Air Toxics Hot Spots

Program Risk Assessment

Guidelines

The Air Toxics Hot Spots

Program Guidance Manual

for Preparation of Health

Risk Assessments

August 2003

Secretary for Environmental Protection

California Environmental Protection Agency

Winston H. Hickox

Director

Office of Environmental Health Hazard Assessment

Joan E. Denton, Ph.D.

Office of Environmental Health Hazard Assessment 

California Environmental Protection Agency 

1515 Clay Street, 16th Floor 

Oakland, California 94612 (510) 622-3200 

The Air Toxics Hot Spots Program Guidance Manual for 

Preparation of Health Risk Assessments 

August 2003 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Editors and Project Leads: Robert Blaisdell, Ph.D.a

James F. Collins, Ph.D., D.A.B.T.a

Daryn E. Dodge, Ph.D.a

 Gregory Harrisb

Melanie A. Marty, Ph.D.a

Contributing Authors: 

 

Robert Blaisdell, Ph.D.a

 

 Renee Coad b 

James F. Collins, Ph.D., D.A.B.T.a

Daryn E. Dodge, Ph.D. a

 Gregory Harris b

 Michele Houghton b 

 Michelle Komlenic b 

Melanie A. Marty, Ph.D.a

Carol McLaughlin b 

M. Beth Schwehr, M.S. b

Anthony Servin, P.E. b

 

 Glen Villab

Departmental Reviewers: George V. Alexeeff, Ph.D., D.A.B.T.a

 

 

Support Staff: Laurie Blissa

 Michelle Johnsona

 Leon Surgeona

 

a

Office of Environmental Health Hazard Assessment 

b

California Air Resources Board 

i 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

ii 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

The Air Toxics Hot Spots Program 

Guidance Manual for Preparation of Health Risk Assessments 

Executive Summary ........................................................................................................... ES-1 

1. Introduction ........................................................................................................................ 1-1 

1.1 Development of Guidelines .................................................................................... 1-2 

1.2 Use of the Guidance Manual ................................................................................. 1-2 

1.3 Who is Required to Conduct a Risk Assessment .................................................... 1-3 

1.4 The Hot Spots Analysis and Reporting Program (HARP) Software ...................... 1-3 

1.5 Risk Assessment Review Process ........................................................................... 1-4 

1.6 Uncertainty in Risk Assessment ............................................................................. 1-4 

2. Overview of Health Risk Assessment ................................................................................. 2-1 

2.1 The Model for Risk Assessment ............................................................................. 2-1 

2.2 Hazard Identification .............................................................................................. 2-1 

2.3 Exposure Assessment ............................................................................................ 2-1 

2.4 Dose-Response Assessment ................................................................................... 2-2 

2.5 Risk Characterization ............................................................................................. 2-3 

2.5.1 Point Estimate Approach ............................................................................ 2-4 

2.5.2 Stochastic Exposure Assessment ................................................................ 2-4 

2.5.3 Tiered Approach to Risk Assessment ......................................................... 2-5 

3. Hazard Identification – Air Toxics Hot Spots Emissions ................................................... 3-1 

3.1 The Air Toxics Hot Spots List of Substances and Emissions Inventory ................. 3-1 

4. Air Dispersion Modeling .......................................................................................... 4-1 

4.1 Air Dispersion Modeling in Exposure Assessment: Overview ............................. 4-1 

4.2 Emission Inventories .............................................................................................. 4-3 

i
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

4.2.1 Air Toxics Hot Spots Emissions ................................................................ 4-3 

4.2.1.1 Emission Estimates Used in Risk Assessment ......................... 4-4 

A. Molecular Weight Adjustments for the Emissions of 

Metal Compounds .......................................................... 4-4 

4.2.1.2 Release Parameters .................................................................. 4-5 

4.2.1.3 Operation Schedule .................................................................. 4-6 

4.2.1.4 Emission Controls .................................................................... 4-6 

4.2.2 Landfill Emissions ................................................................................. 4-6 

4.3 Source Characterization .................................................................................. 4-7 

4.3.1 Classification According to Source Type ............................................... 4-7 

4.3.1.1 Point Sources ........................................................................... 4-7 

4.3.1.2 Line Sources ............................................................................ 4-7 

4.3.1.3 Area Sources ............................................................................ 4-7 

4.3.1.4 Volume Sources ....................................................................... 4-8 

4.3.2 Classification According to Quantity of Sources ...................................... 4-8 

4.4 Terrain Characterization ......................................................................................... 4-8 

4.4.1 Land Type Classification ....................................................................... 4-8 

4.4.1.1 Land Use Procedure .................................................................. 4-9 

4.4.1.2 Population Density Procedure ................................................... 4-9 

4.4.2 Terrain Topography Classification .................................................... 4-11 

4.4.2.1 Simple Terrain (also referred to as “Rolling Terrain”) .............. 4-11 

4.4.2.2 Complex Terrain ........................................................................ 4-11 

4.5 Level of Detail: Screening vs. Refined Analysis .................................................... 4-11 

4.6 Population Exposure ................................................................................................ 4-12 

4.6.1 Zone of Impact ............................................................................................. 4-12 

4.6.2 Screening Population Estimates for Risk Assessments ............................... 4-13 

ii
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

4.6.3 Refined Population Estimates for Risk Assessments .................................. 4-15 

4.6.3.1 Census Tracts ............................................................................. 4-15 

4.6.3.2 Subcensus Tract ......................................................................... 4-15 

4.6.4 Sensitive Receptor Locations ...................................................................... 4-16 

4.7 Receptor Siting ........................................................................................................ 4-17 

4.7.1 Receptor Points ............................................................................................ 4-17 

4.7.2 Centroid Locations ....................................................................................... 4-18 

4.8 Meteorological Data ............................................................................................. 4-18 

4.9 Model Selection ....................................................................................................... 4-20 

4.9.1 Recommended Models ................................................................................ 4-20 

4.9.2 Alternative Models ...................................................................................... 4-20 

4.10 Screening Air Dispersion Models ........................................................................ 4-23 

4.10.1 SCREEN3 ................................................................................................... 4-23 

4.11 Refined Air Dispersion Models ............................................................................... 4-26 

4.11.1 ISCST3 ........................................................................................................ 4-27 

4.11.1.1 Regulatory Options .............................................................. 4-27 

4.11.1.2 Special Cases ....................................................................... 4-28 

a. Building Downwash ................................................ 4-28 

b. Area Sources ............................................................ 4-28 

c. Volume Source ........................................................ 4-28 

d. Intermediate Terrain ................................................ 4-28 

e. Deposition ................................................................ 4-28 

4.11.1.3 HARP Dispersion Analysis ................................................. 4-29 

4.12 Modeling Special Cases; Specialized Models ......................................................... 4-29 

4.13 Interaction with the District ..................................................................................... 4-29 

iii
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

5. Exposure Assessment – Estimation of Air Concentration and Dose ................................. 5-1 

5.1 Introduction ............................................................................................................. 5-1 

5.2 Criteria for Exposure Pathway Evaluation ............................................................. 5-3 

5.3 Estimation of Concentrations in Air, Soil, and Water .......................................... 5-5 

5.3.1 Air ........................................................................................................... 5-5 

5.3.2 Soil ........................................................................................................... 5-5 

5.3.3 In Water .................................................................................................. 5-7 

5.3.4 Estimation of Concentrations in Vegetation and Animal 

Products .................................................................................................. 5-8 

1. Vegetation ................................................................................... 5-8 

2. Animal Products............................................................................5-10 

3. Fish Products............................................................................... 5-15 

5.4 Estimation of Dose ............................................................................................... 5-15 

5.4.1 Estimation of Exposure Through Inhalation ............................................. 5-16 

5.4.2 Estimation of Exposure Through Dermal Absorption .............................. 5-17 

5.4.3 Estimation of Exposure Through Ingestion .............................................. 5-18 

1. Exposure Through Ingestion of Soil ............................................. 5-18 

2. Exposure Through Ingestion of Water ............................................ 5-19 

3. Exposure Through Ingestion of Food ........................................... 5-20 

a. Plant Products ................................................................ 5-21 

b. Animal Products (Including Fisher-caught fish................. 5-24 

c. Mother’s Milk ................................................................... 5-24 

5.5 References for Kow and Doc Values in Table 5.3 

6. Dose-Response Assessment for Noncarcinogenic Endpoints.............................................. 6-1 

6.1 Derivation of Toxicity Criteria ................................................................................ 6-1 

iv
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

6.2 Description of Acute Reference Exposure Levels .................................................. 6-2 

6.3 Description of Chronic Reference Exposure Levels .............................................. 6-5 

6.4 Description of Chronic Oral (Noninhalation) Reference Exposure Levels ........... 6-10 

7. Dose-Response Assessment for Carcinogens ..................................................................... 7-1 

7.1 Introduction ............................................................................................................. 7-1 

7.2 Definition of Carcinogenic Potency ........................................................................ 7-1 

7.2.1 Description of the Inhalation Cancer Potency Factor ................................. 7-1 

7.2.2 Description of the Oral Cancer Potency Factor .......................................... 7-2 

8. Risk Characterization for Carcinogens and Noncarcinogens and the Requirements for 

Hot Spots Risk Assessments .............................................................................................. 8-1 

8.1 Introduction ............................................................................................................. 8-1 

8.2 Risk Characterization for Cancer Health Effects .................................................... 8-2 

8.2.1 Calculating Inhalation Cancer Risk ............................................................. 8-2 

8.2.2 Calculating Cancer Risk Using Different Exposure Durations ................... 8-3 

A. Residential ..................................................................................... 8-5 

B. Worker ............................................................................................ 8-5 

C. Uses of Exposure Duration Adjustments for On-site 

Receptors.....................................................................8-6 

8.2.3 Speciation for Specific Classes of Compounds: Polycyclic Aromatic 

Hydrocarbons (PAHs) and Polychlorinated Dibenzo-p-dioxins (PCDDs) 

and Dibenzofurans (PCDFs) .................................................................... 8-7 

8.2.4 Determination of Noninhalation (Oral) Cancer Risk .................................. 8-8 

8.2.5 Evaluation of Multipathway (Inhalation and Noninhalation) Cancer 

Risk ........................................................................................................... 8-9 

A. Deposition Rate ............................................................. 8-9 

B. Use of Air Dispersion Modeling Results for Pastures and Water 

Risk Assessment and the HARP Software.............................. 8-9 

v
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

C. Summary of the Tiered Approach to Risk Assessment ................ 8-10 

D. Multipathway Cancer Risk Methodology ................................ 8-11 

8.2.6 Risk Characterization for Stochastic Risk Assessment ............................. 8-12 

8.3 Risk Characterization for Noncarcinogens ............................................................. 8-12 

A. Evaluation of Background Criteria Pollutants ............................................ 8-14 

8.3.1 Noncancer Chronic Inhalation Health Impacts ........................................... 8-14 

8.3.2 Noncancer Chronic Health Impacts from the Oral Route ........................... 8-14 

8.3.3 Evaluation of Chronic Noncancer Multipathway Hazard Quotients 

and Hazard Indices ............................................................................... 8-15 

8.3.4 Noncancer Acute Health Impacts ............................................................... 8-15 

8.4 Population-Level Risk Estimates................................................................ 8-17 

8.4.1 Carcinogenic Risk ..........................................................................8-17 

8.4.2 Population Exposure Estimates of Noncancer Health Impacts ................... 8-18 

9. Summary of the Requirements for a Modeling Protocol and a Health Risk 

Assessment Report.............................................................................................................. 9-1 

9.1 Submittal of Modeling Protocol ............................................................................. 9-1 

9.2 Outline for a Health Risk Assessment Report ........................................................ 9-3 

10. References ...........................................................................................................................10-1 

Appendices

Appendix A: Air Toxics Hot Spots Program List of Substances 

A-I. Substances For Which Emissions Must Be Quantified 

 A-II Substances For Which Production, Use, Or Other Presence Must Be Reported 

A-III Substances Which Need Not Be Reported Unless Manufactured By The Facility

Appendix B: Health and Safety Code Related to the Air Toxics Hot Spots Program

Appendix C: Asbestos Quantity Conversion Factors 

vi
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

Appendix D: Risk Assessment Procedures to Evaluate Particulate Emissions from

Diesel-Fueled Engines 

Appendix E: Toxicity Equivalency Factors for Polychlorinated Dibenzo-p-Dioxins, Dibenzofurans 

and Polychlorinated Biphenyls 

Appendix F: Overview of the Lead Risk Assessment Procedures 

Appendix G: PAH Potency Factors and Selection of Potency Equivalency Factors (PEF) for 

PAHs Based on Benzo[a]pyrene Potency 

Appendix H: Recommendations for Estimating Concentrations of Longer Averaging Periods 

from the Maximum One-Hour Concentration for Screening Purposes 

Appendix I: Calculation Examples for Estimating Potential Health Impacts 

Appendix J: Glossary of Acronyms and Definitions of Selected Terms 

Appendix K: HRA Forms and Maps Used with Air Dispersion Modeling 

Appendix L: OEHHA/ARB Approved Health Values for Use in Hot Spot Facility Risk 

Assessments 

List of Tables Page

Table 4.1 Identification and Classification of Land Use Types..............................4-10 

Table 4.2 Recommended Air Dispersion Models.............................................4-22 

Table 4.3 Recommended Factors to Convert Maximum 1-hour Avg. Concentrations 

to Other Averaging Periods...........................................................4-23 

Table 4.4 Required Input Parameters to Model a Point Source Using SCREEN3........4-24 

Table 4.5 Required Input Parameters to Model a Flare Using SCREEN3.................4-25 

Table 4.6 Required Input Parameters to Model an Area Source Using SCREEN3......4-25 

Table 4.7. Required Input Parameters to Model a Volume Source Using SCREEN3....4-26 

Table 4.8 Basic Input Parameters Required to Model a Point Source Using ISCST3...4-27 

Table 5.1 Specific Pathways to be Analyzed for Each Multipathway Substance...... 5-4 

Table 5.2 Point Estimates for Animal Pathway............................................... 5-11 

vii
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

Table 5.3 Substance Specific Default Values for Multipathway Substances.............5-14 

Table 5.4 Point Estimates for Daily Breathing Rate for 9, 30 and 70-Year 

 Exposure Durations (DBR) (L/kg BW * Day) ...................................5-16 

Table 5.5 Breathing Rate Distributions for 9, 30 And 70-Year Exposure 

 Durations for Stochastic Analysis (L/kg BW * Day)...........................5-17 

Table 5.6 Recommended Point Estimate Values for Dermal Pathway 

 for 9, 30 and 70 Year Exposure Durations and Worker.1

..................... 5-18 

Table 5.7 Soil Ingestion Rates (SIR) for 9, 30 And 70-Year 

 Exposure Durations and Off-Site Worker........................................5-19 

Table 5.8 Point Estimate Water Consumption Ingestion Rates (WIR) for 

 9, 30 and 70-Year Exposure Durations (ml/kg BW * Day)....................5-20 

Table 5.9 Water Ingestion Lognormal Distributions for 9, 30 and 70-Year 

 Exposure Durations (ml/kg BW * Day) (Stochastic Analysis).................5-20 

Table 5.10 Point Estimates for Per Capita Food Consumption Rates 

(g/kg BW * Day).....................................................................5-22 

Table 5.11 Parametric Models for Ages 0-9 Food Consumption Distributions 

(g/kg BW * Day) (Stochastic Analysis)..........................................5-22 

Table 5.12 Parametric Models for Ages 0-70 Food Consumption Distributions 

(g/kg BW * Day) (Stochastic Analysis)..........................................5-23 

Table 5.13 Default Values for Fisher–Caught Fish Consumption (g/kg BW * Day)....5-23 

Table 5.14 Parametric Model for Fisher-Caught Fish Consumption Distribution for 

9, 30 and 70-Year Exposure Scenarios (g/kg BW *Day) 

(Stochastic Analysis).................................................................5-23 

Table 5.15 Point Estimate Values for Breast Milk Consumption Rate 

(ml/kg BW * Day)....................................................................5-26 

Table 5.16 Parametric Model for Breast Milk Consumption Rate for 

9, 30 and 70-Year Exposure Durations (Stochastic Analysis)..................5-26 

Table 5.17 References for Kow Values..........................................................5-26 

Table 5.18 References for Koc Values...........................................................5-26 

Table 6.1 Acute Reference Exposure Levels and Target Organ Systems Impacted......6-3 

viii
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

Table 6.2 Chronic Inhalation Reference Exposure Levels (RELs) and Chronic..........6-6 

Hazard Index Target Organ System(s) 

Table 6.3. Chronic Noninhalation ‘Oral’ Reference Exposure Levels (RELs) 

 and Chronic Hazard Index Target Organ System(s)..............................6-12 

Table 7.1 Inhalation and Oral Cancer Potency Factors.......................................7-3 

Table 8.1 Tiers for Cancer and Noncancer Hot Spots Risk Assessments .......... .......8-2 

Table 8.2 Adjustment Factors to Convert Inhalation Based Cancer Risk Estimates 

 for a Residential Receptor to a Worker Receptor................................ 8-6 

Table 8.3 Mandatory and Site/Route Dependant Exposure Pathways .....................8-8 

 

Table E-1 Toxicity Equivalency Factors: Chlorinated Dibenzo-p-Dioxins & 

Dibenzofurans (Relative To 2,3,7,8-TCDD).......................................E-2 

Table F-1 Percentage of Children with Blood Lead Levels ≥ 10 μg/dl for 

 Various Air Lead Concentrations at Two Exposure Scenarios..................F-3 

Table G-1 Potencies of PAHs and Derivatives.................................................G-2 

Table G-2 OEHHA PEF Weighting Scheme for PAHs and Their Resulting 

Cancer Potency Values................................................................G-3 

Table H.1 Recommended Factors to Convert Maximum 1-Hour Average 

Concentrations to Other Averaging Periods........................................H-2 

Table H.2 Recommended Factors to Convert Maximum 1-Hour Average 

Concentrations to Non-Standard Averaging Periods..............................H-3 

Table H.3 Maximum Annual Average Concentration (χ/Q) Above 

Ambient Conditions - Burning Barrel Emissions...................................H-11 

Table I.1 Annual Average Concentrations at the MEIR and Inhalation 

Cancer Potency Factors.................................................................I-2 

Table I.2 Recommended Default Values.........................................................I-3 

Table I.3 Calculated Doses for Substances...................................................... I-3 

ix
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

Table I.4 Hypothetical Facility Inhalation Cancer Risk.......................................I-4 

Table I.5 Concentrations, Acute RELs, and Target Organ System(s) 

for Substances at the MEIR............................................................I-5 

Table I.6 Individual Hazard Quotients and Total Hazard Index..............................I-6 

Table I.7 Annual Average Concentrations, Chronic RELs, and Target 

Organ Systems for Substances at the MEIR.............................. .........I-8 

Table I.8 Substance-Specific Inhalation and Noninhalation Hazard Quotients 

and the Hazard Index by Target Organ System ..................... .............I-10 

Table L1 OEHHA/ARB Health Values for Use in Hot Spot Facility Risk 

 Assessments.............................................................................L-3 

Table L2 OEHHA/ARB Acute Reference Exposure Values for Use in Hot 

Spot Facility Risk Assessments......................................................L-17 

Table L3 OEHHA/ARB Chronic Inhalation And Oral Reference Exposure 

Levels And Target Organs............................................................L-21

List of Figures Page 

Figure 1 Overview of the Air Dispersion Modeling Process.................................4-2 

Figure H.1 Screening Factors to Convert 1-Hr Max. to Longer Avg. Periods...............H-2 

Figure H.2 Hourly Wind Direction - Los Angeles (January and June)........................H-4 

Appendix K 

Figure 1 Census Tracts View 1...................................................................K-2 

Figure 2 Census Tracts View 2...................................................................K-3 

Figure 4 USGS 7.5 MinuteTopographic Map...................................................K-4 

x
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

ES-1

Executive Summary 

The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments (OEHHA, 2003) (Guidance Manual) is a concise description of the algorithms, 

recommended exposure variates, and cancer and noncancer health values needed to perform a health 

risk assessment (HRA) under the Air Toxics Hot Spots Information and Assessment Act of 1987 

(Hot Spots or AB 2588) (AB 2588, Connelly, Statutes of 1987; Health and Safety Code 

Section 44300 et seq.) (see Appendix B). The information presented in the Guidance Manual is a 

compilation of information presented in the four technical support documents (TSDs) released by the 

Office of Environmental Health Hazard Assessment (OEHHA) for the Hot Spots Program. The four 

TSDs underwent public comment and peer review and were adopted for use in the Air Toxics Hot 

Spots program by the Director of OEHHA. These four TSDs present detailed information on cancer 

and noncancer health effects values and exposure pathway information. Excerpts of these four 

documents are presented in this document. All four TSDs are available on OEHHA’s web site at 

www.oehha.ca.gov. There is relatively little new information in the Guidance Manual since the 

previously adopted TSDs form the basis of the Guidance Manual.

The Guidance Manual supercedes the risk assessment methods presented in The California 

Air Pollution Control Officer’s Association (CAPCOA) Air Toxics Hot Spots Program; Revised 

1992; Risk Assessment Guidelines, October 1993 (CAPCOA, 1993). The Guidance Manual 

scientifically updates health effects values, exposure pathway variates (e.g., breathing rates), and 

presents a tiered approach for performing HRAs. The tiered approach provides a risk assessor with 

flexibility and allows consideration of site-specific differences. Furthermore, risk assessors can tailor the 

level of effort and refinement of an HRA by using the point-estimate exposure assumptions or the 

stochastic treatment of data distributions. The four-tiered approach to risk assessment primarily applies 

to residential cancer risk assessment. OEHHA is not recommending a stochastic approach (Tier-3 and 

Tier-4) for worker receptors or for noncancer chronic evaluations. Only Tier-1 applies to acute 

exposure evaluations. Compared to the CAPCOA 1993 document, the exposure pathways in the 

Guidance Manual remain the same, the exposure algorithms are similar, and risk algorithms have been 

revised to accept the data needed for the tiered risk assessment approach. 

The Guidance Manual also contains example calculations and an outline for a modeling protocol and a 

HRA report. A software program, the Hot Spots Analysis and Reporting Program (HARP), has been 

developed by a contractor in consultation with OEHHA, the Air Resources Board (ARB), and the Air 

Pollution Control or Air Quality Management District (District) representatives. The HARP software is 

the recommended model for calculating and presenting HRA results for the Hot Spots Program. 

Information on obtaining the HARP software can be found on the ARB’s web site at www.arb.ca.gov

under the Hot Spots Program. 

The intent in developing this Guidance Manual and the HARP software is to provide consistent 

risk assessment procedures. The use of consistent risk assessment methods and report presentation has 

many benefits, such as, expediting the preparation and review of HRAs, minimizing revision and 

resubmission of HRAs, allowing a format for facility comparisons, and cost-effective implementation of 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

ES-2

HRAs and the Hot Spots Program. Risk assessments prepared with this Guidance Manual may be used 

for permitting new or modified stationary sources, or public notification, and risk reduction requirements 

of the Hot Spots Program.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

1-1

1. Introduction

1.1 Development of Guidelines

The Hot Spots Act is designed to provide information to state and local agencies and to the 

general public on the extent of airborne emissions from stationary sources and the potential public health 

impacts of those emissions. The Hot Spots Act requires that OEHHA develop risk assessment 

guidelines for the Hot Spots program (Health and Safety Code (HSC) Section 44360(b)(2)) (see 

Appendix B for the text of the HSC). In addition, the Hot Spots Act specifically requires OEHHA to 

develop a “likelihood of risks” approach to health risk assessment. In response, OEHHA developed a 

tiered approach to risk assessment where a point-estimate approach is first employed. If a more 

detailed analysis is needed, OEHHA has developed a stochastic, or probabilistic, approach using 

exposure factor distributions that can be applied in a stochastic estimate of the exposure. A detailed 

presentation of the tiered approach, risk assessment algorithms, selected exposure variates 

(e.g., breathing rate), and distributions with a literature review is presented in the Air Toxics Hot Spots 

Risk Assessment Guidelines; Part IV; Exposure Assessment and Stochastic Analysis Technical 

Support Document (OEHHA, 2000b) (Part IV TSD). A summary of this information can be found in 

Chapter 5 of this document. 

Cancer and noncancer (acute and chronic) dose-response relationships (health effects values) 

for many Hot Spots substances are presented in the first three Technical Support Documents. The Air 

Toxics Hot Spots Program Risk Assessment Guidelines; Part I; The Determination of Acute 

Reference Exposure Levels for Airborne Toxicants (OEHHA, 1999a) presents acute Reference 

Exposure Levels (RELs) for 51 toxicants and toxicant compound classes. The Air Toxics Hot Spots 

Program Risk Assessment Guidelines; Part II; Technical Support Document for Describing 

Available Cancer Potency Factors (OEHHA, 1999b and 2002) contains inhalation cancer potency 

factors and oral cancer potency factors for 122 toxicants and toxicant compound classes developed by 

OEHHA or developed by other authoritative bodies and endorsed by OEHHA. The Air Toxics Hot 

Spots Program Risk Assessment Guidelines; Part III; Technical Support Document for the 

Determination of Noncancer Chronic Reference Exposure Levels (OEHHA, 2000a) documents the 

development of chronic noncancer inhalation RELs for 72 toxicants and toxicant classes. The OEHHA 

website (www.oehha.ca.gov) should be consulted for chronic RELs adopted subsequent to (OEHHA, 

2000a). In addition, for a small subset of these substances that are subject to airborne deposition and 

hence human oral and dermal exposure, oral chronic RELs are presented. A summary of cancer and 

noncancer health effects values can be found in Appendix L and Chapters 6 and 7 of the Guidance 

Manual. All four Technical Support Documents have undergone public and peer review and have been 

endorsed by the state’s Scientific Review Panel on Toxic Air Contaminants and adopted by OEHHA. 

The Guidance Manual has also undergone the same public and peer review process.

The Guidance Manual contains a concise description of the algorithms, recommended exposure 

variates, and cancer and noncancer health values needed to perform a Hot Spots risk assessment under 

the Hot Spots Act (see Appendix B). The information for the Guidance Manual is taken from the other 

four TSDs. The Guidance Manual is the successor document to The CAPCOA Air Toxics “Hot 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

1-2

Spots” Program; Revised 1992; Risk Assessment Guidelines, October 1993 prepared by 

CAPCOA (CAPCOA, 1993). The Guidance Manual scientifically updates risk assessment variates 

and presents a tiered approach including a stochastic as well as a point-estimate approach to exposure 

and risk assessment. The exposure pathways remain the same and the algorithms are similar to the 

1993 CAPCOA document. 

The Guidance Manual is intended to address health risks from airborne contaminants released by 

stationary sources. Some of the methodology used is common to other regulatory risk assessment 

applications, particularly for California programs. However, if the reader needs to prepare an HRA under

another program, the HRA may need additional analyses. Therefore, appropriate California and federal 

agencies should be contacted. For example, if a facility must comply with HRA requirements under the 

Resource Conservation and Recovery Act (RCRA) or the Comprehensive Environmental Response, 

Compensation and Liability Act (CERCLA), the California Department of Toxic Substances Control 

(DTSC) must be contacted to determine if an HRA written to comply with AB 2588 will also satisfy 

RCRA/CERCLA requirements.

1.2 Use of the Guidance Manual

The intent in developing this Guidance Manual is to provide HRA procedures for use in the Air 

Toxics Hot Spots Program or for the permitting of new or modified stationary sources. See the ARB’s 

website at www.arb.ca.gov for more information on the Hot Spots Program and for risk management 

guidelines that provide recommendations for permitting new or modified stationary sources. The use of 

consistent risk assessment procedures and report presentation allows comparison of one facility to 

another, expedites the review of HRAs by reviewing agencies, and minimizes revision and resubmission of 

HRAs. However, OEHHA recognizes that no one risk assessment procedure or set of exposure variates 

could perfectly address the many types of stationary facilities in diverse locations in California. Therefore 

a tiered risk assessment approach was developed to provide flexibility and allow consideration of sitespecific differences. 

These guidelines should be used in conjunction with the emission data collected and reported 

pursuant to requirements of the ARB’s Emission Inventory Criteria and Guidelines Regulations (Title 

17, California Code of Regulations, Sections 93300-93300.5), and the Emission Inventory Criteria 

and Guidelines Report for the Air Toxics “Hot Spots” Program (EICG Report), which is 

incorporated by reference therein (ARB, 1997). This regulation outlines requirements for the collection of 

emission data, based on an inventory plan, which must be approved by the Air Pollution Control or Air 

Quality Management District (District). The emissions reported under this program are routine or 

predictable and include continuous and intermittent releases and predictable process upsets or leaks. 

Emissions for unpredictable releases (e.g., accidental catastrophic releases) are not reported under this 

program.

For landfill sites, these guidelines should be applied to the results of the landfill testing required 

under Health and Safety Code Section 41805.5 as well as to any emissions reported under the emission 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

1-3

inventory requirements of the Air Toxics Hot Spots Act (e.g., from flares or other on-site equipment). 

Districts should be consulted to determine the specific landfill testing data to be used.

1.3 Who is Required to Conduct a Risk Assessment

The Hot Spots Act requires that each local District determine which facilities will prepare an 

HRA. As defined under the Hot Spots Act, an HRA includes a comprehensive analysis of the dispersion 

of hazardous substances in the environment, their potential for human exposure, and a quantitative 

assessment of both individual and populationwide health risks associated with those levels of exposure. 

Districts are to determine which facilities will prepare an HRA based on a prioritization process 

outlined in the law. The process by which Districts identify priority facilities for risk assessment involves 

consideration of potency, toxicity, quantity of emissions, and proximity to sensitive receptors such as 

hospitals, daycare centers, schools, work-sites, and residences. The District may also consider other 

factors that may contribute to an increased potential for significant risk to human receptors. As part of this 

process Districts are to categorize facilities as high, intermediate, or low priority. The District prioritization 

process is described in the CAPCOA Air Toxics Hot Spots Program Facility Prioritization 

Guidelines, July 1990 (CAPCOA, 1990). Consult the District for updates to the Prioritization 

Guidelines. See the Hot Spots Program on ARB’s web site at www.arb.ca.gov for more information on 

facility prioritization procedures.

Facilities designated by a District as “high priority” are required to submit an HRA to the District 

within 150 days. Districts may grant a 30-day extension. However, a District may require any facility to 

prepare and submit an HRA according to the District priorities established for purposes of the Hot Spots 

Act. 

1.4 The Hot Spots Analysis and Reporting Program (HARP) Software 

The ARB and the Districts have identified a critical need for software to assist with the 

programmatic aspects of the Hot Spots Program. HARP is a single integrated software package used by 

the ARB, OEHHA, Districts, and facility operators to promote statewide consistency, efficiency, and 

cost-effective implementation of HRAs and the Hot Spots Program. The HARP software package 

consists of three modules that include: 1) the Emissions Inventory Database Module, 2) the Air Dispersion 

Modeling Module, and 3) the Risk Analysis and Mapping Module. The user-friendly Windows-based 

package provides for:

1. Electronic implementation of the risk assessment methods presented in the OEHHA 

guidelines (Guidance Manual);

2. Electronic data transfer from facilities and Districts;

3. The production of reports;

4. Facility prioritization and identification;

5. Air dispersion modeling (ISCST3) of multiple emission releases or facilities for cumulative 

impact evaluations;
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

1-4

6. A summary report of acute and chronic health hazard quotients or indices, and cancer risk 

at the point of maximum impact (PMI), maximally exposed individual resident

(MEIR), and the maximally exposed individual worker (MEIW). (Other receptors may be 

evaluated as needed.); 

7. Mapping displays of facility property boundaries, risk isopleths, street maps, and elevation 

contours;

8. The ability to display combined risk contours from multiple facilities; 

9. Output of data for use in other “off-the-shelf” Geographic Information Systems (GIS) 

programs for additional types of analysis; and

10. Census data for determining the number of people exposed at various cancer risk levels and 

cancer burden.

1.5 Risk Assessment Review Process

The Hot Spots Act risk assessments are reviewed by the local District and by OEHHA. The 

Districts focus their review on the emissions data and the air dispersion modeling. OEHHA provides 

comments on the HRA’s general concordance with the Guidelines Manual and the completeness of the 

reported health risks. The District, taking into account the comments of OEHHA, approves the HRA 

or returns it to the facility for revision and resubmission. If the HRA is not revised and resubmitted by 

the facility within 60 days, the District may modify the HRA and approve it as modified. Based on the 

approved HRA, the District determines if there is a significant health risk associated with emissions from 

the facility. If the District determines that facility emissions pose a significant health risk, the facility 

operator provides notice to all exposed individuals regarding the results of the HRA and may be 

required to take steps to reduce emissions by implementing a risk reduction audit and plan. Notification 

is to be made according to procedures specified by the District. Each District determines its own levels 

of significance for cancer and noncancer health effects for notification and risk reduction. See the Hot 

Spots Program on ARB’s web site at www.arb.ca.gov for more information on significance levels 

selected by each District. 

1.6 Uncertainty in Risk Assessment

OEHHA has striven to use the best science available in developing these risk assessment 

guidelines. However, there is a great deal of uncertainty associated with the process of risk assessment. 

The uncertainty arises from lack of data in many areas necessitating the use of assumptions. The 

assumptions used in these guidelines are designed to err on the side of health protection in order to avoid 

underestimation of risk to the public. Sources of uncertainty, which may either overestimate or 

underestimate risk, include: 1) extrapolation of toxicity data in animals to humans, 2) uncertainty in the 

estimation of emissions, 3) uncertainty in the air dispersion models, and 4) uncertainty in the exposure 

estimates. Uncertainty may be defined as what is not known and may be reduced with further scientific 

studies. In addition to uncertainty, there is a natural range or variability in the human population in such 

properties as height, weight, and susceptibility to chemical toxicants. Scientific studies with representative 

individuals and large enough sample size can characterize this variability. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

1-5

Interactive effects of exposure to more than one carcinogen or toxicant are also not necessarily 

quantified in the HRA. Cancer risks from all emitted carcinogens are typically added, and hazard 

quotients for substances impacting the same target organ/system are added to determine

the hazard index (HI). Many examples of additivity and synergism (interactive effects greater than 

additive) are known. For substances that act synergistically, the HRA could underestimate the risks. 

Some substances may have antagonistic effects (lessen the toxic effects produced by another substance). 

For substances that act antagonistically, the HRA could overestimate the risks. 

Other sources of uncertainty, which may underestimate or overestimate risk, can be found in 

exposure estimates where little or no data are available (e.g., soil half-life and dermal penetration of some 

substances from a soil matrix).

The differences among species and within human populations usually cannot be easily quantified 

and incorporated into risk assessments. Factors including metabolism, target site sensitivity, diet, 

immunological responses, and genetics may influence the response to toxicants. The human population is 

much more diverse both genetically and culturally (e.g., lifestyle, diet) than inbred experimental animals. 

The intraspecies variability among humans is expected to be much greater than in laboratory animals. 

Adjustment for tumors at multiple sites induced by some carcinogens could result in a higher potency. 

Other uncertainties arise 1) in the assumptions underlying the dose-response model used, and 2) in 

extrapolating from large experimental doses, where, for example, other toxic effects may compromise the 

assessment of carcinogenic potential, to usually much smaller environmental doses. Also, only single 

tumor sites induced by a substance are usually considered. When epidemiological data are used to 

generate a carcinogenic potency, less uncertainty is involved in the extrapolation from workplace 

exposures to environmental exposures. However, children, a subpopulation whose hematological, 

nervous, endocrine, and immune systems, for example, are still developing and who may be more sensitive 

to the effects of carcinogens on their developing systems, are not included in the worker population and 

risk estimates based on occupational epidemiological data are more uncertain for children than adults. 

Finally, the quantification of each uncertainty applied in the estimate of cancer potency is itself uncertain. 

Thus, risk estimates generated by an HRA should not be interpreted as the expected rates of 

disease in the exposed population but rather as estimates of potential risk, based on current knowledge 

and a number of assumptions. Additionally, the uncertainty factors integrated within the estimates of 

noncancer RELs are meant to err on the side of public health protection in order to avoid underestimation 

of risk. Risk assessment is best used as a ruler to compare one source with another and to prioritize 

concerns. Consistent approaches to risk assessment are necessary to fulfill this function. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

2-1

2. Overview of Health Risk Assessment

2.1 The Model for Risk Assessment 

The standard approach currently used for health risk assessment (HRA) was originally 

proposed by the National Academy of Sciences in the 1983 book: Risk Assessment in the Federal 

Government: Managing the Process (NAS, 1983) and was updated in the Academy’s 1994 book: 

Science and Judgment in Risk Assessment (NAS, 1994). The four steps involved in the risk 

assessment process are 1) hazard identification, 2) exposure assessment, 3) dose-response assessment, 

and 4) risk characterization. These four steps are briefly discussed below.

2.2 Hazard Identification

For air toxics sources, hazard identification involves identifying if a hazard exists, and if so, what 

are the exact pollutant(s) of concern and whether a pollutant is a potential human carcinogen or is 

associated with other types of adverse health effects. For the Air Toxics Hot Spots Program (Hot 

Spots), the emitted substances that are addressed in a risk assessment are found in the list of hazardous 

substances designated in the ARB’s Emission Inventory Criteria and Guidelines Regulations (Title 

17, California Code of Regulations, Sections 93300-93300.5), and the Emission Inventory 

Criteria and Guidelines Report (EICG Report), which is incorporated by reference therein (ARB, 

1997). This list of substances is contained in Appendix A of this document and the EICG Report. The 

list of substances also identifies those substances that are considered human carcinogens or potential

human carcinogens. 

2.3 Exposure Assessment

The purpose of the exposure assessment is to estimate the extent of public exposure to each 

substance for which potential cancer risk or acute and chronic noncancer effects will be evaluated. This 

involves emission quantification, modeling of environmental transport, evaluation of environmental fate, 

identification of exposure routes, identification of exposed populations, and estimation of short-term and 

long-term exposure levels. These activities are described in Chapters 4 and 5. Chapter 5 also 

discusses the tiered approach to risk assessment. 

The ARB’s EICG Report provides assistance in determining those substances that must be 

evaluated in an HRA and the reporting requirements of facilities, while the Hot Spots Analysis and 

Reporting Program (HARP) software can be used to model ground level concentrations at specific offsite locations resulting from facility emissions. Currently, the most commonly used air modeling software 

is the ISCST3 (Industrial Source Complex Dispersion Model). This air modeling software is 

incorporated into HARP, which allows the user to input all dispersion parameters directly into the 

program to generate air dispersion data. Alternatively, the air dispersion data may be generated 

separately from HARP using other air dispersion models, and then imported into HARP to generate risk 

estimates. Data imported into HARP must already be in the format required by HARP. HARP has the 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

2-2

flexibility to generate a summary of the risk data necessary for an HRA by either of the above 

approaches.

Most of the toxicants assessed under the Hot Spots program are volatile organic compounds 

that remain as gases when emitted into the air. These chemicals are not subject to appreciable 

deposition to soil, surface waters, or plants. Therefore, human exposure does not occur to any 

appreciable extent via ingestion or dermal exposure. Significant exposure to these volatile organic 

toxicants emitted into the air only occurs through the inhalation pathway. A small subset of Hot Spots 

substances, semi-volatile organic and metal toxicants, is emitted partially or totally as particles subject to 

deposition. Ingestion and dermal pathways as well as the inhalation pathway must be evaluated for 

these chemicals. Table 5.1 in Chapter 5, Table 6.3 in Chapter 6, and Table 7.1 in Chapter 7 list the 

substances that must be evaluated for multipathway impacts. HARP is designed to assess potential 

health impacts posed by substances that must be analyzed by a multipathway approach. 

2.4 Dose-Response Assessment

Dose-response assessment is the process of characterizing the relationship between exposure to 

an agent and incidence of an adverse health effect in exposed populations. In quantitative carcinogenic 

risk assessment, the dose-response relationship is expressed in terms of a potency slope that is used to 

calculate the probability or risk of cancer associated with an estimated exposure. Cancer potency 

factors are expressed as the 95th percent upper confidence limit of the slope of the dose response curve 

estimated assuming continuous lifetime exposure to a substance at a dose of one milligram per kilogram 

of body weight-day and commonly expressed in units of inverse dose (i.e., (mg/kg/day)-1). It is 

assumed in cancer risk assessments that risk is directly proportional to dose and that there is no 

threshold for carcinogenesis. The Office of Environmental Health Hazard Assessment (OEHHA) has 

compiled cancer potency factors, which should be used in risk assessments for the Hot Spots program, 

in Table 7.1. For clarity, consistency, and to assure proper use in risk assessment, cancer potencies 

should not be modified. Cancer potency factors listed in Table 7.1 were derived either by the United 

States Environmental Protection Agency (U.S. EPA) or by OEHHA and underwent public and peerreview and were adopted for use in the program. Chapter 8 describes procedures for use of potency 

values in estimating excess cancer risk. For a detailed description of cancer potency factors, refer to 

The Air Toxics Hot Spots Program Risk Assessment Guidelines; Part II; Technical Support 

Document for Describing Available Cancer Potency Factors (OEHHA, 1999b and 2002). 

For noncarcinogenic effects, dose-response data developed from animal or human studies are 

used to develop acute and chronic noncancer Reference Exposure Levels (RELs). The acute and 

chronic RELs are defined as the concentration at which no adverse noncancer adverse health effects are 

anticipated. The most sensitive health effect is chosen to determine the REL if the chemical affects 

multiple organ systems. Unlike cancer health effects, noncancer acute and chronic health effects are 

generally assumed to have thresholds for adverse effects. In other words, acute or chronic injury from a 

pollutant will not occur until exposure to that pollutant has reached or exceeded a certain concentration 

(i.e., threshold). The acute and chronic RELs are intended to be below the threshold for health effects 

for the general population. The actual threshold for health effects in the general population is generally 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

2-3

not known with any precision. Uncertainty factors are applied to the Lowest Observed Adverse Effects 

Level (LOAEL) or No Observed Adverse Effects Level (NOAEL) or Benchmark Concentration 

values from animal or human studies to help ensure that the chronic and acute REL values are below the 

threshold for human health for nearly all individuals. This guidance manual provides the acute and 

chronic Reference Exposure Levels in Tables 6.1 and 6.2, respectively. Some substances that pose a 

chronic inhalation hazard may also present a chronic hazard via non-inhalation routes of exposure (e.g., 

ingestion of contaminated water, foods, or soils, and dermal absorption). The ‘oral’ RELs for these 

substances are presented in Table 6.3. The methodology and derivations for acute and chronic RELs 

are described in the Air Toxics Hot Spots Program Risk Assessment Guidelines; Part I; The 

Determination of Acute Reference Exposure Levels for Airborne Toxicants (Part I TSD) 

(OEHHA 1999a) and Air Toxics Hot Spots Program Risk Assessment Guidelines; Part III; 

Technical Support Document for the Determination of Chronic Reference Exposure Levels (Part 

III TSD)(OEHHA 2000a).

2.5 Risk Characterization

This is the final step of risk assessment. In this step, modeled concentrations and public 

exposure information, which are determined through exposure assessment, are combined with potency 

factors and RELs that are developed through dose-response assessment. The use of cancer potency 

factors to assess total cancer risk and the use of the hazard index approach for evaluating the potential 

for noncarcinogenic health effects are described in Chapter 8. Example calculations for determining 

(inhalation) cancer risk and acute and chronic hazard quotients and hazard indices are presented in 

Appendix I. Chapter 9 provides an outline that specifies the content and recommended format of HRA 

results. 

Under the Hot Spots Act, health risk assessments are to quantify both individual and 

population-wide health impacts (Health and Safety Code, Section 44306). The health risk assessments 

are facility specific and the calculated risk should be combined for all pollutants emitted by a single 

facility. For example, cancer risk from multiple carcinogens is considered additive. For exposures to 

multiple non-carcinogen pollutants, a hazard index approach is applied for air contaminants affecting the 

same organ system. Any emitted toxicant, that is not included in the quantitative analysis due to lack of 

a potency value or REL, should be qualitatively identified. 

For assessing risk, OEHHA has developed two methods for determining dose via inhalation, 

dermal absorption, and ingestion pathways. These two methods, the point-estimate approach and the 

stochastic exposure assessment approach, are described below and in Chapters 5 and 8. Detailed 

presentations of these methods can be found in The Air Toxics Hot Spots Program Risk Assessment 

Guidelines; Part IV; Technical Support Document for Exposure Assessment and Stochastic 

Analysis (OEHHA, 2000b) (Part IV TSD). 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

2-4

2.5.1 Point-Estimate Approach

The traditional approach used in the previous California Air Pollution Control Officer’s 

Association (CAPCOA) Air Toxics Hot Spots Program; Revised 1992; Risk Assessment 

Guidelines, October 1993 (CAPCOA, 1993) (CAPCOA Guidelines) for exposure and risk 

assessment has been to assign a single high-end point-estimate for each exposure pathway 

(e.g., breathing rate). A high-end value was generally chosen so that the potential cancer risk will not be 

underestimated. However, in the past, the high-end point-estimate has not been well defined as to 

where it fell on a data distribution. An improvement over the single point-estimate approach is to select 

two values, one representing an average and another representing a defined high-end value. OEHHA 

provides information in this document on average and high-end values for key exposure pathways (e.g., 

breathing rate). The average and high-end of point-estimates in this document are defined in terms of 

the probability distribution of values for that variate. The mean represents the average values for pointestimates and the 95th percentiles represent the high-end point-estimates from the distributions identified 

in OEHHA (2000b). Thus, within the limitations of the data, average, and high-end point-estimates are 

supported by the distribution.

Tier-1 of the tiered approach to risk assessment, which is briefly discussed in Section 2.5.3 and 

presented in more detail in Chapter 8, utilizes a combination of the average and high-end point-estimates 

to more realistically estimate exposure. This method uses high-end exposure estimates for driving 

exposure pathways and the average point-estimate for non-driving exposure pathways. The HARP 

software can perform this analysis.

In addition to using an estimate of average and high-end consumption rates, cancer risk 

evaluations for 9, 30, and 70-year exposure durations can be presented instead of just a single 70-year 

exposure duration. While 9 and 30-year exposure durations are available to present potential impacts 

over a range of residency periods, all HRAs must present the results based on 70-year exposure. The 

9-and 30-year durations correspond to the central tendency and high-end estimates for residency time 

recommended by (U.S. EPA, 1997b). The parameters used for the 9-year exposure scenario are for 

the first 9-years of life and are thus protective of children. Children have higher intake rates on a per 

kilogram body weight basis and thus receive a higher dose from contaminated media. See Chapter 5 

for the point-estimates that can be used to estimate impacts for children. Chapters 5 and 8 discuss how 

to calculate cancer risk based on various exposure durations and point-estimates. Appendix I contains 

an example calculation and Chapter 9 clarifies how to present the findings in an HRA.

2.5.2 Stochastic Exposure Assessment

OEHHA was directed under Senate Bill (SB) 1731 to develop a “likelihood of risk” approach 

to risk assessment. To satisfy this requirement, OEHHA developed a stochastic approach to risk 

assessment that utilizes distributions for exposure variates such as breathing rate and water consumption 

rate rather than a single point-estimate. The variability in exposure can be propagated through the risk 

assessment model using the distributions as input and a Monte Carlo or similar method. The result of 

such an analysis is a range of risks that at least partially characterizes variability in exposure. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003. 

2-5

Distributions of key exposure variates that are presented in the Part IV TSD were taken from 

the literature, if adequate, or developed from raw data of original studies. Intake variates such as 

vegetable consumption are relatively data rich; for these variates reasonable probability distributions can 

be constructed. However, the data necessary to characterize the variability in risk assessment variates 

are not always available. For example, for the fate and transport parameters (e.g., fish bioconcentration 

factors), there are only a few measurements available which precludes the adequate characterization of 

a probability distribution. We only developed distributions for those key exposure variates that were 

adequately characterized by data. Development of distributions is described in detail in the Part IV 

TSD. 

2.5.3 Tiered Approach to Risk Assessment

OEHHA recommends using a tiered approach to risk assessment. Tier-1 is a standard pointestimate approach using the recommended point-estimates presented in this document. If site-specific 

information is available to modify some point-estimates developed in the Part IV TSD and is more 

appropriate to use than the recommended point-estimates in this document, then Tier-2 allows use of 

that site-specific information. In Tier-3, a stochastic approach to exposure assessment is used with the 

data distributions developed in Part IV TSD and presented in this document. Tier-4 is also a stochastic 

approach but allows for utilization of site-specific distributions, if they are justifiable and more 

appropriate for the site under evaluation than those recommended in this document. Persons preparing 

an HRA that has a Tier-2 through Tier-4 evaluation must also include the results of a Tier-1 evaluation. 

Tier-1 evaluations are required for all HRAs prepared for the Hot Spots Program. Chapter 8 provides 

a summary of the tiered approach and the Part IV TSD discusses it in detail. Chapter 9 provides an 

outline that specifies the content and recommended format of HRA results. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

3-1

3. Hazard Identification - Air Toxics Hot Spots Emissions

3.1 The Air Toxics Hot Spots List of Substances and Emissions Inventory

For air toxics sources, hazard identification involves identifying pollutants of concern and 

whether these pollutants are potential human carcinogens or associated with other types of adverse 

health effects. For the Air Toxics Hot Spots (Hot Spots) Program, the emitted substances that are 

addressed in a health risk assessment (HRA) are found in the list of hazardous substances designated in 

the Air Resources Board’s (ARB’s) Emission Inventory Criteria and Guidelines Regulations (Title 

17, California Code of Regulations, Sections 93300-93300.5), and the Emission Inventory 

Criteria and Guidelines Report (EICG Report), which is incorporated by reference therein (ARB, 

1997). This list of substances is contained in Appendix A of this document and the EICG Report. The 

list of substances also identifies those substances that are considered human carcinogens or potential 

human carcinogens. 

The substances included on the Hot Spots Program list of substances are defined in the statute 

as those substances found on lists developed by the following sources:

• International Agency for Research on Cancer (IARC);

• U.S. Environmental Protection Agency (U.S. EPA);

• U.S. National Toxicology Program (NTP);

• ARB Toxic Air Contaminant Identification Program List;

• Hazard Evaluation System and Information Service (HESIS) (State of California);

• Proposition 65 Safe Drinking Water and Toxic Enforcement Act of 1986 list of carcinogens 

and reproductive toxicants (State of California).

All substances emitted by the facility that are on the Hot Spots Act list of substances must be 

identified in the HRA.

The ARB EICG Report specifies that each facility subject to the Hot Spots Act must submit an 

Emission Inventory Report to the local air pollution control or air quality management district. This 

Emission Inventory Report must identify and account for all listed substances used, manufactured, 

formulated, or released by the facility. All routine, predictable releases must be reported. These 

inventory reports include the emission data necessary to estimate off-site levels of facility-released Hot 

Spots substances. These inventory reports will be discussed in further detail in Chapter 4. See Chapter 9 

for an outline that specifies the content and recommended format for presenting the air dispersion 

modeling and HRA results. As presented in Appendix A, the EICG Report divides the list into three 

groups for reporting purposes. Potency or severity of toxic effects and potential for facility emission were 

considered in placing compounds into the three groups.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

3-2

For the first group (listed in these guidelines in Appendix A-I), all emissions of these substances 

must be quantified in the HRA. For substances in the second group (listed in these guidelines in Appendix 

A-II), emissions are not quantified; however, facilities must report whether the substance is used, 

produced, or otherwise present on-site (i.e., these substances are simply listed in a table in the HRA). 

Lastly, substances in the third group (Appendix A-III) also only need to be reported in a table in the HRA 

if they are manufactured by the reporting facility.

