TOXICOLOGICAL PROFILE FOR

SULFUR DIOXIDE

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Agency for Toxic Substances and Disease Registry

December 1998
SULFUR DIOXIDE ii

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.
SULFUR DIOXIDE iii

UPDATE STATEMENT

Toxicological profiles are revised and republished as necessary, but no less than once every three years. For

information regarding the update status of previously released profiles, contact ATSDR at:

Agency for Toxic Substances and Disease Registry

Division of Toxicology/Toxicology Information Branch

1600 Clifton Road NE, E-29

Atlanta, Georgia 30333



SULFUR DIOXIDE vii

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: Health Effects: Specific health effects of a given hazardous compound are reported by route

of exposure, by type of health effect (death, systemic, immunologic, reproductive), 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 2.6 Children’s Susceptibility

Section 5.6 Exposures of Children

Other Sections of Interest:

Section 2.7 Biomarkers of Exposure and Effect

Section 2.10 Methods for Reducing Toxic Effects

___________________________________________________________________________

ATSDR Information Center

Phone: l-800-447-1544 (to be replaced by l-888-42-ATSDR in 1999)

or 404-639-6357 Fax: 404-639-6359

E-mail: atsdric@,cdc.gov Internet: http://atsdrl.atsdr.cdc.gov:8080

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.
SULFUR DIOXIDE viii

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 l •Phone:

770-488-7000 l 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, #5 13, Washington, DC 20005 •Phone: 202-347-4976 •FAX: 202-

347-4950 •e-mail: aoec@,dgs.dgsys.com • AOEC Clinic Director: http://occ-envmed.mc.duke.edu/oem/aoec.htm.

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, 55 West Seegers Road, Arlington Heights, IL 60005 •Phone: 847-

228-6850 •FAX: 847-228-1856.
SULFUR DIOXIDE ix

CONTRIBUTORS

CHEMICAL MANAGER(S)/AUTHOR(S):

Hana Pohl, M.D., Ph.D.

ATSDR, Division of Toxicology, Atlanta, GA

John Liccione, Ph.D.

Sciences International, Inc., Alexandria, VA

Annette Iannucci, M.S.

Sciences International, Inc., Alexandria, VA

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 . Quality Assurance Review. The Quality Assurance Branch assures that consistency across profiles

is maintained, identifies any significant problems in format or content, and establishes that Guidance

has been followed.

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PEER REVIEW

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

1. Edward Avol, Associate Professor, Department of Preventive Medicine, School of Medicine,

University of Southern California, Los Angeles, CA

2 . Ingeborg Harding-Barlow, Ph.D., Private Consultant, Palo Alto, CA

3 . Clint Skinner, Ph.D., Skinner Associates, Creston, CA.

These experts collectively have knowledge of sulfur dioxide’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(1)(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. A list of databases reviewed and a

list of unpublished documents cited are also included in the administrative record.

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.









SULFUR DIOXIDE 1

1. PUBLIC HEALTH STATEMENT

This public health statement tells you about sulfur dioxide and the effects of exposure.

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

the nation These sites make up the National Priorities List (NPL) and are the sites targeted for

long-term federal cleanup activities. Sulfur dioxide has been found in at least 16 of the

1,467 current or former NPL sites. However, the total number of NPL sites evaluated for this

substance is not known. As more sites are evaluated, the sites at which sulfur dioxide is found

may increase. This information is important because exposure to this substance may harm you and

because these sites may be sources of exposure.

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

such as a drum or bottle, it enters the environment. This release does not always lead to

exposure. You are 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 sulfur dioxide, many factors determine whether you’ll 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 the other chemicals you’re exposed to and your age, sex, diet, family

traits, lifestyle, and state of health.

1 .1 WHAT IS SULFUR DIOXIDE?

Sulfur dioxide is a colorless gas with a pungent odor. It is a liquid when under pressure. Sulfur

dioxide dissolves in water very easily. It cannot catch fire.

Sulfur dioxide in the air results primarily from activities associated with the burning of fossil fuels

(coal, oil) such as at power plants or from copper smelting. In nature, sulfur dioxide can be
SULFUR DIOXIDE 2

1. PUBLIC HEALTH STATEMENT

released to the air, for example, from volcanic eruptions. Chapters 3 and 4 contain further

information.

1.2 WHAT HAPPENS TO SULFUR DIOXIDE WHEN IT ENTERS THE

 ENVIRONMENT?

Once released into the environment, sulfur dioxide moves to the air. In the air, sulfur dioxide can

be converted to sulfuric acid, sulfur trioxide, and sulfates. Sulfur dioxide dissolves in water.

Once dissolved in water, sulfur dioxide can form sulfurous acid. Soil can absorb sulfur dioxide,

but we do not know if or how it moves in soil. Chapters 4 and 5 contain further information.

1.3 HOW MIGHT I BE EXPOSED TO SULFUR DIOXIDE?

You may be exposed to sulfur dioxide mainly by breathing air that contains it. You may also be

exposed to sulfur dioxide by skin contact with it.

The people most often exposed to sulfur dioxide are workers in plants where sulfur dioxide

occurs as a by-product, such as in the copper smelting industry and in the processing or burning

of coal or oil. Other exposures occur in the manufacture of sulfuric acid, paper, food preservatives,

and fertilizers. The primary way that workers are exposed to sulfur dioxide is through the

air. Workers may be exposed to concentrations of sulfur dioxide that are higher than typical

outdoor air levels. People living near heavily industrial activities that involve smelting copper or

the processing or burning of coal or oil are also likely to be exposed to sulfur dioxide by breathing

it. Chapter 5 contains further information.

1.4 HOW CAN SULFUR DIOXIDE ENTER AND LEAVE MY BODY?

If you breathe air containing sulfur dioxide, you may absorb it into your body through your nose

and lungs. Sulfur dioxide can easily and rapidly enter your bloodstream through your lungs.

Once in the body, it breaks down to sulfate and leaves through the urine. Chapter 2 contains

further information.
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1. PUBLIC HEALTH STATEMENT

1.5 HOW CAN SULFUR DIOXIDE AFFECT MY HEALTH?

To protect the public from the harmful effects of toxic chemicals and to find ways to treat people

who have been harmed, scientists use many tests.

One way to see if a chemical will hurt people is to learn how the chemical is absorbed, used, and

released by the body; for some chemicals, animal testing may be necessary. Animal testing may

also be used to identify health effects such as cancer or birth defects. Without laboratory animals,

scientists would lose a basic method to get information needed to make wise decisions to protect

public health. Scientists have the responsibility to treat research animals with care and

compassion. Laws today protect the welfare of research animals, and scientists must comply with

strict animal care guidelines.

Short-term exposures to high levels of sulfur dioxide can be life-threatening. Exposure to

100 parts of sulfur dioxide per million parts of air (ppm) is considered immediately dangerous to

life and health. Previously healthy nonsmoking miners who breathed sulfur dioxide released as a

result of an explosion in an underground copper mine developed burning of the nose and throat,

breathing difficulties, and severe airway obstructions. Long-term exposure to persistent levels of

sulfur dioxide can also affect your health. Lung function changes have been observed in some

workers exposed to 0.4-3.0 ppm sulfur dioxide for 20 years or more. However, these workers

were also exposed to other chemicals, making it difficult to attribute their health effects to sulfur

dioxide exposure alone. Additionally, exercising asthmatics are sensitive to the respiratory effects

of low concentrations (0.25 ppm) of sulfur dioxide.

For comparative purposes, typical outdoor concentrations of sulfur dioxide may range from 0 to

1 ppm. Occupational exposures to sulfur dioxide may lawfully range from 0 to 5 ppm as enforced

by your state OSHA (Occupational Safety and Health Administration). During any 8-hour

workshift of a 40-hour workweek, the average concentration of sulfur dioxide in the workplace
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1. PUBLIC HEALTH STATEMENT

may not exceed 5 ppm. However, during system malfunctions or unforseen events, levels

approaching 50 ppm or more have been reported.

Studies in animals support the human data regarding respiratory effects of sulfur dioxide. At low

levels (less than 1 ppm) of sulfur dioxide exposure, guinea pigs displayed changes in their ability

to breathe as deeply or as much air per breath. More severe symptoms seen in animals exposed to

high concentrations of sulfur dioxide include decreased respiration, inflammation or infection of

the airways, and destruction of areas of the lung.

For more information refer to Chapter 2.

1.6 HOW CAN SULFUR DIOXIDE AFFECT CHILDREN?

This section discusses potential health effects from exposures during the period from conception

to maturity at 18 years of age in humans. Potential effects on children resulting from exposures of

the parents are also considered.

Since sulfur dioxide is primarily present as a gas, the general public is exposed to it mostly by

breathing contaminated air. Levels of sulfur dioxide in the atmosphere vary from region to region

and are mainly influenced by the intensity of industry and development usually associated with

cities. Therefore, children with the highest exposure to sulfur dioxide are those living near

industrial sources (i.e., industries that process or burn coal or oil, copper smelting plants, sulfuric

acid manufacturers, fertilizer factories, or paper pulp factories).

Members of the general public may also have additional risk for exposure if they live near a

hazardous waste site contaminated with sulfur dioxide. At 16 of the 1,467 NPL hazardous waste

sites, sulfur dioxide has been identified in air, surface water, groundwater, soil, or sediment.

Children, as well as adults, living near these sites are most likely to be exposed by breathing

contaminated air.
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1. PUBLlC HEALTH STATEMENT

Most of the effects of sulfur dioxide exposure that occur in adults (i.e., difficulty breathing,

changes in the ability to breathe as deeply or take in as much air per breath, and burning of the

nose and throat) are also of potential concern in children, but it is unknown whether children are

more vulnerable to exposure. Children may be exposed to more sulfur dioxide than adults

because they breathe more air for their body weight than adults do. Children also exercise more

frequently than adults. Exercise increases breathing rate. This increase results in both a greater

amount of sulfur dioxide in the lungs and enhanced effects on the lungs. One study suggested that

a person’s respiratory health, and not his or her age, determines vulnerability to the effects of

breathing sulfur dioxide. This study implies that healthy adolescents (ages 12-17) are no more

vulnerable to the effects of breathing sulfur dioxide than healthy senior citizens.

Long-term studies surveying large numbers of children have indicated possible associations

between sulfur dioxide pollution and respiratory symptoms or reduced breathing ability. Children

who have breathed sulfur dioxide pollution may develop more breathing problems as they get

older, may make more emergency room visits for treatment of wheezing fits, and may get more

respiratory illnesses than is typical for children. However, studies like these are unable to provide

conclusive evidence about sulfur dioxide’s effects on children’s health because many other

pollutants are also present in the air.

It is known that exercising asthmatics are sensitive to low concentrations of sulfur dioxide.

Therefore, increased susceptibility is expected in children with asthma, but it is not known

whether asthmatic children are more sensitive than asthmatic adults. Additionally, asthma occurs

most often in African Americans, children between the ages of 8 and 11, and people living in

cities. For unknown reasons, the death rates associated with asthma are also higher in nonCaucasian people. Therefore, it is expected that asthmatic, African American children living in

urban areas have increased sensitivity to sulfur dioxide.

There are few studies which provide evidence of reproductive or developmental effects of sulfur

dioxide exposure in humans. One study found no relationship between spontaneous abortion and

exposure to sulfur dioxide among women living in an industrial community in Finland. However,
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1. PUBLIC HEALTH STATEMENT

another study in China showed a relationship between decreased infant birth weight and exposure

to sulfur dioxide pollution during pregnancy. Another study, in the Czech Republic, showed that

18- year-old males who were exposed to high levels of sulfur dioxide had sperm with more

abnormalities and reduced abilities to move. Studies like these, though, are often hard to

interpret. It can be difficult to distinguish among the effects of individual pollutants within air

pollution mixtures. Tests on laboratory mice have shown that sulfur dioxide exposure did not

affect reproductive function

Only a small number of developmental studies have been done in animals, and of these, in only

one study were serious effects on development observed. Developmental studies are designed to

determine how a pregnant female’s exposure to a chemical might affect the normal processes that

take place in her child as it grows (i.e., adverse effects might include a child with learning deficits

or problems in social behavior). In one study the offspring of mice who breathed sulfur dioxide

during their pregnancy were born small and had some abnormal reflexes. Three other studies in

mice and one in rabbits showed no serious effects. Minor variations in the skeleton did occur in

the offspring of the rabbits exposed to sulfur dioxide during pregnancy, and delayed bone

hardening occurred in the offspring of treated mice. Due in large part to these conflicting results,

conclusions about the effects of sulfur dioxide on unborn children cannot be drawn from the

available studies.

A study in rats indicated that poor maternal nutrition might cause offspring to be more susceptible

later in life to the damage that can occur from breathing sulfur dioxide. In rats, offspring whose

mothers were fed a low protein diet while they were pregnant showed greater lung damage from

breathing sulfur dioxide when they were older. It is unknown if similar conclusions about human

maternal nutrition can be drawn.

It is not likely that parental exposure to sulfur dioxide will cause any changes in a mother’s eggs

or father’s sperm that could affect their unborn children. It is not known if sulfur dioxide or the

products to which it is broken down within the body can cross the placenta or accumulate in

breast milk; further, it is also unknown whether these resulting breakdown products would be
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1. PUBLIC HEALTH STATEMENT

harmful Accumulation of sulfur dioxide in maternal tissues and then, movement during

pregnancy, is unlikely because this compound is water soluble.

Sections 2.6 and 5.6 contain more information about the effects of sulfur dioxide on children.

1.7 HOW CAN FAMILIES REDUCE THE RISK OF EXPOSURE TO SULFUR

 DIOXIDE?

If your doctor finds that you have been exposed to significant amounts of sulfur dioxide, ask if

children may also be exposed. When necessary your doctor may need to ask your State

Department of Public Health to investigate.

Because exposure to sulfur dioxide is most likely to occur by breathing contaminated air, families

should try to limit their outdoor activities during times of high air pollution. While levels of sulfur

dioxide in the air are typically highest during the winter months, human exposure to sulfur dioxide

has been shown to be greatest during the summer months. This result is most likely seen because

people enjoy being outdoors in warm weather and often leave their household windows open for

ventilation.

By paying attention to news bulletins and air pollution advisories, families can control their

amount of exposure. The EPA (Environmental Protection Agency) issues the vast majority of air

quality alerts. Such warnings might reach the public when ATSDR (Agency for Toxic Substances

and Disease Registry) issues a Public Health Advisory and notifies the EPA of a public health

threat caused by high levels of atmospheric sulfur dioxide. State and local health and

environmental agencies will also be notified and will, in turn, notify their communities. People

with respiratory difficulties should pay special attention to these warnings. Additionally,

asthmatic children’s outdoor exercise should be limited when high levels of sulfur dioxide are

present in the air.
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1. PUBLIC HEALTH STATEMENT

Since exposure to sulfur dioxide occurs primarily through direct breathing of contaminated air,

workers in plants where sulfur dioxide occurs as a by-product will not expose their family

members at home through residues on their skin or clothes.

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

 EXPOSED TO SULFUR DIOXIDE?

Sulfur dioxide in the body is changed into other sulfur-containing chemicals in the body. These

breakdown products can be found and measured in the blood and urine. However, their

measurement requires special equipment which is not routinely available in a doctor’s office.

Furthermore, exposure to chemicals other than sulfur dioxide can also produce sulfate, so, the

presence of sulfate breakdown in your body does not necessarily mean you have been exposed to

sulfur dioxide. Lung function tests can be used to examine potential respiratory effects of sulfur

dioxide. However, tests of lung function changes cannot determine whether or not you have been

specifically exposed to sulfur dioxide because other chemicals can produce similar lung function

changes. Chapters 2 and 6 contain further information.

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. Federal agencies that develop regulations for toxic

substances include the Environmental Protection Agency (EPA), the Occupational Safety and

Health Administration (OSHA), and the Food and Drug Administration (FDA).

Recommendations provide valuable guidelines to protect public health but cannot be enforced by

law. Federal organizations that develop recommendations for toxic substances include the

Agency for Toxic Substances and Disease Registry (ATSDR) and the National Institute for

Occupational Safety and Health (NIOSH).

Regulations and recommendations can be expressed in not-to-exceed levels in air, water, soil, or

food that are usually based on levels that affect animals, then they are adjusted to help protect
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1. PUBLIC HEALTH STATEMENT

people. Sometimes these not-to-exceed levels differ among federal organizations because of

different exposure times (an 8-hour workday or a 24-hour day), the use of different animal

studies, or other factors.

Recommendations and regulations are also periodically updated 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 sulfur dioxide include the following:

The federal government has set regulations to protect individuals from the possible health effects

of breathing sulfur dioxide. EPA recommends that the long-term, 1-year average concentrations

of sulfur dioxide should not exceed 0.03 ppm. The short-term, 24-hour average concentration

should not exceed 0.14 ppm more than once a year. OSHA regulates levels of sulfur dioxide in

the workplace. This regulation states that workroom air should contain no more than an average

of 2 ppm sulfur dioxide over an 8-hour working shift for 5 consecutive days in a workweek.

NIOSH recommends that the average workroom air levels of sulfur dioxide not exceed 2 ppm

over a 10-hour period. The 15-minute average exposure in air that should not be exceeded at any

time during a workday is 5 ppm. For more information on rules and standards for sulfur dioxide,

see Chapter 7.

1.1 0 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:

Agency for Toxic Substances and Disease Registry

Division of Toxicology

1600 Clifton Road NE, Mailstop E-29

Atlanta, GA 30333
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1. PUBLIC HEALTH STATEMENT

* Information line and technical assistance

Phone: l-800-447- 1544

Fax: (404) 639-6359

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

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

hazardous substances.

* To order toxicological profiles. contact:

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Phone: (800) 553-6847 or (703) 4874650
SULFUR DIOXIDE 11

2. HEALTH EFFECTS

2.1 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 of the toxicology of sulfur dioxide. 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.

2.2 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 lowest-observed-adverseeffect 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
SULFUR DIOXIDE 12

2. HEALTH EFFECTS

determining whether or not 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 (LOAEL) 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 sulfur dioxide are

indicated in Table 2- 1 and Figure 2- 1.

Estimates of exposure levels posing minimal risk to humans (Minimal Risk Levels or MRLs) have been made

for sulfur dioxide. 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.

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.
SULFUR DIOXIDE 13

2. HEALTH EFFECTS

2.2.1 Inhalation Exposure

Table 2-l and Figure 2-l summarize the available quantitative information on the health effects that have

been observed in humans and animals following inhalation exposure to sulfur dioxide. All exposure levels

are expressed in parts per million (ppm).

2.2.1.1. Death

There have been several case reports of human deaths following acute exposure to high concentrations of

sulfur dioxide (Atkinson et al. 1993; Charan et al. 1979; Harkonen et al. 1983; Huber and Loving 1991;

Rabinovitch et al. 1989). In most studies, concentrations were not measured. In one study, analysis of gas

samples at the time of rescue showed sulfur dioxide concentrations greater than 40 ppm (Rabinovitch et al.

1989). A sulfur dioxide level of 150 ppm was measured during the reenactment of an incident in which a 76-

year-old asthmatic woman died of an asthma attack after inhaling vapors from a sulfite-based derusting agent

used in her dishwasher (Huber and Loving 1991) Actual sulfur dioxide levels were probably higher since the

quantity of derusting agent used in the investigation was approximately 7-10% of the amount originally used

by the woman. A concentration of 100 ppm is considered immediately dangerous to life and health (HSDB

1998).

Excess mortality among humans occurred following exposure to high concentrations of sulfur dioxide and

suspended particulate matter during acute smog episodes in London during the 1950s (Amdur et al. 1991;

Mazumdar et al. 1982; WHO 1979). Peak daily concentrations as high as 4,000 μg/m3

(1.5 ppm) sulfur

dioxide and 6,000 μg/m3

 of smoke were recorded during some of the pollution episodes. The available

evidence suggested that excess mortality may occur at sulfur dioxide levels at or above 500 μg/m3 (0.2 ppm)

(24-hour mean concentration) in combination with elevated particle levels. The increases in mortality were

attributed to bronchitis and other causes of impairment of respiratory function (WHO 1979). Increased

mortality from heart disease was also noted. The mortalities occurred mainly among the elderly and among

those with pre-existing cardiac and/or respiratory disorders (WHO 1979).

A reanalysis of the mortality and London pollution data revealed that the mortalities were due almost entirely

to smoke (Mazumdar et al. 1982). This study applied three types of analyses, including year-by-year multiple

regression; stratification using nested quartiles of one pollutant with quantities of the other; and multiple













SULFUR DIOXIDE 27

2. HEALTH EFFECTS

regression of a special subset of high-pollution days. However, a later reexamination of mortality and

London pollution data revealed that the log of acid aerosol concentrations was more strongly associated with

raw total mortality in bivariate analyses than with smoke or sulfur dioxide (Thurston et al. 1989).

Associations have also been reported between daily acute mortality and pollution episodes involving sulfur

dioxide and suspended particulate matter in Philadelphia, PA, (1973-1988) and Steubenville, OH,

(1974-1984) (Moolgavkar et al. 1995a, 1995b). Although the mortality has been attributed to the particulate

component of air pollution, recent reanalyses of data have indicated that it is premature to single out one

specific component as being responsible for the observed association between air pollution and mortality

(Moolgavkar et al. 1995a, 1995b, 1996).

In eastern and western Europe, associations between short-term increases in air pollutant levels and daily

mortality rates have recently been evaluated by the Air Pollution and Health: a European Approach (APHEA)

project (Katsouyanni et al. 1997). The purpose of the project was to provide a standardized method of

evaluation between 15 European cities, and the primary pollutants examined were sulfur dioxide and

particulates. Effects were evaluated for both current day increases in air pollution levels and for lag periods

of 1-5 days past exposure. Statistical techniques were used to control for confounding factors such as

various air pollutants, temperature, relative humidity, influenza epidemics, season, year, month, holidays, and

day of the week. Despite efforts to ensure consistency in evaluation methods, the sulfur dioxide findings were

variable. For instance, an association between short-term increases of sulfur dioxide levels in air and

increased daily mortality was not observed in some cities (Bacharova 1996; Ballester et al. 1996; Verhoeff et

al. 1996). In another city, analysis of sulfur dioxide alone indicated an association with daily mortality, but

the effect was not observed when sulfur dioxide was analyzed in combination with other air pollutants

(Anderson et al. 1996). Weak but significant associations between short term increases in sulfur dioxide

levels and daily mortality were noted in other cities (Spix and Wichmann 1996; Sunyer et al. 1996; Touloumi

et al. 1996).

Results from the pooling and metaanalysis of data from 12 cities suggested a weak but statistically significant

association between increased sulfur dioxide levels and daily mortality in western European cities but not in

central eastern Europe (Katsouyanni et al. 1997). In most studies sulfur dioxide concentrations were highest

during the winter, but effects were noted primarily during the summer. A possible explanation may be an

increased exposure to outdoor air during the summer months (Sunyer 1996).
SULFUR DIOXIDE 28

2. HEALTH EFFECTS

Inconsistencies in the various APHEA studies may have resulted from several limitations with

epidemiological studies of air pollutant mixtures. For example, there may be confounding effects due to air

pollutants that were not measured. Also, concentrations were obtained from monitoring sites and this

provides no information about personal exposures within individuals of a population. Lastly, there is little

understanding about the mechanism of toxicity associated with air pollutants. Therefore, separating the

effects of individual air pollutants within a mixture is very unlikely with the technology available at this time

(Ballester et al. 1996; Katsouyanni et al. 1997; Loomis et al. 1996; Moolgavkar et al. 1996).

The approximate LC50 for a 30minute exposure in Swiss albino mice was 3,000 ppm (Hilado and Machado

1977). In this study, there were no deaths during or after 30 minutes of exposure to 1,190 ppm sulfur

dioxide. However, the study is limited because it did not indicate the age, sex, or the number of animals used

in the experiment. The LC50 value associated with death in mice is shown in Table 2-1 and plotted in

Figure 2- 1.

The time course of mortality in groups of 8 rats was examined following continuous exposure to

590-500,000 ppm sulfur dioxide (Cohen et al. 1973). Average mortality times were 1866,750, 176, <10

and <2 minutes for rats exposed to 590,925,2350,50,000, or 500,000 ppm sulfur dioxide respectively.

Mortality in groups of 8 rats was monitored for two weeks following a 4-hour exposure to 224-1,319 ppm

sulfur dioxide (Cohen et al. 1973). None of the rats died following exposure to 224 or 593 ppm sulfur

dioxide. Three rats exposed to 965 ppm died. Five rats exposed to 1,168 ppm died 1 to 48 hours after

exposure, and all rats exposed to 1,3 19 ppm died 1 to 24 hours after exposure. Results of these studies are

illustrated in Table 2- 1 and plotted in Figure 2-l.

2.2.1.2 Systemic Effects

Respiratory Effects. In humans, and in particular asthmatics, respiratory changes are a primary response

following acute exposure to sulfur dioxide. Numerous controlled clinical studies have examined pulmonary

lung function, usually assessed by measurement of increases in specific airway resistance and/or decreases in

forced expiratory volume or forced expiratory flow, in human subjects exposed to sulfur dioxide. The

methods of exposure to sulfur dioxide usually involved oronasal, nose-only, or mouth breathing techniques.

Several chamber studies have been performed in a number of investigations in various laboratories.
SULFUR DIOXIDE 29

2. HEALTH EFFECTS

In 10 healthy, nonasthmatic individuals, exposure to sulfur dioxide (via a chamber) at concentrations of up to

1 .0 ppm for up to 40 minutes was associated with only a slight increase in subjective, mild, upper-respiratory

symptoms such as sore throat and ability to taste and smell sulfur dioxide (Schachter et al. 1984). There was

no effect on lung function parameters. However, reductions in forced expiratory volume and forced

expiratory flow and an increase in nasal airflow resistance were observed in 15 healthy subjects exposed

nose-only to >1.0 ppm sulfur dioxide for 1-6 hours (Andersen et al. 1974). Decreased tidal volume and

increased respiratory rate were observed in 14 healthy subjects exposed to 1-8 ppm sulfur dioxide for

10 minutes (Amdur et al. 1953). Specific airway resistance was increased in 26 healthy individuals exposed

to 0.6-0.8 ppm sulfur dioxide for 5 minutes (Islam et al. 1992). A significant decrease in nasal mucus flow

was seen in 15 healthy subjects at 5 and 25 ppm sulfur dioxide (Anderson et al. 1974). Reduced bronchial

clearance was observed at 5 ppm (Wolff et al. 1986). Exercise increased the rate of bronchial clearance

(Wolff et al. 1986). Increased airway resistance during rest was observed in 7 healthy subjects exposed to

4-6 ppm sulfur dioxide in a body plethysmograph for 10 minutes (Nadel et al. 1965). When 11 healthy

subjects were exposed mouth only to 5 ppm sulfur dioxide for 10-30 minutes, increased flow resistance was

noted (Frank et al. 1962). Cough, a sense of irritation, and increased salivation were also seen at 5 ppm. No

effects were observed at 1 ppm. Erythema of the trachea and main bronchi was seen in 22 healthy individuals

exposed to 8 ppm sulfur dioxide for 20 minutes (Sandstrom et al. 1989a, 1989b). This effect was

accompanied by an increase in inflammatory cells in bronchoalveolar lavage fluid. Increased numbers of

inflammatory cells were also observed in the bronchoalveolar lavage fluid in groups of 4-10 subjects exposed

to 4 ppm sulfur dioxide for 20 minutes (Sandstrom et al. 1989a, 1989b).

A series of inhalation exposure studies were conducted in healthy subjects with various concentrations of

sulfur dioxide (Lawther et al. 1975). Pulmonary function changes were not observed in 13 subjects following

quiet nasal breathing of 1 ppm sulfur dioxide for 1 hour. However, significant increases in specific airway

resistance were noted in 12 subjects who inhaled 25 deep breaths of 1 ppm sulfur dioxide. Increases in

specific airway resistance were also noted after the subjects took 25 deep breaths of filtered air, but the

response was greater with sulfur dioxide exposure. In 17 subjects, significant increases in specific airway

resistance were observed after inhaling 16 deep breaths of filtered air. Additional dose-related increases in

resistance were observed after inhaling 8, 16, and 32 deep breaths of 3 ppm sulfur dioxide. Specific airway

resistance increased significantly in 14 subjects who breathed 5 ppm sulfur dioxide quietly by mouth for 10

minutes. Resistance was highest immediately after exposure and, depending on the sensitivity of the subject,

lasted from 5 to 65 minutes following exposure. Additional dose-related increases in resistance were not
SULFUR DIOXIDE 30

2. HEALTH EFFECTS

observed following exposure to l0-30 ppm sulfur dioxide. Subjects could detect higher levels of sulfur

dioxide and it appears that they tried to protect themselves with shallow breathing.

The results of controlled laboratory studies in humans have established that asthmatics are particularly

sensitive to the respiratory effects of sulfur dioxide. In a study of 21 asthmatics exposed for 10 minutes to

sulfur dioxide by mouthpiece while at rest, increases in airway resistance were observed at 1,3, and 5 ppm

(Sheppard et al. 1980).

Controlled studies have indicated that the pulmonary effects of sulfur dioxide can be significantly enhanced

by exercise. Significant increases in airway resistance have been clearly demonstrated in moderately

exercising asthmatics (ventilation range: 35-60 L/minute) exposed briefly (3-10 minutes) to 0.40-1.0 ppm

sulfur dioxide (Bethel et al. 1985; Linn et al. 1983a, 1984a; Roger et al. 1985; Schachter et al. 1984).

Significant changes in airway resistance were observed in young adult mild asthmatics exposed, while

exercising, to a concentration of sulfur dioxide as low as 0.25 ppm through a mouthpiece (Sheppard et al.

1981). The two most sensitive asthmatics exhibited some degree of bronchoconstriction, as evidenced by a

slight increase in specific airway resistance (SRaw) following inhalation of 0.1 ppm sulfur dioxide through a

mouthpiece for 10 minutes (Sheppard et al. 1981). A dose-response relationship, as measured by airway

resistance, was observed in the two sensitive subjects following exposures to 0.25 and 0.5 ppm sulfur

dioxide. At 0.25 ppm, the difference between baseline specific airway resistance and specific airway

resistance (∆SR,) after inhalation of sulfur dioxide was approximately 5 Lxcm H2O/L/s (units of SRaw). At

0.5 ppm, the ∆SRaw exceeded 15 Lxcm H,O/L/s.

Lung function changes in asthmatics exposed by inhalation to 0.25 ppm sulfur dioxide have been reported by

other investigators. In a chamber study of moderately exercising asthmatics, the concentration of sulfur

dioxide required to produce an increase in airway resistance 100% greater than the response to clean air

[designated as PC(SO2)] has been determined (Horstman et al. 1986). Analysis of the cumulative percentage

of subjects plotted as a function of PC(SO2,) revealed that 25% of the subjects exhibited a PC(SO2) of

0.25-0.5 ppm sulfur dioxide. The study authors suggested that the 25% of the mild asthmatics who were

very sensitive to sulfur dioxide could possibly exhibit bronchoconstriction if they were to perform normal

exercise routines in some highly industrialized areas of the United States. Increases in specific airway

resistance were observed in 9-19 moderately exercising asthmatics exposed oronasally to 0.25 ppm sulfur

dioxide for 5 minutes (Bethel et al. 1985). A dose-related increase in specific airway resistance was seen in
SULFUR DIOXIDE 31

2. HEALTH EFFECTS

asthmatics following a 3-minute exposure (via mouthpiece) to > 0.25 ppm sulfur dioxide (Myers et al. 1986a,

1986b).

