadly defined as indicators signaling events in biologic systems or samples. They have 

been classified as markers of exposure, markers of effect, and markers of susceptibility. 

Cancer Effect Level (CEL)—The lowest dose of chemical in a study, or group of studies, that produces 

significant increases in the incidence of cancer (or tumors) between the exposed population and its 

appropriate control. 

Carcinogen—A chemical capable of inducing cancer. 

Case-Control Study—A type of epidemiological study that examines the relationship between a 

particular outcome (disease or condition) and a variety of potential causative agents (such as toxic 

chemicals). In a case-controlled study, a group of people with a specified and well-defined outcome is 

identified and compared to a similar group of people without outcome. 

Case Report—Describes a single individual with a particular disease or exposure. These may suggest 

some potential topics for scientific research, but are not actual research studies. 

Case Series—Describes the experience of a small number of individuals with the same disease or 

exposure. These may suggest potential topics for scientific research, but are not actual research studies. 
BENZENE 378 

10. GLOSSARY 

Ceiling Value—A concentration of a substance that should not be exceeded, even instantaneously. 

Chronic Exposure—Exposure to a chemical for 365 days or more, as specified in the Toxicological 

Profiles. 

Cohort Study—A type of epidemiological study of a specific group or groups of people who have had a 

common insult (e.g., exposure to an agent suspected of causing disease or a common disease) and are 

followed forward from exposure to outcome. At least one exposed group is compared to one unexposed 

group. 

Cross-sectional Study—A type of epidemiological study of a group or groups of people that examines 

the relationship between exposure and outcome to a chemical or to chemicals at one point in time. 

Data Needs—Substance-specific informational needs that if met would reduce the uncertainties of human 

health assessment. 

Developmental Toxicity—The occurrence of adverse effects on the developing organism that may result 

from exposure to a chemical prior to conception (either parent), during prenatal development, or 

postnatally to the time of sexual maturation. Adverse developmental effects may be detected at any point 

in the life span of the organism. 

Dose-Response Relationship—The quantitative relationship between the amount of exposure to a 

toxicant and the incidence of the adverse effects. 

Embryotoxicity and Fetotoxicity—Any toxic effect on the conceptus as a result of prenatal exposure to 

a chemical; the distinguishing feature between the two terms is the stage of development during which the 

insult occurs. The terms, as used here, include malformations and variations, altered growth, and in utero 

death. 

Environmental Protection Agency (EPA) Health Advisory—An estimate of acceptable drinking water 

levels for a chemical substance based on health effects information. A health advisory is not a legally 

enforceable federal standard, but serves as technical guidance to assist federal, state, and local officials. 

Epidemiology—Refers to the investigation of factors that determine the frequency and distribution of 

disease or other health-related conditions within a defined human population during a specified period. 

Genotoxicity—A specific adverse effect on the genome of living cells that, upon the duplication of 

affected cells, can be expressed as a mutagenic, clastogenic, or carcinogenic event because of specific 

alteration of the molecular structure of the genome. 

Half-life—A measure of rate for the time required to eliminate one half of a quantity of a chemical from 

the body or environmental media. 

Immediately Dangerous to Life or Health (IDLH)—The maximum environmental concentration of a 

contaminant from which one could escape within 30 minutes without any escape-impairing symptoms or 

irreversible health effects. 

Immunologic Toxicity—The occurrence of adverse effects on the immune system that may result from 

exposure to environmental agents such as chemicals. 

Immunological Effects—Functional changes in the immune response. 
BENZENE 379 

10. GLOSSARY 

Incidence—The ratio of individuals in a population who develop a specified condition to the total 

number of individuals in that population who could have developed that condition in a specified time 

period. 

Intermediate Exposure—Exposure to a chemical for a duration of 15–364 days, as specified in the 

Toxicological Profiles. 

In Vitro—Isolated from the living organism and artificially maintained, as in a test tube. 

In Vivo—Occurring within the living organism. 

Lethal Concentration(LO) (LCLO)—The lowest concentration of a chemical in air that has been reported 

to have caused death in humans or animals. 

Lethal Concentration(50) (LC50)—A calculated concentration of a chemical in air to which exposure for 

a specific length of time is expected to cause death in 50% of a defined experimental animal population. 

Lethal Dose(LO) (LDLo)—The lowest dose of a chemical introduced by a route other than inhalation that 

has been reported to have caused death in humans or animals. 

Lethal Dose(50) (LD50)—The dose of a chemical that has been calculated to cause death in 50% of a 

defined experimental animal population. 

Lethal Time(50) (LT50)—A calculated period of time within which a specific concentration of a chemical 

is expected to cause death in 50% of a defined experimental animal population. 

Lowest-Observed-Adverse-Effect Level (LOAEL)—The lowest exposure level of chemical in a study, 

or group of studies, that produces statistically or biologically significant increases in frequency or severity 

of adverse effects between the exposed population and its appropriate control. 

Lymphoreticular Effects—Represent morphological effects involving lymphatic tissues such as the 

lymph nodes, spleen, and thymus. 

Malformations—Permanent structural changes that may adversely affect survival, development, or 

function. 

Minimal Risk Level (MRL)—An estimate of daily human exposure to a hazardous substance that is 

likely to be without an appreciable risk of adverse noncancer health effects over a specified route and 

duration of exposure. 

Modifying Factor (MF)—A value (greater than zero) that is applied to the derivation of a Minimal Risk 

Level (MRL) to reflect additional concerns about the database that are not covered by the uncertainty 

factors. The default value for a MF is 1. 

Morbidity—State of being diseased; morbidity rate is the incidence or prevalence of disease in a specific 

population. 

Mortality—Death; mortality rate is a measure of the number of deaths in a population during a specified 

interval of time. 
BENZENE 380 

10. GLOSSARY 

Mutagen—A substance that causes mutations. A mutation is a change in the DNA sequence of a cell’s 

DNA. Mutations can lead to birth defects, miscarriages, or cancer. 

Necropsy—The gross examination of the organs and tissues of a dead body to determine the cause of 

death or pathological conditions. 

Neurotoxicity—The occurrence of adverse effects on the nervous system following exposure to a 

chemical. 

No-Observed-Adverse-Effect Level (NOAEL)—The dose of a chemical at which there were no 

statistically or biologically significant increases in frequency or severity of adverse effects seen between 

the exposed population and its appropriate control. Effects may be produced at this dose, but they are not 

considered to be adverse. 

Octanol-Water Partition Coefficient (Kow)—The equilibrium ratio of the concentrations of a chemical 

in n-octanol and water, in dilute solution. 

Odds Ratio (OR)—A means of measuring the association between an exposure (such as toxic substances 

and a disease or condition) that represents the best estimate of relative risk (risk as a ratio of the incidence 

among subjects exposed to a particular risk factor divided by the incidence among subjects who were not 

exposed to the risk factor). An OR of greater than 1 is considered to indicate greater risk of disease in the 

exposed group compared to the unexposed group. 

Organophosphate or Organophosphorus Compound—A phosphorus-containing organic compound 

and especially a pesticide that acts by inhibiting cholinesterase. 

Permissible Exposure Limit (PEL)—An Occupational Safety and Health Administration (OSHA) 

allowable exposure level in workplace air averaged over an 8-hour shift of a 40-hour workweek. 

Pesticide—General classification of chemicals specifically developed and produced for use in the control 

of agricultural and public health pests. 

Pharmacokinetics—The dynamic behavior of a material in the body, used to predict the fate 

(disposition) of an exogenous substance in an organism. Utilizing computational techniques, it provides 

the means of studying the absorption, distribution, metabolism, and excretion of chemicals by the body. 

Pharmacokinetic Model—A set of equations that can be used to describe the time course of a parent 

chemical or metabolite in an animal system. There are two types of pharmacokinetic models: data-based 

and physiologically-based. A data-based model divides the animal system into a series of compartments, 

which, in general, do not represent real, identifiable anatomic regions of the body, whereas the 

physiologically-based model compartments represent real anatomic regions of the body. 

Physiologically Based Pharmacodynamic (PBPD) Model—A type of physiologically based doseresponse model that quantitatively describes the relationship between target tissue dose and toxic end 

points. These models advance the importance of physiologically based models in that they clearly 

describe the biological effect (response) produced by the system following exposure to an exogenous 

substance. 

Physiologically Based Pharmacokinetic (PBPK) Model—Comprised of a series of compartments 

representing organs or tissue groups with realistic weights and blood flows. These models require a 

variety of physiological information: tissue volumes, blood flow rates to tissues, cardiac output, alveolar 
BENZENE 381 

10. GLOSSARY 

ventilation rates, and possibly membrane permeabilities. The models also utilize biochemical 

information, such as air/blood partition coefficients, and metabolic parameters. PBPK models are also 

called biologically based tissue dosimetry models. 

Prevalence—The number of cases of a disease or condition in a population at one point in time. 

Prospective Study—A type of cohort study in which the pertinent observations are made on events 

occurring after the start of the study. A group is followed over time. 

q1*—The upper-bound estimate of the low-dose slope of the dose-response curve as determined by the 

multistage procedure. The q1* can be used to calculate an estimate of carcinogenic potency, the 

incremental excess cancer risk per unit of exposure (usually μg/L for water, mg/kg/day for food, and 

μg/m3 for air). 

Recommended Exposure Limit (REL)—A National Institute for Occupational Safety and Health 

(NIOSH) time-weighted average (TWA) concentration for up to a 10-hour workday during a 40-hour 

workweek. 

Reference Concentration (RfC)—An estimate (with uncertainty spanning perhaps an order of 

magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups) 

that is likely to be without an appreciable risk of deleterious noncancer health effects during a lifetime. 

The inhalation reference concentration is for continuous inhalation exposures and is appropriately 

expressed in units of mg/m3 or ppm. 

Reference Dose (RfD)—An estimate (with uncertainty spanning perhaps an order of magnitude) of the 

daily exposure of the human population to a potential hazard that is likely to be without risk of deleterious 

effects during a lifetime. The RfD is operationally derived from the no-observed-adverse-effect level 

(NOAEL, from animal and human studies) by a consistent application of uncertainty factors that reflect 

various types of data used to estimate RfDs and an additional modifying factor, which is based on a 

professional judgment of the entire database on the chemical. The RfDs are not applicable to 

nonthreshold effects such as cancer. 

Reportable Quantity (RQ)—The quantity of a hazardous substance that is considered reportable under 

the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). Reportable 

quantities are (1) 1 pound or greater or (2) for selected substances, an amount established by regulation 

either under CERCLA or under Section 311 of the Clean Water Act. Quantities are measured over a 

24-hour period. 