Facilities that must comply with the Resource Conservation and Recovery Act and 

Comprehensive Environmental Response, Compensation and Liability Act (RCRA/CERCLA) 

requirements for risk assessment need to consult the California Department of Toxic Substances Control 

(DTSC) Remedial Project Manager to determine which substances must be evaluated in their risk 

assessment. Some RCRA/CERCLA facilities may emit substances which are not currently listed under 

the Hot Spots Program but which may require evaluation in a RCRA/CERCLA risk assessment.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-1

4. Air Dispersion Modeling

The information contained in this section is primarily an abbreviated version of the material found 

in Chapter II of the Air Toxics Hot Spots Risk Assessment Guidelines; Part IV; Exposure 

Assessment and Stochastic Analysis Technical Support Document (OEHHA, 2000b) (Part IV 

TSD). Several references have been included in this section to indicate those areas that are covered in 

more detail in the Part IV TSD. However, some air dispersion concepts and procedures have been 

added or updated to assist the reader in the health risk assessment (HRA) process. In particular, a brief 

summary of the Hot Spots Analysis and Reporting Program (HARP) software applicability to air 

dispersion analysis has been included. The HARP software has been developed by a contractor 

through the consultation of OEHHA, Air Resources Board (ARB), and Air Pollution Control or Air 

Quality Management District (District) representatives. The HARP software is the recommended 

model for calculating and presenting HRA results for the Air Toxics Hot Spots Program (Hot Spots). 

Information on obtaining the HARP software can be found under the Hot Spots Program on the ARB’s 

web site at www.arb.ca.gov. See Chapter 9 for an outline that specifies the content and recommended 

format for presenting the air dispersion modeling and HRA results. 

Additionally, there are many direct references to the United States Environmental Protection 

Agency (U.S. EPA) ISCST3 air dispersion model. Recently the U.S. EPA has been promoting a new 

air dispersion model to effectively replace the ISCST3 model. Currently this new model, AERMOD, is 

available for testing and review. Once the U.S. EPA adopts the AERMOD air dispersion model into 

their list of regulatory approved models, the references and recommendations to specific models in this 

document are likely to change. 

4.1 Air Dispersion Modeling in Exposure Assessment: Overview

The concentration of pollutants in ambient air is needed to characterize both inhalation and 

noninhalation exposure pathways. Pollutant concentrations are required in HRA calculations to estimate 

the potential cancer risk or hazard indices associated with the emissions of any given facility. Although 

monitoring of a pollutant provides excellent characterization of its concentrations, it is time consuming, 

costly, and typically limited to a few receptor locations and snapshots in time. Air dispersion modeling 

has the advantage of being relatively inexpensive and is less time consuming, provided that all the model 

inputs are available. In addition, air dispersion modeling provides greater flexibility for placement of 

receptors, assessment of individual and cumulative source contributions, and characterization of 

concentration over greater spatial extents. 

Air dispersion modeling requires the execution of the following steps (see Fig 1):

1. Complete an emission inventory of the toxic releases (Section 4.2);

2. Classify the emissions according to source type and source quantity (Section 4.3);

3. Classify the analysis according to terrain (Section 4.4);
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-2

Figure 1. Overview of the Air Dispersion Modeling Process.

1. Collect Data from Emissions Inventory (Section 4.2)

2. Classify Emissions According to Source Type and Quantity (Section 4.3)

3. Classify Analysis According to Terrain (Section 4.4)

4. Determine Level of Detail for Analysis: Screening or Refined (Section 4.5)

5. Describe Population Exposure (Section 4.6)

6. Determine Receptor Locations (Section 4.7)

7. Obtain Meteorological Data (Section 4.8)*

8. Select an Air Dispersion Model (Section 4.9)

9. Prepare Modeling Protocol and Submit to District (Chapter 9)**

10. Perform Air Dispersion Modeling

Obtain Concentration Field 11. If Necessary, Change 

 Level of Detail for Analysis

12. Estimate Health Risks

13. If Necessary,

 Change Level of Detail for Analysis

14. Prepare HRA Report and Submit to District (Chapter 9)

* Some screening models do not require any meteorological data. 

Reference Exposure 

Levels

Cancer Potency Factors

Other Survey Data
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-3

** Optional but strongly recommended.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-4

4. Determine level of detail for the analysis: refined or screening analysis (Section 4.5);

5. Identify the population exposure (Section 4.6);

6. Determine the receptor locations where impacts need to be analyzed (Section 4.7);

7. Obtain meteorological data (for refined air dispersion modeling only) (Section 4.8);

8. Select an air dispersion model (Section 4.9);

9. Prepare modeling protocol and submit to the local Air District (Chapter 9);

10. Perform an air dispersion analysis;

11. If necessary, redefine the receptor network and return to Step 10;

12. Perform HRA;

13. If necessary, change from screening to refined model and return to Step 8; and

14. Present the HRA results (Chapter 9 provides an outline that specifies the content and 

recommended format of HRA results). 

The output of an air dispersion modeling analysis will be a receptor field of concentrations of the 

pollutant in ambient air. These concentrations in air need to be coupled with Reference Exposure 

Levels and cancer potency factors to estimate the hazard indices and potential carcinogenic risks. It 

should be noted that in the Hot Spots program emissions are considered inert for the purpose of

transport and dispersion towards downwind receptors. Atmospheric transformations are not currently 

estimated. 

4.2 Emission Inventories

The Emission Inventory Reports (Inventory Reports) developed under the Hot Spots Program 

provide data to be used in the HRA and in the air dispersion modeling process. The Inventory Reports 

contain information regarding emission sources, emitted substances, emission rates, emission factors, 

process rates, and release parameters (area and volume sources may require additional release data 

beyond that generally available in Emissions Inventory reports). This information is developed 

according to the ARB’s Emission Inventory Criteria and Guidelines Regulations (Title 17, 

California Code of Regulations, Sections 93300-93300.5), and the Emission Inventory Criteria 

and Guidelines Report (EICG Report), which is incorporated by reference therein (ARB, 1997). 

4.2.1 Air Toxics Hot Spots Emissions

As noted in Chapter 3, Hazard Identification, the HRA should identify all substances emitted by 

the facility, which are on the Hot Spots Act list of substances (see Appendix A of the Guidance Manual 

or the EICG Report). The EICG Report specifies that Inventory Reports must identify and account for 

all listed substances used, manufactured, formulated, or released by the facility. All routine, predictable 

releases must be reported. Substances on the “list to be quantified” must be listed with emission 

quantities in a table in the HRA. For substances in the second and third groups, emissions do not need 

to be quantified; these substances should be listed in a separate table in the HRA. Chapter 9 provides 

an outline that specifies the content and recommended format of HRA results.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-5

4.2.1.1 Emission Estimates Used in the Risk Assessment

The HRA must include emission estimates for all substances that are required to be quantified in 

the facility’s emission inventory report. Specifically, HRAs should include both annual average 

emissions and maximum 1-hour emissions for each pollutant. Emissions for each substance must be 

reported for individual emitting processes associated with unique devices within a facility. Total facility 

emissions for an individual air contaminant will be the sum of emissions, reported by process, for that 

facility. Information on daily and annual hours of operation, and relative monthly activity, must be 

reported for each emitting process. Devices and emitting processes must be clearly identified and 

described and must be consistent with those reported in the emissions inventory report.

The HRA should include tables that present the emission information (i.e., emission rates for 

each substance released from each process) in a clear and concise manner. The District may allow the 

facility operator to base the HRA on more current emission estimates than those presented in the 

previously submitted emission inventory report (i.e., actual enforceable emission reductions realized by 

the time the HRA is submitted to the District). If the District allows the use of more current emission 

estimates, the District must review and approve the new emissions estimates prior to use in the HRA. 

The HRA report must clearly state what emissions are being used and when any reductions became 

effective. Specifically, a table presenting emission estimates included in the previously submitted 

emission inventory report as well as those used for the HRA should be presented. The District should 

be consulted concerning the specific format for presenting the emission information. Chapter 9 provides 

an outline that specifies the content and recommended format of HRA results. A revised emission 

inventory report must be submitted to the District prior to submitting the HRA and forwarded by the 

District to the ARB, if revised emission data are used. 

Facilities that must also comply with RCRA/CERCLA requirements for HRAs need to consult 

the Cal/EPA Department of Toxic Substances Control (DTSC) Remedial Project Manager to 

determine what constitutes appropriate emissions data for use in the HRA. Source testing may be 

required for such facilities even if it is not required under the Hot Spots Program. Additional 

requirements for statistical treatment of source test results may also be imposed by DTSC on 

RCRA/CERCLA facilities.

A. Molecular Weight Adjustments for the Emissions of Metal Compounds

For most of the Hot Spots toxic metals, the OEHHA cancer potency factors apply to the 

weight of the toxic metal atom contained in the overall compound. Some of the Hot Spots compounds 

contain various elements along with the toxic metal atom (e.g., “Nickel hydroxide”, CAS number 

12054-48-7, has a formula of H2NiO2). Therefore, an adjustment to the reported pounds of the overall 

compound is needed before applying the OEHHA cancer potency factor for “Nickel and compounds” 

to such a compound. This ensures that the cancer potency factor is applied only to the fraction of the 

overall weight of the emissions that are associated with health effects of the metal. In other cases, the 

Hot Spots metals are already reported as the metal atom equivalent (e.g., CAS 7440-02-0, “Nickel”), 

and these cases do not use any further molecular weight adjustment. (Refer to Note [7] in Appendix A, 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-6

List of Substances in the EICG Report for further information on how the emissions of various 

Hot Spots metal compounds are reported.)

The appropriate molecular weight adjustment factors (MWAF) to be used along with the 

OEHHA cancer potency factors for Hot Spots metals can be found in the MWAF column1 of the table 

containing OEHHA/ARB Approved Health Values For Use In Hot Spots Facility Risk Assessments 

that is in Appendix L of this document.

As an example, the compound “Nickel hydroxide” has a molecular formula of H2NiO2. The 

atomic weight of each of the elements in this compound, and the fraction they represent of the total 

weight, are therefore as follows:

 Element Atomic Weight Fraction of Total Weight = MWAF

1 x Nickel (Ni) 1 x 58.70 58.70 / 92.714 = 0.6332 ( MWAF for Nickel)

2 x Oxygen (O) 2 x 15.999

2 x Hydrogen (H) 2 x 1.008

-------------------------------------------------

Total Molecular 

Weight of H2NiO2: 92.714

So, for example, assume that 100 pounds of “Nickel hydroxide” emissions are reported under CAS 

number 12054-48-7. To get the Nickel atom equivalent of these emissions, multiply by the listed 

MWAF (0.6332) for Nickel hydroxide: 

• 100 pounds x 0.6332 = 63.32 pounds of Nickel atom equivalent.

This step should be completed prior to applying the OEHHA cancer potency factor for “Nickel and 

compounds” in a calculation for a prioritization score or risk assessment calculation. Note, however, 

that the HARP software automatically applies the appropriate MWAF for each Hot Spots 

chemical (by CAS number), so the emissions should not be manually adjusted when using 

HARP. Therefore, if using HARP, you would use 100 pounds for Nickel hydroxide and HARP 

will make the MWAF adjustment for you. 

4.2.1.2 Release Parameters

In order to use air dispersion models, release parameters (e.g., stack height and inside diameter, 

stack gas exit velocity, release temperature, and emission source location in actual UTM coordinates) 

need to be reported. The EICG Report specifies that the release parameters must be reported for each 

 

1

 The value listed in the MWAF column for Asbestos is not a molecular weight adjustment. This is a conversion 

factor for adjusting mass and fibers or structures. See Appendix C for more information on Asbestos or the EICG 

report for reporting guidance.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-7

stack, vent, ducted building, exhaust site, or other site of exhaust release. Additional information may 

be required to characterize releases from non-stack (volume and area) sources; see U.S. EPA air 

dispersion modeling guidelines or specific user's manuals. This information should also be included in 

the air dispersion portion of the HRA. This information must be presented in tables included in the 

HRA. Note that some dimensional units needed for the dispersion model may require conversion from 

the units reported in the Inventory Report (e.g., degrees K vs. degrees F). Chapter 9 provides an 

outline that specifies the content and recommended format of HRA results. 

4.2.1.3 Operation Schedule

The HRA should include a discussion of the facility operation schedule and daily emission 

patterns. Special weekly or seasonal emission patterns may vary and should be discussed. This is 

especially important in a refined HRA. Diurnal emission patterns should match the diurnal dispersion 

characteristics of the ambient air. Hourly emission scalars are needed to best represent emissions from 

facilities, especially for diurnal pattern. Air dispersion models, such as ISCST3, readily accept hourly 

emissions scalars and these scalars are fully functional in the HARP software with ISCST3. In addition, 

for the purposes of exposure adjustment for an off-site work receptor the emission schedule and 

exposure schedule should corroborate any exposure adjustment factors. (For example, no exposure 

adjustment factor should be made when an off-site receptor and the emissions are on a coincident 

schedule.) Some fugitive emission patterns may be continuous. Additionally, these data are used for 

adjustments in a screening air dispersion analysis (see Appendix H for further details). A table should 

be included with the emission schedule on an hourly, weekly and yearly basis. Chapter 9 provides an 

outline that specifies the content and recommended format of HRA results. 

4.2.1.4 Emission Controls

The HRA should include a description of control equipment, the emitting processes it serves, 

and its efficiency in reducing emissions of substances on the Air Toxics Hot Spots list. The EICG 

Report requires that this information be included in the Inventory Reports, along with the emission data 

for each emitting process. If the control equipment did not operate full-time, the reported overall 

control efficiency must be adjusted to account for downtime of control equipment. Any entrainment of 

toxic substances to the atmosphere from control equipment should be accounted for; this includes 

fugitive releases during maintenance and cleaning of control devices (e.g., baghouses and cyclones). 

Contact the District for guidance with control equipment adjustments. Recommended default deposition 

rates that are used when calculating potential noninhalation health impacts are listed in Section 8.2.4. 

Chapter 9 provides an outline that specifies the content and recommended format of HRA results.

4.2.2 Landfill Emissions

Emission estimates for landfill sites should be based on testing required under Health and Safety 

Code, Section (HSC) 41805.5 (AB 3374, Calderon) and any supplemental AB 2588 source tests or 

emission estimates used to characterize air toxics emissions from landfill surfaces or through off-site 

migration. The District should be consulted to determine the specific Calderon data to be used in the 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-8

HRA. The Hot Spots Program HRA for landfills should also include emissions of listed substances for 

all applicable power generation and maintenance equipment at the landfill site. Processes that need to 

be addressed include stationary internal combustion engines, flares, evaporation ponds, composting 

operations, boilers, and gasoline dispensing systems.

4.3 Source Characterization

The types of sources and quantity of sources at a facility need to be characterized in order to 

select an appropriate air dispersion model.

4.3.1 Classification According to Source Type

Air dispersion models can be classified according to the type of source that they are designed to 

simulate, including, but not limited to, point, line, area, and volume sources. Several models have the 

capability to simulate more than one type of source.

4.3.1.1 Point Sources

Point sources are probably the most common type of source and most air dispersion models 

have the capability to simulate them. Typical examples of point sources include isolated vents from 

buildings and exhaust stacks from facility processes.

4.3.1.2 Line Sources

In practical terms, line sources are a special case of either an area or a volume source. 

Consequently, they are normally modeled using either an area or volume source model as described 

below. Examples of line sources include conveyor belts and rail lines. A roadway is a unique line 

source. Models designed to simulate the enhanced mixing due to motor vehicle movements have been 

developed (i.e., CALINE4 and CAL3QHCR).

4.3.1.3 Area Sources

Emissions, that are to be modeled as area sources, include fugitive sources characterized by 

non-buoyant emissions containing negligible vertical extent of release (e.g., no plume rise or distributed 

over a fixed level).

Fugitive particulate (PM2.5, PM10, TSP) emission sources include areas of disturbed ground 

(open pits, unpaved roads, parking lots), which may be present during operational phases of a facility’s 

life. Also included are areas of exposed material (e.g., storage piles and slag dumps) and segments of 

material transport where potential fugitive emissions may occur (uncovered haul trucks or rail cars, 

emissions from unpaved roads). Fugitive emissions may also occur during stages of material handling 

where particulate material is exposed to the atmosphere (uncovered conveyors, hoppers, and crushers).
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-9

Other fugitive emissions emanating from many points of release at the same elevation may be 

modeled as area sources. Examples include fugitive emissions from valves, flanges, venting, and other 

connections that occur at ground level, or at an elevated level or deck if on a building or structure.

4.3.1.4 Volume Sources

Non-point sources with emissions containing an initial vertical extent should be modeled as 

volume sources. The initial vertical extent may be due to plume rise or a vertical distribution of 

numerous smaller sources over a given area. Examples of volume sources include buildings with natural 

fugitive or passive ventilation, and line sources such as conveyor belts and rail lines.

4.3.2 Classification According to Quantity of Sources

The selection of an air dispersion model also requires the consideration of the number of distinct 

sources. Some dispersion models are capable of simulating only one source at a time, and therefore are 

referred to as single-source models (e.g., SCREEN3).

In some cases, for screening purposes, single-source models may be used in situations involving 

more than one source using one of the following approaches:

1. Combining all sources into one single “representative” source.

In order to be able to combine all sources into one single source, the individual sources must 

have similar release parameters. For example, when modeling more than one stack as a single 

“representative” stack, the stack gas exit velocities and temperatures must be similar. In order 

to obtain a conservative estimate, the values leading to the higher concentration estimates should 

typically be used (e.g., the lowest stack gas exit velocity and temperature, the height of the 

shortest stack, and the shortest distance from the receptor to the nearest stack).

2. Run the model separately for each individual source and superimposing the results.

Superposition of results from each source is the approach used by all the Gaussian models 

capable of simulating more than one source. Simulating sources in this manner may lead to 

conservative estimates if worst-case meteorological data are used or if the approach is used 

with a model that automatically selects worst-case meteorological conditions, especially wind 

direction. The approach will typically be more conservative the farther apart the sources are, 

because each run would use a different worst-case wind direction.

Additional guidance regarding source merging is provided by the U.S. EPA (1995a).

4.4 Terrain Characterization
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-10

Two types of terrain characterizations are required to select the appropriate model. One 

classification is made according to land type and another one according to terrain topography.

4.4.1 Land Type Classification

Most air dispersion models use different dispersion coefficients (sigmas) depending on the land 

use over which the pollutants are being transported. The type of land use is also used by some models 

to select appropriate wind profile exponents. Traditionally, the land type has been categorized into two 

broad divisions for the purposes of dispersion modeling: urban and rural. Accepted procedures for 

determining the appropriate category are those suggested by Irwin (1978): one based on land use 

classification and the other based on population. AERMOD does not depend on the dispersion 

coefficients used by models such as ISCST3. Therefore AERMOD does not need to classify the land 

type into urban or rural. When AERMOD becomes adopted as a Guideline model and is more widely 

used, these recommendations on land use classifications will need to be modified. Until that time, the 

following recommendations are relevant.

The land use procedure is generally considered more definitive. Population density should be 

used with caution and should not be applied to highly industrialized areas where the population density 

may be low. For example, in low population density areas a rural classification would be indicated, but 

if the area is sufficiently industrialized the classification should already be “urban” and urban dispersion 

parameters should be used.

If the facility is located in an area where land use or terrain changes abruptly (e.g., on the coast) 

the District should be consulted concerning the classification. The District may require a classification 

that biases estimated concentrations towards over-prediction. As an alternative, the District may 

require that receptors be grouped according to the terrain between source and receptor.

4.4.1.1 Land Use Procedure

1. Classify the land use within the total area ‘A’, circumscribed by a 3 km radius circle 

centered at the source, using the meteorological land use typing scheme proposed by 

Auer (1978) and shown in Table 4.1.

2. If land use types I1, I2, C1, R2 and R3 account for 50 percent or more of the total area 

‘A’ described in (1), use urban dispersion coefficients. Otherwise, use appropriate rural 

dispersion coefficients.

4.4.1.2 Population Density Procedure

1. Compute the average population density (p) per square kilometer with ‘A’ as defined in 

the Land Use procedure described above. (Population estimates are also required to 

determine the exposed population; for more information see Section 4.6.2 and 4.6.3.).
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-11

2. If p is greater than 750 people/km2 use urban dispersion coefficients; otherwise, use 

appropriate rural dispersion coefficients.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-12

Table 4.1 Identification and classification of land use types (Auer, 1978).

Type Use and Structures Vegetation

I1 Heavy Industrial

Major chemical, steel and fabrication 

industries; generally 3-5 story buildings, flat 

roofs

Grass and tree growth extremely rare; <5% 

vegetation

I2 Light-moderate industrial

Rail yards, truck depots, warehouses, 

industrial parks, minor fabrications; generally 

1-3 story buildings, flat roofs

Very limited grass, trees almost totally 

absent; <5% vegetation

C1 Commercial

Office and apartment buildings, hotels; >10 

story heights, flat roofs

Limited grass and trees; <15% vegetation

R1 Common residential

Single family dwelling with normal 

easements; generally one story, pitched roof 

structures; frequent driveways

Abundant grass lawns and light-moderately 

wooded; >70% vegetation

R2 Compact residential

Single, some multiple, family dwelling with 

close spacing; generally <2 story, pitched 

roof structures; garages (via alley), no 

driveways

Limited lawn sizes and shade trees; <30% 

vegetation

R3 Compact residential

Old multi-family dwellings with close (<2 m) 

lateral separation; generally 2 story, flat roof 

structures; garages (via alley) and ash pits, 

no driveways

Limited lawn sizes, old established shade 

trees; <35% vegetation

R4 Estate residential

Expansive family dwelling on multi-acre 

tracts

Abundant grass lawns and lightly wooded; 

>80% vegetation

A1 Metropolitan natural

Major municipal, state, or federal parks, golf 

courses, cemeteries, campuses; occasional 

single story structures

Nearly total grass and lightly wooded; >95% 

vegetation

A2 Agricultural rural Local crops (e.g., corn, soybean); >95% 

vegetation

A3 Undeveloped

Uncultivated; wasteland

Mostly wild grasses and weeds, lightly 

wooded; >90% vegetation

A4 Undeveloped rural Heavily wooded; >95% vegetation

A5 Water surfaces

Rivers, lakes
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-13

4.4.2 Terrain Topography Classification

Surface conditions and topographic features generate turbulence, modify vertical and horizontal 

winds, and change the temperature and humidity distributions in the boundary layer of the atmosphere. 

These in turn affect pollutant dispersion and various models differ in their needs to adjust for these 

variables.

The classification according to terrain topography should ultimately be based on the topography 

at the receptor location with careful consideration of the topographical features between the receptor 

and the source. The ISCST3 model uses a screening approach to complex terrain. AERMOD also 

provides algorithms for complex terrain.

Topography can be classified according to the following sections.

4.4.2.1 Simple Terrain (also referred to as “Rolling Terrain”)

Simple terrain is all terrain located below stack height including gradually rising terrain (i.e., 

rolling terrain). Note that Flat Terrain also falls in the category of simple terrain.

4.4.2.2 Complex Terrain

Complex terrain is terrain located above plume height. Complex terrain models are necessarily 

more complicated than simple terrain models. There may be situations in which a facility is “overall” 

located in complex terrain but in which the nearby surroundings of the facility can be considered simple 

terrain. In such cases, receptors close to the facility in this area of simple terrain will “dominate” the risk 

analysis and there may be no need to use a complex terrain model.

4.5 Level of Detail: Screening vs. Refined Analysis

Air dispersion models can be classified as “screening” or “refined” according to the level of 

detail that is used in the assessment of the concentration estimates. Refined air dispersion models use 

more robust algorithms that are capable of using representative meteorological data to predict more 

representative and usually less conservative estimates. Refined air dispersion models are, however, 

more resource intensive than their screening counterparts. It is advisable to first use a screening model 

to obtain conservative concentration estimates and calculate health risks. If the health risks are 

estimated to be above the threshold of concern, then use of a refined model to calculate more 

representative concentrations and health risk estimates would be warranted. There are situations when 

screening models represent the only viable alternative (e.g., when representative meteorological data are 

not available). The HARP software addresses these situations by incorporating the capability of using 

either representative meteorological data or the default meteorological conditions from the SCREEN3 

model as inputs to the ISCST3 air dispersion model. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-14

It is acceptable to use a refined air dispersion model in a “screening” mode for this program’s 

HRAs. In this case, worst-case hourly meteorological data are used to estimate the maximum 1-

hour concentration with the ISCST3 model. Conservative conversion factors are used to estimate 

longer term averaging periods based on the maximum 1-hour concentration. (See Table 4.3 and 

Appendix H for guidance on the use of the conversion factors.) 

4.6 Population Exposure

Population exposure can be assessed by determining the number of people at a particular 

cancer risk level such as 1 x 10-5 or 1 x 10-6. For noncancer risk it can be the number of people 

exposed to the Hazard Index over a certain level such as one or five. The traditional way of estimating 

population exposure for cancer has been the cancer burden or the number of excess cancer cases in the 

exposed population. 

The detail required for the analysis (e.g., screening or refined), and the procedures to be used in 

determining geographic resolution and exposed population, require case-by-case analysis and 

professional judgment. The District or reviewing authority should be consulted before beginning the 

population exposure estimates. As results are generated, further consultation may be necessary. Some 

suggested approaches and methods for handling the breakdown of population and performance of a 

screening or detailed risk analysis are provided in this section. In addition, the HARP software can 

provide population exposure estimates as cancer burden or as the number of persons exposed to a 

selected potential (user identified) health risk/impact level. Information on obtaining the HARP software 

can be found under the Hot Spots Program on the ARB’s web site at www.arb.ca.gov. Chapter 9 

provides an outline that specifies the content and recommended format of HRA results.

4.6.1 Zone of Impact

The first step of population exposure estimate in an HRA is to define the zone of impact. The 

zone of impact is the area around the facility that is affected by the facility’s emissions. This zone is 

commonly defined as the area surrounding the facility where receptors have a potential multipathway 

(inhalation and noninhalation exposure) cancer risk greater than 10-6 (one in a million), an acute 

(inhalation) hazard index (HI) of 1.0, and/or a chronic multipathway HI of 1.0. Some Districts may 

prefer to use a cancer risk of 10-7 or an HI of 0.5 as the zone of impact. Therefore, the District should 

be consulted before modeling efforts are initiated. If the zone of impact is greater than 25 km from the 

facility at any point, the District should be consulted. The District may specify limits on the area of the 

zone of impact. Ideally, these preferences would be discussed with the District before being presented 

in the modeling protocol and HRA.

Note that when depicting the HRA results, potential cancer and noncancer isopleths must 

present the total cancer and noncancer health impacts from both inhalation and noninhalation pathways, 

when appropriate. The zone of impact should be clearly shown on a map with geographic markers of 

adequate resolution (see Section 4.6.3.1). The text below discusses methodology for defining the zone 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-15

of impact and has format recommendations. Chapter 9 provides an outline that specifies the content 

and recommended format of all HRA results. 

The zone of impact can be defined once the exposure assessment (air dispersion modeling) 

process has determined the pollutant concentrations at each designated off-site receptor and a risk 

analysis (see Chapter 8) has been performed. For clarity, the cancer and noncancer zone(s) of impact 

should be presented on separate maps. A map illustrating the carcinogenic zone of impact is required. 

The District may at their discretion ask for the map illustrating the potential carcinogenic zone of impact 

to identify the zone of impact for the minimum exposure pathways (inhalation, soil, dermal, and mothers 

milk) and the zone of impact for all applicable pathways of exposure (minimum pathways plus site/route 

dependent pathways). Two maps may be needed to accomplish this. The legend of these maps should 

state the level(s) used for the zone of impact and identify the exposure pathways that were included in 

the assessment.

The noncancer maps should also clearly identify the noncancer zones of impact. These include 

the acute (inhalation) zone of impact and the chronic (including both inhalation, multipathway) zone of 

impact. The District may at its discretion require separate chronic inhalation and chronic multipathway 

zones of impact maps. For clarity, presentation of the two chronic zones of impact may also require 

two or more maps. The legend of these maps should state the level(s) used for the zone of impact and 

identify the exposure pathways (and target organs) that were included in the assessment. Further 

information regarding the methods for determination of hazard indices and cancer risk are discussed in 

Chapter 8 and Appendices I.

4.6.2 Screening Population Estimates for Risk Assessments

Not all HRAs require refined population exposure assessments and at times a screening estimate 

may be appropriate. A screening population estimate should include an estimate of the maximum 

exposed population. The impact area to be considered should be selected to be health protective (i.e., 

will not underestimate the number of exposed individuals). A health-protective assumption is to assume 

that all individuals within a large radius of the facility are exposed to the maximum concentration. If a 

facility must also comply with the RCRA/CERCLA HRA requirements, health effects to on-site 

workers may also need to be addressed. The DTSC’s Remedial Project Manager should be consulted 

on this issue. The District should be consulted to determine the population estimate to be used for 

screening purposes. Guidance for one screening method is presented here. 

1. Use a screening dispersion model (e.g., SCREEN3) to obtain concentration estimates for each 

emitted pollutant at varying receptor distances from the source. Several screening models 

feature the generation of an automatic array of receptors that is particularly useful for 

determining the zone of impact. In order for the model to generate the array of receptors, the 

user needs to provide some information normally consisting of starting distance, increment, and 

number of intervals.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-16

2. Calculate the potential cancer risk and hazard index for each receptor location by using the 

methods provided in the risk characterization sections of this document (Chapter 8).

3. Find the distance where the potential cancer risk is equal to District specified levels (e.g., 10-6); 

this may require redefining the receptor array in order to have two receptor locations that bound 

a total cancer risk of 10-6. This exercise should be repeated for the noncancer health impacts.

4. Calculate cancer burden by estimating the number of people in the grid and stipulate that all are 

exposed at the highest level.

4.6.3 Refined Population Estimates for Risk Assessments

The refined HRA requires a more detailed analysis of the population distribution that is exposed 

to emissions from the facility. These populations can include exposure estimates for workers and 

residents through the use of land use maps. The District may require that locations with high densities of 

sensitive individuals be identified (e.g., schools, daycare centers, hospitals). The overall exposed 

residential and worker populations should be apportioned into smaller geographic subareas. The 

information needed for each subarea is:

1. the number of exposed persons, and 

2. the receptor location at which the calculated ambient air concentration is assumed to be 

representative of the exposure to the entire population in the subarea.

A multi-tiered approach is suggested for the population analysis. Census tracts, which the 

facility could significantly impact, should be identified (see Section 4.6.3.1). A census tract should be 

divided into smaller subareas if it is close to the facility where ambient concentrations vary widely. The 

District may determine that census tracts provide sufficient resolution near the facility to adequately 

characterize population exposure or they may prefer the census information to be evaluated using 

smaller blocks. Further downwind where ambient concentrations are less variable, the census tract level 

may be acceptable to the District. The District may determine that the aggregation of census tracts 

(e.g., when the census tracts making up a city are combined) is appropriate for receptors that are 

considerable distances from the facility. 

If a facility must also comply with the RCRA/CERCLA HRA requirements, health effects to onsite workers may also need to be addressed. The DTSC’s Remedial Project Manager should be 

consulted on this issue. In some cases it may be appropriate to evaluate risks to on-site receptors. The 

district should be consulted about special cases for which evaluation of on-site receptors is appropriate, 

such as facilities frequented by the public or where people may reside (e.g., military facilities).
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-17

4.6.3.1 Census Tracts

For a refined HRA, the boundaries of census tracts can be used to define the geographic area 

to be included in the population exposure analysis. Maps showing census tract boundaries and numbers 

can be obtained from “The Thomas Guide® - Census Tract Edition”. Statistics for each census tract 

can be obtained from the U.S. Census Bureau. Numerous additional publicly accessible or 

commercially available sources of census data can be found on the World Wide Web. A specific 

example of a census tract is given in Appendix K. 

The two basic steps in defining the area under analysis are:

1. Identify the “zone of impact” (as defined previously in Section 4.6.1) on a map detailed 

enough to provide for resolution of the population to the subcensus tract level. (The 

U.S. Geological Survey (USGS) 7.5-minute series maps provide sufficient detail.) This is 

necessary to clearly identify the zone of impact, location of the facility, and sensitive 

receptors within the zone of impact. If significant development has occurred since the 

USGS survey, this should be indicated. A specific example of a 7.5-minute series map is 

given in Appendix K.

2. Identify all census tracts within the zone of impact using a U.S. Bureau of Census or 

equivalent map (e.g., Thomas Brothers). If only a portion of the census tract lies within the 

zone of impact, the population used in the burden calculation should include the proportion 

of the population in that isopleth zone. The census tract boundaries should be transferred to 

a map, such as a USGS map (referred to hereafter as the “base map”).

An alternative approach for estimating population exposure in heavily populated urban areas is 

to apportion census tracts to a Cartesian grid cell coordinate system. This method allows a Cartesian 

coordinate receptor concentration field to be merged with the population grid cells. Each receptor 

located on the Cartesian grid must be identified with actual UTM coordinates. This process may be 

computerized and minimizes manual mapping of centroids and census tracts. The HARP software can 

provide population exposure estimates as cancer burden or as the number of persons exposed at the 

block level to a selected potential (user identified) health risk/impact level. 

The District may determine that aggregation of census tracts (e.g., which census tracts making 

up a city can be combined) is appropriate for receptors that are located at considerable distances from 

the facility. If the District permits such an approach, it is suggested that the census tract used to 

represent the aggregate be selected in a manner to ensure that the approach is health protective. For 

example, the census tract included in the aggregate that is nearest (downwind) to the facility should be 

used to represent the aggregate.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-18

Subcensus Tract

Within each census tract are smaller population units. These units (urban block groups (BG) 

and rural enumeration districts (ED)) contain about 1,100 persons. BGs are further broken down into 

statistical units called blocks. Blocks are generally bounded by four streets and contain an average of 

70 to 100 persons. However, the populations presented above are average figures and population units 

may vary significantly. In some cases, the EDs are very large and identical to a census tract.

The area requiring detailed (subcensus tract) resolution of the exposed residential and worker 

population will need to be determined on a case-by-case basis through consultation with the District. 

The District may determine that census tracts provide sufficient resolution near the facility to adequately 

characterize population exposure.

It is necessary to limit the size of the detailed analysis area because inclusion of all subcensus 

tracts would greatly increase the resource requirements of the analysis. For example, an urban area of 

100,000 persons would involve approximately 25 census tracts, approximately 100 to 150 block 

groups, and approximately 1,000 to 1,400 blocks. Furthermore, a high degree of resolution at large 

distances from a source would not significantly affect the analysis because the concentration gradient at 

these distances is generally small. Thus, the detailed analysis of census tracts within several kilometers 

of a facility should be sufficient. The District should be consulted to determine the area that requires 

detailed analysis.

The District should also be consulted to determine the degree of resolution required. In some 

cases, resolution of residential populations to the BG/ED level may be sufficient. However, resolution to 

the block level may also be required for those BG/EDs closest to the facility or those having maximum 

concentration impacts. The identified employment subareas should be resolved to a similar degree of 

resolution as the residential population. For each subarea analyzed, the number of residents and/or 

workers exposed should be estimated.

Employment population data can be obtained at the census tract level from the U.S. Census 

Bureau or from local planning agencies. This degree of resolution will generally not be sufficient for 

most HRAs. For the area requiring detailed analysis, zoning maps, general plans, and other planning 

documents should be consulted to identify subareas with worker populations.

The boundaries of each residential and employment population area should be transferred to the 

base map.

4.6.4 Sensitive Receptor Locations

Individuals who may be more sensitive to toxic exposures than the general population are 

distributed throughout the total population. Sensitive populations may include young children and 

chronically ill individuals. The District may require that locations with high densities of sensitive 

individuals be identified (e.g., schools, nursing homes, residential care facilities, daycare centers, 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-19

hospitals). The HRA should state what the District requirements are regarding identification of sensitive 

receptor locations.

Although sensitive individuals are protected by general assumptions made in the dose response 

assessment, their identification may be useful to assure the public that such individuals are being 

considered in the analysis. For cancer and noncancer effects, the identification of sensitive receptor 

locations may be crucial in evaluating the potential impact of the toxic effect.

4.7 Receptor Siting

4.7.1 Receptor Points

The modeling analysis should contain a network of receptor points with sufficient detail (in 

number and density) to permit the estimation of the maximum concentrations. Locations that must be 

identified include the maximum estimated off-site impact or point of maximum impact (PMI), the 

maximum exposed individual at an existing residential receptor (MEIR), and the maximum exposed 

individual at an existing occupational worker receptor (MEIW). Note, however, some situations may 

require that on-site receptor (worker or residential) locations be evaluated. Some examples where the 

health impacts of on-site receptors may be appropriate could be military base housing, prisons, 

universities, or locations where the public may have regular access for the appropriate exposure period 

(e.g., a lunch time café or museum for acute exposures). The risk assessor should contact the District 

for guidance if on-site exposure situations are present at the emitting facility. These on-site locations 

should be included in the HRA. All of these locations (i.e., PMI, MEIR, and MEIW) must be identified 

for potential multipathway carcinogenic and noncarcinogenic effects. Some facilities will not have offsite workers in the vicinity of the facility and will not need to evaluate worker exposure. The approval 

to omit the MEIW receptor should be verified in writing with the District or reviewing authority and 

included in the HRA. 

Other sensitive receptor locations may also be of interest and required to be included in the 

HRA. The District or reviewing authority should be consulted to determine which sensitive receptor 

locations must be included. It is possible that the estimated PMI, MEIR, and MEIW risk for 

carcinogenic, chronic noncarcinogenic, and acute noncarcinogenic health effects occur at different 

locations. Methods used to determine dose are provided in Chapter 5 and methods for calculating 

potential health impacts are included in Chapter 8 and Appendix I . 

The results from a screening model (if available) can be used to identify the area(s) where the 

maximum concentrations are likely to occur. Receptor points should also be located at the population 

centroids (see Section 4.7.2) and sensitive receptor locations (see Section 4.6.4). The exact 

configuration of the receptor array used in an analysis will depend on the topography, population 

distribution patterns, and other site-specific factors. All receptor locations should be identified in the 

HRA using actual UTM (Universal Transverse Mercator) coordinates and receptor number. The 

receptor numbers in the summary tables should match receptor numbers in the computer output. In 

addition to actual UTM coordinates, the block/street locations (i.e., north side of 3,000 block of Smith 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-20

Street) should be provided for the PMI, MEIR, and MEIW for carcinogenic and noncarcinogenic 

health effects. Chapter 9 provides an outline that specifies the content and recommended format of 

HRA results.

To evaluate localized impacts, receptor height should be taken into account at the point of 

maximum impact on a case-by-case basis. For example, receptor heights may have to be included to 

account for receptors significantly above ground level. Flagpole receptors to represent the breathing 

zone, or direct inhalation, of a person may need to be considered when the source to receptor distance 

is less than a few hundred meters. Consideration must also be given to the multipathway analysis, which 

requires the deposition at ground level. A health protective approach is to select a receptor height from 

0 meters to 1.8 meters that will result in the highest predicted downwind concentration. Final approval 

lies with the District. 

4.7.2 Centroid Locations

For each subarea analyzed, a centroid location (the location at which a calculated ambient 

concentration is assumed to represent the entire subarea) should be determined. When population is 

uniformly distributed within a population unit, a geographic centroid based on the shape of the 

population unit can be used. Where population is not uniformly distributed, a population-weighted 

centroid is needed. Another alternative could be to use the concentration at the point of maximum 

impact (PMI) within that census tract as the concentration to which the entire population of that census 

tract is exposed. 

The centroids represent locations that should be included as receptor points in the dispersion 

modeling analysis. Annual average concentrations should be calculated at each centroid using the 

modeling procedures presented in this chapter.

For census tracts and BG/EDs, judgments can be made using census tracts maps and street 

maps to determine the centroid location. At the block level, a geographic centroid is sufficient.

4.8 Meteorological Data

Refined air dispersion models require hourly meteorological data. The first step in obtaining 

meteorological data should be to check with the District for data availability. Other sources of data 

include the National Weather Service (NWS); National Climatic Data Center (NCDC) in Asheville, 

North Carolina; military stations; and private networks. Meteorological data for a subset of NWS 

stations are available from the U.S. EPA Support Center for Regulatory Air Models (SCRAM). The

SCRAM can be accessed at www.epa.gov/scram001/main.htm. All meteorological data sources 

should be approved by the District. Data not obtained directly from the District should be checked for 

quality, representativeness, and completeness. U.S. EPA provides guidance (U.S. EPA, 1995e) for 

these data. The HRA should indicate if the District required the use of a specified meteorological data 

set. All memos indicating District approval of meteorological data should be attached in an appendix. 

The argument that “this is the nearest available meteorological data” does not justify that the data are 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-21

representative. If no representative meteorological data are available, screening procedures should be 

used as indicated in Section 4.10. 

The analyst should acquire enough meteorological data to ensure that the worst-case 

meteorological conditions are represented in the model results. The period of record, recommended for 

use in the air dispersion model, is five years. If it is desired to use a single year to represent long-term 

averages (i.e., chronic exposure), then the worst-case year should be used. The worst-case year 

should be the year that yields the greatest maximum chronic off-site risk. If the only adverse health 

effects associated with all emitted pollutants from a given facility are acute, the worst-case year should 

be the year that yields the greatest maximum acute off-site risk. With the increasing speeds of today’s 

desktop computers, processing five years of data should be relatively fast. Therefore, we strongly 

encourage the use of five years of meteorological data when available. However, the District may 

determine that one year of representative meteorological data is sufficient to adequately characterize the 

facility’s impact.

Otherwise, to determine annual average concentrations for analysis of chronic health effects, the 

data can be averaged, if a minimum of three years of meteorological data is available. For calculation of 

the one-hour maximum concentrations needed to evaluate acute effects, the worst-case year should be 

used in conjunction with the maximum hourly emission rate. For example, the annual average 

concentration and one-hour maximum concentration at a single receptor for five years of meteorological 

data are calculated below:

Year Annual Average 

(μg/m3)

Maximum One-Hour 

(μg/m3)

1 7 100

2 5 80

3 9 90

4 8 110

5 6 90

5-year average 7

In the above example, the long-term average concentration over five years is 7.0 μg/m3. Therefore, 7 

μg/m3 should be used to evaluate carcinogenic and chronic effects (i.e., annual average concentration). 

The one-hour maximum concentration is the highest one-hour concentration in the five-year period. 

Therefore, 110 μg/m3 is the peak one-hour concentration that should be used to evaluate acute effects.

During the transitional period from night to day (i.e., the first one to three hours of daylight) the 

meteorological processor may interpolate some very low mixing heights. This is a period of time in 

which the mixing height may be growing rapidly. When predicted concentrations are high and the mixing 

height is very low for the corresponding averaging period, the modeling results deserve additional 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-22

consideration. For receptors in the near field, it is within the model formulation to accept a very low 

mixing height for short durations. However, it would be unlikely that the very low mixing height would 

persist long enough for the pollutants to travel into the far field. In the event that the analyst identifies 

any of these time periods, they should be discussed with the District on a case-by-case basis. 

More information on sources of meteorological data, as well as representativeness and 

completeness of meteorological data, can be found in Chapter 2 of the Part IV TSD. 

4.9 Model Selection

There are several air dispersion models that can be used to estimate pollutant concentrations 

and new ones are likely to be developed. U.S. EPA is in the process of adding new models to the

preferred list of models: ISC-PRIME, AERMOD, AERMOD-PRIME, and CalPuff. The latest version 

of the U.S. EPA recommended models can be found at the SCRAM Bulletin Board located at 

www.epa.gov/scram001. However, any model, whether a U.S. EPA guideline model or otherwise, 

must be approved for use by the local air district. Recommended models and guidelines for using 

alternative models are presented in this section. New models placed on U.S. EPA’s preferred list of 

models (i.e., ISC-PRIME, AERMOD, AERMOD-PRIME, and CalPuff) can be considered at that 

time. All air dispersion models used to estimate pollutant concentrations for HRA analyses must be in 

the public domain. Classification according to terrain, source type, and level of analysis is necessary 

before selecting a model (see Section 4.4). The selection of averaging times in the modeling analysis is 

based on the health effects of concern. Annual average concentrations are required for an analysis of 

carcinogenic or other chronic effects. One-hour maximum concentrations are generally required for 

analysis of acute effects. There are a few pollutants that require averaging times up to 7 hours; these 

can be found in Table 6.1.

4.9.1 Recommended Models

Recommended air dispersion models to estimate concentrations for HRA analyses are shown in 

Table 4.2. Currently, SCREEN3 and ISCST3 are the two preferred models for HRAs. This could 

change when the U.S. EPA places ISC-PRIME, AERMOD, AERMOD-PRIME, and CalPuff on the 

preferred list. Some of the names of the air dispersion models reflect the version number at the time of 

the writing of this document. The most current version of the models should be used for the HRA 

analysis. More than one model may be necessary in some situations, for example, when modeling 

scenarios have receptors in simple and complex terrain. Some facilities may also require models 

capable of handling special circumstances such as building downwash, dispersion near coastal areas, 

etc. See Chapter 2 of the Part IV TSD for more information on modeling special cases and for specific 

information including inputs and default option settings for most of the models presented in Table 4.2. 

To further facilitate the model selection, the District should be consulted for additional 

recommendations on the appropriate model(s) or a protocol can be submitted for District review and 

approval (see Chapter 9). A brief description of the preferred screening model, SCREEN 3, and the 

preferred refined model, ISCST3, are discussed below.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-23

4.9.2 Alternative Models

Alternative models are acceptable if applicability is demonstrated or if they produce results 

identical or superior to those obtained using one of the preferred models shown in Table 4.2. For more 

information on the applicability of alternative models refer to the following documents:

• U.S. EPA (1986) Guideline on Air Quality Models (Revised)

• U.S. EPA (1992a) Protocol for Determining the Best Performing Model

• U.S. EPA (1985a) Interim Procedures for Evaluating Air Quality Models – Experience 

with Implementation

• U.S. EPA (1984) Interim Procedures for Evaluating Air Quality Models (Revised)
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-24

TABLE 4.2 Recommended Air Dispersion Models

AVERAGING TERRAIN SINGLE SOURCE MULTIPLE SOURCE

PERIOD TYPE RURAL URBAN RURAL URBAN

SHORT 

TERM

SIMPLE ISCST3

RAM

ISCST3

ISCST3

RAM

ISCST3

(1-24 hour avg)

COMPLEX CTDMPLUS CTDMPLUS CTDMPLUS CTDMPLUS

LONG TERM

SIMPLE ISCST3

ISCLT3

RAM

 ISCST3 ISCLT3 ISCST3

ISCLT3

CDM20 / RAM

ISCST3

ISCLT3

(MonthlyAnnual) COMPLEX CTDMPLUS CTDMPLUS CTDMPLUS CTDMPLUS

SHORT 

TERM

SIMPLE SCREEN3 SCREEN3 SCREEN3 SCREEN3

(1-24 hour avg)

COMPLEX

ISCST3

RTDM, 

CTSCREEN

VALLEY SCRN

SHORTZ

CTSCREEN

VALLEY SCRN

ISCST3

CTSCREEN*

VALLEY SCRN

SHORTZ

CTSCREEN*

VALLEY SCRN

LONG TERM

SIMPLE SCREEN3 SCREEN3 SCREEN3 SCREEN3

(MonthlyAnnual) COMPLEX ISCST3

RTDM

LONGZ ISCST3 LONGZ

Generally speaking, ISCST3 and SCREEN3 are the models that are used in most cases in the Hot 

Spots Program. Other models in this list may be considered on a case-by-case basis. Additionally, 

newer models (e.g., ISC-PRIME, AERMOD, AERMOD-PRIME, and/or CalPuff) may be added to 

this list at a future date.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-25

4.10 Screening Air Dispersion Models

A screening model may be used to estimate a maximum concentration that is biased toward 

overestimation of public exposure. Use of screening models in place of refined modeling procedures is 

optional unless the District specifically requires the use of a refined model. Screening models are 

normally used when no representative meteorological data are available and may be used as a 

preliminary estimate to determine if a more detailed assessment is warranted. 

Some screening models provide only 1-hour average concentration estimates. Maximum 

1-hour concentration averages can be converted to other averaging periods through consultation and 

approval by the District. Appendix H describes the use of the conversion factors. Because of 

variations in local meteorology and source types, the exact factor selected may vary from one district to 

another. Table 4.3 provides guidance on the range and typical values applied. The conversion factors 

are designed to bias predicted longer-term averaging periods towards overestimation. 

Table 4.3. Recommended Factors to Convert Maximum 1-hour Avg. Concentrations 

to Other Averaging Periods (U.S. EPA, 1995a; ARB, 1994).

Averaging Time Range Typical Recommended

3 hours 0.8 - 1.0 0.9

8 hours 0.5 - 0.9 0.7

24 hours 0.2 - 0.6 0.4

30 days 0.2 - 0.3 0.3

Annual 0.06 - 0.1 0.08

4.10.1 SCREEN3

The SCREEN3 model is among the most widely used model primarily because it has been 

periodically updated to reflect changes in air dispersion modeling practices and theories. The 

SCREEN3 model represents a good balance between ease of use and the capabilities and flexibility of 

the algorithms. In addition, the calculations performed by the model are very well documented (U.S. 

EPA, 1995a). The SCREEN3 User’s Guide (U.S. EPA, 1995d) also presents technical information 

and provides references to other support documents. The dispersion algorithms used in SCREEN3 are 

consistent with ISCST3. (With the implementation of AERMOD, which is expected in the future, 

SCREEN3 may need to be superseded with a model that is compatible with AERMOD.)

The most important difference between the SCREEN3 model and refined models such as 

ISCST3 is the meteorological data used to estimate pollutant concentrations. The SCREEN3 model 

can assume worst-case meteorology, which greatly simplifies the resources and time normally 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-26

associated with obtaining meteorological data. Consequently, more conservative (higher concentration) 

estimates are normally obtained. Alternatively, a single stability class and wind speed may also be 

entered.

Number of Sources and Type

SCREEN3 was designed to simulate only a single source at a time. However, more than one 

source may be modeled by consolidating the emissions into one emission point or by individually running 

each point source and adding the results. SCREEN3 can be used to model point sources, flare 

releases, and simple area and volume sources. Input parameters required for various source-types are 

shown in Tables 4.4 (point), 4.5 (flare release), 4.6 (area), and 4.7 (volume). 

Table 4.4. Required Input Parameters to Model a Point Source Using SCREEN3.

Emission Rate (g/s)

Stack Height (m)

Stack Inside Diameter (m)

Stack Gas Exit Velocity (m/s) or Volumetric Flow Rate (ACFM, m3/s)

Stack Gas Temperature (K)

Ambient Temperature (K)

Receptor Height Above Ground (m)

Receptor Distance from the Source (m) [discrete distance or automated array]

Land Type [urban or rural]

Meteorology [option “1” (full meteorology) is normally selected]

In Addition, for building downwash calculations

Building Height (m)

Minimum Horizontal Dimension (m)

Maximum Horizontal Dimension (m)
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-27

Table 4.5. Required Input Parameters to Model a Flare Using SCREEN3.

Emission Rate (g/s)

Flare Stack Height (m)

Total Heat Release (cal/s)

Receptor Height Above Ground (m)

Receptor Distance from the Source (m)

Land Type [urban or rural]

Meteorology [option “1” (full meteorology) is normally selected]

In Addition, for building downwash calculations

Building Height (m)

Minimum Horizontal Dimension (m)

Maximum Horizontal Dimension (m)

Table 4.6. Required Input Parameters to Model an Area Source Using SCREEN3.

Emission Rate (g/s-m2)

Source Release Height (m)

Length of Larger Side of the Rectangular Area (m)

Length of Smaller Side of the Rectangular Area (m)

Receptor Height Above Ground (m)

Receptor Distance from the Source (m)

Land Type [urban or rural]

Meteorology [option “1” (full meteorology) is normally selected]

[wind direction optional]
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-28

Regulatory Options

SCREEN3 algorithms contain all regulatory options internally coded including stack-tip 

downwash and buoyancy-induced dispersion. These regulatory options are the default settings of the 

parameters so the user does not need to set any switches during a run.