Some studies of asthmatics have reported a lack of significant lung function changes in asthmatics following

exposures to 0.1-0.5 ppm (Jorres and Magnussen 1990; Koenig et al. 1990). Bronchoconstrictive responses

to sulfur dioxide are highly variable among individual asthmatics (Horstman et al. 1986). In some studies,

asthmatics were preselected for sensitivity to sulfur dioxide and this may explain the range of sulfur

dioxide-induced responses obtained by different investigators.

In addition to exercise, sulfur dioxide-induced bronchoconstriction can be increased by cold or dry air

(Sheppard et al. 1984). For instance, the concentration resulting in a 100% increase in specific airway

resistance in 8 asthmatics exposed to sulfur dioxide in dry, cold air was 0.51±0.2 ppm, whereas it was

0.6±0.41 ppm for dry, warm air and 0.87±0.41 ppm humid, warm air.

In several controlled acute studies, the authors have reported that, because of the severity of the pulmonary

response, exposures to sulfur dioxide had to be terminated for some of the asthmatic subjects while other

subjects required medical attention. Two out of seven asthmatics required a bronchodilator after exposure to

cold air and 0.5 ppm sulfur dioxide (Bethel et al. 1984). Two of 10 subjects exposed to 0.5 ppm sulfur

dioxide were unable to complete the experiment (Koenig et al. 1985). Similar events were reported by other

investigators (Balmes et al. 1987; Horstman et al. 1986, 1988; Linn et al. 1984a, 1984b, 1984c; Roger et al.

1985).

Controlled studies in asthmatic subjects have demonstrated that repeated exposures to sulfur dioxide reduce

the responsiveness of asthmatics to the chemical. For instance, the pulmonary response of 10 exercising

asthmatics exposed to 1.0 ppm sulfur dioxide in a chamber was attenuated after repeated exercise (Kehrl et

al. 1987). Bronchoconstriction was less severe in 14 exercising asthmatics exposed to 0.6 ppm sulfur dioxide

on the second day of a 2-day exposure period (Linn et al. 1984a). Specific airway resistance in 28 asthmatics

was significantly less after second and third exercises when compared with the first exercise (Roger et al.

1985).

A series of studies was conducted to examine respiratory function in healthy, atopic, or asthmatic adolescents

(n=8-9, aged 12-17 years) and 10 healthy male seniors (aged 55-73 years) following exposure by mouth

only to a mixture of 1 ppm sulfur dioxide and 1 mg/m3

 sodium chloride aerosol (Koenig et al. 1982a, 1982b,
SULFUR DIOXIDE 32

2. HEALTH EFFECTS

1983; Rondinelli et al. 1987). Exposures consisted of 30 minutes of rest and 10 minutes of exercise for

adolescents and 20 minutes of rest and 10 minutes of exercise for seniors. Statistically significant decreases

in forced expiratory volume in 1 second (FEV,) were observed in all groups of subjects following exposure to

the sulfur dioxide and sodium chloride mixture during exercise. The magnitude of decreases were 23% in

asthmatic adolescents, 18% in atopic adolescents, 8% in seniors, and 6% in healthy adolescents (Koenig et al.

1982a, 1982b, 1983; Rondinelli et al. 1987). Based on a comparison of FEV1 values, it was concluded that

respiratory health status and not age is the primary factor in determining susceptibility to sulfur dioxide

(Rondinelli et al. 1987). Asthmatic adolescents were most sensitive to sulfur dioxide and normal adolescents

were least sensitive. Responses in seniors and healthy adolescents were similar. However, the doses inhaled

by healthy adolescents were about 20% greater because of a longer exposure period and a more strenuous

exercise period. A further decrease in FEV1 may have occurred in seniors had they received the same doses

as adolescents. In two of the studies, it was verified that exposure to a mixture of sulfur dioxide and sodium

chloride aerosol resulted in effects which did not differ significantly from those obtained from exposure to

sulfur dioxide alone (Koenig et al. 1982b, 1982a). The authors stated that in the environment sulfur dioxide

occurs together with one or more droplet aerosols and that the studies represented a realistic exposure

scenario.

Adaptation to irritant concentrations of sulfur dioxide is a recognized occurrence in workers (Department of

Labor 1975). The potential tolerance to the pulmonary effects of sulfur dioxide has been studied in

asthmatics. Eight asthmatic subjects were exposed to 0.5 ppm sulfur dioxide with a mouthpiece for

3 minutes while performing voluntary eucapnic kyperpnea (rapid deep breathing to deplete arterial carbon

dioxide) (Sheppard et al. 1983). The subjects received three 3minute exposures with a 30 minute rest period

between exposures. Specific airway resistance was increased significantly more after the first exposure to

sulfur dioxide than after the second or third exposures. Tolerance was not observed when the exposures were

repeated 24 hours later and then 7 days later.

A case study describes a sulfide dust explosion in a copper mine that liberated large amounts of sulfur

dioxide (Rabinovitch et al. 1989). Analysis of the gas sample obtained at the time of rescue showed sulfur

dioxide concentrations >40 ppm, with no other toxic gases present. Effects resulting from the exposure

consisted of burning of the nose and throat, dyspnea, and severe airway obstruction that was only partially

reversed 2 years after the exposure. In another case report of an industrial accident at a paper mill involving

exposure to high but unmeasured concentrations of sulfur dioxide, histological examination of the lungs of

two subjects who died revealed extensive sloughing of the mucosa of large and small airways along with
SULFUR DIOXIDE 33

2. HEALTH EFFECTS

hemorrhagic alveolar edema. Acute symptoms observed in the survivors included irritation of the nose and

throat, tightness in the chest, and intense dyspnea. Serial pulmonary function tests revealed severe airway

obstruction in one subject on the first day of examination Another survivor who used a protective mask

developed mild airway obstruction by day 116. No pulmonary function abnormalities were noted in the third

survivor, a fireman who arrived later on the scene.

Bronchial hypersensitivity can develop following a single exposure to very high concentrations of sulfur

dioxide, a syndrome referred to as reactive airway dysfunction syndrome or RADS (Brooks et al. 1985;

Brooks 1992; Brooks et al. 1985; Goldstein et al. 1979; Harkonen et al. 1983). In this syndrome, the

bronchial epithelial damage results in increased sensitization and nonspecific hypersensitivity to a wide range

of other irritant stimuli.

Bronchitis has been reported in pulp mill workers following brief accidental exposures to 100 ppm sulfur

dioxide (Skalpe 1964).

Epidemiological studies on the relationship between sulfur dioxide exposure and respiratory effects have been

conducted. However, these studies are limited because of the difficulties in separating potential effects of

sulfur dioxide from those of particulates and other air pollutants. A longitudinal study of 343 children

exposed to sulfur dioxide and particulate sulfate was conducted in three towns in Arizona, two of which have

smelters (Dodge et al. 1985). The study period was from 1979 to 1982. The annual average sulfur dioxide

concentrations at two monitoring sites were 0.005 ppm and 0.04 ppm. However, in the area of highest

pollution, children were exposed intermittently to high levels of sulfur dioxide (peak 3-hour mean exceeded

1 ppm) and moderate levels of particulate sulfate. A significant correlation between the prevalence of cough

and pollution levels was noted. There were no significant changes in lung function over a 3-year period.

Lung function has been studied in 200 school-age children following several acute air pollution episodes in

Steubenville, OH (Dockery et al. 1982). One day in 1979, the children were exposed to mean daily

concentrations of go.17 ppm sulfur dioxide and 0.27 mg/m3

 total suspended particles, levels which exceeded

the 24-hour standard in 1979. The children were examined in three weekly visits following each pollution

alert. The children were measured again in five weekly examinations in the spring and fall of 1980. Slight

(2-3%) decreases in forced vital capacity (FVC) and forced expiratory volume in 0.75 seconds (FEV0.75) were

noted in the children the day following the acute pollution episode in 1979 and 1-2 weeks later. The
SULFUR DIOXIDE 34

2. HEALTH EFFECTS

decreases were statistically significant and reversible after 2-3 weeks. The effects of total suspended

particles cannot be separated from those of sulfur dioxide.

In New York, the association between seasonal air pollution levels and forced expiratory volume in

0.75 seconds (FEV0.75) was examined in two age groups of children (5-8 and 9-13 years-old) (Shy et

al. 1973). At the time of the study, sulfur dioxide levels in polluted areas ranged from 63-71 μg/m3

(0.024-0.027 ppm) and levels of total suspended particulates (TSPs), suspended sulfates, and nitrogen

dioxide were elevated in comparison to non-polluted areas Levels of sulfur dioxide, TSPs, and suspended

sulfates were greatly reduced three years prior to the initiation of the study. Previous sulfur dioxide levels

ranged from 364-435 μg/m3

 (0.14-0.17 ppm). Associations between pollutant levels and reduced FEV0.75

were only observed in the older group of children. The authors concluded that persistent respiratory effects

may result from 5-10 year exposures to high air pollution levels during early childhood. Effects associated

with individual air pollutants were not separated in this study.

The association between peak expiratory flow rates (PEFR) and sulfur dioxide and particulate air pollution

was examined in 60 asthmatic children (aged 9-14 years) living in Budapest during the period of September

13 through December 5, 1993 (Agocs et al. 1997). Sulfur dioxide concentrations ranged from 11-185 μg/m3

(0.004-0.071), and levels exceeded World Health Organization (WHO) limits on several days. Confounding

effects such as temperature, humidity, weekday, and time trends were controlled and each air pollutant was

assessed individually. There were no consistent associations between air pollution levels and PEER. Study

limitations were addressed by the authors and included medication use by many of the children, incomplete

control of seasonal factors, and a lack of personal exposure data.

In London, increases in sulfur dioxide levels by 14 μg/m3

 (0.005 ppm) were associated with a 12% increase

in the number of emergency room visits for wheezing episodes in children (Buchdahl et al. 1996).

Confounding effects from season, temperature, and wind speed were controlled, but effects from the other air

pollutants, including ozone and nitrogen dioxide, were not separated. In the evaluation of ozone, effects from

sulfur dioxide and nitrogen dioxide were controlled, and ozone was found to have the strongest association

with wheezing episodes in children.

Difficulty breathing and cough were reported by 91-94% of workers in a Yugoslavian broom manufacturing

plant who were exposed to average sulfur dioxide concentrations of 0.29 and 57 ppm in the summer and

winter, respectively (Savic et al. 1987). There appeared to be no control for confounding factors such as
SULFUR DIOXIDE 35

2. HEALTH EFFECTS

occupational exposure to dust and smoking. A study was also conducted of workers exposed to sulfur

dioxide in a refrigerant manufacturing plant (Kehoe et al. 1932). Prior to 1927 concentrations of sulfur

dioxide averaged between 80-100 ppm. After the installation of a ventilation system, sulfur dioxide levels

typically ranged between 5-35 ppm with occasional peaks as high as 50-70 ppm. The incidence of

respiratory irritation and shortness of breath during heavy activity were significantly increased in

100 exposed workers. Chest X-rays revealed no differences between exposed and unexposed workers at the

same plant Occupational exposure to other chemicals was not described and there was no control for

smoking.

A significant decrease in forced expiratory volume in 1 second (FEV,) was observed in 113 workers exposed

to >1 ppm sulfur dioxide for an unspecified time in a copper smelter in Salt Lake City (Smith et al. 1977).

Smoking status was assessed in the study. However, workers were also exposed to respirable dust. A study

conducted years later at the same copper smelter found no significant relationships between pulmonary

function and exposure to >5 ppm sulfur dioxide in 430 workers (Lebowitz et al. 1979). Exposure durations

ranged from 0 to >20 years and confounding factors such as age, smoking, and exposure to dust were

corrected. The authors suggested that the previous findings of Smith et al. (1977) may have resulted from

limitations of the study, such as a small sample size and inadequate correction for age.

In another study of 953 copper smelter workers in Garfield, Utah, a significant reduction in FVC and FEV1

was associated with long-term (>20 years) exposure to 0.4-3.0 ppm sulfur dioxide (Archer and Gillan 1978).

Smoking status was assessed in the study. The reduction was observed with increasing duration of exposure

in both smokers and nonsmokers but was not observed in controls. However, workers were also exposed to

arsenic, copper, manganese, iron, and other trace metals.

Workers (4,506 and 5,943) exposed to mean concentrations of approximately 0.84-1.2 ppm sulfur dioxide

for an unspecified time period at two British steel plants did not experience an increase in respiratory

symptoms or reduction in respiratory performance (Lowe et al. 1970). Confounding factors such as smoking

and occupational exposure to dust were controlled.

In a study of 56 workers exposed to 2-36 ppm sulfur dioxide in pulp mills for 1 month to 40 years, cough

was reported by 56% of workers, sputum production by 46%, and difficulty breathing by 22% (Skalpe 1964).

Symptoms were reported in higher percentages by workers <50-years-old. Maximal expiratory flow rate was

significantly reduced in workers <50-years-old. The author speculated the most likely reason for increased
SULFUR DIOXIDE 36

2. HEALTH EFFECTS

effects in the younger workers was that minor respiratory effects are more likely detected due to decreased

prevalence of respiratory symptoms at a younger age. Occupational exposure to other chemicals was not

reported but smoking status was controlled.

Variability in the findings of occupational studies may have resulted from common limitations in each study.

Personal exposure levels were not measured in any of the studies but were estimated from area samples.

Levels of sulfur dioxide may vary widely within a small area of a plant and exposures could differ

significantly between workers in the area. Also, workers are usually exposed to multiple toxic substances and

it is difficult to separate the effects of individual compounds.

Though erratic results were obtained in the occupational studies, it is evident that effects occurred at higher

concentrations than in controlled chamber studies of asthmatics. Workers represent a subgroup which is

healthier than the general population. Due to breathing difficulties, individuals with respiratory disorders

such as asthma would generally be excluded from working in areas with high sulfur dioxide concentrations.

Therefore, it is expected that changes in lung function would be observed at lower concentrations in

controlled studies of asthmatics than in occupational studies.

Studies in experimental animals have supported the findings of pulmonary effects of sulfur dioxide in humans

following inhalation exposure. Sixteen guinea pigs exposed to 2.6 ppm sulfur dioxide in an exposure

chamber for 1 hour showed a 20% increase in resistance accompanied by a 10% decrease in compliance

(Amdur 1959). The response to sulfur dioxide increased when exposure was increased to 3 hours. Increased

pulmonary flow resistance was noted in guinea pigs exposed to 0.16-835 ppm sulfur dioxide (Amdur 1966).

In a nose-only inhalation exposure study, 6-9 guinea pigs exposed to 1 .0 ppm sulfur dioxide for 1 hour did

not display any effects on airway responsiveness (Chen et al. 1992b). In a study designed to assess the

bronchial sensitization to ovalbumin in 6 guinea pigs following exposure to 5 ppm sulfur dioxide for 6 hours

per day for 5 days, serious pulmonary effects, including severe destruction of ciliated epithelium of the

bronchioles, partial collapse of alveoli, and polymorphonuclear infiltrates were observed (Riedel et al. 1992).

Decreased compliance and increased resistance were noted in 12 anesthetized dogs exposed to 1.1-141 ppm

sulfur dioxide for 2040 minutes using a mask fitted so that the dogs breathed through the nose and the

mouth (Balchum et al. 1959). The average decrease in compliance for 11 dogs was 5%. One dog exposed to

141 ppm for 40 minutes displayed a 121% decrease in compliance. Dogs exposed to 45 ppm for 16 minutes

displayed increased resistance and decreased compliance of the lung (Balchum et al. 1960b). Decreased
SULFUR DIOXIDE 37

2. HEALTH EFFECTS

compliance and increased resistance were also seen in 10 anesthetized dogs exposed to 1.8-148 ppm sulfur

dioxide through a tracheal cannula (Balchum et al. 1960b). Compliance was decreased from control levels at

an average of 8.5% (statistically significant at p<0.05). Resistance was increased by 150-300%. The

increase in resistance occurred within 9 seconds after the onset of sulfur dioxide exposure and disappeared

quickly at the end of exposure.

Acute-duration exposure to high concentrations of sulfur dioxide can result in biochemical, clinical, and

histological changes in the respiratory systems of the mouse and the rabbit. A transient decrease in cough

reflex and the Hering-Breuer inflation reflex were observed in 18 rabbits exposed to 200-300 ppm sulfur

dioxide for 10-20 minutes (Hanacek et al. 1991).

Four mice exposed to 20 ppm sulfur dioxide for up to 120 minutes exhibited degenerative changes in the

olfactory epithelium (Min et al. 1994). A substantially increased number of polymorphonuclear lymphocytes

in the trachea was noted in 3-5 rats exposed to 230 ppm sulfur dioxide for 5 hours (Farone et al. 1995). A

loss of cilia and cell necrosis in the trachea and main bronchus were observed in 5 rats exposed to 800 ppm

sulfur dioxide for 8 hours (Stratmann et al. 1991).

Rats exposed to 25 ppm sulfur dioxide for up to 5 days displayed nasal epithelial metaplasia and basal cell

hyperplasia (Fowlie et al. 1990).

Respiratory effects from exposure to sulfur dioxide for intermediate time periods have also been studied in

animals. Mild bronchitic lesions were seen in 72 hamsters exposed to 650 ppm sulfur dioxide for

4 hours/day, 5 days/week, for 19-74 days (Goldring et al. 1970). Decreased respiratory rate, rhinitis,

tracheitis, and bronchopneumonia were observed in 6 rabbits exposed to 70-300 ppm sulfur dioxide for

6 weeks (Miyata et al. 1990). Nasopharyngitis and lipid peroxidation of lung tissue were observed in guinea

pigs exposed to 10 ppm sulfur dioxide for 1 hour/day, for 30 days (Haider 1985).

Significantly increased activities of acid phosphatase and alkaline phosphatase, decreased numbers of

epithelial cells, and increased numbers of leukocytes were observed in the bronchial lavage fluid of rats

exposed to 30-40 ppm sulfur dioxide for 1 hour/day, 5 days/week, for 12 weeks (Krasnowska et al. 1998). A

histopathological evaluation of the bronchial mucosa revealed damage to the epithelium accompanied by

infiltration of leukocytes, destruction of cilia, and squamous cell metaplasia, which were more pronounced in

rats examined three weeks past exposure compared to those examined immediately after exposure.
SULFUR DIOXIDE 38

2. HEALTH EFFECTS

Exposure of 12 rats to 400 ppm sulfur dioxide for 3 hours/day, 5 days/week, for up to 42 days resulted in

airway effects similar to those observed in humans with chronic bronchitis (Lamb and Reid 1968). A loss of

cilia, epithelial necrosis, and a reduction in goblet cell numbers were observed during the first 2-4 days of

exposure. By 3-6 weeks of exposure, signs of healing were evident and included a thickened epithelium and

the reappearance of shortened cilia. During the same time period, goblet cells increased in size and number

and appeared in distal airways, areas in which they are normally lacking. The sialidase-sensitive mucous in

distal airways was replaced by sialidase-resistant mucous within three weeks of exposure. Tracheal gland

size continually increased throughout the exposure period. Similar effects were observed in another study in

which rats were exposed to 400 ppm sulfur dioxide for 3 hours/day, 5 days/week, for up to 3 weeks

(Basbaum et al. 1990). An 8-9 fold increase in mucin mRNA transcripts was also observed.

The respiratory toxicity of sulfur dioxide following chronic-duration inhalation exposure has been studied in

animals” No effects on lung function were observed in 50 guinea pigs exposed to 5.72 ppm sulfur dioxide for

22 hours/day, 7 days/week, for 52 weeks (Alarie et al. 1972). Likewise, no lung function changes or

histopathological alterations in the lung were observed in 9 monkeys exposed to 5.12 ppm sulfur dioxide for

23.3 hours/day, 7 days/week, for 78 weeks (Alarie et al. 1975).

In summary, the available data indicate that sensitive asthmatics may respond to concentrations of sulfur

dioxides as low as 0.1 ppm. Healthy nonasthmatics respond to higher concentrations of sulfur dioxide

(≥1.0 ppm). Factors that can exacerbate the respiratory effects of sulfur dioxide include exercise and

breathing of dry or cold air. Animal data support the human data on respiratory effects of sulfur dioxide.

Cardiovascular Effects. Human, nonasthmatic subjects (n=≤14) exposed to 1-8 ppm sulfur dioxide

showed increased pulse rate (Amdur et al. 1953).

No evidence of histological lesions in the heart was found in 9 monkeys following exposure to 5.12 ppm

sulfur dioxide for 23.3 hours/day, 7 days/week, for 78 weeks (Alarie et al. 1975). Likewise, no microscopic

lesions were detected in the hearts of 50 guinea pigs that were exposed by inhalation to 5.72 ppm sulfur

dioxide for 22 hours/day, 7 days/week, for 52 weeks (Alarie et al. 1972). Increased lipid peroxidation was

observed in hearts of 6 guinea pigs exposed to 10 ppm sulfur dioxide for 1 hour/day, for 30 days (Haider

1985). The biological significance of the result is unknown.
SULFUR DIOXIDE 39

2. HEALTH EFFECTS

Gastrointestinal Effects. Nausea and vomiting were observed in 3 humans exposed to >40 ppm sulfur

dioxide during an accident at a copper mine (Rabinovitch et al. 1989).

No studies were located regarding gastrointestinal effects in animals after inhalation exposure to sulfur

dioxide.

Hematological Effects. Blood samples were taken from PO0 workers exposed to sulfur dioxide for a

minimum of 2 years in a refrigerant manufacturing plant (Kehoe et al. 1932). Prior to 1927, the sulfur

dioxide concentration averaged between 80-100 ppm. After the installation of a ventilation system, sulfur

dioxide levels typically ranged between 5-35 ppm with occasional peaks as high as 50-70 ppm. Small but

significant differences in the numbers of polymorphonuclear leukocytes and lymphocytes were observed. The

authors questioned the biological significance of the hematological findings and concluded that it was

probably due to the high rate of respiratory infection in both exposure and control groups.

An increase in blood levels of methemoglobin was observed in 45-59 workers in a broom manufacturing

factory who were exposed to average sulfur dioxide concentrations of 0.29 and 57.0 ppm in the summer and

winter, respectively (Savic et al. 1987). The mean percentage of methemoglobin was elevated during the

winter when windows were closed and sulfur dioxide levels were highest. With the exception of dust,

exposure to other workplace chemicals was not described.

Numerous studies in animals have demonstrated oxidative effects on erythrocytes. An increase in erythrocyte

deformability was observed in 14 rats exposed to 1 ppm sulfur dioxide for 24 hours (Baskurt et al. 1990).

Osmotic hemolysis and sulfhemoglobin levels were increased in erythrocytes of 50 rats exposed to 0.9 ppm

sulfur dioxide for 24 hours (Baskurt et al. 1988). An increase in the erythrocyte deformability index and lipid

peroxidation were noted in the erythrocytes of 12 guinea pigs exposed to 10 ppm sulfur dioxide for 30 days

(Dikmenoglu et al. 1991). Significantly increased ratios of methemoglobin and sulfhemoglobin, lipid

peroxidation, and increased fragility of erythrocytes, were observed in 7 rats exposed to 10 ppm sulfur

dioxide for 1 hour/day, for 45 days (Etlik et al. 1997). Significant changes in antioxidant enzyme activities

and increased lipid peroxidation were observed in the erythrocytes of 15 rats exposed to 10 ppm sulfur

dioxide for 1 hour/day, 7 days/week, for 8 weeks (Gumusht et al. 1998).

Mixed results have been obtained for sulfur dioxide-induced effects on blood cell numbers in animals. No

hematological effects were observed in a chronic toxicity study of 9 monkeys exposed to 5.12 ppm sulfur
SULFUR DIOXIDE 40

2. HEALTH EFFECTS

dioxide for 23.3 hours/day, 7 days/week, for 78 weeks (Alarie et al. 1975). Likewise, no hematological

effects were noted in guinea pigs exposed to 5.72 ppm sulfur dioxide for 22 hours/day, 7 days/week, for

52 weeks (Alarie et al. 1972). However, statistically significant increases in white and red blood cell counts,

as well as hematocrit and hemoglobin levels were observed in 7 rats exposed to 10 ppm sulfur dioxide for

1 hour/day for 45 days (Etlik et al. 1997). Increased hematocrit levels were also observed in 50 rats exposed

to 0.9 ppm sulfur dioxide for 24 hours (Baskurt et al. 1988).

Animal studies support the finding in humans that sulfur dioxide induces oxidative effects in erythrocytes.

However, both human and animal studies are limited. In the occupational studies, levels of sulfur dioxide

varied greatly and individual exposure levels were not measured. In addition, confounding factors such as

smoking or exposure to other chemicals were not controlled. Because multiple doses were not tested in any

of the animal studies, the dose response relationship is unknown. Data from well designed occupational or

animal studies are required before the impact of sulfur dioxide exposure on blood can be assessed.

Musculoskeletal Effects. No studies were located regarding musculoskeletal effects in humans or

animals after inhalation exposure to sulfur dioxide.

Hepatic Effects. No studies were located regarding hepatic effects in humans after inhalation exposure to

sulfur dioxide.

Information on hepatic effects in animals from inhalation exposure is very limited. In a chronic toxicity study

in monkeys, no histological lesions were detected in 9 monkeys exposed to 5.12 ppm sulfur dioxide for

23.3 hours/day, 7 days/week, for 78 weeks (Alarie et al. 1975). In contrast, microscopic examination of

50 guinea pigs exposed to 5.72 ppm sulfur dioxide for 22 hours/day, 7 days/week, for 52 weeks, showed an

increase in the size of hepatocytes accompanied by cytoplasmic vacuolation (Alarie et al. 1972).

Liver triglyceride and esterfied cholesterol levels were increased in 9 rats fed either a standard diet or a high

cholesterol diet and exposed to 10 ppm sulfur dioxide for 24 hours/day, 7 days/week, for 15 days (Lovati et

al. 1996). However, exposure of 9 diabetic rats to 5 or 10 ppm sulfur dioxide under the same conditions

resulted in a significant reduction in liver triglyceride levels. Additional experiments demonstrated that

triglyceride levels increased in nondiabetic rats due to a reduction in lipid catabolism. Because insulin

regulates the activity of lipases, it appears that a concurrent drop in insulin levels was responsible for the

decreased catabolism in the nondiabetic rats. However, in diabetic rats it was postulated that triglyceride
SULFUR DIOXIDE 41

2. HEALTH EFFECTS

catabolism may have increased to provide an alternate energy source for glucose. Contrary findings were

observed in 6 guinea pigs whose liver cholesterol level decreased following exposure to 10 ppm sulfur

dioxide for 1 hour/day for 30 days (Haider 1985). Liver triglyceride levels were not measured, but total lipid

concentrations in livers of exposed animals were unaffected. Additional studies are needed to define the role

of sulfur dioxide in the hepatic metabolism of lipids.

Renal Effects. No studies were located regarding renal effects in humans after inhalation exposure to

sulfur dioxide.

Information on renal effects in animals from inhalation exposure is very limited. No renal effects were

observed in a chronic toxicity study of 9 monkeys exposed to 5.12 ppm sulfur dioxide for 23.3 hours/day,

7 days/week, for 78 weeks (Alarie et al. 1975). Likewise, no renal effects were noted in 50 guinea pigs

exposed to 5.72 ppm sulfur dioxide for 22 hours/day, 7 days/week, for 52 weeks (Alarie et al. 1972).

Endocrine Effects. No studies were located regarding endocrine effects in humans after inhalation

exposure to sulfur dioxide.

Significant dose-related decreases in plasma insulin levels were observed in 9 rats exposed to 5 or 10 ppm

sulfur dioxide for 24 hours/day, 7 days/week, for 15 days, but significant effects on insulin levels were not

noted in diabetic rats exposed to sulfur dioxide under the same conditions (Lovati et al 1996).

Dermal Effects. No studies were located regarding dermal effects in humans after inhalation exposure to

sulfur dioxide. Skin irritation was observed in 6 rats and 12 guinea pigs exposed to 10 ppm sulfur dioxide

for 1 hour/day for 30 days (Haider 1982, 1985).

Ocular Effects. In a case report of a paper mill accident in which five persons were exposed to high

concentrations (not specified) of sulfur dioxide for less than 5 minutes, reversible conjunctivitis and

superficial cornea1 burns were noted (Charan et al. 1979). In another case report dealing with a pyrite dust

explosion that resulted in nine persons being exposed to high levels of sulfur dioxide, conjunctival irritation

and cornea1 erosion were observed (Harkonen et al. 1983). The concentration of sulfur dioxide was not

measured, but based on experimental explosions, the concentration was estimated to be 30-1,600 ppm.

Burning of the eyes was reported in a case study of an accident at a copper mine in which 3 miners were

exposed to >40 ppm sulfur dioxide (Rabinovitch et al. 1989). Irritation and tearing were reported by 65% of
SULFUR DIOXIDE 42

2. HEALTH EFFECTS

workers in a Yugoslavian broom manufacturing plant who were exposed to sulfur dioxide levels averaging

between 0.29 ppm in summer and 57 ppm in winter (Savic et al. 1987).

Eye irritation was observed in 6 rats and 12 guinea pigs exposed to 10 ppm sulfur dioxide for 1 hour/day for

30 days (Haider 1982,1985).

Body Weight Effects. No studies were located regarding body weight effects in humans after inhalation

exposure to sulfur dioxide.

Following 5 weeks of exposure to 70-300 ppm sulfur dioxide, body weight gains of the 6 treated rabbits

were 25% lower than those of controls (Miyata et al. 1990). No body weight effects were observed in a

chronic toxicity study of 9 monkeys exposed to 5.12 ppm sulfur dioxide for 23.3 hours/day, 7 days/week, for

78 weeks (Alarie et al. 1975). Likewise, no body weight effects were noted in 50 guinea pigs exposed to

5.72 ppm sulfur dioxide for 22 hours/day, 7 days/week, for 52 weeks (Alarie et al. 1972) or in 15 rats

exposed to 10 ppm sulfur dioxide for 1 hour/day, 7 days/week, for 8 weeks (Gumuslu et al. 1998).

2.2.1.3 Immunological and Lymphoreticular Effects

In a study of 10 mild asthmatic subjects, prior inhalation of 0.2 ppm sulfur dioxide for 6 hours did not

significantly affect the provocation dose of Dermatophagoides pteronyssinus allergen required to produce a

20% decrease in forced expiratory volume in 1 second (Devalia et al. 1994). However, prior inhalation

exposure to a combination of 200 ppb sulfur dioxide and 400 ppb nitrogen dioxide significantly reduced the

provocation dose of allergen required to cause a 20% decrease in forced expiratory volume in 1 second.