Reproductive Toxicity—The occurrence of adverse effects on the reproductive system that may result 

from exposure to a chemical. The toxicity may be directed to the reproductive organs and/or the related 

endocrine system. The manifestation of such toxicity may be noted as alterations in sexual behavior, 

fertility, pregnancy outcomes, or modifications in other functions that are dependent on the integrity of 

this system. 

Retrospective Study—A type of cohort study based on a group of persons known to have been exposed 

at some time in the past. Data are collected from routinely recorded events, up to the time the study is 

undertaken. Retrospective studies are limited to causal factors that can be ascertained from existing 

records and/or examining survivors of the cohort. 

Risk—The possibility or chance that some adverse effect will result from a given exposure to a chemical. 
 

 

BENZENE 382 

10. GLOSSARY 

Risk Factor—An aspect of personal behavior or lifestyle, an environmental exposure, or an inborn or 

inherited characteristic that is associated with an increased occurrence of disease or other health-related 

event or condition. 

Risk Ratio—The ratio of the risk among persons with specific risk factors compared to the risk among 

persons without risk factors. A risk ratio greater than 1 indicates greater risk of disease in the exposed 

group compared to the unexposed group. 

Short-Term Exposure Limit (STEL)—The American Conference of Governmental Industrial 

Hygienists (ACGIH) maximum concentration to which workers can be exposed for up to 15 minutes 

continually. No more than four excursions are allowed per day, and there must be at least 60 minutes 

between exposure periods. The daily Threshold Limit Value-Time Weighted Average (TLV-TWA) may 

not be exceeded. 

Standardized Mortality Ratio (SMR)—A ratio of the observed number of deaths and the expected 

number of deaths in a specific standard population. 

Target Organ Toxicity—This term covers a broad range of adverse effects on target organs or 

physiological systems (e.g., renal, cardiovascular) extending from those arising through a single limited 

exposure to those assumed over a lifetime of exposure to a chemical. 

Teratogen—A chemical that causes structural defects that affect the development of an organism. 

Threshold Limit Value (TLV)—An American Conference of Governmental Industrial Hygienists 

(ACGIH) concentration of a substance to which most workers can be exposed without adverse effect. 

The TLV may be expressed as a Time Weighted Average (TWA), as a Short-Term Exposure Limit 

(STEL), or as a ceiling limit (CL). 

Time-Weighted Average (TWA)—An allowable exposure concentration averaged over a normal 8-hour 

workday or 40-hour workweek. 

Toxic Dose(50) (TD50)—A calculated dose of a chemical, introduced by a route other than inhalation, 

which is expected to cause a specific toxic effect in 50% of a defined experimental animal population. 

Toxicokinetic—The absorption, distribution, and elimination of toxic compounds in the living organism. 

Uncertainty Factor (UF)—A factor used in operationally deriving the Minimal Risk Level (MRL) or 

Reference Dose (RfD) or Reference Concentration (RfC) from experimental data. UFs are intended to 

account for (1) the variation in sensitivity among the members of the human population, (2) the 

uncertainty in extrapolating animal data to the case of human, (3) the uncertainty in extrapolating from 

data obtained in a study that is of less than lifetime exposure, and (4) the uncertainty in using lowestobserved-adverse-effect level (LOAEL) data rather than no-observed-adverse-effect level (NOAEL) data. 

A default for each individual UF is 10; if complete certainty in data exists, a value of 1 can be used; 

however, a reduced UF of 3 may be used on a case-by-case basis, 3 being the approximate logarithmic 

average of 10 and 1. 

Xenobiotic—Any chemical that is foreign to the biological system. 
BENZENE A-1 

APPENDIX A. ATSDR MINIMAL RISK LEVELS AND WORKSHEETS 

The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) [42 U.S.C. 

9601 et seq.], as amended by the Superfund Amendments and Reauthorization Act (SARA) [Pub. L. 99– 

499], requires that the Agency for Toxic Substances and Disease Registry (ATSDR) develop jointly with 

the U.S. Environmental Protection Agency (EPA), in order of priority, a list of hazardous substances most 

commonly found at facilities on the CERCLA National Priorities List (NPL); prepare toxicological 

profiles for each substance included on the priority list of hazardous substances; and assure the initiation 

of a research program to fill identified data needs associated with the substances. 

The toxicological profiles include an examination, summary, and interpretation of available toxicological 

information and epidemiologic evaluations of a hazardous substance. During the development of 

toxicological profiles, Minimal Risk Levels (MRLs) are derived when reliable and sufficient data exist to 

identify the target organ(s) of effect or the most sensitive health effect(s) for a specific duration for a 

given route of exposure. An MRL is an estimate of the daily human exposure to a hazardous substance 

that is likely to be without appreciable risk of adverse noncancer health effects over a specified duration 

of exposure. MRLs are based on noncancer health effects only and are not based on a consideration of 

cancer effects. These substance-specific estimates, which are intended to serve as screening levels, are 

used by ATSDR health assessors to identify contaminants and potential health effects that may be of 

concern at hazardous waste sites. It is important to note that MRLs are not intended to define clean-up or 

action levels. 

MRLs are derived for hazardous substances using the no-observed-adverse-effect level/uncertainty factor 

approach. They are below levels that might cause adverse health effects in the people most sensitive to 

such chemical-induced effects. MRLs are derived for acute (1–14 days), intermediate (15–364 days), and 

chronic (365 days and longer) durations and for the oral and inhalation routes of exposure. Currently, 

MRLs for the dermal route of exposure are not derived because ATSDR has not yet identified a method 

suitable for this route of exposure. MRLs are generally based on the most sensitive chemical-induced end 

point considered to be of relevance to humans. Serious health effects (such as irreparable damage to the 

liver or kidneys, or birth defects) are not used as a basis for establishing MRLs. Exposure to a level 

above the MRL does not mean that adverse health effects will occur. 

MRLs are intended only to serve as a screening tool to help public health professionals decide where to 

look more closely. They may also be viewed as a mechanism to identify those hazardous waste sites that 
BENZENE A-2 

APPENDIX A 

are not expected to cause adverse health effects. Most MRLs contain a degree of uncertainty because of 

the lack of precise toxicological information on the people who might be most sensitive (e.g., infants, 

elderly, nutritionally or immunologically compromised) to the effects of hazardous substances. ATSDR 

uses a conservative (i.e., protective) approach to address this uncertainty consistent with the public health 

principle of prevention. Although human data are preferred, MRLs often must be based on animal studies 

because relevant human studies are lacking. In the absence of evidence to the contrary, ATSDR assumes 

that humans are more sensitive to the effects of hazardous substance than animals and that certain persons 

may be particularly sensitive. Thus, the resulting MRL may be as much as 100-fold below levels that 

have been shown to be nontoxic in laboratory animals. 

Proposed MRLs undergo a rigorous review process: Health Effects/MRL Workgroup reviews within the 

Division of Toxicology and Environmental Medicine, expert panel peer reviews, and agency-wide MRL 

Workgroup reviews, with participation from other federal agencies and comments from the public. They 

are subject to change as new information becomes available concomitant with updating the toxicological 

profiles. Thus, MRLs in the most recent toxicological profiles supersede previously published levels. 

For additional information regarding MRLs, please contact the Division of Toxicology and 

Environmental Medicine, Agency for Toxic Substances and Disease Registry, 1600 Clifton Road NE, 

Mailstop F-32, Atlanta, Georgia 30333. 
BENZENE	 A-3 

APPENDIX A 

MINIMAL RISK LEVEL (MRL) WORKSHEET 

Chemical Name: Benzene 

CAS Numbers: 71-43-2 

Date: August 2007 

Profile Status: Post Public, Final Draft 

Route: [x] Inhalation [ ] Oral 

Duration: [x] Acute [ ] Intermediate [ ] Chronic 

Graph Key: 46 

Species: Mouse 

Minimal Risk Level: 0.009 [ ] mg/kg/day [x] ppm 

Reference: Rozen MG, Snyder CA, Albert RE. 1984. Depression in B- and T-lymphocyte mitogeninduced blastogenesis in mice exposed to low concentrations of benzene. Toxicol Lett 20:343-349. 

Experimental design: Male C57BL/6J mice (7–8/group) were exposed to benzene (0, 10.2, 31, 100, or 

301 ppm) in whole-body dynamic inhalation chambers for 6 hours/day for 6 consecutive days. Control 

mice were exposed to filtered, conditioned air only. Erythrocyte counts were depressed in C57BL/6 mice 

only at 100 and 301 ppm. The 10.2 ppm exposure level resulted in significant depression of femoral 

lipopolysaccharide-induced B-colony-forming ability in the absence of a significant depression of total 

numbers of B cells. At 31 ppm, splenic phytohemagglutinin-induced blastogenesis was significantly 

depressed without a concomitant significant depression in numbers of T-lymphocytes. Peripheral 

lymphocyte counts were depressed at all exposure levels. These results demonstrate that short-term 

inhaled benzene even at low exposure concentrations can alter certain immune associated processes. 

Effect noted in study and corresponding doses: 

10.2 ppm = No adverse effect on erythrocytes, depressed peripheral lymphocytes and mitogeninduced blastogenesis of femoral B-lymphocytes (less serious LOAEL). 

31 ppm = No adverse effect on erythrocytes, depression of mitogen-induced blastogenesis of 

 splenic T-cells. 

100 ppm = Depressed erythrocyte counts. 

Dose and end point used for MRL derivation: 

[ ] NOAEL [x] LOAEL 

Uncertainty Factors used in MRL derivation: 300 

[ ] 1 [ ] 3 [x] 10 (for use of a LOAEL)
 

[ ] 1 [x] 3 [ ] 10 (for extrapolation from animals to humans using dosimetric conversion) 

[ ] 1 [ ] 3 [x] 10 (for human variability)
 

Was a conversion factor used from ppm in food or water to a mg/body weight dose? Not applicable. 
BENZENE A-4 

APPENDIX A 

If an inhalation study in animals, list conversion factors used in determining human equivalent dose: The 

concentration was adjusted for intermittent exposure by multiplying the LOAEL (10.2 ppm) by 6/24 to 

correct for less than a full day of exposure. The resulting LOAELADJ is 2.55 ppm. 

According to current EPA (1994b) methodology for calculating a human equivalent concentration (HEC) 

for extrarespiratory effects of a category 3 gas (such as benzene): 

LOAELHEC = LOAELADJ x ([Hb/g]A/[Hb/g]H) 

where: 

LOAELHEC = The LOAEL dosimetrically adjusted to a human equivalent concentration 

LOAELADJ = The LOAEL adjusted from intermittent to continuous exposure 

[Hb/g]A/[Hb/g]H = The ratio of the blood:gas partition coefficient of the chemical for the laboratory

 animal species to the human value 

If the animal blood:gas partition coefficient is greater than the human blood:gas partition coefficient, a 

default value of 1 is used for the ratio. According to Wiester et al. (2002), benzene blood:gas partition 

coefficients for mice and humans are 17.44 and 8.12, respectively. Therefore the default value of 1 is 

applied, in which case, the LOAELHEC is equivalent to the LOAELADJ = 2.55 ppm. 