Special Cases

SCREEN3 has the capability to model several special cases by setting switches in the input file 

or by responding to on-screen questions (if run interactively). The special cases include:

• simple elevated terrain

• plume impaction in complex terrain using VALLEY model 24-hr screening procedure

• building downwash (only for flat and simple elevated terrain)

• cavity region concentrations (The PRIME algorithms included with ISCST3-PRIME should be 

used for estimates in the cavity zone)

• inversion break-up fumigation (only for rural inland sites with stack heights greater than or equal 

to 10 m and flat terrain)

• shoreline fumigation (for sources within 3,000 m from a large body of water)

• plume rise for flare releases

4.11 Refined Air Dispersion Models

Refined air dispersion models are designed to provide more representative concentration 

estimates than screening models. In general, the algorithms of refined models are more robust and have 

the capability to account for site-specific meteorological conditions. For more information regarding 

general aspects of model selection see Section 4.9.

Table 4.7. Required Input Parameters to Model a Volume Source Using SCREEN3.

Emission Rate (g/s)

Source Release Height (m)

Initial Lateral Dimension of Volume (m)

Initial Vertical Dimension of Volume (m)

Receptor Height Above Ground (m)

Receptor Distance from the Source (m)

Land Type [urban or rural]

Meteorology [option “1” (full meteorology) is normally selected]
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-29

4.11.1 ISCST3

The ISCST3 model (U.S. EPA, 1995b; 1995c) is a steady-state Gaussian plume model, which 

can be used to assess pollutant concentrations from a wide variety of sources associated with an 

industrial source complex. The ISCST3 model can be used for multiple sources in urban or rural 

terrain. The model includes the algorithms of the complex terrain model COMPLEX I. The user can 

specify if calculations are to be made for simple terrain, complex terrain, or both. However since 

COMPLEX 1 is a screening model, the ISCST3 model is only a screening tool for receptors in 

complex terrain. The ISCST3 model can calculate concentration averages for 1-hour or for the entire 

meteorological data period (e.g., annual or intermediate time periods such as 24-hour averages). A 

summary of basic input parameters needed to model a point source is shown in Table 4.8. Guidance on 

additional input requirements (e.g., for area and volume sources) may be found in the ISC Users Guide. 

(ISCST3 may be replaced with AERMOD in the future pending promulgation by the U.S. EPA.) 

Table 4.8. Basic Input Parameters Required to Model a Point Source Using ISCST3.

Land Use Urban or Rural

Averaging Period

Emission Rate (g/s)

Stack Height (m)

Stack Gas Exit Temperature (K)

Stack Gas Exit Velocity (m/s)

Stack Diameter (m)

Receptor Locations (x,y) coordinates (m) dis crete points; polar array; Cartesian array;

Meteorology may be supplied by preprocessor, e.g., PCRAMMET

Anemometer Height (m)

4.11.1.1 Regulatory Options

Regulatory application of the ISCST3 model requires the selection of specific switches 

(i.e., algorithms) during a model run. All the regulatory options can be set by selecting the DFAULT 

keyword. The regulatory options, automatically selected when the DFAULT keyword is used, are:

• Stack-tip downwash (except for Schulman-Scire downwash)

• Buoyancy-induced dispersion (except for Schulman-Scire downwash)

• Final plume rise (except for building downwash)

• Treatment of calms

• Default values for wind profile exponents
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-30

• Default values for vertical potential temperature gradients

• Use upper-bound concentration estimates for sources influenced by building downwash from 

super-squat buildings

4.11.1.2 Special Cases

a. Building Downwash

The ISC models automatically determine if the plume is affected by the wake region of buildings 

when their dimensions are given. Including building dimensions in the model input does not necessarily 

mean that there will be downwash. See Chapter 2 of the Part IV TSD for guidance on how to 

determine when downwash is likely to occur.

b. Area Sources

The area source algorithms in ISCST3 use an integration technique that allows placement of 

receptors within the area source. Additionally, initial dispersion in the vertical can be included to 

simulate sources with vertical extent. 

c. Volume Sources

The volume source algorithms in ISCST3 require an estimate of the initial distribution of the 

emission source in the horizontal and the vertical. Tables that provide information on how to estimate 

the initial distribution for different sources are given in the ISC3 User’s Guide (U.S. EPA, 1995b; 

1995c).

d. Intermediate Terrain

When simple and complex terrain algorithms are selected by the user, ISCST3 will select the 

higher impact from the two algorithms on an hour-by-hour, source-by-source, and receptor-byreceptor basis for all receptors located in intermediate terrain (U.S. EPA, 1995b).

Alternatively, the pollution concentrations in the receptor field may be generated separately from 

HARP using other approved air dispersion models. HARP has the flexibility to generate a summary of 

the risk data necessary for an HRA by either approach: ISCST3 internal to HARP or the use of other 

approved models outside of HARP.

In addition, the HARP software also incorporates the capability of using either user supplied 

representative meteorological data or the worst-case meteorological conditions from the SCREEN3 

model as inputs to the ISCST3 air dispersion model. Information on obtaining the HARP software can 

be found on the ARB’s web site at www.arb.ca.gov. Chapter 9 provides an outline that specifies the 

content and recommended format of HRA results.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

4-31

e. Deposition

The ISC models contain algorithms to model settling and deposition and require additional 

information such as the particle size distribution. For more information consult the ISC3 User’s Guide 

(U.S. EPA, 1995b). Note that, when performing the HRA modeling, a deposition rate will be 

requested and used for the noninhalation pathway exposure (see Section 8.2.5.A).

4.11.1.3 HARP Dispersion Analysis 

It is highly recommended that air dispersion analysis be performed using the HARP software. 

HARP can perform refined dispersion analysis by utilizing the U.S. EPA standard program ISCST3 

(Industrial Source Complex – Short Term 3). In addition, HARP directly links the ISCST3 outputs 

with risk assessment modules eliminating the need for intermediate processing by the user. 

 

4.12 Modeling Special Cases; Specialized Models

Special situations arise in modeling some sources that require considerable professional 

judgment; these include building down-wash effects, wet and dry deposition, short term emissions (i.e., 

significantly less than 1-hour), fumigation effects, rain-cap on stack, and landfill sites. Details for these 

special modeling situations and specific models can be found in Chapter 2 of the Part IV TSD. It is 

recommended that the reader consider retaining professional consultation services if the procedures are 

unfamiliar. Some models have been developed for application to very specific conditions. Examples 

include models capable of simulating sources where both land and water surfaces affect the dispersion 

of pollutants and models designed to simulate emissions from specific industries. 

4.13 Interaction with the District

The risk assessor must contact the District to determine if there are any specific modeling 

requirements. Examples of such requirements may include specific receptor location guidance, specific 

usage of meteorological data, and specific report format (input and output). See Chapter 9 for 

information on the format and content of modeling protocols and HRAs.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

5. Exposure Assessment - Estimation of Concentration and Dose 

5.1 Introduction 

This chapter provides a summary of how toxicant ground level air concentrations 

estimated from air dispersion modeling or monitoring results are used to determine dose at 

receptors of interest. This chapter includes all the algorithms and data (e.g., point-estimates, 

distributions, and transfer factors) that are needed to determine the substance-specific 

concentration in exposure media and the dose at a receptor of interest. The determination of 

exposure concentrations and dose precede the calculations of potential health impacts. See 

Chapter 8 and Appendix I for information on calculating potential health impacts. 

At minimum, three receptors are evaluated in Hot Spots health risk assessments (HRA) 

(see Section 4.7);, these are: 

1) the Point of Maximum Impact (PMI), 

2) the Maximally Exposed Individual Resident (MEIR), and 

3) the Maximally Exposed Individual Worker (MEIW). 

The PMI is defined as the receptor point(s) with the highest acute, chronic, or cancer 

health impacts outside the facility boundary. The facility boundary is defined as the property 

line. Often the fence is on the property line. The MEIR is defined as the existing off-site 

residence(s) (e.g., house or apartment) with the highest acute, chronic, or cancer health impacts. 

The MEIW is defined as the highest acute, chronic, or cancer health impacts at an existing 

off-site workplace. Note, however, that occasionally some situations may require that on-site 

receptor (worker or residential) locations be evaluated. Some examples where the health impacts 

of on-site receptors may be appropriate could be military base housing, prisons, universities, or

locations where the public may have regular access for the appropriate exposure period (e.g., a 

lunch time café or museum for acute exposures). The risk assessor should contact the Air 

Pollution Control or Air Quality Management District (District) for guidance if on-site exposure 

situations exist at the emitting facility. These on-site locations should be included in the health 

risk assessment (HRA). 

If the facility emits multiple substances from two or more stacks, the acute, chronic, and 

cancer health impacts at the PMI may be located at different physical locations. The MEIR or 

MEIW cancer, acute, and chronic receptors may also be at different locations. In addition, it 

may be necessary to determine risks at sensitive receptors (e.g., schools, daycare, eldercare, and 

hospitals). The District or reviewing authority should be consulted in order to determine the 

appropriate sensitive receptors for evaluation. 

The process for determining dose at the receptor location, and ultimately potential health 

impacts, will likely include air dispersion modeling, and, with less frequency, air monitoring 

data. Air dispersion modeling combines the facility emissions and release parameters and uses 

default or site-specific meteorological conditions to estimate downwind, ground-level 

concentrations at various (user-defined) receptor locations. Air dispersion modeling is described 

in Chapter 4 and is presented in detail in the Air Toxics Hot Spots Program Risk Assessment 

5-1 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

5-2 

Guidelines; Part IV; Technical Support Document for Exposure Assessment and Stochastic 

Analysis (OEHHA, 2000b) (Part IV TSD).

In summary, the process of using air dispersion modeling results as the basis of an HRA 

follows these four steps. 

• Air dispersion modeling is used to estimate an annual-average and maximum one, four, 

six, and seven-hour ground level concentrations. The air dispersion modeling results are 

expressed as an air concentration or in terms of (Chi over Q) for each receptor point. 

(Chi over Q) is the modeled downwind air concentration based on an emission rate of one 

gram per second. (Chi over Q) is expressed in units of micrograms per cubic meter per 

gram per second, or (μg/m3

)/(g/s). (Chi over Q) is sometimes written as (χ/Q) and is 

sometimes referred to as the dilution factor. 

• When multiple substances are evaluated, the χ/Q is normally utilized since it is based on 

an emission rate of one gram per second. The χ/Q at the receptor point of interest is 

multiplied by the substance-specific emission rate (in g/s) to yield the substance-specific 

ground-level concentration (GLC) in units of μg/m3

. The following equations illustrate 

this point. 

( ) over in , from model results with unit emission rate

3





















=

s

g

m

g

Chi Q Q

μ

χ

( ) GLC Q Qsubstance 

 



 = χ





 



 = s

g substance specific emission rate Qsubstance

• The applicable exposure pathways (e.g., inhalation, soil, fish) are identified for the 

emitted substances and the receptor locations are identified. This determines which 

exposure algorithms in this chapter are ultimately used to estimate dose. After the 

exposure pathways are identified, the fate and transport algorithms described in this 

chapter are used to estimate concentrations in the applicable exposure media (e.g., soil or 

water) and the exposure algorithms are used to determine the substance-specific dose. 

• The dose is used with cancer and noncancer health values to calculate the potential health 

impacts for the receptor (Chapter 8). An example calculation using the high-end pointestimates for the inhalation (breathing) exposure pathway can be found in Appendix I. 

The algorithms in this chapter are also used to calculate media concentrations and dose in 

the rare instance for the Hot Spots program when monitoring equipment were used rather than 

air dispersion modeling to obtain a receptor’s substance-specific GLC. One situation that is 

specific to monitored data is the treatment of results below the sampling method level of 

detection (LOD). In short, it is standard risk assessment practice when monitoring results are 

reported both above and below the LOD to use one-half of the LOD for those sample 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

concentrations reported below the LOD. If all testing or monitoring results fall below the LOD, 

then assessors should contact the District for appropriate procedures. For more information 

about reporting emissions under the Hot Spots Program, see the ARB’s Emission Inventory 

Criteria and Guidelines Regulations (Title 17, California Code of Regulations, Sections 93300-

93300.5), and the Emission Inventory Criteria and Guidelines Report (EICG Report), which is 

incorporated by reference therein (ARB, 1997). 

The HARP software is the recommended model for calculating and presenting HRA 

results for the Hot Spots Program. A contractor, through consultation with OEHHA, Air 

Resources Board (ARB), and District representatives, developed the HARP software. 

Information on obtaining the HARP software can be found on the ARB’s web site at 

www.arb.ca.gov under the Hot Spots Program. 

5.2 Criteria for Exposure Pathway Evaluation 

In order to determine total dose to the receptor the applicable pathways of exposure need 

to be identified. The inhalation pathway must be evaluated for all Hot Spots substances emitted 

by the facility. A small subset of Hot Spots substances is subject to deposition on to the soil, 

plants, and water bodies. These substances need to be evaluated by the appropriate 

noninhalation pathways, as well as by the inhalation pathway, and the results must be presented 

in all HRAs. These substances include semi-volatile organic chemicals and heavy metals. Such 

substances are referred to as multipathway substances. Two steps are used to determine if a 

substance should be evaluated for multipathway impacts: 

• Step one is to see if the substance or its group (e.g., dioxins, PAHs) is listed in Table 5.1. 

• Step two is to determine if the substance has an oral reference exposure level (REL) 

listed in Table 6.3, or if it has an oral cancer slope factor listed in Table 7.1. Oral or 

noninhalation exposure pathways include the ingestion of soil, fisher caught fish, 

drinking water from surface waters, mother’s milk, homegrown produce, beef, pork, 

chicken, eggs and cow’s milk. The dermal pathway is also evaluated via contact with 

contaminated soil. 

For all multipathway substances, the minimum exposure pathways that must be evaluated 

at every residential site (in addition to inhalation) are soil ingestion and dermal exposure. If 

dioxins, furans, or PCBs are emitted, then the breast-milk consumption pathway also becomes 

mandatory. The other exposure pathways (e.g., the ingestion of homegrown produce or fish) are 

evaluated on a site-by-site basis. If the resident can be exposed through an impacted exposure 

pathway, then it must be included in the HRA. However, if there were no vegetable gardens or 

fruit trees within the zone of impact for a facility, for example, then the produce pathways would 

not be evaluated. Note that on-site residential receptors are potentially subject to inhalation and 

noninhalation exposure pathways. Table 8.2 identifies the residential and worker receptor 

exposure pathways that are mandatory and those that are dependent on the site-specific 

decisions. While residents can be exposed though several exposure pathways, worker receptors 

are only evaluated for inhalation, soil ingestion, and dermal exposure using single 

point-estimates. 

5-3 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 5.1 shows the multipathway substances that, based on available scientific data, can 

be considered for each noninhalation exposure pathway. The exposure pathways that are 

evaluated for a substance depend on two factors: 1) whether the substance is considered a 

multipathway substance for the Hot Spots Program (Table 5.1), and 2) what the site-specific 

conditions are. A multipathway substance may be excluded from a particular exposure pathway 

because its physical-chemical properties can preclude significant exposure via the pathway. For 

example, some water-soluble chemicals do not appreciably bioaccumulate in fish; therefore, the 

fish pathway is not appropriate. In addition, if a particular exposure pathway is not impacted by 

the facility or is not present at the receptor site, then the pathway is not evaluated. For example, 

if surface waters are not impacted by the facility, or the water source is impacted but never used 

for drinking water, then the drinking water pathway is not evaluated. 

Table 5.1 Specific Pathways to be Analyzed for each Multipathway Substance 

Substance 

Soil Ingestion 

Dermal 

Meat, Milk &Egg Ingestion

Fish 

Ingestion

Exposed 

Vegetable

Ingestion 

Leafy

Vegetable

Ingestion 

Protected

Vegetable

Ingestion 

Root

Vegetable

Ingestion 

Water

Ingestion 

Breast Milk 

Ingestion 

4,4'-Methylene dianiline X X X X X X X X 

Creosotes X X X X X X X

Diethylhexylphthalate X X X X X X X X

Hexachlorocyclohexanes X X X X X X

PAHs X X X X X X X

PCBs X X X X X X X X

Cadmium & compounds X X X X X X X X X

Chromium VI & compounds X X X X X X X X X

Inorganic arsenic & compounds X X X X X X X X X

Beryllium & compounds X X X X X X X X X

Lead & compounds X X X X X X X X X

Mercury & compounds X X X X X X X X X

Nickel X X X X X X X X

Fluorides (Including hydrogen 

fluoride) 

 To be determined 

Dioxins & furans X X X X X X X X

5-4 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

5.3 Estimation of Concentrations in Air, Soil, and Water

Once emissions exit the source, the substances will be dispersed in the air. The 

substances in the exhaust gas with high vapor pressures will remain largely in the vapor phase, 

and substances with lower vapor pressures will tend to adsorb to fly ash or other particulate 

matter. The emission plume may contain both vapor phase substances and particulates. A single 

semivolatile organic toxicant can partition as a vapor and into a particulate. Particulates will 

deposit at a rate that is dependent on the particle size. The substances will deposit on vegetation, 

on soil, and in water. Use the 0.02 m/s factor for emission sources that have verifiable 

particulate matter control devices or for emission sources that may be uncontrolled but only emit 

particulate matter that is less than 2.5 microns (e.g., internal combustion engines powered by 

compressed natural gas). The following algorithms are used to estimate concentrations in 

environmental media including air, soil, water, vegetation, and animal products.

5.3.1 Air 

The concentration of the substance in air at ground level (GLC) is a function of the 

facility emission rate and the dilution factor (χ/Q) at the points under evaluation. 

a. Formula 5.3.1 A: GLC = E-rate * χ/Q (EQ 5.3.1 A)

1> GLC = Ground-level concentration (μg/m3

) 

2> E-rate = Substance emission rate (g/sec) 

 3> χ /Q = Dilution factor provided by dispersion modeling (μg/m3

/g/sec) 

b. Recommended values for EQ 5.3.1 A:

1> E-rate = Facility specific, substance emission rate 

 2> χ/Q = For point of interest, site specific, from dispersion modeling 

c. Assumptions for EQ 5.3.1 A:

1> No plume depletion 

2> Emission rate is constant, i.e., assumes steady state 

5.3.2 Soil 

The average concentration of the substance in soil (Cs) is a function of the deposition, 

accumulation period, chemical specific soil half-life, mixing depth, and soil bulk density. 

a. Formula 5.3.2 A: Cs = Dep * X / (Ks * SD * BD * Tt) (EQ 5.3.2 A)

 1> Cs = Average soil concentration over the evaluation period (μg/kg)

2> Dep = Deposition on the affected soil area per day (μg/m2

/d) 

5-5 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

 a> Formula 5.3.2 B: Dep = GLC * Dep-rate * 86,400 (EQ 5.3.2 B)

1: GLC = Ground-level concentration (μg/m3

) 

 2: Dep-rate = Vertical rate of deposition (m/sec) 

3: 86,400 = Seconds per day conversion factor (sec/d) 

 b> Recommended default values for EQ 5.3.2 B: 

1: GLC = Calculated above in EQ 5.3.1 A 

2: Dep-rate = Use 0.02 meters/second for controlled or 

 0.05 meters/second for uncontrolled sources. 

 c> Assumptions for EQ 5.3.2 B: 

 1: Deposition rate remains constant 

 3> X = Integral function 

 a> Formula 5.3.2 C: X = [{e-Ks * Tf - e-Ks * To} / Ks] + Tt (EQ 5.3.2 C)

1: e = 2.718 

 2: Ks = Soil elimination constant

 3: Tf = End of evaluation period (d) 

 4: To = Beginning of evaluation period (d) 

 5: Tt = Total days of exposure period Tf-To (d) 

 a: Formula 5.3.2 D: Ks = 0.693 / t1/2 (EQ 5.3.2 D)

1) 0.693 = Natural log of 2 

 2) t

1/2 = Chemical specific soil half-life (d) 

 b: Recommended default values for EQ 5.3.2 D: 

 1) t

1/2 = See Table 5.3 

 b> Recommended default values for EQ 5.3.2 C: 

 1: Ks = Calculated above in EQ 5.3.2 D 

 2: Tf = 25,550 (d) = 70 yr (for 9, 30 and 70 years). Identifies the total 

number of days of soil deposition. 

= 9,490 (d) = 26 years for nursing mother in mother’s milk pathway 

 3: To = 0 (d) The initial time (start period) of exposure to all receptors that 

are impacted by the soil pathway. Used for direct soil 

exposure to a worker, residential adults (9, 30, and 

70-years), and children. Also used as the initial time for 

5-6 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

determining the concentration in soil that is used for 

estimating the dose from the ingestion of breast milk. 

 

4> SD = Soil mixing depth (m) 

5> BD = Soil bulk density (kg/m3

) 

b. Recommended default values for EQ 5.3.2 A: 

1> Dep = Calculated above in EQ 5.3.2 B 

2> X = Calculated above in EQ 5.3.2 C 

 3> Ks = Calculated above in EQ 5.3.2 D 

4> SD = 0.01 (m) for playground setting (soil ingestion and dermal pathways) 

 and 0.15 (m) for agricultural setting (produce and meat pathways). 

5> BD = 1,333 (kg/m3

) 

 6> Tt = 25,550 (d) = 70 (yr) for 9, 30 and 70 year exposure 

 durations and mother’s milk pathway 

= 25,550 (d) for adult in mother's milk pathway 

c. Assumptions for EQ 5.3.2 A: 

1> Substances are uniformly mixed in soil. 

2> Substances are not leached or washed away, except where evidence exists 

 to the contrary. 

3> For a receptor ingesting mother's milk, the mother is exposed for 26 years, the 

child receives milk for one year (the last year of maternal exposure), and then is 

exposed to all other pathways for 9, 30 or 70 years. 

4> It is assumed that toxicants accumulate in the soil for 70 years from deposition. 

5.3.3 In Water 

The average concentration of the substance in water (Cw) is a function of direct 

deposition and material carried in by surface run-off. However, only the contribution from direct 

deposition will be considered at this time. 

a. Formula 5.3.3 A: Cw = Cdepw (EQ 5.3.3 A)

 1> Cw = Average concentration in water (μg/kg) 

 2> Cdepw = Contribution due to direct deposition (μg/kg) 

 a> Formula 5.3.3 B: Cdepw = Dep * SA * 365 / (WV * VC) (EQ 5.3.3 B)

1: Dep = Deposition on water body per day (μg/m2

/d) 

 2: SA = Water surface area (m2

) 

 3: 365 = Days per year (d/yr) 

5-7 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

 4: WV = Water volume (kg) 

5: VC = Number of volume changes per year 

 b> Recommended default values for EQ 5.3.3 B: 

1: Dep = Calculated above in EQ 5.3.2 B 

2: SA = Site specific water surface area (m2

) 

 3: WV = Site specific water volume in (kg) 

 4: VC = Site specific number of volume changes per year 

(SA, WV, and VC values can be acquired from the applicable 

 Department of Water Resources (DWR) Regional office) 

 c> Assumptions for EQ 5.3.3 B: 

 1: All material deposited into the water remains suspended or dissolved 

in the water column and is available for bioconcentration in fish. 

5.3.4 Estimation of Concentrations in Vegetation and Animal Products 

Estimates of the concentration of the substance in vegetation and animals require the use 

of the results of the air, water, and soil environmental fate evaluation. Plants and animals will be 

exposed to the substances at the concentrations previously calculated in Section 5.31 to 5.33 

above. 

 1. Vegetation

The average concentration of a substance in and on vegetation (Cf) is a function of direct 

deposition of the substance onto the vegetation and of root translocation or uptake from soil 

contaminated by the substance. 

 a. Formula 5.3.4.1 A: Cv = Cdepv * GRAF + Ctrans (EQ 5.3.4.1 A)

 1> Cv = Average concentration in and on 

 specific types of vegetation (μg/kg) 

 2> Cdepv = Concentration due to direct deposition (μg/kg) 

 3> GRAF = Gastrointestinal Relative Absorption Fraction 

 a> Formula 5.3.4.1 B: 

 Cdepv = [Dep * IF / (k * Y)] * (1 - e-kT)] (EQ 5.3.4.1 B)

1: Dep = Deposition on affected vegetation per day (μg/m2

/d) 

 2: IF = Interception fraction 

 3: k = Weathering constant (d-1) 

 4: Y = Yield (kg/m2

) 

5: e = Base of natural logarithm (2.718) 

5-8 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

 6: T = Growth period (d) 

 b> Recommended default values for EQ 5.3.4.1 B: 

1: Dep = Calculated above in EQ 5.3.2 B 

 2: IF = Crop specific

 a: Root crops = 0 

 b: Leafy crops = 0.2 

 c: Protected crops = 0 

 d: Exposed crops = 0.1 

 3: k = 0.1 (d-1 ) 

 4: Y = 2 (kg/m2

) for root, leafy, protected, exposed and 

pasture [CA Department of Food and Agriculture dot 

maps]

5: T = 45 (d) for leafy crops 

 T = 90 (d) for exposed crops 

 c> Assumptions for EQ 5.3.4.1 B: 

 1: No deposition on root or protected crops 

 3> GRAF = Gastrointestinal Relative Absorption Fraction 

 0.43 for dioxins; 1.0 for all other chemicals 

 The term GRAF, or gastrointestinal relative absorption factor, is defined as the fraction 

of contaminant absorbed by the GI tract relative to the fraction of contaminant absorbed from the 

matrix (feed, water, other) used in the study(ies) that is the basis of either the cancer potency 

factor (CPF) or the reference exposure level (REL). If no data are available to distinguish 

absorption in the toxicity study from absorption from the environmental matrix in question, 

i.e., soil, then GRAF = 1. The GRAF allows for adjustment for absorption from a soil matrix if

it is known to be different from absorption across the GI tract in the study used to calculate the 

CPF or REL. In most instances, the GRAF will be 1 (Table 5.3). 

 4> Ctrans = Concentration due to root translocation or uptake (μg/kg) 

 a> Formula 5.3.4.1 C: Ctrans = Cs * UF2 (EQ 5.3.4.1 C)

 1: Cs = Average soil concentration (μg/kg) 

 2: UF2 = Uptake factor based on soil concentration 

 b> Recommended default values for EQ 5.3.4.1 C: 

 1: Cs = Calculated above in EQ 5.3.2 A 

 2: UF2 = Inorganic compounds--see Table 5.3 

 

5-9 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

 1) Formula 5.3.4.1 D: (for organic compounds) 

 UF2 = [(0.03 * Kow

0.77

) + 0.82] / [(Koc)(Foc)] (EQ 5.3.4.1 D)

 a) 0.03 = Empirical constant 

 b) Kow = Octanol: water partition factor 

 c) 0.77 = Empirical constant 

 d) 0.82 = Empirical constant 

 e) Koc = Organic carbon partition coefficient 

 f) Foc = Fraction organic carbon in soil 

 2) Recommended default values for EQ 5.3.4.1 D: 

 a) Kow = Chemical specific, see Table 5.3 

 b) Koc = Chemical specific, see Table 5.3 

 c) Foc = 0.1 

 2. Animal Products

The average concentration of the substance in animal products (Cfa) depends on which 

routes of exposure exist for the animals. Animal exposure routes include inhalation, soil 

ingestion, ingestion of contaminated feed and pasture, and ingestion of contaminated water. 

 a. Formula EQ 5.3.4.2 E: 

 Cfa = (Inhalation + Water ingestion + Feed ingestion + (EQ 5.3.4.2 E)

 Pasture/Grazing ingestion + Soil ingestion) * Tco

 1> Cfa = Average concentration in farm animals 

 and their products (μg/kg) 

 2> Inhalation= Dose through inhalation (μg/d) 

 a> Formula 5.3.4.2 F: Inhalation = BRA * GLC (EQ 5.3.4.2 F)

 1: BRA = Inhalation rate for animal (m3

/d) 

 2: GLC = Ground-level concentration (μg/m3

) 

 b> Recommended default values for EQ 5.3.4.2 F: 

 1: BRA = See Table 5.2 

2: GLC = Calculated above in EQ 5.3.1 A 

 c> Assumptions for EQ 5.3.4.2 F: 

 1: All material inhaled is 100% absorbed 

5-10 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

 3> Water ingestion = Dose through water ingestion (μg/d) 

 a> Formula EQ 5.3.4.2 G: 

 Water ingestion = WIRa * FSW * Cw (EQ 5.3.4.2 G)

 1: WIRa = Water ingestion for animal (kg/d) 

 2: FSW = Fraction of water ingested from a 

contaminated body of water 

 3: Cw = Average concentration in water (μg/kg) 

 For water 1 kg = 1 L 

b>Recommended default values for EQ 5.3.4.2 G: 

 1: WIRa = See Table 5.2 

2: FSW = Site specific, need to survey, fraction of 

water ingestion practices in affected area 

 

 3: Cw = Calculated above in EQ 5.3.3 A 

Table 5.2 Point Estimates for Animal Pathway*

Parameter Beef Cattle Lactating Dairy 

Cattle Pigs Poultry 

BW (body weight) (kg) 500 500 60 2

BRA (inhalation rate) (m3/d) 100 100 7 0.4

WIRa (water ingestion) (kg/d)** 40 80 8 0.2

FIR (feed ingestion) (kg/d) 8 16 2 0.1

FSf (soil fraction of feed) 0.01 0.01 NA NA

FSp (soil fraction of pasture) 0.05 0.05 0.04 0.02

Beef and dairy cattle food from pasture grazing is assumed to be leafy vegetation (grass) and account for 0.5 of the cattle’s diet. 

For pigs, the default assumes a pig’s diet consists of equal portions of all plant types exposed, leafy, protected and root. The default 

assumption is that 0.1 of the diet is homegrown. The default assumption for chickens is that pasture is composed of equal 

proportions all plant types with 0.05 homegrown.

Agricultural mixing depth should be used for calculating soil concentration for feed and pasture contamination. 

NA Not applicable. Assume F Sf is equal to zero. 

* See Section 7 of Technical Support Document for Exposure Assessment and Stochastic Analysis (OEHHA, 2000b) for source of 

these values. 

** 1 kg=1 L for water 

 4> Feed ingestion = Dose through the ingestion of feed (μg/d) that is 

harvested after it is impacted by source emissions

 a> Formula EQ 5.3.4.2 H: 

Feed ingestion = (1 - FG) * FIR * L * Cf (EQ 5.3.4.2 H)

5-11 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

 1: FG = Fraction of Diet provided by grazing 

 2: FIR = Feed ingestion rate (kg/d) 

3: L = fraction of locally grown (source impacted) feed 

that is not pasture 

 4: Cf = Concentration in feed (μg/kg) 

 b> Recommended default values EQ 5.3.4.2 H: 

 1: FG = Site specific fraction of diet provided by grazing 

(need to survey) 

 2: FIR = See Table 5.2 

 3 L = Site specific, fraction of locally grown (source 

impacted) feed that is not pasture 

 4: Cf = As calculated above in EQ 5.3.4.1 A 

 5> Pasture/Grazing ingestion = Dose through pasture/grazing (μg/d) 

 a> Formula EQ 5.3.4.2 I: 

 Pasture/Grazing ingestion = FG * Cv * FIR (EQ 5.3.4.2 I)

 1: FG = Fraction of Diet provided by grazing 

 2: Cv = Concentration in pasture/grazing material (μg/kg) 

 3: FIR = Feed ingestion rate (kg/d) 

 b> Recommended default values EQ 5.3.4.2 J: 

 1: FG = Site specific fraction of diet provided by grazing 

(need to survey) 

 2: Cv = As calculated above in EQ 5.3.4.1 A 

 3: FIR = See Table 5.2 

 6> Soil ingestion= Dose through soil ingestion (μg/kg) 

 a> Formula EQ 5.3.4.2 K: Soil ingestion = SIa * Cs (EQ 5.3.4.2 K)

 1: SIa = Soil ingestion rate for animal (kg/d)

 a: Formula EQ 5.3.4.2 L: 

 SIa = [(1 - FG) * FSf * FIR] +[ FG * FSp * FIR] (EQ 5.3.4.2 L)

 1) FG = Fraction of diet provided by grazing 

 2) FSf = Soil ingested as a fraction of feed 

 ingested 

 3) FIR = Feed ingestion rate (kg/d) 

5-12 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

 4) FSp = Soil ingested as a fraction of pasture 

ingested 

 b: Recommended default values for EQ 5.3.4.2 L: 

 1) FG = Site specific fraction of diet provided 

by grazing 

 2) FSf = See Table 5.2 

 3) FIR = See Table 5.2

 4) FSp = See Table 5.2 

 2: Cs = Average soil concentration (μg/kg) 

 

b> Recommended default values for EQ 5.3.4.2 K: 

 1: SIa = Calculated above 

 2: Cs = Calculated above in EQ 5.3.2 A 

 7> Tco = Transfer coefficient of contaminant from diet to 

animal product (d/kg) 

 a> Recommended default values: 

 1: TCO = SEE TABLE 5.3 

 b> Recommended default values EQ 5.3.4.2 J: 

 1: FG = Site specific fraction of diet provided by grazing 

(need to survey) 

 2: Cf = As calculated above in EQ 5.3.4.1 A 

 3: FIR = See Table 5.2 

5-13 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003

Table 5.3 Substance Specific Default Values for Multipathway Substance

Feed to meat, milk, eggs 

Transfer Coefficients 3[Tco 

(d/kg)] 

Root Uptake Factors (for

inorganic compounds) 

Multipathway 

Substance 

Log 

Koc2

Log 

Kow2

Fish 

Biocon. 

Factor 

Tco

Meat

Tco

Milk 

Tco3

Egg Root Leafy 

Exposed

& 

Protected 

GRAF

Arsenic (inorganic) NA6 NA 4.0 x 10+0 2.0 x 10-3 6.2 x 10-5 2.0 x 10-3 4.0 x 10-4 4.0 x 10-3 9.0 x 10-4 1.0 Beryllium & Compounds NA NA 1.9 x 10+1 1.0 x 10-3 9.1 x 10-7 1.0 x 10-3 2.0 x 10-3 1.0 x 10-3 2.0 x 10-4 1.0 Cadmium &Compounds NA NA 3.66 x 10+2 5.5 x 10-4 1.0 x 10-3 5.5 x 10-4 4.0 x 10-2 6.0 x 10-2 2.0 x 10-2 1.0 Creosotes NA NA 5.83 x 10+2 3.4 x 10-2 1.6 x 10-2 3.4 x 10-2 NA NA NA 1.0 Chromium VI & Cmpds NA NA 2.0 x 10+0 9.2 x 10-3 1.0 x 10-5 9.2 x 10-3 1.0 x 10-3 8.0 x 10-4 7.0 x 10-4 1.0 Diethylhexylphthalate 4.72 5.11 4.83 x 10+2 NA NA NA NA NA NA 1.0 Dioxins and Furans NA NA 1.9 x 10+4 4.0 x 10-1 4.0 x 10-2 4.0 x 10-1 NA NA NA 0.43 Hexachlorocyclohexanes NA NA 4.56 x 10+2 NA NA NA NA NA NA 1.0 Lead & Compounds (inorganic) NA NA 1.55 x 10+2 4.0 x 10-4 2.6 x 10-4 4.0 x 10-4 2.0 x 10-3 5.0 x 10-3 1.0 x 10-3 1.0 Mercury (inorganic) NA NA 5.0 x 10+3 2.7 x 10-2 9.7 x 10-6 2.7 x 10-2 5.0 x 10-2 9.0 x 10-2 3.0 x 10-2 1.0 Nickel and compounds NA NA NA 2.0 x 10-3 1.0 x 10-3 2.0 x 10-3 2.0 x 10-2 6.0 x 10-3 9.0 x 10-3 1.0 4,4’-Methylene dianiline 2.24 1.59 1.11 x 10+1 NA NA NA NA NA NA 1.0 PAH as Benzo(a)pyrene NA NA 5.83 x 10+2 3.4 x 10-2 1.6 x 10-2 3.4 x 10-2 NA NA NA 1.0 Polychlorinated Biphenyls NA NA 9.97 x 10+4 5.0 x 10-2 1.0 x 10-2 5.0 x 10-2 NA NA NA 1.0 

(1) Values based on South Coast AQMD Multi-Pathway Assessment Input Parameters Guidance Document as adapted and modified by OEHHA.

(2) See Tables 5.17 and 5.18 for derivation and references for Kow and Koc values. (3) Values for the Egg Transfer Coefficients have not been developed but are assumed to be similar to meat transfer coefficients cited in the SCAQMD document. 

(4) GRAF (Gastrointestinal Relative Absorption Factor). The guidelines allow for adjusting for bioavailability where the evidence warrants. For example, there are good data 

available to an organism when bound to soil or fly ash matrices relative to when it is in solution or in food. Therefore, a bioavailability factor is incorporated into the model to ainformation becomes available for other chemicals of concern, this type of bioavailability will be incorporated into the model.(5) Dermal absorption of many compounds is limited. The guidelines have incorporated dermal absorption factors to account for the decreased absorption relative to other rout

dermal dose used to assess both cancer and noncancer health hazards. The dermal absorption values come from literature describing absorption of chemicals across the skin. In 

available for specific compounds. In other cases, an absorption fraction is inferred from data for similar chemicals. In a few cases the effects of adsorption to a soil or fly ash mhave been studied. In these rare instances, the dermal absorption factor used in the guidelines accounts for this decreased bioavailability (e.g., the dermal absorption value for d

decreased bioavailability). NA - Data Not Available or Not Applicable. 

5-14 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

 b> Assumptions: 

1: The transfer coefficient is the same for all exposure routes.

 2: The transfer coefficient for all meat is the same. 

3: The transfer coefficient for eggs is the same as for meat. 

3. Fish Products

The average concentration in fish (Cf) is based on the concentration in water and a 

bioconcentration factor. 

 a. Formula EQ 5.3.4.3 M: Cf = Cw * BCF (EQ 5.3.4.3 M)

 1> Cf = Concentration in fish (μg/kg) 

 2> Cw = Concentration in water (μg/kg) 

3> BCF = Bioconcentration factor 

 b. Recommended default values for EQ 5.3.4.3 M: 

 1> Cw = Calculated above in EQ 5.3.3 A 

2> BCF = See Table 5.3 

 c. Assumptions for EQ 5.3.4.3 M: 

1> All contaminants in water are available for bioconcentration. 

2> Contaminant is present in a soil or fly ash matrix. 

Contaminant concentrations are uniform in water based on dispersion. 

4> Only bioconcentration is currently considered. Bioaccumulation from 

the food chain is not considered. 

5.4 Estimation of Dose 

Once the concentrations of substances are estimated in air, soil, water, plants, and animal 

products, they are used to evaluate estimated exposure to people. Exposure is evaluated by 

calculating the lifetime average daily dose (LADD). The following algorithms calculate this 

dose for exposure through inhalation, dermal absorption, and ingestion pathways. This section 

contains average and high-end point-estimates and data distributions for adults and children for 

many exposure pathways. The point-estimates and data distributions that should be used for 

children are listed under the nine-year exposure duration. The point-estimates and data 

distributions that should be used for adults are listed under the 30 and 70-year exposure duration. 

Workers are addressed as adults using single point-estimates for three exposure pathways. 

Point-estimates for workers are listed under “worker (single value).” 

OEHHA has not generated or endorsed distributions for worker exposure. Therefore there is no 

Tier 3 stochastic approach for offsite worker cancer risk assessment. 

 

5-15 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

5.4.1 Estimation of Exposure Through Inhalation 

Exposure through inhalation (Dose-inh) is a function of the respiration rate and the 

concentration of a substance in the air. 

1. Formula EQ 5.4.1 A: 

Dose-inh = Cair *{DBR} * A * EF * ED *10-6 (EQ 5.4.1 A)

 AT

 where: 

Dose-inh = Dose through inhalation (mg/kg/d) 

 10-6 = Micrograms to milligrams conversion, Liters to cubic meters 

 conversion 

 Cair = Concentration in air (μg/m3

) 

 {DBR} = Daily breathing rate (L/kg body weight - day) 

A = Inhalation absorption factor 

 EF = Exposure frequency (days/year) 

ED = Exposure duration (years) 

 AT = Averaging time period over which exposure is averaged, 

 in days (e.g., 25,550 d for 70 yr for cancer risk) 

2. Recommended default values for EQ 5.4.1 A: 

a. EF = 350 d/y 

b. ED = 9; 30; or 70 yr 

 c. AT = 25,550 days 

 d. A = 1

e. {DBR} 9, 30 & 70 year exposure = see Table 5.4 

f. {DBR} 30 and 70 year exposure = see Table 5. 5 for parametric models 

(distributions for Tier 3 stochastic risk assessment) 

Table 5.4 Point Estimates for Daily Breathing Rate for 9, 30, and 70-year 

Exposure Durations (DBR) (L/kg BW * Day) 

9-Year 

Exposure Duration 

30 & 70-Year 

Exposure Duration 

Off-site1

Worker

Average High End Average High End (Single Value) 

452 581 271 393 149

1

This value corresponds to a 70 kg worker breathing 1.3 m3/hour for an eight hour day. 1.3 m3/hr is the breathing

rate recommended by U.S.EPA, (1997a) as an hourly average for outdoor workers. 

5-16 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 5.5 Breathing Rate Distributions for 9, 30, and 70-Year 

Exposure Durations for Stochastic Analysis (L/kg BW * Day) 

 

9-Year 

Exposure Duration

30 & 70-Year 

Exposure Duration 

Distribution Type Gamma Gamma 

Location 301.67 193.99

Scale 29.59 31.27

Shape 5.06 2.46

3. Assumption for EQ 5.4.1 A: 

a. The fraction of chemical absorbed (A) is the same fraction absorbed in the study 

on which the cancer potency or Reference Exposure Level is based. 

5.4.2 Estimation of Exposure Through Dermal Absorption 

Exposure through dermal absorption (Dose-dermal) is a function of the soil or dust loading 

of the exposed skin surface, the amount of skin surface area exposed, and the concentration and 

availability of the substance. Distributions are not available for stochastic analysis. Tier III 

stochastic risk assessments should include the dermal pathway as a high end point estimate. 

1. Formula EQ 5.4.2 A: 

 Dose-dermal = Cs * SA * SL * Ef * ABS * 10- 9 * ED/ BW* AT(EQ 5.4.2 A)

 Where: 

 Dose-dermal = Exposure dose through dermal absorption (mg/kg/d) 

 Cs = Average soil concentration (μg/kg) 

SA = Surface area of exposed skin (cm2

) 

SL = Soil loading on skin (mg/cm2

-d) 

 ABS = Fraction absorbed across skin 

 BW = Body weight (kg) 

 10-9 = Micrograms to kilogram conversion factor (μg/kg) 

EF = (EF defined in Table 5.6) (days/year) 

AT = 25,550 days (70 years) 

 ED = Exposure Duration (years) 

2. Recommended default values for EQ 5.4.2 A: 

 a. Cs = Calculated above in EQ 5.3.2 A 

b. SA = See Table 5.6 

c. SL = See Table 5.6 

d. ABS = See Table 5.3 

e. BW = See Table 5.6 

f. f = See Table 5.6 

5-17 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 5.6 Recommended Point Estimate Values for Dermal Pathway 

for 9, 30, and 70 Year Exposure Durations and Worker1

9 Year1

Exposure Duration 

30 & 70 Year 

Exposure Duration 

Worker

2

(Single Value) 

BW Body Weight (kg) 18 63 70 

Average High End Average High End 

SL Soil Loading (mg/cm2-

day)3

0.2 1.0 0.2 1.0 1.0

EF Exposure Frequency 

(d/yr) 

228 350 121 350 245

SA Surface Area Exposed 

(cm2) 

2,778 3,044 4,700 5,500 5,800

1. OEHHA, 2000b, page 6-10 contains surface area exposed and exposure frequency recommended values for 

children (1- 6) and adults (>6). For the 9 year average surface area exposed, a time weighted average value for ages

0-9 was derived with following formula (5/9 x 2000) + (3/9 x 5000) = 2,778 cm2

. For the 9 year high-end surface 

area exposed, (5/9 x 2000) + (3/9 x 5800) = 3,044 cm2

. It is assumed that dermal exposure to outdoor soil does not 

occur the first year of life. For exposure frequency the same approach was used: 

(5/9 x 350) + (3/9 x 100) = 228 (d/yr) for average. 

2. Worker values for surface area exposed and soil loading are the high end adult values from page 6-10, OEHHA, 

2000b. The exposure frequency assumes that the worker works 49 weeks per year, 5 days per week and that he or 

she is exposed everyday at work. 

3. For Hot Spots risk assessments it is assumed that one event occurs per day. 

 

5.4.3 Estimation of Exposure Through Ingestion 

Exposure through ingestion is a function of the concentration of the substance in the 

substance ingested (soil, water, and food), the gastrointestinal absorption of the substance in a 

soil or fly ash matrix, and the amount ingested. 

1. Exposure through Ingestion of Soil 

There are no distributions for soil ingestion currently recommended. Tier III stochastic 

risk assessments should include a high-end point estimate of soil ingestion, soil loading , 

exposure frequency and soil area. The dose from inadvertent soil ingestion can be estimated by 

the point estimate approach using the following general equation: 

Dose = Csoil x GRAF x SIR x EF x ED x 10-9 (EQ 5.4.3.1 A)

 AT 

 where: 

Dose = dose from soil ingestion (mg/kg BW *day) 

 10-9 = conversion factor (mg/μg) (kg/mg) 

 Csoil = concentration of contaminant in soil (μg/g)

5-18 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

GRAF = gastrointestinal relative absorption fraction, unitless; chemical-specific 

(see Table 5.3) 

SIR = soil ingestion rate (mg/kg BW * day) (see Table 5.7) 

EF = exposure frequency (days/year) 

ED = exposure duration (years) 

AT = averaging time, period of time over which exposure is averaged (days); 

for noncancer endpoints, AT = ED x 365 d/yr; for cancer risk estimates, AT 

= 70 yr x 365 d/yr = 25,550 days 

b. Recommended default values for EQ 5.4.3.1 A: 

a. GRAF = Table 5.3 

b. SIR = Table 5.7 

c. EF = 350 d/year resident, 245 d/year worker 

d. ED = 9, 30, or 70 yr 

e. AT = 25,550 days 

Table 5.7 Soil Ingestion Rates (SIR) for 9, 30 and 70-Year 

Exposure Durations and Off-site Worker. 

9-Year 

Exposure Duration 

30 & 70-Year 

Exposure Duration 

Off-site1

Worker

Soil Ingestion Rate 

(mg/kg BW *Day) 8.7 1.7 1.4

1. The soil ingestion rate of 1.4 (mg/kg BW * day) corresponds to the OEHHA, 2000b recommendation of

100 mg/day for a 70 kg adult.

In this approach, it is assumed that the soil ingested contains a representative 

concentration of the contaminant(s) and the concentration is constant over the exposure period. 

 The term GRAF, or gastrointestinal relative absorption factor, is defined as the fraction 

of contaminant absorbed by the GI tract relative to the fraction of contaminant absorbed from the 

matrix (feed, water, other) used in the study(ies) that is the basis of either the cancer potency 

factor (CPF) or the Reference Exposure Level (REL). If no data are available to distinguish 

absorption in the toxicity study from absorption from the environmental matrix in question, 

i.e., soil, then GRAF = 1. The GRAF allows for adjustment for absorption from a soil matrix if

it is known to be different from absorption across the GI tract in the study used to calculate the 

CPF or REL. In most instances, the GRAF will be 1. 

2. Exposure through Ingestion of Water 

 a. Formula EQ 5.4.3.2 B: 

 Dose-w = Cw * WIR * ABing * Fdw * EF * ED * 10-6 /AT (EQ 5.4.3.2 B)

5-19 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

 where: 

Dose-w = Exposure dose through ingestion of water (mg/kg/d) 

 Cw = Water concentration (μg/kg) 

WIR = Water ingestion rate (ml/kg BW/day) 

 ABing = Gastrointestinal absorption factor

 Fdw = Fraction of drinking water from contaminated source 

 EF = Exposure frequency (days/year) 

 ED = Exposure duration (years) 

 10-6 = Conversion factor (μg/mg)(L/ml) 

 b. Recommended default values for EQ 5.4.3.2 B: 

 1> Cw = Calculated above 5.3.3 A 

2> WIR = See Tables 5.8 and 5.9 

 3> ABing = Default set to 1 

 4>EF = 350 d/yr 

 5>ED = 9, 30, or 70 yrs 

 6>AT = 25,550 days 

Table 5.8 

Point Estimate Water Consumption Ingestion Rates (WIR) for 

9, 30, and 70-Year Exposure Durations (ml/kg BW * day) 

9-Year Exposure 

Duration 

30 and 70-Year Exposure 

Duration 

Average High End Average High End 

40 81 24 54

Table 5.9 

Water Ingestion Lognormal Distributions for 9, 30, and 70-Year 

Exposure Durations (ml/kg BW * day) (Stochastic Analysis) 

Distribution

Type 

9-Year 

Exposure Duration 

30 & 70-Year 

Exposure Duration 

Lognormal Mean ± S.D. μ ± σ Mean ± S.D. μ ± σ

Lognormal 40.03 ±

21.45 

3.57 ± 0.50 24.2 ± 17.0 2.99 ± 0.63 

5-20 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

3. Exposure through Ingestion of Food 

The exposure through food ingestion can be through ingestion of plant products, animal 

products (including fish), and mother's milk. 

a. Plant products 

Exposure through ingesting plants (Dose-p) is a function of the type of plant, 

gastrointestinal absorption factor, bioavailability and the fraction of plants ingested that are 

homegrown. The calculation is done for each type of plant, then summed to get total dose for 

this pathway.

 1> Formula EQ 5.4.3.3.a C: 

 Dose-p = (Cf * IP * GRAF * L * EF * ED * 10-6) /AT (EQ 5.4.3.3.a C)

 a> Dose-p = Exposure dose through ingestion of plant 

 products (mg/kg/d) 

 b> Cf = Concentration in plant type (μg/kg) 

 c> IP = Consumption of exposed, leafy, protected, or root 

 produce (g/kg*day) 

d> GRAF = Gastrointestinal relative absorption factor 

 e> L = Fraction of exposed, leafy, protected, or root produce 

 homegrown

 f> EF = Exposure frequency (days/year) 

 g> ED = Exposure duration (years) 

 h> 10-6 = Conversion factor (μg/kg to mg/g) 

i> AT = Averaging time, period over which exposure is 

averaged (days) 

 2> Recommended default values for EQ 5.4.3.3.a C: 

 a> Cf = Calculated above in EQ 5.3.4.1 A 

 b> IP = See Tables 5.10 to 5.12 

c> GRAF = See Table 5.3 

 d> L = Site specific fraction of produce homegrown or 

 locally produced. For nonurban sites 0.15 may 

be used as a default. For urban sites 0.052 may be 

 used (USEPA, 1997b). 

 e> EF = 350 d/yr 

 f> ED = 70 yrs

 g> AT = 25,550 days 

5-21 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 5.10 

Point Estimates for Per Capita Food 

Consumption Rates (g/Kg BW * Day)

9-Year 

Exposure Duration 

30 & 70-Year 

 Exposure Durations 

Average High End Average High End 

Produce 

 Exposed 4.16 15.7 3.56 12.1

 Leafy 2.92 10.9 2.90 10.6

 Protected 1.63 6.66 1.39 4.88

 Root 4.08 14.9 3.16 10.5

Meat 

 Beef 2.24 7.97 2.25 6.97

 Chicken 1.80 4.77 1.46 5.02

 Pork 1.31 5.10 1.39 4.59

Dairy 12.0 51.9 5.46 17.4

Eggs 3.21 10.3 1.80 5.39

Table 5.11 

Parametric Models for Ages 0-9 Food Consumption 

Distributions (g/kg BW * Day) (Stochastic Analysis) 

Food 

Category 

Distribution

Type Mean Std. Dev. Location Scale Shape μ ± σ

Produce 

 Exposed Lognormal 3.93 5.49 exp(0.83±1.04)

 Leafy Lognormal 2.83 3.89 exp(0.43±1.03)

 Protected Weibull 0.13 1.21 0.71

 Root Lognormal 4.08 5.91 exp(0.84±1.06)

Meat 

 Beef Weibull 0.24 1.72 0.77

 Chicken Gamma 0.25 2.94 0.53

 Pork Weibull 0.18 0.97 0.78

Dairy Lognormal 11.32 18.3 exp(1.78±1.13)

Eggs Weibull 0.26 2.67 0.82

5-22 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 5.12 

Parametric Models for Ages 0-70 Food Consumption Distributions 

(g/kg BW * Day) (Stochastic Analysis) 

Category of Food Mean Standard 

Deviation 

Distribution 

Type μ ± σ

Produce 

 Exposed 3.43 6.16 Lognormal Exp (0.51±1.20) 

 Leafy 2.97 4.95 Lognormal Exp (0.42±1.15) 

 Protected 1.39 2.43 Lognormal Exp (-0.37±1.18) 

 Root 3.07 5.23 Lognormal Exp (0.44±1.17) 

Meat 

 Beef 2.32 3.50 Lognormal Exp (0.25±1.09) 

 Chicken 1.44 2.19 Lognormal Exp (-0.23±1.09) 

 Pork 1.42 2.30 Lognormal Exp (-0.29±1.13) 

Dairy 5.57 10.5 Lognormal Exp (0.96±1.23) 

Eggs 1.84 2.60 Lognormal Exp (0.061±1.05) 

Table 5.13 

Default Values for Fisher–caught Fish Consumption (g/kg BW * Day)

9, 30, & 70-Year 

Exposure Scenario 

Average 0.48 

High-End 1.35 

Table 5.14 

Parametric Model for Fisher-caught Fish Consumption Distribution for 

9, 30 and 70-Year Exposure Scenarios (g/kg BW *Day) (stochastic analysis). 