Increased prevalence of allergies was observed in 556 children (aged 7-13 years) living near an aluminum

smelter in Norway for seven years or more (Soyseth et al. 1996). Prevalence was highest in children who

lived in the area between the ages of 19-36 months and were exposed to sulfur dioxide levels of 20-24 μg/m3

(0.008-0.009 ppm). However, there was no evidence of a dose response relationship. Fluoride levels were

also increased in the area, but other air pollutants were not discussed. The data was controlled for age, sex,

and in utero exposure to cigarette smoke. However, there was no control for possible confounders such as

fluoride or other air pollutants.
SULFUR DIOXIDE 43

2. HEALTH EFFECTS

The incidence of acute respiratory disease was compared in groups of 2,705-8,991 children (aged

1-1,2 years) living in high and low pollution areas (based on 1973 national primary standards) within four

regions of the United States (French et al. 1973). All polluted areas had high concentrations of sulfur

dioxide ranging from 63-275 μg/m3

 (0.024-0. 1 ppm). Contaminants which were also present in high

concentrations in polluted areas included sulfates in the Salt Eake Basin, metals in the Rocky Mountain

communities, and total suspended particulates (TSPs) in New York and Chicago. Generally, increases in

incidence of respiratory infection were observed in children who lived in polluted areas for a minimum of 3

years. In the Salt Lake Basin and Rocky Mountain region, effects were greatest in children 4 years-old and

younger. However, in Chicago, there was no association between respiratory illness and air pollution in

children who were too young to attend nursery school. The discrepancy in results may have resulted from

differences in the pollution mixtures or from inadeqluate control of confounding factors.

Increased sensitization to antigen was reported in a study of guinea pigs exposed by inhalation to sulfur

dioxide (Riedel et al. 1992). In this experiment, groups of six guinea pigs were exposed to either clean air or

5 ppm sulfur dioxide for 8 hours/day for 5 days, with intermittent inhalation of ovalbumin (8 mg aerosolized)

in order to sensitize the animals. Control animals were also sensitized with ovalbumin. Antibodies against

ovalbumin were measured before and after sham and sulfur dioxide exposures. Animals were exposed to the

ovalbumin for 45 minutes following sham or sulfur dioxide exposure on days 3,4, and 5. Seven days after

the last exposure, the guinea pigs were tested for bronchial sensitization to ovalbumin. Sulfur dioxide-treated

animals were found to be sensitized to ovalbumin, while pretreatment with anti-inflammatory drugs (methylprednisolone, indomethacin, or nedocromilsodium) for 6 days beginning 12 hours before the first sulfur

dioxide exposure prevented ovalbumin sensitization.

In an acute-duration inhalation study with hamsters (≤12/group), there was a significant reduction in

endocytosis by pulmonary macrophage (process used in defending lung against pathogens and foreign bodies)

following exposure to 50 ppm sulfur dioxide for 4 hours while exercising (Skornik and Brain 1990).

However, exposure of rats and mice to 0.32-0.43 ppm sulfur dioxide together with 87-113 μg/m3

 sulfate for

4 hours prior to or 17 hours following infection with StaphyZococcus aureus or Group C Streptococci had no

effect on clearance or phagocytosis of S. aureus or rates of Streptococcal killing and proliferation (Goldstein

et al. 1979).
SULFUR DIOXIDE 44

2. HEALTH EFFECTS

2.2.1.4 Neurological Effects

Reflexes were examined in 100 workers exposed to sulfur dioxide in a refrigerant manufacturing facility for a

minimum of 2 years (Kehoe et al. 1932). Prior to 1927, the sulfur dioxide concentration averaged between

80-100 ppm. After the installation of a ventilation system, sulfur dioxide levels typically ranged between

5-35 ppm with occasional peaks as high as 50-70 ppm. A significant number of workers had reflex

response times that were slower or faster than normal The authors stated that the effect did not indicate

neurological injury but reflected differences in irritability. Neurological effects of sulfur dioxide exposure

have been described as psychological responses to general toxicity (Parmeggiani 1983). The Kehoe et al.

(1932) study is limited because occupational exposure to other chemicals was not well characterized.

Seizures and prostration were observed in groups of 8 rats prior to death, which occurred following exposure

to 2,350,50,000 and 500,000 ppm sulfur dioxide for an average of 176 minutes, <10 minutes, and <2

minutes respectively (Cohen 1973). Transient changes in the frequency of certain behavioral patterns, such

as grooming and digging, were noted in 10 male and 10 female mice continuously exposed to 30 ppm sulfur

dioxide and observed during the first 9 days prior to mating in a reproductive study (Petruzzi et al. 1996).

There was no evidence of neurological effects in the offspring of the mice.

Increased lipid peroxidation was observed in the brains of 6 guinea pigs exposed to 10 ppm sulfur dioxide for

1 hour/day for 30 days (Haider et al. 1982). However, the study is limited due to small sample size and

administration of only one dose. In addition similar studies are not available and the reproducibility of results

cannot be verified. Therefore, the biological significance of the results is not known.

2.2.1.5 Reproductive Effects

In a cross-sectional study of spontaneous abortions in an industrial community in Finland, no evidence was

found that exposure to sulfur dioxide was associated with a risk of spontaneous abortions (Hemminki and

Niemi 1982). Types of industries in the area included pulp paper, metal, viscose rayon, and chemical.

Hydrogen sulfide and carbon disulfide were industry-related pollutants which were also present in the area.

High sulfur dioxide levels in air were associated with abnormal sperm morphology and decreased motility in

325 18-year-old males in the Czech Republic (Selevan et al. 1995). Because sulfur dioxide was used as a

surrogate for all air pollutants, it cannot be determined which chemical or mixture of chemicals was

responsible for the effect on sperm.
SULFUR DIOXIDE 45

2. HEALTH EFFECTS

Reproductive effects, as measured by completed pregnancies, litter size, sex ratio, and neonatal mortality,

were not observed in 10 male and 10 female mice continuously exposed to 5, 12, or 30 ppm sulfur dioxide for

9 days prior to mating until days 12-14 of pregnancy (Petruzzi et al. 1996). Pregnancy rate and the number

of implants, resorptions, and live fetuses were not affected in 32 mice exposed to 25 ppm sulfur dioxide for

7 hours/day on gestation days 6-16 or in 20 rabbits exposed to 70 ppm sulfur dioxide for 7 hours/day on

gestation days 6-18 (Murray et al. 1979). The offspring were not assessed for reproductive function in these

studies.

2.2.1.6 Developmental Effects

An association between exposure to increased levels of air pollution during pregnancy and a reduction in birth

weight has been reported in humans (Wang et al. 1997) Decreased birth weights were noted for exposure to

both sulfur dioxide and TSPs. However, evaluation of these results is complicated by limitations such as

confounding effects of various air pollutants and a lack of personal exposure data. Therefore, differentiation

of potential developmental effects associated with individual pollutants of a mixture is not possible.

No developmental effects were observed in a study in which 13 pregnant mice were exposed to 32-250 ppm

sulfur dioxide on gestation days 7-17 (Singh 1982). Neurological effects, measured by reflexes and learning

ability, were not observed in the offspring of 10 mice continuously exposed to 5-30 ppm sulfur dioxide

9 days prior to mating through gestation days 12-14 (Petruzzi et al. 1996). Although there were some

transient behavioral effects in dams, there were no signs of systemic maternal toxicity. In another

developmental study in mice in which pregnant females were exposed to 32 ppm or 65 ppm on gestation days

7-18, increased time for righting reflex on postnatal day 1 and increased negative geotaxis on postnatal

day 10 were reported in offspring (Singh 1989). The duration of exposure for each day was not stated. No

visible signs of maternal toxicity and no effect on the number of live births were observed. Sulfur dioxide at

a concentration of 65 ppm significantly decreased the birth weight (about 89% of controls) of the pups.

Reduced body weight and delayed ossification in sternebrae and occipital bones were observed in the

offspring of 32 mice exposed to 25 ppm sulfur dioxide for 7 hours/day on gestation days 6-15 (Murray et al.

1979). Increased numbers of skeletal variations, such as non-ossified sections of frontal bones, fusion of

occipital and parietal bones, and extra ribs, were also observed in the offspring of 20 rabbits exposed to

70 ppm sulfur dioxide for 7 hours/day on gestation days 6-l 8 (Murray et al. 1979). Decreased food intake

was the only sign of maternal toxicity in mice and rabbits.
SULFUR DIOXIDE 46

2. HEALTH EFFECTS

2.2.1.7 Genotoxic Effects

Assays of clastogenic effects in humans following occupational exposure to sulfur dioxide via inhalation

show mostly positive results. Increases in chromosome aberrations and sister chromatid exchanges were

detected in lymphocytes from 42 workers at an Indian fertilizer plant who were exposed to an average

concentration of 41.7 mg/m3

 (15.92 ppm) of sulfur dioxide (Yadav and Kaushik 1996). Similar findings

were observed in another study of 40 workers exposed to sulfur dioxide in a Chinese sulfuric acid factory

(Meng and Zhang 1990a). Exposure concentrations ranged from 0.34 to 11.97 mg/m3 (0.13 to 4.57 ppm).

In addition, increases in the frequencies of lymphocytes with micronuclei were noted (Meng and Zhang

1990b). Confounding exposures were not discussed in these studies” A significant increase in the frequency

of chromosomal aberrations was found among 19 workers at a sulfite pulp factory (Nordenson et al. 1980).

Exposure concentrations were not reported and workers were also exposed to chlorine and dust. One study of

potential chromosomal abnormalities in workers exposed to sulfur dioxide while working in the aluminum

industry revealed that sulfur dioxide did not cause an effect (Sorsa et al. 1982).

No studies were located regarding genotoxic effects in animals after inhalation exposure to sulfur dioxide.

Other genotoxicity studies are discussed in Section 2.5.

2.2.1.8 Cancer

There is no definitive evidence for an increased cancer potential from sulfur dioxide in humans. Several

epidemiological studies have been conducted on copper smelter workers and pulp and paper workers who can

be exposed to sulfur dioxide (IARC 1992). However, the studies conducted in copper smelters focused

primarily on the association between arsenic exposure and cancer. It has also been difficult to separate the

potential effects of sulfur dioxide and arsenic exposures in the studies of copper smelter workers. Studies of

the potential carcinogenicity of sulfur dioxide are discussed in the following paragraphs.

A cohort study of 5,403 male copper smelter workers showed that respiratory cancer risks in workers

exposed to sulfur dioxide for 212 months were not significantly increased when controls for exposure to

arsenic were in place (Lubin et al. 1981). The concentration of sulfur dioxide was not specified in the study.

In a follow-up mortality study in the same cohort of smelter workers, a clear dose-response relationship

between arsenic exposure and respiratory cancer was demonstrated (Welch et al. 1982). Although an
SULFUR DIOXIDE 47

2. HEALTH EFFECTS

apparent relationship of mortality to sulfur dioxide exposure was observed, workers in the medium and high

sulfur dioxide exposure categories had higher exposures to arsenic than those in the low sulfur dioxide

exposure groups. The concentrations of sulfur dioxide associated with each exposure category were not

specified. The studies by Lubin et al. (1981) and Welch et al. (1982) are limited because exposures to sulfur

dioxide and arsenic could not be completely separated.

The results of a case-cohort study of mortality in workers from eight copper smelters in the United States

revealed that arsenic was the primary cause of lung cancer in copper smelter workers after adjusting for

cigarette smoking and sulfur dioxide exposure (Enterline et al. 1987).

A nested case-control study of lung cancer among 308 workers in a large chemical facility revealed

significantly elevated risks for workers with moderate and high potential exposure (≥1 year) to sulfur dioxide

(Bond et al. 1986). For workers who had been exposed to sulfur dioxide, the odds ratio for lung cancer

was 1.40. Application of multivariate analyses showed a significant trend (p=0.003) of increasing lung

cancer risk associated with increasing intensity of sulfur dioxide exposure for the comparison with the

decedent controls. The odds ratios for lung cancer were 0.48 for low exposure, 1.69 for moderate exposure,

and 1.45 for high exposure. No trend was apparent for the comparison with the living controls. Also, the

risk of lung cancer did not increase with duration of exposure to sulfur dioxide. Confounding effects from

exposure to numerous chemicals was also possible. Products manufactured at the facility included

chlorinated solvents, plastics, chlorine, caustic soda, ethylene, styrene, epoxy, latex, magnesium metal,

chlornitrogen products, and glycols. Employees were exposed to an average of 7.5 chemicals during their

career, and the most common exposures included chlorine, sulfur dioxide, hydrogen chloride, and carbon

tetrachloride. The study authors indicated that the findings must be interpreted with caution because such

findings have not been reported among other workers with similar exposures.

The relationship between ambient air pollution (including sulfur dioxide) and lung cancer has been examined.

Lung cancer cases (2,439 males, 765 females) were identified in Helsinki, Finland, and standardized

incidence ratios were calculated for 33 subareas of Helsinski for 1975-1978, 1979-1982, and 1983-1986

(Ponka et al. 1993). Mean annual concentrations of sulfur dioxide were 0.005-0.008 ppm. After adjustment

for age, sex, and level of education, the lung cancer risk increased slightly, but nonsignificantly, with

increasing sulfur dioxide concentration. Lung cancer was 1.3% higher in the subareas with the highest sulfur

dioxide concentrations (≥ 0.008 ppm) compared with the subareas with the lowest concentrations

(<.005 ppm). There was no consistent relationship between the concentration of nitrogen dioxide and the
SULFUR DIOXIDE 48

2. HEALTH EFFECTS

incidence of lung cancer. The study authors concluded that sulfur dioxide had little, if any, effect on the risk

of lung cancer.

A retrospective cohort study employing a Poisson regression model for time trends of mortality to detect

subtle effects of air pollution on lung cancer mortality in Japan was conducted (Tango 1994). The trend of

mortality in females, aged 40-79, was examined in 23 wards of the Tokyo metropolitan area. The size of the

cohort was not specified. The ward-specific time trend of nitrogen dioxide and sulfur dioxide concentrations

for the years 1972 through 1988 was estimated. The result of this study showed a statistically significant

(p=0.0055) association between exposure to nitrogen dioxide and an increased trend of lung cancer mortality.

The association for sulfur dioxide exposure was nonsignificant (p=0.0655).

One chronic-duration animal study investigated the potential carcinogenicity of inhaled sulfur dioxide in mice

(Peacock and Spence 1967). An experimental group of 35 male and 30 female mice was exposed to 500 ppm

sulfur dioxide for 5 minutes/day, 5 days/week, for 2 years. The control group consisted of 41 males and

39 females. Female mice exposed to sulfur dioxide exhibited a significant increase in the incidence of lung

tumors (13/30 adenomas and carcinomas versus 5130 in controls; 4/30 primary carcinomas versus none in the

controls). The incidence of lung adenomas and carcinomas was also higher in the treated males (15/28 versus

11/35 in controls), but the increase was not significant. The incidence of primary carcinomas in treated males

was similar to that of the controls. These data provide limited evidence for the carcinogenicity of sulfur

dioxide in the mice. However, the determination of carcinogenic potential is complicated by study

limitations. A dose-response relationship could not be assessed because multiple doses were not tested. Due

to the small group sizes, it cannot be concluded that increased tumor incidences did not result from chance

alone. Quality studies in additional species are reqluired before the carcinogenicity status of the compound

can be determined.

2.2.2 Oral Exposure

2.2.2.1 Death

No studies were located regarding death in humans or animals after oral exposure to sulfur dioxide.
SULFUR DIOXIDE 49

2. HEALTH EFFECTS

2.2.2.2 Systemic Effects

No studies were located regarding respiratory, cardiovascular, gastrointestinal, hematological,

musculoskeletal, hepatic, renal, endocrine, dermal, ocular, body weight, or other systemic effects after oral

exposure to sulfur dioxide.

No studies were located regarding the following health effects in humans or animals after oral exposure to

sulfur dioxide:

2.2.2.3 Immunological and Lymphoreticular Effects

2.2.2.4 Neurological Effects

2.2.2.5 Reproductive Effects

2.2.2.6 Developmental Effects

2.2.2.7 Genotoxic Effects

Genotoxicity studies are discussed in Section 2.5.

2.2.2.8 Cancer

No studies were located regarding cancer in humans or animals after oral exposure to sulfur dioxide.

2.2.3 Dermal Exposure

2.2.3.1 Death

No studies were located regarding death in humans or animals after dermal exposure to sulfur dioxide.

2.2.3.2 Systemic Effects

No studies were located regarding respiratory, cardiovascular, gastrointestinal, hematological,

musculoskeletal, hepatic, renal, endocrine, body weight, or other systemic effects in humans or animals after

dermal exposure to sulfur dioxide.
SULFUR DIOXIDE 50

2. HEALTH EFFECTS

Dermal Effects. Sulfur dioxide is a severe irritant to the skin (Department of Labor 1975). Sulfur dioxide

is a liquid under pressure at 0°C. On the skin, the liquid produces burns from the freezing effect of rapid

evaporation (Department of Labor 1975).

No studies were located regarding dermal effects in animals after dermal exposure to sulfur dioxide.

Ocular Effects. Sulfur dioxide is a severe irritant to the eyes (Department of Labor 1975). The eye

irritation level is 10 ppm. Exposure of the eyes to liquid sulfur dioxide from pressurized containers can cause

cornea1 burns and opacification, resulting in loss of vision It has been reported that ocular damage from

exposure to liquid sulfur dioxide is not due to freezing but is characteristic of chemical burns (Grant 1974).

Sulfurous acid, which is formed when sulfur dioxide contacts moist surfaces, appears to be the primary cause

of eye injury.

Application of pure sulfur dioxide gas (room temperature and atmospheric pressure) directly to the eyes of

rabbits for 5 seconds has produced severe damage to the cornea and conjunctiva, similar to effects observed

with acid burns (Grant 1974). Opacity of corneas was noted immediately after and 6 months following

exposure.

No studies were located regarding the following health effects in humans or animals after dermal exposure to

sulfur dioxide.

2.2.3.3 Immunological and Lymphoreticular Effects

2.2.3.4 Neurological Effects

2.2.3.5 Reproductive Effects

2.2.3.6 Developmental Effects

2.2.3.7 Genotoxic Effects

Genotoxicity studies are discussed in Section 2.5.
SULFUR DIOXIDE 51

2. HEALTH EFFECTS

2.2.3.8 Cancer

No studies were located regarding cancer in humans or animals after dermal exposure to sulfur dioxide.

2. 3 TOXICOKINETICS

Sulfur dioxide, a highly water-soluble gas, is rapidly absorbed by the mucosa of the nose and upper

respiratory tract. Absorption in the lower respiratory tract is increased with enhanced ventilation associated

with a transition from nasal to oronasal breathing. Upon contact with moist mucous membranes, sulfur

dioxide is hydrolyzed to sulfites which are taken up by the blood and readily distributed throughout the body.

Sulfites can either be oxidized to sulfates by sulfite oxidase, primarily in the liver, or they can react with

proteins to form S-sulfonate. Sulfates are excreted in the urine.

2.3.1 Absorption

2.3.1.1 Inhalation Exposure

Studies in humans have shown that sulfur dioxide, a highly water-soluble gas, is rapidly absorbed by the

mucosa of the nose and upper respiratory tract (Kleinman 1984; Speizer and Frank 1966). Absorption of

sulfur dioxide in the upper respiratory tract is more efficient during nose breathing than during mouth

breathing (WHO 1979). Absorption of sulfur dioxide by the mucosa of the lower respiratory tract is

minimal because of minimal delivery to this region, but is increased with increased ventilation associated with

a transition from nasal to oronasal breathing at a mean minute ventilation of 30 L/mm (EPA 1986d).

Studies in animals indicate that sulfur dioxide is rapidly absorbed by mucosa following inhalation exposure.

In rabbits exposed to 100,200, or 300 ppm sulfur dioxide, 90-95% absorption of sulfur dioxide by tissues in

the upper respiratory tract was observed (Dalhamn and Strandberg 1961). The rate of absorption of sulfur

dioxide by tissues in the nasal cavity was found to be higher than that of tissues of the mouth or pharynx

(Dalhamn and Strandberg 1961). A subsequent study in rabbits revealed that absorption of sulfur dioxide is

dependent on concentration. At high concentrations (≥100 ppm), sulfur dioxide absorption in tissues of the

respiratory tract was ≥90%, while at low concentrations (≤0.1 ppm), absorption was approximately 40%

(Strandberg 1964). Studies in dogs have supported the findings that sulfur dioxide is readily absorbed by
SULFUR DIOXIDE 52

2. HEALTH EFFECTS

mucosa in the upper respiratory tract and that the nose is more efficient than the mouth in removing sulfur

dioxide (Balchum et al. 1959, 1960a; Frank et al. 1969).

Studies in humans have indicated that 12-15 % of sulfur dioxide absorbed on nasal mucosa is desorbed and

exhaled (Speizer and Frank 1966). The remaining sulfur dioxide metabolites are absorbed into the systemic

circulation or are delivered to the lower respiratory system by repeated absorption and desorption from

mucosa (Frank et al. 1969). Systemic absorption of sulfur dioxide metabolites from tissues of the upper

respiratory tract has been demonstrated in animals. In dogs a small segment of trachea was isolated and

perfused with radiolabeled sulfur dioxide (35SO2) while the lungs were ventilated with air to prevent entry of

the 35S02 (Balchum et al. 1960a). Detection of 35S in lungs, liver, spleen, and kidneys indicated systemic

absorption from the tracheal mucosa. It appears that systemic absorption is more efficient from the lower

respiratory tract. Blood and organ levels of 35S were compared in dogs following oronasal or tracheal

exposure to 35SO2 (Balchum et al. 1959). Following oronasal exposure, levels of 35S were lower in the blood

and several organs including the liver, spleen, and lymph nodes. In most dogs the level of 35S remained

steady, thus suggesting a slow continued absorption.

2.3.1.2 Oral Exposure

No studies were located regarding absorption of sulfur dioxide after oral exposure in humans or animals.

2.3.1.3 Dermal Exposure

No studies were located regarding absorption of sulfur dioxide after dermal exposure in humans or animals.

2.3.2 Distribution

2.3.2.1 Inhalation Exposure

S-sulfonate (R- S- SO3), which forms from the reaction of the sulfur dioxide metabolite, sulfite, and proteins,

was measured in the plasma of human subjects following continuous exposure to 0.3, 1.0, 3.0,4.2, or

6.0 ppm sulfur dioxide in a chamber for periods of up to 120 hours (Gunnison and Palmes 1974). Plasma

levels of S-sulfonate showed a positive correlation with air concentrations of sulfur dioxide.
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2. HEALTH EFFECTS

The results of studies in dogs indicate that absorbed sulfur dioxide metabolites are taken up by the blood and

are readily distributed throughout the body (Balchum et al. 1960a; Frank et al. 1967; Yokoyama et al. 1971).

The results of a study in dogs exposed to 22±2 ppm radiolabelled sulfur dioxide (35SO,) for 30-60 minutes

showed that the radioactivity levels appeared in blood within 5 minutes of the onset of exposure. It was

estimated that from 5% to 18% of the radioactivity administered to dogs was contained in the blood by the

end of exposure (Frank et al. 1969). In a subsequent study in dogs designed to further examine the

distribution of radioactivity in the blood, blood-radioactivity levels increased progressively when the isolated

upper airways were exposed to 35SO2 for 30-60 minutes (Yokoyama et al. 1971). Blood-radioactivity levels

decreased slightly during a postexposure period that lasted up to 3 hours. The radiolabel was more

concentrated in the plasma than in the red blood cells. Also, approximately one-third of the radioactivity in

plasma was associated with proteins, especially y-globulins. About two-thirds of the radioactivity associated

with the red blood cells was intracellular.

The tissue distribution of radioactivity has been studied in dogs administered 35SO2 as a gas to either the

intact upper airways or to an isolated segment of the trachea (Balchum et al. 1959, 1960a, 1960b). In dogs

that breathed 0.47-148 ppm 35SO2, via a tracheal canuula, the radioactivity levels were highest in the trachea

and lungs, followed by the hilar lymph nodes, kidneys, and esophagus (Balchum 1960b). A lower

concentration of radiolabel was noted in other tissues such as the heart muscle, liver, spleen, striated muscle,

brain, ovaries, stomach, pancreas, eye, skin, and submaxillary gland. Experiments in dogs in which 35SO2

was inhaled through the nose and mouth at concentrations of 1-141 ppm, showed that a large proportion was

deposited in the upper airways (Balchum et al. 1959,1960a). Also, dogs breathing 35SO2 through the nose

and mouth retained a smaller portion of the radiolabel in the trachea, lungs, hilar lymph nodes, liver, and

spleen than those breathing similar concentrations via a tracheostomy (Balchum et al. 1959).

Following oronasal inhalation of radiolabeled sulfur dioxide by dogs, 0.01-0.18% of the 35S dose was

measured in ovaries and 0.006-0.1% was measured in testicles (Balchum et al. 1959).

2.3.2.2 Oral Exposure

No studies were located regarding distribution of sulfur dioxide after oral exposure.
SULFUR DIOXIDE 54

2. HEALTH EFFECTS

2.3.2.3 Dermal Exposure

No studies were located regarding distribution of sulfur dioxide after dermal exposure.

2.3.3 Metabolism

Following absorption, inhaled sulfur dioxide dissolves on the walls of the moist airways producing a mixture

of sulfite, bisulfite, and hydrogen ions (Gunnison et al. 1987). The formation of sulfite (SO32-), bisulfite

(HSO3-), and hydrogen ions occurs in accordance with the equilibria depicted below:

+H +H+

SO2 + H2O ↔ H2SO3↔ HSO3- ↔ SO3

2-

-H+ -H+

pKa = 1.8 pKa = 7.2

Once formed, sulfite can be oxidized to sulfate, a reaction catalyzed by sulfite oxidase which occurs primarily

in the liver (Ellenhorn and Barceloux 1988; Gunnison et al. 1987). Sulfite oxidase is present in mitochondria

and has been detected in most tissues (Cabre et al. 1990). High sulfite oxidase activity has been measured in

the liver, kidney, and heart, but the brain, spleen, lungs, and testis have been found to have low sulfite oxidase

activity. Low pulmonary sulfite oxidase activity may explain the findings of a study in which the distribution

of sulfite from inhaled sulfur dioxide was restricted largely to the major airways of the lung (Gunnison et al.

1987).

Experiments utilizing sulfite oxidase-competent rats have demonstrated an absence of sulfite in the plasma of

rats following inhalation exposure to sulfur dioxide (Gunnison et al. 1987). Decreased activity of sulfite

oxidase in sulfite oxidase-deficient rats results in higher in vivo concentrations of sulfite. S-sulfonate is the

primary chemical form of absorbed sulfur dioxide in the bloodstream (Sheppard 1988; Yokohama et al.

1971).

Sulfite oxidase activity was found to be lower in the livers of young versus mature rats (Cohen 1974). In one

day-old rats, sulfite oxidase activity was approximately 1/10 the level of adult rats. Activity increased as the

rats matured and at 32 days of age, the liver sulfite oxidase activity was approximately one half the level of
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2. HEALTH EFFECTS

adult rats. The efficiency of the sulfite oxidation reaction depends primarily on the activity of sulfite oxidase

(Gunnison and Palmes 1976). However, detoxification of sulfites is not solely dependent on sulfite oxidase

activity. It has been demonstrated that rabbits clear injected sulfites at a faster rate than rhesus monkeys, but

that the in vitro activity of sulfite oxidase in the primary metabolizing organs is greater for monkeys than

rabbits (Gunnison et al. 1977). The study indicates that metabolism of sulfites is dependent on factors in

addition to sulfite oxidase activity.

Sulfites can react and become associated with disulfide bonds of plasma proteins, resulting in the formation

of S-sulfonates (Gunnison et al. 1987). Further metabolism and subsequent elimination of S-sulfonates is not

known However, it has been postulated that disulfide bonds may reassociate resulting in the release of

sulfite (Gumnison and Palmes 1973). Another possibility is that glutathione may react with S-sulfonates to

either separate S-sulfonates from proteins or form mixed disulfides and sulfites. It is expected that sulfites

produced from such reactions would then be metabolized by sulfite oxidase. Reaction of sulfites with plasma

proteins may protect tissues against injury, but may also prolong exposure to very low levels of sulfite

(Gunnison and Benton 1971).

There is evidence of age related differences in tbe metabolism of sulfite to sulfate and in the formation of a

sulfur trioxide radical intermediate. Levels of sulfur trioxide radicals and sulfite oxidase activity were

measured in polymorphonuclear leukocytes (PMN’s) obtained from healthy young adults (average age 25),

healthy older adults (average age 65), 3 centenarians (≥100 years), and 3 Down’s syndrome patients

(Constantin et al. 1996). Significantly increased amounts of sulfur trioxide radicals were observed in PMN’s

from healthy adults who had low sulfite oxidase activity. In centenarians and Down’s syndrome patients,

generation of the sulfur trioxide radical appeared to be the primary mechanism for detoxification of sulfite,

and the sulfur trioxide radical formation was not correlated with the sulfite oxidase activity level.

Sulfur dioxide is excreted primarily in the urine as sulfate (Yokoyama et al. 197 1). Increased levels of

sulfates have been detected in the urine of dogs (Yokoyama et al. 1971) and humans (Savic et al. 1987)

exposed to sulfur dioxide.

Decreased glutathione levels in the lungs of rats exposed to sulfur dioxide suggest that glutathione may be

involved in the detoxification process (Langley-Evans et al. 1996). In vitro experiments have demonstrated

that sulfites, metabolites of sulfur dioxide, react with reduced glutathione to form S-sulfoglutathione in a

reaction which is catalyzed by thiol transferase (Kagadel et al. 1986). Conversion of S-sulfoglutathione by
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2. HEALTH EFFECTS

γ-glutamyltranspeptidase yields S-sulfocysteinylglycine which is hydrolyzed to S-sulfocysteine by renal

peptides. S-sulfoglutathione has been detected in lenses and intestinal mucosa of animals and S-sulfocysteine

has been observed in body fluids.

2.3.4 Elimination and Excretion

2.3.4.1 Inhalation Exposure

The excretion of inhaled sulfur dioxide has been studied in dogs. In dogs exposed by inhalation to 35SO2, 35S

was excreted primarily in the urine as sulfate (Yokoyama et al. 197 1). An average of 84.4% of the urinary

radioactivity was in the form of inorganic sulfate, while 92.4% was present as total sulfate (Yokoyama et al.

1971). In dogs exposed to 35SO2, the rate of excretion of the radiolabel by the kidney has been shown to be

approximately proportional to the level of radiolabel in plasma and whole blood (Frank et al. 1967;

Yokoyama et al. 1971).

In humans it is estimated that 12-15% of sulfur dioxide absorbed to mucous membranes is desorbed and

exhaled (Speizer and Frank 1966). In an experimental study in which the airways were surgically isolated in

anesthetized dogs, 35S was detected in expired gas samples following exposure to 22 ppm 35SO2 (Frank et al.

1967). The concentration of 35SO2 in the expired gas samples was low, representing 1% of the exposure

level. The study authors stated that the lungs were releasing the gas during expiration, presumably from

pulmonary capillaries.