Was a conversion used from intermittent to continuous exposure? The concentration was adjusted for 

intermittent exposure by multiplying the LOAEL (10.2 ppm) by 6/24 to correct for less than a full day of 

exposure. The resulting LOAELADJ is 2.55 ppm. 

Other additional studies or pertinent information that lend support to this MRL: Increased number of 

micronucleated polychromatic erythrocytes (MN-PCEs), decreased numbers of granulopoietic stem cells 

(Toft et al. 1982), lymphopenia (Cronkite et al. 1985), lymphocyte depression, and increased 

susceptibility to bacterial infection (Rosenthal and Snyder 1985) are among the adverse hematological 

and immunological effects observed in several other acute-duration inhalation studies. The study by 

Rozen et al. (1984) shows benzene immunotoxicity (reduced mitogen-induced lymphocyte proliferation) 

at a slightly lower exposure level than these other studies. C57BI/6J mice were exposed to 0, 10.2, 31, 

100, and 301 ppm benzene for 6 days at 6 hours/day. Lymphocyte counts were depressed at all exposure 

levels while erythrocyte counts were elevated at 10.2 ppm, equal to controls at 31 ppm, and depressed at 

100 and 301 ppm. Femoral B-lymphocyte and splenic B-lymphocyte numbers were reduced at 100 ppm. 

Levels of circulating lymphocytes and mitogen-induced blastogenesis of femoral B-lymphocytes were 

depressed after exposure to 10.2 ppm benzene for 6 days. Mitogen-induced blastogeneses of splenic 

T-lymphocytes were depressed after exposure to 31 ppm of benzene for 6 days. In another study, mice 

exhibited a 50% decrease in the population of erythroid progenitor cells (CFU-E) after exposure to 

10 ppm benzene for 5 days, 6 hours/day (Dempster and Snyder 1991). In a study by Wells and Nerland 

(1991), groups of 4–5 male Swiss-Webster mice were exposed to 0, 3, 25, 55, 105, 199, 303, 527, 1,150, 

or 2,290 ppm benzene for 6 hours/day for 5 days. The number of leukocytes in peripheral blood and 

spleen weights were significantly decreased compared with untreated controls at all concentrations 

≥25 ppm. Therefore, 3 ppm was the NOAEL and 25 ppm was the LOAEL for these effects. Other end 

points were not monitored in this study. These data support the choice of Rozen et al. (1984) as a critical 

study. 

Agency Contacts (Chemical Managers): Sharon Wilbur, M.A., Sam Keith, M.S., C.H.P., Obaid Faroon, 

Ph.D. 
BENZENE	 A-5 

APPENDIX A 

MINIMAL RISK LEVEL (MRL) WORKSHEET 

Chemical Name: Benzene 

CAS Numbers: 71-43-2 

Date: August 2007 

Profile Status: Post Public, Final Draft 

Route: [x] Inhalation [ ] Oral 

Duration: [ ] Acute [x] Intermediate [ ] Chronic 

Graph Key: 126 

Species: Mouse 

Minimal Risk Level: 0.006 [ ] mg/kg/day [x] ppm 

Reference: Rosenthal GJ, Snyder CA. 1987. Inhaled benzene reduces aspects of cell-mediated tumor 

surveillance in mice. Toxicol Appl Pharmacol 88:35-43. 

Experimental design: Male C57Bl/6 mice were exposed to 10, 30, or 100 ppm of benzene by inhalation 

6 hours/day, 5 days/week for 20 exposure days. The number of lymphocytes and their functional 

capacities were evaluated in spleens of exposed mice. Following the 20 days of exposure, functional 

capacity of splenic lymphocytes was evaluated in two in vitro assays: mixed-lymphocyte culture (MLC) 

and 51Cr-release cytotoxicity assay. Measured mean daily benzene concentrations in the 10, 30, and 

100 ppm groups were 11.1 (±1.5) ppm, 29.5 (±4.4) ppm, and 99.7 (±7.0) ppm, respectively. No changes 

were observed in the relative proportions of splenic leukocytes, in the percentage of T-cell subsets or in 

the ratio of T-helper and T-suppressor cells, even at the highest exposure level (100 ppm). Therefore, the 

functional assays could be normalized for particular lymphocyte populations by using equal numbers of 

splenic cells. MLC is an in vitro measure of alloreactivity (capacity to mount an immune response 

against foreign antigens). The MLC activity of spleen lymphocytes from 10- and 100-ppm mice was 

delayed on days 2–4 of culture (relative to air-exposed controls), indicating that benzene exposure causes 

impaired in vitro alloreactivity (data for the 30-ppm mice were not included in the reported results). This 

delayed alloreactivity was not due to spleen suppressor cells. The lymphocyte cytotoxic function 

evaluated in the 51Cr-release assay was also altered; splenic lymphocytes from 100-ppm mice had a 

significantly reduced lysing capacity. The results indicate that inhalation exposure of mice to benzene has 

an immunodepressive effect on in vitro alloreactivity and cytotoxicity of splenic lymphocytes. 

Effect noted in study and corresponding doses: 

10 ppm = Significantly delayed splenic lymphocyte reaction to foreign antigens evaluated in in 

vitro mixed lymphocyte reaction (less serious LOAEL). 

30 ppm = Results not reported. 

100 ppm = Significantly delayed splenic lymphocyte reaction to foreign antigens evaluated in in 

vitro mixed lymphocyte reaction. 

Dose and end point used for MRL derivation: 

[ ] NOAEL [x] LOAEL 
BENZENE A-6 

APPENDIX A 

Uncertainty Factors used in MRL derivation: 300 

[ ] 1 [ ] 3 [x] 10 (for use of a LOAEL)
 

[ ] 1 [x] 3 [ ] 10 (for extrapolation from animals to humans using dosimetric conversion) 

[ ] 1 [ ] 3 [x] 10 (for human variability)
 

Was a conversion factor used from ppm in food or water to a mg/body weight dose? Not applicable. 

If an inhalation study in animals, list conversion factors used in determining human equivalent dose: 

According to current EPA (1994b) methodology for calculating a human equivalent concentration (HEC) 

for extrarespiratory effects of a category 3 gas (such as benzene): 

LOAELHEC = LOAELADJ x ([Hb/g]A/[Hb/g]H) 

where: 

LOAELHEC = The LOAEL dosimetrically adjusted to a human equivalent concentration 

LOAELADJ = The LOAEL adjusted from intermittent to continuous exposure 

[Hb/g]A/[Hb/g]H = The ratio of the blood:gas partition coefficient of the chemical for the laboratory

 animal species to the human value 

If the animal blood:gas partition coefficient is greater than the human blood:gas partition coefficient, a 

default value of 1 is used for the ratio. According to Wiester et al. (2002), benzene blood:gas partition 

coefficients for mice and humans are 17.44 and 8.12, respectively. Therefore, the default value of 1 is 

applied, in which case, the LOAELHEC is equivalent to the LOAELADJ. 

Was a conversion used from intermittent to continuous exposure? The concentration was adjusted for 

intermittent exposure by multiplying the LOAEL (10 ppm) by 6 hours/24 hours to correct for less than a 

full day of exposure and 5 days/7 days to correct for less than a full week of exposure. The resulting 

LOAELADJ is 1.8 ppm. 

Other additional studies or pertinent information that lend support to this MRL: Exposure of C57BL 

mice to 10 ppm benzene for 6 hours/day, 5 days/week caused significant depressions in numbers of 

lymphocytes (ca. 30% lower than controls) as early as exposure day 32; this effect was also noted at the 

other scheduled periods of testing (exposure days 66 and 178) (Baarson et al. 1984). Splenic red blood 

cells were significantly reduced (ca. 15% lower than controls) at exposure days 66 and 178. The failure 

of the erythrons of benzene-exposed mice to support normal red cell mass was illustrated by the 

significant reduction in peripheral red cell numbers in these animals at 66 and 178 days of benzene 

exposure. Green et al. (1981a, 1981b) exposed male CD-1 mice to benzene vapors at concentrations of 

0 or 9.6 ppm for 6 hours/day, 5 days/week for 50 days and assessed the effects of exposure on cellularity 

in the spleen, bone marrow, and peripheral blood. Exposure-related effects included a 90% increase in 

numbers of multipotential hematopoietic stem cells (CFU-S) (Green et al. 1981a), approximately 25% 

increase in spleen weight and total splenic nucleated cellularity (Green et al. 1981b), and 80% increase in 

nucleated RBCs (Green et al. 1981b). The results of Baarson et al. (1984) and Green et al. (1981a, 

1981b) are limited for purposes of quantitative risk assessment because a single exposure level was 

employed. However, they support the choice of Rosenthal and Snyder (1987) as the critical study, which 

serves as the basis for the intermediate-duration inhalation MRL. 
BENZENE A-7 

APPENDIX A 

Agency Contacts (Chemical Managers): Sharon Wilbur, M.A., Sam Keith, M.S., C.H.P., Obaid Faroon, 

Ph.D. 
BENZENE A-8 

APPENDIX A 

MINIMAL RISK LEVEL (MRL) WORKSHEET 

Chemical Name: Benzene 

CAS Numbers: 71-43-2 

Date: August 2007 

Profile Status: Post Public, Final Draft 

Route: [x] Inhalation [ ] Oral 

Duration: [ ] Acute [ ] Intermediate [x ] Chronic 

Graph Key: 161 

Species: Human 

Minimal Risk Level: 0.003 [ ] mg/kg/day [x] ppm [ ] mg/m3 

Reference: Lan Q, Zhang L, Li G, et al. 2004a. Hematotoxicity in workers exposed to low levels of 

benzene. Science 306:1774-1776. 

Experimental design: A cross-sectional study was performed on 250 workers (approximately two-thirds 

female) exposed to benzene at two shoe manufacturing facilities in Tianjin, China, and 140 age- and 

gender-matched workers in clothing manufacturing facilities that did not use benzene. The benzeneexposed workers had been employed for an average of 6.1±2.9 years. Benzene exposure was monitored 

by individual organic vapor monitors (full shift) 5 or more times during 16 months prior to phlebotomy. 

Post-shift urine samples were collected from every worker. Urinary benzene concentrations were highly 

correlated with mean individual air levels. Benzene was not found (detection limit 0.04 ppm) in 

workplace and home air samples of control workers taken at three different time periods. Study subjects 

were categorized into four groups (140 controls, 109 at <1 ppm, 110 at 1–<10 ppm, and 31 at ≥10 ppm) 

according to mean benzene exposure levels measured twice during the month prior to phlebotomy. Of the 

250 exposed workers, 109 were exposed to <1 ppm benzene. Each of these individuals worked at the 

larger of the two facilities included in the study. Exposure concentrations were generally higher at the 

smaller facility due to a less adequate ventilation system. Complete blood count (CBC) and differential 

were analyzed mechanically. Coefficients of variation for all cell counts were <10%. 