Mean Standard. 

Deviation 

Distribution

Type μ ± σ

0.48 0.71 Lognormal exp(-1.31 ± 1.08)

5-23 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

b. Animal Products (Including Fisher-caught Fish) 

Exposure through animal product ingestion (Dose-ap) is a function of what type of meat 

and/or fish is ingested, as well as animal milk products and eggs. The calculation is done for 

each type and then summed to get the total dose for this pathway. 

 1> Formula 5.4.3.3.b D: 

 Dose-ap = Cfa * If * GI * L * EF * ED * 10-6 /AT (EQ 5.4.3.3.b D)

a> Dose-ap = Exposure dose through ingestion of animal or fish 

 products (mg/kg BW * day) 

 b> Cfa = Concentration in animal product (μg/kg) 

 c> If = Consumption of animal product (g/kg BW per day), 

 e.g, beef, chicken, pork, diary, eggs, fish 

d> GI = Gastrointestinal absorption factor

e> L = Fraction of animal product homegrown 

 f> EF = Exposure frequency (days/year) 

 g> ED = Exposure duration (years) 

h> AT = Averaging time (days) 

 i> 10-6 = Conversion factor (μg/kg to mg/g) for Cf term

 2> Recommended default values for EQ 5.4.3.3.b D: 

 a> Cfa = Calculated above in EQ 5.3.4.2 E 

b> If = See Tables 5.10, 5.11, and 5.12. For fish ingestion rates see Table 

5.13. For distributions (parametric models) for Tier 3 risk 

assessments see Tables 5.11, 5.12, and 5.14. 

c> GI = Default set to 1. 

d> L = Site specific fraction of product locally produced. 

 e> EF = 350 d/yr 

 f> ED = 70 yrs 

 G> AT = 25,550 DAYS

c. Mother's Milk 

Exposure through mother's milk ingestion (Dose-Im) is a function of the average 

substance concentration in mother's milk and the amount of mother's milk ingested. The 

minimum pathways that the nursing mother is exposed to include inhalation, soil ingestion and 

dermal, since the chemicals evaluated by the mother’s milk pathway are multipathway

chemicals. Other pathways may be appropriate depending on site conditions (e.g. presence of 

vegetable gardens or home grown chickens). The nursing mother in the mother’s milk pathway 

is not herself subject to the mother’s milk pathway. The summed average daily dose (mg/kg 

BW-day) from all pathways is calculated for the nursing mother using the equations on pages 20-

26. 

 

5-24 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

1> Formula 5.4.3.3.c E: 

 Dose-Im = Cm * BMIbw * F * yr / 25,550 (EQ 5.4.3.3.c E)

 a> Dose-Im = Exposure dose through ingestion of mother’s milk (mg/kg 

 BW/d)

 b> Cm = Concentration of contaminant in mother's milk is a function 

 of the mother's exposure through all routes and the 

 contaminant half-life in the body (mg/g milk) 

 1: Formula 5.4.3.3.c F: 

 Cm = Emi * t1/2 * f1 * f3 * 10-3 / (f2 * 0.693) (EQ 5.4.3.3.c F)

 a: Emi = Average daily maternal intake of contaminant 

 from all routes (mg/kg/d) 

 b: t

1/2 = Half-life of contaminant in mother (d) 

 c: fl = Fraction of contaminant that partitions to mother's fat 

 d: f3 = Fraction of fat of mother's milk (kg fat/kg milk) 

 e: f2 = Fraction of mother's weight that is fat(kg fat/kg bw) 

 f: 10-3 = Conversion factor (g to kg milk) 

g: 0.693 = Natural log of 2 

 2: Recommended default values for EQ 5.4.3.3.c F: 

 a: Emi = Sum of doses 

 b: t

1/2 = 2,117 (d) for PCDDs/PCDFs = 5.8 yr 

 1,460 (d) for both PCBs 

 c: fl = 0.8 

 d: f3 = 0.04 (kg fat/kg milk) 

 e: f2 = 0.33 (kg fat/kg BW) 

 c> BMIbw = Daily breast-milk ingestion rate (g/kg BW*day) 

d> F = Frequency of exposure (d/yr) 

 e> yr = Breast-feeding period (yr) 

 

f> 25,550 = Exposure period (d) 

 

2> Recommended default values for EQ 5.4.3.3.c E:

 a> BMIbw = see Table 5.15 

For distribution (parametric model) for Tier 3 stochastic 

5-25 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

 risk assessments see Table 5.16 

 b> F = 365 (d) 

 c> yr = 1(yr) 

 

 3> Assumptions for EQ 5.4.3.3.c E: 

a> For the MEIR, mother is exposed for 25 years, the child receives milk for 

another year, and then the nursing infant is exposed for 9, 30, or 70 years. 

b> For the 9, 30, and 70 year exposure duration scenarios, the total toxicant 

dose from the breast-feeding in the first year of life is assumed to be 

spread over 70 years in order to calculate an average daily dose. 

Table 5.15 

Point Estimate Values for Breast Milk Consumption Rate 

(g/kg BW *day) 

9, 30, and 70-Year 

Exposure Durations 

Average 102 

High End 138 

 

Table 5.16 

Parametric Model for Breast Milk Consumption Rate for 

9, 30, and 70 Year Exposure Durations (Stochastic Analysis) (g/kg BW *day)

Distribution Type Mean ± S.D. 

Normal 102 ± 21.8 

5.5 References for Kow and Koc Values in Table 5.3 

Table 5.17 References for Kow Values 

Compound Notes Reference

Diethylhexlyphthalate Level 1 calculation Mackay et al. (1995)

4,4’-Methylene dianiline Measured Hansch et al. (1985)

5-26 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 5.18 References for Koc Values

Compound Notes Reference

Diethylhexylphthalate Level 1 calculation Mackay et al. (1995)

4,4’-Methylene dianiline Estimated according to methodology of Lyman 

et al. (1990)

Lyman et al. (1990)

5-27 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

6. Dose-Response Assessment for Noncarcinogenic Endpoints 

6.1 Derivation of Toxicity Criteria 

Dose-response assessment describes the quantitative relationship between the amount of 

exposure to a substance (the dose) and the incidence or occurrence of an adverse health impact 

(the response). For noncarcinogens, dose-response information is presented in the form of 

Reference Exposure Levels (RELs). RELs are concentrations or doses at or below which 

adverse effects are not likely to occur following specified exposure conditions. The 

methodology for developing chronic RELs is fundamentally the same as that used by U.S. EPA 

in developing the inhalation Reference Concentrations (RfCs) and oral Reference Doses (RfDs). 

Acute and chronic RELs are frequently calculated by dividing the no observed adverse 

effect level (NOAEL) or lowest observed adverse effect levels (LOAEL) in human or animal 

studies by uncertainty factors. Uncertainty factors are applied to account for interspecies 

extrapolation, intraspecies variability, the use of subchronic studies to extrapolate to chronic 

effects, and use of a LOAEL instead of a NOAEL. Total uncertainty factors range from one to 

three thousand for current RELs. Haber’s equation is used, where needed, to adjust studies with 

different exposure times to the one-hour period needed for most acute RELs. Currently, there 

are eight acute RELs with reproductive health endpoints, which have exposure time periods 

different from one-hour; these alternative exposure periods include four, six, and seven hours. 

The most sensitive toxicological end point is selected as the basis for the REL when there are 

multiple adverse health effects. A slightly more complicated methodology, the Benchmark 

Concentration approach, is described in OEHHA, 1999a. The selection of the most sensitive 

endpoint as the basis for a REL helps ensure that the REL is protective for all health effects. The 

use of uncertainty factors helps ensure that the REL is protective for nearly all individuals, 

including sensitive subpopulations, within the limitations of current scientific knowledge. 

It should be emphasized that exceeding the acute or chronic REL does not necessarily 

indicate that an adverse health impact will occur. However, levels of exposure above the REL 

have an increasing but undefined probability of resulting in an adverse health impact, 

particularly in sensitive individuals (e.g., depending on the toxicant, the very young, the elderly, 

pregnant women, and those with acute or chronic illnesses). The significance of exceeding the 

REL is dependent on the seriousness of the health endpoint, the strength and interpretation of the 

health studies, the magnitude of combined safety factors, and other considerations. In addition, 

there is a possibility that an REL may not be protective of certain small, unusually sensitive 

human subpopulations. Such subpopulations can be difficult to identify and study because of 

their small numbers, lack of knowledge about toxic mechanisms, and other factors. It may be 

useful to consult OEHHA staff when an REL is exceeded (hazard quotient or hazard index is 

greater than 1.0). Chapter 8 discusses the methods used for determining potential noncancer 

health impacts and Appendix I presents example calculations used to determine a hazard 

quotient (HQ) and hazard indices (HI). For detailed information on the methodology and 

derivations for acute RELs, see the Air Toxics Hot Spots Program Risk Assessment Guidelines; 

Part I; The Determination of Acute Reference Exposure Levels for Airborne Toxicants (OEHHA 

1999a) (Part I TSD). For information on chronic RELS see the Air Toxics Hot Spots Program 

6-1
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Risk Assessment Guidelines; Part III; Technical Support Document for the Determination of 

Chronic Reference Exposure Levels (OEHHA 2000a) (Part III TSD). 

Tables 6.1 and 6.2 list the currently adopted acute and chronic inhalation RELs. Some

substances that pose a chronic inhalation hazard may also present a chronic hazard via 

non-inhalation (oral) routes of exposure. The oral RELs for these substances are presented in 

Table 6.3. Appendix L provides a consolidated listing of all the acute and chronic RELs and 

target organs that are approved for use by OEHHA and ARB for the Hot Spots Program. 

Periodically, new or updated RELs are adopted by OEHHA and these guidelines will be updated 

to reflect those changes. See OEHHA’s web site at www.oehha.ca.gov (look under “Air”, then 

select “Hot Spots Guidelines”) to determine if any new or updated RELs have been adopted 

since the last guideline update. 

6.2 Description of Acute Reference Exposure Levels 

OEHHA developed acute RELs for assessing potential noncancer health impacts for 

short-term, generally one-hour peak exposures to facility emissions. (A few RELs are for 4 to 

7-hour peak exposures.) By definition, an acute REL is an exposure that is not likely to cause 

adverse health effects in a human population, including sensitive subgroups, exposed to that 

concentration (in units of micrograms per cubic meter or μg/m3

) for the specified exposure 

duration on an intermittent basis. Many acute RELs are based on mild adverse effects, such as 

mild irritation of the eyes, nose, or throat, or may result in other mild adverse physiological 

changes. For most individuals, it is expected that the mild irritation and other adverse 

physiological changes will not persist after exposure ceases. Some acute RELs are based on 

reproductive/developmental endpoints, such as teratogenicity or fetotoxicity, which are 

considered severe adverse effects. The RELs, target organ systems, and the averaging time for 

substances that can present a potential acute hazard from inhalation are presented in Table 6.1. 

Unlike the chronic RELs discussed in the following section, there are no acute noninhalation 

RELs. Chapter 8 discusses the methods used for determining noncancer acute health impacts. 

Appendix I presents an example calculation used to determine an HQ and HI. 

6-2
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

 

Table 6.1 Acute Reference Exposure Levels and Target Organ Systems Impacted 

Substance 

Chemical 

Abstract 

Service 

Number 

(CAS) 

Acute 

Inhalation 

REL 

(μg/m3) 

Averaging a

Time 

(hour) 

Acute Hazard Index 

Target Organ Systems(s) 

Acrolein 107-02-8 1.9 x 10-1 1 Eyes; Respiratory System

Acrylic Acid 79-10-7 6.0 x 10+3 1 Eyes; Respiratory System

Ammonia 7664-41-7 3.2 x 10+3 1 Eyes; Respiratory System

Arsenic and Inorganic 

Arsenic Compounds 7440-38-2 1.9 x 10-1 4 Reproductive/Developmental

Arsine 7784-42-1 1.6 x 10+2 1 Hematologic System 

Benzene 71-43-2 1.3 x 10+3 6

Hematologic System; Immune 

System; 

Reproductive/Developmental

Benzyl Chloride 100-44-7 2.4 x 10+2 1 Eyes; Respiratory System

Carbon Disulfide 75-15-0 6.2 x 10+3 6 Nervous System;

Reproductive/Developmental

Carbon Monoxide b 630-08-0 2.3 x 10+4 1 Cardiovascular System

Carbon Tetrachloride 56-23-5 1.9 x 10+3 7

Alimentary Tract; 

Nervous System;

Reproductive/Developmental

Chlorine 7782-50-5 2.1 x 10+2 1 Eyes; Respiratory System

Chloroform 67-66-3 1.5 x 10+2 7 Nervous System;

Reproductive/Developmental

Chloropicrin 76-06-2 2.9 x 10+1 1 Eyes; Respiratory System

Copper and Compounds 7440-50-8 1.0 x 10+2 1 Respiratory System

1,4-Dioxane 123-91-1 3.0 x 10+3 1 Eyes; Respiratory System

Epichlorohydrin 106-89-8 1.3 x 10+3 1 Eyes; Respiratory System

Ethylene Glycol 

Monobutyl Ether 111-76-2 1.4 x 10+4 1 Eyes; Respiratory System

Ethylene Glycol 

Monoethyl Ether 110-80-5 3.7 x 10+2 6 Reproductive/Developmental

Ethylene Glycol 

Monoethyl Ether Acetate 

111-15-9 1.4 x 10+2 6 Nervous System;

Reproductive/Developmental

Ethylene Glycol 

Monomethyl Ether 109-86-4 9.3 x 10+1 6 Reproductive/Developmental

Formaldehyde 50-00-0 9.4 x 10+1 1 Eyes; Immune System; 

Respiratory 

Hydrogen Chloride 7647-01-0 2.1 x 10+3 1 Eyes; Respiratory System

Hydrogen Cyanide 74-90-8 3.4 x 10+2 1 Nervous System

Hydrogen Fluoride 7664-39-3 2.4 x 10+2 1 Eyes; Respiratory System

Hydrogen Selenide 7783-07-5 5.0 x 10+0 1 Eyes; Respiratory System

Hydrogen Sulfide 

b 7783-06-4 4.2 x 10+1 1 Nervous System

Isopropyl Alcohol 67-63-0 3.2 x 10+3 1 Eyes; Respiratory System

Mercury (Inorganic) 7439-97-6 1.8 x 10+0 1 Reproductive/Developmental

Methanol 67-56-1 2.8 x 10+4 1 Nervous System

6-3
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 6.1 Acute Reference Exposure Levels and Target Organ Systems Impacted 

Substance 

Chemical 

Abstract 

Service 

Number 

(CAS) 

Acute 

Inhalation 

REL 

(μg/m3) 

Averaging a

Time 

(hour) 

Acute Hazard Index 

Target Organ Systems(s) 

Methyl Bromide 74-83-9 3.9 x 10+3 1

Nervous System; Respiratory

Irritation;

Reproductive/Developmental

Methyl Chloroform 71-55-6 6.8 x 10+4 1 Nervous System

Methyl Ethyl Ketone 78-93-3 1.3 x 10+4 1 Eyes; Respiratory System

Methylene Chloride 75-09-2 1.4 x 10+4 1 Nervous System

Nickel and Nickel 

Compounds 7440-02-0 6.0 x 10+0 1 Immune System; 

Respiratory System

Nitric Acid 7697-37-2 8.6 x 10+1 1 Respiratory System

Nitrogen Dioxide b 10102-44-0 4.7 x 10+2 1 Respiratory System

Ozone b 10028-15-6 1.8 x 10+2 1 Eyes; Respiratory System

Perchloroethylene 127-18-4 2.0 x 10+4 1 Eyes; Nervous System;

Respiratory System 

Phenol 108-95-2 5.8 x 10+3 1 Eyes; Respiratory System

Phosgene 75-44-5 4.0 x 10+0 1 Respiratory System

Propylene Oxide 75-56-9 3.1 x 10+3 1 Eyes; Respiratory System; 

Reproductive/Developmental

Sodium Hydroxide 1310-73-2 8.0 x 10+0 1 Eyes; Skin; 

Respiratory System

Styrene 100-42-5 2.1 x 10+4 1 Eyes; Respiratory System

Sulfates b N/A 1.2 x 10+2 1 Respiratory System

Sulfur Dioxide b 7446-09-5 6.6 x 10+2 1 Respiratory System

Sulfuric Acid and Oleum

7664-93-9 

8014-95-7 1.2 x 10+2 1 Respiratory System

Toluene 108-88-3 3.7 x 10+4 1 

Nervous System; Eyes;

Respiratory System; 

Reproductive/Developmental

Triethylamine 121-44-8 2.8 x 10+3 1 Nervous System; Eyes 

Vanadium Pentoxide 1314-62-1 3.0 x 10+1 1 Eyes; Respiratory System

Vinyl Chloride 75-01-4 1.8 x 10+5 1 Nervous System; Eyes;

Respiratory System

Xylenes (m,o,p-isomers) 1330-20-7 2.2 x 10+4 1 Eyes; Respiratory System

a. The averaging period of noncancer acute RELs is generally a one-hour exposure. However, some are based on 

several hour exposure for reproductive/developmental endpoints (see section 1.6 of the Part I TSD). The 

RELs for the following substances must be compared to modeled emission concentrations of the same

duration rather than maximum one-hour concentrations (e.g., a 4-hour REL should be compared to the 

maximum 4-hour average concentration from the air dispersion model). 

b. California Ambient Air Quality Standard

6-4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

6.3 Description of Chronic Reference Exposure Levels 

OEHHA has developed chronic RELs for assessing noncancer health impacts from

long-term exposure. (See the Part III TSD for detailed information on the development of 

noncancer chronic inhalation and oral RELs.) A chronic REL is a concentration level (that is 

expressed in units of micrograms per cubic meter (μg/m3

) for inhalation exposure and in a dose 

expressed in units of milligram per kilogram-day (mg/kg-day) for oral exposures), at or below 

which no adverse health effects are anticipated following long-term exposure. Long-term

exposure for these purposes has been defined as 12% of a lifetime, or about eight years for 

humans. Table 6.2 lists the chronic noncancer RELs that should be used in the assessment of 

chronic health effects from inhalation exposure. Appendix L provides a consolidated listing of 

all the acute and chronic RELs and target organs that are approved for use by OEHHA and ARB 

for the Hot Spots Program. Periodically, new or updated RELs are adopted by OEHHA and 

these guidelines will be updated to reflect those changes. See OEHHA’s web site at 

www.oehha.ca.gov (look under “Air”, then select “Hot Spots Guidelines”) to determine if any 

new or updated RELs have been adopted since the last guideline update. 

The most sensitive organ system(s) associated with each chronic REL are also presented 

in Table 6.2. Chapter 8 discusses the methods used for determining potential noncancer health 

impacts and Appendix I presents example calculations used to determine a HQ and HI. 

6-5
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 6.2 Chronic Inhalation Reference Exposure Levels (RELs) 

And Chronic Hazard Index Target Organ System(s)

Substance 

Chemical 

Abstract 

Service 

Number 

(CAS) 

Chronic 

Inhalation

REL 

(μg/m3

) 

Chronic Inhalation Hazard Index 

Target Organ System(s) 

Acetaldehyde a 75-07-0 9.0 x 10+0 Respiratory System

Acrolein 107-02-8 6.0 x 10-2 Eyes; Respiratory System

Acrylonitrile 107-13-1 5.0 x 10+0 Respiratory System

Ammonia 7664-41-7 2.0 x 10+2 Respiratory System

Arsenic & Inorganic Arsenic Compounds 7440-38-2 3.0 x 10-2 Cardiovascular System; Developmental;

Nervous System

Benzene 71-43-2 6.0 x 10+1 Developmental; Hematopoietic System;

Nervous System

Beryllium and Beryllium Compounds 7440-41-7 7.0 x 10-3 Immune System; Respiratory System

Butadiene 106-99-0 2.0 x 10+1 Reproductive System

Cadmium and Cadmium Compounds 7440-43-9 2.0 x 10-2 Kidney; Respiratory System

Carbon Disulfide 75-15-0 8.0 x 10+2 Nervous System; Reproductive System

Carbon Tetrachloride 56-23-5 4.0 x 10+1 Alimentary System; Developmental; 

Nervous System

Chlorine 7782-50-5 2.0 x 10-1 Respiratory System

Chlorine Dioxide 10049-04-4 6.0 x 10-1 Respiratory System

Chlorinated Dibenzo-p-dioxins 

b

2,3,7,8-Tetrachlorodibenzo-p-dioxin 

b

1746-01-6 4.0 x 10-5

1,2,3,7,8-Pentachlorodibenzo-p-dioxin 

b

40321-76-4 4.0 x 10-5

1,2,3,4,7,8-Hexachlorodibenzo-p-dioxin 

b

39227-28-6 4.0 x 10-4

1,2,3,6,7,8-Hexachlorodibenzo-p-dioxin 

b

57653-85-7 4.0 x 10-4

1,2,3,7,8,9-Hexachlorodibenzo-p-dioxin 

b 19408-74-3 4.0 x 10-4

1,2,3,4,6,7,8-Heptachlorodibenzo-p-dioxin 

b 35822-46-9 4.0 x 10-3

1,2,3,4,6,7,8,9-Octachlorodibenzo-p-dioxin 

b 3268-87-9 4.0 x 10-1

Alimentary System; Developmental; 

Endocrine System; Hematopoietic System;

Reproductive System; Respiratory System

Chlorinated Dibenzofurans b

2,3,7,8-Tetrachlorodibenzofuran b 5120-73-19 4.0 x 10-4

1,2,3,7,8-Pentachlorodibenzofuran b 57117-41-6 8.0 x 10-4

2,3,4,7,8-Pentachlorodibenzofuran b 57117-31-4 8.0 x 10-5

1,2,3,4,7,8-Hexachlorodibenzofuran b 70648-26-9 4.0 x 10-4

1,2,3,6,7,8-Hexachlorodibenzofuran b 57117-44-9 4.0 x 10-4

Alimentary System; Developmental; 

Endocrine System; Hematopoietic System;

Reproductive System; Respiratory System

6-6
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 6.2 Chronic Inhalation Reference Exposure Levels (RELs) 

And Chronic Hazard Index Target Organ System(s)

Substance 

Chemical 

Abstract 

Service 

Number 

(CAS) 

Chronic 

Inhalation

REL 

(μg/m3

) 

Chronic Inhalation Hazard Index 

Target Organ System(s) 

1,2,3,7,8,9-Hexachlorodibenzofuran b 72918-21-9 4.0 x 10-4

2,3,4,6,7,8-Hexachlorodibenzofuran b 60851-34-5 4.0 x 10-4

1,2,3,4,6,7,8-Heptachlorodibenzofuran b 67562-39-4 4.0 x 10-3

1,2,3,4,7,8,9-Heptachlorodibenzofuran b 55673-89-7 4.0 x 10-3

1,2,3,4,6,7,8,9-Octachlorodibenzofuran b 39001-02-0 4.0 x 10-1

Alimentary System; Developmental; 

Endocrine System; Hematopoietic System;

Reproductive System; Respiratory System

Chlorobenzene 108-90-7 1.0 x 10+3 Alimentary System; Kidney; Reproductive 

System

Chloroform 67-66-3 3.0 x 10+2 Alimentary System; Developmental; 

Kidney

Chloropicrin 76-06-2 4.0 x 10-1 Respiratory System

Chromium VI & Soluble Chromium VI 

Compounds (except chromic trioxide) 18540-29-9 2.0 x 10-1 Respiratory System

Chromic Trioxide (as chromic acid mist) 1333-82-0 2.0 x 10-3 Respiratory System

Cresol Mixtures 1319-77-3 6.0 x 10+2 Nervous System

1,4-Dichlorobenzene 106-46-7 8.0 x 10+2 Alimentary System; Kidney; Nervous 

System; Respiratory System; 

1,1-Dichloroethylene (Vinylidene Chloride) 75-35-4 7.0 x 10+1 Alimentary System

Diesel Exhaust a N/A 5.0 x 10+0 Respiratory System

Diethanolamine 111-42-2 3.0 x 10+0 Cardiovascular System; Nervous System

N,N-Dimethylformamide 68-12-2 8.0 x 10+1 Alimentary System; Respiratory System

1,4-Dioxane 123-91-1 3.0 x 10+3 Alimentary System; Cardiovascular 

System; Kidney

Epichlorohydrin 106-89-8 3.0 x 10+0 Eyes; Respiratory System

1,2-Epoxybutane 106-88-7 2.0 x 10+1 Cardiovascular System; Respiratory

System

Ethylbenzene 100-41-4 2.0 x 10+3

Alimentary System (Liver); 

Developmental; Endocrine System;

Kidney 

Ethyl Chloride 75-00-3 3.0 x 10+4 Alimentary System; Developmental 

Ethylene Dibromide 106-93-4 8.0 x 10-1 Reproductive 

Ethylene Dichloride 107-06-2 4.0 x 10+2 Alimentary System (Liver) 

Ethylene Glycol 107-21-1 4.0 x 10+2 Developmental; Kidney; Respiratory

System

Ethylene Glycol Monoethyl Ether 110-80-5 7.0 x 10+1 Hematopoietic System; Reproductive 

System 

Ethylene Glycol Monoethyl Ether Acetate 111-15-9 3.0 x 10+2 Developmental

6-7
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 6.2 Chronic Inhalation Reference Exposure Levels (RELs) 

And Chronic Hazard Index Target Organ System(s)

Substance 

Chemical 

Abstract 

Service 

Number 

(CAS) 

Chronic 

Inhalation

REL 

(μg/m3

) 

Chronic Inhalation Hazard Index 

Target Organ System(s) 

Ethylene Glycol Monomethyl Ether 109-86-4 6.0 x 10+1 Reproductive System 

Ethylene Glycol Monomethyl Ether Acetate 110-49-6 9.0 x 10+1 Reproductive System

Ethylene Oxide 75-21-8 3.0 x 10+1 Nervous System

Formaldehyde 50-00-0 3.0 x 10+0 Eyes; Respiratory System

Fluorides 1.3 x 10+1 Bone and Teeth, Respiratory System

Glutaraldehyde 111-30-8 8.0 x 10-2 Respiratory System

Hexane (n-) 110-54-3 7.0 x 10+3 Nervous System

Hydrazine 302-01-2 2.0 x 10-1 Alimentary System; Endocrine System

Hydrogen Chloride 7647-01-0 9.0 x 10+0 Respiratory System

Hydrogen Cyanide 74-90-8 9.0 x 10+0 Cardiovascular System; Endocrine System;

Nervous System

Hydrogen Fluoride 7664-39-3 1.4 x 10+1 Bone and Teeth, Respiratory System

Hydrogen Sulfide 7783-06-4 1.0 x 10+1 Respiratory System

Isophorone 78-59-1 2.0 x 10+3 Alimentary System; Developmental 

Isopropanol 67-63-0 7.0 x 10+3 Developmental; Kidney

Maleic Anhydride 108-31-6 7.0 x 10-1 Respiratory System

Manganese & Manganese Compounds 7439-96-5 2.0 x 10-1 Nervous System

Mercury & Mercury Compounds (inorganic) 7439-97-6 9.0 x 10-2 Nervous System

Methanol 67-56-1 4.0 x 10+3 Developmental

Methyl Bromide 74-83-9 5.0 x 10+0 Developmental; Nervous System;

Respiratory System

Methyl tertiary-Butyl Ether 1634-04-4 8.0 x 10+3 Alimentary System; Eyes; Kidney 

Methyl Chloroform 71-55-6 1.0 x 10+3 Nervous System

Methyl Isocyanate 624-83-9 1.0 x 10+0 Reproductive; Respiratory System

Methylene Chloride 75-09-2 4.0 x 10+2 Cardiovascular System; Nervous System

4,4’-Methylene Dianiline (and its dichloride) 101-77-9 2.0 x 10+1 Alimentary System; Eyes 

Methylene Diphenyl Isocyanate 101-68-8 7.0 x 10-1 Respiratory System

Naphthalene 91-20-3 9.0 x 10+0 Respiratory System

Nickel & Nickel Compounds 

(except nickel oxide) 7440-02-0 5.0 x 10-2 Hematopoietic System; Respiratory

System

6-8
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 6.2 Chronic Inhalation Reference Exposure Levels (RELs) 

And Chronic Hazard Index Target Organ System(s)

Substance 

Chemical 

Abstract 

Service 

Number 

(CAS) 

Chronic 

Inhalation

REL 

(μg/m3

) 

Chronic Inhalation Hazard Index 

Target Organ System(s) 

Nickel Oxide 1313-99-1 1.0 x 10-1 Hematopoietic System; Respiratory

System

Phenol 108-95-2 2.0 x 10+2 Alimentary System; Cardiovascular 

System; Kidney; Nervous System

Phosphine 7803-51-2 8.0 x 10-1

Alimentary System; Hematopoietic

System; Kidney; Nervous System;

Respiratory System

Phosphoric Acid 7664-38-2 7.0 x 10+0 Respiratory System

Phthalic Anhydride 85-44-9 2.0 x 10+1 Respiratory System

Polychlorinated biphenyls

P4 (PCBs) (speciated)b

3,3’,4,4’-Tetrachlorobiphenyl (77)b 35298-13-3 4.0 x10-1

3,4,4’,5-Tetrachlorobiphenyl (81)b 70362-50-4 4.0 x 10-1

2,3,3’,4,4’- Pentachlorobiphenyl (105)b 32598-14-4 4.0 x 10-1

2,3,4,4’5- Pentachlorobiphenyl (114)b 74472-37-0 8.0 x 10-2

2,3’4,4’,5- Pentachlorobiphenyl (118)b 31508-00-6 4.0 x 10-1

2’,3,4,4’,5- Pentachlorobiphenyl (123)b 65510-44-3 4.0 x 10-1

3,3’,4,4’,5- Pentachlorobiphenyl (126)b 57465-28-8 4.0 x 10-4

2,3,3’,4,4’,5-Hexachlorobiphenyl (156)b 38380-08-4 8.0 x 10-2

2,3,3’,4,4’,5’-Hexachlorobiphenyl (157)b 69782-90-7 8.0 x10-2

2,3’,4,4’,5,5’-Hexachlorobiphenyl (167)b 52663-72-6 4.0 x10-0

3,3’,4,4’5,5’- Hexachlorobiphenyl (169)b 32774-16-6 4.0 x 10-3

2,3,3’4,4’,5,5’- Heptachlorobiphenyl (189)b 39635-31-9 4.0 x 10-1

Alimentary System; Developmental; 

Endocrine System; Hematopoietic System;

Reproductive System; Respiratory System

Propylene 115-07-1 3.0 x 10+3 Respiratory System

Propylene Glycol Monomethyl Ether 107-98-2 7.0 x 10+3 Alimentary System 

Propylene Oxide 75-56-9 3.0 x 10+1 Respiratory System

Selenium and Selenium compounds (other 

than Hydrogen Selenide) 7782-49-2 2.0 x 10+1 Alimentary System; Cardiovascular 

System; Nervous System

Styrene 100-42-5 9.0 x 10+2 Nervous System

Sulfuric Acid 7664-93-9 1.0 x 10+0 Respiratory System

Tetrachloroethylene a (Perchloroethylene) 127-18-4 3.5 x 10+1 Alimentary System; Kidney 

6-9
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 6.2 Chronic Inhalation Reference Exposure Levels (RELs) 

And Chronic Hazard Index Target Organ System(s)

Substance 

Chemical 

Abstract 

Service 

Number 

(CAS) 

Chronic 

Inhalation

REL 

(μg/m3

) 

Chronic Inhalation Hazard Index 

Target Organ System(s) 

Toluene 108-88-3 3.0 x 10+2 Developmental; Nervous System;

Respiratory System

2,4-Toluene Diisocyanate 584-84-9 7.0 x 10-2 Respiratory System

2,6-Toluene Diisocyanate 91-08-7 7.0 x 10-2 Respiratory System

Trichloroethylene a 79-01-6 6.0 x 10+2 Eyes; Nervous System

Triethylamine 121-44-8 2.0 x 10+2 Eyes

Vinyl Acetate 108-05-4 2.0 x 10+2 Respiratory System

Xylenes (m, o, p-isomers) 1330-20-7 7.0 x 10+2 Nervous System; Respiratory System

a These peer-reviewed values were developed under the Toxic Air Contaminant (TAC) Program mandated by

AB1807 (California Health and Safety Code Sec. 39650 et seq.). 

N/A Not Applicable 

b The OEHHA has adopted the World Health Organization 1997 Toxicity Equivalency Factor (WHO97-TEF) 

scheme for evaluating the cancer risk and noncancer risk due to exposure to samples containing mixtures of 

polychlorinated dibenzo-p-dioxins (PCDD) (also referred to as chlorinated dioxins and dibenzofurans), 

polychlorinated dibenzofurans (PCDF) and polychlorinated biphenyls (PCBs). See Appendix E for more 

information about the scheme and for the methodology for calculating 2,3,7,8-equivalents for PCDD and 

PCDFs. For convenience, OEHHA has calculated chronic REL values for speciated PCDDs, PCDFs and 

PCBs based on the WHO97 TEF values and the chronic REL for 2,3,7,8-tetrachlorodibenzo-p-dioxin using 

the procedure discussed in Appendix E. The chronic REL values can be used to calculate a hazard index 

when the mixtures are speciated from individual congener ground level concentrations. 

6-10
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

6.4 Description of Chronic Oral (Noninhalation) Reference Exposure Levels

As specified throughout the guidelines, estimates of long-term exposure resulting from

facility air emissions of specific compounds must be analyzed for both inhalation and 

noninhalation (multipathway) pathways of exposure for humans. Facilities often emit substances 

under high temperature and pressure in the presence of particulate matter. While some of these 

substances are expected to remain in the vapor phase, other substances such as metals and semivolatile organics can be either emitted as particles, form particles after emission from the facility, 

or adhere to existing particles. Some substances will partition between vapor and particulate 

phases. Substances in the particulate phase can be removed from the atmosphere by settling and, 

thus, potentially present a significant hazard via noninhalation pathways. 

Particulate-associated chemicals can be deposited directly onto soil, onto the leaves or 

fruits of crops, or onto surface waters. Exposure via the oral route is the predominant 

noninhalation pathway, resulting in the noninhalation RELs being referred to as ‘oral RELs’ in 

this document. The oral RELs are expressed as doses in milligrams of substance (consumed and 

dermally absorbed) per kilogram body weight per day (mg/kg-day). 

Table 6.3 lists the chronic noncancer RELs to be used in the assessment of chronic health 

effects from noninhalation pathways of exposure. Appendix L provides a consolidated listing of 

all chronic RELs and target organs that are approved for use by OEHHA and ARB for the Hot 

Spots Program. Periodically, new or updated RELs are adopted by OEHHA and these guidelines 

will be updated to reflect those changes. See OEHHA’s web site at www.oehha.ca.gov (look 

under “Air”, then select “Hot Spots Guidelines”) to determine if any new or updated RELs have 

been adopted since the last guideline update. Chapter 8 discusses the methods used for 

determining potential noncancer health impacts and Appendix I presents example calculations 

used to determine a HQ and HI. 

 

6-11
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 6.3 Chronic Noninhalation ‘Oral’ Reference Exposure Levels (RELs) 

And Chronic Hazard Index Target Organ System(s)

Substance 

Chemical 

Abstract 

Service 

Number 

(CAS) 

Chronic 

Oral 

REL 

(mg/kg-day)

Chronic Oral Hazard Index 

Target Organ System(s) 

Arsenic & Inorganic Arsenic Compounds 7440-38-2 3.0 x 10-4 Cardiovascular System; Skin 

Beryllium and Beryllium Compounds 7440-41-7 2.0 x 10-3 Alimentary System

Cadmium and Cadmium Compounds 7440-43-9 5.0 x 10-4 Kidney

Chlorinated Dibenzo-p-dioxins a

2,3,7,8-Tetrachlorodibenzo-p-dioxin a 1746-01-6 1.0 x 10-8

1,2,3,7,8-Pentachlorodibenzo-p-dioxin a 40321-76-4 1.0 x 10-8

1,2,3,4,7,8-Hexachlorodibenzo-p-dioxin a 39227-28-6 1.0 x 10-7

1,2,3,6,7,8-Hexachlorodibenzo-p-dioxin a 57653-85-7 1.0 x 10-7

1,2,3,7,8,9-Hexachlorodibenzo-p-dioxin a 19408-74-3 1.0 x 10-7

1,2,3,4,6,7,8-Heptachlorodibenzo-p-dioxin a 35822-46-9 1.0 x 10-6

1,2,3,4,6,7,8,9-Octachlorodibenzo-p-dioxin a 3268-87-9 1.0 x 10-4

Alimentary System; 

Developmental; Endocrine 

System; Hematopoietic System;

Reproductive System; Respiratory

System

Chlorinated Dibenzofurans a

2,3,7,8-Tetrachlorodibenzofuran a 5120-73-19 1.0 x 10-7

1,2,3,7,8-Pentachlorodibenzofuran a 57117-41-6 5.0 x 10-7

2,3,4,7,8-Pentachlorodibenzofuran a 57117-31-4 5.0 x 10-8

1,2,3,4,7,8-Hexachlorodibenzofuran a 70648-26-9 1.0 x 10-7

1,2,3,6,7,8-Hexachlorodibenzofuran a 57117-44-9 1.0 x 10-7

1,2,3,7,8,9-Hexachlorodibenzofuran a 72918-21-9 1.0 x 10-7

2,3,4,6,7,8-Hexachlorodibenzofuran a 60851-34-5 1.0 x 10-7

1,2,3,4,6,7,8-Heptachlorodibenzofuran a 67562-39-4 1.0 x 10-6

1,2,3,4,7,8,9-Heptachlorodibenzofuran a 55673-89-7 1.0 x 10-6

1,2,3,4,6,7,8,9-Octachlorodibenzofuran a 39001-02-0 1.0 x 10-4

Alimentary System; 

Developmental; Endocrine 

System; Hematopoietic System;

Reproductive System; Respiratory

System

Chromium VI & Soluble Chromium VI 

Compounds (except chromic trioxide) 18540-29-9 2.0 x 10-2 Hematologic

Fluorides (including hydrogen fluoride) 4.0 x 10-2 Bones and Teeth 

Mercury & Mercury Compounds (inorganic) 7439-97-6 3.0 x 10-4 Immune System; Kidney

Nickel & Nickel Compounds (except nickel

oxide) 7440-02-0 5.0 x 10-2 Alimentary System

Nickel Oxide 1313-99-1 5.0 x 10-2 Alimentary System

Polychlorinated biphenyls

P4 (PCBs) (speciated)b

3,3’,4,4’-Tetrachlorobiphenyl (77)b 35298-13-3 1.0 x 10-4

3,4,4’,5-Tetrachlorobiphenyl (81)b 70362-50-4 1.0 x 10-4

2,3,3’,4,4’- Pentachlorobiphenyl (105)b 32598-14-4 1.0 x 10-4

Alimentary System; 

Developmental; Endocrine 

System; Hematopoietic System;

Reproductive System; Respiratory

System

6-12
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

Table 6.3 Chronic Noninhalation ‘Oral’ Reference Exposure Levels (RELs) 

And Chronic Hazard Index Target Organ System(s)

Substance 

Chemical 

Abstract 

Service 

Number 

Chronic 

Oral 

REL 

(mg/kg-day)

Chronic Oral Hazard Index 

Target Organ System(s) 

(CAS) 

2,3,4,4’5- Pentachlorobiphenyl (114)b 74472-37-0 2.0 x 10-5

2,3’4,4’,5- Pentachlorobiphenyl (118)b 31508-00-6 1.0 x 10-4

2’,3,4,4’,5- Pentachlorobiphenyl (123)b 65510-44-3 1.0 x 10-4

3,3’,4,4’,5- Pentachlorobiphenyl (126)b 57465-28-8 1.0 x 10-7

2,3,3’,4,4’,5-Hexachlorobiphenyl (156)b 38380-08-4 2.0 x 10-5

2,3,3’,4,4’,5’-Hexachlorobiphenyl (157)b 69782-90-7 2.0 x 10-5

2,3’,4,4’,5,5’-Hexachlorobiphenyl (167)b 52663-72-6 1.0 x 10-3

3,3’,4,4’5,5’- Hexachlorobiphenyl (169)b 32774-16-6 1.0 x 10-6

2,3,3’4,4’,5,5’- Heptachlorobiphenyl (189)b 39635-31-9 1.0 x 10-4

Alimentary System; 

Developmental; Endocrine 

System; Hematopoietic System;

Reproductive System; Respiratory

System

a The OEHHA has adopted the World Health Organization 1997 Toxicity Equivalency Factor (WHO97-TEF) 

scheme for evaluating the cancer risk due to exposure to samples containing mixtures of polychlorinated 

dibenzo-p-dioxins (PCDD) (also referred to as chlorinated dioxins and dibenzofurans), polychlorinated 

dibenzofurans (PCDF) and polychlorinated biphenyls (PCBs). For convenience, OEHHA has calculated 

chronic REL values for speciated PCDDs, PCDFs and PCBs based on the WHO97 TEF values and the 

chronic REL for 2,3,7,8-tetrachlorodibenzo-p-dioxin using the procedure discussed in Appendix E. See 

Appendix E for more information about the scheme and for the methodology for calculating 

2,3,7,8-equivalents for PCDD, PCDFs and PCBs. The oral chronic RELs for these compounds may be 

used if the mixtures are speciated to calculate a hazard index from individual congener doses. 

6-13
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

7-1

7. Dose-Response Assessment for Carcinogens

7.1 Introduction

Dose-response assessment describes the quantitative relationship between the amount of 

exposure to a substance (the dose) and the incidence or occurrence of injury (the response). The 

process often involves establishing a toxicity value or criterion to use in assessing potential health risk. 

The toxicity criterion, or health guidance value, for carcinogens is the cancer potency slope (potency 

factor), which describes the potential risk of developing cancer per unit of average daily dose over a 

70-year lifetime. Cancer inhalation and oral potency factors have been determined by the Office of 

Environmental Health Hazard Assessment (OEHHA) or by the United States Environmental Protection 

Agency (U.S. EPA) and endorsed by OEHHA. They are available for many of the substances listed in 

Appendix A (List of Substances) as carcinogens. Table 7.1 and Appendix L list the inhalation and oral 

cancer potency factors that should be used in multipathway health risk assessments (HRAs) for the Hot 

Spots Program. 

The details on the methodology of dose-response assessment for carcinogens are provided in 

the 1985 California Department of Health Services publication Guidelines for Chemical Carcinogen 

Risk Assessments and their Scientific Rationale (CDHS, 1985). Substance-by-substance 

information is presented in OEHHA’s document entitled, The Air Toxics Hot Spots Program Risk 

Assessment Guidelines; Part II; Technical Support Document for Describing Available Cancer 

Potency Factors (OEHHA 1999b) (Part II TSD). 

7.2 Definition of Carcinogenic Potency

Cancer potency factors are expressed as the upper bound probability of developing cancer 

assuming continuous lifetime exposure to a substance at a dose of one milligram per kilogram of body 

weight, and are expressed in units of inverse dose as a potency slope [i.e., (mg/kg/day)-1]. Another 

common potency expression is in units of inverse concentration [(μg/m3)

-1

)] when the slope is based on 

exposure concentration rather than dose; this is termed the unit risk factor. It is assumed in cancer risk 

assessments that risk is directly proportional to dose and that there is no threshold for carcinogenesis. 

The derivation of carcinogenic inhalation and oral cancer potency factors takes into account the 

available information on pharmacokinetics and on the mechanism of carcinogenic action. These values 

are generally the 95% upper confidence limits (UCL) on the dose-response slope. Table 7.1 and 

Appendix L list inhalation and oral cancer potency factors that should be used in risk assessments for 

the Hot Spots Program. Chapter 8 describes procedures for use of potency factors in estimating 

potential cancer risk. 

7.2.1 Description of the Inhalation Cancer Potency Factor

Under the new risk assessment methodology and algorithms presented in Chapters 5 and 8, 

inhalation cancer slope factors must be expressed in units of inverse dose (i.e., (mg/kg/day)-1). Unit 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

7-2

risk factors, in the units of inverse concentration as micrograms per cubic meter (i.e., (μg/m3)

-1

), which 

have been used in previous guidelines for the Hot Spots program, can also be used for assessing cancer 

inhalation risk directly from air concentrations. However, breathing rates, expressed in units of liters per 

kilogram of body weight-day (L/kg*BW-day or L/kg-day), can be coupled with the air concentrations 

to estimate dose in mg/kg-day. This allows estimation of average, high-end, and distributions of cancer 

risk. Therefore for the Hot Spots Program, inhalation cancer potency factors are now recommended 

for determining cancer risk instead of unit risk factors. Unit risk factors are still listed in the Part II TSD 

and may prove useful in other risk assessment applications. 

Multiplication of the average daily inhalation dose over 70 years (mg/kg-day) with the cancer 

potency factor (mg/kg-day)-1 will give inhalation cancer risk (unitless). A more complete description of 

how cancer risk is calculated from the exposure dose and cancer potency factors is provided in Chapter 

8. Appendix I presents an example calculation for determining potential (inhalation) cancer risk. A list 

of current inhalation potency factors is provided in Table 7.1. Periodically, new or revised cancer 

potency factors will be peer reviewed by the State’s Scientific Review Panel on Toxic Air Contaminants 

and adopted by the Director of OEHHA. At that time, these guidelines will be updated to reflect those 

changes. However, in the interim between the adoption of new or updated numbers and a guideline 

update, consult the OEHHA web site at www.oehha.ca.gov (look under “Air”, then select “Hot Spots 

Guidelines”) to determine if any new or updated cancer potency factors have been adopted since the 

last guideline update. If so, these too should be used in the HRA.

7.2.2 Description of the Oral Cancer Potency Factor

Under the Hot Spots Program, a few substances are considered multipathway substances. 

Multipathway substances have the potential to impact a receptor through inhalation and noninhalation 

(oral) exposure routes. These substances include heavy metals and semi-volatile organic substances 

such as dioxins, furans, and polycyclic aromatic hydrocarbons (PAHs). These substances commonly 

exist in the particle phase or partially in the particle phase when emitted into the air. They can therefore 

be deposited onto soil, vegetation, and water. Noninhalation exposure pathways considered under the 

Hot Spots Program include the ingestion of soil, homegrown produce, meat, milk, surface water, breast 

milk, and fish as well as dermal exposure to contaminants deposited in the soil. See Table 5.1 for a list 

of substances that must be evaluated for multipathway exposure. 

Table 7.1 and Appendix L list oral cancer potency factors in units of (mg/kg-day)-1 that should 

be used for assessing the potential cancer risk for these substances through noninhalation exposure 

pathways. The cancer risk from these individual pathways is calculated by multiplying the dose (mg/kgday) times the oral cancer potency factor (mg/kg-day)-1

 to yield oral potential cancer risk (unitless). 

Chapter 5 provides all of the algorithms to calculate exposure dose through all of the individual exposure 

pathways. Appendix I provides a sample calculation for dose and cancer risk using the inhalation 

exposure pathway. 

Four carcinogens (cadmium, hexavalent chromium, beryllium, and nickel), although subject to 

deposition, are only treated as carcinogenic by the inhalation route and not by the oral route. Therefore, 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

7-3

there are no oral cancer potency factors for these substances. However, the oral doses of these 

substances need to be estimated because of their noncancer toxicity. See Chapters 6 and 8, and 

Appendices I, J, and L for dose-response factors, and calculations to address these substances. 