2.3.4.2 Oral Exposure

No studies were located regarding the elimination and excretion of sulfur dioxide after oral exposure.

2.3.4.3 Dermal Exposure

No studies were located regarding the elimination and excretion of sulfur dioxide after dermal exposure.
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2. HEALTH EFFECTS

2.3.5 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 et al. 1987; Andersen and

Krishnan 1994). 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 parametrization, (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 (Ktishnan and Andersen 1994; Leung 1993).

PBPK models for a particular substance require estimates of the chemical substance-specific 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.

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) is adequately described,

however, this simplification is desirable because data are often unavailable for many biological processes. A
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2. HEALTH EFFECTS

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 2-2 shows a

conceptualized representation of a PBPK model

At the current time, PBPK models for sulfur dioxide have not been developed.

2.4 MECHANISMS OF ACTION

2.4.1 Pharmacokinetic Mechanisms

Sulfur dioxide, a water-soluble gas, is readily absorbed through the upper respiratory tract of both animal

species and humans. Once absorbed, sulfur dioxide is mostly metabolized in the liver by sulfite oxidase

(Ellenhorn and Barceloux 1988). Sulfites are readily distributed throughout the body. Although significant

species-specific differences in sulfite oxidase activity in the liver and kidney have been reported (Tejnorova

1978), potential species-specific differences in the activity of this enzyme in the respiratory tract (the target

of sulfur dioxide toxicity in both humans and animals) have not been reported. Sulfur dioxide is excreted in

the urine as sulfate.

2.4.2 Mechanisms of Toxicity

Sulfur dioxide-induced increase in airway resistance is due to reflex bronchoconstriction (Frank et al. 1962;

Nadel et al. 1965). The reflex nature of bronchoconstriction during inhalation of sulfur dioxide has been

demonstrated in humans and in cats (Nadel et al. 1965). Injection of atropine prevented the increase in

airway resistance in healthy subjects exposed by inhalation to 4-6 ppm sulfur dioxide for 10 minutes. In

another study, atropine increased airway conductance in 11 healthy subjects exposed to 8 ppm sulfur dioxide

for 3 minutes but had no effect on 4 asthmatics (Snashall and Baldwin 1982). The finding suggested minimal

contribution of cholinergic mechanisms in sulfur dioxide-induced bronchoconstriction in asthmatics. In

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2. HEALTH EFFECTS

anesthetized, paralyzed, artificially ventilated cats, sulfur dioxide increased pulmonary resistance, an effect

prevented by complete cold block of the cervical vagosympathetic nerves or by the injection of atropine.

Studies indicate that sulfur dioxide-induced bronchoconstriction in cats arises from the activation of a

muscarinic (cholinergic) reflex via the vagus nerves (Nadel et al. 1965; Sheppard 1988). However, there is

some evidence from studies of human asthmatics for the existence of both muscarinic and nonmuscarinic

components (Sheppard 1988), with the nonmuscarinic component possibly involving an effect of sulfur

dioxide on airway mast cells.

Induction of sulfur dioxide-induced bronchoconstriction by non-cholinergic mechanisms has been

demonstrated in humans. The role of prostaglandins in sulfur dioxide-induced bronchoconstriction was

investigated by measuring airway responses in asthmatic subjects administered indomethacin, a prostaglandin

synthetase inhibitor (Field et al. 1996). A small but significant reduction in airway responsiveness to a sulfur

dioxide challenge was noted following administration of indomethacin, thus suggesting a minor role of

grostaglandins.

Leukotrienes, which are released by mast cells, may also contribute to sulfur dioxide-induced

bronchoconstriction. Administration of zafirlukast, a leukotriene receptor antagonist, to asthmatics resulted in a

significant decrease in sulfur-dioxide induced airway responsiveness in 10 out of 12 subjects (Lazarus et al.

1997).

Evidence of noncholinergic mechanisms of sulfur dioxide-induced bronchoconstriction has also been obtained

in various animal studies. In guinea pigs partial decreases in response to a sulfur dioxide challenge were

noted in 5 out of 7 animals following atropine administration, but results were not significant (Hajj et al.

1996). Results suggested that cholinergic mechanisms may play a minor role in sulfur dioxide responses.

However, tachykinins, neuropeptides which are present in the C-fibers of the vagus nerve, were demonstrated

to play a larger role in sulfur dioxide-induced bronchoconstriction. Blocking of tachykinin receptors with

antagonists, primarily the neurokinin-2 receptor, resulted in a significant attenuation of sulfur dioxideinduced

bronchoconstriction in guinea pigs.

Afferent impulses in vagal fibers of the lower respiratory tract were measured in dogs exposed to

200-500 ppm sulfur dioxide for 2 minutes (Roberts et al. 1982). Fifteen out of 34 vagal C-fibers were
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2. HEALTH EFFECTS

stimulated, and the impulses were consistent with sulfur dioxide-induced contraction of tracheal smooth

muscle.

It has been postulated that treatment of neonatal rats with capsaicin to eliminate tachykinins through the

destruction of C-fibers would result in an attenuation of sulfur dioxide-induced respiratory effects.

Responses in isolated lungs from capsaicin-treated and control guinea pigs were compared following

exposure to 250 ppm sulfur dioxide for 15 minutes (Bannenberg et al. 1994). Capsaicin treatment eliminated

the sulfur dioxide-induced release of tachykinins and significantly reduced bronchoconstriction. Minimal

bronchoconstriction was present in the lungs of the capsaicin-treated guinea pigs, thus suggesting effects

from additional, unknown mediators. However, different results were obtained in intact rats treated with

capsaicin shortly after birth (Long et al. 1997). Exposure to sulfur dioxide at 3 months of age resulted in

increased bronchonstriction and airway responsiveness in the capsaicin treated rats. Enhanced in vitro

responses of tracheal rings to cholinergic substances in the capsaicin group supported the theory that sulfur

dioxide effects are primarily mediated through cholinergic components. Another possible explanation is that

destruction of the C-fibers resulted in the loss of a protective mechanism in which tachykinins alter breathing

patterns to prevent the entry of irritants into the lungs.

The ability of tachykinins to alter breathing patterns in response to a respiratory irritant has been

demonstrated in rats. Acute exposure of rats to 5% sulfur dioxide through tracheal cannulas was

characterized by an immediate reduction in breathing rate (Wang et al. 1996). However, the effect was no

longer noted following treatment of the vagal nerves with capsaicin to destroy C-fibers or following a

vagotomy.

The involvement of a cholinergic component in the alteration of breathing patterns following exposure to

sulfur dioxide has also been demonstrated. Mice (4/group) who breathed various concentrations of sulfur

dioxide between 17-298 ppm experienced a rapid reduction in breathing rate at each concentration (Alarie et

al. 1973). When the mice were exposed to the same concentrations via a tracheal cannula, the respiratory

rate was unaffected. It was therefore concluded that respiratory suppression occurs through the stimulation of

cholinergic trigeminal nerve endings in the nasal mucosa.

In conclusion, sulfur dioxide-induced bronchoconstriction and respiratory inhibition appear to be mediated

through vagal reflexes by both cholinergic and non-cholinergic mechanisms. Non-cholinergic components

include but are not limited to tachykinins, leukotrienes, and prostaglandins. The extent to which cholinergic
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or non-cholinergic mechanisms contribute to sulfur dioxide-induced effects is not known and may vary

between asthmatic and healthy individuals and between animal species.

The chemical mechanism underlying the bronchoconstrictive effect of sulfur dioxide has been examined

(Sheppard 1988). Sulfur dioxide can dissolve in water to form bisulfite ion, sulfite ion, and hydrogen ion.

Studies on the bronchoconstrictor effects of inhaled sulfur dioxide and acidic and basic sulfite solution

suggest that sulfite ion is not likely to mediate sulfur dioxide-induced bronchoconstriction. However, the

bisulfite ion, which is present at a ratio of 5:1 (bisulfite:sulfite) at physiologic pH and more chemically

reactive than sulfite, might mediate the bronchoconstrictor effect. Bisulfite is a nucleophile that can react

with many biomolecules by substitution at electrophilic sites (Sheppard 1988). Such reactions can cause

disruption of disulfide bonds It has been postulated that bisulfite formed at the airway surface during

inhalation of sulfur dioxide may initiate bronchoconstriction by disrupting disulfide bonds present in tissue

proteins (Sheppard 1988). Hydrogen ion production is not considered a likely factor since the concentration

of hydrogen ions generated by inhaled sulfur dioxide is regarded to be insufficient to cause

bronchoconstriction (Sheppard 1988).

Possible mechanisms for the augmentation of sulfur-dioxide bronchoconstriction by cold, dry air have been

suggested (Sheppard et al. 1984). Dry air may contribute to bronchoconstriction through heat loss and

subsequent airway cooling. Increased osmolarity from water evaporation in airways is another possible

mechanism. Induction of bronchoconstriction through the inhalation of hypertonic aerosols has been

demonstrated. It is also likely that drying of the upper airways decreases sulfur dioxide absorption in those

areas, thus allowing greater concentrations to reach the lower airways.

Bronchial hyperactivity can develop after a single exposure to a very high concentration of sulfur dioxide

(Brooks et al. 1985; Harkonen et al. 1983). Respiratory injury includes epithelial cell injury and destruction,

airway mucosal edema and inflammation, and airway smooth muscle bronchospasm. This hyperactivity has

been termed reactive airway dysfunction syndrome (RADS) (Brooks 1992; Brooks et al. 1985; Goldstein

et al. 1979). Subjects with RADS react positively to methacholine challenge. The bronchial epithelial

damage results in nonspecific hyperesponsiveness to a wide range of other irritant stimuli.

Inhalation exposure to sulfur dioxide has been shown to alter mucus secretion in animals (Sheppard 1988).

There is some evidence that this effect may be mediated by a vagal reflex (Sheppard 1988). Support for this

mechanism has been obtained from a study in dogs which showed that a sulfur dioxide-induced increase in
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submucosal gland secretion can be abolished by vagal cooling (Sheppard 1988). The mechanism differs from

that of acidic aerosols in which toxicity is dependent on hydrogen ion content (Holma 1985). In vitro studies

have shown that mucus viscosity increases when its pH is lowered below approximately 7.4. The available

literature indicates that mucus of asthmatics has a lower pH and that toxicity of acid aerosols is related to a

reduction in the buffering and hydrogen ion absorption capacity of the mucus. There is no evidence which

indicates that sulfur dioxide-induced changes in mucous secretion are dependent upon hydrogen ion

concentration.

Mechanisms which could contribute to clastogenic effects in human lymphocytes (Meng and Zhang 1990a,

1990b; Nordenson et al. 1980; Yadav and Kaushik 1996) have been discussed (Shapiro et al. 1977).

Sulfites, metabolites of sulfur dioxide, can inhibit DNA synthesis and induce chromosomal aberrations in

human lymphocytes (Yadav and Kaushik 1996). Bisulfite has been shown to deaminate cytosine in bacteria,

a reaction which can lead to mutations by the replacement of Guanine-Cytosine (GC) sites with AdenineThymine (AT). Bisulfite can also catalyze the crosslinking of proteins and nucleic acids, such as DNA and

histones (proteins to which the DNA is bound). Lastly, numerous free radicals are generated during the

oxidation of bisulfite to sulfate, and interaction of the free radicals with nucleic acids is possible. Though

many of these mechanisms have been demonstrated in vitro, it has yet to be verified that the mechanisms are

plausible in humans.

2.4.3 Animal-to-Human Extrapolations

The acute pulmonary effect of sulfur dioxide following inhalation exposure has been consistent in both

humans and animals. Dose-related increases in airway resistance have been demonstrated in guinea pigs,

dogs, and human subjects. There appears to be a good correlation between the dose-response in guinea pigs

and asthmatic subjects; therefore, the guinea pig is considered a good model that is predictive of the response

of asthmatics (Amdur et al. 1991).

2.5 RELEVANCE TO PUBLIC HEALTH

Overview

The inhalation route is the primary route of exposure to sulfur dioxide. In the atmosphere, sulfur dioxide

occurs with other air pollutants including sulfuric acid, sulfur trioxide, ozone, nitrogen dioxide, and
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2. HEALTH EFFECTS

particulates. Sulfur dioxide can undergo transformation to sulfur trioxide and sulfuric acid. Sulfur dioxide

tends to be a problem in urban industrial areas, particularly those where industrial activities utilize the

combustion of fuels.

Table 2-2 summarizes the health effects of sulfur dioxide. The results of controlled exposure studies in

humans have established that exposure to sulfur dioxide can result in lung function changes indicative of

bronchoconstriction. Stimulation of sensory receptors in the airways by irritants and chemicals such as sulfur

dioxide produces reflex bronchoconstriction that is mediated primarily via cholinergic pathways. Cool, dry

air and exercise also cause bronchoconstriction possibly through increased respiration associated with cooling

and drying of the airways (Sheppard et al. 1984). Exercising asthmatics, in particular, are sensitive to the

pulmonary effects of sulfur dioxide at concentrations as low as 0.1 ppm (Sheppard et al. 1981). It has been

estimated that asthmatics represent a subset of approximately 10 million people, or 4% of the population

(EPA 1994a, 1994b). The true prevalence of asthmatics though, may be as high as 7-10% of the population.

Responses to sulfur dioxide are variable among asthmatics. It is estimated that exposure to 0.2-0.5 ppm

sulfur dioxide during moderate exercise would lead to substantial respiratory effects, in only about l0-20%

of mild to moderate asthmatics, but the most sensitive individuals could likely experience incapacitating

effects (EPA 1994b). The susceptibility of asthmatics to sulfur dioxide-induced respiratory effects is not

clear. Severe asthmatics may actually be less prone to adverse respiratory effects because they are less likely

to exercise outdoors and more likely to take medication (EPA 1994b). Healthy nonasthmatic individuals can

show lung function changes following inhalation exposure to sulfur dioxide concentrations of ≥1.0 ppm.

Sulfur dioxide levels in the most polluted urban industrial areas can exceed 1 ppm. Clinical studies have

demonstrated that in adolescents and senior citizens, susceptibility to sulfur dioxide is determined primarily

by respiratory health status and not age (Koenig et al. 1982a, 1982b, 1983; Rondinelli et al. 1987).

Most clinical studies in humans have demonstrated statistically significant but subtle effects (i.e., in the

normal range) which are not considered pathological. Therefore the effects were classified as minimal

according to the ATSDR definition which describes minimal effects as “those that reduce the capacity of an

organ or organ system to absorb additional toxic stress but will not necessarily lead to the inability of the

organ or organ system to function normally.”

Sulfur dioxide was administered through a mouthpiece in many of the clinical studies. A mouth-only

exposure eliminates scrubbing by the nasal mucosa and likely results in the delivery of larger doses to the

lower airways. Mouth breathing is common during exercise and in individuals with blocked nasal passages,

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such as asthmatics. However, one group of investigators demonstrated similar results following oral and

oronasal exposure of asthmatics to sulfur dioxide (Bethel et al. 1985; Sheppard et al. 1981). A need has been

expressed for additional research in the distribution of inhaled air under various conditions (e.g., rest and

exercise) and in healthy versus asthmatic subjects (Sheppard et al. 1981).

Acute studies in animals have supported the human data on pulmonary effects of sulfur dioxide (Amdur

1959; Balchum et al. 1959, 1960b). Guinea pigs, in particular, are considered a suitable animal model

(Amdur 1966). The dose-response relationship in guinea pigs correlates well with that observed for

exercising asthmatics. Exposure to high concentrations of sulfur dioxide has been shown to result in

degenerative changes in the olfactory epithelium in mice (Min et al. 1994), cellular necrosis in the trachea and

bronchus of rats (Stratmann et al. 1991), nasal epithelial metaplasia and basal cell hyperplasia in rats (Fowlie

et al. 1990), bronchitic lesions in hamsters (Goldring et al. 1970), increased goblet cell numbers in rats

(Lamb and Reid 1968) and decreased respiratory rate and bronchopneumonia in rabbits (Miyata et al. 1990).

The association between mortality and/or lung function changes and acute exposures to ambient levels of

sulfur dioxide that have been examined in epidemiology studies is not clear. In the atmosphere, sulfur

dioxide occurs with other pollutants that may also be associated with similar toxicological effects. It has been

difficult to separate the potential effects of sulfur dioxide from those of other pollutants in epidemiological

studies.

The 1990 Amendments to the Clean Air Act require that emissions of sulfur dioxide be decreased. In

accordance with these Amendments, EPA has reviewed the human health effects data on this chemical and

has established National Ambient Air Quality Standards (NAAQS). The annual arithmetic mean NAAQS is

0.03 ppm, while the 24-hour limit is 0.14 ppm.

Minimal Risk Levels for Sulfur Dioxide

Inhalation MRLs

• An MRL of 0.01 ppm has been derived for acute-duration exposure (14 days or less) to sulfur dioxide.

This MRL is derived from the study by Sheppard et al. (1981) in which exercising mild asthmatics

were exposed to ≥0.1 ppm sulfur dioxide for 10 minutes. The two most sensitive subjects developed

slight bronchoconstriction after inhaling 0.1 ppm sulfur dioxide. These two subjects showed doserelated

increases in airway resistance after exposure to 0.1-0.5 ppm sulfur dioxide. Significant increases in airway

resistance were observed in other asthmatics exposed to ≥0.25 ppm sulfur dioxide
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during moderate exercise. Other studies in asthmatics have shown lung function changes at

concentrations as low as 0.25 ppm (Bethel et al. 1985; Horstman et al. 1986; Myers et al. 1986a,

1986b). A few studies have not reported respiratory effects in asthmatics at 0.25 ppm (Linn et al.

1983b, 1987,1990).

The dose level of 0.1 ppm sulfur dioxide can be considered a minimal LOAEL. This concentration was

divided by an uncertainty factor of 9 (3 for the use of a minimal LOAEL and 3 for human variability)

to yield a calculated MRL of 0.01 ppm. The uncertainty factor for human variability addresses varying

sensitivity among asthmatics and possible increased sensitivity in children. There is concern of

increased sensitivity in young children but there is not sufficient data to confirm it (See Section 2.6).

No intermediate-duration or chronic-duration MRLs were derived because the data are insufficient.

Oral MRLS

No oral MRLs were derived since data were not sufficient, and this is not a clinically relevant route of

exposure in humans.

Death. Several case studies of human deaths following acute inhalation exposures to high concentrations of

sulfur dioxide have been reported (Atkinson et al. 1993; Charan et al. 1979; Harkonen et al. 1983; Huber and

Loving 199 1; Rabinovitch et al. 1989). It is not possible, however, to draw accurate or definitive conclusions

from these reports as to the levels of exposure. Effects that were described in these reports included

asphyxia, secondary to passive pulmonary and visceral congestion, lungs filled with proteinaceous edema

fluid, and irreversible airway obstruction. A concentration of 100 ppm is considered immediately dangerous

to life and health (HSDB 1998). Excess mortality among the elderly or the chronically sick has been

observed in humans exposed to sulfur dioxide, smoke, and particulate matter during acute air pollution

episodes (WHO 1979). The deaths were due primarily to respiratory failure. Associations between daily

acute mortality and exposure to ambient levels of sulfur dioxide are difficult to assess because of confounders

such as the influence of other air pollutants.

In one limited acute study in the mouse, an acute LC50 of 3,000 ppm was reported (Hilado and Machado

1977). For rats (n=8/group) continuously exposed to 590, 925, 2350, 50,000, or 500,000 ppm sulfur

dioxide, average times of death were 1866,750, 176, <10 and <2 minutes respectively (Cohen et al. 1973).

The numbers of rats (n=8/group) which died within 2 weeks following exposure to 224,593,965, 1,168, or

1,319 ppm sulfur dioxide for 4 hours were 0, 0,3,5, and 8 respectively (Cohen et al. 1973).
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There are no reports of fatalities in humans or animals exposed by the oral route or solely by the dermal

route.

Systemic Effects

Respiratory Effects. Available data indicate that the respiratory system is the primary target system for

sulfur dioxide toxicity following inhalation exposure at low levels (EPA 1994a, 1994b; Sheppard et al.

198 1). Numerous controlled studies in humans have documented that acute-duration inhalation exposure to

sulfur dioxide causes constriction of the airways, particularly in asthmatics and other sensitive individuals

(Sheppard et al. 1981). Bronchoconstriction has been observed in asthmatics exposed to sulfur dioxide at

concentrations as low as 0.1 ppm (Sheppard et al. 1981). Studies have also shown that exercise enhances the

responsiveness to sulfur dioxide. Other factors that can exacerbate the respiratory effects of sulfur dioxide

include exposure to cold or dry air and other pollutants such as sulfuric acid, nitrogen dioxide, and ozone.

Healthy nonasthmatics respond to higher concentrations of sulfur dioxide (i.e., ≥1 .0 ppm) (EPA 1994a,

1994b).

Acute exposures to high concentrations of sulfur dioxide can result in severe and irreversible airway

obstruction and in edematous lung tissue (Charan et al. 1979; Harkonen et al. 1983). However, actual

concentrations of sulfur dioxide causing these effects have not been reported. Bronchitis has been reported

following brief, accidental exposures to 100 ppm sulfur dioxide (Skalpe 1964).

Epidemiological studies on the relationship between sulfur dioxide and respiratory effects are limited because

of the difficulties in separating potential effects of sulfur dioxide from those of particulates and other

pollutants. Some epidemiological studies of children reveal reversible lung function changes such as

decreases in FVC and FEV1 but no effects on respiratory performance were noted in other studies (Agocs et

al. 1997; Buchdahl et al. 1996; Dockery et al. 1982; Dodge et al. 1985; Shy et al. 1973). The effects of

particulates cannot be separated from those of sulfur dioxide. Likewise, although lung function changes were

reported in several occupational studies in which workers were also exposed to other chemicals such as

arsenic (Archer and Gillan 1978; Smith et al. 1977). No significant relationship between pulmonary function

and exposure to ≥5 ppm sulfur dioxide was noted in studies of potential lung function changes in copper

smelter workers (Lebowitz et al. 1979) and steel plant workers (Lowe et al. 1970). Breathing difficulties and

cough were reported by workers in a broom manufacturing facility who were exposed to 0.29-57 ppm sulfur

dioxide and also to high concentrations of dust (Savic et al. 1987). Irritation and shortness of breath upon
SULFUR DIOXIDE 69

2. HEALTH EFFECTS

exertion were reported by workers in a refrigeration unit plant who were exposed to sulfur dioxide levels of

5-100 ppm (Kehoe et al. 1932). Cough, sputum production, and difficulty breathing were reported by pulp

mill workers exposed to 2-36 ppm sulfur dioxide (Skalpe 1964).

Studies in experimental animals have indicated pulmonary effects of sulfur dioxide following inhalation

exposure. Increased airway resistance and decreased compliance were noted in groups of guinea pigs

exposed to 2.6 ppm sulfur dioxide for 1 hour (Amdur 1959). Increased pulmonary flow resistance was noted

in guinea pigs exposed to 0.16-835 ppm sulfur dioxide (Amdur 1966). Similar effects were observed in

dogs exposed to 1.1-141 ppm sulfur dioxide (Balchum et al. 1959). Acute-duration exposure to high

concentrations of sulfur dioxide can result in biochemical, clinical, and histological changes in the respiratory

systems of the mouse and rabbit. Degenerative changes in the olfactory epithelium were seen in mice

exposed to 20 ppm sulfur dioxide for 120 minutes (Min et al. 1994). Rats exposed to 25 ppm sulfur dioxide

for up to 5 days displayed nasal epithelial metaplasia and basal cell hyperplasia (Fowlie et al. 1990).

In an intermediate-duration study, mild bronchitic lesions were seen in hamsters exposed to 650 ppm sulfur

dioxide for 19-74 days (Goldring et al. 1970). Inflarmnation of the bronchial mucosa was observed in rats

exposed to 3040 ppm sulfur dioxide for 1 hour/day, 5 days/week, for 12 weeks (Krasnowska et al. 1998).

Nasopharyngitis and lipid peroxidation of lung tissue were observed in guinea pigs exposed to 10 ppm sulfur

dioxide for 1 hour/day for 30 days (Haider 1985). Increased numbers of goblet cells were observed in the

airways of rats exposed to 400 ppm sulfur dioxide for 3 hours/day, 5 days/week (Basbaum et al. 1990; Lamb

and Reid 1968).

The respiratory toxicity of sulfur dioxide following chronic-duration inhalation exposure has been studied in

animals. No effects on lung function were observed in guinea pigs exposed to 5.72 ppm sulfur dioxide for

22 hours/day, 7 days/week, for 52 weeks (Alarie et al. 1972). Likewise, no lung function changes or

histopathological alterations in the lung were observed in monkeys exposed to 5.12 ppm sulfur dioxide for

23.3 hours/day, 7 days/week, for 78 weeks (Alarie et al. 1975).

Cardiovascular Effects. Human, nonasthmatic subjects exposed by inhalation to 1-8 ppm sulfur dioxide for

acute durations showed increased pulse rate (Amdur et al. 1953). No studies were located regarding

cardiovascular toxicity after oral or dermal exposure.
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2. HEALTH EFFECTS

No evidence of histological lesions in the heart was found in monkeys following exposure to 5.12 ppm sulfur

dioxide for 23.3 hours/day, 7 days/week, for 78 weeks (Alarie et al. 1975). Likewise, no microscopic lesions

were detected in the hearts of guinea pigs exposed by inhalation to 5.72 ppm sulfur dioxide for 22 hours/day,

7 days/week, for 52 weeks (Alarie et al. 1972). Lipid peroxidation was observed in the hearts of guinea pigs

exposed to 10 ppm sulfur dioxide for 1 hour/day for 30 days (Haider 1985).

Gastrointestinal Effects. Nausea and vomiting were observed in humans exposed to >40 ppm sulfur dioxide

during an accident at a copper mine (Rabinovitch et al. 1989). No studies were located regarding

gastrointestinal effects after oral or dermal exposure. No studies were located regarding gastrointestinal

effects in animals.

Hematological Effects. Exposure to sulfur dioxide levels of 5-100 ppm did not appear to affect blood cell

numbers of workers in a refrigeration unit plant (Kehoe et al. 1932). Methemoglobin levels were increased in

the blood of workers exposed to 0.29-57 ppm sulfur dioxide in a broom manufacturing facility (Savic et al.

1987).

An increase in erythrocyte deformability was observed in rats exposed to 1 ppm sulfur dioxide for 24 hours

(Baskurt et al. 1990). Lipid peroxidation has been observed in the erythrocytes of rats exposed to 10 ppm

sulfur dioxide for 1 hour/day for 45 days (Etlik et al. 1997) or for 1 hour/day, 7 days/week, for 8 weeks

(Gumuslu et al. 1998). Osmotic hemolysis and sulfhemoglobin levels were increased in erythrocytes of rats

exposed to 0.9 ppm sulfur dioxide for 24 hours (Baskurt et all 1988). An increase in the erythrocyte

deformability index and lipid peroxidation was noted in the erythrocytes of guinea pigs exposed to 10 ppm

sulfur dioxide for 30 days (Dikmenoglu et al. 1991). No hematological effects were observed in a chronic

toxicity study of monkeys exposed to 5.12 ppm sulfur dioxide for 23.3 hours/day, 7 days/week, for 78 weeks

(Alarie et al. 1975). Likewise, no hematological effects were noted in guinea pigs exposed to 5.72 ppm

sulfur dioxide for 22 hours/day, 7 days/week, for 52 weeks (Alarie et al. 1972). However, statistically

significant increases in white and red blood cell counts, as well as hematocrit and hemoglobin levels were

observed in rats exposed to 10 ppm sulfur dioxide for 1 hour/day for 45 days (Etlik et al. 1997). Increased

hematocrit levels were also observed in rats exposed to 0.9 ppm sulfur dioxide for 24 hours (Baskurt et al.

1988).

Musculoskeletal Effects. No studies were located regarding musculoskeletal effects in humans or animals

after exposure to sulfur dioxide.
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2. HEALTH EFFECTS

Hepatic Effects. No adverse hepatic effects have been reported in humans exposed by inhalation to sulfur

dioxide No studies were located regarding hepatic effects in humans or animals after oral or dermal

exposure.

Information on hepatic effects in animals following inhalation of sulfur dioxide is very limited. In a chronic

toxicity study in monkeys, no histological lesions were detected in monkeys exposed to 5.12 ppm sulfur

dioxide for 23.3 hours/day, 7 days/week, for 78 weeks (Alarie et al. 1975). In contrast, microscopic

examination of guinea pigs exposed to 5.72 ppm sulfur dioxide for 22 hours/day, 7 days/week, for 52 weeks

showed an increase in the size of hepatocytes accompanied by cytoplasmic vacuolation (Alarie et al. 1972).

Liver triglyceride and esterfied cholesterol levels were increased in rats fed either a standard diet or a high

cholesterol diet and exposed to 10 ppm sulfur dioxide for 24 hours/day, 7 days/week, for 15 days (Lovati et

al. 1996). However, exposure of diabetic rats to 5 or 10 ppm sulfur dioxide under the same conditions

resulted in a significant reduction of liver triglyceride levels Liver cholesterol levels decreased in guinea pigs

exposed to 10 ppm sulfur dioxide for 1 hour/day for 30 days (Haider 1985).

Renal Effects. No adverse renal effects have been reported in humans exposed by inhalation to sulfur

dioxide. No studies were located regarding renal effects in humans or animals after oral or dermal exposure.

Information on renal effects in animals following inhalation of sulfur dioxide is very limited. No renal effects

were observed in a chronic toxicity study of monkeys exposed to 5.12 ppm sulfur dioxide for 23.3 hours/day,

7 days/week, for 78 weeks (Alarie et al. 1975). Likewise, no renal effects were noted in guinea pigs exposed

to 5.72 ppm sulfur dioxide for 22 hours/day, 7 days/week, for 52 weeks (Alarie et al. 1972).

Dermal Effects. Liquid sulfur dioxide from pressurized containers is a severe skin irritant in humans

(Department of Labor 1975). Skin irritation has been observed in rats and guinea pigs exposed to 10 ppm

sulfur dioxide for 1 hour/day for 30 days (Haider 1982, 1985).

Ocular Effects. In a case report of an industrial accident in which five persons were exposed by inhalation to

high concentrations (not specified) of sulfur dioxide for less than 5 minutes, reversible conjunctivitis and

superficial cornea1 burns were noted (Charan et al. 1979). In another case report dealing with a pyrite dust

explosion that resulted in nine persons being exposed to high levels of sulfur dioxide, conjunctival irritation

and cornea1 erosion were observed (Harkonen et al. 1983). The concentration of sulfur dioxide was not

measured, but, in experimental explosions, the concentration was estimated to be 30-1,600 ppm. Burning of
SULFUR DIOXIDE 72

2. HEALTH EFFECTS

the eyes was reported in a case study of an accident at a copper mine in which miners were exposed to

>40 ppm sulfur dioxide (Rabinovitch et al. 1989).