Mean 1-month benzene exposure levels in the four groups (controls, <1 ppm, 1–<10 ppm, and ≥10 ppm) 

were <0.04, 0.57±0.24, 2.85±2.11, and 28.73±20.74 ppm, respectively (see Table A-1). An evaluation of 

potential confounding factors showed that age, gender, cigarette smoking, alcohol consumption, recent 

infection, and body mass index were associated with at least one hematological end point. The values in 

Table A-1 represent values that were adjusted to account for these variables. All types of white blood 

cells (WBCs) and platelets were significantly decreased in the lowest exposure group (<1 ppm), ranging 

in magnitude from approximately 8 to 15% lower than controls. Although similar statistical analyses for 

the mid- and high-exposure groups were not included in the study report, decreases in all types of WBCs 

and platelets were noted at these exposure levels as well; the decreases in the highest exposure group 

ranged in magnitude from 15 to 36%. Lymphocyte subset analysis revealed significantly decreased 

CD4+-T cells, CD4+/CD8+ ratio, and B cells. Hemoglobin concentrations were significantly decreased 

only within the highest (≥10 ppm) exposure group. Tests for a linear trend using benzene air level as a 

continuous variable were significant for platelets and all WBC measures except monocytes and 

CD8+-T cells. Upon restricting the linear trend analyses to workers exposed to <10 ppm benzene, 

excluding controls, inverse associations remained for total WBCs, granulocytes, lymphocytes, B cells, 

and platelets. In order to evaluate the effect of past benzene exposures on the hematological effects 

observed in this study, the authors compared findings for a group of workers who had been exposed to 

<1 ppm benzene over the previous year (n=60) and a subset who also had <40 ppm-years lifetime 

cumulative benzene exposure (n=50). The authors stated that the same cell types were significantly 

reduced in these groups, but did not provide further information of the magnitude (i.e., percent change) of 
BENZENE A-9 

APPENDIX A 

the hematological effects observed. These data suggest that the 1-month benzene exposure results could 

be used as an indicator of longer term low-level benzene hematotoxicity. To demonstrate that the 

observed effects were attributable to benzene, significantly decreased levels of WBCs, granulocytes, 

lymphocytes, and B cells were noted in a subgroup (n=30; mean 1-month exposure level of 

0.29±0.15 ppm) of the <1 ppm group for which exposure to other solvents was negligible. 

Table A-1. Significantly Reduced Blood Values in Workers Exposed to Benzene 

in Tianjin, China (Adapted from Lan et al. 2004a) 

Mean exposure level in ppma (number of subjects) 

End point <0.04 (140) 0.57±0.24 (109) 2.85±2.11 (110) 28.73±20.74 (31) 

WBCsb 

Granulocytesb 

Monocytesb 

Lymphocytesb

 CD4+-T cellsb

 CD4+/CD8+ ratio 

 B cellsb 

Plateletsc 

6,480±1,710 

4,110±1,410 

241±92 

2,130±577 

742±262 

1.46±0.58 

218±94 

230±59.7 

5,540±1,220d 

3,360±948d 

217±97d 

1,960±541d 

635±187d 

1.26±0.41d 

186±95d 

214±48.8d 

5,660±1,500 

3,480±1,170 

224±93 

1,960±533 

623±177 

1.22±0.45 

170±75 

200±53.4 

4,770±892 

2,790±750 

179±74 

1,800±392 

576±188 

1.09±0.35 

140±101 

172±44.8 

a

Arithmetic mean of an average of two measurements per subject collected during the month prior to phlebotomy b

Mean cell numbers per microliter blood±standard deviation 

c

Mean number of platelets (x103) d

Statistically significantly lower than controls (p<0.05) by linear regression on ln of each end point 

Effect noted in study and corresponding doses: As shown in Table A-1, exposure-response relationships 

were noted for several blood factors. Benzene-induced decreased B cell count was selected as the critical 

effect for benchmark dose (BMD) modeling because it represented the highest magnitude of effect (i.e., B 

cell count in the highest exposure group was approximately 36% lower than that of controls). A BMD 

modeling approach was selected to identify the point of departure because the critical study (Lan et al. 

2004a) identified a LOAEL in the absence of a NOAEL. 

Dose and end point used for MRL derivation: 0.10 ppm (BMCL0.25sd) for decreased B cell count. 

All continuous variable models in the EPA Benchmark Dose Software (Version 1.3.2) were fit to the 

B cell count data shown in Table A-1. Visual inspection of the plots of observed versus expected values 

for B cell counts indicated that the Hill model provided the only adequate fit of the data set (see 

Figure A-1). A benchmark response (BMR) of 0.25 sd below the control mean B cell count was selected 

because it resulted in a BMC0.25sd of 0.42 ppm and its lower 95% confidence limit (BMCL0.25sd) of 

0.10 ppm (Figure A-1), which are below the mean exposure level of the lowest exposure group 

(0.57 ppm) for which a statistically significant decrease in mean B cell count (186 versus 218 in controls, 

see Table A-1) was observed. Although Lan et al. (2004a, 2004b) noted significantly decreased levels of 

WBCs, granulocytes, lymphocytes, and B cells in a subgroup (n=30; mean 1-month exposure level of 

0.29±0.15 ppm) of the 0.57 ppm exposure group, this subgroup could not be included in the BMD 

analysis because the study authors did not include the means and standard deviations for the decreased 

blood factors, nor did they provide quantitative information regarding the remaining 70 subjects in the 

0.57 ppm exposure group (n=109). Assuming that the 0.29 ppm exposure level may represent a 

minimally adverse exposure level, it seems reasonable to accept the BMCL0.25sd of 0.10 ppm as the point 

of departure for deriving a chronic-duration inhalation MRL for benzene. 
BENZENE A-10 

APPENDIX A 

Figure A-1. Observed and Predicted B Cell Counts in Human Subjects 

Occupationally Exposed to Benzene. BMD=BMC0.25sd=0.42 ppm; 

BMDL=BMCL0.25sd=0.10 ppm 

Hill Model with 0.95 Confidence Level 

100 

120 

140 

160 

180 

200 

220 

240 

Mean Response 

BMDL BMD

Hill 

0 5 10 15 20 25 30 

dose 

08:53 04/11 2005 

The computer output for fitting of the Hill model to B cell counts in human subjects occupationally 

exposed to benzene (Lan et al. 2004a) follows. 

==================================================================== 

Hill Model. $Revision: 2.1 $ $Date: 2000/10/11 21:21:23 $

Input Data File: C:\ATSDR\BENZENE\BMD FILES\BENZENELANBCELLS.(d)

Gnuplot Plotting File: C:\ATSDR\BENZENE\BMD 

FILES\BENZENELANBCELLS.plt 

 Mon Nov 20 09:27:13 2006 

==================================================================== 

BMDS MODEL RUN 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

The form of the response function is: 

Y[dose] = intercept + v*dose^n/(k^n + dose^n) 

Dependent variable = MEAN

Independent variable = ppm

rho is set to 0 

Power parameter restricted to be greater than 1

A constant variance model is fit 

Total number of dose groups = 4

Total number of records with missing values = 0

Maximum number of iterations = 250 

Relative Function Convergence has been set to: 1e-008

Parameter Convergence has been set to: 1e-008 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

------ --- -------- ----------- -------- ----------- ---------- 

BENZENE A-11 

APPENDIX A 

Default Initial Parameter Values 

alpha = 8224.64 

rho = 0 Specified

intercept = 218 

v = -78 

n = 0.572459 

k = 1.5675 

Asymptotic Correlation Matrix of Parameter Estimates 

( *** The model parameter(s) -n have been estimated at a 

boundary point, or have been specified by the user, and do not appear in the

correlation matrix ) 

alpha rho intercept v k 

alpha 1 0 0 0 0 

rho 0 1 0 0 0 

intercept 0 0 1 0 0 

v 0 0 0 1 0 

k 0 0 0 0 1 

Parameter Estimates 

Variable Estimate Std. Err. 

alpha 8027.09 1 

rho 0 1 

intercept 217.113 1 

v -69.0144 1 

n 1 NA 

k 0.878186 1 

NA - Indicates that this parameter has hit a bound

implied by some inequality constraint and thus

has no standard error. 

Table of Data and Estimated Values of Interest 

Dose N Obs Mean Obs Std Dev Est Mean Est Std Dev Chi^2 Res. 

0 140 218 94 217 89.6 0.0099 

0.57 109 186 95 190 89.6 -0.0441 

2.85 110 170 75 164 89.6 0.063 

28.73 31 140 101 150 89.6 -0.113 

Model Descriptions for likelihoods calculated 

Model A1: Yij = Mu(i) + e(ij) 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

BENZENE A-12 

APPENDIX A 

Var{e(ij)} = Sigma^2 

Model A2: Yij = Mu(i) + e(ij)

Var{e(ij)} = Sigma(i)^2 

Model R: Yi = Mu + e(i)

Var{e(i)} = Sigma^2 

Degrees of freedom for Test A1 vs fitted <= 0 

Likelihoods of Interest 

Model 

A1 

Log(likelihood)

-1947.632025 

DF 

5 

AIC 

3905.264050 

A2 -1943.411648 8 3902.823297 

fitted -1948.162584 4 3904.325168 

R -1962.157799 2 3928.315597 

Test 1: Does response and/or variances differ among dose levels

(A2 vs. R)

Test 2: Are Variances Homogeneous (A1 vs A2)

Test 3: Does the Model for the Mean Fit (A1 vs. fitted) 

Tests of Interest 

Test -2*log(Likelihood Ratio) Test df p-value 

Test 1 37.4923 6 <.0001 

Test 2 8.44075 3 0.03773 

Test 3 1.06112 0 NA 

The p-value for Test 1 is less than .05. There appears to be a

difference between response and/or variances among the dose levels.

It seems appropriate to model the data 

The p-value for Test 2 is less than .05. Consider running a

non-homogeneous variance model 

NA - Degrees of freedom for Test 3 are less than or equal to 0. The ChiSquare test for fit is not valid 

Benchmark Dose Computation

Specified effect = 0.25 

Risk Type = Estimated standard deviations from the control mean 

Confidence level = 0.95 

BMC = 0.42196 

BMCL = 0.104163 

Although Test 3 (mean fit) produced an invalid Chi-Square test (degrees of freedom ≤0), visual inspection 

of the observed vs expected B cell counts from the Hill model output (Figure A-1) resulted in the 

determination that the predicted B cell counts adequately reflect the observed values and that the 
BENZENE A-13 

APPENDIX A 

associated BMCL0.25sd of 0.104163 provides an appropriate point of departure for deriving a chronicduration inhalation MRL for benzene. 