 

Table 7.1 Inhalation and Oral Cancer Potency Factors

Substance

Chemical 

Abstract 

Service

Number

(CAS)

Inhalation

Potency 

Factor 

(mg/kg-day)-1

Oral Slope 

Factor

(mg/kg-day)-1

Acetaldehyde 75-07-0 1.0 x 10-2

Acetamide 60-35-5 7.0 x 10-2

Acrylamide 79-06-1 4.5 x 10+0

Acrylonitrile 107-13-1 1.0 x 10+0

Allyl chloride 107-05-1 2.1 x 10-2

2-Aminoanthraquinone 117-79-3 3.3 x 10-2

Aniline 62-53-3 5.7 x 10-3

Arsenic (inorganic) 7440-38-2 1.2 x 10+1 1.5 x 10+0

Asbestos # 1332-21-4 1.9 x 10-4 #

Benz[a]anthracene BaP 56-55-3 3.9 x 10-1

1.2 x 10+0

Benzene 71-43-2 1.0 x 10-1

Benzidine 92-87-5 5.0 x 10+2

Benzo[a]pyrene 50-32-8 3.9 x 10+0 1.2 x 10+1

Benzo[b]fluoranthrene BaP 205-99-2 3.9 x 10-1

1.2 x 10+0

Benzo[j]fluoranthrene BaP 205-82-3 3.9 x 10-1

1.2 x 10+0

Benzo[k]fluoranthrene BaP 207-08-9 3.9 x 10-1

1.2 x 10+0

Benzyl chloride 100-44-7 1.7 x 10-1

Beryllium 7440-41-7 8.4 x 10+0

Bis(2-chloroethyl) ether 111-44-4 2.5 x 10+0

Bis(chloromethyl)ether 542-88-1 4.6 x 10++1

1,3-Butadiene 106-99-0 6.0 x 10-1

Cadmium (and compounds) 7440-43-9 1.5 x 10+1

Carbon tetrachloride 56-23-5 1.5 x 10-1

Chlorinated Dibenzo-p-dioxins A

2,3,7,8-Tetrachlorodibenzo-p-dioxin 1746-01-6 1.3 x 10+5 1.3 x 10+5

1,2,3,7,8-Pentachlorodibenzo-p-dioxin 40321-76-4 1.3 x 10+5 1.3 x 10+5

1,2,3,4,7,8-Hexachlorodibenzo-p-dioxin 39227-28-6 1.3 x 10+4 1.3 x 10+4

1,2,3,6,7,8-Hexachlorodibenzo-p-dioxin 57653-85-7 1.3 x 10+4 1.3 x 10+4

1,2,3,7,8,9-Hexachlorodibenzo-p-dioxin 19408-74-3 1.3 x 10+4 1.3 x 10+4

1,2,3,4,6,7,8-Heptachlorodibenzo-p-dioxin 35822-46-9 1.3 x 10+3 1.3 x 10+3

1,2,3,4,,6,7,8,9-Octachlorodibenzo-p-dioxin 3268-87-9 1.3 x 10+1 1.3 x 10+1

Chlorinated Dibenzofurans A

2,3,7,8-Tetrachlorodibenzofuran 5120-73-19 1.3 x 10+4 1.3 x 10+4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

7-4

Table 7.1 Inhalation and Oral Cancer Potency Factors

Substance

Chemical 

Abstract 

Service

Number

(CAS)

Inhalation

Potency 

Factor 

(mg/kg-day)-1

Oral Slope 

Factor

(mg/kg-day)-1

1,2,3,7,8-Pentachlorodibenzofuran 57117-41-6 6.5 x 10+3 6.5 x 10+3

2,3,4,7,8-Pentachlorodibenzofuran 57117-31-4 6.5 x 10+4 6.5 x 10+4

1,2,3,4,7,8-Hexachlorodibenzofuran 70648-26-9 1.3 x 10+4 1.3 x 10+4

1,2,3,6,7,8-Hexachlorodibenzofuran 57117-44-9 1.3 x 10+4 1.3 x 10+4

1,2,3,7,8,9-Hexachlorodibenzofuran 72918-21-9 1.3 x 10+4 1.3 x 10+4

2,3,4,6,7,8-Hexachlorodibenzofuran 60851-34-5 1.3 x 10+4 1.3 x 10+4

1,2,3,4,6,7,8-Heptachlorodibenzofuran 67562-39-4 1.3 x 10+3 1.3 x 10+3

1,2,3,4,7,8,9-Heptachlorodibenzofuran 55673-89-7 1.3 x 10+3 1.3 x 10+3

1,2,3,4,,6,7,8,9-Octachlorodibenzofuran 39001-02-0 1.3 x 10+1 1.3 x 10+1

Chlorinated paraffins 108171-26-2 8.9 x 10-2

Chloroform 67-66-3 1.9 x 10-2

4-Chloro-o-phenylenediamine 95-83-0 1.6 x 10-2

p-Chloro-o-toluidine 95-69-2 2.7 x 10-1

Chromium (hexavalent) 18540-29-9 5.1 x 10+2

Chrysene BaP 218-01-9 3.9 x 10-2

1.2 x 10-1

Creosote 8001-58-9 *

p-Cresidine 120-71-8 1.5 x 10-1

Cupferron 135-20-6 2.2 x 10-1

2,4-Diaminoanisole 615-05-4 2.3 x 10-2

2,4-Diaminotoluene 95-80-7 4.0 x 10+0

Dibenz[a,h]acridine BaP 226-36-8 3.9 x 10-1

1.2 x 10+0

Dibenz[a,j]acridine BaP 224-42-0 3.9 x 10-1

1.2 x 10+0

Dibenz[a,h]anthracene BaP 53-70-3 4.1 x 10+0 4.1 x 10+0

Dibenzo[a,e]pyrene BaP 192-65-4 3.9 x 10+0 1.2 x 10+1

Dibenzo[a,h]pyrene BaP 189-64-0 3.9 x 10+1 1.2 x 10+2

Dibenzo[a,I]pyrene BaP 189-55-9 3.9 x 10+1 1.2 x 10+2

Dibenzo[a,l]pyrene BaP 191-30-0 3.9 x 10+1 1.2 x 10+2

7H-Dibenzo[c,g]carbazole BaP 194-59-2 3.9 x 10+0 1.2 x 10+1

1,2-Dibromo-3-chloropropane 96-12-8 7.0 x 10+0

1,4-Dichlorobenzene 106-46-7 4.0 x 10-2

3,3'-Dichlorobenzidine 91-94-1 1.2 x 10+0

1,1-Dichloroethane 75-34-3 5.7 x 10-3

Diesel exhaust B NA 1.1 x 10+0

Diethylhexylphthalate 117-81-7 8.4 x 10-38.4 x 10-3

p-Dimethylaminoazobenzene 60-11-7 4.6 x 10+0

7,12-Dimethylbenz[a]anthracene BaP 57-97-6 2.5 x 10+2 2.5 x 10+2
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

7-5

Table 7.1 Inhalation and Oral Cancer Potency Factors

Substance

Chemical 

Abstract 

Service

Number

(CAS)

Inhalation

Potency 

Factor 

(mg/kg-day)-1

Oral Slope 

Factor

(mg/kg-day)-1

1,6-Dinitropyrene BaP 42397-64-8 3.9 x 10+1 1.2 x 10+2

1,8-Dinitropyrene BaP 42397-65-9 3.9 x 10+0 1.2 x 10+1

2,4-Dinitrotoluene 121-14-2 3.1 x 10-1

1,4-Dioxane 123-91-1 2.7 x 10-2

Epichlorohydrin 106-89-8 8.0 x 10-2

Ethylene dibromide 106-93-4 2.5 x 10-1

Ethylene dichloride 107-06-2 7.2 x 10-2

Ethylene oxide 75-21-8 3.1 x 10-1

Ethylene thiourea 96-45-7 4.5 x 10-2

Formaldehyde 50-00-0 2.1 x 10-2

Hexachlorobenzene 118-74-1 1.8 x 10+0

Hexachlorocyclohexanes (technical grade) 608-73-1 4.0 x 10+0 4.0 x 10+0

Hydrazine 302-01-2 1.7 x 10+1

Indeno[1,2,3-cd]pyrene BaP 193-39-5 3.9 x 10-1

1.2 x 10+0

Lead and lead compounds 7439-92-1 4.2 x 10-28.5 x 10-3

Lindane 58-89-9 1.1 x 10+0

Methyl tertiary-butyl ether 1634-04-4 1.8 x 10-3

3-Methylcholanthrene BaP 56-49-5 2.2 x 10+1 2.2 x 10+1

5-Methylchrysene BaP 3697-24-3 3.9 x 10+0 1.2 x 10+1

4, 4'-Methylene bis(2-chloroaniline) (MOCA) 101-14-4 1.5 x 10+0

Methylene chloride 75-09-2 3.5 x 10-3

4,4'-Methylenedianiline 101-77-9 1.6 x 10+0 1.6 x 10+0

Michler's ketone 90-94-8 8.6 x 10-1

Nickel (and compounds) 7440-02-0 9.1 x 10-1

5-Nitroacenaphthene BaP 602-87-9 1.3 x 10-1

1.3 x 10-1

6-Nitrochrysene BaP 7496-02-8 3.9 x 10+1 1.2 x 10+2

2-Nitrofluorene BaP 607-57-8 3.9 x 10-2

1.2 x 10-1

1-Nitropyrene BaP 5522-43-0 3.9 x 10-1

1.2 x 10+0

4-Nitropyrene BaP 57835-92-4 3.9 x 10-1

1.2 x 10+0

N-Nitroso-n-butylamine 924-16-3 1.1 x 10+1

N-Nitroso-N-methylethylamine 10595-95-6 3.7 x 100

N-Nitrosodi-n-propylamine 621-64-7 7.0 x 10+0

N-Nitrosodiethylamine 55-18-5 3.6 x 10+1

N-Nitrosodimethylamine 62-75-9 1.6 x 10+1

N-Nitrosodiphenylamine 86-30-6 9.0 x 10-3

p-Nitrosodiphenylamine 156-10-5 2.2 x 10-2

N-Nitrosomorpholine 59-89-2 6.7 x 10+0
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

7-6

Table 7.1 Inhalation and Oral Cancer Potency Factors

Substance

Chemical 

Abstract 

Service

Number

(CAS)

Inhalation

Potency 

Factor 

(mg/kg-day)-1

Oral Slope 

Factor

(mg/kg-day)-1

N-Nitrosopiperidine 100-75-4 9.4 x 10+0

N-Nitrosopyrrolidine 930-55-2 2.1 x 10+0

Pentachlorophenol 87-86-5 1.8 x 10-2

Perchloroethylene 127-18-4 2.1 x 10-2

Polychlorinated biphenyls (PCBs) (unspeciated 

mixture)

1336-36-3

(high risk) P1 2.0 x 10+0 2.0 x 10+0

(mediumlow risk) P2 4.0 x 10-1

4.0 x 10-1

(lowest risk) P3 7.0 x 10-2

7.0 x 10-2

Polychlorinated biphenylsP4 (PCBs) (speciated)

3,3’,4,4’-Tetrachlorobiphenyl (77) 35298-13-3 1.3 x 10+1 1.3 x 10+1

3,4,4’,5-Tetrachlorobiphenyl (81) 70362-50-4 1.3 x 10+1 1.3 x 10+1

2,3,3’,4,4’- Pentachlorobiphenyl (105) 32598-14-4 1.3 x 10+1 1.3 x 10+1

2,3,4,4’5- Pentachlorobiphenyl (114) 74472-37-0 6.5 x 10+1 6.5 x 10+1

2,3’4,4’,5- Pentachlorobiphenyl (118) 31508-00-6 1.3 x 10+1 1.3 x 10+1

2’,3,4,4’,5- Pentachlorobiphenyl (123) 65510-44-3 1.3 x 10+1 1.3 x 10+1

3,3’,4,4’,5- Pentachlorobiphenyl (126) 57465-28-8 1.3 x 10+4 1.3 x 10+4

2,3,3’,4,4’,5-Hexachlorobiphenyl (156) 38380-08-4 6.5 x 10+1 6.5 x 10+1

2,3,3’,4,4’,5’-Hexachlorobiphenyl (157) 69782-90-7 6.5 x 10+1 6.5 x 10+1

2,3’,4,4’,5,5’-Hexachlorobiphenyl (167) 52663-72-6 1.3 x 10+0 1.3 x 10+0

3,3’,4,4’5,5’- Hexachlorobiphenyl (169) 32774-16-6 1.3 x 10+3 1.3 x 10+3

2,3,3’4,4’,5,5’- Heptachlorobiphenyl (189) 39635-31-9 1.3 x 10+1 1.3 x 10+1

Potassium bromate 7758-01-2 4.9 x 10-1

1,3-Propane sultone 1120-71-4 2.4 x 10+0

Propylene oxide 75-56-9 1.3 x 10-2

1,1,2,2-Tetrachloroethane 79-34-5 2.0 x 10-1

Thioacetamide 62-55-5 6.1 x 10+0

2,4-Toluene diisocyanate 584-84-9 3.9 x 10-2

2,6-Toluene diisocyanate 91-08-7 3.9 x 10-2

1,1,2-Trichloroethane (vinyl trichloride) 79-00-5 5.7 x 10-2

Trichloroethylene 79-01-6 7.0 x 10-3

2,4,6-Trichlorophenol 88-06-2 7.0 x 10-2

Urethane 51-79-6 1.0 x 10+0

Vinyl chloride 75-01-4 2.7 x 10-1
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

7-7

# Asbestos: [100 PCM fibers/m3]

-1

 A unit risk factor of 2.7 x 10-6 (μg/m3)

-1

 and an inhalation cancer 

potency factor of 2.2 x 10+2 (mg/kg BW*day)-1 are available (see Appendix C for explanation ).

BaP PAHs and PAH Derivatives: Many have potency equivalency factors relative to benzo[a]pyrene 

(see Appendix G).

A Polychlorinated Dibenzo-p-dioxins and Polychlorinated Dibenzofurans: The World Health 

Organization 1997 (WHO-97) Toxicity Equivalency Factors are used for polychlorinated dibenzo-pdioxins, polychlorinated dibenzofurans and polychlorinated biphenyls. (see Appendix E). For 

convenience, OEHHA has calculated cancer potency factors for speciated poly chlorinated 

biphenyls congeners using the procedure in Appendix E. 

B Diesel Exhaust is listed as a Toxic Air Contaminant by the Air Resources Board as “Particulate 

Matter from Diesel-Fueled Engines”. (See Appendix D)

* Creosote: Can be calculated using Potency Equivalency Factors contained in the benzo[a]pyrene 

Toxic Air Contaminant document and in Appendix G of these guidelines.

P1 Polychlorinated Biphenyls (PCBs): High Risk is for use in cases where congeners with more than 

four chlorines do not comprise less (are greater) than one-half percent of total PCBs. The high risk 

number is the default for Polychlorinated Biphenyls (PCBs).

P2 The low risk number is generally not applicable to the Hot Spots program. The Hot Spots program 

addresses PCBs emitted by stationary facilities. It cannot be assumed that such emissions would 

occur by simple evaporation. There is a dermal absorption factor applied in evaluation of the 

dermal pathway for PCBs so the medium risk would not apply to dermal exposure. The water 

pathway does not include an assumption that PCB isomers are water soluble, so the medium 

number would not apply to the water pathway. 

P3 Polychlorinated Biphenyls (PCBs): Lowest Risk is for use in cases where congeners with more than 

four chlorines comprise less than one-half percent of total PCBs. In order for the low number to be 

used, scientific justification needs to be presented. 

P4 Number in parentheses is the IUPAC #, the PCB nomenclature is IUPAC. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-1

8. Risk Characterization for Carcinogens and Noncarcinogens and the 

Requirements for Hot Spots Risk Assessments

8.1 Introduction

Risk characterization is the final step of the health risk assessment (HRA). In this step, 

information developed through the exposure assessment (e.g., monitored or modeled concentrations, 

inhalation or oral doses, and exposure pathway information) is combined with cancer potency factors 

and Reference Exposure Levels (RELs) to quantify the cancer risk and noncancer health impacts, 

respectively. Under the Air Toxics Hot Spots (Hot Spots) Act, comprehensive risk assessments should 

quantify both individual and population-wide health risks (Health and Safety Code Section (HSC) 

44306). Persons preparing HRAs for the Hot Spots Program should consult the local Air Pollution 

Control or Air Quality Management District (District) to determine if the District has special guidelines 

to assist with HRA format or other requirements of the Hot Spots Program. Note that, for the Hot 

Spots Program, the 70-year exposure duration should continue to be used as the basis for estimating 

risk.

This chapter provides guidance on how to evaluate the risk characterization components 

required by the Hot Spots Program. A general summary of the HRA components includes the following 

items or information. This information should be clearly presented in cross-referenced text, tables, 

figures, and/or maps. 

• The location and potential acute noncancer, and multipathway (inhalation and noninhalation) 

cancer and noncancer chronic health impacts at the point of maximum impact (PMI), at the 

maximum exposed individual resident (MEIR), at the maximum exposed individual worker 

(MEIW), and at specified (contact District or reviewing authority) sensitive receptors 

(e.g., schools, hospitals, daycare, or eldercare facilities). 

• Estimates of population exposure for potential cancer risk and noncancer acute and chronic 

health impacts. 

To perform the HRA and create the information listed above, OEHHA recommends using a 

tiered approach to risk assessment. The tiered approach provides a risk assessor with flexibility and 

allows consideration of site-specific differences. Furthermore, risk assessors can tailor the level of effort 

and refinement of an HRA by using the point-estimate exposure assumptions or the stochastic treatment 

of exposure factor distributions. Tier-1 evaluations are required for all HRAs prepared for the Hot 

Spots Program. Persons preparing an HRA using Tier-2 through Tier-4 evaluations must also include 

the results of a Tier-1 evaluation in the HRA. The four-tiered approach to risk assessment is intended 

to primarily apply to residential cancer risk assessment, both for inhalation and noninhalation pathways. 

OEHHA is not recommending a stochastic approach (Tier-3) for worker exposure, or noncancer 

inhalation chronic evaluations. A Tier-2 evaluation could be used for off-site worker risk assessments. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-2

There is only a Tier-1 option for determining acute noncancer risks since calculating the hazard quotient 

only involves the acute REL and short-term maximum ground level air concentrations. There is only a 

Tier-1 option for evaluating inhalation noncancer chronic risks since calculating the chronic hazard 

quotient only involves the chronic Reference Exposure Level and the annual average concentration (not 

exposure parameter distributions). Chronic noninhalation noncancer risks involve a calculation of dose 

from oral pathways. It is possible that site-specific intake variates (e.g., fish consumption) could be 

appropriate for a particular site and therefore a Tier-2 analysis could be useful. See the Air Toxics Hot 

Spots Program Risk Assessment Guidelines; Part IV; Technical Support Document for Exposure 

Assessment and Stochastic Analysis (OEHHA, 2000b) (Part IV TSD) for a detailed discussion of the 

tiered approach. Table 8.1 summarizes OEHHA’s recommendations for the four Tiers. 

Table 8.1 Tiers for Cancer and Noncancer Hot Spots Risk Assessments

Tier Cancer Chronic Non Cancer Acute

Inhalation Noninhalation Inhalation Noninhalation Inhalation

Tier-1 X X X X X

Tier-2 X X X

Tier-3 X X

Tier-4 X X

Cancer risk assessment as currently practiced involves estimating exposure to carcinogenic 

chemicals and multiplying the dose times the cancer potency factor. There are often questions regarding 

the validity of applying the cancer potency factors to less than lifetime exposures. The cancer potency or 

unit risk factors are estimated from long-term animal studies approaching lifetime, or from worker 

epidemiological studies involving long term exposure usually over decades. 

8.2 Risk Characterization for Cancer Health Effects

8.2.1 Calculating Inhalation Cancer Risk 

A 70-year inhalation cancer risk evaluation is required for all carcinogenic risk assessments (see 

Sections 8.2.2 and 8.2.3 for exposure duration information). There are two pieces of information 

needed to assess inhalation cancer risk. These are the inhalation cancer potency for the substance, 

expressed in units of inverse dose as a potency slope (i.e., (mg/kg/day)-1) from Table 7.1, and an 

estimate of average daily inhalation dose in units of milligram per kilogram-day (mg/kg-day) (see 

Chapters 4 and 5). Cancer risk is calculated by multiplying the inhalation dose by the inhalation cancer 

potency factor to yield the potential inhalation excess cancer risk. The following equation illustrates the 

formula for calculating cancer risk. See Appendix I for an example calculation.

(Inhalation Dose (mg/kg-day)) x (Cancer Potency (mg/kg-day)-1) = Cancer Risk
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-3

To convert this to chances per million of developing cancer, multiply the potential cancer risk by 106. 

This result is useful as a risk communication tool.

Tier-1 is a standard point-estimate approach that uses the recommended exposure pathway 

(e.g., breathing rate) point-estimates presented in this document. A Tier-1 evaluation must use the 

high-end point-estimate for the inhalation pathway to present the inhalation cancer risk. For the Hot 

Spots Program, the 70-year exposure duration should be used as the basis for public notification and 

risk reduction audits and plans. Sections 8.2.2 and 8.2.3 describe the use of exposure duration 

adjustment factors for residential and worker receptors. As supplemental information, the assessor may 

wish to evaluate the cancer risk by using the average point-estimate to provide a range of cancer risk to 

the risk manager. The assessor may also decide to further supplement the HRA by performing a Tier-3 

evaluation using the daily breathing rate data distribution in a stochastic analysis. See Chapter 5 for the 

algorithms and exposure information used for all exposure pathways for Tier-1 and Tier-3 evaluations. 

The HARP software will perform all of these analyses. Specifically, the required high-end, 70-year 

inhalation cancer risk evaluation can be performed in HARP by selecting either the high-end pointestimate/cancer risk analysis or by selecting the derived/70-year cancer risk analysis. 

The risk assessment guidelines require the use of the 95th percentile (i.e., high end) breathing 

rate for all assessments of cancer risk by the inhalation route in Tier-1 risk assessments in order to avoid 

underestimating risk to the public, including children. In general, the risk management of facilities in the 

Air Toxics Hot Spots program is based on the 70-year risk at the highest exposed receptor point using 

high-end estimates of breathing rate. Some facilities subject to the Air Toxics Hot Spots Act (e.g., some 

in the industry-wide categories) have very small zones of impact. In some of these instances, there will 

be very few receptors within the zone of impact. It isn’t possible to develop special recommendations 

for all possible exposure scenarios. Alternative breathing rates (point estimates or distributions) may be 

used as part of Tier-2 or Tier-4 risk assessments. Thus, the risk manager should take this into account 

during any risk management decisions. OEHHA is willing to work with risk managers at ARB and the 

Districts on this issue. Further examination of the issue is warranted. 

8.2.2 Calculating Cancer Risk Using Different Exposure Durations 

A. Residential 

OEHHA recommends the 70-year exposure duration (ED) be used for determining residential 

cancer risks. For the Hot Spots Program, the 70-year exposure duration should be used as the basis 

for public notification and risk reduction audits and plans. This will ensure that a person residing in the 

vicinity of a facility for a lifetime will be included in the evaluation of risk posed by that facility. Exposure 

durations of 9-years and 30-years may also be evaluated as supplemental information to show the range 

of cancer risk based on residency periods. Lifetime or 70-year exposure is the historical benchmark for 

comparing facility impacts on receptors and for evaluating the effectiveness of air pollution control 

measures. Although it is not likely that most people will reside at a single residence for 70 years, it is 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-4

common that people will spend their entire lives in a major urban area. While residing in urban areas it 

is very possible to be exposed to the emissions of another facility at the next residence. In order to help 

ensure that people do not accumulate an excess unacceptable cancer risk from cumulative exposure to 

stationary facilities at multiple residences, OEHHA recommends the 70-year exposure duration for risk 

management decisions. However, if a facility is notifying the public regarding cancer risk, it is useful 

information for a person who has resided in his current residence for less than 70 years to know that the 

calculated estimate of his or her cancer risk is less than that calculated for a 70-year risk.

Cancer risk assessment as currently practiced involves estimating exposure to carcinogenic 

chemicals and multiplying the dose times the cancer potency factor. There are often questions 

regarding the validity of applying the cancer potency factors to less than lifetime exposures. 

The cancer potency or unit risk factors are estimated from long-term animal studies 

approaching lifetime, or from worker epidemiological studies involving long term exposure 

usually over decades. 

OEHHA has presented in this document exposure variates for estimating 9, 30 and 70-year exposures. 

These exposures are chosen to coincide with U.S. EPA’s estimates of the average (9 years), high-end 

estimates (30-years) of residence time, and a typical lifetime (70 years). We support the use of cancer 

potency factors for estimating cancer risk for these exposure durations. However, as the exposure 

duration decreases the uncertainties introduced by applying cancer potency factors derived from very 

long term studies increases. Short-term high exposures are not necessarily equivalent to longer-term 

lower exposures even when the total dose is the same. OEHHA therefore does not support the use of 

current cancer potency factor to evaluate cancer risk for exposures of less than 9 years. If such risk 

must be evaluated, we recommend assuming that average daily dose for short-term exposure is 

assumed to last for a minimum of 9 years. OEHHA is evaluating cancer risk assessment 

methodologies over the next several years to address a number of issues including methods to evaluate 

short-term exposures to carcinogens. 

If children younger than age 9 can be exposed to the emissions of a short term project, then the point 

estimates for a child should be used for an exposure period of 9 years to calculate a child’s potential 

cancer risk. OEHHA is evaluating cancer risk assessment methodologies over the next several years to 

address a number of issues including methods to evaluate short-term exposures to carcinogens.

As presented in Chapter 5 and explained in the Part IV TSD, the 9-year (child) exposure 

duration is intended to represent the first 9-years of life. Children, for physiological as well as 

behavioral reasons, have higher rates of exposure (mg/kg-day) than adults. Therefore, the daily pointestimate (e.g., inhalation rate, soil ingestion rates) for the 9-year exposure duration is higher than for the 

30 and 70-year (adult) exposure durations. When assessing the impacts specifically for children, the 

9-year point-estimates and exposure factor distributions should be used. If a 9-year adult exposure 

duration is desired, then the 30 and 70-year point-estimates could be used and the cancer risk is 

adjusted using a factor of 9/70.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-5

The 30 and 70-year exposure durations are intended to represent the first 30 and first 70 years 

of life, respectively. However, in the interest of simplicity, the 30-year exposure duration scenario uses 

the same exposure point-estimates and data distributions as the 70-year exposure duration scenario. 

This assumption to use the 70-year exposure point-estimate for both 30 and 70-year exposures 

probably results in a small underestimation of dose for the 30-year exposure scenario, since the 

exposure parameters for earlier years are higher than years spent as an adult. 

The mother’s milk pathway is unlike other pathways because the (entire) dose to the breastfed infant is 

received in the first year of life. In evaluating risk from the pathway for 9, 30 and 70 years, it is 

assumed that the cancer risk from the one-year exposure to contaminants in mother’s milk is equally 

spread over 70 years to obtain a lifetime risk. If an assessor wants to calculate the multipathway risk 

for a 9-year exposure duration, then the cancer risk for this exposure pathway is adjusted using a 9/70th

factor.

B. Worker

The general approach for estimating the potential health impacts to an offsite worker (e.g., 

MEIW) includes estimating the concentration at the receptor and identifying the duration of that 

exposure. The best way to determine potential impacts for a worker is to use the algorithms and 

exposure information in Chapter 5 and the HARP software. 

There are three factors that affect worker exposure for cancer risk determination. The first is 

the offsite worker’s schedule. For example, some workers such as teachers have three months off 

during the summer and some workers work throughout the year except for weekends, holidays and 

vacation. The second factor is the operating schedule of the emitting facility under consideration. This is 

important because the ISCST-3 air dispersion computer model, or other models typically calculate an 

annual average air concentration based on actual operating conditions. For example, the facility may 

operate 365 days a year, 24 hours a day or may operate eight hours a day, five days a week. The third 

factor is the coincidence of the offsite worker’s schedule with the time that the facility is emitting. For 

example, if the facility emits during the day, five days a week, and the offsite worker is working only at 

night, then no inhalation exposure would occur.

If an adjustment needs to be made for the time that the worker is present (coincident with the 

emissions), then the standard default assumption is the worker is present for 5 days per week, 49 

weeks per year, for 40 years. The 40-year working lifetime is the same assumption used under the 

Proposition 65 Regulation. The worker is assumed to breath 149 L/kg BW* day for an 8-hour 

workday. Other adjustments may be appropriate, such as for teachers or other workers. If the offsite 

worker only works part time, for example 4 hours per day, a factor of 0.5 (4/8) may be used to adjust 

the daily inhalation exposure proportionally. 

If the annual average concentration of pollutants from the emitting facility (determined by the air 

model) is different than the air concentration that the worker breathes when present at the site, then the 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-6

annual average concentration for the worker inhalation pathway will need to be adjusted. For example, 

if the offsite worker and emitting facility are on concurrent schedules (i.e., the worker has a standard 

working schedule of eight hours per day, 5 days a week, and the facility emits 5 days a week, 8 hours 

per day), then the annual average air concentrations for the worker inhalation pathway would need be 

approximated by adjusting it upward using a factor of 4.2 (7/5 x 24/8). The annual average determined 

by the air modeling program is a 24 hour per day , 7 days per week, 365 days per year regardless of 

the actual operating schedule of the facility. The adjustment simply reflects the air concentration that the 

worker breathes. If the worker is only present some of the time that the facility is operating, then the 

average concentration that the worker breathes over his or her working day may be used. 

For the chemicals where noninhalation pathways (e.g., soil ingestion and dermal exposure) need 

to be evaluated for workers, the annual average concentration should not be adjusted to account for the 

operating schedule of the emitting facility or the worker schedule (even if the facility emits only 5 days 

per week 8 hours per day while the offsite worker is present). The pollutant will be deposited and 

accumulate in the soil in the absence or presence of the worker; therefore, the total deposition and soil 

concentration will be dependent on the annual average air concentration.

If the calculation for determining a MEIW inhalation risk are not able to be performed using the 

original algorithms or the HARP software, then the adjustment factors in Table 8.2 may be of use for 

inhalation assessments only. The algorithms and assumptions in Chapter 5 must be used to determine 

multipathway impacts to a worker receptor. 

Table 8.2: Adjustment Factors to Convert Inhalation Based Cancer Risk Estimates for a 

Residential Receptor to a Worker Receptor

Worker Receptor Typ Adjustment Factor e

(Hrs/Days/Weeks/Years

)

Facility Operating Schedule

(Hrs/Days/Weeks/Years) (High End)* (Average)*

Worker (8/5/49/40) Continuous (24/7/52/70) 0.1516 0.2199

Worker (8/5/49/40) Standard (8/5/52/70) 0.6366 0.9234

Teacher (8/5/36 T/40) Continuous (24/7/52/70) 0.1114 0.1616

Teacher (8/5/36 T/40) Standard (8/5/52/70) 0.4679 0.6787

* High End adjustment factors convert the residential receptor risk based on the high-end breathing rate point-estimate to a 

worker receptor risk. Average adjustment factors convert the residential receptor risk based on the average breathing rate 

point-estimate to a worker receptor risk. 

T Number of weeks is based on school days per year reported by school district representatives.

C. Uses of Exposure Duration Adjustments for On-site Receptors

On-site workers are protected by CAL OSHA and do not have to be evaluated under the Hot 

Spots program, unless the worker also lives on the facility site, or property. Occasionally, facilities like 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-7

prisons, military bases, and universities have worker housing within the facility. In these situations the 

evaluation of on-site cancer risks, and/or acute and chronic noncancer hazard indices is appropriate 

under the Hot Spots program. 

If the receptor lives and works on the facility, site, or property, then they should be evaluated 

under both scenarios and the one that is most health protective should be used. 

The cancer risk estimates for the onsite residents may be done using the 70-year exposure 

variates and 40-year exposure duration. The use of the 70 year exposure variates will overestimate 

exposure to adult workers to a small extent because higher inhalation rates, etc., during the portion of a 

70 year lifetime that a person is a child are incorporated. If the on-site resident under evaluation can be 

exposed through an impacted exposure pathway (other than inhalation), then that exposure pathway 

must be included. Other situations that may require on-site receptor assessment include the presence of 

locations where the public may have regular access for the appropriate exposure period 

(e.g., a lunchtime café, store, or museum for acute exposures). No exposure adjustments apply to 

acute exposure analyses. The District may be consulted on the appropriate evaluations for the risk 

assessment. 

8.2.3 Speciation for Specific Classes of Compounds: Polycyclic Aromatic Hydrocarbons 

(PAHs),Polychlorinated Dibenzo-p-dioxins (PCDDs) and Dibenzofurans (PCDFs), and 

Polychlorinated Biphenyls (PCBs)

Health values and potency equivalency factors (PEFs) have been developed for approximately 26 

PAHs (see Appendix G). When speciation of PAHs has been performed on facility emissions, these 

health values and PEFs should be used. In those cases where speciation of PAHs has not been 

performed, then benzo(a)pyrene or B(a)P serves as the surrogate carcinogen for all PAH emissions. A 

similar method has been developed for PCDDs and PCDFs, and PCBs known as toxicity equivalency 

factors (WHO TEFs), based on the number of chlorines and their position on the molecule (see Appendix 

E). Where speciation of PCDDs and PCDFs, and PCBs has been performed on facility emissions, the 

WHO TEFs should be used. In those cases where speciation of PCDDs and PCDFs has not been 

performed, then 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) serves as the surrogate for PCDD and 

PCDF emissions. Similarly, where only total PCBs are available, then the cancer potency factor for 

PCBs should be applied. 

When using the HARP software, the emission contribution of speciated PAHs and 

PCDDs/PCDFs that have health values can be entered into the software. Unknown contributions of the 

PAH or PCDD/PCDF mixtures, or PAHs without a health value, should be assigned the appropriate 

surrogate. If a surrogate substance is used in the report, the facility-emitted substance (PAH mixture or 

PCDDs/PCDF mixture) must also be clearly indicated in the risk assessment as the actual substance 

emitted. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-8

Since the surrogates for total PAH (B(a)P) and total PCDD/PCDF (2,3,7,8-tetrachlorodibenzop-dioxin) are the most or nearly-the-most potent carcinogens in the class, use of the cancer potency 

factors for these with total emissions will overestimate the risk. 

Given that speciation data on these classes of compounds can result in significant capital 

investment, it may be reasonable to run a screening estimate of risk on the unknown mixture using the 

appropriate surrogate compound to represent the class. If the resulting risk estimate is deemed 

significant enough to trigger health concerns, it would then be advisable to speciate the mixture and run a 

screening estimate using the speciated data. 

8.2.4 Determination of Noninhalation (Oral) Cancer Risk

A small subset of Hot Spots substances is subject to deposition onto the soil, plants, and water 

bodies. These substances need to be evaluated by the appropriate noninhalation pathways, as well as 

by the inhalation pathway, and the results must be presented in all HRAs. These substances include 

semi-volatile organic chemicals and heavy metals. 

For all multipathway substances, the minimum exposure pathways that must be evaluated at 

every residential site (in addition to inhalation) are soil ingestion and dermal exposure. If dioxins, furans, 

or PCBs are emitted, then the breast-milk consumption pathway becomes mandatory. The other 

exposure pathways (e.g., ingestion of homegrown produce or fish) are only evaluated if the facility 

impacts that exposure medium and the receptor under evaluation can be exposed to that medium or 

pathway. For example, if the facility does not impact a fishable body of water within the isopleth of the 

facility, or the impacted water body does not sustain fish, then the fish pathway will not be considered 

for that facility or receptor. Table 5.1 lists the multipathway substances and the pathways that can be 

considered for each substance. Table 8.3 identifies the residential receptor exposure pathways that are 

mandatory and those that are dependent on the available routes of exposure. Table 8.3 also identifies 

the three exposure pathways that are appropriate for a worker receptor. 

Table 8.3 Mandatory and Site/Route Dependant Exposure Pathways

Mandatory Exposure Pathways Site/Route Dependent Exposure 

Pathways

• Inhalationw

• Soil Ingestionw

• Dermal Exposurew

• Breast-Milk or Mother’s Milk 

Consumption*

• Homegrown Produce Ingestion

• Fish Ingestion 

• Drinking Water Ingestion

• Dairy (Cow’s) Milk Ingestion

• Meat (Beef, Pork, Chicken, and Egg) 

Ingestion

(*) If dioxins, furans, or PCBs are emitted, then the breast-milk consumption pathway becomes mandatory.

(w) Identifies the only appropriate exposure pathways that should be evaluated for a worker. These pathways are 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-9

 inhalation, dermal exposure, and the soil ingestion pathways. 

The oral cancer risk is calculated using the same steps as inhalation cancer risk described in 

Section 8.2.1. The only difference is that the inhalation dose is replaced by a noninhalation pathway 

dose (e.g., soil ingestion) and consideration is given to determining the dominant exposure pathways for 

the proper use of point-estimates (see Section 8.2.5).

In summary, an oral dose (see Chapters 4 and 5) from the pathway under evaluation (e.g., soil 

ingestion) is multiplied by the substance-specific oral slope factor, expressed in units of inverse dose as 

a potency slope (i.e., (mg/kg/day)-1) from Table 7.1 or Appendix L, to yield the soil ingestion cancer 

risk. The following equation illustrates the formula for calculating cancer risk. Details (data, algorithms, 

and guidance) for each exposure pathway are presented in Chapter 5 and the Part IV TSD. See the 

discussion of Tier-1 in Section 8.2.6 or the Part IV TSD for the method used to determine the 

multipathway cancer risk. See Appendix I for an example calculation for the inhalation exposure 

pathway.

To convert this to chances per million of developing cancer, multiply the cancer risk by 106. This result 

is useful as a risk communication tool.

Cancer risk x 106 = chances per million

8.2.5 Evaluation of Multipathway (Inhalation and Noninhalation) Cancer Risk 

A. Deposition Rate

A deposition rate must be used when determining potential noninhalation health impacts. In the 

absence of facility specific information on the size of the emitted particles, the default values for 

deposition rate should be used. Currently, the default value of 0.02 meters per second is used for 

emission sources that have verifiable particulate matter control devices or for emission sources that may 

be uncontrolled but only emit particulate matter that is less than 2.5 microns (e.g., internal combustion 

engines powered by compressed natural gas). The 0.05 meters per second default value is used for 

risk assessment if the emissions are uncontrolled. If other deposition rate factors are used, sufficient 

support documentation must be included with the HRA. 

= 











 −

















− mg

kg day

kg day

mg Oral Dose OralSlope Factor Potential Cancer Risk 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-10

B. Use of Air Dispersion Modeling Results for Pastures and Water Bodies in Risk 

Assessment and the HARP Software 

The substance or pollutant deposition to a drinking or pasture water body source and 

pastureland will be evaluated if an HRA includes the drinking water, fish ingestion, and cow’s milk or 

meat (beef) exposure pathways. Two approaches are recommended for determining the deposition 

impacts to water bodies and pastureland. A simple approach is to select the results from a single 

receptor point on the grid laid over the area covered by the water body or pasture and assume that the 

modeled concentration at that grid-point is uniform across the water or pasture area. To make this first 

approach health protective, the grid-point within the area of the water body or pastureland with the 

highest modeled concentration should be used. A more refined approach is to average the air 

dispersion modeling results for all of the grid-points covering the area of the pasture or water body.

C. Summary of the Tiered Approach to Risk Assessment 

The tiered approach for risk assessment that is presented in detail in the Part IV TSD and 

summarized here should be reviewed prior to estimating multipathway cancer risk. The tiered approach 

to risk assessment and the evaluation described here are included in the HARP software. The HARP 

software is the recommended model for calculating HRA results for the Hot Spots Program. 

Information on obtaining the HARP software can be found under the Air Toxics Program on the ARB’s 

web site at www.arb.ca.gov.

Tier-1 is a standard point-estimate approach that uses the recommended exposure variate (e.g., 

breathing or water ingestion rate) point-estimates presented in this document. If an HRA cancer risk 

assessment involves multipathway residential exposures, then the risk assessor needs to first calculate 

the cancer risk from each pathway using the high-end exposure variates for all pathways. Then a 

second calculation is performed in which the pathways with the two highest cancer risks are added to 

the cancer risks from the rest of the pathways (if any) calculated with the average exposure variates. 

Dominant pathways are defined as the two exposure pathways that contribute the most to the total 

cancer risk estimate when using high-end point-estimates for all the exposure pathways under 

consideration. The final cancer risk calculation using a combination of high end and average exposure 

variates is referred to as derived risk in the HARP software and applies only to the residential receptor. 

There are only single values for exposure variates for the worker for the three pathways considered. 

A similar procedure is used to determine the hazard index for the noncancer noninhalation 

pathways. The doses from all pathways (noninhalation) are calculated using the high-end exposure 

variate. The dose is used to calculate the hazard quotient for all noninhalation pathways. The hazard 

quotient for the inhalation pathway is calculated from the ground level concentration and the chronic 

REL. The three pathways with the highest hazard quotient are the dominant pathways. The 

remaining noninhalation pathways (if any) hazard quotients may be recalculated using the average 

exposure variates. The total hazard quotient for the chemical may be calculated by adding the 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-11

individual hazard quotients from the dominant pathways and those calculated with the average exposure 

variates.

Using the derived estimate of dose and risk will lessen the issue of compounding high-end 

exposure estimates, while retaining a health-protective approach for the more important exposure 

pathway(s). It is unlikely that an individual receptor would be on the high-end of exposure for all the 

intake variates (exposure pathways). Usually, inhalation is the dominant pathway posing the most 

cancer risk and noncancer chronic health impacts in the HRAs prepared for the Hot Spots Program. 

Occasionally, risks from other exposure pathways may also be dominant for lipophilic (fat-loving) 

compounds or metals. Therefore, for many facilities emitting volatile and multipathway chemicals, the 

inhalation pathway will be at least one of the two exposure pathways for which cancer risks are 

assessed using a high-end estimate (see Section 8.2.1). 

The relatively health-protective assumptions incorporated into the Tier-1 risk assessment (e.g., 

70-year exposure duration (for cancer) and the high-end values for key variates in the driving pathways) 

make it unlikely that the risks are underestimated for the general population. If the results indicate that a 

facility’s estimated cancer risk and noncancer hazard are below the level of regulatory concern, further 

analysis may not be warranted. If the results are above a regulatory level of concern, the risk assessor 

may want to proceed with further analysis as described in Tier-2, or use a more resource-intensive 

stochastic modeling effort described in Tier-3 and Tier-4. While further evaluation may provide more 

information to the risk manager on which to base decisions, the Tier-1 evaluation is useful in comparing 

risks among a large number of facilities and must be included in all HRAs. 

Tier-2 analysis allows the use of available site-specific information to develop point-estimates 

that are more appropriate to use in the site-specific HRA than the recommended point-estimates. In 

Tier-3, a stochastic approach to exposure assessment is taken using the exposure factor distributions 

presented in the Part IV TSD and in Chapter 5. The Part IV TSD exposure factor distributions apply 

only to a residential receptor and are used only for the determination of cancer risk. Tier-4 is also a 

stochastic approach but allows for utilization of site-specific distributions if they are justifiable and more 

appropriate for the site under evaluation than those recommended in this document. 

Tier-3 and Tier-4 analyses show a distribution of cancer risk indicating the percent of the 

population exposed to various levels of risk. This type of analysis provides an illustration of population 

risk. The results from this type of analysis can also be used to show what percentage of the population 

would be protected with various risk management options. 

OEHHA is not recommending a stochastic approach (Tier-3) for worker exposure, or 

noncancer inhalation chronic evaluations. A Tier-2 evaluation could be used for off-site worker risk 

assessments. There is only a Tier-1 option for determining acute noncancer risks since calculating the 

hazard quotient only involves the acute REL and short-term maximum ground level air concentrations. 

In addition, no exposure duration adjustment should be made for noncancer assessments.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-12

D. Multipathway Cancer Risk Methodology

In order to characterize total substance risk for a single multipathway substance the inhalation 

risk is calculated by multiplying the inhalation dose (mg/kg-day) times the inhalation cancer potency 

factor to give the inhalation cancer risk (Section 8.2.1). Using Tier-1, the dermal and oral dose from 

each relevant exposure pathway is multiplied times the substance-specific oral potency factor to give the 

oral (noninhalation) cancer risk (see Sections 8.2.4 and 8.2.5). The inhalation cancer risk and oral 

cancer risk are then summed to give the multipathway cancer risk for that substance. Many facilities will 

emit multiple carcinogenic substances. If multiple substances are emitted, the cancer risk from each of 

the individual substances (including multipathway and volatile, inhalation-only substances) is summed to 

give the (total) multipathway cancer risk for the entire facility at the receptor location.

Cancer risks from different substances are treated additively in the Hot Spots Program in part 

because many carcinogens act through the common mechanism of DNA damage. However, this 

assumption fails to take into account the limited information on substance interactions. However, the 

overall uncertainty in the cancer potency factors and the variability in the human population is probably 

far greater than the uncertainty from the assumption of additivity. In addition, cancers are life threatening 

serious diseases so it is not unreasonable to consider total additive risk. Therefore, the additive 

assumption is reasonable from a public health point of view. Other possible interactions of multiple 

carcinogens include synergism (effects are greater than additive) or antagonism (effects are less than 

additive). The type of interaction is substance dependent and can be dose dependent. All three types 

of interactions have been demonstrated scientifically. 

8.2.6 Risk Characterization for Stochastic Risk Assessment.

Risk characterization for a stochastic risk assessment is similar to that described for the pointestimate approach. However, the results of the stochastic risk assessment is a distribution of risk which 

accounts for some of the variability in cancer risk that results from natural variability in exposure, such as 

breathing rates or water intake. The cancer risk distribution for inhalation cancer risk, for example, is 

generated by multiplying random values from the breathing rate distribution times the ground level air 

concentration, and the cancer potency factor. A variation of the Monte Carlo method called Latin 

hypercube sampling is the method by which the values from the breathing rate distribution are selected. 

If noninhalation pathways need to be evaluated, the same process is followed for each pathway and the 

risk is summed to give an overall inhalation and noninhalation cancer risk distribution. Distributions are 

only available for some of the exposure variates and none are currently recommended for the fate and 

transport algorithms. As more data become available for exposure variates and fate and transport 

variates, OEHHA will expand the number of distributions in our model to better capture the variability in 

exposure and risk. 

The HARP software will perform an HRA using either OEHHA or user-provided data 

distributions using a Monte Carlo analysis and include the statistics on the distributions. The 70-year 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-13

exposure duration should be used as the basis for public notification and risk reduction audits and plans. 

If an assessor would prefer to evaluate 9 or 30-year exposure durations, then a cancer risk distribution 

for 9 or 30-year exposure duration would be presented in addition to the 70-year exposure duration. 

An adult’s analysis would use the 30 and 70-year data distributions. If a stochastic analysis is 

performed for a child, then the child’s (9-year) distribution must be used. A stochastic approach for 

acute and chronic health impacts and worker (MEIW) exposures are not currently recommended. 

Information on obtaining the HARP software can be found under the Air Toxics Program on the ARB’s 

web site at www.arb.ca.gov.

8.3 Risk Characterization for Noncarcinogens

Noncancer impacts are determined for acute (inhalation) exposure and for both inhalation and 

oral chronic exposure. Estimates of health impacts for noncancer endpoints are expressed as a hazard 

quotient (for individual substances) or a hazard index (for multiple substances). In addition, all hazard 

quotients (HQ) and hazard indices (HI) must be determined by target organ system. An HQ of one or 

less indicates that adverse health effects are not expected to result from exposure to emissions of that 

substance. As the HQ increases above one, the probability of human health effects increases by an 

undefined amount. However, it should be noted that a hazard index above 1 is not necessarily indicative 

of health impacts due to the application of uncertainty factors in deriving the Reference Exposure Levels. 

There are limitations to this method of assessing cumulative noncancer chronic health impacts. The 

impact on organ systems may not be additive if health effects occur by different mechanisms. However, 

the impact on organ systems could also be synergistic. An analysis by a trained health professional 

familiar with the substance’s toxicological literature is usually needed to determine the public health 

significance of an HQ or HI above one. It is recommended that the Air District contact OEHHA if this 

situation presents itself. For assessing the noncancer health impacts of lead, different procedures are 

used; please see Appendix F. 

There is only one approach to calculating the acute HI because the calculation is based on the 

highest short-term ground level air concentrations and the acute Reference Exposure Level. Likewise 

the chronic inhalation HI calculation is performed using the annual average ground level concentration 

and the chronic REL. Therefore no Tier-2, Tier-3 or Tier-4 options are available for acute or chronic 

noncancer inhalation hazard evaluation. However, there may be cases in which site specific fate and 

transport variates or exposure variates may be more appropriate to determine dose (mg/kg-day) for the 

noninhalation chronic HI; therefore, in some cases a Tier-2 evaluation may be appropriate for the 

noninhalation pathways. 

Generally, the inhalation pathway is the largest contributor to the total dose. However, there are 

situations where a noninhalation pathway of exposure contributes substantially to a noncancer chronic 

HI. In these cases, the high-end point-estimate of dose is appropriate to use for the three dominant 

pathways and the average point-estimate for the non-dominant pathways. Dominant pathways are 

defined as the three pathways that contribute the most to the total hazard quotient for a chemical 

noncancer HI result when using high-end point-estimates for all the exposure pathways under 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-14

consideration. Typically inhalation would be one of these three pathways. In addition, no exposure 

duration adjustment (e.g., 9/70 or 30/70) should be made for noncancer assessments. See the Part IV 

TSD for a detailed discussion of the tiered approach, or Section 8.2.5 for a short overview of each tier.

 

Information contained in the following locations is needed to evaluate noncancer health impacts. 

Chapter 4 describes air dispersion modeling and both Chapter 6 and Appendix L list all the needed 

dose-response information. Appendix I presents sample calculations for determining chronic 

multipathway noncancer HQs and HIs and acute (inhalation) HQs and HIs. Chapter 9 provides an 

outline of information required for risk characterization. The HARP software is the recommended 

model for calculating and presenting HRA results for the Hot Spots Program. Information on obtaining 

the HARP software can be found under the Air Toxics Program on the ARB’s web site at 

www.arb.ca.gov.

A. Evaluation of Background Criteria Pollutants

The District should be contacted to determine if the contribution of background criteria 

pollutants to respiratory health effects is required to be included in an HRA for the Hot Spots Program. 

If inclusion is required, the method for calculating the health impact from both acute and chronic 

exposure (respiratory endpoint) is the standard HI approach (see Sections 8.3.1 and 8.3.4). The 

background criteria pollutant contribution should be calculated if the HI from the facility’s emissions 

exceeds 0.5 in either the acute or chronic assessment for the respiratory endpoint. 

The most recent criteria pollutant concentration data should be obtained from the ARB’s 

ambient air monitoring network and can be found in the California Almanac of Emissions and Air 

Quality on their web site at www.arb.ca.gov. For determining the criteria pollutant contribution in both 

the chronic and acute HI calculations, annual average concentration data should be taken from a 

monitoring site near the facility. If background contributions are unavailable, the District may direct the 

risk assessor to make an alternative assumption. The criteria pollutants that should be included in both 

the acute and chronic assessments for the respiratory endpoint are ozone, nitrogen dioxide, sulfur 

dioxide, sulfates, and hydrogen sulfide.

8.3.1 Noncancer Chronic Inhalation Health Impacts

All substances in the Hot Spots Program must be evaluated through the inhalation pathway. 

Noncancer chronic inhalation health impacts are calculated by dividing the substance-specific annual 

average air concentration in micrograms per cubic meter (μg/m3) by the chronic inhalation REL (μg/m3) 

(Table 6.2). An REL is used as an indicator of potential noncancer health impacts and is defined as the 

concentration at which no adverse noncancer health effects are anticipated. If this calculation is 

performed for a single substance, then it is called the hazard quotient (HQ). The following equation 

illustrates how to calculate the HQ for chronic inhalation exposure.

Chronic Reference Exposure Level ( g/m )

Annual Average Concentrat ion ( g/m )

3

3

m

m

Hazard Quotient =
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-15

The risk characterization of cumulative noncancer chronic health impacts from the emissions of 

multiple substances by the inhalation route is accomplished by determining the HI. The HI is calculated 

by summing the HQs from all of the substances that affect the same organ system. Note, do not add the 

HQs or HIs for different target organs together (e.g., do not add the impacts for the eye to the 

cardiovascular system). Table 6.2 and Appendix L have a list of the organ systems affected by each 

substance. No exposure duration adjustment (e.g., 9/70) should be made for noncancer assessments. 

The following equation illustrates how to calculate the HI for chronic exposure for the eye (target organ) 

from two substances. See Appendix I for an example calculation.

Hazard Index (HIeye) = HQ substance 1(eye) + HQ substance 2(eye) 

8.3.2 Noncancer Chronic Health Impacts from the Oral Route

Risk characterization for chronic health effects from exposure via the oral route is also 

conducted using the hazard index approach. The hazard quotient is obtained by dividing the oral dose 

(derived from the annual average concentration) in milligrams per kilogram-day (mg/kg-day) by the oral 

chronic REL, expressed in units of (mg/kg-day) (Table 6.3). The point-estimates and algorithms for 

calculating the oral dose for all applicable exposure pathways and receptors (e.g., workers or residents) 

are explained in Chapter 5. 

The high-end point-estimates are used for all exposure pathways to determine which exposure 

pathways are dominant. Once the dominant exposure pathways are decided, the assessor uses the 

high-end point-estimates for the two dominant noninhalation pathways and the average point estimates 

for the rest of the non-dominant exposure pathways to determine the dose and chronic health impacts at 

the residential receptor. The 70-year exposure duration point-estimates are used for residential 

receptors and the worker (single) point-estimates are used for the MEIW in this calculation. No 

exposure duration adjustment (e.g., 9/70) should be made for noncancer assessments. The oral HQ is 

calculated by dividing the oral dose by the oral chronic REL. The significance of oral HQs greater or 

less than one are the same as explained for the chronic inhalation chronic HQ in Section 8.3.1. The 

following equation illustrates how to calculate the HQ for chronic noninhalation exposure.

To estimate the hazard index from noninhalation exposures when multiple pollutants impact the same 

target organ, the oral HQ’s are summed (See Section 8.3.3 below).

8.3.3 Evaluation of Chronic Noncancer Multipathway Hazard Quotients and Hazard 

Indices

Hazard Quotient oral Chronic (oral) Reference Exposure Level (mg/kg - day)

Exposure Pathway Dose (mg/kg - day)

=
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-16

To determine multipathway chronic noncancer health impacts, it is necessary to calculate the 

total hazard index from both inhalation and noninhalation exposures. First, the inhalation HQ is 

calculated (Section 8.3.1). Second, if the substance has an oral REL, then the oral HQ is calculated 

(see Sections 8.2.5, 8.3, and 8.3.2). For a residential receptor, the oral HQ is calculated using the 

70-year high-end point-estimates for the two dominant noninhalation pathways and the average 

point-estimates for the rest of the pertinent exposure pathways. If a worker is under evaluation, then the 

worker single point-estimates are used for the soil and dermal pathways. The third step is to add the 

HQs together for each exposure pathway to give the substance’s total multipathway HQ by target 

organ. If there is only one substance, then the multipathway HQ is the same as the HI. 