Sulfur dioxide gas is an eye irritant, causing burning and lacrimation (Department of Labor 1975). A

concentration of 20 ppm is considered definitely irritating to the eyes (Ellenhom and Barceloux 1988). Direct

application of sulfur dioxide gas to the eyes of rabbits has resulted in severe damage to the cornea and

conjunctiva, similar to that observed with chemical bums (Grant 1974).

Body Weight Effects- No body weight effects have been reported in humans following exposure to sulfur

dioxide.

Following 5 weeks of inhalation exposure to 70-300 ppm sulfur dioxide, body weight gains of treated

animals were 25% lower than those of controls (Miyata et al. 1990). No body weight effects were observed

in a chronic toxicity study of monkeys exposed to 5.12 ppm sulfur dioxide for 23.3 hours/day, 7 days/week,

for 78 weeks (Alarie et al. 1975). Likewise, no body weight effects were noted in guinea pigs exposed to

5.72 ppm sulfur dioxide for 22 hours/day, 7 days/week, for 52 weeks (Alarie et al. 1972) or in rats exposed

to 10 ppm sulfur dioxide for 1 hour/day, 7 days/week for 8 weeks (Gumuslu et al. 1998).

Endocrine Effects. No studies were located regarding endocrine effects in humans after inhalation

exposure to sulfur dioxide.

Significant dose-related decreases in plasma insulin levels were observed in rats exposed to 5 or 10 ppm

sulfur dioxide for 24 hours/day, 7 days/week, for 15 days, but significant effects on insulin levels were not

noted in diabetic rats exposed to sulfur dioxide under the same conditions (Lovati et al 1996).

Immunological and Lymphoreticular Effects. One study was located that examined the effect of

sulfur dioxide on the airway response of human subjects with mild asthma to inhaled house dust mite allergen

(Devalia et al. 1994). Prior inhalation of 0.2 ppm sulfur dioxide for 6 hours did not significantly affect the

airway response to inhaled antigen, as measured by the dose of allergen required to produce a 20% decrease

in forced expiratory volume in 1 second (PD20 FEV1). However, exposure to both 0.2 ppm sulfur dioxide and

0.4 ppm nitrogen dioxide significantly reduced the PD20 FEV1.
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2. HEALTH EFFECTS

Epidemiological studies have indicated that sulfur dioxide air pollution may increase the prevalence of

allergies (Soyseth et al. 1996) and incidence of respiratory infections (French et al. 1973) in children.

However, both studies were limited by inadequate control of confounding factors and a lack of personal

exposure data.

Sulfites are potent sensitizers, and anaphylaxis can occur from exposure to residues in food (IARC 1992;

Sheppard 1988; WHO 1979). Since sulfite-containing foods can off-gas sulfur dioxide, it is possible that

some of the effects of sulfite ingestion may be due to inhalation of sulfur dioxide (Sheppard 1988).

Increased sensitization to antigen was reported in a study of guinea pigs exposed by inhalation to sulfur

dioxide (Riedel et al. 1992). In this experiment, groups of 6 guinea pigs were exposed to air or 5 ppm sulfur

dioxide for 8 hours/day for 5 days, with intermittent inhalation of ovalbumin (8 mg aerosolized) in order to

sensitize the animals. The guinea pigs were also given two sensitizing injections of Canida albicans prior

to sulfur dioxide exposure. Animals were exposed to the ovalbumin for 45 minutes following sham or sulfur

dioxide exposure on days 3,4, and 5. Seven days after the last exposure, the guinea pigs were tested for

bronchial sensitization to ovalbumin. Antibodies against ovalbumin were measured before and after sham

and sulfur dioxide exposures. Sulfur dioxide-treated animals were sensitized to ovalbumin. Pretreatment

with anti-inflammatory drugs (methylprednisolone, indomethacin, or nedocromilsodium) for 6 days beginning

12 hours before the first sulfur dioxide exposure prevented the sensitization to ovalbumin. Exposure of

guinea pigs to 5 ppm sulfur dioxide for 4 hours/day, 5 days/week, for 6 weeks resulted in increased delayedtype

dyspneic symptoms (Kitabatake et al. 1995).

In an acute-duration inhalation study with hamsters, there was a significant reduction in endocytosis by

pulmonary macrophage following exposure to 50 ppm sulfur dioxide for 4 hours while exercising (Skornik

and Brain 1990). However, pulmonary defense mechanisms were not affected in rats and mice exposed to

0.32-0.43 ppm sulfur dioxide together with 87-113 μg/m3

 sulfate for 4 hours prior to or 17 hours following

infection with Staphylococcus aureus or Group C Streptococci (Goldstein et al. 1979).

Neurological Effects. Variations in reflex response time were observed in workers of a refrigeration unit

plant who were exposed to sulfur dioxide concentrations ranging from 5-100 ppm (Kehoe et al. 1932). The

effect was not consistent with neurological injury but may have been a psychological response to general

sulfur dioxide toxicity. Seizures and prostration were observed in rats following exposure to 2350, 50,000

and 500,000 ppm sulfur dioxide for an average of 176 minutes, <10 minutes, and <2 minutes respectively
SULFUR DIOXIDE 74

2. HEALTH EFFECTS

(Cohen 1973). Lipid peroxidation has been observed in the brains of guinea pigs exposed to sulfur dioxide

for 1 hour/day for 30 days (Haider et al. 1982). Transient changes in the frequency of certain behavioral

patterns, such as grooming and digging, were noted in male and female mice continuously exposed to 30 ppm

sulfur dioxide and observed during the first 9 days of a reproductive study. There was no evidence of

neurological effects in the offspring of the mice (Petruzzi et al. 1996).

Reproductive Effects. In a cross-sectional study of spontaneous abortions in an industrial community in

Finland, no evidence was found that exposure to sulfur dioxide was associated with a risk of spontaneous

abortions (Hemminki and Niemi 1982). A study conducted in the Czech Republic found that abnormal sperm

morphology was associated with sulfur dioxide, which was used as a surrogate for all air pollutants (Selevan

et al. 1995). Reproductive effects, as measured by completed pregnancies, litter size, sex ratio, and neonatal

mortality, were not observed in male and female mice continuously exposed to 5, 12, or 30 ppm sulfur

dioxide for 9 days prior to mating until days 12-14 of pregnancy (Petruzzi et al. 1996). The offspring were

not assessed for reproductive function.

Developmental Effects. An association between exposure to increased levels of air pollution during

pregnancy and a reduction in birth weight has been reported in humans (Wang et al. 1997). Decreased birth

weights were noted for exposure to both sulfur dioxide and total suspended particulates (TSPs). However

evaluation of these results is complicated by limitations such as confounding effects of various air pollutants

and a lack of personal exposure data. Therefore, differentiation of potential developmental effects associated

with individual pollutants of a mixture is not possible.

No developmental effects were observed in a study in which pregnant mice were exposed to 32-250 ppm

sulfur dioxide on gestation days 7-17 (Singh 1982). Neurological effects, measured by reflexes and learning

ability, were not observed in the offspring of mice continuously exposed to 5-30 ppm sulfur dioxide 9 days

prior to mating through pregnancy days 12-14 (Petruzzi et al. 1996). Although there were some transient

behavioral effects in dams, there were no signs of systemic maternal toxicity. In another developmental study

in mice in which pregnant females were exposed to 32 ppm or 65 ppm sulfur dioxide on gestation days 7-l 8,

increased time for righting reflex on postnatal day 1 and increased negative geotaxis on postnatal day 10 were

reported (Singh 1989). The duration of exposure for each day was not stated. No visible signs of maternal

toxicity and no effect on the number of live births were observed. Sulfur dioxide at a concentration of

65 ppm significantly decreased the birth weight (about 89% of controls) of the pups. Minor skeletal

variations were reported in offspring of mice exposed to 25 ppm sulfur dioxide for 7 hours/day on gestation
SULFUR DIOXIDE 75

2. HEALTH EFFECTS

days 6-l 5 and rabbits exposed to 70 ppm sulfur dioxide for 7 hours/day on gestation days 6-l 8 (Murray et

al. 1979). Fetal body weights were also lower in exposed mice. Reduced food intake was the only sign of

maternal toxicity.

Genotoxic Effects. There are some limited data showing clastogenic effects in humans following

occupational exposure to sulfur dioxide. Increases in chromosome aberrations and sister chromatid

exchanges were detected in lymphocytes from 42 workers of an Indian fertilizer factory who were exposed to

au average concentration of 41.7 mg/m3 (15.92 ppm) of sulfur dioxide (Yadav and Kaushik 1996).

Confounding factors were not discussed. Similar findings were observed in another study of 40 workers

occupationally exposed to sulfur dioxide at a Chinese sulfuric acid plant (Meng and Zhang 1990a). Exposure

concentrations ranged from 0.34 mgm3

 to 11.97 mg/m3 (0.13-4.57 ppm). In addition, increases in the

frequencies of lymphocytes with micronuclei were noted (Meng and Zhang 1990b). A significant increase in

the frequency of chromosomal aberrations was found among 19 workers at a sulfite pulp factory (Nordenson

et al. 1980). Exposure concentrations were not reported and workers were also exposed to chlorine and dust.

One study of potential chromosomal abnormalities in workers exposed to sulfur dioxide in the aluminum

industry revealed that sulfur dioxide was witbout effect (Sorsa et al. 1982).

Mutations were not observed in germ cells following in viva exposure of animals to sulfites (metabolites of

sulfur dioxide). In a dominant lethal test, mutations were not observed in offspring of male mice treated by

peritoneal injection with 300 mg/Kg sodium bisulfite on 38 out of 54 days or 400 mg/kg sodium bisulfite 5

days/week for a total of 20 treatments (Russell and Kelly 1975; Shapiro et al. 1977). A specific-locus test

was conducted in the same wild type mice from the 400 mg/kg treatment group, which were mated to females

with homozygous marker genes for seven loci (Russell and Kelly 1975). Mutations associated with

abnormalities in sperm were not observed in 13,568 offspring which were scored for mutations at seven loci.

Table 2-3 summarizes data on genotoxicity of sulfur dioxide in vivo.

Table 2-4 summarizes data on the genotoxicity of sulfur dioxide in vitro. Chromosomal effects were

observed following in vitro testing with sulfur dioxide. Lymphocytes from healthy donors were cultured and

treated with 100 cc of 5.7 ppm sulfur dioxide, which was bubbled through the culture media (Schneider and

Calkins 1970). Twenty percent of the hypotonic smears obtained from treated cells contained abnormalities

which were characterized primarily by clumping of chromosomes. Abnormalities were not observed in any of

the air-treated controls.


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2. HEALTH EFFECTS

Chromosomes from mouse, cow, and ewe oocytes appeared fuzzy following in vitro exposure to 250 μg/m3

sodium sulfite and fragmentation and clumping were observed in the ewe and cow oocytes (Jagiello et al.

1975). Meiosis was inhibited in mouse and cow oocytes treated with 150 and 500 μg/m3

 sodium sulfite

respectively. However, oocyte meiosis was not inhibited in mice administered 1-5 mg sodium sulfite

intravenously, thus supporting the conclusion that in vivo exposure of sulfur dioxide will not result in

genotoxic effects of germ cells (Jagiello et al. 1975).

Sulfur dioxide induced micronuclei in eukaryotic plants (Ma et al. 1973, 1984). Sulfur dioxide caused gene

mutation in Saccharomyces cerevisiae (Guerra et al. 198 1).

The available in vivo and in vitro data indicate a genotoxic potential for sulfur dioxide. The mechanism of

the potential genotoxicity of sulfur dioxide is unknown; however, a metabolite of sulfur dioxide, i.e., sulfite,

can cause an inhibition of DNA synthesis and induce chromosomal aberrations in human lymphocytes

(Yadav and Kaushik 1996).

Cancer. There are no definitive data in humans or animals that indicate a carcinogenic potential for sulfur

dioxide. Several epidemiological studies have been conducted on workers in the copper smelting and pulp

and paper industries in which exposure to sulfur dioxide can occur (Enter-line et al. 1987; IARC 1992; Lubin

et al. 198 1; Welch et al. 1982). However, many of the copper smelter studies focused primarily on the

association between arsenic exposure and respiratory cancer (Enterline et al. 1987; Lubin et al. 198 1; Welch

et al. 1982). It has been difficult to separate the potential effects of sulfur dioxide and arsenic exposures in

the copper smelter studies. IARC has classified sulfur dioxide as a Group 3 carcinogen, i.e., not classifiable

as to its carcinogenicity. EPA has not assigned sulfur dioxide a cancer classification.

One chronic-duration animal study investigated the potential carcinogenicity of inhaled sulfur dioxide

in mice (Peacock and Spence 1967). Male and female mice were exposed to 500 ppm sulfur dioxide for

5 minutes/day, 5 days/week, for 2 years. A significant increase in the incidence of lung tumors

(13/30 adenomas and carcinomas versus 5/30 in controls; 4/30 primary carcinomas versus none in controls)

was observed in the female mice. The incidence of lung adenomas and carcinomas was also higher in the

treated males (15/28 versus 11/35 in controls), but the increase was not significant. The incidence of primary

carcinomas in treated males was similar to that of the controls. Although limited by small sample size and

testing of just one dose, the study suggests that sulfur dioxide may be a carcinogen in mice. Studies in

additional species are required before a conclusion can be made about carcinogenic potential.
SULFUR DIOXIDE 79

2. HEALTH EFFECTS

2.6 CHILDREN’S SUSCEPTIBILITY

This section discusses potential health effects from exposures during the period from conception to maturity

at I8 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 due to 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 5.6 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 4993). 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 pre-natal and post-natal life and a particular structure or

function will be most sensitive to disruption during its critical period(s). Damage 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 (Widdowson and Dickerson

1964; Foman et al. 1982; Owen and Brozek 1966; Altman and Dittmer 1974; Foman 1966). 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 and 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

(Leeder and Kearns 1997; Komori 1990; Vieira et al. 1996; NRC 1993). Whether differences in xenobiotic

metabolism make the child more or less susceptible also depend 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 the newborn who has a low glomerular filtration rate and has not
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2. HEALTH EFFECTS

developed efficient tubular secretion and resorption capacities (Altman and Dittmer 1974; West et al. 1948;

NRC 1993). Children and adults may differ in their capacity to repair damage from chemical insults.

Children also have a longer 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 while others may

decrease susceptibility to the same chemical. For example, the fact that infants breathe more air per kilogram

of body weight than adults may be somewhat counterbalanced by their alveoli being less developed, so there

is a disproportionately smaller surface area for absorption (NRC 1993).

Clinical studies were conducted to examine respiratory function in healthy, atopic, or asthmatic adolescents

(n=8-9, aged 12-17 years) and 10 healthy male seniors (aged 55-73 years) following a mouth only exposure

to a mixture of 1 ppm sulfur dioxide and 1 mg/m3 sodium chloride aerosol (Koenig et al. 1982a, 1982b,

1983; Rondinelli et al. 1987). Adolescents exercised for 10 minutes of a 40 minute exposure period and the

seniors exercised for 10 minutes of a 30 minute exposure period. Statistically significant decreases in forced

expiratory volume in 1 second (FEV1) were observed in asthmatic adolescents (-23%), atopic adolescents

(-18%), seniors (-8%), and healthy adolescents (-6%). This study concluded that respiratory health status

and not age is the primary factor in determining susceptibility to sulfur dioxide in adolescents and adults

because asthmatic adolescents were most sensitive to sulfur dioxide and normal adolescents were least

sensitive (Rondinelli et al. 1987). However, the study did not examine susceptibility to sulfur dioxide in

younger children and adults, so conclusions about sensitivity in infants and preadolescent children cannot be

made. Subjects were exposed to sulfur dioxide in combination with sodium chloride aerosols because sulfur

dioxide is found together with one or more droplet aerosols in the atmosphere. In two studies, it was verified

that exposure to a mixture of sulfur dioxide and sodium chloride aerosol resulted in effects which did not

differ significantly from those obtained from exposure to sulfur dioxide alone (Koenig et al. 1982b, 1982a).

Exposure of 9 atopic adolescents (aged 12-18 years) to 0.1 ppm sulfur dioxide by mouth did not result in

statistically significant changes in respiratory function parameters (Koenig et al. 1989). However, there was

some evidence that exposure to 0.1 ppm sulfur dioxide may potentiate sulfuric acid-induced respiratory

effects. Exposure to 0.068 mg/m3

 sulfuric acid by itself resulted in a nonsignificant, 12% increase in total

respiratory resistance. However, a statistically significant, 15% increase in respiratory resistance was

observed following exposure to both 0.1 ppm sulfur dioxide and 0.068 mg/m3 sulfuric acid.
SULFUR DIOXIDE 81

2. HEALTH EFFECTS

Epidemiological evidence indicates a possible association between sulfur dioxide levels in air and respiratory

symptoms in children. In a longitudinal study of children in Arizona, increased prevalence of coughing was

noted following exposure to high intermittent levels of sulfur dioxide with a peak 3 hour mean >2.5 mg/m3

(0.95 ppm) and average levels of particulates (Dodge et al. 1985). In Steubenville, OH, small transient

reductions in forced vital capacity (FVC) and forced expiratory volume in 0.75 seconds (FEV0.75) were

observed in children following acute episodes of air pollution with sulfur dioxide levels ≥0.17 ppm and total

suspended particulate levels that exceeded the 1979 standard (Dockery et al. 1982).

In New York seasonal levels of sulfur dioxide, 63-71 μg/m3

 (0.024-0.027 ppm), and other air pollutants

(total suspended particulates (TSPs), suspended sulfates, and nitrogen dioxide) were associated with reduced

FEV0.75, in children aged 9-13 years but not in children aged 5-8 years (Shy et al. 1973). Three years prior to

the study, levels of sulfur dioxide, TSPs, and suspended sulfates were greatly reduced. Sulfur dioxide levels

previously ranged between 364-435 μg/m3 (0.14-0.17 ppm). It was concluded that persistent respiratory

effects in the older children resulted from exposure to high levels of sulfur dioxide, TSPs, and suspended

sulfates during the first 5-10 years of life. Effects associated with individual air pollutants were not

separated and there was no control for other confounding factors.

There was no association between peak expiratory flow rates (PEFR) and seasonal levels of sulfur dioxide,

11-185 μg/m3

 (0.004-0.071 ppm), and particulate pollution in relation to 60 asthmatic children (aged

9-14 years) residing in Budapest (Agocs et al. 1997). Air pollutants were evaluated separately and numerous

confounding factors were controlled. However, the authors discussed limitations such as medication use by

subjects and a lack of personal exposure data.

In London, emergency room visits for acute wheezing episodes in children increased by 12% for each 14

μg/m3

 (0.005 ppm) increase in sulfur dioxide level (Buchdahl et al. 1996). Effects from other air pollutants

(ozone and nitrogen dioxide) were not separated in the evaluation of sulfur dioxide but were controlled for in

the evaluation of ozone. The association with wheezing episodes in children was strongest for ozone.

Increased prevalence of allergies was observed in 556 children (aged 7-13 years) living near an aluminum

smelter in Norway for 7 years or more (Soyseth et al. 1996). Prevalence was highest in children who were

exposed to 20-24 μg/m3

 (0.008-0.009 ppm) sulfur dioxide between the ages of 19-36 months. However,

there was no evidence of a dose response relationship and fluoride levels were also increased in the area.
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2. HEALTH EFFECTS

There was some evidence of increased incidence of respiratory infection in children who lived within four

regions of the United States with high levels of sulfur dioxide, 63-275 μg/m3

 (0.024-0.1 ppm) and other air

pollutants (French at al. 1973). In the Salt Lake Basin and Rocky Mountain region, the highest incidence of

respiratory infection was observed in children 4 years-old and younger. However, in Chicago, there was no

association between respiratory illness and air pollution in children who were too young to attend nursery

school. Polluted areas of the Salt Lake Basin, Rocky Mountain regions, and Chicago were also characterized

by high levels of sulfates, metals, and TSPs, respectively. Differences in the chemical composition of air

pollutants or inadequate control of confounding factors are possible reasons for the different findings in very

young children.

As previously mentioned, epidemiological studies are limited due to confounding effects of particulates and

other air pollutants. It has been reported that particulate sulfates, which form when sulfur oxides are released

into the atmosphere, have a greater impact on health than sulfur dioxide (Shy 1977). Sulfate levels as low as

0.009-0.013 mg/m3 may adversely affect lung function and increase the incidence of acute respiratory disease

in children. The exposure levels are similar to those associated with adverse cardiopulmonary effects in adults

(0.006-0.010 mg/m3

). Additional research is required to assess the impact of individual air pollutants on

children’s health.

Asthmatics in general are most susceptible to sulfur dioxide exposure, especially during periods of exercise

(EPA 1994a, 1994b). Prevalence of asthma is highest in African Americans, children between the ages of

8 to 11, and urban residents (EPA 1994b). Mortality rates associated with asthma are higher in nonwhite

individuals, but the reason is not known (EPA 1994b). Increased susceptibility to sulfur dioxide is therefore

expected in asthmatic, minority children living in urban areas.

Following inhalation of radiolabeled sulfur dioxide by dogs, a very small percentage of radiolabel was

detected in gonads, 0.01-0.18% in ovaries, and 0.006-0.1% in testicles, but the chemical form of the

radiolabel was not known (Balchum et al. 1959). There are no known studies which examine whether sulfur

dioxide metabolites can pass the placenta or be excreted into breast milk. Because sulfur dioxide and its

metabolites are water soluble, accumulation in maternal tissues and subsequent mobilization during

pregnancy or lactation is not likely.

Evidence of reproductive and developmental effects in humans is limited. No association was observed

between exposure to sulfur dioxide and spontaneous abortion in 1,792 cases of a cross-sectional study of an
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industrial community in Finland (Hemminki and Niemi 1982). In the Czech Republic an association was

noted between a high sulfur dioxide level (as a marker for all air pollutants) and an increased incidence of

abnormality and reduced motility of sperm in 325 18-year-old males (Selevan et al. 1995). A developmental

study conducted in China indicated an association between exposure to sulfur dioxide pollution during

pregnancy and decreased infant birth weight in 74,671 women (Wang et al. 1997). However, interpretation

of such studies is complicated by limitations such as confounding effects of various air pollutants and a lack

of personal exposure data.

Reproductive effects following exposure to sulfur dioxide have not been observed in animals. Exposure of

male and female mice (10/sex/group) to 5-30 ppm sulfur dioxide for 9 days prior to mating until days

12-14 of pregnancy had no effect upon completed pregnancies, litter size, sex ratio, and neonatal mortality

(Petruzzi et al. 1996). Significant differences in pregnancy rate, implantations, resorptions, or live fetuses

were not observed in 20 rabbits exposed to 70 ppm sulfur dioxide on gestation days 6-18 or 32 mice exposed

to 25 ppm sulfur dioxide on gestation days 5-15 for 7 hours/day (Murray et al. 1977, 1979).

Serious developmental effects following exposure to sulfur dioxide have only been observed in one study.

Increased time for righting reflex on postnatal day 1 and negative geotaxis on postnatal day 10 were observed

in the offspring of mice exposed to 32 or 65 ppm sulfur dioxide on gestation days 7-18 (Singh 1989).

However, neurological effects were not noted in the offspring of 10 mice exposed to 5-30 ppm sulfur dioxide

9 days prior to mating until days 12-14 of pregnancy (Petruzzi et al. 1996). Exposure of mice

(13-17/group) to 32-250 ppm sulfur dioxide on gestation days 7-17 did not result in adverse developmental

effects (Singh 1982). Serious malformations were not observed in 20 rabbits exposed to 70 ppm sulfur

dioxide on gestation days 6-18 and 32 mice exposed to 25 ppm sulfur dioxide on gestation days 5-15 for

7 hours/day, but minor skeletal variations were noted (Murray et al. 1977, 1979). Effects in rabbits included

nonossified sections of bone, fusion of cranial bones, and extra ribs. Delayed ossification and decreased fetal

weight were observed in treated mice.

Genotoxic compounds could potentially affect children’s health through the induction of mutations in the

parental germ cells. In vitro experiments have demonstrated a genotoxic effect of sulfur dioxide metabolites

on animal germ cells, but adverse effects have not been observed following in vivo exposure of animals.

Mouse, cow, and ewe oocytes were incubated in cell culture media containing sodium sulfite (Jagiello et. al.

1975). An inhibitory effect on meiosis was noted in the mouse and cow oocytes at 150 and 500 μm3

sodium sulfite, respectively. At 250 μg/cm3 sodium sulfite, the chromosomes of all three species appeared
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fuzzy. Fragmentation and clumping were also observed in the chromosomes of ewe and cow oocytes.

Genetic disorders can occur in the offspring which develop from oocytes containing fragmented

chromosomes. Therefore, the effects of sodium sulfite on oocytes were studied following intravenous

administration of l-5 mg sodium sulfite to 6 mice (Jagiello et. al. 1975). In vivo administration of sodium

sulfite had no effect on the meiotic division of the oocytes.

In a dominant-lethal test, mutations were not observed in the offspring of 10 male mice injected peritoneally

with 300 mg/kg sodium bisulfite on 38 out of 54 days or 400 mg/kg sodium bisulfite 5 days/week for a total

of 20 treatments (Russell and Kelly 1975; Shapiro et al. 1977). Seven and a half weeks later, the mice (wildtype)

from the latter treatment group were mated to females with homozygous marker genes for seven loci

(Russell and Kelly 1975). No mutations that may have resulted from an abnormality in sperm were observed

in a total of 13,568 offspring which were scored for mutations at the seven loci. The authors estimated the

upper limit of genetic risk to humans and concluded that exposure to sulfur dioxide would not likely result in

adverse genetic effects in germ cells. Studies which examined effects of in utero sulfur dioxide exposure on

developing germ cells and possible effects on future generations were not located.

One study has also indicated that maternal nutrition may affect the offspring’s susceptibility to sulfur dioxide

later in life. Increased evidence of pulmonary injury, measured by protein in bronchoalveolar lavage fluid,

was observed in 10 rats exposed to a low protein diet (60 g caisin/kg) in utero and 0.29 mg/m3

 (0.11 ppm)

sulfur dioxide later in life (Langley-Evans et al. 1997).

Studies which specifically examine metabolism in children were not identified. Upon contact with moisture

in airways, sulfur dioxide is converted to a mixture of sulfite, bisulfite, and hydrogen ions (Gunnison et al.

1987). Sulfites are then converted to sulfates by sulfite oxidase. Age-related differences in sulfite oxidase

activity have been demonstrated in rats (Cohen 1974). In 1-day-old rats, liver sulfite oxidase activity was

approximately 1/10 the level of adult rats. Activity increased as the rats matured and at 32 days of age, the

liver sulfite oxidase activity was approximately one half the level of adult rats. Decreased sulfite oxidase

activity may result in increased susceptibility to oxidative effects due to increased formation of a sulfur

trioxide radical intermediate (Constantin 1996). Animal studies have demonstrated an inverse relationship

between sulfite oxidase activity and susceptibility to intraperitoneally administered bisulfite as measured by

lethality (Tejnorova 1978).
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Pharmacokinetic studies or PBPK models which specifically look at absorption, distribution, and excretion in

children or immature animals were not identified. The solubility of gases determine where in the respiratory

tract absorption will occur and absorption in the lungs is based upon the blood-to-gas partition coefficient

(Amdur et al. 1991). Because solubility and blood-to-gas partition coefficient are constants, absorption of

sulfur dioxide in children is expected to be similar to that of adults. Distribution and excretion are also

expected to be similar in children and adults. General information about pharmacokinetics can be obtained in

Section 2.3.

Studies which determine if mechanisms of toxicity differ in children were not identified. Evidence indicates

that sulfur dioxide-induced bronchoconstriction occurs through reflexes which are mediated by cholinergic

and noncholinergic factors The percent contribution from cholinergic versus noncholinergic mechanisms is

not known and it could potentially differ between children and adults. However, this has not been verified

througb research. General mechanisms of toxicity are discussed in Section 2.4.

There are no studies which look at interactions with other chemicals that are unique to children. Studies in

animals have indicated that exposure to other air pollutants may potentiate reproductive or developmental

effects of sulfur dioxide. Exposure of 20 rabbits to a mixture of 70 ppm sulfur dioxide and 250 ppm carbon

monoxide 7 hours/day on gestation days 6-18 resulted in a significant increase in resorptions/litter, which

was not observed with exposure to sulfur dioxide or ozone alone (Murray et al. 1977, 1979). Exposure of

32 mice to a combination of 25 ppm sulfur dioxide and 250 ppm carbon monoxide 7 hours/day on gestation

days 5-l 5 potentiated the decrease in fetal weight gain which was observed with exposure to sulfur dioxide

alone (Murray et al. 1977,1979). Exposure of pregnant mice to ozone alone had the opposite effect and

resulted in a significant increase in fetal weight gain. A significant decrease in fetal crown-rump length was

also observed in the same mice following exposure of dams to the mixture of pollutants but not sulfur dioxide

alone. Effects on fetal crown-rump length following exposure to ozone alone were not discussed.

It is not known whether biomarkers or methods for reducing toxicity to exposure differ in children.

In conclusion, studies of susceptibility in children are limited. Clinical studies have indicated that compared

to senior citizens, adolescents are not at increased risk of respiratory effects following inhalation of sulfur

dioxide. Adverse respiratory effects may be associated with exposure of children to air pollution, but it is

difficult to determine if sulfur dioxide is directly responsible for those effects. The genotoxic potential of

sulfur dioxide metabolites has been demonstrated through in vitro studies, but in vivo exposure of animals
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2. HEALTH EFFECTS

has indicated that mutations in germ cells are unlikely following inhalation exposure of sulfur dioxide.

Difficulties in separating the effects of individual air pollutants also complicate the evaluation of sulfur

dioxide pollution on human reproduction and gestational development. Results from animal developmental

studies are equivocal. Most animal studies have indicated a lack of serious developmental effects but other

studies have reported developmental delays which might have resulted from decreased food intake by dams.

Lastly, decreased sulfite oxidase activity in young animals has been demonstrated. If the same is true for

children, they may be at increased risk for oxidative damage.

2.7 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 (NAUNRC 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 (NRC 1989). The preferred

biomarkers of exposure are generally the substance itself or 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 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 sulfur dioxide are discussed in Section 2.7.1.

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
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2. HEALTH EFFECTS

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 sulfur dioxide are discussed in

Section 27.2.

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 2.9, Populations That Are Unusually Susceptible.

2.7.1 Biomarkers Used to Identify or Quantify Exposure to Sulfur Dioxide

No studies pertaining to specific biomarkers used to identify or quantify exposure to sulfur dioxide were

located.

Sulfur dioxide metabolites are rapidly absorbed from the upper respiratory passages and are readily

distributed throughout the body (Balchum et al. 1960a; Frank et al. 1967; Kleinman 1984; Speizer and Frank

1966; Yokoyama et al. 197 1). Sulfur dioxide reacts with plasma proteins to form S-sulfonates. S-Sulfonate

levels in human plasma showed a positive correlation with atmospheric levels (> 0.3 ppm) of sulfur dioxide

(Gunnison and Palmes 1974). Therefore, plasma S-sulfonate levels may serve as a potential indicator of

exposure to sulfur dioxide.