[ ] NOAEL [ ] LOAEL 

Uncertainty Factors used in MRL derivation: 10 

[ ] 1 [ ] 3 [ ] 10 (for use of a LOAEL)
 

[ ] 1 [ ] 3 [ ] 10 (for extrapolation from animals to humans) 

[ ] 1 [ ] 3 [x] 10 (for human variability)
 

Was a conversion factor used from ppm in food or water to a mg/body weight dose? Not applicable. 

If an inhalation study in animals, list conversion factors used in determining human equivalent dose: Not 

applicable. 

Was a conversion used from intermittent to continuous exposure? The BMCL0.25sd of 0.10 ppm was 

adjusted from the 8-hour TWA to a continuous exposure concentration (BMCL0.25sdADJ) as follows: 

BMCL0.25sdADJ = BMCL0.25sd x (8 hours/24 hours) x (6 days/7 days) 

Therefore: 

BMCL0.25sdADJ = 0.10 ppm x (8 hours/24 hours) x (6 days/7 days) 

BMCL0.25sdADJ = 0.03 ppm 

Other additional studies or pertinent information that lend support to this MRL: Lan et al. (2004a, 2004b) 

was selected as the critical study for derivation of a chronic-duration inhalation MRL because it (1) was 

well designed, (2) provided adequate exposure-response information, (3) employed individual exposure 

monitoring data collected for up to 16 months prior to blood testing, (4) demonstrated effects that did not 

appear to be significantly influenced by previous high-level exposures, and (5) included larger numbers of 

subjects than previous studies (Qu et al. 2002, 2003; Rothman et al. 1996a, 1996b; Ward et al. 1996). In 

addition, Lan et al. (2004a, 2004b) measured lymphocyte subsets and colony formation from 

hematopoietic progenitor cells as measures of toxicity. 

Previously conducted epidemiology studies provide support to the findings of Lan et al. (2004a). Qu et 

al. (2002, 2003) compared hematologic values among 105 healthy workers (51 men, 54 women) in 

industries with a history of benzene usage (Tianjin, China) and 26 age- and gender-matched workers in 

industries that did not use benzene. Benzene-exposed workers were chosen based on at least 3 years of 

exposure history. The mean duration of occupational exposure to benzene was 9.7 years (SD=6.2 years). 

At the time of the study, benzene exposure was monitored by individual organic vapor monitors at 

1-week intervals for 4 weeks prior to collection of blood samples for analysis. Measured benzene levels 

were averaged for each individual to produce a 4-week mean exposure level. Exposure-response 

relationships were assessed according to ranges of benzene levels (unexposed, >0–5, >5–15, >15–30, and 

>30 ppm). Benzene hematotoxicity was assessed by mechanical counts of total WBCs, red blood cells 

(RBCs), and platelets. The WBC differential was hand-counted on a total of 900 cells. Calculations of 

the numbers of various WBC types were based on total WBCs and differential counts. The mean 4-week 

benzene level in the control group was 0.004±0.003 ppm. Among all the benzene-exposed workers, the 

mean 4-week benzene exposure level was 5.2±7.3 ppm. Within the >0–5, >5–15, >15–30, and >30 ppm 

exposure categories, mean 4-week benzene levels were 2.26±1.35, 8.67±2.44, 19.9±3.1, and 
BENZENE A-14 

APPENDIX A 

51.8±43.3 ppm, respectively. A significant exposure-related reduction in the numbers of neutrophils 

(ranging in magnitude from 12% in the 2.26 ppm exposure group to 31% in the 51.8 ppm exposure 

group) was observed in all four groups of benzene-exposed workers, relative to controls. Significantly 

reduced numbers of RBCs (approximately 11–16% lower than controls) were also noted in all benzeneexposed groups. Significantly reduced total WBCs were seen in the highest (>30 ppm) exposure group. 

The study authors identified a subgroup (within the >0–5 ppm exposure group) of 16 women with no 

measured exposure levels exceeding 0.5 ppm (4-week mean benzene exposure level of 0.14±0.04 ppm) 

and reported significantly reduced total WBCs, neutrophils, and RBCs in this subgroup as well. 

However, these results are based on a small number of workers within the larger group and the reduced 

counts of total WBCs, neutrophils, and RBCs within this subgroup are –much greater in magnitude than 

those reported for the main (>0–5 ppm) exposure group, rendering the results in this subgroup of 

questionable value for purposes of risk assessment. Qu et al. (2002, 2003) clearly identified a LOAEL of 

2.26 ppm for significantly reduced total WBCs, neutrophils, and RBCs, and provided indication of 

benzene-induced changes in some hematological values at exposure levels lower than the current industry 

8-hour TWA of 1 ppm. 

Rothman et al. (1996a, 1996b) performed a cross-sectional study in 1992 on 44 healthy workers 

(23 males, 21 females) in Chinese (Shanghai) industries with a history of benzene usage and 44 age- and 

gender-matched workers in industries that did not use benzene. The mean duration of occupational 

exposure to benzene was 6.3 years (SD=4.4 years). At the time of the study, benzene exposure was 

monitored by individual organic passive dosimetry badges on 5 separate days during 1 to 2 weeks prior to 

the collection of blood and urine samples for analysis. Benzene hematotoxicity was assessed by 

mechanical counts of total WBCs, absolute lymphocytes (ALC), RBCs, and platelets, as well as 

hemoglobin value and mean corpuscular volume (MCV). The WBC differential was also hand-counted 

on 100 cells. Abnormal counts were reviewed by hand. Mean (geometric mean of the five exposure 

samples) 8-hour TWAs for the benzene-exposed workers ranged from 1 to 238 ppm (median 8-hour 

TWA of 31 ppm). Benzene-exposed workers exhibited statistically significantly reduced numbers of total 

WBCs, ALC, RBCs, and platelets (approximately 12, 21, 6, and 23% lower, respectively) and 

significantly increased MCV (approximately 3% higher), relative to unexposed workers. The results were 

comparable in both men and women. Among the benzene-exposed workers whose mean exposure levels 

were >31 ppm (median 8-hour TWA of 91.9 ppm; n=22), all measured blood parameters were 

significantly different from controls; only ALC, RBCs, and platelets were significantly lower in benzene 

workers with mean exposures of <31 ppm (median 8-hour TWA of 13.6; n=22), compared with controls. 

In a subgroup of benzene-exposed workers whose measured benzene exposure levels did not exceed 

31 ppm on any of the five sampling days (median 8–hour TWA of 7.6 ppm; n=11), significantly reduced 

ALC (approximately 16% lower than controls) was noted. 

In a nested case-control study of a cohort of workers in the Pliofilm production departments of a rubber 

products manufacturer in Ohio (Ward et al. 1996), incident cases were defined as the first occurrence of a 

low WBC or RBC count, and matched controls were chosen from those tested within approximately 

6 months of the case’s blood test date. Hematologic screening data were available for 657 of 

1,037 individuals employed at the plant from 1939 through 1976. A total of 21,710 blood test records 

were identified; the number of blood tests per individual ranged from 1 to 354, but the majority of 

subjects had five or fewer blood tests. All blood tests were taken from 1940 through 1975, the majority 

of which were routine hematological screening tests. Benzene exposures were estimated using a job 

exposure matrix developed by Rinsky et al. (1987). The effects of benzene exposure in the 30, 90, and 

180 days prior to the blood test date, as well as cumulative exposure up until the blood test date, were 

examined using conditional logistic regression. A total of 78 cases and 5,637 controls were included in 

the WBC analysis and 105 cases and 8,489 controls in the RBC analysis, all of whom had worked only 

within the rubber hydrochloride departments during the 180 days prior to the selected blood sample date. 

The maximum daily benzene exposure estimate was 34 ppm. A strong exposure-response relationship 
BENZENE A-15 

APPENDIX A 

was noted for WBCs, and all of the exposure metrics selected showed a significant relationship with low 

blood count. A weak positive exposure-response relationship was observed for RBCs, which was 

significant for the dose metric of cumulative exposure up until the blood test date. The study authors 

noted that there was no evidence for a threshold for hematologic effects and suggested that exposure to 

benzene levels <5 ppm may result in hematologic suppression. 

Agency Contacts (Chemical Managers): Sharon Wilbur, M.A., Sam Keith, M.S., C.H.P., Obaid Faroon, 

Ph.D. 
BENZENE A-16 

APPENDIX A 

MINIMAL RISK LEVEL (MRL) WORKSHEET 

Chemical Name: Benzene 

CAS Numbers: 71-43-2 

Date: August 2007 

Profile Status: Post Public, Final Draft 

Route: [ ] Inhalation [x] Oral 

Duration: [ ] Acute [ ] Intermediate [x] Chronic 

Graph Key: 45 

Species: Human 

Minimal Risk Level: 0.0005 [x] mg/kg/day [ ] ppm 

Reference: Lan Q, Zhang L, Li G, et al. 2004a. Hematotoxicity in workers exposed to low levels of 

benzene. Science 306:1774-1776. 

Experimental design: The chronic-duration oral MRL for benzene is based on route-to-route 

extrapolation of the results of benchmark dose analysis of a hematological endpoint (B cell count) 

assessed in 250 workers (approximately two-thirds female) exposed to benzene at two shoe 

manufacturing facilities in Tianjin, China, and 140 age- and gender-matched workers in clothing 

manufacturing facilities that did not use benzene. See the MRL worksheet for the chronic-duration 

inhalation MRL for details of study design. 

Effect noted in study and corresponding doses: As described in the MRL worksheet for the chronicduration inhalation MRL (see also Table A-1), exposure-response relationships were noted for several 

blood factors. Benzene-induced decreased B cell count was selected as the critical effect for benchmark 

dose (BMD) modeling because it represented the highest magnitude of effect (i.e., B cell count in the 

highest exposure group was approximately 36% lower than that of controls). A BMD modeling approach 

was selected to identify the point of departure because the critical study (Lan et al. 2004a) identified a 

LOAEL in the absence of a NOAEL. 

Dose and end point used for MRL derivation: BMCL0.25sdADJ of 0.014 mg/kg/day for decreased B cell 

count, resulting from route-to-route extrapolation of the BMCL0.25sdADJ of 0.03 ppm described in the MRL 

worksheet for the chronic-duration inhalation MRL. 

Results of toxicokinetic studies of inhaled benzene in humans (Nomiyama and Nomiyama 1974a; Pekari 

et al. 1992; Srbova et al. 1950) and inhaled and orally-administered benzene in rats and mice (Sabourin et 

al. 1987) indicate that absorption of benzene at relatively low levels of exposure is approximately 50% of 

an inhaled dose and essentially 100% of an oral dose. Based on these assumptions, inhalation data can be 

used to estimate equivalent oral doses that would be expected to similarly affect the critical targets of 

benzene toxicity. Therefore, the point of departure for the chronic-duration inhalation MRL for benzene, 

namely the BMCL0.25sdADJ of 0.03 ppm for decreased B cell counts in benzene-exposed workers (Lan et al. 