• If there are multiple substances emitted, then the fourth step is to total the HQs for all the 

individual substances by each target organ. For example, add the HQs for all substances 

that impact the respiratory system, then repeat this step for the next target organ system 

(e.g., cardiovascular system). This step is repeated until all target organs (for the substances 

emitted) are individually totaled. These impacts by target organ are now referred to as the 

HI. Note, do not add the HQs or HIs for different target organ together (e.g., do not add 

the impacts for the respiratory system to the cardiovascular system). No exposure duration 

adjustment (e.g., 9/70) should be made for noncancer assessments. See Appendix I for an 

example calculation.

• For respiratory irritants, do not add in an oral contribution to the HI for the respiratory 

system for chemicals with both inhalation and oral RELs. 

8.3.4 Noncancer Acute Health Impacts

Risk characterization for acute health effects uses the same principles (HQ, for an individual 

substance, and HI, for multiple substances) as the chronic noncancer inhalation methodology (see 

Section 8.3.1). All acute substances are evaluated through the inhalation pathway only. 

• Noncancer acute health impacts are calculated by dividing the substance-specific shortterm maximum concentration in micrograms per cubic meter (μg/m3

) by the acute REL 

(also in units of μg/m3) (Table 6.1) for each substance. If this calculation is performed 

for a single substance, then it is called the HQ. The HQ should be applied to all 

appropriate target organs for a given substance.

• If multiple substances are emitted, then the next step is to total the individual substance’s 

HQs by each target organ. For example, add the HQs for all substances that impact 

the respiratory system, then repeat this step for the next target organ system. This step 

is repeated until all target organs (for the substances emitted) are individually totaled. 

These impacts by target organ are now referred to as the HI. Note, do not add the 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-17

HQs or HIs for different target organs together (e.g., do not add the impacts for the 

respiratory system to immune system). 

There are no oral acute RELs since it is anticipated that health effects from such a brief 

exposure via the oral route would be insignificant relative to the inhalation route. No exposure duration 

adjustment should be made for noncancer assessments. See Appendix I for an example calculation. 

HQs calculated using one, four, six, and seven hour exposure duration RELs may be added together for 

calculation of an acute HI. This would only occur in evaluating reproductive and developmental 

toxicants, since all other endpoints have only one hour acute RELs.

The HARP software incorporates two procedures for determining an acute HI. Both 

procedures use the calculations for HQ and HI described above. These two procedures make a 

difference when a facility has two or more separated emission points or for HRAs involving multiple 

facilities. The first procedure is a more simplistic approach (consistent with previous CAPCOA HRA 

methods) where the maximum concentrations from each emission source are superimposed to impact 

receptors at the same time, irrespective of wind direction and/ or atmospheric stability. This procedure 

is a simple, health protective approach to assess acute impacts. The second procedure is more refined 

than the first and improves on previous HRA methods. This second procedure takes into account 

meteorology and relative source positions by superimposing results from multiple sources with 

concurrent wind direction and atmospheric conditions, thereby computing a more refined maximum 

impact by hour at each receptor. This refined HI procedure may decrease the concentrations at many 

receptor locations when compared to the simplistic approach, but should not underestimate potential

health impacts (i.e., HQs or HIs). This dual procedure approach is another way the new HRA 

guidelines are building flexibility into the HRA methods.

8.4 Population-Level Risk Estimates 

8.4.1 Carcinogenic Risk

There are basically two ways to provide population-level risk estimates, namely cancer burden 

estimates and estimates of the number of people exposed at specific cancer risk levels. 

1. The cancer burden is calculated by multiplying the number of people exposed (census 

information) by the cancer risk at either the MEIR or the population centroid of each 

census block. The result of this calculation is an estimate of the number of cancer cases 

expected from a 70-year exposure to current estimated facility emissions. 

2. An estimate of the number of people exposed at various cancer risk levels can provide 

perspective on the magnitude of the potential public health threat posed by a facility. 

This approach is intended as a replacement for the cancer burden calculation used by 

some Districts in the past. The new approach provides a much easier way to interpret 

results when compared to cancer burden estimates. A facility in a sparsely populated 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-18

area can have a public health impact different from the same facility in a highly populated 

area. Such information can be useful in risk management decisions. The level of detail 

required for the population analysis (e.g., screening or refined) and the procedures to be 

used in determining geographic resolution and exposed population require case-by-case 

analysis and professional judgment. Some suggested approaches and methods for 

handling the breakdown of population and performance of a screening or refined 

population exposure analyses are provided in Section 4.6.

The population estimates should be based on the latest available census results. The population 

of the census block may be assumed to be equally distributed over the census block, unless for some 

reason more refined information is available. The population in census blocks cut by two or more risk 

isopleths can thus be apportioned based on the area in each isopleth. The isopleths needed should be 

drawn using the smallest practical grid size. The Districts may ask facilities to use the new procedure or 

the cancer burden approach. The District or reviewing authority should be consulted before beginning 

the population exposure estimates and, as results are generated, further consultation may be necessary.

A fundamental first step in estimating the number of people at risk from facility emissions is to 

define the zones of impact (see Section 4.6.1). This zone is commonly defined as the area within the 

isopleth surrounding the facility where receptors have a multipathway cancer risk greater than 10-6. 

Some Districts may prefer to use a cancer risk of 10-7 to define the carcinogenic zone of impact. The 

total number of persons exposed to a series of potential risk levels can be presented to aid risk 

managers in understanding the magnitude of the potential public health impacts. See Table 8.3 for an 

example of data summarizing population exposure estimates for cancer risk.

Table 8.3 Example of Estimates of Population Risk 

Estimated Number of 

Persons Exposed 

Cancer Risk N

(chances per million) 

X 1 to 10

Y 10 to 100

Z >100

(N) Column would be titled to reflect acute or chronic noncancer health impacts.

The HARP software can provide population-level risk estimates as cancer burden or as the 

number of persons exposed to a selected (user-identified) cancer risk level at block level centroids. 

Information on obtaining the HARP software can be found under the Air Toxics Program on the ARB’s 

web site at www.arb.ca.gov. Chapter 9 provides an outline that specifies the content and 

recommended format of HRA results. 

8.4.2 Population Estimates of Noncancer Health Impacts
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. July 

2003.

8-19

A noncancer chronic and acute population estimate of the number of people exposed to acute 

and chronic HQs or HIs exceeding 0.5 or 1.0, in increments of 1.0, should also be presented. For 

example, a facility with a maximum chronic HI of 4.0 would present the number of people exposed to a 

chronic HI of 0.5, 1.0, 2.0, 3.0, and 4.0. The isopleths used in this determination should be drawn 

using the smallest feasible grid size. The same methods that are described in Chapter 4 and Section 

8.4.1 (for the population exposure estimate for cancer risk) should be used in the chronic and acute 

population estimates. Population estimates for acute and chronic health impacts should be presented 

separately and in a format consistent with Table 8.3.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-1

9. Summary of the Requirements for a Modeling Protocol and a Health 

Risk Assessment Report

The purpose of this chapter is to clarify the type of information that is expected to be included in 

modeling protocols and health risk assessments (HRAs). These outlines are intended to promote 

transparent, consistent presentation and efficient review of these products. It is possible that protocols 

and HRAs that do not include all the information presented in these outlines may be considered deficient 

by the reviewing authority. We recommend that persons preparing these products consult with the local 

Air Pollution Control or Air Quality Management District (District) to determine if the District has 

modeling or HRA guidelines that supercede these outlines. If the District does not have guidelines for 

these products, then we recommend Section 9.1 be used for modeling protocols and Section 9.2 be 

used for the presentation of HRAs. Persons preparing modeling protocols and HRAs should specify 

the guidelines that were used to prepare their products. 

9.1 Submittal of Modeling Protocol

It is strongly recommended that a modeling protocol be submitted to the District for review and 

approval prior to extensive analysis with an air dispersion model. The modeling protocol is a plan of the 

steps to be taken during the air dispersion modeling and risk assessment process. We encourage 

people who are preparing protocols to take advantage of the protocol step and fully discuss anticipated 

methodologies for any portion of your project that may need special consideration. Below, we have 

provided an example of the format that may be followed in the preparation of the modeling protocol. 

Consult with the District to confirm format and content requirements or to determine the 

availability of District modeling guidelines before submitting the protocol.

I. Introduction

• Include the facility name, address, and a brief overview describing the facility’s 

operations. 

• Provide a description of the terrain and topography surrounding the facility and potential 

receptors.

• Indicate the format in which data will be provided. Ideally, the report and summary of 

data will be on paper and all data and model input and output files will be provided 

electronically (e.g., compact disk or CD).

• Identify the guidelines used to prepare the protocol. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-2

II. Emissions

• For each pollutant and process whose emissions are required to be quantified in the 

HRA, list the annual average emissions (pounds/year and grams/second) and maximum 

one-hour emissions (pounds/hour and grams/second)*.

• Identify the reference and method(s) used to determine emissions (e.g., source tests, 

emission factors, etc.). Clearly indicate any emission data that are not reflected in the 

previously submitted emission inventory report. In this event, a revised emission 

inventory report will need to be submitted to the District.

III. Models / Modeling Assumptions

• Identify the model(s) to be used, including the version number.

• Identify the model options that will be used in the analysis.

• Indicate complex terrain options that may be used, if applicable.

• Identify the source type(s) that will be used to represent the facility’s operations (e.g., 

point, area, or volume sources, flare options or other).

• Indicate the preliminary source characteristics (e.g., stack height, gas temperature, exit 

velocity, dimensions of volume source, etc.).

• Identify and support the use of urban or rural dispersion coefficients for those models 

that require dispersion coefficients. For other models, identify and support the 

parameters required to characterize the atmospheric dispersion due to land 

characteristics (e.g., surface roughness, Monin-Obukhov length).

IV. Meteorological Data

• Specify the type, source, and year(s) of hourly meteorological data (e.g., hourly surface 

data, upper air mixing height information).

• State how the data are representative for the facility site.

• Describe QA/QC procedures.

• Identify any gaps in the data; if gaps exist, describe how the data gaps are filled.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-3

*Except radionuclides, for which annual and hourly emissions are reported in Curies/year and millicuries/hour, 

respectively.

V. Deposition

• Specify the method to calculate deposition (if applicable).

VI. Receptors

• Identify the method that will be used to determine the location of sensitive receptors, the 

point of maximum impact (PMI), and the maximum exposed individual residential 

(MEIR) and worker (MEIW) receptors (e.g., fine receptor spacing of 20 meters at the 

fenceline and centered on the maximum impacts; coarse receptor spacing of 100 meters 

out to 2,000 meters; extra coarse spacing of 1,000 meters out to 20,000 meters).

• Identify the method that will be used to evaluate potential cancer risk in the vicinity of 

the facility for purposes of calculating cancer burden or population impact estimates. 

Clarify the same information for the presentation of noncancer impacts (e.g., centroids 

of the census tracts in the area within the zone of impact).

• Specify that actual UTM coordinates and the block/street locations (i.e., north side of 

3,000 block of Smith Street), where possible, will be provided for specified receptor 

locations.

• Identify and support the use of any exposure adjustments. 

• Identify if sensitive receptors are present and which receptors will be evaluated in the 

HRA. 

VII. Maps

• Indicate which cancer risk isopleths will be plotted for the cancer zone of impact (e.g., 

10-7, 10-6 see Section 4.6.1).

• Indicate the hazard quotients or hazard indices to be plotted for the noncancer acute 

and chronic zones of impact (e.g., 0.5, 1.0, etc.).

9.2 Outline for a Health Risk Assessment Report

The purpose of this section is to provide an outline to assist with the preparation and review of 

heath risk assessments (HRAs). This outline specifies the key components that should be included in 

HRAs. All information used for the report must be presented in the HRA. Ideally, the HRA report and 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-4

a summary of data used in the HRA will be on paper and all data and model input and output files will 

be provided electronically (e.g., CD). Persons preparing HRAs for the Hot Spots Program should 

consult the District to determine if HRA guidelines or special formats are to be followed when preparing 

and presenting the HRA’s results. If District guidelines or formats do not exist that supersede this 

outline, then the HRA should follow the format presented here. If the HRA is prepared for other 

programs, the reviewing authority should be consulted for clarification of format and content. We 

recommend that those persons preparing HRAs specify the guidelines that were used to prepare their 

product. The HRA may be considered deficient by the reviewing authority if components that are listed 

here are not included.

I. Table of Contents

• Section headings with page numbers indicated.

• Tables and figures with page numbers indicated.

• Appendices with page numbers indicated.

II. Executive Summary

• Name of the facility including the complete address.

• Facility identifier number (consult the District). 

• Description of facility operations and a list identifying emitted substances including 

table of maximum 1-hour and annual average emissions.

• Provide a brief definition of acute, chronic, and cancer health impacts and 

multipathway substances.

• Text presenting overview of dispersion modeling and exposure assessment.

• Text defining dose-response assessment for cancer and noncancer health impacts 

and a table showing target organ systems by substance for noncancer impacts.

• Summary of results, including:

• Location block/street location; e.g., north side of 3,000 block of Smith 

Street) and description of the off-site point of maximum impact (PMI), 

maximum exposed individual resident (MEIR), and maximum exposed 

individual worker (MEIW). 

• Location block/street location; e.g., north side of 3,000 block of Smith 

Street) and description of any on-site receptors that were evaluated at the 

facility (consult District or agency).

• Location (block/street location; e.g., north side of 3,000 block of Smith 

Street) and description of any sensitive receptors that are required by the 

district or reviewing authorities (consult District or agency).
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-5

NOTE: When presenting the following information, potential cancer risk 

should be presented for a 70-year, Tier–1 analysis. Results of other exposure 

assumptions or tier evaluations can be presented, but must be clearly labeled. 

For the Hot Spots Program, the 70-year exposure duration should be used as 

the basis for public notification and risk reduction audits and plans.

• Text presenting an overview of the (total) potential multipathway cancer risk 

at the PMI, MEIR, MEIW, and sensitive receptors. Provide a table of 

cancer risk by substance for the MEIR and MEIW (if applicable). Include a 

statement indicating which of the substances appear to contribute most to 

(drive) the potential health impacts. In addition, identify the exposure 

pathways evaluated in the HRA.

• Provide a map of the facility and surroundings and identify the location of the 

MEIR, MEIW, and PMI. 

• Provide a map of 70-year lifetime cancer risk zone of impact, if applicable. 

• Text presenting an overview of the acute and chronic noncancer hazard 

quotients or the (total) hazard indices for the PMI, MEIR, MEIW, and 

sensitive receptors. Include separate statements (for acute and chronic 

exposures) indicating which of the substances appear to drive the potential 

health impacts. In addition, clearly identify the primary target organ(s) that are 

impacted from acute and chronic exposures. 

• Identify any subpopulations (e.g., subsistence fishers) of concern.

• Table and text presenting an overview of estimates of population exposure 

(e.g., cancer burden or population estimates from HARP) (consult District or 

agency) (see Section 8.4).

• Version of the Risk Assessment Guidelines and computer program(s) used to 

prepare the risk assessment.

III. Risk Assessment Procedures

A. Hazard identification

• Table and text identifying all substances emitted from the facility, plus any other 

substances required by the District or reviewing authority. Include the CAS number 

of the substance and the physical form of the substance if possible. [The Hot Spots 

substances are listed in Appendix A, and also in the ARB’s Emission Inventory 

Criteria and Guidelines Regulations (Title 17, California Code of Regulations, 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-6

Sections 93300-93300.5), and the Emission Inventory Criteria and Guidelines 

Report (EICG Report), which is incorporated by reference therein (ARB, 1997)]. 

• Table and text identifying all substances that are evaluated for cancer risk and/or 

noncancer acute and chronic health impacts. In addition, identify any substances 

that present a potential cancer risk or chronic noncancer hazard via noninhalation 

routes of exposure. 

• Describe the types and amounts of continuous or intermittent predictable emissions 

from the facility that occurred during the reporting year. As required by statute, 

releases from a facility include spilling, leaking, pumping, pouring, emitting, 

emptying, discharging, injecting, escaping (fugitive), leaching, dumping, or disposing 

of a substance into ambient air. Include the substance(s) released and a description 

of the processes that resulted in long-term and continuous releases.

B. Exposure assessment

This section describes the information related to the air dispersion modeling process that 

should be reported in the risk assessment. In addition, doses calculated by pathway of 

exposure for each substance should be included in this section. The District may have specific 

requirements regarding format and content (see Section 4.13). Sample calculations may 

need to be provided (in an appendix) for each step to indicate how the reported 

emissions data were used, if software other than HARP is used. The educated reader 

should be able to reproduce the risk assessment without the need for clarification. The 

location of any information that is presented in appendices, on electronic media, or 

attached documents that supports information presented in this section, must be 

clearly identified by title and page number in this section’s text and in the document’s 

table of contents.

1. Information on the Facility and its Surroundings

• Report the following information regarding the facility and its surroundings:

• Facility name

• Facility identifier number (consult the District).

• Location (use actual UTM coordinates and street address)

• Land use type (see Section 4.4)

• Local topography.

• Facility plot plan identifying†

• emission source locations

• property line

• horizontal scale

• building heights and dimensions

• complex terrain if applicable
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-7

• Description of the site/route dependent exposure pathways. Provide a 

summary of the site-specific inputs used for each pathway (e.g., water or 

grazing intake assumptions). This information may be presented in the 

appendix with the information clearly presented and cross-referenced to the 

text.

2. Source and Emission Inventory Information†

Source Description and Release Parameters

• Report the following information for each source in table format:

• Source identification number used by the facility

• Source name

• Source location using actual UTM coordinates (m)

• Source base elevation (m)

• Source height (m)

• Source dimensions (e.g., stack diameter, building dimensions, area size) 

(m)

• Exhaust gas exit velocity (m/s)

• Exhaust gas volumetric flow rate (ACFM)

• Exhaust gas exit temperature (K)

(See Appendix K for an example.) 

Source Operating Schedule

• The operating schedule for each source should be reported in table form 

including the following information:

• Number of operating hours per day and per year (e.g., 0800-1700, 

2700 hr/yr)

• Number of operating days per week (e.g., Mon-Sat)

• Number of operating days or weeks per year (e.g., 52 wk/yr excluding 

major holidays)

(See Appendix K for an example.)

Emission Control Equipment and Efficiency

• Report emission control equipment and efficiency by source and by 

substance. The description should be brief.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-8

Emissions Data Grouped By Source

• Report emission rates for each toxic substance, grouped by source 

(i.e., emitting device or process identified in Inventory Report), in table form 

including the following information (see Appendix K):

• Source name

• Source identification number

• Substance name and CAS number (Emittent ID from Inventory 

Guidelines)

• Annual average emissions for each substance (lb/yr & g/s)*

• Maximum one-hour emissions for each substance (lb/hr & g/s)*

*Except radionuclides, for which annual and hourly emissions are reported in Curies/year and millicuries/hour, 

respectively.

Emissions Data Grouped by Substance

• Report facility total emission rate by substance for all emitted substances 

listed in the Air Toxics Hot Spots Program including the following 

information (see Appendix K):

• Substance name and CAS number (Emittent ID from Inventory 

Guidelines)

• Annual average emissions for each substance (lb/yr & g/s)

• Maximum one-hour emissions for each substance (lb/hr & g/s)

Emission Estimation Methods

• Report the methods used in obtaining the emissions data indicating whether 

emissions were measured or estimated. Clearly indicate any emission data 

that are not reflected in the previously submitted emission inventory report 

and submit a revised emission inventory report to the District. A reader 

should be able to reproduce the risk assessment without the need for 

clarification.

3. Meteorological Data

• The HRA should indicate the source and time period of the meteorological 

data used. Include the meteorological data (electronically) with the HRA.

• Include proper justification for using this data including information regarding 

appropriateness and quality assurance/quality control.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-9

• Identify any gaps in the data; if gaps exist, describe how the data gaps are 

filled.

• Provide a wind rose for a minimum of the entire time period of the 

meteorological data used, and time period coincident with operating 

schedule. (Other wind roses may be useful as well, such as a stability rose 

or a day/night wind rose.)

• The HRA should indicate if the District required the use of a specified 

representative meteorological data set or the use of default meteorological 

conditions from SCREEN3. All memos indicating the District’s approval of 

meteorological data should be attached in an appendix.

4. Model Selection and Modeling Rationale

• The report should include an explanation of the model chosen to perform 

the analysis and any other decisions made during the modeling process. 

The report should clearly indicate the name of the model used, the level of 

detail (screening or refined analysis) and the rationale behind the selection.

• Table and text that specifies the following information for each air dispersion 

model used:

• version number

• selected options and parameters 

• receptor grid spacing

5. Air Dispersion Modeling Results

• All information used for the report must be presented in the HRA. Ideally, 

a summary of data used in the HRA will be on paper and all data and model 

input and output (e.g., the ISCST3 input file containing the regulatory 

options and emission parameters, receptor locations, meteorology, etc) files 

will be provided electronically (e.g., CD). 

• For the PMI, MEIR, MEIW, and any sensitive receptors required by the 

District, include tables that summarize the annual average concentrations 

that are calculated for all the substances at each site. We recommend the 

use of tables to present the relative contribution of each emission point to 

the receptor concentration. (These tables should have clear reference to the 

computer model that generated the data. It should be made clear to any 

reader how data from the computer output was transferred to these tables).
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-10

• For the PMI, MEIR, MEIW, and any sensitive receptors required by the 

District, include tables that summarize the maximum one-hour; four, six, or 

seven-hour (for those substance with RELs based on those averaging 

periods); and 30-day average (lead only‡) concentrations. (These tables 

should have clear reference to the computer model that generated the data. 

It should be made clear to any reader how data from the computer output 

was transferred to these tables).

• If proprietary software is used, all algorithms and parameters should be 

included with the HRA in a clear, easy to use format.

C. Dose-Response

• Provide tables of the inhalation and oral RELs and cancer potency factors for each 

substance that is quantified in the HRA.

• Identify the guidelines (title and date) that were used to obtain these factors. 

• Provide a table of target organ systems for each noncancer substance, including 

chronic inhalation, chronic oral (if applicable), and acute.

D. Risk Characterization

The Hot Spots Analysis and Reporting Program (HARP) will generate the risk

characterization data needed for the outline below. Any data needed to support the risk 

characterization findings should be clearly presented and referenced in the text and appendices. 

A listing of HARP output files that meet these HRA requirements are provided in this outline 

under the section entitled “Appendices”. All HARP files should be included in the HRA. 

Ideally, the HRA report and a summary of data used in the HRA will be on paper and 

all data and model input and output files will be provided electronically (e.g., CD). 

Information on obtaining copies of HARP is available on the California Air Resources 

Board’s Internet web site under the Air Toxics Program at www.arb.ca.gov.

NOTE: The potential cancer risk for the PMI, MEIR and sensitive receptors of 

interest must be presented in the HRA’s text, tables, and maps using a (lifetime) 

70-year exposure period. MEIW location should use appropriate exposure periods. 

For the Hot Spots Program, the 70-year exposure duration should be used as the basis 

for residential public notification and risk reduction audits and plans. All HRAs must 

include the results of a Tier-1 exposure assessment (see Chapter 2 and 8, or Part IV 

TSD). If the reviewing authority specifies that additional exposure periods should be 

presented, or if persons preparing the HRA would like to present additional 

information (i.e., exposure duration adjustments or the inclusions of risk 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-11

characterizations using Tier-2 through Tier-4 exposure data), then this information 

should be presented in separate, clearly titled, sections, tables, and text. 

The following information should be presented in this section of the HRA. If not fully 

presented here, then by topic, clearly identify the section(s) and pages within the HRA 

where this information is presented.

• Description of receptors to be quantified.

• Table and text providing the location [UTM coordinates and the block/street 

address (e.g., north side of 3,000 block of Smith Street)] and description of the 

PMI, MEIR, and MEIW for both cancer and noncancer risks.

• Table and text providing description of the PMI and MEIR for 9-and 30-year 

cancer risk.

• Table and text providing the location [UTM coordinates and the block/street 

address (e.g., north side of 3,000 block of Smith Street)] and description of any 

sensitive receptors that are of interest to the District or reviewing authorities (consult 

District or agency).

• Provide any exposure information that is used for risk characterization 

(e.g., concentrations at receptors, emissions information, census information, figures, 

zone of impact maps, etc.). Identify the site/route dependent exposure pathways 

(e.g., water ingestion) for the receptor(s), where appropriate (e.g., MEIR). Provide 

a summary of the site-specific inputs used for each exposure pathway (e.g., water 

or grazing intake assumptions). This information may be presented in the appendix 

with the information clearly presented and cross-referenced to the text. In addition, 

provide reference to the appendix (section and page number) that contains the 

modeling (i.e., HARP/dispersion modeling) files that show the same information.

• If any exposure parameters were used other than those provided in the Air Toxics 

Risk Assessment Guidelines; Part IV; Technical Support Document for 

Exposure Assessment and Stochastic Analysis (2000b) (Part IV TSD), they must 

be presented in detail. The derivation and data used must be presented so that it is 

clear to the reviewer. The justification for using site-specific exposure parameters 

must be clearly presented. 

• Include tables of the estimated dose for each substance by each exposure pathway 

at the PMI, MEIR, MEIW, and at any sensitive receptor locations (required by the 

District).
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-12

• Table and text presenting the potential multipathway cancer risk by substance, by 

pathway, and total, at the PMI, MEIR, MEIW, and sensitive receptor locations 

(required by the District). 

• Table and text presenting the acute (inhalation) and chronic noncancer (inhalation 

and oral) hazard quotients (by substance, exposure pathways, and target organs) 

and the (total) hazard indices by substance and target organs for the PMI, MEIR, 

MEIW, and sensitive receptors. Note: chronic noncancer results should be shown 

with inhalation and oral contributions (shown separately) and for the combined 

(multipathway) impact. 

• Identify any subpopulations (e.g., subsistence fishers) of concern. 

• Table and text presenting estimates of population exposure (e.g., population 

exposure estimates or cancer burden from HARP) (consult District or agency). 

Tables should indicate the number of persons exposed to a (total) cancer risk 

greater than 10-7, 10-6, 10-5, 10-4, etc., and total hazard quotient or hazard index 

greater than 0.5, 1.0, 2.0, and 3.0, etc. Provide a table that shows excess cancer 

burden for each population unit and the total excess cancer burden, if cancer burden 

calculation is required.

• Provide maps that illustrate the HRA results for the three bullet points below. These 

maps should be an actual street map of the area impacted by the facility with 

elevation contours and actual UTM coordinates, and the facility boundaries clearly 

labeled. In some cases the elevation contours will make the map too crowded and 

should therefore not appear. This should be a true map (one that shows roads, 

structures, etc.), drawn to scale, and not just a schematic drawing. USGS 

7.5-minute maps are usually the most appropriate choice (see Section 4.6). Note 

that the HARP program contains a mapping feature. 

• The facility (emission points and boundaries), the locations of the PMI, 

MEIR, MEIW, and sensitive receptors.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-13

• Maps of the cancer zone of impacts (e.g., 10-6 or 10-7 levels - consult 

District or Agency). The map should clearly identify the zone of impact for 

the minimum exposure pathways (inhalation, soil ingestion, dermal exposure, 

and breast-milk consumption) and the zone of impact for all the applicable 

exposure pathways (minimum exposure pathways plus additional site/route 

specific pathways). Two maps may be needed to accomplish this. The 

legend of these maps should state the level(s) used for the zone of impact 

and identify the exposure pathways that were included in the assessment.

• Maps of the noncancer hazard index (HI) zone of impacts (e.g., 0.5 or 1.0 

- consult District or Agency). The noncancer maps should clearly identify 

the noncancer zones of impact. These include the acute (inhalation), chronic 

(inhalation), and chronic (multipathway) zones of impact. For clarity, 

presentation of the noncancer zones of impact may require two or more 

maps. The legend of these maps should state the level(s) used for the zone 

of impact and identify the exposure pathways. 

• The risk assessor may want to include a discussion of the strengths and weaknesses 

of the risk analyses and associated uncertainty directly related to the facility HRA.

• If appropriate, comment on the possible alternatives for control or remedial 

measures. How do the risks compare?

• If possible, identify any community concerns that influence public perception of risk.

• Sample calculations may be needed for all analyses in the HRA if proprietary 

software other than HARP was used. The District should be consulted. These 

calculations should be clearly presented and referenced to the findings they are 

supporting in the HRA text.

• Version of the Risk Assessment Guidelines and computer program used to prepare 

the risk assessment.

• If software other than HARP is used for the heath assessment modeling, all 

supporting material must be included with the HRA (e.g., all algorithms and 

parameters used in a clear, easy to review format).

E. References

F. Appendices

The appendices should contain all data, sample calculations, assumptions, and all 

modeling and risk assessment files that are needed to reproduce the HRA results. Ideally, a 

summary of data used in the HRA will be on paper and all data and model input and output files 

will be provided electronically (e.g., CD), unless otherwise specified by the district or reviewing 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-14

authority. All appendices and the information they contain should be referenced, clearly titled, 

and paginated. The HARP program (input and output) files will include many of the items listed 

below.

• Potential Appendix Topics (if not presented elsewhere in the HRA report):

• List of all receptors locations (UTM coordinates and the block/street address 

(e.g., north side of 3,000 block of Smith Street)) for the PMI, MEIR, MEIW, 

and sensitive receptors.

• List of all emitted substances. 

• All emissions files.

• List of dose-response factors.

• All air dispersion modeling input and output files. Detailed discussions of 

meteorological data, regulatory options, emission parameters, receptor 

locations, etc. 

• Census data.

• Maps.

• Identify the site/route dependent exposure pathways for the receptor(s), where 

appropriate (e.g., MEIR). Provide a summary of the site-specific inputs used 

for each pathway (e.g., water or grazing intake assumptions) and the data to 

support them.

• All calculations used to determine emissions, concentrations, and potential 

health impacts at the PMI, MEIR, MEIW, and sensitive receptors.

• All HRA model input and output (HARP) files for receptors of concern. 

• (Total) cancer and noncancer impacts by receptor, substance, and exposure 

pathway (by endpoint for noncancer) at all receptors.

• Presentation of alternate risk assessment methods (e.g., alternate exposure 

durations, or Tier-2 to Tier-4 evaluations with supporting information).

• List of HARP files that meet the Submittal Requirements

• ISC workbook file with all ISC parameters (filename.ISC).

• ISC input file generated by HARP when ISC is run (filename.INP)

• ISC output file generated by HARP when ISC in run (filename.OUT)

• ISC binary output file; holds χ/Q for data for each hour (filename.BIN)

• List of error messages generated by ISC (filename.ERR)

• Sources receptor file; contains list of sources and receptors for the ISC run; 

generated by HARP when you set up ISC (filename.SRC)

• Point estimate risk values generated by HARP; this file is updated automatically 

each time you perform one of the point estimate risk analysis functions 

(filename.RSK)

• Average and maximum χ/Q values for each source-receptor combination; 

generated by ISC (filename.XOQ)

• Plot file generated by ISC (filename.PLT)
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

9-15

• Representative meteorological data used for the facility air dispersion modeling 

(filename.MET)

• Site-specific parameters used for all receptor risk modeling (filename.SIT)

• Map file used to overlay facility and receptors (filename.DEB)

————————

(†) Health and Safety Code (HSC) Section 44346 authorizes facility operators to designate certain 

Hot Spots information as trade secret. HSC Section 44361(a) requires Districts to make health 

risk assessments available for public review upon request. HSC Section 44346 specifies 

procedures to be followed upon receipt of a request for the release of trade secret information. 

See also the Inventory Guidelines Report regarding the designation of trade secret information in 

the Inventory Reports.

(‡) Please see Appendix F or contact the Office of Environmental Health Hazard Assessment for 

information on calculating and presenting chronic lead results.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

10. References 

Auer Jr AH. (1978). Correlation of Land Use and Cover with Meteorological Anomalies. J. 

Appl. Meteorology 17:(5):636-643. 

ARB (1994). ARB memorandum dated 4/11/94 from A. Ranzieri to J. Brooks on the subject, 

"One-hour to Thirty-day Average Screening Factor." 

ARB (1997). California Air Resources Board. Emission Inventory Criteria and Guidelines for 

the Air Toxics “Hot Spots” Program. Including the Final Regulation and the Emission 

Inventory Criteria and Guidelines Report May 15, 1997. Effective May 15, 1997. 

CAPCOA,(1990) CAPCOA Air Toxics Hot Spots Program Facility Prioritization Guidelines, 

July, 1990. 

CAPCOA (1993) CAPCOA Air Toxics Hot Spots Program Revised 1992 Risk Assessment 

Guidelines. California Air Pollution Control Officers Association. October, 1993. 

CDHS (1985). California Department of Health Services. Guidelines for Chemical Carcinogen 

Risk Assessments and their Scientific Rationale. Sacramento. 

Hausch C, Leo AJ, Medchem Project Issue No. 26, Claremont CA: Pomona College (1985) 

Irwin JS. (1978). Proposed Criteria for Selection of Urban Versus Rural Dispersion Coefficients. 

(Draft Staff Report). Meteorology and Assessment Division. U.S. Environmental 

Protection Agency, Research Triangle Park, NC. (Docket No. A-80-46, II-B-8). 

Lyman WJ, Reehl WF, Rosenblatt DH et al., 1990. Handbook of Chemical Property Estimation 

Methods. American Chemical Society, Wash. DC 

NAS. (1983). National Academy of Sciences. Risk Assessment in the Federal Government: 

Managing the Process. National Reasearch Council. National Academy Press, 

Washington D.C. 

NAS. (1994). National Academy of Sciences. Science and Judgment in Risk Assessment 

National Reasearch Council. National Academy Press, Washington D.C. 

OEHHA. (1999a). Air Toxics Hot Spots Program Risk Assessment Guidelines. Part I. The 

Determination of Acute Reference Exposure Levels for Airborne Toxicants. Available 

online at http://www.oehha.ca.gov

OEHHA. (1999b). Air Toxics Hot Spots Program Risk Assessment Guidelines. Part II. 

Technical Support Document for Describing Available Cancer Potency Factors. Available 

online at http://www.oehha.ca.gov

10-1 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

OEHHA. (2000a). Air Toxics Hot Spots Program Risk Assessment Guidelines. Part III. 

Technical Support Document for the Determination of Noncancer Chronic Reference 

Exposure Levels. Available online at http://www.oehha.ca.gov

OEHHA. (2000b). Air Toxics Hot Spots Program Risk Assessment Guidelines. Part IV. 

Technical Support Document for Exposure Assessment and Stochastic Analysis. 

Available online at http://www.oehha.ca.gov

OEHHA, 2002 Part II, Technical Support Document for Describing Available Cancer Potency 

Factors. Available online at http://www.oehha.ca.gov 

OEHHA; 2003 The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health 

Risk Assessments. Available online at http://www.oehha.ca.gov

U.S. EPA (1984). Interim Procedures for Evaluating Air Quality Models (Revised). EPA-450/4-

84-023. U.S. Environmental Protection Agency, Research Triangle Park, NC. (NTIS No. 

PB 85-106060) 

U.S. EPA (1985a). Interim Procedures for Evaluating Air Quality Models: Experience with 

Implementation. U.S. EPA Publication No. EPA-450/4-85-006. U.S. Environmental 

Protection Agency, Research Triangle Park, NC. (NTIS No. PB 85-242477) 

U.S. EPA (1986). Guideline on Air Quality Models (Revised). EPA 450/2-78-027R. 

U.S. Environmental Protection Agency, Research Triangle Park, NC. 

U.S. EPA (1992a). Protocol for Determining the Best Performing Model. U.S. EPA Publication 

No. EPA-454/R-92-025. U.S. Environmental Protection Agency, Research Triangle Park, 

NC. 

U.S. EPA (1995a). Screening Procedures for Estimating the Air Quality Impact of Stationary 

Sources, Revised. EPA-450/R-92-019. U.S. Environmental Protection Agency, Research 

Triangle Park, NC. 

U.S. EPA (1995b). User’s Guide for the Industrial Source Complex (ISC) Dispersion Models. 

Volume I: User Instructions. EPA-454/B-95-003a. U.S. Environmental Protection 

Agency, Research Triangle Park, NC. 

U.S. EPA (1995c). User’s Guide for the Industrial Source Complex (ISC) Dispersion Models. 

Volume II: User Instructions. EPA-454/B-95-003a. U.S. Environmental Protection 

Agency, Research Triangle Park, NC. 

U.S. EPA (1995d). SCREEN3 Model User’s Guide. EPA-454/B-95-004. U.S. Environmental 

Protection Agency. Research Triangle Park, NC. 

10-2 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003. 

U.S. EPA (1995e). On-Site Meteorological Program Guidance For Regulatory Modeling 

Applications. EPA-450/4-87-013. U.S. Environmental Protection Agency, Research 

Triangle Park, NC. 

U.S. EPA (1997a), Exposure Factors Handbook, Vol I, General Factors. EPA/600/P-95/002Fa 

U.S. EPA (1997b), Exposure Factors Handbook, Vol II, Food Ingestion Factors. EPA/600/P95/002Fb 

van den Berg, M., Birnbaum, L., Bosveld, A. T. C., Brunstrom, B., Cook, P., Feeley, M., Giesy, 

J. P., Hanberg, A., Hasegawa, R., Kennedy, S. W., Kubiak, T., Larsen, J. C., Van Leeuwen, 

F. X. R., Liem, A. K. D., Nolt, C., Peterson, R. E., Poellinger, L., Safe, S., Schrenk, D., 

Tillitt, D., Tysklind, M., Younes, M., Waern, F., and Zacharewski, T. (1998). Toxic 

equivalency factors (TEFs) for PCBs, PCDDs, PCDFs for humans and wildlife. Environ. 

Health Perspec. V106, 775-792. 

10-3 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

Appendix A

Air Toxics Hot Spots Program

List Of Substances*

*The List of Substances presented in Appendix A is periodically updated by the California Air 

Resources Board. The last update was July 1, 1997. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

Appendix A:1 - 1

Appendix A-I

Substances For Which

Emissions Must Be Quantified

July 1, 1997
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

Appendix A:1 - 2
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 3

 

APPENDIX A-I

Substances For Which Emissions Must Be Quantified

 Applicable

 Degree of

Emittent ID Add Date Carcinogen Accuracy (lb/yr) Source List(s) 

Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [5]) (Note [6]) 

Notes(s)

 75070 Acetaldehyde c 20. 1 2 3 4

 60355 Acetamide c 2. 1 2 3 4

 75058 Acetonitrile 06/91 200. 1 2

 98862 Acetophenone 06/91 100. 1 2

 53963 2-Acetylaminofluorene [PAH-Derivative, POM] c 100. 1 2 4 5

 107028 Acrolein 0.05 1 2

 79061 Acrylamide c 0.01 1 2 3 4

 79107 Acrylic acid 06/91 5. 1 2

 107131 Acrylonitrile c 0.1 1 2 3 4 5

 107051 Allyl chloride c 5. 1 2 4

 7429905 Aluminum 06/91 100. 1

 1344281 Aluminum oxide (fibrous forms) 06/91 100. 7

 117793 2-Aminoanthraquinone [PAH-Derivative, POM] c 5. 1 2 4 5

 92671 4-Aminobiphenyl [POM] c 100. 1 2 3 4 5

 61825 Amitrole c 0.1 3 4 5

 7664417 Ammonia 200. 1 2

 6484522 Ammonium nitrate 06/91 100. 1

 7783202 Ammonium sulfate 06/91 100. 1

 62533 Aniline 09/90 c 5. 1 2 4

 90040 o-Anisidine c 100. 1 2 3 4 5

 - Anthracene [PAH, POM], (see PAH)

 7440360 Antimony 06/91 1. 7

 * Antimony compounds 06/91 1. 1 2 

 [7]

 including but not limited to:

 1309644 Antimony trioxide 09/90 c 1. 1 2 3 4 

 [7]

 7440382 Arsenic c 0.01 1 2 3 4 5

 1016 Arsenic compounds (inorganic) c 0.01 1 2 3 4 5 

 [7]

 including but not limited to:

 7784421 Arsine 0.01 1 2 7 

 [7]

 1017 Arsenic compounds (other than inorganic) 06/91 0.1 1 

 [7]

 7440393 Barium 06/91 1. 7

 * Barium compounds 06/91 1. 1 

 [7]

 - Benz[a]anthracene [PAH, POM], (see PAH)

 71432 Benzene c 2. 1 2 3 4 5
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 4

 92875 Benzidine (and its salts) [POM] c 0.0001 1 2 3 4 5

 1020 Benzidine-based dyes [POM] c 0.0001 1 2 3

 including but not limited to:

 1937377 Direct Black 38 [PAH-Derivative, POM] c 0.0001 1 2 4 5

 2602462 Direct Blue 6 [PAH-Derivative, POM] c 0.0001 1 2 4 5

 16071866 Direct Brown 95 (technical grade) [POM] 09/89 c 0.0001 1 2 4

 - Benzo[a]pyrene [PAH, POM], (see PAH)

 - Benzo[b]fluoranthene [PAH, POM], (see PAH)

 271896 Benzofuran 06/91 c 100. 4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 5

 

APPENDIX A-I

Substances For Which Emissions Must Be Quantified

 Applicable

 Degree of

Emittent ID Add Date Carcinogen Accuracy (lb/yr) Source List(s) 

Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [5]) (Note [6]) 

Notes(s)

 98077 Benzoic trichloride {Benzotrichloride} c 10. 1 2 4 5

 - Benzo[j]fluoranthene [PAH, POM], (see PAH)

 - Benzo[k]fluoranthene [PAH, POM], (see PAH)

 98884 Benzoyl chloride 06/91 100. 1

 94360 Benzoyl peroxide 06/91 100. 7

 100447 Benzyl chloride c 50. 1 2 4

 7440417 Beryllium c 0.001 1 2 3 4 5

 * Beryllium compounds 09/89 c 0.001 1 2 3 4 5 

 [7]

 92524 Biphenyl [POM] 06/91 0.5 1 2

 111444 Bis(2-chloroethyl) ether {DCEE} 09/89 c 0.05 1 2 4

 542881 Bis(chloromethyl) ether c 0.001 1 2 3 4 5

 103231 Bis(2-ethylhexyl) adipate 06/91 100. 1

 7726956 Bromine 0.5 2

 * Bromine compounds (inorganic) 100. 1 2 

 [7]

 including but not limited to:

 7758012 Potassium bromate 0.1 1 3 4 

 [7]

 75252 Bromoform 06/91 100. 1 2 4

 106990 1,3-Butadiene c 0.1 1 2 3 4 5

 141322 Butyl acrylate 06/91 100. 1

 71363 n-Butyl alcohol 06/91 100. 1

 78922 sec-Butyl alcohol 06/91 100. 1

 75650 tert-Butyl alcohol 06/91 100. 1

 85687 Butyl benzyl phthalate 06/91 100. 1

 7440439 Cadmium c 0.01 1 2 3 4 5

 * Cadmium compounds c 0.01 1 2 3 4 5 

 [7]

 156627 Calcium cyanamide 06/91 100. 1 2

 105602 Caprolactam 06/91 100. 1 2

 2425061 Captafol 09/89 c 100. 4

 133062 Captan 09/90 c 100. 1 2 4

 63252 Carbaryl [PAH-Derivative, POM] 06/91 100. 1 2

 1050 Carbon black extracts c 2. 1 3 4

 75150 Carbon disulfide 09/89 200. 1 2 4

 56235 Carbon tetrachloride c 1. 1 2 3 4 5

 463581 Carbonyl sulfide 06/91 100. 1 2

 1055 Carrageenan (degraded) c 100. 3 4

 120809 Catechol 06/91 100. 1 2
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 6

 133904 Chloramben 06/91 100. 1 2

 57749 Chlordane 09/89 c 10. 1 2 4

 108171262 Chlorinated paraffins (average chain length, C12; 09/89 c 2. 3 4 5

 approximately 60% chlorine by weight)

 7782505 Chlorine 0.5 1 2

 10049044 Chlorine dioxide 06/91 1. 1

 79118 Chloroacetic acid 06/91 100. 1 2

 532274 2-Chloroacetophenone 06/91 0.1 1 2

 106478 p-Chloroaniline 07/96 100. 4 7
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 7

 

APPENDIX A-I

Substances For Which Emissions Must Be Quantified

 Applicable

 Degree of

Emittent ID Add Date Carcinogen Accuracy (lb/yr) Source List(s) 

Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [5]) (Note [6]) 

Notes(s)

 1058 Chlorobenzenes 06/91 100. 1

 including but not limited to:

 108907 Chlorobenzene 200. 1 2

 25321226 Dichlorobenzenes (mixed isomers) 06/91 100. 1 7

 including:

 95501 1,2-Dichlorobenzene 06/91 200. 1 7

 541731 1,3-Dichlorobenzene 06/91 100. 1 7

 106467 p-Dichlorobenzene {1,4-Dichlorobenzene} c 5. 1 2 3 5

 120821 1,2,4-Trichlorobenzene 06/91 200. 1 2

 510156 Chlorobenzilate [POM] {Ethyl-4,4'- 09/90 c 100. 1 2 4

 dichlorobenzilate}

 67663 Chloroform c 10. 1 2 3 4 5

 107302 Chloromethyl methyl ether (technical grade) c 100. 1 2 4 5

 1060 Chlorophenols c 100. 1 3

 including but not limited to:

 120832 2,4-Dichlorophenol 06/91 c 100. 1 7

 87865 Pentachlorophenol 09/90 c 10. 1 2 4

 58902 2,3,4,6-Tetrachlorophenol 07/96 c 100. 1 7 

 

 95954 2,4,5-Trichlorophenol 06/91 c 100. 1 2

 88062 2,4,6-Trichlorophenol c 2. 1 2 4

 95830 4-Chloro-o-phenylenediamine c 10. 3 4 5

 76062 Chloropicrin 2. 7

 126998 Chloroprene 5. 1 2

 95692 p-Chloro-o-toluidine c 0.5 3 4

 7440473 Chromium 06/91 0.001 7

 * Chromium compounds (other than hexavalent) 06/91 0.001 1 2 

 [7]

 18540299 Chromium, hexavalent (and compounds) c 0.0001 1 2 3 4 5 

 [7]

 including but not limited to:

 10294403 Barium chromate 06/91 c 0.001 1 2 5 

 [7]

 13765190 Calcium chromate 06/91 c 0.001 1 2 5 

 [7]

 1333820 Chromium trioxide 06/91 c 0.0001 1 2 5 

 [7]

 7758976 Lead chromate 06/91 c 0.001 1 2 5 

 [7]
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 8

 10588019 Sodium dichromate 06/91 c 0.0001 1 2 5 

 [7]

 7789062 Strontium chromate 06/91 c 0.001 1 2 5 

 [7]

 - Chrysene [PAH, POM], (see PAH)

 7440484 Cobalt 06/91 0.5 7

 * Cobalt compounds 06/91 0.5 1 2 

 [7]

 1066 Coke oven emissions c 0.05 1 2 3 4 5

 7440508 Copper 0.1 2

 * Copper compounds 09/89 0.1 1 2 

 [7]

 1070 Creosotes c 0.05 1 3 4

 120718 p-Cresidine c 1. 3 4 5

 1319773 Cresols (mixtures of) {Cresylic acid} 5. 1 2

 including:

 108394 m-Cresol 06/91 5. 1 2

 95487 o-Cresol 06/91 5. 1 2

 106445 p-Cresol 06/91 5. 1 2

 4170303 Crotonaldehyde 07/96 c 50. 7
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 9

 

APPENDIX A-I

Substances For Which Emissions Must Be Quantified

 Applicable

 Degree of

Emittent ID Add Date Carcinogen Accuracy (lb/yr) Source List(s) 

Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [5]) (Note [6]) 

Notes(s)

 98828 Cumene 06/91 200. 1 2

 80159 Cumene hydroperoxide 06/91 100. 1

 135206 Cupferron c 0.5 4 5

 1073 Cyanide compounds 06/91 0.05 1 2 

 [8]

 including but not limited to:

 74908 Hydrocyanic acid 10. 2

 110827 Cyclohexane 06/91 200. 1

 108930 Cyclohexanol 07/96 200. 7

 66819 Cycloheximide 2. 6

 1163195 Decabromodiphenyl oxide [POM] 06/91 100. 1 2

 1075 Dialkylnitrosamines 0.001 1

 including but not limited to:

 924163 N-Nitrosodi-n-butylamine c 0.0001 1 3 4 5

 1116547 N-Nitrosodiethanolamine c 100. 1 3 4 5

 55185 N-Nitrosodiethylamine c 0.001 1 3 4 5

 62759 N-Nitrosodimethylamine c 0.01 1 2 3 4 5

 621647 N-Nitrosodi-n-propylamine c 0.01 1 3 4 5

 10595956 N-Nitrosomethylethylamine c 0.001 1 3 4

 615054 2,4-Diaminoanisole c 5. 3 4

 1078 Diaminotoluenes (mixed isomers) 09/90 c 100. 1 4

 including but not limited to:

 95807 2,4-Diaminotoluene {2,4-Toluenediamine} c 0.05 1 2 3 4 5

 334883 Diazomethane 06/91 c 5. 1 2

 226368 Dibenz[a,h]acridine [POM] c 0.5 1 2 3 4 5

 224420 Dibenz[a,j]acridine [POM] c 0.5 1 2 3 4 5

 - Dibenz[a,h]anthracene [PAH, POM], (see PAH)

 194592 7H-Dibenzo[c,g]carbazole c 0.05 1 2 3 4 5

 - Dibenzo[a,e]pyrene [PAH, POM], (see PAH)

 - Dibenzo[a,h]pyrene [PAH, POM], (see PAH)

 - Dibenzo[a,i]pyrene [PAH, POM], (see PAH)

 - Dibenzo[a,l]pyrene [PAH, POM], (see PAH)

 132649 Dibenzofuran [POM] 06/91 100. 1 2

 - Dibenzofurans (chlorinated) (see Polychlorinated

 dibenzofurans) [POM]

 96128 1,2-Dibromo-3-chloropropane {DBCP} c 0.01 1 2 3 4 5

 96139 2,3-Dibromo-1-propanol 07/96 c 50. 4

 84742 Dibutyl phthalate 06/91 100. 1 2

 - p-Dichlorobenzene {1,4-Dichlorobenzene} (see

 Chlorobenzenes)
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 10

 91941 3,3'-Dichlorobenzidine [POM] c 0.1 1 2 3 4 5

 72559 Dichlorodiphenyldichloroethylene {DDE} [POM] 09/89 c 100. 1 2 4

 75343 1,1-Dichloroethane {Ethylidene dichloride} 09/90 c 20. 1 2 4

 94757 Dichlorophenoxyacetic acid, salts and esters 06/91 100. 1 2

 {2,4-D}

 78875 1,2-Dichloropropane {Propylene dichloride} 09/90 c 20. 1 2 4

 542756 1,3-Dichloropropene c 10. 1 2 3 4 5

 62737 Dichlorovos {DDVP} 09/89 c 0.5 1 2 4

 115322 Dicofol [POM] 06/91 100. 1 2
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 11

 

APPENDIX A-I

Substances For Which Emissions Must Be Quantified

 Applicable

 Degree of

Emittent ID Add Date Carcinogen Accuracy (lb/yr) Source List(s) 

Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [5]) (Note [6]) 

Notes(s)

 - - Diesel engine exhaust 09/90 c 1 3 4 

 [9]

 9901 Diesel engine exhaust, particulate matter 09/90 c 10. 1 3 4 

 [9]

 9902 Diesel engine exhaust, total organic gas 09/90 c 10. 1 3 4 

 [9]