Inhaled sulfur dioxide is excreted in the urine as sulfate. However, sulfur dioxide is not the only source of

sulfate in the urine. Sulfates are also metabolites of sulfur-containing amino acids and are therefore normal

constituents of intracellular fluids and urine (Vander et al. 1975; Lentner 1981). Concentrations of sulfates in

urine range from 53 μmol/dL/Kg in l-day-old infants to 500 μmol/dL/Kg in young men (Lentner 1981).

The level of urinary sulfate is a measure of the quantity and quality of proteins in the diet.

2.7.2 Biomarkers Used to Characterize Effects Caused by Sulfur Dioxide

No specific biomarkers used to characterize effects caused by sulfur dioxide were located.
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2. HEALTH EFFECTS

The upper respiratory system is the primary target of sulfur dioxide. Alterations in pulmonary function

parameters, such as increased airway resistance, decreased airway conductance, and decreased expiratory

volume, might be considered potential biomarkers to characterize effects caused by sulfur dioxide, but

alterations in pulmonary function parameters are not specific for sulfur dioxide and may indicate exposure to

other air pollutants such as nitrogen dioxide or ozone.

An increase in the number of leukocytes in bronchoalveolar lavage fluid is a biomarker for inflammatory

effects of sulfur dioxide exposure (Sandstrom 1989a, 1989b). However, the effect is not specific to sulfur

dioxide because inflammation can occur following exposure to numerous irritating substances.

DNA adduct formation in the nasal mucosa is a potential biomarker which could be used in the future. The

formation of adducts between the DNA base cytosine and bisulfite, a metabolite of sulfur dioxide, has been

demonstrated in vitro (Shapiro 1977). Adduct formation in placentas of women from industrial and

agricultural areas in the Czech Republic has been compared in order to assess toxicity from exposure to

pollution, and no differences were found between the two groups (Topinka et al. 1995). However, the nasal

mucosa is the optimal tissue for the development of biomarker techniques because it is the first to contact

inhaled toxicants (Flato et al. 1996). A technique for measuring adduct formation in cells obtained from

nasal lavage has been demonstrated (Flato et al. 1996). However, studies are needed to verify the formation

of bisulfite adducts in nasal mucosa. Adduct formation would not be specific to sulfur dioxide because

adducts can form after exposure to numerous carcinogenic and mutagenic compounds.

Clastogenic effects in humans following occupational exposure to sulfur dioxide via the inhalation route have

been reported. Increases in chromosome aberrations and sister chromatid exchanges were detected in

lymphocytes from 42 workers who were occupationally exposed to an average concentration of 41.7 mg/m3

(15.92 ppm) of sulfur dioxide (Yadav and Kaushik 1996). Similar findings were observed in another study

of 40 workers occupationally exposed to sulfur dioxide (Meng and Zhang 1990a). Exposure concentrations

ranged from 0.34 to 11.97 mg/m3

 (0.13 to 4.57 ppm). In addition, increases in the frequencies of

lymphocytes with micronuclei were noted (Meng and Zhang 1990b). Such clastogenic effects may serve as

potential biomarkers for genotoxic effects of sulfur dioxide. However, these clastogenic effects are not

specific for sulfur dioxide and could indicate exposure to other chemicals.

For more information on biomarkers for renal and hepatic effects of chemicals see ATSDR/CDC

Subcommittee Report on Biological Indicators of Organ Damage (1990).
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2.8 INTERACTIONS WITH OTHER CHEMICALS

Epidemiological studies of the associations between effects of sulfur dioxide and exposure to other chemicals

are limited because of the presence of other air pollutants as confounders. A number of other air pollutants

such as nitrogen dioxide, sulfuric acid, particulate matter, and ozone can result in respiratory effects similar to

those of sulfur dioxide. There is some evidence that particulate matter, which is capable of oxidizing sulfur

dioxide to sulfuric acid, can cause a three- to four-fold potentiation of the irritant response in guinea pigs

(Amdur 1969). It is postulated that binding of acids to carbon particles may impede neutralization by

ammonia in the airways or buffering system in epithelium (Jakab 1996). Under conditions of high relative

humidity, sulfate is produced from mixtures of sulfur dioxide and carbon particles. Acute exposure of mice

to a mixture of carbon particles and sulfur dioxide at 85% relative humidity resulted in an inhibition of

phagocytosis by alveolar macrophages, which was not observed from exposure to sulfur dioxide alone (Jakab

1996).

Concurrent exposures to sulfur dioxide, smoke, and particulates have been associated with symptoms

including: increased respiratory effects, increased frequencies of respiratory illness, excess mortality, and

worsening of existing respiratory disease (WHO 1979). Table 2-5 summarizes the effects of concurrent

exposure to sulfur dioxide, smoke, and particulates, and it designates which portions of the population are

affected.

In a controlled study of asthmatic subjects, an enhanced responsiveness to 0.75 ppm sulfur dioxide after

exposure to 0.25 ppm nitrogen dioxide for 30 minutes was reported (Jorres and Magnussen 1990). The

authors suggested that acute exposure of asthmatics to nitrogen dioxide at rest enhances airway

responsiveness to hyperventilation of sulfur dioxide without altering airway tone. In another study of

asthmatics, the influence of prior exposure to a low concentration of ozone (0.12 ppm) on the pulmonary

response to a subthreshold concentration of sulfur dioxide (0.1 ppm) was examined (Koenig et al. 1990).

Prior exposure to ozone potentiated subsequent responses to sulfur dioxide in asthmatic subjects.

The effects of exposure to a combination of sulfur dioxide (0.2 ppm) and nitrogen dioxide (0.4 ppm) on the

airway response of mild asthmatic patients to allergen inhalation have been investigated (Devalia et al. 1994).

Subjects were exposed for 6 hours to sulfur dioxide and nitrogen dioxide alone, or in combination, in

exposure chambers. The subjects were then challenged with predetermined concentrations of Dermato

phagoides pteronyssinus allergen 10 minutes after each exposure. The cumulative breath units of allergen

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2. HEALTH EFFECTS

required to produce a 20% fall in forced expiratory volume in 11 second (PD20 FEV1) were measured after

each exposure. The results showed that only the combination of sulfur dioxide and nitrogen dioxide

significantly (p=0.005) decreased PD20 FEV1.

Exposure to sulfur dioxide may enhance ozone absorption in the lung. Absorption of ozone boluses were

compared in healthy nonsmokers before and after exposure to 0.36 ppm sulfur dioxide for 2 hours (Rigas et

al. 1997). Sulfur dioxide exposure resulted in an increased absorption of ozone in males, but the results were

not statistically significant. It was postulated that anatotical variations, such as differences in dead space

volume, may have contributed to the sex-specific differences in absorption.

Sulfur dioxide (0.1 ppm) by itself had no statistically significant effect on forced expiratory volume in one

second (FEV1) or total respiratory resistance in 9 atopic adolescents (aged 12-18 years) who were exposed

by mouth, while exercising for 10 minutes of a 40 minute exposure period (Koenig et al. 1989). However,

sulfur dioxide may potentiate sulfuric acid-induced respiratory effects. Exposure to 0.068 mg/m3

 sulfuric

acid (0.6 μm aerosol) by itself resulted in a nonsignificant 12% increase in total respiratory resistance.

However, a statistically significant 15% increase in respiratory resistance was observed following exposure to

both 0.1 ppm sulfur dioxide and 0.068 mg/m3 sulfuric acid.

A synergistic toxic action between sulfur dioxide and sulfuric acid has been observed in studies of

bronchoconstriction in guinea pigs (Amdur 1959, 1974). Particle size of the aerosol was found to be an

important factor in the potentiation of the response to sulfur dioxide. For instance, a potentiation of the

response to sulfur dioxide was noted when the sulfuric acid particle size was 0.8 microns (μ) but not when the

particle size was 2.5 μ.

Sulfite may potentiate effects induced by peroxynitrite, a compound commonly found in lungs of individuals

with inflammatory diseases such as asthma (Reist et al. 1998). Peroxynitrite inactivates al-antiproteinase,

which inhibits lung damaging enzymes such as elastase. In vitro experiments have demonstrated potentiation

by sulfite only if its concentration does not exceed that of peroxynitrite.

2.9 POPULATIONS THAT ARE UNUSUALLY SUSCEPTIBLE

A susceptible population will exhibit a different or enhanced response to sulfur dioxide than will most

persons exposed to the same level of sulfur dioxide in the environment. Reasons may include genetic
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makeup, age, health and nutritional status, and exposure to other toxic substances (e.g., cigarette smoke).

These parameters may result in reduced detoxification or excretion of sulfur dioxide, or compromised

function of target organs affected by sulfur dioxide. Populations who are at greater risk due to their unusually

high exposure to sulfur dioxide are discussed in Section 5.7, Populations With Potentially High Exposure.

Exercising asthmatics are recognized as the most susceptible group to sulfur dioxide inhalation (EPA 1994a,

1994b). Some sensitive asthmatics have been shown to respond to sulfur dioxide at a concentration as low as

0.1 ppm (Sheppard et al. 1981). In particular, physically active asthmatics would be at special risk (EPA

1994a, 1994b). In addition, based on data pertaining to the prevalence of asthma and mortality rates from

asthma, certain minority group individuals (e.g., African American, Hispanic) may also represent population

segments at increased potential risk for sulfur dioxide respiratory effects (EPA 1994a, 1994b) given the

higher rates of asthma mortality in nonwhite populations. A sulfite-sensitive subpopulation of asthmatics,

who have a relative deficiency of sulfite oxidase, has been postulated to exist (IARC 1992). In contrast,

healthy nonasthmatic individuals do not experience respiratory effects at concentrations of up to 1.0 ppm.

Nonasthmatic individuals who are unusually sensitive to cold air may also be more susceptible to the

respiratory effects of sulfur dioxide.

Elderly adults with preexisting respiratory or cardiovascular disease may be susceptible to the increased risk

of mortality associated with acute-duration exposure to sulfur dioxide (WHO 1979). Children may be

particularly susceptible to increased frequencies of respiratory illness following chronic-duration exposure to

sulfur dioxide (WHO 1979).

Individuals who are deficient in sulfite oxidase may be susceptible to oxidative effects following exposure to

sulfur dioxide. In vitro studies have demonstrated increased formation of a sulfur trioxide radical

intermediate in polymorphonuclear leukocytes of individuals with decreased sulfite oxidase activity

(Constantin 1996). Elderly adults and individuals with Down’s syndrome may also be susceptible to

oxidative stress because limited evidence indicates increased formation of the sulfur trioxide radical

regardless of sulfite oxidase activity (Constantin, 1996). Animal studies have demonstrated an inverse

relationship between sulfite oxidase activity and susceptibility to intraperitoneally administered bisulfite

(Tejnorova 1978).
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2.10 METHODS FOR REDUCING TOXIC EFFECTS

This section will describe clinical practice and research concerning methods for reducing toxic effects of

exposure to sulfur dioxide. 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 sulfur dioxide. When

specific exposures have occurred, poison control centers and medical toxicologists should be consulted for

medical advice. For specific information about treatment following exposures to sulfur dioxide, the reader

should consult the text of Bronstein AC and Currance PL (1994): Emergency Care for Hazardous Materials

Exposure.

2.10.1 Reducing Peak Absorption Following Exposure

There are no specific methods available to reduce the absorption of sulfur dioxide following exposure.

Supportive treatment includes administration of 100% humidified supplemental oxygen with assisted

ventilation as required, endotracheal intubation or tracheostomy if upper airway obstruction is present, and

the use of inhaled sympathomimetic bronchodilators, such as albuterol, metaproterenol, or cromolyn sodium,

for bronchoconstriction (HSDB 1996). For eye contamination, copious irrigation is recommended (HSDB

1998).

2.10.2 Reducing Body Burden

There are no known methods for reducing the body burden of sulfur dioxide. Following absorption, inhaled

sulfur dioxide dissolves on the walls of the moist airways to form sulfite and bisulfite, which in turn, can be

oxidized to sulfates, a reaction catalyzed by sulfite oxidase (Gunnison et al. 1987). An inverse correlation

between sulfite oxidase activity and sensitivity to bisulfite toxicity has been noted (IARC 1992; Tejnorova

1978).

2.10.3 Interfering with the Mechanism of Action for Toxic Effects

Sulfur dioxide-induced increase in airway resistance is due to reflex bronchoconstriction (Frank et al. 1962;

Nadel et al. 1965). Injection of atropine prevented the increase in airway resistance in healthy subjects

exposed by inhalation to 4-6 ppm sulfur dioxide for 10 minutes. Indomethacin, a prostaglandin synthetase

inhibitor, and zafirlukast, a leukotriene receptor antagonist, were demonstrated to reduce airway
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2. HEALTH EFFECTS

responsiveness in asthmatics challenged with sulfur dioxide (Field et al. 1996; Lazarus et al. 1997). Inhaled

sympathomimetic bronchodilators can also be used to reverse bronchospasm. Inhaled corticosteroids may be

used to alleviate bronchoconstriction associated with reactive airways dysfunction syndrome (RADS)

(Kennedy et al. 1992).

Several techniques have been demonstrated to reduce sulfur dioxide-induced toxicity in animals. Administration

of low molecular weight heparin to rats during or after exposure to 30-40 ppm sulfur dioxide for

1 hour/day, 5 days/week, for 12 weeks reduced inflammation of the bronchial mucosa (Krasnowska et al.

1998). Treatment with nebulized S-carboxymethylcysteine, a medication prescribed for sinusitis in the

United Kingdom and Japan, restored ciliary function and accelerated mucosal repair in the sinuses of rabbits

exposed to 20 ppm sulfur dioxide 4 hours/day for 4 weeks (Sugiura et al. 1997).

Administration of vitamins E and C to 7 rats exposed to 10 ppm sulfur dioxide for 1 hour/day for 45 days

reduced oxidative effects such as lipid peroxidation and membrane damage in erythrocytes (Etlik et al. 1997).

Once absorbed by the respiratory tract, sulfur dioxide can form sulfite and bisulfite. The enzyme sulfite

oxidase can oxidize sulfite and bisulfite to sulfates. Sulfite and/or bisulfite may be the chemicals that play a

role in the sulfur dioxide-induced bronchoconstriction (Sheppard 1988). Interference with the formation of

sulfite and/or bisulfite may be a potential strategy, albeit untested, to reduce bronchoconstriction from sulfur

dioxide toxicity.

2.11 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 sulfur dioxide is available. Where adequate information is not 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 sulfur dioxide.

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
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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.

2.11.l Existing Information on Health Effects of Sulfur Dioxide

The existing data on health effects of inhalation, oral, and dermal exposure of humans and animals to sulfur

dioxide are summarized in Figure 2-3. The purpose of this figure is to illustrate the existing information

concerning the health effects of sulfur dioxide. 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 (ATSDR 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.

There are human data on inhalation exposure to sulfur dioxide that provide information on acute and chronic

systemic effects. There are also limited data on genotoxic and carcinogenic effects. Data on potential chronic

systemic, genotoxic, and carcinogenic effects are Bimited because of confounding factors such as multiple

exposure. There are no oral or dermal exposure data.

Animal data on inhalation exposure cover primarily acute respiratory effects. Data on other acute-duration

effects in animals are limited. There are limited data concerning intermediate- and chronic-duration systemic

effects in animals. One chronic study is available that examines potential carcinogenicity of sulfur dioxide in

mice. Studies which examine ocular and dermal effects from exposure to the liquid and gaseous form of

sulfur dioxide are available. There are no oral exposure data.

2.11.2 Identification of Data Needs

Acute-Duration Exposure. There are several case reports of deaths in humans (Atkinson et al. 1993;

Charan et al. 1979; Harkonen et al. 1983; Huber and Loving 199 1; Rabinovitch et al. 1989). However,

estimates of exposure concentrations were not often reported in these studies.

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There are numerous acute-duration controlled inhalation studies in humans concerning respiratory effects.

These studies have established that acute-duration exposure to sulfur dioxide causes constriction of the

airways, especially in exercising asthmatics (Bethel et al. 1985; EPA 1994a, 1994b; Horstman et al. 1986;

Linn et al. 1983a, 1983b, 1987, 1990; Roger et al. 1985; Sheppard et al. 1980, 1981). Pulmonary function

tests in exercising, mild asthmatics indicate that 0.1 ppm sulfur dioxide may be close to the threshold for

bronchoconstriction (Sheppard et al. 1981). Based on a minimal LOAEL of 0.1 ppm for increased airway

resistance observed in the Sheppard et al. (1981) study, an acute-duration MRL of 0.01 ppm was derived.

Studies in experimental animals have supported the pulmonary effects of sulfur dioxide following inhalation

exposure. Increased airway resistance and decreased compliance were noted in guinea pigs exposed to

2.6 ppm sulfur dioxide for 1 hour (Amdur 1959).

In recent years, there has been concern about the potential health significance of 5-10 minute exposures to

peak levels of sulfur dioxide currently occurring near heavy industrial areas (EPA 1994a, 1994b). Additional

studies on the frequency of occurrence of 5-10 minute peak sulfur dioxide levels in the ambient air and the

number of asthmatics that may be potentially exposed to such peak levels of sulfur dioxide would be useful in

assessing the health effects of exposure to peak levels of sulfur dioxide. In addition, information on the

frequency at which asthmatics would be potentially exposed to peak levels of sulfur dioxide would be helpful.

Although epidemiological studies on the association between short-term peaks of sulfur dioxide and potential

health effects in asthmatics, including exercising asthmatics, would be useful, these kinds of studies are most

likely difficult to design and conduct.

The controlled human exposure studies of sulfur dioxide are typically restricted to mild asthmatics. Thus, it

is not certain if such studies, although consistent in demonstrating the sensitivity of asthmatics to sulfur

dioxide, reflect the characteristics of the asthmatic population as a whole. Individuals with severe asthma

may be more susceptible to the effects of sulfur dioxide because of their lower reserve of lung function.

However, severe asthmatics may actually be protected against sulfur dioxide effects because they are less

prone to strenuous outdoor exercise and more likely to take medication before participating in outdoor

activities (EPA 1994b). Additional information on this issue would be useful.

Although oral and dermal data regarding the effects of sulfur dioxide were not identified, human exposure

would be expected to be principally by inhalation. Oral and dermal exposure studies may be clinically

relevant but are not a high priority.
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2. HEALTH EFFECTS

Intermediate-Duration Exposure. There are no human data on intermediate-duration inhalation

exposures to sulfur dioxide. Studies on intermediate-duration exposures to sulfur dioxide would be useful for

assessing potential health risks to humans. No intermediate-duration oral or dermal studies of sulfur dioxide

were identified. Because inhalation is the primary route of concern, oral and dermal exposure studies should

not be a high priority.

Intermediate-duration inhalation exposure studies in animals are very limited. In an intermediate-duration

study, mild bronchitic lesions were seen in hamsters exposed to 650 ppm sulfur dioxide for 19-74 days

(Goldring et al. 1970). Decreased respiratory rate, rhinitis, tracheitis, and bronchopneumonia were observed

in rabbits exposed to 70-300 ppm sulfur dioxide for 6 weeks (Miyata et al. 1990). Inflammation of the

bronchial mucosa was observed in rats exposed to 30-40 ppm sulfur dioxide for 1 hour/day, 5 days/weeks,

for 12 weeks (Krasnowska et al. 1998). Increased numbers of goblet cells were observed in the airways of

rats exposed to 400 ppm sulfur dioxide for 3 hours/day, 5 days/week, for 3 weeks (Basbaum et al. 1990;

Lamb and Reid 1968). Nasopharyngitis and lipid peroxidation of lung tissue were observed in guinea pigs

exposed to 10 ppm sulfur dioxide for 1 hour/day for 30 days (Haider 1985). Systemic effects have also been

noted following inhalation of sulfur dioxide. Lipid peroxidation was observed in erythrocytes of rats exposed

to 10 ppm sulfur dioxide for 1 hour/day for 45 days (Etlik et al. 1997) or for 1 hour/day, 7 days/week, for

8 weeks (Gumuslu et al. 1998). A reduction in insulin plasma levels and increase in liver triglyceride levels

were observed in rats exposed to 10 ppm sulfur dioxide for 24 hours/day, 7 days/week, for 15 days (Lovati et

al. 1996). Studies were inadequate for the development of an MRL. Doses administered in animal studies

were well above levels which produce toxic effects in ‘humans. In addition, multiple doses were not

administered and a dose-response relationship could not be assessed. Additional well designed animal studies

on the respiratory and systemic effects following intermediate-duration exposure, and the relevance of the

effects to exposed humans, would be useful.

Chronic-Duration Exposure and Cancer. There are limited data on chronic occupational exposures to

sulfur dioxide. These studies indicate a potential association between sulfur dioxide exposure and respiratory

effects (lung cancer and lung function changes). However, these occupational studies have limitations

concerning accurate exposure assessments and concomitant exposures. Epidemiological studies on the

relationship between ambient air pollution and lung cancer (Ponka et al. 1993; Tango 1994), although

limited, have suggested that the risk of lung cancer from sulfur dioxide exposure in the environment is

nonsignificant. Epidemiological studies of children and acute air episodes of sulfur dioxide are confounded

by the presence of other air pollutants that are also associated with lung function changes. Additional
SULFUR DIOXIDE 99

2. HEALTH EFFECTS

supporting data from well-designed epidemiological studies would be useful. There are no known data on

chronic human oral or dermal exposures to sulfur dioxide. Oral and dermal routes may be clinically relevant

to humans, but inhalation is the primary route of concern. Therefore, oral and dermal data should not be high

priority.

There are only a few limited chronic inhalation studies in two species of animals. No effects on lung function

were observed in guinea pigs exposed to 5.72 ppm sulfur dioxide for 22 hours/day, 7 days/week, for

52 weeks (Alarie et al. 1972). Likewise, no lung function changes or histopathological alterations in the lung

were observed in monkeys exposed to 5.12 ppm sulfur dioxide for 23.3 hours/day, 7 days/week, for 78 weeks

(Alarie et al. 1975). Additional chronic-duration animal studies following inhalation exposure would be

useful for providing supporting data regarding potential chronic-duration effects of sulfur dioxide. The

chronic-duration studies are too limited to be used for developing a chronic-duration inhalation MRL;

because only one dose was administered, a dose response relationship could not be assessed.

Several epidemiological studies were conducted to address cancer associated with occupational or

environmental exposure to sulfur dioxide pollution, but definitive conclusions could not be drawn because of

confounding factors (Bond et al. 1986; Enterline et al. 1987; Lubin et al. 1981; Ponka et al. 1993; Tango et

al. 1994; Welch et al. 1982). A chronic cancer study in mice was also limited due to small sample size and

the administration of only one dose. Well-designed studies which evaluate carcinogenicity potential

following inhalation exposure in additional species are needed. Oral and dermal exposure studies are not of

high priority because inhalation is the primary route of concern

Genotoxicity. In vivo clastogenic effects of sulfur dioxide in humans have been reported. Increases in

chromosome aberrations and sister chromatid exchanges were detected in lymphocytes from 42 workers who

were occupationally exposed to an average concentration of 41.7 mg/m3

 (15.9 ppm) of sulfur dioxide, a level

which is eight times greater than the TLV (Yadav and Kaushik 1996). Similar findings have been observed

in other studies of workers (Meng and Zhang 1990a; Nordenson et al. 1980). In addition, increases in the

frequencies of lymphocytes with micronuclei were noted (Meng and Zhang 1990b). One study of potential

chromosomal abnormalities in workers exposed to 8-hour workshift levels of about 1 ppm (2.62 mg/m3

)

sulfur dioxide in the aluminum industry revealed that sulfur dioxide was without effect (Sorsa et al. 1982).

Studies which examine genotoxic effects in animals following inhalation exposure were not located but would

be useful Studies with animal germ cells have indicated that genotoxicity is possible following in vitro

exposure but unlikely from intraperitoneal administration of sulfur dioxide metabolites (Jagiello et al. 1975;
SULFUR DIOXIDE 100

2. HEALTH EFFECTS

Russell and Kelly 1975). Further human and animal studies to explore possible dose-response relationships

and to provide mechanistic data would be useful.

Reproductive Toxicity. In a cross-sectional study of spontaneous abortions in an industrial community in

Finland, no evidence was found that would associate sulfur dioxide exposure with a risk of spontaneous

abortions (Hemminki and Niemi 1982). However, abnormalities in sperm have been associated with sulfur

dioxide, as a surrogate of all air pollutants, in a limited study conducted in the Czech Republic (Selevan et al.

1995).

Reproductive effects were not observed in rats exposed to 5-30 ppm sulfur dioxide for 9 days prior to mating

until 12-14 days of pregnancy (Petruzzi et al. 1996) in mice exposed to 25 ppm sulfur dioxide 7 hours/day

on gestation days 6-15 or in rabbits exposed to 70 ppm sulfur dioxide 7 hours/day on gestation days 6-18

(Murray et al. 1979). The studies are limited because the reproductive function of the offspring were not

assessed. A well-designed multigeneration reproductive study would be useful to assess the potential

reproductive toxicity of sulfur dioxide. Because inhalation is the primary route of concern, oral and dermal

exposure studies should not be a high priority.

Developmental Toxicity. There is evidence of an association between environmental exposure to sulfur

dioxide during pregnancy and reduced birth weight. Like other epidemiological studies of air pollution

mixtures, interpretation of the results is complicated by confounding factors such as other air pollutants and a

lack of personal exposure information. Though additional, well-designed epidemiological studies would be

useful, separation of effects associated with individual components of an air pollution mixture is unlikely

considering the currently available technology.

Numerous developmental studies have been conducted in animals. One study reported no developmental

effects in the offspring of pregnant mice that were exposed to 32-250 ppm sulfur dioxide on gestation

days 7-17 (Singh 1982). In another developmental study in mice in which pregnant females were exposed to

32 ppm or 65 ppm on gestation days 7-18, increased time for righting reflex on postnatal day 1 and increased

negative geotaxis on postnatal day 10 were reported (Singh 1989). The duration of exposure for each day

was not stated. No visible signs of maternal toxicity and no effect on the number of live births were

observed. Sulfur dioxide at a concentration of 65 ppm significantly decreased the birth weight (about 89% of

controls) of the pups. Minor skeletal variations were reported in offspring of mice exposed to 25 ppm sulfur

dioxide 7 hours/day on gestation days 6-15 and rabbits exposed to 70 ppm sulfur dioxide 7 hours/day on
SULFUR DIOXIDE 101

2. HEALTH EFFECTS

gestation days 6-l 8 (Murray et al. 1979). Fetal body weights were also lower in exposed mice. Reduced

food intake was the only sign of maternal toxicity. Neurological effects were not observed in the offspring of

mice exposed to 5-30 ppm sulfur dioxide for 9 days prior to mating until 12-14 days of pregnancy (Petruzzi

et al. 1996). Studies which examine developmental effects following oral or dermal exposure should not be

high priority since inhalation is the primary route of concern.

Immunotoxicity. Epidemiological studies have indicated that sulfur dioxide air pollution may increase the

prevalence of allergies (Soyseth et al. 1996) and incidence of respiratory infections (French et al. 1973) in

children. However, both studies were limited by inadequate control of confounding factors and a lack of

personal exposure data. Increased sensitization to antigen was reported in a study of guinea pigs exposed by

inhalation to sulfur dioxide (Riedel et al. 1992). In an acute-duration inhalation study with hamsters, there

was a significant reduction in endocytosis by pulmonary macrophage following exposure to 50 ppm sulfur

dioxide for 4 hours while exercising (Skornik and Brain 1990). However, pulmonary defense mechanisms

were not affected in rats and mice exposed to 0.32-0.43 ppm sulfur dioxide together with 87-113 μg/m3

sulfate for 4 hours prior to or 17 hours following infection with Stuphylococcus aureus or Group C

Streptococci (Goldstein et al. 1979). Additional studies to determine if inhalation of sulfur dioxide increases

susceptibility to infection or allergen sensitization in humans or animals would be useful. Because inhalation

is the primary route of concern, studies of oral or dermal exposure should not be high priority.

Neurotoxicity. One study has indicated that reflex response times may have been affected in workers

exposed to sulfur dioxide, and was most likely due to a psychological response to systemic toxicity (Kehoe et

al. 1932). Seizures and prostration were observed in rats following exposure to 2,350,50,000 and

500,000 ppm sulfur dioxide for an average of 176 minutes, <10 minutes, and <2 minutes respectively (Cohen

1973). Lipid peroxidation has been observed in the brains of guinea pigs exposed to sulfur dioxide for

1 hour/day for 30 days (Haider et al. 1982). Additional neurotoxicity studies would be useful to characterize

the effects of sulfur dioxide on the nervous system.

Epidemiological and Human Dosimetry Studies. There are several epidemiological studies that

have examined the potential association between respiratory effects (i.e., lung cancer and lung function

changes) and inhalation exposure to sulfur dioxide (Agocs et al. 1997; Archer and Gillan 1978; Buchdahl et

al. 1996; Dockery et al. 1982; Dodge et al. 1985; Lebowitz et al. 1979; Lowe et al. 1970; Shy et al. 1973;

Skalpe 1964; Smith et al. 1977). There are also epidemiological studies that have examined the association

between daily acute mortality and sulfur dioxide exposure (Anderson et al. 1996; Bacharova, 1996; Ballester
SULFUR DIOXIDE 102

2. HEALTH EFFECTS

et al. 1996; Katsouyanni et al. 1997; Loomis et al. 1996; Mazumdar et al. 1982; Moolgavkar et al. 1995a,

1995b; Spix, 1996; Sunyer et al. 1996; Thurston et al. 1989; Touloumi et al. 1996; Verhoeff et al. 1996).

Reproductive effects have also been examined in epidemiological studies (Selevan et al. 1995; Wang et al.

1997). However, these epidemiological studies are limited by confounders. Further studies that employ more

precise measurements of exposure, control of exposure to other chemicals, and follow-up of occupational

cohorts would be useful. Monitoring of populations around industrial areas where there are exposures to

peak levels of sulfur dioxide would also be useful.

Biomarkers of Exposure and Effect

Exposure. No specific biomarkers of exposure for sulfur dioxide have been identified. Potential biomarkers

include plasma S-sulfonate levels and urinary levels of sulfate (Balchum et al. 1960a; Frank et al. 1967;

Kleinman 1984; Speizer and Frank 1966; Yokoyama et al. 197 1). Further studies examining the suitability

of these potential biomarkers would be useful.

Effect. No specific biomarkers of effect for sulfur dioxide have been identified. Potential nonspecific

biomarkers include lung function changes (EPA 1994a, 1994b; Sheppard et al. 1981) and clastogenic effects

(Meng and Xhang 1990a, 1990b; Yadav and Kaushik 1996), leukocytes in bronchioalveolar lavage fluid

(Sandstrom 1989a, 1989b), or adduct formation in nasal mucosa (Topinka et al. 1995). Further studies

examining the suitability of these potential biomarkers, and identification of potential specific biomarkers,

would be useful.