2004a, 2004b), serves as the point of departure for deriving the chronic-duration oral MRL as well. 

The point of departure (in ppm) was converted to mg/m3

 using the molecular weight of 78.11 for benzene 

and assuming 25 °C and 760 mm Hg: 

BMCL0.25sdADJ of 0.03 ppm x 78.11/24.45 = 0.096 mg/m3 

The BMCL0.25sdADJ of 0.096 mg/m3

 for inhaled benzene was converted to an equivalent BMDL0.25sdADJ for 

ingested benzene using EPA (1988b) human reference values for inhalation rate (20 m3

/day) and body 
BENZENE A-17 

APPENDIX A 

weight (70 kg) and a factor of 0.5 to adjust for differences in absorption of benzene following inhalation 

versus oral exposure (50 versus 100%, respectively) as follows: 

BMDL0.25sdADJ = BMCL0.25sdADJ of 0.096 mg/m3

 x 20 m3/day x 0.5 ÷ 70 kg = 0.014 mg/kg/day 

[ ] NOAEL [ ] LOAEL 

Uncertainty Factors used in MRL derivation: 30 

[ ] 1 [ ] 3 [ ] 10 (for use of a LOAEL)
 

[ ] 1 [ ] 3 [ ] 10 (for extrapolation from animals to humans) 

[ ] 1 [ ] 3 [x] 10 (for human variability)
 

[ ] 1 [x] 3 [ ] 10 (for uncertainty in route-to-route extrapolation) 

Was a conversion factor used from ppm in food or water to a mg/body weight dose? Not applicable. 

If an inhalation study in animals, list conversion factors used in determining human equivalent dose: Not 

applicable. 

Was a conversion used from intermittent to continuous exposure? The BMCL0.25sd of 0.10 ppm was 

adjusted from the 8-hour TWA to a continuous exposure concentration (BMCL0.25sdADJ) as follows: 

BMCL0.25sdADJ = BMCL0.25sd x (8 hours/24 hours) x (6 days/7 days) 

Therefore: 

BMCL0.25sdADJ = 0.10 ppm x (8 hours/24 hours) x (6 days/7 days) 

BMCL0.25sdADJ = 0.03 ppm 

Other additional studies or pertinent information that lend support to this MRL: Results of toxicokinetic 

studies of inhaled benzene in humans (Nomiyama and Nomiyama 1974a; Pekari et al. 1992; Srbova et al. 

1950) and inhaled and orally-administered benzene in rats and mice (Sabourin et al. 1987) indicate that 

absorption of benzene at relatively low levels of exposure is approximately 50% of an inhaled dose and 

essentially 100% of an oral dose. Based on these assumptions, inhalation data can be used to estimate 

equivalent oral doses that would be expected to similarly affect the critical targets of benzene toxicity. 

See the chronic-duration inhalation MRL worksheet for additional information that supports the selection 

of the principal study and critical effect for deriving the chronic-duration inhalation MRL. 

Agency Contacts (Chemical Managers): Sharon Wilbur, M.A., Sam Keith, M.S., C.H.P., Obaid Faroon, 

Ph.D. 
BENZENE A-18 

APPENDIX A 

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BENZENE	 B-1 

APPENDIX B. USER'S GUIDE 

Chapter 1 

Public Health Statement 

This chapter of the profile is a health effects summary written in non-technical language. Its intended 

audience is the general public, especially people living in the vicinity of a hazardous waste site or 

chemical release. If the Public Health Statement were removed from the rest of the document, it would 

still communicate to the lay public essential information about the chemical. 

The major headings in the Public Health Statement are useful to find specific topics of concern. The 

topics are written in a question and answer format. The answer to each question includes a sentence that 

will direct the reader to chapters in the profile that will provide more information on the given topic. 

Chapter 2 

Relevance to Public Health 

This chapter provides a health effects summary based on evaluations of existing toxicologic, 

epidemiologic, and toxicokinetic information. This summary is designed to present interpretive, weightof-evidence discussions for human health end points by addressing the following questions: 

1.	 What effects are known to occur in humans? 

2. What effects observed in animals are likely to be of concern to humans? 

3. What exposure conditions are likely to be of concern to humans, especially around hazardous 

waste sites? 

The chapter covers end points in the same order that they appear within the Discussion of Health Effects 

by Route of Exposure section, by route (inhalation, oral, and dermal) and within route by effect. Human 

data are presented first, then animal data. Both are organized by duration (acute, intermediate, chronic). 

In vitro data and data from parenteral routes (intramuscular, intravenous, subcutaneous, etc.) are also 

considered in this chapter. 

The carcinogenic potential of the profiled substance is qualitatively evaluated, when appropriate, using 

existing toxicokinetic, genotoxic, and carcinogenic data. ATSDR does not currently assess cancer 

potency or perform cancer risk assessments. Minimal Risk Levels (MRLs) for noncancer end points (if 

derived) and the end points from which they were derived are indicated and discussed. 

Limitations to existing scientific literature that prevent a satisfactory evaluation of the relevance to public 

health are identified in the Chapter 3 Data Needs section. 

Interpretation of Minimal Risk Levels 

Where sufficient toxicologic information is available, ATSDR has derived MRLs for inhalation and oral 

routes of entry at each duration of exposure (acute, intermediate, and chronic). These MRLs are not 

meant to support regulatory action, but to acquaint health professionals with exposure levels at which 

adverse health effects are not expected to occur in humans. 
BENZENE B-2 

APPENDIX B 

MRLs should help physicians and public health officials determine the safety of a community living near 

a chemical emission, given the concentration of a contaminant in air or the estimated daily dose in water. 

MRLs are based largely on toxicological studies in animals and on reports of human occupational 

exposure. 

MRL users should be familiar with the toxicologic information on which the number is based. Chapter 2, 

"Relevance to Public Health," contains basic information known about the substance. Other sections such 

as Chapter 3 Section 3.9, "Interactions with Other Substances,” and Section 3.10, "Populations that are 

Unusually Susceptible" provide important supplemental information. 

MRL users should also understand the MRL derivation methodology. MRLs are derived using a 

modified version of the risk assessment methodology that the Environmental Protection Agency (EPA) 

provides (Barnes and Dourson 1988) to determine reference doses (RfDs) for lifetime exposure. 

To derive an MRL, ATSDR generally selects the most sensitive end point which, in its best judgement, 

represents the most sensitive human health effect for a given exposure route and duration. ATSDR 

cannot make this judgement or derive an MRL unless information (quantitative or qualitative) is available 

for all potential systemic, neurological, and developmental effects. If this information and reliable 

quantitative data on the chosen end point are available, ATSDR derives an MRL using the most sensitive 

species (when information from multiple species is available) with the highest no-observed-adverse-effect 

level (NOAEL) that does not exceed any adverse effect levels. When a NOAEL is not available, a 

lowest-observed-adverse-effect level (LOAEL) can be used to derive an MRL, and an uncertainty factor 

(UF) of 10 must be employed. Additional uncertainty factors of 10 must be used both for human 

variability to protect sensitive subpopulations (people who are most susceptible to the health effects 

caused by the substance) and for interspecies variability (extrapolation from animals to humans). In 

deriving an MRL, these individual uncertainty factors are multiplied together. The product is then 

divided into the inhalation concentration or oral dosage selected from the study. Uncertainty factors used 

in developing a substance-specific MRL are provided in the footnotes of the levels of significant exposure 

(LSE) tables. 

Chapter 3 

Health Effects 

Tables and Figures for Levels of Significant Exposure (LSE) 

Tables and figures are used to summarize health effects and illustrate graphically levels of exposure 

associated with those effects. These levels cover health effects observed at increasing dose 

concentrations and durations, differences in response by species, MRLs to humans for noncancer end 

points, and EPA's estimated range associated with an upper- bound individual lifetime cancer risk of 1 in 

10,000 to 1 in 10,000,000. Use the LSE tables and figures for a quick review of the health effects and to 

locate data for a specific exposure scenario. The LSE tables and figures should always be used in 

conjunction with the text. All entries in these tables and figures represent studies that provide reliable, 

quantitative estimates of NOAELs, LOAELs, or Cancer Effect Levels (CELs). 

The legends presented below demonstrate the application of these tables and figures. Representative 

examples of LSE Table 3-1 and Figure 3-1 are shown. The numbers in the left column of the legends 

correspond to the numbers in the example table and figure. 
BENZENE	 B-3 

APPENDIX B 

LEGEND 

See Sample LSE Table 3-1 (page B-6) 

(1) Route of Exposure. One of the first considerations when reviewing the toxicity of a substance 

using these tables and figures should be the relevant and appropriate route of exposure. Typically 

when sufficient data exist, three LSE tables and two LSE figures are presented in the document. 

The three LSE tables present data on the three principal routes of exposure, i.e., inhalation, oral, 

and dermal (LSE Tables 3-1, 3-2, and 3-3, respectively). LSE figures are limited to the inhalation 

(LSE Figure 3-1) and oral (LSE Figure 3-2) routes. Not all substances will have data on each 

route of exposure and will not, therefore, have all five of the tables and figures. 

(2) Exposure Period. Three exposure periods—acute (less than 15 days), intermediate (15– 

364 days), and chronic (365 days or more)—are presented within each relevant route of exposure. 

In this example, an inhalation study of intermediate exposure duration is reported. For quick 

reference to health effects occurring from a known length of exposure, locate the applicable 

exposure period within the LSE table and figure. 

(3) Health Effect. The major categories of health effects included in LSE tables and figures are 

death, systemic, immunological, neurological, developmental, reproductive, and cancer. 

NOAELs and LOAELs can be reported in the tables and figures for all effects but cancer. 

Systemic effects are further defined in the "System" column of the LSE table (see key number 

18). 

(4) Key to Figure. Each key number in the LSE table links study information to one or more data 

points using the same key number in the corresponding LSE figure. In this example, the study 

represented by key number 18 has been used to derive a NOAEL and a Less Serious LOAEL 

(also see the two "18r" data points in sample Figure 3-1). 

(5) Species. The test species, whether animal or human, are identified in this column. Chapter 2, 

"Relevance to Public Health," covers the relevance of animal data to human toxicity and 

Section 3.4, "Toxicokinetics," contains any available information on comparative toxicokinetics. 

Although NOAELs and LOAELs are species specific, the levels are extrapolated to equivalent 

human doses to derive an MRL. 