 # Diesel fuel (marine) 06/91 c

 111422 Diethanolamine 06/91 20. 1 2

 117817 Di(2-ethylhexyl) phthalate {DEHP} c 20. 1 2 3 4 5

 64675 Diethyl sulfate c 100. 1 2 3 4 5

 119904 3,3'-Dimethoxybenzidine [POM] c 100. 1 2 3 4 5

 60117 4-Dimethylaminoazobenzene [POM] c 0.01 1 2 3 4 5

 121697 N,N-Dimethylaniline 06/91 200. 1 2

 57976 7,12-Dimethylbenz[a]anthracene [PAH-Derivative, 09/90 c 0.0001 1 2 4

 POM]

 119937 3,3'-Dimethylbenzidine {o-Tolidine} [POM] c 10. 1 2 3 4 5

 79447 Dimethyl carbamoyl chloride c 100. 1 2 3 4 5

 68122 Dimethyl formamide 09/90 c 100. 1 2 3

 57147 1,1-Dimethylhydrazine c 0.1 1 2 3 4 5

 131113 Dimethyl phthalate 06/91 50. 1 2

 77781 Dimethyl sulfate c 0.01 1 2 3 4 5

 534521 4,6-Dinitro-o-cresol (and salts) 06/91 100. 1 2

 51285 2,4-Dinitrophenol 06/91 100. 1 2

 42397648 1,6-Dinitropyrene [PAH-Derivative, POM] 06/91 c 0.001 1 2 3 4

 42397659 1,8-Dinitropyrene [PAH-Derivative, POM] 06/91 c 0.05 1 2 3 4

 25321146 Dinitrotoluenes (mixed isomers) 06/91 100. 7

 including but not limited to:

 121142 2,4-Dinitrotoluene 09/89 c 0.5 1 2 4

 606202 2,6-Dinitrotoluene 06/91 100. 7

 123911 1,4-Dioxane c 5. 1 2 3 4 5

 - Dioxins (Chlorinated dibenzodioxins) (see

 Polychlorinated dibenzo-p-dioxins) [POM]

 630933 Diphenylhydantoin [POM] c 100. 1 2 4

 122667 1,2-Diphenylhydrazine {Hydrazobenzene} [POM] c 100. 1 2 4 5

 1090 Environmental Tobacco Smoke c 2. 1 3 4

 106898 Epichlorohydrin c 2. 1 2 3 4 5

 106887 1,2-Epoxybutane 06/91 100. 1 2

 1091 Epoxy resins 09/89 100. 6

 140885 Ethyl acrylate c 200. 1 2 3 4 5

 100414 Ethyl benzene 06/91 200. 1 2
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 12

 75003 Ethyl chloride {Chloroethane} 200. 1 2 4

 - Ethyl-4,4'-dichlorobenzilate (see Chlorobenzilate)

 74851 Ethylene 06/91 200. 7

 106934 Ethylene dibromide {1,2-Dibromoethane} c 0.5 1 3 4 5 6

 107062 Ethylene dichloride {1,2-Dichloroethane} c 2. 1 2 3 4 5

 107211 Ethylene glycol 06/91 200. 1 2

 151564 Ethyleneimine {Aziridine} 06/91 100. 1 2

 75218 Ethylene oxide c 0.5 1 2 3 4 5 6

 96457 Ethylene thiourea c 2. 1 2 3 4 5
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 13

 

APPENDIX A-I

Substances For Which Emissions Must Be Quantified

 Applicable

 Degree of

Emittent ID Add Date Carcinogen Accuracy (lb/yr) Source List(s) 

Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [5]) (Note [6]) 

Notes(s)

 1101 Fluorides and compounds 09/89 100. 2

 including but not limited to:

 7664393 Hydrogen fluoride 50. 1 2 7

 1103 Fluorocarbons (brominated) 200. 6 

 [10]

 1104 Fluorocarbons (chlorinated) 200. 1 6 

 [10]

 including but not limited to:

 76131 Chlorinated fluorocarbon {CFC-113} 200. 1 2 6

 {1,1,2-Trichloro-1,2,2-trifluoroethane}

 75456 Chlorodifluoromethane {Freon 22} 07/96 200. 1 6 7 

 

 75434 Dichlorofluoromethane {Freon 12} 07/96 200. 1 6 7

 75694 Trichlorofluoromethane {Freon 11} 07/96 200. 1 6 7

 50000 Formaldehyde c 5. 1 2 3 4 5 6

 110009 Furan 07/96 c 5. 4

 - - Gasoline engine exhaust 09/90 c 3 

 [9]

 including but not limited to:

 - - Gasoline engine exhaust (condensates & extracts) 06/91 c 4 

 [9]

 9910 Gasoline engine exhaust, particulate matter 09/90 c 100. 3 4 

 [9]

 9911 Gasoline engine exhaust, total organic gas 09/90 c 100. 3 4 

 [9]

 1110 Gasoline vapors c 200. 1 2 3 4 

 [11]

 111308 Glutaraldehyde 0.1 1 6

 1115 Glycol ethers and their acetates 100. 1 2 6

 including but not limited to:

 111466 Diethylene glycol 09/90 100. 1 6

 111966 Diethylene glycol dimethyl ether 09/90 100. 1 2 6

 112345 Diethylene glycol monobutyl ether 09/90 100. 1 2 6

 111900 Diethylene glycol monoethyl ether 09/90 100. 1 2 6

 111773 Diethylene glycol monomethyl ether 09/90 100. 1 2 6

 25265718 Dipropylene glycol 09/90 100. 1 6

 34590948 Dipropylene glycol monomethyl ether 09/90 100. 1 6

 629141 Ethylene glycol diethyl ether 09/90 100. 1 2 6

 110714 Ethylene glycol dimethyl ether 09/90 100. 1 2 6

 111762 Ethylene glycol monobutyl ether 09/90 200. 1 2 6
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 14

 110805 Ethylene glycol monoethyl ether 09/89 50. 1 2 6

 111159 Ethylene glycol monoethyl ether acetate 09/90 100. 1 2 6

 109864 Ethylene glycol monomethyl ether 09/89 10. 1 2 6

 110496 Ethylene glycol monomethyl ether acetate 09/90 200. 1 2 6

 2807309 Ethylene glycol monopropyl ether 09/90 100. 1 2 6

 107982 Propylene glycol monomethyl ether 09/90 200. 1 6

 108656 Propylene glycol monomethyl ether acetate 09/90 100. 1 6

 112492 Triethylene glycol dimethyl ether 09/90 100. 1 2 6

 76448 Heptachlor 09/89 c 100. 1 2 4

 118741 Hexachlorobenzene c 0.1 1 2 3 5

 87683 Hexachlorobutadiene 06/91 0.1 1 2

 1120 Hexachlorocyclohexanes(mixed or technical grade) c 0.05 1 3 4 5 

 including but not limited to:

 319846 alpha-Hexachlorocyclohexane 07/96 c 0.1 1 3 4 5 7

 319857 beta-Hexachlorocyclohexane 07/96 c 0.1 1 3 4 5 7

 58899 Lindane {gamma-Hexachlorocyclohexane} 09/90 c 0.1 1 2 4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 15

 

APPENDIX A-I

Substances For Which Emissions Must Be Quantified

 Applicable

 Degree of

Emittent ID Add Date Carcinogen Accuracy (lb/yr) Source List(s) 

Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [5]) (Note [6]) 

Notes(s)

 77474 Hexachlorocyclopentadiene 2. 1 2

 67721 Hexachloroethane 09/90 c 200. 1 2 4

 680319 Hexamethylphosphoramide c 100. 1 2 3 4 5

 110543 Hexane 06/91 200. 1 2

 302012 Hydrazine c 0.01 1 2 3 4 5

 7647010 Hydrochloric acid 20. 1 2

 - Hydrocyanic acid (see Cyanide compounds)

 7783064 Hydrogen sulfide 5. 1 2

 123319 Hydroquinone 06/91 100. 1 2

 - Indeno[1,2,3-cd]pyrene [PAH, POM], (see PAH)

 13463406 Iron pentacarbonyl 07/96 5. 7

 1125 Isocyanates 0.05 6

 including but not limited to:

 822060 Hexamethylene-1,6-diisocyanate 06/91 0.05 1 2

 101688 Methylene diphenyl diisocyanate {MDI} [POM] 06/91 0.1 1 2

 624839 Methyl isocyanate 1. 1 2

 - Toluene-2,4-diisocyanate (see Toluene

 diisocyanates)

 - Toluene-2,6-diisocyanate (see Toluene

 diisocyanates)

 78591 Isophorone 06/91 200. 1 2

 78795 Isoprene, except from vegetative emission sources 07/96 c 200. 3

 67630 Isopropyl alcohol 06/91 200. 1

 80057 4,4'-Isopropylidenediphenol [POM] 06/91 100. 1 2

 7439921 Lead c 0.5 1 4 6

 1128 Lead compounds (inorganic) c 0.5 1 3 

 [7]

 including but not limited to:

 301042 Lead acetate c 1. 1 2 4 5 

 [7] [12]

 - Lead chromate (see Chromium, hexavalent)

 7446277 Lead phosphate c 2. 1 4 5 

 [7]

 1335326 Lead subacetate 09/90 c 2. 1 2 4 

 [7] [12]

 1129 Lead compounds (other than inorganic) 06/91 5. 1 2 

 [7]

 108316 Maleic anhydride 0.5 1 2

 7439965 Manganese 0.1 1 2
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 16

 * Manganese compounds 09/89 0.1 1 2 

 [7]

 7439976 Mercury 1. 1 2 4 6

 * Mercury compounds 09/89 1. 1 2 4 

 [7]

 including but not limited to:

 7487947 Mercuric chloride 1. 2 

 [7]

 593748 Methyl mercury {Dimethylmercury} 1. 2 

 [7]

 67561 Methanol 200. 1 2

 72435 Methoxychlor [POM] 06/91 100. 1 2

 75558 2-Methylaziridine {1,2-Propyleneimine} c 100. 1 2 3 4

 74839 Methyl bromide {Bromomethane} 20. 1 2 6

 74873 Methyl chloride {Chloromethane} 06/91 20. 1 2

 71556 Methyl chloroform {1,1,1-Trichloroethane} 200. 1 2 6

 56495 3-Methylcholanthrene [PAH-Derivative, POM] 09/90 c 0.001 1 2 4

 3697243 5-Methylchrysene [PAH-Derivative, POM] c 0.05 1 2 3 4 5
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 17

 

APPENDIX A-I

Substances For Which Emissions Must Be Quantified

 Applicable

 Degree of

Emittent ID Add Date Carcinogen Accuracy (lb/yr) Source List(s) 

Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [5]) (Note [6]) 

Notes(s)

 101144 4,4'-Methylene bis(2-chloroaniline) {MOCA} [POM] c 0.1 1 2 3 4 5

 75092 Methylene chloride {Dichloromethane} c 50. 1 2 3 4 5 6

 101779 4,4'-Methylenedianiline (and its dichloride) [POM] c 0.1 1 2 3 4 5

 78933 Methyl ethyl ketone {2-Butanone} 06/91 200. 1 2

 60344 Methyl hydrazine 06/91 100. 1 2

 74884 Methyl iodide {Iodomethane} c 100. 1 2 4 5

 108101 Methyl isobutyl ketone {Hexone} 06/91 20. 1 2

 75865 2-Methyllactonitrile {Acetone cyanohydrin} 07/96 50. 7

 80626 Methyl methacrylate 200. 1 2 6

 109068 2-Methylpyridine 07/96 100. 7

 1634044 Methyl tert-butyl ether 06/91 200. 1 2

 90948 Michler's ketone [POM] c 0.1 1 2 4 5

 1136 Mineral fibers (fine, manmade) 06/91 c 100. 1 2 7

 (fine mineral fibers which are manmade and are

 airborne particles of a respirable size greater

 than 5 microns in length, less than or equal to

 3.5 microns in diameter, with a length to

 diameter ratio of 3:1)

 including but not limited to:

 1056 Ceramic fibers 09/89 c 100. 1 2 3 4

 1111 Glasswool fibers 09/89 c 100. 1 2 3 4

 1168 Rockwool fibers 09/89 c 100. 1 2 3

 1181 Slagwool fibers 09/89 c 100. 1 2 3

 1135 Mineral fibers (other than manmade) 100. 2 7

 including but not limited to:

 1332214 Asbestos c 0.0001 1 2 3 4 5

 12510428 Erionite c 100. 2 3 4

 1190 Talc containing asbestiform fibers c 100. 2 3 4

 1313275 Molybdenum trioxide 06/91 100. 1

 - Naphthalene [PAH, POM], (see PAH)

 7440020 Nickel c 0.1 1 2 3 4 5

 * Nickel compounds c 1. 1 2 3 4 5 

 [7]

 including but not limited to:

 373024 Nickel acetate 06/91 c 0.1 1 2 5 

 [7]

 3333393 Nickel carbonate 06/91 c 0.1 1 2 5 

 [7]

 13463393 Nickel carbonyl c 0.1 1 2 4 5 

 [7]
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 18

 12054487 Nickel hydroxide 06/91 c 0.1 1 2 5 

 [7]

 1271289 Nickelocene 06/91 c 0.1 1 2 5 

 [7]

 1313991 Nickel oxide 06/91 c 0.1 1 2 5 

 [7]

 12035722 Nickel subsulfide c 0.1 1 2 4 5 

 [7]

 1146 Nickel refinery dust from the pyrometallurgical 09/89 c 0.1 4

 process

 7697372 Nitric acid 06/91 50. 1

 139139 Nitrilotriacetic acid c 100. 1 4 5

 98953 Nitrobenzene 0.5 1 2

 92933 4-Nitrobiphenyl [POM] 09/89 c 100. 1 2 4

 7496028 6-Nitrochrysene [PAH-Derivative, POM] 06/91 c 0.001 1 2 3 4

 607578 2-Nitrofluorene [PAH-Derivative, POM] 06/91 c 5. 1 2 3 4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 19

 

APPENDIX A-I

Substances For Which Emissions Must Be Quantified

 Applicable

 Degree of

Emittent ID Add Date Carcinogen Accuracy (lb/yr) Source List(s) 

Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [5]) (Note [6]) 

Notes(s)

 302705 Nitrogen mustard N-oxide c 0.05 3 4

 100027 4-Nitrophenol 06/91 100. 1 2

 79469 2-Nitropropane c 0.01 1 2 3 4 5

 5522430 1-Nitropyrene [PAH-Derivative, POM] 06/91 c 0.5 1 2 3 4

 156105 p-Nitrosodiphenylamine [POM] c 5. 1 2 4 5

 684935 N-Nitroso-N-methylurea c 100. 1 2 4 5

 59892 N-Nitrosomorpholine c 0.01 1 2 3 4 5

 100754 N-Nitrosopiperidine c 200. 3 4 5

 930552 N-Nitrosopyrrolidine c 0.05 3 4 5

 - - PAHs (Polycyclic aromatic hydrocarbons) [POM] 1 2 

 [13]

 including but not limited to:

 1151 PAHs, total, w/o individ. components reported 50. 1 2

 1150 PAHs, total, with individ. components also 50. 1 2

 reported

 83329 Acenaphthene [PAH, POM] 07/96 50. 1

 208968 Acenaphthylene [PAH, POM] 07/96 50. 1

 120127 Anthracene [PAH, POM] 06/91 50. 1 2 7

 56553 Benz[a]anthracene [PAH, POM] c 0.5 1 2 3 4 5

 50328 Benzo[a]pyrene [PAH, POM] c 0.05 1 2 3 4 5

 205992 Benzo[b]fluoranthene [PAH, POM] c 0.5 1 2 3 4 5

 192972 Benzo[e]pyrene [PAH, POM] 07/96 0.5 1

 191242 Benzo[g,h,i]perylene [PAH, POM] 07/96 0.5 1

 205823 Benzo[j]fluoranthene [PAH, POM] c 0.5 1 2 3 4 5

 207089 Benzo[k]fluoranthene [PAH, POM] c 0.5 1 2 3 4 5

 218019 Chrysene [PAH, POM] 09/90 c 5. 1 2 4

 53703 Dibenz[a,h]anthracene [PAH, POM] c 0.1 1 2 3 4 5

 192654 Dibenzo[a,e]pyrene [PAH, POM] c 0.05 1 2 3 4 5

 189640 Dibenzo[a,h]pyrene [PAH, POM] c 0.001 1 2 3 4 5

 189559 Dibenzo[a,i]pyrene [PAH, POM] c 0.001 1 2 3 4 5

 191300 Dibenzo[a,l]pyrene [PAH, POM] c 0.001 1 2 3 4 5

 206440 Fluoranthene [PAH, POM] 07/96 0.5 1

 86737 Fluorene [PAH, POM] 07/96 0.5 1

 193395 Indeno[1,2,3-cd]pyrene [PAH, POM] c 0.5 1 2 3 4 5

 91576 2-Methyl naphthalene [PAH, POM] 07/96 50. 1

 91203 Naphthalene [PAH, POM] 50. 1 2

 198550 Perylene [PAH, POM] 07/96 0.5 1

 85018 Phenanthrene [PAH, POM] 07/96 0.5 1

 129000 Pyrene [PAH, POM] 07/96 0.5 1
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 20

 # PAH-Derivatives (Polycyclic aromatic hydrocarbon 06/91 

 [14]

 derivatives) [POM]

 (including but not limited to those substances

 listed in Appendix A with the bracketed

 designation [PAH-Derivative, POM])

 56382 Parathion 06/91 100. 1 2

 1336363 PCBs (Polychlorinated biphenyls) [POM] c 0.01 1 2 3 4 5 6

 82688 Pentachloronitrobenzene {Quintobenzene} 06/91 100. 1 2

 79210 Peracetic acid 06/91 100. 1

 127184 Perchloroethylene {Tetrachloroethene} c 5. 1 2 3 4 5 6
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 21

 

APPENDIX A-I

Substances For Which Emissions Must Be Quantified

 Applicable

 Degree of

Emittent ID Add Date Carcinogen Accuracy (lb/yr) Source List(s) 

Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [5]) (Note [6]) 

Notes(s)

 108952 Phenol 200. 1 2

 106503 p-Phenylenediamine 06/91 100. 1 2

 90437 2-Phenylphenol [POM] 06/91 100. 1 2

 75445 Phosgene 2. 1 2

 7723140 Phosphorus 0.1 1 2

 - - Phosphorus compounds: 09/89 2

 7803512 Phosphine 0.01 1 2 7

 7664382 Phosphoric acid 09/89 50. 1 2

 10025873 Phosphorus oxychloride 09/89 0.1 2

 10026138 Phosphorus pentachloride 09/89 0.1 2

 1314563 Phosphorus pentoxide 09/89 0.1 2

 7719122 Phosphorus trichloride 09/89 0.1 2

 126738 Tributyl phosphate 09/89 100. 2

 78400 Triethyl phosphine 09/89 100. 2

 512561 Trimethyl phosphate 09/89 100. 2

 78308 Triorthocresyl phosphate [POM] 09/89 0.5 1 2

 115866 Triphenyl phosphate [POM] 09/89 100. 1 2

 101020 Triphenyl phosphite [POM] 09/89 100. 1 2

 85449 Phthalic anhydride 0.01 1 2

 - - Polychlorinated dibenzo-p-dioxins {PCDDs or c 1 2

 Dioxins} [POM]

 including but not limited to:

 1086 Dioxins, total, w/o individ. isomers reported c 0.00002 1 2

 {PCDDs}

 1085 Dioxins, total, with individ. isomers also c 0.00002 1 2

 reported {PCDDs}

 1746016 2,3,7,8-Tetrachlorodibenzo-p-dioxin {TCDD} [POM] c 0.000001 1 2 3 4 5

 40321764 1,2,3,7,8-Pentachlorodibenzo-p-dioxin [POM] c 0.000001 1 2

 39227286 1,2,3,4,7,8-Hexachlorodibenzo-p-dioxin [POM] c 0.000001 1 2 4

 57653857 1,2,3,6,7,8-Hexachlorodibenzo-p-dioxin [POM] c 0.000001 1 2

 19408743 1,2,3,7,8,9-Hexachlorodibenzo-p-dioxin [POM] c 0.000001 1 2

 35822469 1,2,3,4,6,7,8-Heptachlorodibenzo-p-dioxin [POM] c 0.000001 1 2

 3268879 1,2,3,4,5,6,7,8-Octachlorodibenzo-p-dioxin [POM] 07/96 c 0.000001 1 2

 41903575 Total Tetrachlorodibenzo-p-dioxin [POM] 07/96 c 0.000001 1 2

 36088229 Total Pentachlorodibenzo-p-dioxin [POM] 07/96 c 0.000001 1 2

 34465468 Total Hexachlorodibenzo-p-dioxin [POM] 07/96 c 0.000001 1 2

 37871004 Total Heptachlorodibenzo-p-dioxin [POM] 07/96 c 0.000001 1 2
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 22

 

APPENDIX A-I

Substances For Which Emissions Must Be Quantified

 Applicable

 Degree of

Emittent ID Add Date Carcinogen Accuracy (lb/yr) Source List(s) 

Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [5]) (Note [6]) 

Notes(s)

 - - Polychlorinated dibenzofurans {PCDFs or c 1 2

 Dibenzofurans} [POM]

 including but not limited to:

 1080 Dibenzofurans (Polychlorinated dibenzofurans) c 0.00002 1 2

 {PCDFs} [POM]

 51207319 2,3,7,8-Tetrachlorodibenzofuran [POM] c 0.000001 1 2

 57117416 1,2,3,7,8-Pentachlorodibenzofuran [POM] c 0.000001 1 2

 57117314 2,3,4,7,8-Pentachlorodibenzofuran [POM] c 0.000001 1 2

 70648269 1,2,3,4,7,8-Hexachlorodibenzofuran [POM] c 0.000001 1 2

 57117449 1,2,3,6,7,8-Hexachlorodibenzofuran [POM] c 0.000001 1 2

 72918219 1,2,3,7,8,9-Hexachlorodibenzofuran [POM] c 0.000001 1 2

 60851345 2,3,4,6,7,8-Hexachlorodibenzofuran [POM] c 0.000001 1 2

 67562394 1,2,3,4,6,7,8-Heptachlorodibenzofuran [POM] c 0.000001 1 2

 55673897 1,2,3,4,7,8,9-Heptachlorodibenzofuran [POM] c 0.000001 1 2

 39001020 1,2,3,4,5,6,7,8-Octachlorodibenzofuran [POM] 07/96 c 0.000001 1 2

 55722275 Total Tetrachlorodibenzofuran [POM] 07/96 c 0.000001 1 2

 30402154 Total Pentachlorodibenzofuran [POM] 07/96 c 0.000001 1 2

 55684941 Total Hexachlorodibenzofuran [POM] 07/96 c 0.000001 1 2

 38998753 Total Heptachlorodibenzofuran [POM] 07/96 c 0.000001 1 2

 # POM (Polycyclic organic matter) 09/89 1 2 

 [15]

 (including but not limited to those substances

 listed in Appendix A with the bracketed

 designation of [POM], [PAH, POM], or

 [PAH-Derivative, POM])

 1120714 1,3-Propane sultone c 0.05 1 2 3 4 5

 57578 beta-Propiolactone c 10. 1 2 3 4 5

 123386 Propionaldehyde 06/91 200. 1 2

 114261 Propoxur {Baygon} 06/91 100. 1 2

 115071 Propylene 200. 1 2

 75569 Propylene oxide c 10. 1 2 3 4 5

 - 1,2-Propyleneimine (see 2-Methylaziridine)

 110861 Pyridine 06/91 100. 7

 91225 Quinoline 06/91 100. 1 2

 106514 Quinone 06/91 100. 1 2

 1165 Radionuclides c 100. 1 2 4 

 [16]

 including but not limited to:

 24267569 Iodine-131 09/89 c 100. 1 2 4

 1166 Radon and its decay products 09/89 c 100. 1 4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 23

 50555 Reserpine [POM] c 100. 1 2 4 5

 # Residual (heavy) fuel oils 06/91 c

 7782492 Selenium 0.5 2

 * Selenium compounds 0.5 1 2 

 [7]

 including but not limited to:

 7446346 Selenium sulfide 09/90 c 0.1 2 4 5 

 [7]

 1175 Silica, crystalline c 0.1 1 3 4

 7440224 Silver 06/91 2. 7

 * Silver compounds 06/91 2. 1 

 [7]
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 24

 

APPENDIX A-I

Substances For Which Emissions Must Be Quantified

 Applicable

 Degree of

Emittent ID Add Date Carcinogen Accuracy (lb/yr) Source List(s) 

Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [5]) (Note [6]) 

Notes(s)

 1310732 Sodium hydroxide 2. 1 2

 100425 Styrene c 100. 1 2 3 6

 96093 Styrene oxide c 100. 1 2 3 4

 7664939 Sulfuric acid 06/91 2. 1

 100210 Terephthalic acid 06/91 100. 1

 79345 1,1,2,2-Tetrachloroethane 09/90 c 1. 1 2 4

 7440280 Thallium 06/91 100. 7

 * Thallium compounds 06/91 100. 7 

 [7]

 62555 Thioacetamide c 0.01 3 4 5

 62566 Thiourea c 0.1 1 3 4 5

 7550450 Titanium tetrachloride 06/91 100. 1 2

 108883 Toluene 200. 1 2 4 6

 - 2,4-Toluenediamine (see 2,4-Diaminotoluene)

 1204 Toluene diisocyanates 06/91 c 0.1 1 3

 including but not limited to:

 584849 Toluene-2,4-diisocyanate c 0.1 1 2 3 5

 91087 Toluene-2,6-diisocyanate c 0.1 1 2 3 5

 95534 o-Toluidine c 10. 1 2 3 4 5

 8001352 Toxaphene {Polychlorinated camphenes} c 100. 1 2 3 4 5

 79005 1,1,2-Trichloroethane {Vinyl trichloride} 06/91 c 50. 1 2 4

 - 1,1,1-Trichloroethane (see Methyl chloroform)

 79016 Trichloroethylene c 20. 1 2 4

 - 2,4,6-Trichlorophenol (see Chlorophenols)

 96184 1,2,3-Trichloropropane 07/96 c 200. 3 4 7

 121448 Triethylamine 06/91 20. 1 2

 1582098 Trifluralin 06/91 100. 1 2

 95636 1,2,4-Trimethylbenzene 06/91 5. 1

 540841 2,2,4-Trimethylpentane 06/91 100. 1 2

 51796 Urethane {Ethyl carbamate} c 0.1 1 2 3 4 5

 7440622 Vanadium (fume or dust) 06/91 10. 7 

 [17]

 108054 Vinyl acetate 06/91 200. 1 2

 593602 Vinyl bromide c 20. 1 2 3 4

 75014 Vinyl chloride c 0.5 1 2 3 4 5

 100403 4-Vinylcyclohexene 07/96 c 5. 3

 75025 Vinyl fluoride 07/96 c 200. 3

 75354 Vinylidene chloride 20. 1 2

 1206 Wood preservatives (containing arsenic and 09/89 100. 6

 chromate)
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. August 2003.

Appendix A:1 - 25

 1210 Xylenes (mixed xylenes) 200. 1 2 6

 including:

 108383 m-Xylene 06/91 200. 1 2

 95476 o-Xylene 06/91 200. 1 2

 106423 p-Xylene 06/91 200. 1 2

 7440666 Zinc 2. 2

 * Zinc compounds 09/89 2. 1 2 

 [7]

 including but not limited to:

 1314132 Zinc oxide 2. 2 

 [7]
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:2 - 1

Appendix A-II

Substances For Which Production, Use,

Or Other Presence Must Be Reported

July 1, 1997
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:2 - 2
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:2 - 2

 

APPENDIX A-II

Substances For Which Production, Use, Or Other Presence Must Be Reported

Emittent ID Add Date Carcinogen Source List(s) Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [6]) Notes(s)

 26148685 A-alpha-C {2-Amino-9H-pyrido[2,3-b]indole} 09/89 c 3 4 [18]

 34256821 Acetochlor 09/89 c 4

 62476599 Acifluorfen [POM] 09/90 c 1 2 4

 3688537 AF-2 c 3 4

 1000 Aflatoxins c 3 4 5

 15972608 Alachlor 09/89 c 4

 309002 Aldrin 09/89 c 4

 107186 Allyl alcohol 06/91 7

 60093 p-Aminoazobenzene {4-Aminoazobenzene} [POM] c 1 2 3 4

 97563 o-Aminoazotoluene [POM] c 1 2 3 4 5

 6109973 3-Amino-9-ethylcarbazole hydrochloride [POM] 09/89 c 1 2 4 5

 125848 Aminoglutethimide 09/90 4

 82280 1-Amino-2-methylanthraquinone [PAH-Derivative, c 1 2 4 5

 POM]

 68006837 2-Amino-3-methyl-9H-pyrido(2,3-b) indole {MeA- 09/89 c 3 4

 alpha-C}

 712685 2-Amino-5-(5-nitro-2-furyl)-1,3,4-thiadiazole c 3 4

 - 2-Amino-9H-pyrido(2,3-b)indole (see A-alpha-C)

 134292 o-Anisidine hydrochloride c 4 5

 104949 p-Anisidine 06/91 7

 140578 Aramite c 3 4

 492808 Auramine [POM] c 1 2 3 4 5

 446866 Azathioprine c 3 4 5

 103333 Azobenzene [POM] 09/90 c 1 2 4

 98873 Benzal chloride 06/91 7

 55210 Benzamide 06/91 7

 1694093 Benzyl violet 4B [POM] c 1 2 3 4

 1025 Betel quid with tobacco c 3 4

 494031 N-N-Bis(2-chloroethyl)-2-naphthylamine c 1 2 3 4 5

 {Chlornaphazine} [PAH-Derivative, POM]

 108601 Bis(2-chloro-1-methylethyl) ether 06/91 7

 1030 Bitumens, extracts of steam-refined and air- c 3 4

 refined bitumens

 1035 Bleomycins c 3

 75274 Bromodichloromethane 09/90 c 4

 1689845 Bromoxynil 06/91 4

 25013165 Butylated hydroxyanisole {BHA} c 3 4

 123728 Butyraldehyde 06/91 7

 3068880 beta-Butyrolactone c 3 4

 630080 Carbon monoxide 09/89 4

 143500 Chlordecone {Kepone} c 3 4

 6164983 Chlordimeform 09/89 c 4

 115286 Chlorendic acid 09/89 c 3 4 5

 124481 Chlorodibromomethane 09/90 c 4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:2 - 3

 563473 3-Chloro-2-methylpropene 09/89 c 4 5

 1065 Chlorophenoxy herbicides c 3

 1897456 Chlorothalonil 09/89 c 4

 1059 p-Chloro-o-toluidine (strong acid salts) 06/91 c 3
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:2 - 4

 

APPENDIX A-II

Substances For Which Production, Use, Or Other Presence Must Be Reported

Emittent ID Add Date Carcinogen Source List(s) Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [6]) Notes(s)

 4680788 C. I. Acid Green 3 [POM] 06/91 1 2 7

 569642 C. I. Basic Green 4 [POM] 06/91 1 2 7

 989388 C. I. Basic Red 1 [POM] 06/91 1 2 7

 569619 C. I. Basic Red 9 monohydrochloride [POM] 09/89 c 1 2 4 5

 2832408 C. I. Disperse Yellow 3 [POM] 06/91 1 2 7

 (NOTE: "C. I." means "color index")

 87296 Cinnamyl anthranilate [POM] 09/89 c 1 2 4 5

 6358538 Citrus Red No. 2 [POM] c 1 2 3 4

 8007452 Coal tars 09/89 c 3 4 5

 21725462 Cyanazine 09/90 4

 14901087 Cycasin c 3 4

 13121705 Cyhexatin 09/89 4

 3468631 D and C Orange No. 17 [PAH-Derivative, POM] 09/90 c 1 2 4

 81889 D and C Red No. 19 [POM] 09/90 c 1 2 4

 2092560 D and C Red No. 8 [PAH-Derivative, POM] 06/91 c 1 2 4

 5160021 D and C Red No. 9 [PAH-Derivative, POM] 09/90 c 1 2 4

 1596845 Daminozide 09/90 c 4

 50293 DDT {1,1,1-Trichloro-2,2-bis(p- c 1 2 3 4 5

 chlorophenyl)ethane} [POM]

 613354 N,N'-Diacetylbenzidine [POM] c 1 2 3 4

 2303164 Diallate 06/91 7

 39156417 2,4-Diaminoanisole sulfate c 4 5

 101804 4,4'-Diaminodiphenyl ether [POM] c 1 2 3 4 5

 764410 1,4-Dichloro-2-butene 09/90 c 4

 28434868 3,3'-Dichloro-4,4'-diaminodiphenyl ether [POM] 09/89 c 1 2 3 4

 72548 Dichlorodiphenyldichloroethane {DDD} [POM] 09/89 c 1 2 4

 540590 1,2-Dichloroethylene 06/91 7

 78886 2,3-Dichloropropene 06/91 7

 60571 Dieldrin 09/89 c 4

 1464535 Diepoxybutane c 3 4 5

 1615801 1,2-Diethylhydrazine c 3 4

 84662 Diethyl phthalate 06/91 7

 101906 Diglycidyl resorcinol ether {DGRE} c 3 4 5

 94586 Dihydrosafrole c 3 4

 20325400 3,3'-Dimethoxybenzidine dihydrochloride [POM] 06/91 c 1 2 4

 55738540 trans-2-[(Dimethylamino)methylimino]-5-[2-(5- c 3 4

 nitro-2-furyl)vinyl-1,3,4-oxadiazol

 540738 1,2-Dimethylhydrazine c 3 4

 105679 2,4-Dimethylphenol {2,4-Xylenol} 06/91 7

 513371 Dimethylvinylchloride {DMVC} 09/89 c 4 5

 25154545 Dinitrobenzenes (mixtures of) 09/90 4 7

 including:

 99650 m-Dinitrobenzene 06/91 7

 528290 o-Dinitrobenzene 06/91 7
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:2 - 5

 100254 p-Dinitrobenzene 06/91 7

 39300453 Dinocap 09/90 4

 88857 Dinoseb 09/89 4

 117840 n-Dioctyl phthalate 06/91 7
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:2 - 6

 

APPENDIX A-II

Substances For Which Production, Use, Or Other Presence Must Be Reported

Emittent ID Add Date Carcinogen Source List(s) Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [6]) Notes(s)

 2475458 Disperse Blue 1 [PAH-Derivative, POM] 06/91 c 1 2 3 4

 541413 Ethyl chloroformate 06/91 7

 62500 Ethyl methanesulfonate c 3 4

 2164172 Fluometuron 06/91 7

 133073 Folpet 09/89 c 4

 3570750 2-(2-Formylhydrazino)-4-(5-nitro-2-furyl)thiazole c 3 4

 60568050 Furmecyclox 09/90 c 4

 67730114 Glu-P-1 {2-Amino-6-methyldipyrido[1,2-a:3',2'- c 3 4

 d]imidazole}

 67730103 Glu-P-2 {2-Aminodipyrido[1,2-a:3',2'-d]imidazole} c 3 4

 765344 Glycidaldehyde c 3 4

 556525 Glycidol 09/90 c 4

 16568028 Gyromitrin {Acetaldehyde methylformylhydrazone} c 4

 2784943 HC Blue 1 09/89 c 4 5

 1024573 Heptachlor epoxide 09/89 c 4

 1335871 Hexachloronaphthalene [PAH-Derivative, POM] 06/91 1 2 7

 10034932 Hydrazine sulfate c 4 5

 76180966 IQ {2-Amino-3-methylimidazo[4,5-f]quinoline} c 3 4

 78842 Isobutyraldehyde 06/91 7

 120581 Isosafrole 09/90 c 4

 4759482 Isotretinoin 4

 77501634 Lactofen [POM] 09/89 c 1 2 4

 1131 Lubricant base oils and derived products, 09/89 c 3 4 5

 specifically vacuum distillates, acid treated

 oils, aromatic oils, mildly solvent-refined

 oils, mildly hydrotreated-oils and

 used engine oils.

 8018017 Mancozeb 09/90 c 4

 12427382 Maneb 09/90 c 4

 59052 Methotrexate 09/89 4

 96333 Methyl acrylate 06/91 7

 590965 Methylazoxymethanol 09/90 c 4

 592621 Methylazoxymethanol acetate 09/89 c 3 4

 101611 4,4'-Methylene bis (N,N-dimethyl) benzenamine c 1 2 4 5

 [POM]

 838880 4,4'-Methylene bis(2-methylaniline) [POM] 09/89 c 1 2 3 4

 74953 Methylene bromide 06/91 7

 66273 Methyl methanesulfonate c 3 4

 129157 2-Methyl-1-nitroanthraquinone (uncertain purity) c 1 2 3 4

 [PAH-Derivative, POM]

 70257 N-Methyl-N'-nitro-N-nitrosoguanidine c 3 4

 - N-Methyl-N-nitrosourethane (see N-Nitroso-N-

 methylurethane)

 924425 N-Methyloacrylamide 09/90 c 4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:2 - 7

 9006422 Metiram 09/90 4

 1140 Mineral oils (untreated and mildly treated oils; c 3 4 5

 and those used in occupations such as

 mulespinning, metal machining, and

 jute processing).
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:2 - 8

 

APPENDIX A-II

Substances For Which Production, Use, Or Other Presence Must Be Reported

Emittent ID Add Date Carcinogen Source List(s) Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [6]) Notes(s)

 2385855 Mirex c 3 4 5

 315220 Monocrotaline c 3 4

 505602 Mustard gas {Sulfur mustard} c 3 4 5

 134327 1-Naphthylamine [PAH-Derivative, POM] 09/90 c 1 2 4

 91598 2-Naphthylamine [PAH-Derivative, POM] c 1 2 3 4 5

 54115 Nicotine 09/90 4

 1148 Nitrilotriacetic acid (salts) 06/91 c 3

 including but not limited to:

 18662538 Nitrilotriacetic acid, trisodium salt 06/91 c 4

 monohydrate

 602879 5-Nitroacenaphthene [PAH-Derivative, POM] c 1 2 3 4

 99592 5-Nitro-o-anisidine c 4 5

 1836755 Nitrofen (technical grade) c 3 4 5

 51752 Nitrogen mustard {Mechlorethamine} c 3 4 5

 55867 Nitrogen mustard hydrochloride 09/89 c 4 5

 55630 Nitroglycerin 06/91 7

 88755 2-Nitrophenol 06/91 7

 57835924 4-Nitropyrene [PAH-Derivative, POM] 06/91 c 1 2 3 4

 86306 N-Nitrosodiphenylamine [POM] 09/89 c 1 2 4

 759739 N-Nitroso-N-ethylurea c 4 5

 60153493 3-(N-Nitrosomethylamino)propionitrile 09/89 c 3 4

 64091914 4-(N-Nitrosomethylamino)-1-(3-pyridyl)-1-butanone 09/89 c 3 4

 {NNK}

 615532 N-Nitroso-N-methylurethane {N-Methyl-N- c 3 4

 nitrosourethane}

 4549400 N-Nitrosomethylvinylamine c 3 4 5

 16543558 N-Nitrosonornicotine c 3 4 5

 13256229 N-Nitrososarcosine c 3 4 5

 303479 Ochratoxin A [POM] 09/90 c 1 2 4

 2234131 Octachloronaphthalene [PAH-Derivative, POM] 06/91 1 2 7

 2646175 Oil Orange SS [PAH-Derivative, POM] c 1 2 3 4

 20816120 Osmium tetroxide 06/91 7

 794934 Panfuran S {Dihydroxymethylfuratrizine} c 3 4

 122601 Phenyl glycidyl ether 09/90 c 3 4

 57410 Phenytoin [POM] c 1 2 3 4 5

 88891 Picric acid 06/91 7

 1155 Polybrominated biphenyls {PBBs} [POM] c 1 2 3 4 5

 53973981 Polygeenan 09/89 c 4

 3761533 Ponceau MX [PAH-Derivative, POM] c 1 2 3 4

 3564098 Ponceau 3R [PAH-Derivative, POM] c 1 2 3 4

 36791045 Ribavirin 09/90 4

 94597 Safrole c 3 4 5

 1180 Shale oils c 3 4

 132274 Sodium o-phenylphenate [POM] c 1 2 3 4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:2 - 9

 128449 Sodium saccharin 09/89 c 4

 1185 Soots c 3 4

 10048132 Sterigmatocystin [POM] c 1 2 3 4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:2 - 10

 APPENDIX A-II

 Substances For Which Production, Use, Or Other Presence Must Be Reported

Emittent ID Add Date Carcinogen Source List(s) Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [6]) Notes(s)

 95067 Sulfallate c 3 4 5

 5216251 p-alpha,alpha,alpha-Tetrachlorotoluene 09/90 c 4

 961115 Tetrachlorvinphos 06/91 7

 509148 Tetranitromethane 09/90 c 4

 139651 4,4'-Thiodianiline [POM] c 1 2 3 4

 1314201 Thorium dioxide c 4 5

 1200 Tobacco products, smokeless c 3 4

 1205 alpha-chlorinated Toluenes c 3

 636215 o-Toluidine hydrochloride c 4 5

 106490 p-Toluidine 09/90 c 4

 52686 Trichlorfon 06/91 7

 68768 Tris(aziridinyl)-p-benzoquinone {Triaziquone} 09/90 c 4

 52244 Tris(1-aziridinyl) phosphine sulfide {Thiotepa} c 3 4 5

 126727 Tris(2,3-dibromopropyl)phosphate 09/89 c 4

 62450060 Trp-P-1 {3-Amino-1,4-dimethyl-5H-pyrido[4,3- c 3 4

 b]indole}

 62450071 Trp-P-2 {3-Amino-1-methyl-5H-pyrido[4,3-b]indole} c 3 4

 72571 Trypan blue [PAH-Derivative, POM] c 1 2 3 4

 106876 4-Vinyl-1-cyclohexene diepoxide {Vinyl cyclohexene 09/90 c 4

 dioxide}

 81812 Warfarin [POM] 1 2 4

 87627 2,6-Xylidene 06/91 4

 12122677 Zineb 09/90 c 4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

September 2002.

Appendix A:3 - 1

Appendix A-III

Substances Which Need Not Be Reported

Unless Manufactured By The Facility

July 1, 1997
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

September 2002.

Appendix A:3 - 2
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:3 - 2

APPENDIX A-III

Substances Which Need Not Be Reported Unless Manufactured By the Facility

Emittent ID Add Date Carcinogen Source List(s) Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [6]) Notes(s)

 546883 Acetohydroxamic acid 09/90 4

 50760 Actinomycin D 09/90 c 4

 23214928 Adriamycin [PAH-Derivative, POM] c 1 2 3 4 5

 28981977 Alprazolam [POM] 09/90 1 2 4

 39831555 Amikacin sulfate 09/90 4

 54626 Aminopterin 4

 1005 Analgesic mixtures containing phenacetin c 3 4 5

 1010 Androgenic (anabolic) steriods c 3 4

 including but not limited to:

 58184 Methyltestosterone 09/90 4

 434071 Oxymetholone c 4 5

 58220 Testosterone and its esters 09/89 4

 including but not limited to:

 315377 Testosterone enanthate 09/90 4

 50782 Aspirin 06/91 4

 115026 Azaserine c 3 4

 5411223 Benzphetamine hydrochloride [POM] 09/90 1 2 4

 154938 Bischloroethyl nitrosourea c 3 4

 55981 1,4-Butanediol dimethanesulfonate {Busulfen/ c 3 4 5

 Myleran}

 41575944 Carboplatin 09/90 4

 474259 Chenodiol 09/90 4

 305033 Chlorambucil c 3 4 5

 56757 Chloramphenicol c 3 4

 1620219 Chlorcyclizine hydrochloride [POM] 1 2 4

 13010474 1-(2-Chloroethyl)-3-cyclohexyl-1-nitrosourea c 3 4 5

 {CCNU}

 13909096 1-(2-Chloroethyl)-3-(4-methylcyclohexyl)-1- c 3

 nitrosourea {Methyl CCNU}

 15663271 Cisplatin c 3 4

 50419 Clomiphene citrate [POM] 09/90 1 2 4

 50180 Cyclophosphamide c 3 4

 147944 Cytarabine 09/89 4

 4342034 Dacarbazine c 3 4 5

 17230885 Danazol 09/90 4

 20830813 Daunomycin [PAH-Derivative, POM] c 1 2 3 4

 23541506 Daunorubicin hydrochloride [PAH-Derivative, POM] 09/90 1 2 4

 84173 Dienestrol [POM] 09/90 c 1 2 4

 564250 Doxycycline 09/90 4

 379793 Ergotamine tartrate [POM] 09/90 1 2 4

 1095 Estrogens, non-steroidal c 3 5

 including but not limited to:

 56531 Diethylstilbestrol [POM] c 1 2 3 4 5

 1100 Estrogens, steroidal c 3 5
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:3 - 3

 including but not limited to:

 1068 Conjugated estrogens 09/90 c 4

 50282 Estradiol 17 beta c 4 5

 53167 Estrone c 4 5
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:3 - 4

 

APPENDIX A-III

Substances Which Need Not Be Reported Unless Manufactured By the Facility

Emittent ID Add Date Carcinogen Source List(s) Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [6]) Notes(s)

 57636 Ethinyl estradiol c 4 5

 72333 Mestranol c 3 4 5

 33419420 Etoposide [POM] 09/90 2

 54350480 Etretinate 4

 51218 Fluorouracil 09/89 4

 76437 Fluoxymesterone 09/90 4

 13311847 Flutamide 09/90 4

 67458 Furazolidone 09/90 c 4

 126078 Griseofulvin c 3 4

 23092173 Halazepam [POM] 09/90 1 2 4

 3778732 Ifosfamide 09/90 4

 9004664 Iron dextran complex c 3 4 5

 303344 Lasiocarpine 09/89 c 3 4

 554132 Lithium carbonate 06/91 4

 919164 Lithium citrate 06/91 4

 846491 Lorazepam [POM] 09/90 1 2 4

 595335 Megestrol acetate 06/91 4

 148823 Melphalan c 3 4 5

 9002680 Menotropins 09/90 4

 6112761 Mercaptopurine 09/90 4

 531760 Merphalan 09/89 c 4

 3963959 Methacycline hydrochloride 06/91 4

 60560 Methimazole 09/90 4

 15475566 Methotrexate sodium 09/90 4

 484208 5-Methoxypsoralen c 3

 56042 Methylthiouracil c 3 4

 443481 Metronidazole c 3 4 5

 59467968 Midazolam hydrochloride [POM] 09/90 1 2 4

 62015398 Misoprostol 09/90 4

 50077 Mitomycin C c 3 4

 70476823 Mitoxantrone hydrochloride [PAH-Derivative, POM] 09/90 1 2 4

 139913 5-(Morpholinomethyl)-3-[(5- c 3 4

 nitrofurfurylidene)amino]-2-oxazolidinone

 86220420 Nafarelin acetate [PAH-Derivative, POM] 09/90 1 2 4

 3771195 Nafenopin [POM] c 1 2 3 4

 1405103 Neomycin sulfate 09/90 4

 56391572 Netilmicin sulfate 09/90 4

 61574 Niridazole c 3 4

 67209 Nitrofurantoin 06/91 c 4

 59870 Nitrofurazone 09/90 c 4

 555840 1-[(5-Nitrofurfurylidene)amino]-2-imidazolidinone c 3 4

 531828 N-[4-(5-Nitro-2-furyl)-2-thiazolyl]acetamide c 3 4

 6533002 Norgestrel 09/90 4

 79572 Oxytetracycline 06/91 4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:3 - 5

 115673 Paramethadione 09/90 4

 52675 Penicillamine 06/91 4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Appendix A:3 - 6

 

APPENDIX A-III

Substances Which Need Not Be Reported Unless Manufactured By the Facility

Emittent ID Add Date Carcinogen Source List(s) Other

(Note [1]) Substance Name (Note [2]) (Note [3]) (Note [4]) (Note [6]) Notes(s)

 57330 Pentobarbital sodium 09/90 4

 63989 Phenacemide 09/90 4

 62442 Phenacetin c 3 4 5

 94780 Phenazopyridine hydrochloride c 3 4 5

 3546109 Phenesterin 09/89 c 4 5

 50066 Phenobarbital c 3 4

 59961 Phenoxybenzamine [POM] 09/89 c 1 2 4

 63923 Phenoxybenzamine hydrochloride [POM] 09/90 c 1 2 3 4 5

 54911 Pipobroman 09/90 4

 18378897 Plicamycin [PAH-Derivative, POM] 09/90 1 2 4

 366701 Procarbazine hydrochloride c 3 4 5

 57830 Progesterone c 3 4 5

 1160 Progestins c 3

 including but not limited to:

 71589 Medroxyprogesterone acetate c 3 4

 68224 Norethisterone c 4 5

 51525 Propylthiouracil c 3 4 5

 302794 all-trans-Retinoic acid 09/89 4

 1167 Retinol/retinyl esters 09/89 c 4

 81072 Saccharin c 3 4 5

 3810740 Streptomycin sulfate 06/91 4

 18883664 Streptozotocin c 3 4 5

 54965241 Tamoxifen citrate [POM] 09/90 1 2 4

 846504 Temazepam [POM] 09/90 1 2 4

 64755 Tetracycline hydrochloride 06/91 4

 50351 Thalidomide 4

 154427 Thioguanine 09/90 4

 49842071 Tobramycin sulfate 09/90 4

 299752 Treosulfan c 3 4

 28911015 Triazolam [POM] 09/90 1 2 4

 13647353 Trilostane 09/90 4

 127480 Trimethadione 06/91 4

 66751 Uracil mustard c 3 4

 26995915 Urofollitropin 09/90 4

 99661 Valproate 4

 143679 Vinblastine sulfate [POM] 09/90 1 2 4

 2068782 Vincristine sulfate [POM] 09/90 1 2 4
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

NOTES TO APPENDIX A

Note Text of Note

 [ 1] Emittent ID (the emittent identification number) is the Chemical Abstract Service (CAS) number where available, or an ARB-assigned 4-digit emittent ID 

code. 

A dash ("-") is shown for the Emittent ID for substances which are alphabetized under a group header or synonym elsewhere on the list. Refer to the 

cross reference indicated in parenthesis, "( )". 

A double dash ("- -") is shown for the Emittent ID to indicate that the entry is a non-reportable group header for the substances immediately following 

it. 

 

An asterisk ("*") is shown for the Emittent ID to indicate that the emissions of unspecified metal compounds shall be reported as the metal atom 

equivalent. See Note [7]. 

A pound sign ("#") is shown for the Emittent ID to indicate that the individual, component listed substances must be reported for this mixture or group. 

 

 

[ 2] Individual substances listed under a group heading must be reported individually. Other, unspecified substances in the group must be summed and 

reported using the emittent ID of the group heading. 

The square bracket designation, "[ ]", indicates that the substance is a component of the chemical group heading(s) within the brackets. 

 

The braces designation, "{ }", indicates a synonym for the substance listed. 

 

[ 3] The date the Board approved addition of the substance to the original list. The original list was approved by the Board in July 1988. 

 

[ 4] The letter "c" indicates that for purposes of this section the substance shall be treated as a human carcinogen or potential human carcinogen. 

[ 5] Applicable degree of accuracy (in lbs/year except where noted). Radionuclides must be reported in Curie units, and the accuracy must be considered 

accordingly. Refer to Section VII.E. and Appendix B. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Note Text of Note

[ 6] Substances are required to be included on the Hot Spots list based on the following lists cited in Health & Safety Code Section 44321: 

1 = California Air Resources Board (44321(c)); 2 = Environmental Protection Agency (44321(e)); 

3 = International Agency for Research on Cancer; 4 = Governor's List of Carcinogens and Reproductive Toxicants;

(44321(a); Labor Code section 6382(b)(1)) (44321(b); HSC Section 25249.8) 

5 = National Toxicology Program (44321(a)); 6 = Hazard Evaluation System and Information Service (44321(d))

7 = Added pursuant to HSC Section 44321 (f). 

[ 7] Emissions of unspecified metal compounds shall be reported as the amount of the metal atom equivalent, using the metal emittent identification number 

for the metal itself (or the emittent identification number indicated on the table, such as for reporting inorganic versus other-than-inorganic arsenic 

compounds). 