Absorption, Distribution, Metabolism, and Excretion. Sulfur dioxide, a highly water-soluble gas, is

rapidly absorbed by the mucosa of the nose and upper respiratory tract (Kleinman 1984; Speizer and Frank

1966). Absorption in the lower respiratory tract is increased with enhanced ventilation associated with a

transition from nasal to oronasal breathing (EPA 1986d). Absorbed sulfur dioxide metabolites are taken up

by the blood and are readily distributed throughout the body (Balchum et al. 1960a; Frank et al. 1967;

Yokoyama et al. 1971). Once absorbed, sulfur dioxide is rapidly metabolized to sulfates in the liver by the

enzyme, sulfite oxidase (Gunnison et al. 1987) or through the generation of sulfur trioxide radical

intermediate (Constantin et al. 1996). Sulfur dioxide may also react with proteins to form S-sulfonate

(Gunnison and Palmes 1974). Sulfur dioxide is excreted primarily in the urine as sulfate. There is also

evidence that glutathione is involved in the detoxification of sulfur dioxide (Kagadel et al. 1986; LangleyEvans et al. 1996). Studies which compare the quantitative distribution of sulfur dioxide within the
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2. HEALTH EFFECTS

respiratory tract during nasal, oronasal, and oral breathing would be useful. Studies which examine the

relationship between sulfite oxidase activity and levels of sulfur dioxide metabolites in blood would also be

helpful. Identification of additional factors involved in the metabolism of sulfites would also be useful

information.

Comparative Toxicokinetics. There are no studies that directly compare the toxicokinetics across

species. PBPK models have not been developed. Sulfite oxidase activity varies among species. Therefore,

studies providing quantitative data necessary to develop PBPK models would be useful.

Methods for Reducing Toxic Effects. Other than removing the subject from exposure, there is no

specific method to reduce the absorption of suIfur dioxide. There are no known methods for specifically

reducing the body burden of sulfur dioxide. Supportive treatment includes administration of 100%

humidified supplemental oxygen with assisted ventilation as recprired, endotracheal intubation or

tracheostomy if upper airway obstruction is present, and the use of inhaled sympathomimetic bronchodilators

for bronchoconstriction (HSDB 1998). Studies examining methods to enhance the oxidation of sulfur

dioxide to increase elimination might be useful. Studies to determine effects of antioxidant therapy in

humans may also be useful.

Children’s Susceptibility. Clinical studies have indicated that compared to healthy senior citizens,

healthy adolescents are not at increased risk of respiratory effects following inhalation of sulfur dioxide

(Koenig et al. 1982b; Rondinelli et al. 1987). Epidemiological studies indicate associations between sulfur

dioxide pollution and respiratory symptoms (Buchdahl et al. 1996; Dodge et al. 1985; WHO 1979) and

transient effects on respiratory function in children (Dockery et al. 1982; Shy et al. 1973). However, such

studies are limited due to confounding effects of other air pollutants. Asthmatics in general are most

susceptible to sulfur dioxide exposure, but it is not known if asthmatic children are more sensitive than

asthmatic adults (EPA 1994a, 1994b). It is unlikely that additional epidemiological studies would provide

conclusive information. However, additional controlled studies in asthmatic and healthy children would be

useful in determining doses which produce effects, and whether these children are more susceptible to sulfur

dioxide-induced respiratory effects than asthmatic and healthy adults. Controlled studies in young versus

mature animals would also be useful.

Studies in humans and animals indicate that serious developmental effects are not likely from maternal

inhalation of sulfur dioxide. However, developmental delays, such as reduced birth weight in humans and
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2. HEALTH EFFECTS

decreased fetal weight and delayed ossification in animals, have been observed (Murray et al. 1977, 1979;

Wang 1997). Data requirements for developmental effects are discussed under the Developmental Toxicity

heading of this section (2.11.2 Identification of Data Needs).

Metabolic studies have demonstrated reduced liver sulfite oxidase activity in young rats (Cohen 1974).

Comparisons of sulfite oxidase activity in infants, children, and adults and also in young and mature animals

would be useful. Measurement of liver sulfite oxidase activity in young versus mature animals of other

species would also be useful. Studies in which liver sulfite oxidase activity in young and mature animals is

compared in relation to blood levels of sulfates, sulfites, and S-sulfonates following sulfur dioxide inhalation

would also be useful. Pharmacokinetic studies in dogs have demonstrated that following inhalation of radiolabeled sulfur dioxide, a small amount of radio-label reaches the ovaries and testes (Balchum 1959).

Experiments to identify the sulfur dioxide metabolite present in gonads would be useful. Studies which

examine pharmacokinetics in immature or pregnant animals were not identified. Studies which examine the

distribution of sulfur dioxide metabolites in pregnant animals to determine if they cross the placenta or are

transferred to breast milk might be useful. Accumulation in maternal tissues is not expected for sulfur

dioxide or its metabolites due to its high water solubility. Studies which examine absorption, metabolism,

distribution, and excretion in immature animals would also be helpful in evaluating the impact of sulfur

dioxide exposure in children.

Studies of genotoxicity in germ cells have demonstrated that mutations and clastogenic effects occur with in

vitro exposure but not in vivo exposure (Iagiello et. al. 1975; Russell and Kelly 1975; Shapiro et al. 1977).

Although genotoxicity has been observed in lymphocytes of workers exposed to sulfur dioxide (Meng and

Zhang 1990a, 1990b; Yadav and Kaushik 1996), available data in animals indicates that genotoxicity in germ

cells is unlikely from exposure to sulfur dioxide metabolites (Russell and Kelly 1975). Inhalation studies in

animals could be done to verify that genotoxicity in germ cells does not occur following inhalation exposure

to sulfur dioxide.

Child health data needs relating to exposure are discussed in 5.8.1 Data Needs: Exposures of Children.

2.11.3 Ongoing Studies

Ongoing animal, occupational, or epidemiological studies of the health effects of sulfur dioxide were not

identified.
SULFUR DIOXIDE 105

3. CHEMICAL AND PHYSICAL INFORMATION

3.1 CHEMICAL IDENTITY

Information regarding the chemical identity of sulfur dioxide is located in Table 3- 1.

3.2 PHYSICAL AND CHEMICAL PROPERTIES

Information regarding the physical and chemical properties of sulfur dioxide is located in Table 3-2.



SULFUR DIOXIDE 109

4. PRODUCTION, IMPORT/EXPORT, USE, AND, DISPOSAL

4.1 PRODUCTION

Sulfur dioxide has been produced commercially from the following raw materials: elemental sulfur; pyrites;

sulfide ores of non-ferrous metals; waste sulfuric acid and sulfates; gypsum and anhydrite; hydrogen sulfidecontaining waste gases; and flue gases from the combustion of sulfurous fossil fuels (IARC 1992). It is most

commonly produced by burning sulfur but can also be produced by burning pyrites in a special furnace or by

purifying and compressing sulfur dioxide gas from smelting operations.

Sulfur dioxide has been produced by burning molten sulfur in a special burner with a controlled amount of

air. The burner gas, free of dust and cooled, is dissolved in water in a series of two towers. In a third tower,

the solution is sprayed at the top and flows down while steam is injected at the base. The gas issuing from

the third tower is then cooled to remove most moisture and passed up a fourth tower against a countercurrent

of sulfuric acid. The dried gas is liquefied by compression (IARC 1992).

Sulfur dioxide can also be recovered commercially by liquefying gas obtained during smelting of non-ferrous

metals such as lead, copper, and nickel. Much of this smelter by-product is recovered and oxidized to sulfur

trioxide for producing sulfuric acid. Sulfur dioxide recovery, however, usually occurs only for environmental

reasons (IARC 1992).

Sulfur dioxide was produced for sale at levels of 64,000 tons in 1960,99,000 tons in 1970, 124,000 tons in

1980, and 227,000 tons in 1987. Production was 1.39x1011 g in 1977, 1.18x1011 g in 1982, and 1.18x1011 g

in 1985 (HSDB 1998). Most of the sulfur dioxide produced is for captive use in the sulfuric acid and wood

pulp industries (IARC 1992). It is also used for refrigeration (HSDB 1998). The major producers of sulfur

dioxide in 1989 included ChemDesign Corporation, Coulton Chemical Corporation, Dow Chemical,

Hoeschst Celenese Corporation, Industrial Chemicals Corporation, Rhone-Poulenc, Inc., Tennessee Chemical

Company, and Phelps Dodge Corporation (HSDB 1998).
SULFUR DIOXIDE 110

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

4.2 IMPORT/EXPORT

Imports of sulfur dioxide were 5.17x1010 g in 1977, 2.26x1010 g in 1982,2.33x107

 g in 1985, and

5.72x107

 kg in 1986 (HSDB 1998). The U.S. import of sulfur dioxide was 68,772,164 kg in 1994 (NTDB

1996).

Exports of sulfur dioxide were 1.62x109

 g in 1978, 5.38x109 g in 1983, 1.60x109 gin 1985, and 8.54x107 g

in 1987 (HSDB 1998). The U.S. export of sulfur dioxide was 1,173,002 kg in 1996 (NTDB 1996).

4.3 USE

Sulfur dioxide has numerous commercial uses which are based on its function as an acid, as a reducing or

oxidizing agent, or as a catalyst. Sulfur dioxide is used in large quantities as a captive intermediate in the

production of sulfuric acid and in the pulp and paper industry. Other common uses of sulfur dioxide include

the following: fumigant, preservative, bleach, and steeping agent for grain in food processing; catalyst or

extraction solvent in the petroleum industry; flotation depressant for sulfide ores in the mining industry;

intermediate for bleach production; and reducing agent in several industrial processes (IARC 1992).

4.4 DISPOSAL

Sulfur dioxide is listed as a toxic substance under Section 3 13 of the Emergency Planning and Community

Right to Know Act (EPCRA) under Title III of the Super-fund Amendments and Reauthorization Act (SARA)

(EPA 19988). Disposal of wastes containing sulfur dioxide is controlled by a number of federal regulations

(see Chapter 7).
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5. POTENTIAL FOR HUMAN EXPOSURE

5.1 OVERVIEW

Sulfur dioxide has been identified in at least 16 of the 1,467 current or former EPA National Priorities List

(NPL) hazardous wastes sites (HazDat 1998). However, the number of sites evaluated for sulfur dioxide is

not known. The frequency of these sites within the United States can be seen in Figure 5-l.

Atmospheric sulfur dioxide is formed as a by-product of the combustion of fuel from power generation and

industrial activities, and by the oxidation of reduced gases in the atmosphere. Volcanic activity also

contributes to the levels of atmospheric sulfur dioxide. The atmospheric lifetime of sulfur dioxide is about

10 days (IARC 1992).

Sulfur dioxide is oxidized rapidly by both homogeneous and heterogeneous reactions and is removed from the

atmosphere by precipitation and by dry deposition on surfaces, mainly as sulfuric acid.

Inhalation of sulfur dioxide, by the general population residing near industrial sources and by workers

exposed to sulfur dioxide, is generally the main route of human exposure to the chemical.

It should be noted that the amount of sulfur dioxide detected by chemical analysis is not necessarily the

amount that is bioavailable.

5. 2 RELEASES TO THE ENVIRONMENT

Releases of sulfur dioxide to the environment from large processing facilities are not required to be reported

to the Toxics Release Inventory (TRI).

Releases of sulfur dioxide are not required to be reported under SARA Section 3 13. Consequently, there are

no data for this compound in the current TRI. Sulfur dioxide has been identified in a variety of environmental

media (air, surface water, groundwater, soil, and sediment) collected at 16 of the 1,467 NPL hazardous waste

sites (HazDat 1998).

SULFUR DIOXIDE 113

5. POTENTIAL FOR HUMAN EXPOSURE

5.2.1 Air

Releases of sulfur dioxide to air from large processing facilities are not required to be reported to the Toxics

Release Inventory (TRI).

Sulfur dioxide has been identified in air samples collected at 8 of the 16 NPL hazardous waste sites where it

was detected in some environmental media (HazDat 1998).

Atmospheric sulfur dioxide, a major oxide of sulfur, can be formed from both anthropogenic and natural

sources. On a global scale, the total annual atmospheric flux of sulfur has been estimated to be 140-350

million tons (of which less than 30% is anthropogenic sulfur) in the form of sulfur dioxide, sulfuric acids, and

sulfate (HSDB 1998). The primary anthropogenic source of sulfur dioxide gas is fuel combustion from

power generation and industrial processes. Fossil fuel accounts for 75-85% of man-made sulfur dioxide

emissions on a global scale; industrial processes such as refining and smelting account for the remainder

(HSDB 1998). Almost all of the man-made sulfur dioxide emissions (93.5%) are released in the Northern

Hemisphere (HSDB 1998). With regard to the United States, EPA National Air Pollutant Emission Trends

estimate that in 1994 a total of 2 1.1 million tons of sulfur dioxide was emitted into the atmosphere in the

United States from point and area sources (EPA 1994a). The Air Toxicities Program is striving to reduce

toxic air pollutants emissions in the United States by 1.5 million tons annually over the next 10 years (EPA

1995). Of the total sulfur dioxide emissions, about 18.5 million tons or 87.6% was attributed to fuel

combustion, of which electricity utilities and industrial combustion constituted about 70% and 14%,

respectively (EPA 1994a). The Utility Air Toxicities Study examines hazardous air pollutant emissions from

coal, oil, and gas (fossil fuel) electric utilities and associated public health hazards. The predictions for the

next two decades are a 30% increase in hazardous air pollutant emissions from coal utilities and a 50%

decrease in emissions from oil utilities (EPA 1995). Sulfur dioxide emission from fuel combustion have

come primarily from coal burning, with coal combustion producing 96% of the electric utility emissions. A

similar trend was observed in England (Lee and Longhurst 1993), but not in Denmark where road traffic was

considered the most prevalent source of air pollution for sulfur dioxide (Jensen and Fenger 1994).

According to EPA’s National Air Pollutant Emission Trends (EPA 1994a), other sources of sulfur dioxide

include emissions from chemical and allied product manufacturing, metal processing, petroleum and related

industries, other industrial processes, and on-road vehicles. These sources are, however, of less importance
SULFUR DIOXIDE 114

5. POTENTIAL FOR HUMAN EXPOSURE

as they collectively contributed less than 13% of the total emissions (EPA 1994a). Since sulfur dioxide is the

major substance used for manufacturing sulfuric acid, it is not surprising that a significant source of industrial

emissions is acid manufacturing and processing facilities.

Data are available at the state-level for sulfur dioxide emissions and rank by major category (EPA 1994a).

Ohio and Indiana ranked first and second in the total sulfur dioxide emissions. Fuel combustion for electrical

utilities accounted for the greatest portion of total emissions in all states. On the national level, sulfur dioxide

emissions have shown a steady decrease in the United States since the 1970s (EPA 1994a; Lefohn and

Shadwick 1991).

A study was conducted among 24 United States communities to study air pollution patterns (Spengler et al.

1996). A strong correlation between particle mass and sulfate concentrations and sulfate and hydrogen ion

concentrations was found in Ohio, Pennsylvania, Virginia, West Virginia, Tennessee, and Kentucky.

Concentrations in these areas ranged between 85 and 126 nmol/m3

 in the summer, the highest being in Ohio,

Pennsylvania, and Kentucky. Due to the meteorological conditions, acidic pollution is highest during summer

months in these areas. Sulfur dioxide is converted to acid sulfates without the presence of ammonia during

this time (Spengler et al. 1996).

The Acid Rain Program projects a 40% reduction in SO2 annual emissions in the United States between 1980

and 2010. The U.S. Geological Survey reports a 10 to 25% reduction in acidic rainfall because of a decline in

emissions due to the Acid Rain Program. This reduction in emissions will also contribute to less sulfate haze

(EPA 1995).

Volcanoes are a sporadic, but significant, natural source of sulfur dioxide. It has been estimated that 1.5x106

tons of sulfur dioxide per year were evolved from worldwide volcanic production between the years 1500 and

1914 (Kellogg 1972). This estimate is about two orders of magnitude lower than the total annual sulfur

dioxide liberated to the atmosphere (Kellogg 1972).

5.2.2 Water

Releases of sulfur dioxide to water from large processing facilities are not required to be reported to the

Toxics Release Inventory (TRI).
SULFUR DIOXIDE 115

5. POTENTIAL FOR HUMAN EXPOSURE

Sulfur dioxide is very soluble in water, and the oceans are generally considered to be a sink for sulfur dioxide

(Kellogg 1972). Surface water bodies can receive sulfur dioxide from the atmosphere by dry and wet

deposition, from surface runoff, and from subsurface drainage (HSDB 1996; IARC 1992; Kellogg 1972;

WHO 1979). It has been estimated that 70% of sulfate in rainwater comes from the washout of sulfur

dioxide (Kellogg 1972). Hydrogen sulfide present in the oceans is probably oxidized to sulfur dioxide within

hours (HSDB 1998). Rivers can transport sulfur compounds to the oceans (HSDB 1996).

It is possible that oceans may be a source of sulfur dioxide, especially during conditions when the equilibrium

vapor pressure of sulfur dioxide in surface water exceeds the partial pressure of sulfur dioxide in the air

immediately above it (Kellogg 1972). Sea salt can contribute to atmospheric levels of sulfate (Kellogg 1972).

There is no information on releases of sulfur dioxide to water from manufacturing and processing facilities

because these releases are not required to be reported (EPA 1997).

5.2.3 Soil

Releases of sulfur dioxide to soil from large processing facilities are not required to be reported to the Toxics

Release Inventory (TRI).

Sulfur dioxide has been identified in soil and sediment samples collected at 5 of the 16 NPL hazardous waste

sites where it was detected in some environmental media (HazDat 1998).

Atmospheric sulfur dioxide can be removed by diffusion to the soil (Kellogg 1972). Sulfur uptake is

dependent upon soil pH and moisture content (Kellogg 1972). One estimate of the uptake of sulfur dioxide

by soil and vegetation is 52x106

 tons per year (Kellogg 1972). A rate of diffusion of 0.9x10-12 g/s

cm2

/second has been calculated for the Northern Hemisphere (Kellogg 1972).

There is no information on releases of sulfur dioxide to soil from manufacturing and processing facilities

because these releases are not required to be reported (EPA 1997).
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5. POTENTIAL FOR HUMAN EXPOSURE

5.3 ENVIRONMENTAL FATE

5.3.1 Transport and Partitioning

Anthropogenic and natural releases of sulfur dioxide to the environment are considered to be primarily to the

atmosphere (HSDB 1998; Kellogg 1972; WHO 1979). Because of its high vapor pressure (3,000 mm Hg at

20°C), sulfur dioxide is typically present in a gaseous phase Some of the sulfur dioxide emitted into the air

moves unchanged to various surfaces including soil, water, grass, and vegetation in general (WHO 1979). In

the atmosphere, sulfur dioxide can be transformed into sulfuric acid or sulfates by a variety of processes

(WHO 1979).

A field deposition study in Canada was conducted to measure the effects of atmospheric stability, rainfall

intensity, and wind speed and direction on SO2 deposition. The object was to compare the field data with the

modeled results from a computation of the SO2 contamination and accumulation by forests downwind from

an SO2 source. The highest levels of SO2 deposition were found along the north-south direction and low

levels were found along the northwest direction. This is due to the fact that the wind direction from northsouth

is consistent with neutral weather, and wind direction from northwest is consistent with rainy weather

and airborne SO2 scavenging. The final results show that the field data is comparable to the modeled data;

the modeled results accurately describe the deposition patterns in relation to the weather patterns and is

therefore considered a reliable source (Bouque et al. 1996).

Sulfur dioxide is very soluble in water, and oceans are generally considered to be a sink for sulfur dioxide

(Kellogg 1972). It is also possible that oceans can be a source of sulfur dioxide if the equilibrium pressure of

sulfur dioxide in surface water exceeds the partial pressure of sulfur dioxide in the air immediately above it.

Any potential releases of sulfur dioxide from water would be expected to partition to the atmosphere as

discussed in Section 5.3.2.1 (Kellogg 1972; WHO 1979).

Soil can absorb sulfur dioxide, with uptake being dependent on the pH and moisture content of the soil

(HSDB 1998). No data were found pertaining to soil adsorption and mobility of sulfur dioxide in soil.

Hill (1971) studied absorption of several gaseous air pollutants by plants and found that the removal rates

were in the following order: hydrogen fluoride (HF) > sulfur dioxide (SO2) > chlorine (Cl2,) > nitrogen

dioxide (NO2) > ozone ( O3) > peroxyacetyl nitrate (PAN) > nitric oxide (NO) > carbon monoxide (CO).
SULFUR DIOXIDE 117

5. POTENTIAL FOR HUMAN EXPOSURE

5.3.2 Transformation and Degradation

5.3.2.1 Air

Sulfur dioxide may be oxidized to sulfur trioxide (SO3) and sulfate in air photochemically or catalytically

(Bufalini 1971; Radojevic 1992). The oxidations generally involve homogeneous-phase reactions (oxidation

in the gas phase) and heterogeneous gas-solid reactions (oxidation on the surface of particles). Gas-phase

reactions include direct photochemical oxidation of sulfur dioxide and oxidation by reacting with other gases

and substances, including O, O2, O3 NO, NO2 NO3 N2O5 OH, hydrocarbons, or some heavy metal ions. In

the direct oxidation pathway, sulfur dioxide gas molecules (at different electronically excited states due to

solar irradiation) react with each other, forming SO3, which may be further oxidized by other gases or water

drops to form sulfate. Reaction pathways with other gases all result in similar end products: SO3, or sulfate.

Rate constants for sulfur dioxide oxidation by these gases vary from 2x 10-24 to 4x10-3 cm3

 molecule-1

second-1, with the reaction with N2O5 being the fastest (Radojevic 1992). Heterogeneous gas-solid reactions

include oxidation of sulfur dioxide on the surfaces of activated carbons, metal oxides, and other particles.

Sulfur dioxide may be photochemically or catalytically oxidized to SO3 and sulfate in air (Bufalini 1971;

Radojevic 1992). The oxidations generally involve homogeneous-phase reactions (oxidation in gas or liquid

phase) and heterogeneous gas-solid reactions (oxidation on the surface of particles). The atmospheric

residence time of sulfur dioxide is about 10 days. Photochemical oxidation involves the reaction of sulfur

dioxide with reactive molecules and free radical species, and with products of O3 and alkene gases.

Photochemical oxidation is thought to be initiated by absorption of solar irradiation energy. However, neither

the detailed mechanism of this process nor the significance of this conversion process is clear. Photochemical

oxidation was previously thought to be insignificant because the maximum conversion rate of sulfur dioxide

was considered to be <0.04% h-l (Calvert et al. 1978; Radojevic 1992). However, much higher conversion

rates (0.65% h-l) were reported by other researchers (Cox and Penkett 1970), and it was suggested that trace

impurities in the ambient air may have contributed to this.

Sulfur dioxide can be oxidized by OH radicals in the gas phase relatively quickly, making it a major

mechanism for sulfur dioxide conversion. This is a three-body addition reaction, and the conversion rates

have been characterized to be >l% hour-1 (Radojevic 1992).
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5. POTENTIAL FOR HUMAN EXPOSURE

In the atmosphere, aqueous-phase oxidation of sulfur dioxide can occur in cloud, fog, rain, deliquescent

aerosol particles, and in surface liquid films on these particles (Benner et al. 1992; McKay 1971). Once

dissolved in these droplets, sulfur dioxide may be oxidized to sulfate via a variety of mechanisms, thus

forming “acid rain.” Such a removal mechanism is commonly termed “wet deposition.” The conditions under

which this oxidation can be enhanced have been studied (Barrie and Georgii 1976). The investigators found

that the absorption of sulfur dioxide was facilitated by manganese and iron ions, high solution pH, and high

temperature. A decrease in temperature from 25 °C to 8°C resulted in a 5-l0-fold decrease in sulfur dioxide

absorption rates. It was estimated that typical sulfur dioxide removal rates through this mechanism were

between 0.08% and 2.0% hour-’ depending on the manganese and iron concentrations and temperature. The

catalytic oxidation of sulfur dioxide in the presence of iron and manganese was, however, not supported by

Fung et al. (1991 ), who investigated the relative importance of three major aqueous reactions thought to be

responsible for the in-cloud conversion of sulfur dioxide: oxidation of sulfur dioxide by H2O2 O3 or O2; in

the presence of iron and manganese. They found that sulfate precipitation concentration is generally more

sensitive to H2O2 than to O3, and that the contribution of iron and manganese in the oxidation of sulfur

dioxide by O2 is insignificant. Some research has shown that catalyzed liquid-phase oxidation in the presence

of iron and manganese is important in urban plumes and fogs where sulfur dioxide concentrations are

sufficiently high; this oxidation may not be significant in cleaner, rural areas (HSDB 1998). The average

overall conversion rate of sulfur dioxide (through all processes) to sulfate in Beijing was estimated to be

4.7% hour-1 during the day and 3.4% hour-1 during the night (Chen et al. 1990).

In a study of acid rain chemistry in the Allegheny Mountains, it was found that on average about half of the

rain sulfate resulted from the scavenging of sulfur dioxide and the other half resulted from aerosol sulfate.

Most of the rain acidity (-80%) arose from gases dissolved in cloudwater and rainwater (Pierson et al. 1987).

Sulfur dioxide can also be oxidized by some particles; this heterogeneous oxidation is affected by oxidants

other than O;, temperature, and humidity. Particles that can react with and oxidize sulfur dioxide include

carbonaceous metal oxides, atmospheric particles, and other particles (Radojevic 1992).

Sulfur dioxide can also be removed from air by uptake of plant leaves. It is reported that direct surface

uptake of sulfur dioxide is the most significant dry removal process for atmospheric sulfur (HSDB 1998). A

study of the absorption of several gaseous air pollutants by plants found that the removal rates were in the

following order: hydrogen fluoride (HF) > sulfur dioxide (SO2) > chlorine (Cl2) > nitrogen dioxide (NO2) >

ozone (O3) > peroxyacetyl nitrate (PAN) > nitric oxide (NO) > carbon monoxide (CO) (Hill 197 1).
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5. POTENTIAL FOR HUMAN EXPOSURE

5.3.2.2 Water

It has been known for a long time that sulfur dioxide in air can be oxidized to sulfate by cloud or rain

droplets, thus forming “acid rain.” Barrie and Georgii (1976) studied the conditions under which this

oxidation can be enhanced. They found that the absorption of sulfur dioxide was facilitated by manganese

and iron ions, high solution pH, and high temperature. A decrease in temperature from 25°C to 8°C resulted

in a 5-10-fold decrease in sulfur dioxide absorption rates. It was estimated that typical sulfur dioxide

removal rates through this mechanism were between 0.08% and 2.0% hour-1, depending on the manganese

and iron concentrations and temperature.

In addition to the above factors, oxidation of sulfur dioxide to sulfate is also affected by many dissolved

gases. The oxidation normally involved reactions with dissolved H2O2,O2,OH-

, HSO3

2-, SO42-,H3COOH,O3,

HO2, NO2 NO3 NH3 NH+

4 NH2 and peroxyacetyl nitrate (PAN), with some reactions catalyzed by F3+ or

Mn2+ (Benner et al. 1992; McKay 1971). All of these reactions are dependent on solution pH because pH

affects the solubility of sulfur dioxide. Benner et al. (1992) reported that oxidation of sulfur dioxide to

sulfate was greatly enhanced when cloud droplets were exposed to sulfur dioxide and NH3. Speculating that

the enhanced surface oxidation may be true for NH3, Benner et al. (1992) further studied the phenomenon and

found that sulfur dioxide can be oxidized to sulfate only when NH3, is present. This led to the conclusion that

sulfur dioxide can be oxidized more rapidly in the presence of both sulfur dioxide and NH3, compared to

sulfur dioxide alone. Fung et al. (1991) determined the relative importance of three major aqueous reactions

through which sulfur dioxide is oxidized to sulfate and found that oxidation by H2O2 was stronger than O3,

oxidation in light precipitation areas and was comparable in heavy precipitation areas. The contribution of

catalytic oxidation of O2

-

; in the presence of manganese and iron was insignificant in any occasion.

The current practices in diluted acids disposal include discharging to waste-water treatment plants or

neutralization by calcium hydroxide, which produces gypsum that can be reused. Stucki et al. (1993)

developed an alternative disposal technique for sulfuric acid and a new combination of recycling processes.

This technique involves reducing sulfuric acid to volatile weak acids by sulfate-reducing bacteria.

Dissolved sulfur dioxide in the surface layer of the ocean may be slowly oxidized to the sulfate anion (SO4

2-)

by the combined presence of dissolved O2 and trace amounts of transition metal salts as catalysts (Kellogg

1972). At ocean depths, dispersed sulfate may be reduced to sulfur dioxide, sulfur, and hydrogen sulfide by

the action of bacteria (Kellogg 1972).
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5. POTENTIAL FOR HUMAN EXPOSURE

Sulfur dioxide absorbed by freshwater lakes is less rapidly oxidized than seawater because of the much lower

salt content of freshwater (Kellogg 1972).

5.3.2.3 Sediment and Soil

Once on the ground, sulfur dioxide may be absorbed by soil, water, and snow-covers (HSDB 1998; WHO

1979). Although snow-covered surfaces are ineffective in absorbing gaseous and particulate sulfur

compounds, the melted snowpack during spring can result in rapid, short-term inputs of high sulfate into

fresh water.

Sulfur dioxide can be reduced to H2S in heat- and alkali-treated sewage sludge by the sulfate-reducing

bacteria, Desulfovbrio desulfuricans or Desulfotomaculum orientis (Deshmane et al. 1993).

Acid rain is the leading cause in an increase in heavy metal mobility in soil. When soil is basic pH, heavy

metals will form insoluble oxides or hydroxides of sulfate, and when soil is acidic, soluble sulfates will form

(Grzesiak et al. 1997).

Hill (1971) and Garland et al. (1973) reported that vegetation could be an important sink for sulfur dioxide

and several other air pollutants. Plants can absorb sulfur dioxide from air. Siebke et al. (1990) developed a

model for simulating uptake and metabolism of sulfur dioxide by different leaf cell compartments.

5.4 LEVELS MONITORED OR ESTIMATED IN THE ENVIRONMENT

Reliable evaluation of the potential for human exposure to sulfur dioxide depends in part on the reliability of

supporting analytical data from environmental samples and biological specimens. In reviewing data on sulfur

dioxide 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 sulfur dioxide in a variety of environmental media are detailed in Chapter 6 (Analytical

Methods).
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5. POTENTIAL FOR HUMAN EXPOSURE

5.4.1 Air

Sulfur dioxide has been detected in urban, rural, and remote areas of the world (Kirk-Othmer 1978).

Worldwide emissions of about 70.7 million tons of sulfur in the form of SO2 were estimated for 1994

(O’Meara 1997). However, sulfur dioxide concentrations in the atmosphere display a very large range,

depending on the intensity of industrial and urban development. It has been estimated that sulfur dioxide

concentrations can range from 1 to 5 μg/m3 (0.0004-0.0019 ppm) in very remote clean areas to at least

6,000 μg/m3 (2.29 ppm) in industrial areas (HSDB 1998). Trends show that in the United States between

1986 and 1995 the national composite SO2 average decreased 37% and SO2 emissions decreased 18%.

Between 1994 and 1995 SO2 emissions decreased 13% and mean national concentrations decreased 17%

(EPA 1995). Spengler et al. (1990) reported that about 80% of U.S. sulfur dioxide emissions come from

within the 31 states bordering on, or located east of the Mississippi River.