(6) Exposure Frequency/Duration. The duration of the study and the weekly and daily exposure 

regimens are provided in this column. This permits comparison of NOAELs and LOAELs from 

different studies. In this case (key number 18), rats were exposed to “Chemical x” via inhalation 

for 6 hours/day, 5 days/week, for 13 weeks. For a more complete review of the dosing regimen, 

refer to the appropriate sections of the text or the original reference paper (i.e., Nitschke et al. 

1981). 

(7) System. This column further defines the systemic effects. These systems include respiratory, 

cardiovascular, gastrointestinal, hematological, musculoskeletal, hepatic, renal, and 

dermal/ocular. "Other" refers to any systemic effect (e.g., a decrease in body weight) not covered 

in these systems. In the example of key number 18, one systemic effect (respiratory) was 

investigated. 

(8) NOAEL. A NOAEL is the highest exposure level at which no harmful effects were seen in the 

organ system studied. Key number 18 reports a NOAEL of 3 ppm for the respiratory system, 

which was used to derive an intermediate exposure, inhalation MRL of 0.005 ppm (see 

footnote "b"). 
BENZENE	 B-4 

APPENDIX B 

(9) LOAEL. A LOAEL is the lowest dose used in the study that caused a harmful health effect. 

LOAELs have been classified into "Less Serious" and "Serious" effects. These distinctions help 

readers identify the levels of exposure at which adverse health effects first appear and the 

gradation of effects with increasing dose. A brief description of the specific end point used to 

quantify the adverse effect accompanies the LOAEL. The respiratory effect reported in key 

number 18 (hyperplasia) is a Less Serious LOAEL of 10 ppm. MRLs are not derived from 

Serious LOAELs. 

(10) Reference. The complete reference citation is given in Chapter 9 of the profile. 

(11) CEL. A CEL is the lowest exposure level associated with the onset of carcinogenesis in 

experimental or epidemiologic studies. CELs are always considered serious effects. The LSE 

tables and figures do not contain NOAELs for cancer, but the text may report doses not causing 

measurable cancer increases. 

(12) Footnotes. Explanations of abbreviations or reference notes for data in the LSE tables are found 

in the footnotes. Footnote "b" indicates that the NOAEL of 3 ppm in key number 18 was used to 

derive an MRL of 0.005 ppm. 

LEGEND 

See Sample Figure 3-1 (page B-7) 

LSE figures graphically illustrate the data presented in the corresponding LSE tables. Figures help the 

reader quickly compare health effects according to exposure concentrations for particular exposure 

periods. 

(13) Exposure Period. The same exposure periods appear as in the LSE table. In this example, health 

effects observed within the acute and intermediate exposure periods are illustrated. 

(14) Health Effect. These are the categories of health effects for which reliable quantitative data 

exists. The same health effects appear in the LSE table. 

(15)	 Levels of Exposure. Concentrations or doses for each health effect in the LSE tables are 

graphically displayed in the LSE figures. Exposure concentration or dose is measured on the log 

scale "y" axis. Inhalation exposure is reported in mg/m3

 or ppm and oral exposure is reported in 

mg/kg/day. 

(16) NOAEL. In this example, the open circle designated 18r identifies a NOAEL critical end point in 

the rat upon which an intermediate inhalation exposure MRL is based. The key number 18 

corresponds to the entry in the LSE table. The dashed descending arrow indicates the 

extrapolation from the exposure level of 3 ppm (see entry 18 in the table) to the MRL of 

0.005 ppm (see footnote "b" in the LSE table). 

(17) CEL. Key number 38m is one of three studies for which CELs were derived. The diamond 

symbol refers to a CEL for the test species-mouse. The number 38 corresponds to the entry in the 

LSE table. 
BENZENE	 B-5 

APPENDIX B 

(18)	 Estimated Upper-Bound Human Cancer Risk Levels. This is the range associated with the upperbound for lifetime cancer risk of 1 in 10,000 to 1 in 10,000,000. These risk levels are derived 

from the EPA's Human Health Assessment Group's upper-bound estimates of the slope of the 

cancer dose response curve at low dose levels (q1*). 

(19)	 Key to LSE Figure. The Key explains the abbreviations and symbols used in the figure. 
SAMPLE 

1 → Table 3-1. Levels of Significant Exposure to [Chemical x] – Inhalation 

LOAEL (effect) Exposure 

Key to frequency/ NOAEL Less serious Serious (ppm) 

figurea Species duration System (ppm) (ppm) Re

2 → INTERMEDIATE EXPOSURE 

5 6 7 8 9 10 

3 → Systemic ↓ ↓ ↓ ↓ ↓ ↓

3b 18 Rat 13 wk Resp 10 (hyperplasia) 

4 → 5 d/wk Nit

6 hr/d 

CHRONIC EXPOSURE Cancer 11 

↓

38 Rat 18 mo 20 (CEL, multiple Wo

5 d/wk organs) 7 hr/d 

39 Rat 89–104 wk 10 (CEL, lung tumors, NT

5 d/wk nasal tumors) 6 hr/d 

40 Mouse 79–103 wk 10 (CEL, lung tumors, NT

5 d/wk hemangiosarcomas) 6 hr/d 

12 → a The number corresponds to entries in Figure 3-1. b Used to derive an intermediate inhalation Minimal Risk Level (MRL) of 5x10-3 ppm; dose adjusted for intermittent ex

by an uncertainty factor of 100 (10 for extrapolation from animal to humans, 10 for human variability). 

BENZENE B-8 

APPENDIX B 

This page is intentionally blank. 
BENZENE C-1 

APPENDIX C. ACRONYMS, ABBREVIATIONS, AND SYMBOLS 

ACGIH American Conference of Governmental Industrial Hygienists 

ACOEM American College of Occupational and Environmental Medicine 

ADI acceptable daily intake 

ADME absorption, distribution, metabolism, and excretion 

AED atomic emission detection 

AFID alkali flame ionization detector 

AFOSH Air Force Office of Safety and Health 

ALT alanine aminotransferase 

AML acute myeloid leukemia 

AOAC Association of Official Analytical Chemists 

AOEC Association of Occupational and Environmental Clinics 

AP alkaline phosphatase 

APHA American Public Health Association 

AST aspartate aminotransferase 

atm atmosphere 

ATSDR Agency for Toxic Substances and Disease Registry 

AWQC Ambient Water Quality Criteria 

BAT best available technology 

BCF bioconcentration factor 

BEI Biological Exposure Index 

BMD benchmark dose 

BMR benchmark response 

BSC Board of Scientific Counselors 

C centigrade 

CAA Clean Air Act 

CAG Cancer Assessment Group of the U.S. Environmental Protection Agency 

CAS Chemical Abstract Services 

CDC Centers for Disease Control and Prevention 

CEL cancer effect level 

CELDS Computer-Environmental Legislative Data System 

CERCLA Comprehensive Environmental Response, Compensation, and Liability Act 

CFR Code of Federal Regulations 

Ci curie 

CI confidence interval 

CL ceiling limit value 

CLP Contract Laboratory Program 

cm centimeter 

CML chronic myeloid leukemia 

CPSC Consumer Products Safety Commission 

CWA Clean Water Act 

DHEW Department of Health, Education, and Welfare 

DHHS Department of Health and Human Services 

DNA deoxyribonucleic acid 

DOD Department of Defense 

DOE Department of Energy 

DOL Department of Labor 

DOT Department of Transportation 

DOT/UN/ Department of Transportation/United Nations/ 

NA/IMCO North America/Intergovernmental Maritime Dangerous Goods Code 
BENZENE C-2 