Higher concentrations of sulfur dioxide have been detected during forest fires and volcano eruptions. In

response to a request from the Hawaii State Health Department, NIOSH made an assessment of exposure to

lava and seawater emissions resulting from lava flowing into the sea, vaporizing the seawater, and releasing

some toxic gases. Sulfur dioxide was detected at about 1.5 ppm in visible plumes (NIOSH 1992a). At the

request of the National Park Services, NIOSH (1992b, 1992c) measured concentrations of several toxic gases

at the personal breathing zone on July 18, 1991, as a result of a forest fire. Sulfur dioxide concentrations

ranged from 0.6 to 3.0 ppm, with three samples above the evaluation criteria. On another occasion,

firefighters’ exposures to chemical contaminants during fire suppression operations were found to be as high

as 9 ppm for sulfur dioxide (NIOSH 1991). These concentrations were close to or above the current NIOSH

sulfur dioxide time-weighted-average (TWA) exposure limit of 2 ppm (NIOSH 1997) and above the shortterm

exposure limit (STEL) of 5 ppm.

5.4.2 Water

No data were located regarding levels of sulfur dioxide monitored or estimated in water.

5.4.3 Sediment and Soil

No data were located regarding levels of sulfur dioxide monitored or estimated in sediment or soil.
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5. POTENTIAL FOR HUMAN EXPOSURE

5.4.4 Other Environmental Media

Sulfur dioxide can be taken up from the atmosphere by sulfate-treated plants. However, no estimates of the

levels of sulfur dioxide are available (HSDB 1998). Sulfur dioxide has been detected in various foods and

beverages (IARC 1992). Table 5-l summarizes the levels of sulfur dioxide in several foods and beverages.

5.5 GENERAL POPULATION AND OCCUPATIONAL EXPOSURE

Since sulfur dioxide is primarily present in gaseous form, the general public is exposed to it mostly by

inhaling contaminated air. Exposures would be expected to be highest near industrial activities that involve

fuel combustion. The well-known air pollution disaster that occurred in London in December of 1952

exemplifies the extent to which the general population has been exposed to adverse levels of sulfur dioxide in

past years (IARC 1992; WHO 1979). Excess mortality was observed during this episode but several other

contaminants were also present in air during the pollution episode. Other well-known examples of serious air

pollution episodes include those that occurred in Meuse Valley, Belgium, and Donora, Pennsylvania (IARC

1992; WHO 1979). Table 5-2 summarizes ambient air levels of sulfur dioxide in various parts of the world.

Occupational exposures to sulfur dioxide are associated with workers in steel processing, refinery, and

chemical plants. These workers are also frequently exposed to acid mists with increased risk of laryngeal

cancer (Ahlborg et al. 1981; Forastiere et al. 1987; Soskolne et al. 1984; Steenland et al. 1988). It has been

estimated that approximately 600,000 American workers may be occupationally exposed to sulfur dioxide

(HSDB 1998). Some of the highest exposures occur when it is a by-product, as in the metal smelting

industry, and in the processing or combustion of high-sulfur coal or oil (HSDB 1998).

Levels of occupational exposure vary from industry to industry. Sulfite pulp worker exposure fluctuates but

reached levels greater than 10 ppm (26 mg/m3

) in the 1950s (studies in Finland, Norway, and the United

States) (IARC 1992). Because of modernization of facilities and processes, these levels have been falling.

Roasting of ores and the combustion of sulfur-containing fuels in the metal industries have resulted in

workplace exposure concentration means of 1-10 ppm (2.6-26 mg/m3

) in copper smelters (studies in the

following countries with corresponding years of measurement: Finland, 1951-1957; Sweden, date not

specified; United States, 1940-1974, 1976, 1982) and 1 ppm (2.6 mg/m3

) or less in other operations.

Occupational exposure levels with a mean >l ppm (2.6 mg/m3

) have been measured during sulfuric acid and

superphosphate fertilizer manufacture as well as during firefighting (IARC 1992).


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5. POTENTIAL FOR HUMAN EXPOSURE

Other probable exposure routes are dermal and/or eye contact (HSDB 1998). Since sulfur dioxide is an

irritating, corrosive gas, direct dermal or ocular contact can cause immediate damage. The general public can

also be exposed to sulfur dioxide through ingestion of some foods that contain it such as onion, garlic, and

wine (see Table 5-1).

According to the National Occupational Exposure Survey (NOES) conducted by NIOSH from 1981 to 1983,

an estimated 55,029 workers were potentially exposed to sulfur dioxide in the workplace (NOES 1990). The

NOES database does not contain information on the frequency, concentration, or duration of exposure; the

survey provides only estimates of the number of workers potentially exposed to chemicals in the workplace.

No information was found regarding the number of people potentially exposed in the vicinity of hazardous

waste sites. However, since sulfur dioxide has been found near hazardous waste sites, people living near

them may be exposed to higher than background levels.

5.6 EXPOSURE OF CHILDREN

This section focuses on exposures from conception to maturity at 18 years in humans and briefly considers

potential pre-conception exposure to germ cells. Differences from adults in susceptibility to hazardous

substances are discussed in 2.6 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, and 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; they put things in their mouths; they may

ingest inappropriate things such as dirt or paint chips; they spend more time outdoors. Children also are

closer to the ground, and they do not have the judgement of adults in avoiding hazards (NRC 1993).

Sulfur dioxide is a common air pollutant, and children of all ages are therefore potentially exposed.

Inhalation is the primary route of exposure and there are no known exposure pathways that are unique to

children. Children living near NPL waste sites or in areas of heavy industry, such as those related to fuel

combustion, metal processing, and paper manufacturing, are typically exposed to the highest concentrations
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5. POTENTIAL FOR HUMAN EXPOSURE

of sulfur dioxide. Levels of sulfur dioxide in air vary between 1-5 μg/m3 (0.0003-0.002 ppm) in rural areas

and 6,000 μg/m3

 (2.3 ppm) in industrial areas (HSDB 1998). Significant exposures are not anticipated

through diet, household products, structural building materials, school activities, or the clothing, skin, or

breath of occupationally exposed parents.

There are no known studies which report levels of sulfur dioxide or its metabolites in children, in maternal

reproductive organs during pregnancy, or in breast milk. The weight adjusted intake of sulfur dioxide in

children compared to adults is not known but may be greater. Children breath more air per kilogram of body

weight than adults. Children exercise more frequently than adults (CDC 1996; Cureton 1987). Exercise

increases breathing rate and results in both an increased dose of sulfur dioxide to the lower respiratory tract

and enhanced pulmonary effects. Studies which measure weight-adjusted intake in children have not been

identified.

Exposure to children can be reduced by limiting the time spent outdoors, especially during periods of high

pollution levels. Studies in animals indicate that small quantities of sulfur dioxide metabolites reach the

gonads following inhalation of sulfur dioxide. The chemical identity of the metabolites is not known (i.e.,

sulfite, S-sulfonate, sulfate), but animal studies indicate that genotoxic effects in germ cells are unlikely

following inhalation of sulfur dioxide. There is no available information on whether sulfur dioxide

metabolites are likely to cross the placenta or be transferred to breast milk.

5.7 POPULATIONS WITH POTENTIALLY HIGH EXPOSURES

In addition to individuals who are occupationally exposed to sulfur dioxide (see Section 5.5), there are several

groups within the general population that have potentially high exposures (higher than background levels) to

sulfur dioxide. These populations include individuals living in proximity to sites where sulfur dioxide was

produced or sites where sulfur dioxide was disposed of, and individuals living near one of the 16 NPL

hazardous waste sites where sulfur dioxide has been detected in some environmental media (HazDat 1998).

Workers involved in industries in which sulfur dioxide is processed or produced are at a potential risk to high

sulfur dioxide exposures. Some of the highest exposures occur during the processing or combustion of highsulfur coal or oil. Members of the general population who live near urban areas with industrial activities

related to the processing or combustion of high-sulfur fuels would be exposed to higher than background

levels of sulfur dioxide.
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5. POTENTIAL FOR HUMAN EXPOSURE

5.8 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 sulfur dioxide is available. Where adequate information is not available,

ATSDR, in conjunction with the 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

sulfur dioxide.

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.

5.8.1 Identification of Data Needs

Physical and Chemical Properties. Adequate information is available on the physical and chemical

properties of sulfur dioxide (HSDB 1998).

Production, Import/Export, Use, Release, and Disposal. According to the Emergency Planning

and Community Right-to-Know Act of 1986,42 USC. Section 11023, industries are required to submit

chemical release and off-site transfer information to the EPA. The Toxics Release Inventory (TRI), which

contains this information for 1993, became available in May of 1995. This database will be updated yearly

and should provide a list of industrial production facilities and emissions.

However, sulfur dioxide is not required to be reported to the TRI. There are some estimates of the release of

sulfur dioxide to the environment from anthropogenic sources on a global scale. There are also some

estimates of national emissions of sulfur dioxide (EPA 1994a). Additional information on the identification

of the sources that are likely to produce 5-and 10-minute peak exposures to high concentrations of sulfur

dioxide, as well as on the frequency of the occurrence of such peak exposures, would be useful.
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5. POTENTIAL FOR HUMAN EXPOSURE

The latest import information reported was for 1994 (NTDB 1996) and the latest export information was for

1996 (NTDB). More recent import/export information would be useful.

Environmental Fate. There are no accurate measurements of the releases of sulfur dioxide from natural

sources such as volcanoes, the ocean, and biogenic sources (Kellogg 1972; WHO 1979). Crude estimates of

the release of sulfur dioxide from natural sources have been based on the amount of sulfur required to balance

the total sulfur budget (Kellogg 1972). Studies that can provide more accurate estimates of the releases of

sulfur dioxide from natural sources would be useful. Also, additional information on the role of sulfur

dioxide released from anthropogenic and natural sources in the formation of acid rain would be useful.

Releases of sulfur dioxide to the environment as a result of industrial activity are expected to be primarily to

the atmosphere (WHO 1979). Atmospheric sulfur dioxide is formed as a by-product of the combustion of

fuel from power generation and industrial activities, and by the oxidation of reduced gases in the atmosphere.

Since water covers 70% of the earth’s surface, biogenic gas emissions are the largest natural source of sulfur

emissions to the atmosphere. Airborne sea spray and marine and coastal organisms are all responsible for

introducing sulfur into the atmosphere. These emissions are estimated at 11.9 and 15.4 Tg S gases/year

(Tg = teragrams = 1012g; S = sulfur) according to the Department of Energy (DOE 1996). Volcanic activity

also contributes to the levels of atmospheric sulfur dioxide (Kellogg 1972). Gas emissions by vulcanism

have shown estimates which vary due to methods of measurement and vulcanism variability. The annual

volcanic sulfur emissions range was between 0.75 and 42 Tg S. However, the more well known median

estimates were 9-24 Tg S/year with the average annual value being ≥9 Tg S (DOE 1996). Sulfur dioxide is

oxidized rapidly by both homogeneous and heterogeneous reactions and is removed from the atmosphere by

precipitation and by dry deposition on surfaces, mainly as sulfuric acid. A 1983 study by Ryaboshapko

found that, based on seasonality, area1 distribution, mixing height and other factors, annual global wind

blown soil sulfur emissions were 3.3-10 Tg S/year with an average of 6.7 Tg S/year. These emissions are

more significant for some arid and semi-arid regions (DOE 1996). Additional information on the half-life of

sulfur dioxide in soils and water would be useful.

Bioavailability from Environmental Media. Sulfur dioxide is likely to be absorbed following

inhalation of contaminated air. However, data are lacking on the bioavailability of sulfur dioxide following

ingestion of contaminated soils and groundwater or foods grown in areas with contaminated air and water.

This information would be useful in determining the importance of these routes of exposure.
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5. POTENTIAL FOR HUMAN EXPOSURE

Food Chain Bioaccumulation. No data pertaining to the potential of sulfur dioxide to bioaccumulate or

biomagnify in the food chain were identified. This information would be useful in assessing the potential

risks associated with the levels of sulfur dioxide in environmental media.

Exposure Levels in Environmental Media. Routine monitoring of sulfur dioxide levels in ambient air

is currently being performed. However, the monitoring data are insufficient to allow adequate

characterization of human exposure to 5- and 10-minute peak levels of sulfur dioxide.

Reliable monitoring data for the levels of sulfur dioxide in contaminated media at hazardous waste sites are

needed so that the information obtained on levels of sulfur dioxide in the environment can be used in

combination with the known body burdens of sulfur dioxide to assess the potential risk of adverse health

effects in populations living in the vicinity of hazardous waste sites.

Exposure Levels in Humans. Occupational studies often do not report adequate exposure levels of

sulfur dioxide. Additional information is needed on the exposure levels among populations living in the

vicinity of hazardous waste sites.

There are several nonspecific biomarkers for sulfur dioxide exposure such as changes in respiratory function

or plasma S-sulfonate levels. Additional studies to identify specific biomarkers may be useful for the

characterization of exposure levels.

This information is necessary for assessing the need to conduct health studies on these populations.

Exposures of Children. Children are primarily exposed to sulfur dioxide through inhalation of polluted

air. There are no pathways which are unique to children and further studies on this subject are not necessary.

The weight-adjusted intake in children is expected to be greater than adults because children breathe more air

per kg of body weight and exercise more frequently. Exercise increases breathing rate, which results in a

greater sulfur dioxide dose to the lower respiratory tract. Studies which measure sulfur dioxide intake in

children as well as data on respiratory parameters, such as tidal volume and breathing rate, are required to

determine actual exposure levels in children. The only way to reduce childhood exposure to sulfur dioxide is

by limiting time spent outdoors during periods of high air pollution. Additional research for the prevention of

exposure is not required.
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5. POTENTIAL FOR HUMAN EXPOSURE

Child health data needs relating to susceptibility are discussed in 2.11.2 Identification of Data Needs:

Children’s Susceptibility.

Exposure Registries. No exposure registries for sulfur dioxide were located. This substance is not

currently one of the compounds for which a subregistry has been established in the National Exposure

Registry. The substance will be considered in the future when chemical selection is made for subregistries to

be established. The information that is amassed in the National Exposure Registry facilitates the

epidemiological research needed to assess adverse health outcomes that may be related to exposure to this

substance.

5.8.2 Ongoing Studies

Ongoing studies of the potential for human exposure to sulfur dioxide were not identified.
SULFUR DIOXIDE 131

6. ANALYTICAL METHODS

The purpose of this chapter is to describe the analytical methods that are available for detecting, and/or

measuring, and/or monitoring sulfur dioxide, its metabolites, and other biomarkers of exposure and effect to

sulfur dioxide. 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.

6.1 BIOLOGICAL SAMPLES

No methods for determining sulfur dioxide in biological materials were located. Most studies concerning

human health effects measure the concentrations of sulfur dioxide in the air or in the water which surrounds

the subject. The measurement of sulfur dioxide in biological materials is not a method commonly used

because of the rapid conversion of sulfur dioxide to sulfur-containing metabolites. Biomarkers can be used to

indirectly measure sulfur dioxide exposure. In a study by Gunnison and Palmes (1974), S-sulfonate levels in

human plasma showed a positive correlation with atmospheric sulfur dioxide. However, the methods for

detecting S-sulfonates in human plasma lacked sensitivity and precision (Gunnison and Palmes 1974). If Ssulfonate is confirmed as a biomarker of exposure to sulfur dioxide and as methods for determining Ssulfonate

in human plasma become more sensitive, detection of S-sulfonates in human plasma may serve as

an adequate method for determining exposure to sulfur dioxide.

6.2 ENVIRONMENTAL SAMPLES

Sulfur dioxide has been measured in air, in water, and in food and beverages. Methods for determining levels

in the air include ion chromatography, titration, calorimetry, mass spectrometry, conductimetry, amperometric

detection, flame photometric detection, and turbidimetry (see Table 6-l). Ion chromatography seems to be

the most sensitive of these methods with a detection limit of 3 μg/sample for sulfur dioxide (NIOSH 1994a).

Sulfur dioxide has also been measured in stack gases. Methods for measuring sulfur dioxide in stack gases





SULFUR DIOXIDE 137

6. ANALYTICAL METHODS

include pulsed-fluorescence detection and titration (Adams et al. 1987; EPA 1995k). Sulfur dioxide is not

found in water because it is reduced to sulfuric acid in water. Colorimetry, titration, and either acid

distillation (AD) or alkali extraction (AE) ion exclusion chromatography (IEC) with electrochemical detection

(ED) can be used to measure sulfur dioxide in food and beer (Helrich 1990; Kim et al. 1990). These methods

are summarized below.

NIOSH recommends ion chromatography (Method 6004) for the determination of sulfur dioxide in ambient

air (NIOSH 1994a). Method 6004 is specific for sulfur dioxide and is applicable to STEL samples; the

working range is 0.5-20 mg/m3

 (0.2-8 ppm) for a 100-L air sample (NIOSH 1994a). Sulfur trioxide may

give a positive interference for sulfur dioxide.

Sulfur dioxide can also be measured in the air and in stack gases by titration. After separation of sulfur

dioxide and sulfuric acid, the quantity of sulfur dioxide can be measured by barium-thorin titration (EPA

1995k). Possible interferents are free ammonia, water-soluble cations, and fluorides.

Colorimetry has been used to measure sulfur dioxide in air and beer (EPA 1986a, 1986b; Kok et al. 1987a,

1987b). Calorimetric analyzers are simple and highly sensitive (Hollowell et al. 1973). Calorimetric

analyzers measure a solution’s optical absorbance spectrophotometrically; the absorbance is proportional to

the concentration of the colored species. However, color intensity is sensitive to temperature, pH,

development time, purity of reagents, age of solutions, and some atmospheric interferents. Specificity may

improve with development time but does not allow a fast response (Hollowell et al. 1973).

Conductimetric analysis measures the increase in conductivity as sulfur dioxide is absorbed into a hydrogen

peroxide solution. This method has also been used to measure sulfur dioxide in the air (Adams et al. 197 1).

Conductimetric analysis is popular because it of its high sensitivity, fast response, minimal maintenance, and

simple operation. The major disadvantage to conductimetric analysis is its susceptibility to interference by

nonsulfur dioxide gases that form or remove ions (Hollowell et al. 1973).

Amperometric analyzers can also be used to measure sulfur dioxide in air. Amperometric analyzers measure

the current necessary to maintain a constant concentration of titrant as sulfur dioxide reduces the titrant. This

method needs minimal maintenance; however, this method is limited by interference from compounds that
SULFUR DIOXIDE 138

6. ANALYTICAL METHODS

react with the titrant (Hollowell et al. 1973). It is applicable to the determination of sulfur dioxide when other

sulfur compounds or other interferents do not exceed 5% of the sulfur dioxide concentration (Kok et al.

1987c).

Sulfur dioxide emissions can be continuously detected and determined in stack gases by pulsed fluorescence

(Adams et al. 1987). Sample gas is irradiated by pulsed ultraviolet illumination that has passed through an

interference filter, while the 90° emitted fluorescent light is passed through a broad band optical filter

(240-420 nm) and is detected by a photomultiplier tube. The emitted light is proportional to the

concentration of the sulfur dioxide in the sample. Interference may be caused by a build-up of particulate

matter and condensed water on all the surfaces in contact with the sample.

Alkalimetric titration is another method for the determination of sulfur dioxide in stack gases (Knapp et al.

1987). This method is applicable to the determination of sulfur dioxide in the range of 26-15,600 mg/m3

 (or

10-6000 ppm). Below these levels, the color change at the end point cannot be visually detected. This lack

of sensitivity and the possible interference caused by ammonia, ammonia compounds, and fluorides limit the

use of this method. Ion chromatography is much more sensitive and allows much lower detection limits for

equivalent sample volumes or better time resolution (shorter sampling times) at the same detection limits.

Sulfur dioxide is used as a fungicide on grapes. Sulfite residues from this use are tolerated up to 10 ppm;

however, it is important that adequate methods are available to measure levels that exceed 10 ppm. Three

methods have been suggested for the determination of sulfite residues on grapes; however, AE/IEC-ED is the

recommended method because it is rapid, straightforward, free from interference, and able to detect sulfite

residues at levels far below the limit of tolerance (Kim et al. 1990).

6.3 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 sulfur dioxide is available. Where adequate information is not available,

ATSDR, in conjunction with the 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

sulfur dioxide.
SULFUR DIOXIDE 139

6. 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.

6.3.1 Identification of Data Needs

Methods for Determining Biomarkers of Exposure and Effect. No specific methods for

determining biomarkers of exposure and effect were identified. Potential biomarkers include plasma

S-sulfonate levels and urinary levels of sulfate (Balchum et al. 1960a; Frank et al. 1967; Kleinman 1984;

Speizer and Frank 1966; Yokoyama et al. 1971). Further studies examining the accuracy and reliability of

these potential biomarkers would be useful.

Methods for Determining Parent Compounds and Degradation Products in Environmental

Media. Methods are available for measuring sulfur dioxide in air (EPA 1995k; NIOSH 1994a; WHO

1979). One difficulty in measuring environmental levels of sulfur dioxide is the interference from other air

pollutants, in particular, other sulfur-containing compounds. Accurate methods that are specific to sulfur

dioxide and that minimize interference from other sulfur-containing compounds would be useful. Also,

research investigating the relationship between observed health effects and levels measured in air, water, soil,

and sediment could increase our confidence in existing methods and/or indicate where improvements are

needed.

6.3.2 Ongoing Studies

No ongoing studies of improved analytical methods were located for sulfur dioxide.

SULFUR DIOXIDE 141

7. REGULATIONS AND ADVISORIES

The international, national, and state regulations and guidelines regarding sulfur dioxide in air, water, and

other media are summarized in Table 7-1.

A NTP cancer classification has not been reported for sulfur dioxide. NTP has not conducted genetic

toxicology or chronic toxicology and carcinogenicity bioassays on sulfur dioxide (ACGIH 1991).

A STEL for occupational exposure has been established at 5 ppm (10 mg/m3

). World Health Organization

(WHO) guidelines for exposure to sulfur dioxide are listed in Table 7-l.

EPA primary and secondary NAAQS are listed in Table 7-1. A 24 hour limit and an annual limit of exposure

have been established. However, despite the effects induced in exercising asthmatics by brief exposure to

sulfur dioxide (Utell and Frampton 1992), no short-term ambient sulfur dioxide standard has been

established.

An acute-duration MRL of 0.01 ppm has been derived based on increased airway resistance in asthmatics

(Sheppard et al. 1981).





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SULFUR DIOXIDE 183

9. GLOSSARY

Acute Exposure-Exposure to a chemical for a duration of 14 days or less, as specified in the Toxicological

Profiles ”

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 a 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.

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.

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.

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.

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.

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 occurred. The terms, as used here, include malformations and variations, altered growth, and in utero

death.

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.

Immediately Dangerous to Life or Health (IDLH)-The maximum environmental concentration of a

contaminant from which one could escape within 30 min without any escape-impairing symptoms or

irreversible health effects.

Intermediate Exposure-Exposure to a chemical for a duration of 15-364 days, as specified in the

Toxicological Profiles.
SULFUR DIOXIDE 184

9. GLOSSARY

Immunologic Toxicity-The occurrence of adverse effects on the immune system that may result from

exposure to environmental agents such as chemicals.

In Vitro-Isolated from the living organism and artificially maintained, as in a test tube.

In Vivo-Occurring within the living organism.

Lethal Concentratioq(L0) (LCLO)-The lowest concentration of a chemical in air which has been reported to

have caused death in humans or animals.

Lethal Concentratioxq(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)(LOLO)-The lowest dose of a chemical introduced by a route other than inhalation that is

expected to have caused death in humans or animals.

Lethal Dose(50)(LD50)-The dose of a chemical which 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 dose 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.

Malformations-Permanent structural changes that may adversely affect survival, development, or function.

Minimal Risk Level-An estimate of daily human exposure to a dose of a chemical that is likely to be

without an appreciable risk of adverse noncancerous effects over a specified duration of exposure.

Mutagen-A substance that causes mutations. A mutation is a change in the genetic material in a body cell.

Mutations can lead to birth defects, miscarriages, or cancer.

Neurotoxicity-The occurrence of adverse effects on the nervous system following exposure to chemical.

No-Observed-Adverse-Effect Level (NOAEL)-The dose of 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.

Permissible Exposure Limit (PEL)-An allowable exposure level in workplace air averaged over an 8-hour

shift.
SULFUR DIOXIDE 185

9. GLOSSARY

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).

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 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

CERCLA. Reportable quantities are (1) 1 pound or greater or (2) for selected substances, an amount

established by regulation either under CERCLA or under Sect. 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.

Short-Term Exposure Limit (STEL)-The maximum concentration to which workers can be exposed for

up to 15 min continually. No more than four excursions are allowed per day, and there must be at least 60

min between exposure periods. The daily TLV-TWA may not be exceeded.

Specific Airway Resistance (SRaw)In physiology, the resistance or opposition to flow of gases during

ventilation due to obstruction or turbulent flow in the upper and lower airways.

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)-A concentration of a substance to which most workers can be exposed

without adverse effect. The TLV may be expressed as a TWA, as a STEL, or as a CL.

Time-Weighted Average (TWA)-An allowable exposure concentration averaged over a normal 8-hour

workday or 40-hour workweek.

Toxic Dose (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.

Uncertainty Factor (UF)-A factor used in operationally deriving the RfD 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 LOAEL data rather

than NOAEL data. Usually each of these factors is set equal to 10.
SULFUR DIOXIDE A-1

APPENDIX A

ATSDR MINIMAL RISK LEVELS AND WORKSHEETS

The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) 142 U.S.C. 9601

et seq.], as amended by the Superfund Amendments and Reauthorization Act (SARA) [Pub. L. 9994991,

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, summaryI 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.
SULFUR DIOXIDE A-2

APPENDIX A

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 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, MREs 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 a hundredfold 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, expert panel peer reviews, and agencywide 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, MRL,s in the

most recent toxicological profiles supersede previously published levels. For additional information

regarding MRLs, please contact the Division of Toxicology, Agency for Toxic Substances and Disease

Registry, 1600 Clifton Road, Mailstop E-29, Atlanta, Georgia 30333.


SULFUR DIOXIDE A-5

APPENDIX A

was seen in asthmatics following a 3 minute exposure (via mouthpiece) to ≥0.25 ppm sulfur dioxide (Myers

et al. 1986a; Myers et al. 1986b).

Increases in specific airway resistance were observed in moderately exercising asthmatics exposed oronasally

to 0.25 ppm sulfur dioxide for 5 minutes (Bethel et al. 1985). This study could have also been used to

develop an MRL. An uncertainty factor of 30 would have been required (10 for the use of a LOAEL and 3

for human variability). Dividing the LOAEL of 0.25 ppm by an uncertainty factor of 30 results in an MRL

of 0.01 ppm, a value consistent with the MRL derived from the Sheppard et al. (1981) study.

Some studies of asthmatics have reported a lack of significant lung function changes in asthmatics following

exposures to 0.1-0.5 ppm (Jorres and Magnussen 1990; Koenig et al. 1990). Bronchoconstrictive responses

to sulfur dioxide are highly variable among individual asthmatics (Horstman et al. 1986). In some studies

asthmatics were preselected for sensitivity to sulfur dioxide and this may explain the range of sulfur dioxideinduced responses obtained by different investigators.

The dose level of 0.1 ppm sulfur dioxide can be considered a minimal LOAEL.

Agency Contact (Chemical Manager): Hana Pohl

Agency Review Date: 1° review:_______

2° review:_______

SULFUR DIOXIDE 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

Tables and Figures for Levels of Significant Exposure (LSE)

Tables (2-1,2-2, and 2-3) and figures (2-1 and 2-2) 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, minimal risk levels (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 No-Observed-Adverse- Effect Levels (NOAELs),

Lowest-Observed-Adverse-Effect Levels (LOAELs), or Cancer Effect Levels (CELs).

The legends presented below demonstrate the application of these tables and figures. Representative

examples of LSE Table 2-1 and Figure 2-1 are shown. The numbers in the left column of the legends

correspond to the numbers in the example table and figure.

LEGEND

See LSE Table 2-l

(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. When sufficient data
SULFUR DIOXIDE B-2

APPENDIX B

exists, 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 Table 2-1,

2-2, and 2-3, respectively). LSE figures are limited to the inhalation (LSE Figure 2-l) and oral (LSE

Figure 2-2) 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 2 ” 18r” data

points in Figure 2-1).

(5) Species The test species, whether animal or human, are identified in this column. Section 2.5,

“Relevance to Public Health,” covers the relevance of animal data to human toxicity and Section 2.3,

“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 regimen

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 1,1,2,2-tetrachloroethane via inhalation for

6 hours per day, 5 days per week, for 3 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, 1 systemic effect (respiratory) was investigated.

(8) NOAEL A No-Observed-Adverse-Effect Level (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
SULFUR DIOXIDE B-3

APPENDIX B

the respiratory system which was used to derive an intermediate exposure, inhalation MRL of 0.005

ppm (see footnote “b”).

(9) LOAEL A Lowest-Observed-Adverse-Effect Level (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

endpoint 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 8 of the profile.

(11) CEL A Cancer Effect Level (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 the NOAEL of 3 ppm in key number 18 was used to derive an MRL

of 0.005 ppm.

LEGEND

See Figure 2-l

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 intermediate and chronic 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, 18r NOAEL is the critical endpoint for which an intermediate inhalation

 exposure MRL is based. As you can see from the LSE figure key, the open-circle symbol indicates to a
SULFUR DIOXIDE B-4

APPENDIX B

NOAEL for the test species-rat. 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 38r is 1 of 3 studies for which Cancer Effect Levels were derived. The diamond

symbol refers to a Cancer Effect Level for the test species-mouse. The number 38 corresponds to the

entry in the LSE table.

(18) Estimated Upper-Bound Human Cancer Risk Levels This is the range associated with the upper-bound

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 (ql*).

(19) Key to LSE Figure The Key explains the abbreviations and symbols used in the figure.

The Relevance to Public Health section provides a health effects summary based on evaluations of existing

toxicologic, epidemiologic, and toxicokinetic information This summary is designed to present interpretive,

weight-of-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 section covers end points in the same order they appear within the Discussion of Health Effects by Route

of Exposure section, by route (inhalation, oral, 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 section. If

data are located in the scientific literature, a table of genotoxicity information is included.

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 Data Needs section.


SULFUR DIOXIDE B-7

APPENDIX B

Chapter 2 (Section 2.5)

Relevance to Public Health

Interpretation of Minimal Risk Levels

Where sufficient toxicologic information is available, we have derived minimal risk levels (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. They 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 2.5,

“Relevance to Public Health,” contains basic information known about the substance. Other sections such as

2.8, “Interactions with Other Substances,” and 2.9, “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 the Environmental Protection Agency (EPA) provides (Barnes

and Dourson 1988) to determine reference doses for lifetime exposure (RfDs).

To derive an MRL, ATSDR generally selects the most sensitive endpoint 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 endpoint are available, ATSDR derives an MRL using the most sensitive species (when

information from multiple species is available) with the highest 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 LSE Tables.