APPENDIX C 

DWEL drinking water exposure level 

ECD electron capture detection 

ECG/EKG electrocardiogram 

EEG electroencephalogram 

EEGL Emergency Exposure Guidance Level 

EPA Environmental Protection Agency 

F Fahrenheit 

F1 first-filial generation 

FAO Food and Agricultural Organization of the United Nations 

FDA Food and Drug Administration 

FEMA Federal Emergency Management Agency 

FIFRA Federal Insecticide, Fungicide, and Rodenticide Act 

FPD flame photometric detection 

fpm feet per minute 

FR Federal Register 

FSH follicle stimulating hormone 

g gram 

GC gas chromatography 

gd gestational day 

GLC gas liquid chromatography 

GPC gel permeation chromatography 

HPLC high-performance liquid chromatography 

HRGC high resolution gas chromatography 

HSDB Hazardous Substance Data Bank 

IARC International Agency for Research on Cancer 

IDLH immediately dangerous to life and health 

ILO International Labor Organization 

IRIS Integrated Risk Information System 

Kd adsorption ratio 

kg kilogram 

kkg metric ton 

Koc organic carbon partition coefficient 

Kow octanol-water partition coefficient 

L liter 

LC liquid chromatography 

LC50 lethal concentration, 50% kill 

LCLo lethal concentration, low 

LD50 lethal dose, 50% kill 

LDLo lethal dose, low 

LDH lactic dehydrogenase 

LH luteinizing hormone 

LOAEL lowest-observed-adverse-effect level 

LSE Levels of Significant Exposure 

LT50 lethal time, 50% kill 

m meter 

MA trans,trans-muconic acid 

MAL maximum allowable level 

mCi millicurie 

MCL maximum contaminant level 

MCLG maximum contaminant level goal 

MF modifying factor 
BENZENE C-3 

APPENDIX C 

MFO mixed function oxidase 

mg milligram 

mL milliliter 

mm millimeter 

mmHg millimeters of mercury 

mmol millimole 

mppcf millions of particles per cubic foot 

MRL Minimal Risk Level 

MS mass spectrometry 

NAAQS National Ambient Air Quality Standard 

NAS National Academy of Science 

NATICH National Air Toxics Information Clearinghouse 

NATO North Atlantic Treaty Organization 

NCE normochromatic erythrocytes 

NCEH National Center for Environmental Health 

NCI National Cancer Institute 

ND not detected 

NFPA National Fire Protection Association 

ng nanogram 

NHANES National Health and Nutrition Examination Survey 

NIEHS National Institute of Environmental Health Sciences 

NIOSH National Institute for Occupational Safety and Health 

NIOSHTIC NIOSH's Computerized Information Retrieval System 

NLM National Library of Medicine 

nm nanometer 

nmol nanomole 

NOAEL no-observed-adverse-effect level 

NOES National Occupational Exposure Survey 

NOHS National Occupational Hazard Survey 

NPD nitrogen phosphorus detection 

NPDES National Pollutant Discharge Elimination System 

NPL National Priorities List 

NR not reported 

NRC National Research Council 

NS not specified 

NSPS New Source Performance Standards 

NTIS National Technical Information Service 

NTP National Toxicology Program 

ODW Office of Drinking Water, EPA 

OERR Office of Emergency and Remedial Response, EPA 

OHM/TADS Oil and Hazardous Materials/Technical Assistance Data System 

OPP Office of Pesticide Programs, EPA 

OPPT Office of Pollution Prevention and Toxics, EPA 

OPPTS Office of Prevention, Pesticides and Toxic Substances, EPA 

OR odds ratio 

OSHA Occupational Safety and Health Administration 

OSW Office of Solid Waste, EPA 

OTS Office of Toxic Substances 

OW Office of Water 

OWRS Office of Water Regulations and Standards, EPA 

PAH polycyclic aromatic hydrocarbon 
BENZENE C-4 

APPENDIX C 

PBPD physiologically based pharmacodynamic 

PBPK physiologically based pharmacokinetic 

PCE polychromatic erythrocytes 

PEL permissible exposure limit 

pg picogram 

PHS Public Health Service 

PID photo ionization detector 

pmol picomole 

PMR proportionate mortality ratio 

ppb parts per billion 

ppm parts per million 

ppt parts per trillion 

PSNS pretreatment standards for new sources 

RBC red blood cell 

REL recommended exposure level/limit 

RfC reference concentration 

RfD reference dose 

RNA ribonucleic acid 

RQ reportable quantity 

RTECS Registry of Toxic Effects of Chemical Substances 

SARA Superfund Amendments and Reauthorization Act 

SCE sister chromatid exchange 

SGOT serum glutamic oxaloacetic transaminase 

SGPT serum glutamic pyruvic transaminase 

SIC standard industrial classification 

SIM selected ion monitoring 

SMCL secondary maximum contaminant level 

SMR standardized mortality ratio 

SNARL suggested no adverse response level 

SPEGL Short-Term Public Emergency Guidance Level 

STEL short term exposure limit 

STORET Storage and Retrieval 

TD50 toxic dose, 50% specific toxic effect 

TLV threshold limit value 

TOC total organic carbon 

TPQ threshold planning quantity 

TRI Toxics Release Inventory 

TSCA Toxic Substances Control Act 

TWA time-weighted average 

UF uncertainty factor 

U.S. United States 

USDA United States Department of Agriculture 

USGS United States Geological Survey 

VOC volatile organic compound 

WBC white blood cell 

WHO World Health Organization 
BENZENE C-5 

APPENDIX C 

> greater than ≥ greater than or equal to 

= equal to 

< less than ≤ less than or equal to 

% percent α alpha β beta γ gamma δ delta μm micrometer μg microgram q1 

* cancer slope factor 

– negative 

+ positive 

(+) weakly positive result 

(–) weakly negative result 
BENZENE C-6 

APPENDIX C 

This page is intentionally blank. 
BENZENE D-1 

APPENDIX D. INDEX 

absorbed dose.................................................................................................... 155, 160, 162, 176, 179, 209
 

acetylcholine ............................................................................................................................................. 134 

acetylcholinesterase .............................................................................................................................. 20, 91 

adipose tissue ............................................................................................................................................ 165 

adrenal gland............................................................................................................................................. 129 

adrenals ....................................................................................................................................................... 73 

adsorbed ............................................................................................................................ 159, 165, 179, 291 

adsorption.................................................................................................................. 162, 241, 258, 292, 300 

aerobic....................................................................................................................... 251, 262, 263, 264, 285 

ambient air .......................................................................... 20, 157, 161, 268, 274, 278, 283, 295, 296, 300 

anaerobic................................................................................................................................... 263, 264, 265 

anemia..........................................5, 12, 13, 17, 19, 73, 74, 77, 78, 79, 86, 90, 127, 213, 215, 221, 224, 229 

bioaccumulation........................................................................................................................................ 259 

bioavailability ................................................................................................................... 159, 162, 165, 286 

bioconcentration factor ............................................................................................................................. 259 

biodegradation................................................................................................... 249, 250, 251, 262, 263, 264
 

biokinetics......................................................................................................................................... 182, 192 

biomarker .................12, 74, 85, 149, 170, 175, 189, 208, 209, 210, 211, 212, 231, 232, 233, 291, 303, 305 

blood cell count................................................................................................................... 85, 141, 212, 303 

body weight effects ............................................................................................... 32, 83, 105, 130, 131, 138 

bone marrow aplasia ................................................................................................................................. 213 

breast milk..........................................176, 177, 179, 182, 185, 186, 192, 193, 207, 235, 271, 281, 282, 291 

cancer ........................................................................... 4, 6, 9, 16, 30, 31, 98, 101, 103, 137, 140, 166, 207, 

208, 219, 221, 223, 224, 230, 273, 289, 305, 310, 311 

carcinogen..................................................6, 15, 16, 103, 136, 137, 223, 224, 248, 249, 289, 305, 310, 311 

carcinogenic ...................................................................................... 6, 16, 21, 29, 30, 86, 98, 103, 310, 311 

carcinogenicity.................................................12, 15, 16, 103, 136, 137, 141, 224, 225, 229, 233, 235, 310 

carcinoma.................................................................................................................................... 15, 103, 136 

cardiovascular ....................................................................................................................... 13, 73, 105, 138 

cardiovascular effects.................................................................................................................... 13, 73, 105 

chromosomal aberrations .................................................................................... 94, 141, 150, 212, 226, 303
 

clearance ................................................................................................................................................... 212 

death.....5, 13, 14, 27, 29, 31, 73, 78, 82, 89, 98, 99, 104, 105, 128, 131, 137, 140, 163, 219, 221, 287, 310 

deoxyribonucleic acid (see DNA)..................................................................................................... 145, 148 

dermal effects........................................................................................................................ 32, 83, 130, 138 

developmental effects ................................................................................... 94, 97, 135, 136, 140, 227, 235 

DNA (see deoxyribonucleic acid).....142, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 168, 174, 

201, 202, 203, 204, 209, 210, 212, 224, 225, 230, 232, 237, 303 

dopamine................................................................................................................................................... 134 

endocrine..................................................................................................................... 32, 105, 138, 205, 206 

endocrine effects ....................................................................................................................................... 129 

erythema.................................................................................................................................................... 138 

fetus............................................................................................... 6, 7, 14, 93, 143, 151, 206, 208, 226, 227
 

gastrointestinal effects ........................................................................................................................ 73, 127 

general population..................................3, 5, 11, 79, 208, 251, 253, 273, 276, 278, 281, 286, 287, 302, 310 

genotoxic....................................................................................... 12, 29, 141, 152, 154, 204, 212, 219, 221 

genotoxicity............................................................... 150, 151, 152, 168, 202, 215, 218, 224, 225, 237, 305 

germinal epithelium .................................................................................................................................... 94 

groundwater .......................................................229, 249, 250, 257, 258, 262, 263, 264, 269, 270, 285, 289 
BENZENE D-2 

APPENDIX D 

growth retardation....................................................................................................................................... 95 

half-life.............................................................................................................. 159, 175, 176, 208, 261, 262
 

hematological effects ...........16, 17, 18, 25, 73, 75, 76, 77, 79, 127, 138, 212, 213, 221, 222, 223, 226, 230 

hematopoietic....................................................................... 24, 73, 80, 85, 96, 97, 133, 136, 140, 155, 172, 

200, 201, 208, 218, 221, 222, 227, 231, 233 

hepatic effects ............................................................................................................................... 13, 82, 128 

hydrolysis.......................................................................................................................... 194, 292, 293, 294 

hydroxyl radical ................................................................................................................ 171, 233, 251, 261 

immune system ........................................................................................... 6, 12, 20, 88, 221, 222, 227, 231 

immunological ...................................................................... 18, 19, 22, 27, 29, 89, 131, 140, 222, 227, 307 

immunological effects......................................................................... 18, 19, 22, 27, 89, 131, 222, 227, 307 

Kow ............................................................................................................................................................ 259 

LD50................................................................................................................................................... 104, 105 

leukemia......................................................6, 9, 12, 14, 16, 17, 74, 78, 79, 97, 98, 100, 103, 104, 127, 137, 

141, 147, 154, 207, 211, 223, 224, 230, 231, 233, 235, 287 

leukopenia................................................................. 12, 27, 73, 77, 78, 79, 85, 86, 127, 132, 211, 221, 233 

lymphatic .......................................................................................................................................... 100, 140 

lymphoreticular......................................................................................................................................... 140 

menstrual................................................................................................................................... 6, 92, 93, 225 

micronuclei ....................................................................................................... 145, 149, 151, 152, 153, 224
 

milk ............................................................................................................................. 91, 192, 216, 271, 282 

musculoskeletal effects ................................................................................................................. 81, 82, 128 

neonatal....................................................................................................................................................... 20 

neoplastic .................................................................................................................... 82, 128, 129, 135, 226 

neurobehavioral......................................................................................................................................... 206 

neurological effects................................................................. 13, 19, 20, 24, 89, 90, 92, 134, 135, 140, 228 

neurotransmitter ........................................................................................................................................ 228 

neutropenia.................................................................................................................................................. 82 

non-Hodgkin’s lymphoma ............................................................................................................ 15, 97, 223 

norepinephrine .......................................................................................................................................... 134 

octanol-water partition coefficient............................................................................................................ 199 

ocular effects......................................................................................................................... 13, 83, 130, 138 

odds ratio................................................................................................................................................... 102 

pancytopenia ................................................................................................... 12, 17, 73, 77, 78, 79, 94, 226 

partition coefficients ..................................................................................................... 22, 23, 182, 192, 199 

pharmacodynamic ............................................................................................................................. 162, 181 

pharmacokinetic................................................................................................ 181, 182, 183, 196, 207, 235
 

photolysis .................................................................................................................................................. 262 

placenta ....................................................................................................................... 94, 163, 164, 207, 235 

rate constant ...................................................................................................................................... 187, 261 

renal effects................................................................................................................................... 13, 82, 129 

reproductive effects....................................................................................................... 92, 94, 135, 140, 225 

respiratory effects................................................................................................................................ 32, 105 

retention .................................................................................................................................................... 204 

solubility ........................................................................................................................... 159, 161, 200, 258 

spermatogonia........................................................................................................................................... 145 

spermatozoa ........................................................................................................................................ 94, 225 

systemic effects..................................................................................... 12, 32, 105, 131, 138, 219, 221, 222 

T4................................................................................................................................................................ 88 

thrombocytopenia ................................................................................................................... 12, 73, 77, 221 

thyroid............................................................................................................................................... 129, 166 
BENZENE D-3 

APPENDIX D 

toxicokinetic........................................................................................................................................ 29, 154 

tremors ........................................................................................................................ 5, 13, 20, 91, 134, 228 

tumors ............................................................................................... 15, 16, 18, 89, 103, 141, 165, 228, 231 

vapor phase ............................................................................................................................... 259, 261, 269 

vapor pressure ........................................................................................................................................... 258 

volatility .................................................................................................................................................... 258 

volatilization ............................................................................................................................. 249, 251, 258 
BENZENE D-4