 

For unspecified metal compounds which contain two or more listed metals (e.g., zinc chromate), each component metal shall be reported as the amount 

of the appropriate metal atom equivalent (i.e., the zinc portion of the weight as zinc equivalent and the chromate portion as hexavalent chromium 

equivalent).

For specific, individually listed metal compounds (e.g., Lead chromate), emissions shall be reported for the compound (as pounds of whole compound), 

using the emittent identification number for that compound.

[ 8] Compounds of the form "X-CN", where formal dissociation can occur. Report as the amount of Cyanide equivalent in the compound using an emittent 

identification code of 1073.

[ 9] Emissions of these mixtures shall be reported as emissions of total particulate matter and total organic gas, using the following emittent identification 

numbers: 

9901 Diesel exhaust, particulate matter 9910 Gasoline exhaust, particulate matter 

9902 Diesel exhaust, total organic gas 9911 Gasoline exhaust, total organic gas 

Individually listed substances from diesel and gasoline exhaust must also be reported. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. September 2002.

Note Text of Note

[10] The emittent identification number 1105 has been discontinued for all facilities reporting for the first time and for all updates. 

Use the listed replacement emittent identification codes 1103 and 1104. 

 

[11] Emissions of the individual, component listed substances must be reported in addition to the total gasoline vapors emissions. 

[12] These lead compounds are listed here so that the inorganic lead fraction will be quantified and reported if these individual compounds cannot be 

quantified. 

[13] PAH: (Polycyclic Aromatic Hydrocarbon) - An organic compound consisting of a fused ring structure containing at least two (2) benzene rings, and 

which may also contain additional fused rings not restricted exclusively to hexagonal rings.

The structure does not include any heteroatoms or substituent groups. The structure includes only carbon and hydrogen. 

PAHs are a subgroup of POM and have a boiling point of greater than or equal to 100 C. 

[14] PAH-DERIVATIVE: (Polycyclic Aromatic Hydrocarbon Derivative) - An organic compound consisting of a fused ring structure containing at least two 

(2) benzene rings, and which may also contain additional fused rings not restricted exc lusively to hexagonal rings. The fused ring structure does not 

contain heteroatoms. The structure does contain one or more substituent groups.

PAH-Derivatives are a subgroup of POM and have a boiling point of greater than or equal to 100 C. 

[15] POM: (Polycyclic Organic Matter) - Includes organic compounds with more than one benzene ring, and which have a boiling point of greater than or 

equal to 100 C. 

[16] Radionuclides and other radioactive substances shall be reported in units of Curies per year (for annual average emissions) and in 

units of milliCuries per hour (for maximum hourly emissions). 

[17] Emissions of Vanadium (fume or dust) shall be reported as the amount of the vanadium atom equivalent, using the identification number 7440622.

[18] The emittent identification number 1001 has been replaced with the CAS number 26148685. 
Appendix B 

Health And Safety Code Related to the 

Air Toxics Hot Spots Program 

Appendix B - 1 
Appendix B 

Health And Safety Code Related To 

The Air Toxics Hot Spots Program1

PART 6. AIR TOXICS "HOT SPOTS" INFORMATION AND ASSESSMENT 

(Part 6 added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to 

Section 44384. Note: Sections 44380 and 44384 became operative Jan. 1, 1988.) 

CHAPTER 1. LEGISLATIVE FINDINGS AND DEFINITIONS 

(Chapter 1 added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to 

Section 44384.) 

44300. This part shall be known and may be cited as the Air Toxics "Hot Spots" Information 

and Assessment Act of 1987. (Added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, 

pursuant to Section 44384.) 

44301. The Legislature finds and declares all of the following: 

(a) In the wake of recent publicity surrounding planned and unplanned releases of toxic 

chemicals into the atmosphere, the public has become increasingly concerned about 

toxics in the air. 

(b) The Congressional Research Service of the Library of Congress has concluded that 

75 percent of the United States population lives in proximity to at least one facility 

that manufactures chemicals. An incomplete 1985 survey of large chemical 

companies conducted by the Congressional Research Service documented that nearly 

every chemical plant studied routinely releases into the surrounding air significant 

levels of substances proven to be or potentially hazardous to public health. 

(c) Generalized emissions inventories compiled by air pollution control districts and air 

quality management districts in California confirm the findings of the Congressional 

Research Service survey as well as reveal that many other facilities and businesses 

which do not actually manufacture chemicals do use hazardous substances in 

sufficient quantities to expose, or in a manner that exposes, surrounding populations 

to toxic air releases. 

(d) These releases may create localized concentrations or air toxics "hot spots" where 

emissions from specific sources may expose individuals and population groups to 

elevated risks of adverse health effects, including, but not limited to, cancer and 

contribute to the cumulative health risks of emissions from other sources in the area. 

In some cases where large populations may not be significantly affected by adverse 

health risks, individuals may be exposed to significant risks. 

________________ 

1

 AB564 Passed in the 1996 legislative session. The text will be added when the code is revised. 

Appendix B - 2 
(e) Little data is currently available to accurately assess the amounts, types, and health 

impacts of routine toxic chemical releases into the air. As a result, there exists 

significant uncertainty about the amounts of potentially hazardous air pollutants 

which are released, the location of those releases, and the concentrations to which 

the public is exposed. 

(f) The State of California has begun to implement a long-term program to identify, 

assess, and control ambient levels of hazardous air pollutants, but additional 

legislation is needed to provide for the collection and evaluation of information 

concerning the amounts, exposures, and short- and long-term health effects of 

hazardous substances regularly released to the surrounding atmosphere from specific 

sources of hazardous releases. 

(g) In order to more effectively implement control strategies for those materials posing 

an unacceptable risk to the public health, additional information on the sources of 

potentially hazardous air pollutants is necessary. 

(h) It is in the public interest to ascertain and measure the amounts and types of 

hazardous releases and potentially hazardous releases from specific sources that may 

be exposing people to those releases, and to assess the health risks to those who are 

exposed. (Added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant 

to Section 44384.) 

44302. The definitions set forth in this chapter govern the construction of this part. (Added by 

Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to Section 44384.) 

44303. "Air release" or "release" means any activity that may cause the issuance of air 

contaminants, including the actual or potential spilling, leaking, pumping, pouring, emitting, 

emptying, discharging, injecting, escaping, leaching, dumping, or disposing of a substance into 

the ambient air and that results from the routine operation of a facility or that is predictable, 

including, but not limited to, continuous and intermittent releases and predictable process upsets 

or leaks. (Added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to 

Section 44384.) 

44304. "Facility" means every structure, appurtenance, installation, and improvement on land 

which is associated with a source of air releases or potential air releases of a hazardous material. 

(Added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to Section 44384.) 

44306. "Health risk assessment" means a detailed comprehensive analysis prepared pursuant to 

Section 44361 to evaluate and predict the dispersion of hazardous substances in the environment 

and the potential for exposure of human populations and to assess and quantify both the 

individual and population wide health risks associated with those levels of exposure. (Added by 

Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to Section 44384.) 

44307. "Operator" means the person who owns or operates a facility or part of a facility. (Added 

by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to Section 44384.) 

Appendix B - 3 
44308. "Plan" means the emissions inventory plan which meets the conditions specified in 

Section 44342. (Added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to 

Section 44384.) 

44309. "Report" means the emissions inventory report specified in Section 44341. (Added by 

Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to Section 44384.) 

CHAPTER 2. FACILITIES SUBJECT TO THIS PART 

(Chapter 2 added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, 

pursuant to Section 44384.) 

44320. This part applies to the following: 

(a) Any facility which manufactures, formulates, uses, or releases any of the substances 

listed pursuant to Section 44321 or any other substance which reacts to form a 

substance listed in Section 44321 and which releases or has the potential to release 

total organic gases, particulates, or oxides of nitrogen or sulfur in the amounts 

specified in Section 44322. 

(b) Except as provided in Section 44323, any facility which is listed in any current toxics 

use or toxics air emission survey, inventory, or report released or compiled by a district. 

A district may, with the concurrence of the state board, waive the application of this part 

pursuant to this subdivision for any facility which the district determines will not release 

any substance listed pursuant to Section 44321 due to a shutdown or a process change. 

(Amended by Stats. 1989, Ch. 1254, Sec. 7). References at the time of publication (see 

page iii): Regulations: 17, CCR, sections 90700-90703, 90704, 93303, 93306 

44321. For the purposes of Section 44320, the state board shall compile and maintain a list of 

substances that contains, but is not limited to, all of the following: 

(a) Substances identified by reference in paragraph (1) of subdivision (b) of Section 6382 

of the Labor Code and substances placed on the list prepared by the National 

Toxicology Program issued by the United States Secretary of Health and Human 

Services pursuant to paragraph (4) of Section 262 of Public Law 95-622 of 1978. For 

the purposes of this subdivision, the state board may remove from the list any 

substance which meets both of the following criteria: 

(1) No evidence exists that it has been detected in air. 

(2) The substance is not manufactured or used in California, or, if manufactured or 

used in California, because of the physical or chemical characteristics of the 

substance or the manner in which it is manufactured or used, there is no 

possibility that it will become airborne. 

(b) Carcinogens and reproductive toxins referenced in or compiled pursuant to 

Section 25249.8, except those which meet both of the criteria identified in 

subdivision (a). 

(c) The candidate list of potential toxic air contaminants and the list of designated toxic 

air contaminants prepared by the state board pursuant to Article 2 (commencing with 

Section 39660) of Chapter 3.5 of Part 2, including, but not limited to, all substances 

currently under review and scheduled or nominated for review and substances 

identified and listed for which health effects information is limited. 

Appendix B - 4 
(d) Substances for which an information or hazard alert has been issued by the 

repository of current data established pursuant to Section 147.2 of the Labor Code. 

(e) Substances reviewed, under review, or scheduled for review as air toxics or potential 

air toxics by the Office of Air Quality Planning and Standards of the Environmental 

Protection Agency, including substances evaluated in all of the following categories 

or their equivalent: preliminary health and source screening, detailed assessment, 

intent to list, decision not to regulate, listed, standard proposed, and standard 

promulgated. 

(f) Any additional substances recognized by the state board as presenting a chronic or 

acute threat to public health when present in the ambient air, including, but not 

limited to, any neurotoxins or chronic respiratory toxins not included within 

subdivision (a), (b), (c), (d), or (e). (Added by Stats. 1987, Ch. 1252, Sec. 1. 

Operative July 1, 1988, pursuant to Section 44384.) 

44322. This part applies to facilities specified in subdivision (a) of Section 44320 in accordance 

with the following schedule: 

(a) For those facilities that release, or have the potential to release, 25 tons per year or 

greater of total organic gases, particulates, or oxides of nitrogen or sulfur, this part 

becomes effective on July 1, 1988. 

(b) For those facilities that release, or have the potential to release, more than 10 but less 

than 25 tons per year of total organic gases, particulates, or oxides of nitrogen or 

sulfur, this part becomes effective July 1, 1989. 

(c) For those facilities that release, or have the potential to release, less than 10 tons per 

year of total organic gases, particulates, or oxides of nitrogen or sulfur, the state 

board shall, on or before July 1, 1990, prepare and submit a report to the Legislature 

identifying the classes of those facilities to be included in this part and specifying a 

timetable for their inclusion. (Amended by Stats. 1989, Ch. 1254, Sec. 8.) 

44323. A district may prepare an industrywide emissions inventory and health risk assessment 

for facilities specified in subdivision (b) of Section 44320 and subdivisions (a) and (b) of 

Section 44322, and shall prepare an industrywide emissions inventory for the facilities specified 

in subdivision (c) of Section 44322, in compliance with this part for any class of facilities that 

the district finds and determines meets all of the following conditions: 

(a) All facilities in the class fall within one four-digit Standard Industrial Classification 

Code. 

(b) Individual compliance with this part would impose severe economic hardships on the 

majority of the facilities within the class. 

(c) The majority of the class is composed of small businesses. 

(d) Releases from individual facilities in the class can easily and generically be 

characterized and calculated. (Amended by Stats. 1989, Ch. 1254, Sec. 9.) 

44324. This part does not apply to any facility where economic poisons are employed in their 

pesticidal use, unless that facility was subject to district permit requirements on or before 

August 1,1987. As used in this section, "pesticidal use" does not include the manufacture or 

formulation of pesticides. (Added by Stats. 1981, Ch. 1252, Sec. 1. Operative July 1, 1988, 

pursuant to Section 44384.) 

Appendix B - 5 
44325. Any solid waste disposal facility in compliance with Section 41805.5 is in compliance 

with the emissions inventory requirements of this part. (Added by Stats. 1987, Ch. 1252, Sec. 1. 

Operative July 1, 1988, pursuant to Section 44384.) 

CHAPTER 3. AIR TOXICS EMISSION INVENTORIES 

(Chapter 3 added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, 

pursuant to Section 44384.) 

44340. (a) The operator of each facility subject to this part shall prepare and submit to the 

district a proposed comprehensive emissions inventory plan in accordance with the 

criteria and guidelines adopted by the state board pursuant to Section 44342. 

(b) The proposed plan shall be submitted to the district on or before August 1, 1989, 

except that, for any facility to which subdivision (b) of Section 44322 applies, the 

proposed plan shall be submitted to the district on or before August 1,1990. The 

district shall approve, modify, and approve as modified, or return for revision and 

resubmission, the plan within 120 days of receipt. 

(c) The district shall not approve a plan unless all of the following conditions are met: 

(1) The plan meets the requirements established by the state board pursuant to Section 

44342. 

(2) The plan is designed to produce, from the list compiled and maintained pursuant to 

Section 44321, a comprehensive characterization of the full range of hazardous 

materials that are released, or that may be released, to the surrounding air from the 

facility. Air release data shall be collected at, or calculated for, the primary locations 

of actual and potential release for each hazardous material. Data shall be collected 

or calculated for all continuous, intermittent, and predictable air releases. 

(3) The measurement technologies and estimation methods proposed provide state-ofthe-art effectiveness and are sufficient to produce a true representation of the types 

and quantities of air releases from the facility. 

(4) Source testing or other measurement techniques are employed wherever necessary to 

verify emission estimates, as determined by the state board and to the extent 

technologically feasible. All testing devices shall be appropriately located, as 

determined by the state board. 

(5) Data are collected or calculated for the relevant exposure rate or rates of each 

hazardous material according to its characteristic toxicity and for the emission rate 

necessary to ensure a characterization of risk associated with exposure to releases of 

the hazardous material that meets the requirements of Section 44361. The source of 

all emissions shall be displayed or described. (Added by Stats. 1987, Ch. 1252, 

Sec. 1. Operative July 1, 1988, pursuant to Section 44384.) 

44341. Within 180 days after approval of a plan by the district, the operator shall implement the 

plan and prepare and submit a report to the district in accordance with the plan. The district shall 

transmit all monitoring data contained in the approved report to the state board. (Added by 

Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to Section 44384.) 

Appendix B - 6 
44342. The state board shall, on or before May 1, 1989, in consultation with the districts, 

develop criteria and guidelines for site-specific air toxics emissions inventory plans which shall 

be designed to comply with the conditions specified in Section 44340 and which shall include at 

least all of the following: 

(a) For each class of facility, a designation of the hazardous materials for which 

emissions are to be quantified and an identification of the likely source types within 

that class of facility. The hazardous materials for quantification shall be chosen from

among, and may include all or part of, the list specified in Section 44321. 

(b) Requirements for a facility diagram identifying each actual or potential discrete 

emission point and the general locations where fugitive emissions may occur. The 

facility diagram shall include any nonpermitted and nonprocess sources of emissions 

and shall provide the necessary data to identify emission characteristics. An existing 

facility diagram which meets the requirements of this section may be submitted. 

(c) Requirements for source testing and measurement. The guidelines may specify 

appropriate uses of estimation techniques including, but not limited to, emissions 

factors, modeling, mass balance analysis, and projections, except that source testing 

shall be required wherever necessary to verify emission estimates to the extent 

technologically feasible. The guidelines shall specify conditions and locations where 

source testing, fence-line monitoring, or other measurement techniques are to be 

required and the frequency of that testing and measurement. 

(d) Appropriate testing methods, equipment, and procedures, including quality assurance 

criteria. 

(e) Specifications for acceptable emissions factors, including, but not limited to, those 

which are acceptable for substantially similar facilities or equipment, and 

specification of procedures for other estimation techniques and for the appropriate 

use of available data. 

(f) Specification of the reporting period required for each hazardous material for which 

emissions will be inventoried. 

(g) Specifications for the collection of useful data to identify toxic air contaminants 

pursuant to Article 2 (commencing with Section 39660) of Chapter 3.5 of Part 2. 

(h) Standardized format for preparation of reports and presentation of data. 

(i) A program to coordinate and eliminate any possible overlap between the 

requirements of this chapter and the requirements of Section 313 of the Superfund 

Amendment and Reauthorization Act of 1986 ( Public Law 99-499). The state board 

shall design the guidelines and criteria to ensure that, in collecting data to be used for 

emissions inventories, actual measurement is utilized whenever necessary to verify

the accuracy of emission estimates, to the extent technologically feasible. (Added by 

Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to Section 44384.) 

44343. The district shall review the reports submitted pursuant to Section 44341 and shall, 

within 90 days, review each report, obtain corrections and clarifications of the data, and notify 

the Office of Environmental Health Hazard Assessment, the Department of Industrial Relations, 

and the city or county health department of its findings and determinations as a result of its 

review of the report. (Added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant 

to Section 44384. Amended by Governor’s Reorganization Plan No. 1 of 1991, §142.) 

Appendix B - 7 
44344. Except as provided in Section 44391, emissions inventories developed pursuant to this 

chapter shall be updated every four years, in accordance with the procedures established by the 

state board. Those updates shall take into consideration improvements in measurement 

techniques and advancing knowledge concerning the types and toxicity of hazardous material 

released or potentially released. (Amended by Stats. 1993, Ch. 1041, Sec. 1. Effective 

January 1, 1994.) 

44344.3. 

(a) A facility shall be granted an exemption by a district from further compliance with 

this part after meeting all of the following criteria: 

(1) The facility was required to comply with this part only as a result of its 

particulate matter emissions. 

(2) The facility has participated in, utilized data derived from, or is eligible to 

utilize data derived from, approved pooled source testing. 

(3) The facility has submitted an emissions inventory plan and report that was 

subsequently accepted and approved. 

(4) The facility has been designated by the district as a low priority facility under 

the guidelines set forth pursuant to this part for facility prioritization, and 

facility emissions do not present a significant health risk as specified in 

subdivision (b) of Section 44362. 

(5) The facility handles, processes, stores, or distributes bulk agricultural 

commodities or handles, feeds, or rears livestock. (b) Subdivision (a) does not 

apply to a facility that, because of information provided pursuant to 

Section 44344.7, is reclassified as an intermediate or high priority facility by the 

district. 

(c) The operator of a facility that has been granted an exemption pursuant to this section 

shall biennially submit a statement to the district for the district's review, with a copy 

of the most recent emissions inventory for the facility, indicating that, except as to 

matters for which an emissions inventory update has been or will be submitted 

pursuant to Section 44344.7, there has been no significant change in facility 

operations or activities. The district shall not impose any fee upon the operator with 

regard to the submission of the statement. (Added by Stats. 1993, Ch. 1037, Sec. 1. 

Effective January 1, 1994.) 

44344.5. The operator of any new facility that previously has not been subject to this part shall 

prepare and submit an emissions inventory plan and report. (Added by Stats. 1993, Ch. 1037, 

Sec. 2. Effective January 1, 1994.) 

44344.7. The operator of a facility exempted pursuant to subdivision (a) of Section 44344.3 

shall submit an emissions inventory update for those sources and substances for which a change 

in activities or operations has occurred, as follows: 

(a) The facility emits a newly listed substance. 

(b) A sensitive receptor has been established or constructed on or after January 1, 1994, 

within 500 meters of the facility. 

(c) The facility emits a substance for which the potency factor has increased. 

Appendix B - 8 
(d) The facility has begun emission of a listed substance not included in the previous 

emissions inventory. (Added by Stats. 1993, Ch. 1037, Sec. 3. Effective 

January 1, 1994.) 

44345. (a) On or before July 1, 1989, the state board shall develop a program to compile and 

make available to other state and local public agencies and the public all data 

collected pursuant to this chapter. 

(b) In addition, the state board, on or before March 1, 1990, shall compile, by district, 

emissions inventory data for mobile sources and area sources not subject to district 

permit requirements, and data on natural source emissions, and shall incorporate 

these data into data compiled and released pursuant to this chapter. (Added by 

Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to Section 44384.) 

44346. (a) If an operator believes that any information required in the facility diagram specified 

pursuant to subdivision (b) of Section 44342 involves the release of a trade secret, 

the operator shall nevertheless make the disclosure to the district, and shall notify the 

district in writing of that belief in the report. 

(b) Subject to this section, the district shall protect from disclosure any trade secret 

designated as such by the operator, if that trade secret is not a public record. 

(c) Upon receipt of a request for the release of information to the public which includes 

information which the operator has notified the district is a trade secret and which is 

not a public record, the following procedure applies: 

(1) The district shall notify the operator of the request in writing by certified mail, 

return receipt requested. 

(2) The district shall release the information to the public, but not earlier than 30 

days after the date of mailing the notice of the request for information, unless, 

prior to the expiration of the 30-day period, the operator obtains an action in an 

appropriate court for a declaratory judgment that the information is subject to 

protection under this section or for a preliminary injunction prohibiting 

disclosure of the information to the public and promptly notifies the district of 

that action. 

(d) This section does not permit an operator to refuse to disclose the information 

required pursuant to this part to the district. 

(e) Any information determined by a court to be a trade secret, and not a public record 

pursuant to this section, shall not be disclosed to anyone except an officer or 

employee of the district, the state, or the United States, in connection with the 

official duties of that officer or employee under any law for the protection of health, 

or to contractors with the district or the state and its employees if, in the opinion of 

the district or the state, disclosure is necessary and required for the satisfactory 

performance of a contract, for performance of work, or to protect the health and 

safety of the employees of the contractor. 

(f) Any officer or employee of the district or former officer or employee who, by virtue 

of that employment or official position, has possession of, or has access to, any trade 

secret subject to this section, and who, knowing that disclosure of the information to 

the general public is prohibited by this section, knowingly and willfully discloses the 

information in any manner to any person not entitled to receive it is guilty of a 

Appendix B - 9 
misdemeanor. Any contractor of the district and any employee of the contractor, 

who has been furnished information as authorized by this section, shall be 

considered an employee of the district for purposes of this section. 

(g) Information certified by appropriate officials of the United States as necessary to be 

kept secret for national defense purposes shall be accorded the full protections 

against disclosure as specified by those officials or in accordance with the laws of 

the United States 

(h) As used in this section, "trade secret" and "public record" have the meanings and 

protections given to them by Section 6254.7 of the Government Code and Section 

1060 of the Evidence Code. All information collected pursuant to this chapter, 

except for data used to calculate emissions data required in the facility diagram, shall 

be considered "air pollution emission data," for the purposes of this section. (Added 

by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to Section 44384.) 

CHAPTER 4. RISK ASSESSMENT 

(Chapter 4 added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, 

pursuant to Section 44384.) 

44360. (a) Within 90 days of completion of the review of all emissions inventory data for 

facilities specified in subdivision (a) of Section 44322, but not later than 

December 1,1990, the district shall, based on examination of the emissions inventory 

data and in consultation with the state board and the State Department of Health 

Services, prioritize and then categorize those facilities for the purposes of health risk 

assessment. The district shall designate high, intermediate, and low priority 

categories and shall include each facility within the appropriate category based on its 

individual priority. In establishing priorities pursuant to this section, the district 

shall consider the potency, toxicity, quantity, and volume of hazardous materials 

released from the facility, the proximity of the facility to potential receptors, 

including, but not limited to, hospitals, schools, day care centers, worksites, and 

residences, and any other factors that the district finds and determines may indicate 

that the facility may pose a significant risk to receptors. The district shall hold a 

public hearing prior to the final establishment of priorities and categories pursuant to 

this section. 

(b) (1) Within 150 days of the designation of priorities and categories pursuant to 

subdivision (a), the operator of every facility that has been included within the 

highest priority category shall prepare and submit to the district a health risk 

assessment pursuant to Section 44361. The district may, at its discretion, grant 

a 30-day extension for submittal of the health risk assessment. 

(2) Health risk assessments required by this chapter shall be prepared in accordance 

with guidelines established by the Office of Environmental Health Hazard 

Assessment. The office shall prepare draft guidelines which shall be circulated 

to the public and the regulated community and shall adopt risk assessment 

guidelines after consulting with the state board and the Risk Assessment 

Committee of the California Air Pollution Control Officers Association and 

after conducting at least two public workshops, one in the northern and one in 

the southern part of the state. The adoption of the guidelines is not subject to 

Appendix B - 10 
Chapter 3.5 (commencing with Section 11340) of Part 1 of Division 3 of Title 2 

of the Government Code. The scientific review panel established pursuant to 

Section 39670 shall evaluate the guidelines adopted under this paragraph and 

shall recommend changes and additional criteria to reflect new scientific data or 

empirical studies. 

(3) The guidelines established pursuant to paragraph (2) shall impose only those 

requirements on facilities subject to this subdivision that are necessary to ensure 

that a required risk assessment is accurate and complete and shall specify the 

type of site-specific factors that districts may take into account in determining 

when a single health risk assessment may be allowed under subdivision (d). 

The guidelines shall, in addition, allow the operator of a facility, at the 

operator's option, and to the extent that valid and reliable data are available, to 

include for consideration by the district in the health risk assessment any or all 

of the following supplemental information: 

(a) Information concerning the scientific basis for selecting risk parameter values that 

are different than those required by the guidelines and the likelihood distributions 

that result when alternative values are used. 

(b) Data from dispersion models, microenvironment characteristics, and population 

distributions that may be used to estimate maximum actual exposure. 

(c) Risk expressions that show the likelihood that any given risk estimate is the correct 

risk value. 

(d) A description of the incremental reductions in risk that occur when exposure is 

reduced. 

(4) To ensure consistency in the use of the supplemental information authorized by 

subparagraphs (A), (B), (C),and (D) of paragraph (3),the guidelines established 

pursuant to paragraph (2) shall include guidance for use by the districts in 

considering the supplemental information when it is included in the health risk 

assessment. (c) Upon submission of emissions inventory data for facilities 

specified in subdivisions (b) and (c) of Section 44322, the district shall 

designate facilities for inclusion within the highest priority category, as 

appropriate, and any facility so designated shall be subject to subdivision (b). 

In addition, the district may require the operator of any facility to prepare and 

submit health risk assessments, in accordance with the priorities developed 

pursuant to subdivision (a). 

(e) The district shall, except where site specific factors may affect the results, allow the 

use of a single health risk assessment for two or more substantially identical facilities 

operated by the same person. 

(f) Nothing contained in this section, Section 44380.5, or Chapter 6 (commencing with 

Section 44390) shall be interpreted as requiring a facility operator to prepare a new 

or revised health risk assessment using the guidelines established pursuant to 

paragraph (2) of subdivision (a) of this section if the facility operator is required by 

the district to begin the preparation of a health risk assessment before those 

guidelines are established. (Amended by Stats. 1992, Ch. 1162, Sec. 1. Effective 

January 1, 1993.) 

Appendix B - 11 
44361. (a) Each health risk assessment shall be submitted to the district. The district shall make 

he health risk assessment available for public review, upon request. After 

preliminary review of the emissions impact and modeling data, the district shall 

submit the health risk assessment to the Office of Environmental Health Hazard 

Assessment for review and, within 180 days of receiving the health risk assessment, 

the office shall submit to the district its comments on the data and findings relating 

to health effects. The district shall consult with the state board as necessary to 

adequately evaluate the emissions impact and modeling data contained within the 

risk assessment. 

(b) For the purposes of complying with this section, the Office of Environmental Health 

Hazard Assessment may select a qualified independent contractor to review the data 

and findings relating to health effects. The office shall not select an independent 

contractor to review a specific health risk assessment who may have a conflict of 

interest with regard to the review of that health risk assessment. Any review by an 

independent contractor shall comply with the following requirements: 

(1) Be performed in a manner consistent with guidelines provided by the office. 

(2) Be reviewed by the office for accuracy and completeness. 

(3) Be submitted by the office to the district in accordance with this section. 

(c) The district shall reimburse the Office of Environmental Health Hazard Assessment 

or the qualified independent contractor designated by the office pursuant to 

subdivision (b), within 45 days of its request, for its actual costs incurred in 

reviewing a health risk assessment pursuant to this section. 

(d) If a district requests the Office of Environmental Health Hazard Assessment to 

consult with the district concerning any requirement of this part, the district shall 

reimburse the office, within 45 days of its request, for the costs incurred in the 

consultation. 

(e) Upon designation of the high priority facilities, as specified in subdivision (a) of 

Section 44360, the Office of Environmental Health Hazard Assessment shall evaluate 

the staffing requirements of this section and may submit recommendations to the 

Legislature, as appropriate, concerning the maximum number of health risk 

assessments to be reviewed each year pursuant to this section. (Added by Stats. 1987, 

Ch. 1252, Sec. l. Operative July 1, 1988, pursuant to Section 

44384. Amended by Governor's Reorganization Plan No. l of l991, §144.) 

44362. (a) Taking the comments of the Office of Environmental Health Hazard Assessment into 

account, the district shall approve or return for revision and resubmission and then 

approve, the health risk assessment within 180 days of receipt. If the health risk 

assessment has not been revised and resubmitted within 60 days of the district's

request of the operator to do so, the district may modify the health risk assessment 

and approve it as modified. 

(b) Upon approval of the health risk assessment, the operator of the facility shall provide 

notice to all exposed persons regarding the results of the health risk assessment 

prepared pursuant to Section 44361 if, in the judgment of the district, the health risk 

assessment indicates there is a significant health risk associated with emissions from

the facility. If notice is required under this subdivision, the notice shall include only 

Appendix B - 12 
information concerning significant health risks attributable to the specific facility for 

which the notice is required. Any notice shall be made in accordance with procedures 

specified by the district. (Added by Stats. 1981, Ch. 1252, Sec. 1. Operative 

July 1, 1988, pursuant to Section 44384. Amended by Governor's Reorganization 

Plan No. 1 of 1991, 145.) 

44363. (a) Commencing July 1, 1991, each district shall prepare and publish an annual report 

which does all of the following: 

(1) Describes the priorities and categories designated pursuant to Section 44360 and 

summarizes the results and progress of the health risk assessment program

undertaken pursuant to this part. 

(2) Ranks and identifies facilities according to the degree of cancer risk posed both 

to individuals and to the exposed population. 

(3) Identifies facilities which expose individuals or populations to any noncancer 

health risks. 

(4) Describes the status of the development of control measures to reduce emissions 

of toxic air contaminants, if any. 

(b) The district shall disseminate the annual report to county boards of supervisors, city 

councils, and local health officers and the district board shall hold one or more public 

hearings to present the report and discuss its content and significance. (Added by 

Stats. 1987, Ch. 1252, Sec. l. Operative July 1, 1988, pursuant to Section 44384.) 

44364. The state board shall utilize the reports and assessments developed pursuant to this part 

for the purposes of identifying, establishing priorities for, and controlling toxic air contaminants 

pursuant to Chapter 3.5 (commencing with Section 39650) of Part 2. (Added by Stats. 1987, 

Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to Section 44384. ) 

44365. (a) If the state board finds and determines that a district's actions pursuant to this part do 

not meet the requirements of this part, the state board may exercise the authority of 

the district pursuant to this part to approve emissions inventory plans and require the 

preparation of health risk assessments. 

(b) This part does not prevent any district from establishing more stringent criteria and 

requirements than are specified in this part for approval of emissions inventories and 

requiring the preparation and submission of health risk assessments. Nothing in this 

part limits the authority of a district under any other provision of law to assess and 

regulate releases of hazardous substances. (Added by Stats. 1987, Ch. 1252, Sec. 1. 

Operative July 1, 1988, pursuant to Section 44384.) 

44366. (a) In order to verify the accuracy of any information submitted by facilities pursuant to 

this part, a district or the state board may proceed in accordance with Section 41510. 

(Added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, pursuant to Section 

44384.) 

CHAPTER 5. FEES AND REGULATIONS 

(Chapter 5 added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, 

pursuant to Section 44384.) 

Appendix B - 13 
44380. (a) The state board shall adopt a regulation which does all of the following: 

(1) Sets forth the amount of revenue which the district must collect to recover the 

reasonable anticipated cost which will be incurred by the state board and the 

Office of Environmental Health Hazard Assessment to implement and administer 

this part. 

(2) Requires each district to adopt a fee schedule which recovers the costs of the 

district and which assesses a fee upon the operator of every facility subject to 

this part. A district may request the state board to adopt a fee schedule for the 

district if the district's program costs are approved by the district board and 

transmitted to the state board by April 1 of the year in which the request is made. 

(3) Requires any district that has an approved toxics emissions inventory compiled 

pursuant to this part by August 1 of the preceding year to adopt a fee schedule, 

as described in paragraph (2), which imposes on facility operators fees which 

are, to the maximum extent practicable, proportionate to the extent of the 

releases identified in the toxics emissions inventory and the level of priority 

assigned to that source by the district pursuant to Section 44360. 

(b) Commencing August 1, 1992, and annually thereafter, the state board shall review 

and may amend the fee regulation. 

(c) The district shall notify each person who is subject to the fee of the obligation to pay 

the fee. If a person fails to pay the fee within 60 days after receipt of this notice, the 

district, unless otherwise provided by district rules, shall require the person to pay an 

additional administrative civil penalty. The district shall fix the penalty at not more 

than 100 percent of the assessed fee, but in an amount sufficient in its determination, 

to pay the district's additional expenses incurred by the person's noncompliance. If a 

person fails to pay the fee within 120 days after receipt of this notice, the district 

may initiate permit revocation proceedings. If any permit is revoked, it shall be 

reinstated only upon full payment of the overdue fee plus any late penalty, and a 

reinstatement fee to cover administrative costs of reinstating the permit. 

(d) Each district shall collect the fees assessed pursuant to subdivision (a). After 

deducting the costs to the district to implement and administer this part, the district 

shall transmit the remainder to the Controller for deposit in the Air Toxics Inventory 

and Assessment Account, which is hereby created in the General Fund. The money 

in the account is available, upon appropriation by the Legislature, to the state board 

and the Office of Environmental Health Hazard Assessment for the purposes of 

administering this part. (Amended by Stats. 1992, Ch. 375, Sec. 1. Effective 

January 1, 1993.) 

44380.1. A facility shall be granted an exemption by a district from paying a fee in accordance 

with Section 44380 if all of the following criteria are met: 

(a) The facility primarily handles, processes, stores, or distributes bulk agricultural 

commodities or handles, feeds, or rears livestock. 

(b) The facility was required to comply with this part only as a result of its particulate 

matter emissions. 

Appendix B - 14 
(c) The fee schedule adopted by the district or the state board for these types of facilities 

is not solely based on toxic emissions weighted for potency or toxicity. (Added by 

Stats. 1993, Ch. 1037, Sec. 4. Effective January 1, 1994.) 

44380.5. In addition to the fee assessed pursuant to Section 44380, a supplemental fee may be 

assessed by the district, the state board, or the Office of Environmental Health Hazard 

Assessment upon the operator of a facility that, at the operator's option, includes supplemental 

information authorized by paragraph (3) of subdivision (b) of Section 44360 in a health risk 

assessment, if the review of that supplemental information substantially increases the costs of 

reviewing the health risk assessment by the district, the state board, or the office. The 

supplemental fee shall be set by the state board in the regulation required by subdivision (a) of 

Section 44380 and shall be set in an amount sufficient to cover the direct costs to review the 

information supplied by an operator pursuant to paragraph (3) of subdivision (b) of 

Section 44360. (Added by Stats. 1992, Ch. 1162, Sec. 2. Effective January 1, 1993.) 

44381. (a) Any person who fails to submit any information, reports, or statements required by 

this part, or who fails to comply with this part or with any permit, rule, regulation, or 

requirement issued or adopted pursuant to this part, is subject to a civil penalty of not 

less than five hundred dollars ($500) or more than ten thousand dollars ($10,000) for 

each day that the information, report, or statement is not submitted, or that the 

violation continues. 

(b) Any person who knowingly submits any false statement or representation in any 

application, report, statement, or other document filed, maintained, or used for the 

purposes of compliance with this part is subject to a civil penalty of not less than one 

thousand dollars ($1,000) or more than twenty-five thousand dollars ($25,000) per 

day for each day that the information remains uncorrected. (Added by Stats. 1987, 

Ch. 1252, Sec. 1. Operative July 1988, pursuant to Section 44384.) 

44382. Every district shall, by regulation, adopt the requirements of this part as a condition of 

every permit issued pursuant to Chapter 4 (commencing with Section 42300) of Part 4 for all 

new and modified facilities. (Added by Stats. 1987, Ch. 1252, Sec. 1. Operative July 1, 1988, 

pursuant to Section 44384. ) 

44384. Except for Section 44380 and this section, all provisions of this part shall become

operative on July 1, 1988. (Added by Stats. 1987, Ch. 1252, Sec. 1. Operative January 1, 1988, 

by its own provisions.) 

CHAPTER 6. FACILITY TOXIC AIR CONTAMINANT RISK REDUCTION 

AUDIT AND PLAN 

(Chapter 6 added by Stats. 1992, Ch. 1162, Sec. 3. Effective January 1, 1993.) 

44390. For purposes of this chapter, the following definitions apply: 

(a) "Airborne toxic risk reduction measure" or "ATRRM" means those in-plant changes 

in production processes or feedstocks that reduce or eliminate toxic air emissions 

subject to this part. ATRRM's may include: 

(1) Feedstock modification. 

Appendix B - 15 
(2) Product reformulations. 

(3) Production system modifications. 

(4) System enclosure, emissions control, capture, or conversion. 

(5) Operational standards and practices modification. 

(b) Airborne toxic risk reduction measures do not include measures that will increase 

risk from exposure to the chemical in another media or that increase the risk to 

workers or consumers. 

(c) "Airborne toxic risk reduction audit and plan'' or "audit and plan" means the audit 

and plan specified in Section 44392. (Added by Stats. 1992, Ch. 1162, Sec. 3. 

Effective January 1, 1993.) 

44391. (a) Whenever a health risk assessment approved pursuant to Chapter 4 (commencing 

with Section 44360) indicates, in the judgment of the district, that there is a 

significant risk associated with the emissions from a facility, the facility operator 

shall conduct an airborne toxic risk reduction audit and develop a plan to implement 

airborne toxic risk reduction measures that will result in the reduction of emissions 

from the facility to a level below the significant risk level within five years of the 

date the plan is submitted to the district. The facility operator shall implement 

measures set forth in the plan in accordance with this chapter. 

(b) The period to implement the plan required by subdivision (a) may be shortened by 

the district if it finds that it is technically feasible and economically practicable to 

implement the plan to reduce emissions below the significant risk level more quickly 

or if it finds that the emissions from the facility pose an unreasonable health risk. 

(c) A district may lengthen the period to implement the plan required by subdivision (a) 

by up to an additional five years if it finds that a period longer than five years will 

not result in an unreasonable risk to public health and that requiring implementation 

of the plan within five years places an unreasonable economic burden on the facility 

operator or is not technically feasible. 

(d) (1) The state board and districts shall provide assistance to smaller businesses that 

have inadequate technical and financial resources for obtaining information, 

assessing risk reduction methods, and developing and applying risk reduction 

techniques. 

(2) Risk reduction audits and plans for any industry subject to this chapter which is 

comprised mainly of small businesses using substantially similar technology 

may be completed by a self-conducted audit and checklist developed by the 

state board. The state board, in coordination with the districts, shall provide a 

copy of the audit and checklist to small businesses within those industries to 

assist them to meet the requirements of this chapter. 

(e) The audit and plan shall contain all the information required by Section 44392. 

(f) The plan shall be submitted to the district, within six months of a district's

determination of significant risk, for review of completeness. Operators of facilities 

that have been notified prior to January 1, 1993, that there is a significant risk 

associated with emissions from the facility shall submit the plan by July 1, 1993. The 

district's review of completeness shall include a substantive analysis of the emission 

reduction measures included in the plan, and the ability of those measures to achieve 

Appendix B - 16 
emission reduction goals as quickly as feasible as provided in subdivisions (a) and 

(b). 

(g) The district shall find the audit and plan to be satisfactory within three months if it 

meets the requirements of this chapter, including, but not limited to, subdivision (f). 

If the district determines that the audit and plan does not meet those requirements, 

the district shall remand the audit and plan to the facility specifying the deficiencies 

identified by the district. A facility operator shall submit a revised audit and plan 

addressing the deficiencies identified by the district within 90 days of receipt of a 

deficiency notice. 

(h) Progress on the emission reductions achieved by the plan shall be reported to the 

district in emissions inventory updates. Emissions inventory updates shall be 

prepared as required by the audit and plan found to be satisfactory by the district 

pursuant to subdivision (g). 

(i) If new information becomes available after the initial risk reduction audit and plan, 

on air toxics risks posed by a facility, or emission reduction technologies that may be 

used by a facility that would significantly impact risks to exposed persons, the 

district may require the plan to be updated and resubmitted to the district. 

(j) This section does not authorize the emission of a toxic air contaminant in violation 

of an airborne toxic control measure adopted pursuant to Chapter 3.5 (commencing 

with Section 39650) or in violation of Section 41700. (Amended by Stats. 1993, 

Ch. 1041, Sec. 2. Effective January 1, 1994.) 

44392. A facility operator subject to this chapter shall conduct an airborne toxic risk reduction 

audit and develop a plan which shall include at a minimum all of the following: 

(a) The name and location of the facility. 

(b) The SIC code for the facility. 

(c) The chemical name and the generic classification of the chemical. 

(d) An evaluation of the ATRRM's available to the operator. 

(e) The specification of, and rationale for, the ATRRMs that will be implemented by the 

operator. The audit and plan shall document the rationale for rejecting ATRRMs 

that are identified as infeasible or too costly. 

(f) A schedule for implementing the ATRRMs. The schedule shall meet the time 

requirements of subdivision (a) of Section 44391 or the time period for 

implementing the plan set by the district pursuant to subdivision (b) or (c) of Section 

44391, whichever is applicable. 

(g) The audit and plan shall be reviewed and certified as meeting this chapter by an 

engineer who is registered as a professional engineer pursuant to Section 6762 of the 

Business and Professions Code, by an individual who is responsible for the processes 

and operations of the site, or by an environmental assessor registered pursuant to 

Section 25570.3. (Added by Stats. 1992, Ch. 1162, Sec. 3. Effective 

January 1, 1993.) 

44393. The plan prepared pursuant to Section 44391 shall not be considered to be the equivalent 

of a pollution prevention program or a source reduction program, except insofar as the audit and 

plan elements are consistent with source reduction, as defined in Section 25244.14, or 

Appendix B - 17 
subsequent statutory definitions of pollution prevention. (Added by Stats. 1992, Ch. 1162, Sec. 

3. Effective January 1, 1993.) 

44394. Any facility operator who does not submit a complete airborne toxic risk reduction audit 

and plan or fails to implement the measures set forth in the plan as set forth in this chapter is 

subject to the civil penalty specified in subdivision (a) of Section 44381, and any facility 

operator who, in connection with the audit or plan, knowingly submits any false statement or 

representation is subject to the civil penalty specified in subdivision (b) of Section 44381. 

(Added by Stats. 1992, Ch. 1162, Sec. 3. Effective January 1, 1993.) 

Appendix B - 18 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

Appendix C

Asbestos Quantity Conversion Factors
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

C-1

Appendix C

Asbestos Quantity Conversion Factors

A. “PCM” versus “TEM”

Two main analytical methods have been used for the analysis of asbestos samples: phase 

contrast microscopy (PCM), the primary method used historically to analyze asbestos samples, 

and transmission electron microscopy (TEM), the current state-of-the-art method.

PCM analysis has been preferred in the past over TEM because it can be done more 

quickly and it is less expensive. One major limitation of PCM analysis, however, especially in 

outdoor environments, is that the analyst cannot distinguish asbestos from non-asbestos fibers, 

such as cellulose, talc, or gypsum. Also, PCM cannot detect fibers that have a diameter of about 

0.3 microns or less, which could substantially underestimate the asbestos fiber concentrations. 

These limitations make PCM impractical for the analysis of ambient asbestos samples.

Transmission electron microscopy (TEM) is the preferred analytical method for outdoor 

asbestos samples because of its ability to detect small fibers (greater than or equal to 

0.0002 microns in diameter) and to distinguish between asbestos fibers and non-asbestos fibers. 

The term “TEM structures” is often used to describe asbestos fibers detected by this method. 

TEM is the method recommended by the Office of Environmental Health Hazard Assessment 

(OEHHA). TEM measurements cannot be directly related to the risk potency factors, however, 

because the studies upon which OEHHA’s risk assessment was based used the less expensive 

PCM analysis. The TEM measurements must be converted to PCM-equivalent units, using the 

following equation (ARB, 1990):

B. Asbestos Inhalation Cancer Potency Factor

The unit risk factor for asbestos fibers is 1.9 x 10-4 in units of (100 PCM fibers/m3)

-1

 and 

the unit risk factor is 6.3 x 10-2 in units of (μg/m3)

-1

. The unit risk factor is based on 

epidemiological studies in which PCM fiber measurements were used. These unit risk factors are 

listed in Chapter 7 and in the Asbestos Toxic Air Contaminant (TAC) identification document 

(CDHS, 1986) and in OEHHA, 1999b. These asbestos cancer potency factors are for 

mesothelioma. Since these cancer potency factors are in units of concentration or dose, 

complications arise when the emitted asbestos quantities are reported in mass units (pounds/year 

and maximum pounds/hour) for the Air Toxics Hot Spots Program (Hot Spots). 

1PCM fiber = 320TEM structures
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

C-2

The TAC inhalation cancer potency factor has been converted from mass to concentration 

using a factor of 0.003 μg asbestos = 100 asbestos PCM fibers. This conversion has been derived 

from information published by the United States Environmental Protection Agency (U.S. EPA) 

(U.S. EPA, 1986). The number of asbestos PCM fibers associated with a given mass of asbestos 

can vary appreciably. Also, U.S. EPA has stated that this conversion factor is the geometric mean 

of measured relationships between optical fiber counts and mass airborne chrysotile in several 

published studies, that the range of the conversion factor between the different studies is large 

(0.0005 - 0.015 μg asbestos/100 asbestos PCM fibers), and that the factor carries with it an 

appreciable uncertainty. 

The current recommendation for Hot Spots risk assessments uses a default breathing rate 

of 393 L/day-kg body weight for a 70 year exposure duration. A dose is calculated from the 

ground level concentration using the following equation:

X (μg/m3) x 393 L/day-kg body weight x 10-6 = dose (mg/kg-day)

The 10-6 term converts the L in the breathing rate to m3 and the μg in the air concentration term to 

mg. 

In order to obtain cancer risk the dose is subsequently multiplied times the cancer potency 

factor as follows:

Dose (mg/kg-body weight) x cancer potency factor (mg/kg-body weight) = Cancer risk (unitless)

For risk communication purposes cancer risk may be converted into chances per million of 

developing cancer. This terminology is often more clearly understood by the public than cancer 

risk. 

Cancer risk x (1 x 106) = chances per million of developing cancer

The cancer potency factor (mg/kg body weight)-1 may be calculated from the fiber cancer potency 

factor using the relationship of 0.003 μg = 100 fibers PCM, 70 kg body weight, 20 m3 breathed 

per day, and a factor of 1000 to convert μg asbestos into mg: 

( ) ( )

2 1

3

1 4 3

2.2 10 /

0.003 /100

1000

20

70 1.9 10 100 /

−

− + −

× × × = × mg kg body weight

m g fibers

kg PCM fibers m

m

The ISCST3 air dispersion modeling program estimates concentrations in units of μg/m3

based on emission estimates in lb/yr. If the ground level concentrations are derived from PCM 

fiber measurements, then no additional uncertainty is introduced by the conversion to μg using the 

factor of 0.003. This is because the factor is effectively cancelled out by its use to derive the 

cancer potency factor in (mg/kg body weight)-1. There is a slight rounding error that may be 

introduced. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

C-3

References

ARB, 1990. Proposed Control Measure for Asbestos-Containing Serpentine Rock in Surfacing 

Applications, Technical Support Document, Air Resources Board, February 1990.

CDHS, (1986) California Department of Health Services (CDHS) 1986. Report to the Air 

Resources Board on Asbestos. Part B. Health Effects of Asbestos. Epidemiological Studies 

Section, Berkeley, CA.

OEHHA. (1999b). Air Toxics Hot Spots Program Risk Assessment Guidelines. Part II. Technical 

Support Document for Describing Available Cancer Potency Factors. Available online at 

http://www.oehha.ca.gov

 

USEPA, 1986. Airborne Asbestos Health Assessment Update. EPA/600/8-84/003F, Office of 

Health and Environmental Assessment, Washington, DC.
Appendix D 

Risk Assessment Procedures to Evaluate 

Particulate Emissions from Diesel-Fueled Engines 

Appendix D - 1 
Appendix D 

Risk Assessment Procedures to Evaluate 

Particulate Emissions from Diesel-Fueled Engines

A. Introduction

The objective of this appendix is to discuss procedures for estimating potential cancer 

and noncancer health risk from exposure to particulate matter (PM) emissions from diesel-fueled 

engines (diesel exhaust). It will also clarify the requirements and recommendations for acute 

noncancer and multipathway cancer and chronic risk assessment for diesel PM. In addition to 

the notification and risk reduction requirements under the Hot Spots Program, this appendix 

should facilitate the use of the Risk Reduction Plan to Reduce Particulate Matter Emissions from 

Diesel-Fueled Engines and Vehicles (ARB, 2000) (Diesel Guidelines). The Diesel Guidelines 

were developed by the Air Resources Board (ARB) with assistance from the Office of 

Environmental Health Hazard Assessment (OEHHA) in October 2000. The Diesel Guidelines 

are intended to assist local Air Pollution Control and Air Quality Management Districts 

(Districts) and sources of diesel PM emissions in making consistent risk management decisions. 

In advance of performing a health risk assessment (HRA), it is recommended that the 

District and the stationary source of diesel emissions reach a consensus on the HRA approach for 

estimating health impacts from diesel exhaust. See Chapter 9 for an outline of a modeling 

protocol. 

B. Calculations/Risk Assessment Procedures 

In August 1998, the ARB identified diesel exhaust as a toxic air contaminant (TAC) 

(ARB, 1998). In the identification report, OEHHA provided an inhalation noncancer chronic 

reference exposure level (REL) of 5 micrograms per cubic meter (μg/m3

) and a range of 

inhalation cancer potency factors of 1.3 x 10-4 to 2.4 x 10-3 (μg/m3

)

-1. The Scientific Review 

Panel on Toxic Air Contaminants recommended a “reasonable estimate” inhalation unit risk 

factor of 3.0 x 10-4 (μg/m3

)

-1. From the unit risk factor an inhalation cancer potency factor of 1.1 

(mg/kg-day)-1

 may be calculated. These noncancer and cancer health factors were developed 

based on whole (gas and particulate matter) diesel exhaust. The surrogate for whole diesel 

exhaust is diesel PM. PM10 (particulate matter, ten microns or less in size) is the basis for the 

potential risk calculations. 

Cancer

When conducting an HRA, the potential cancer risk from inhalation exposure to diesel 

PM will outweigh the potential noncancer health impacts. Therefore, inhalation cancer risk is 

required for every HRA. (The methods for calculating inhalation cancer risk can be found in 

Chapters 5, 7, and 8.) When comparing whole diesel exhaust to speciated diesel exhaust 

(e.g., PAHs, metals), potential cancer risk from inhalation exposure to whole diesel exhaust will 

Appendix D - 2 
outweigh the multipathway cancer risk from the speciated components. For this reason, there 

will be few situations where an analysis of multipathway risk is necessary. 

The District may elect to require a multipathway analysis if reliable data are available and 

the District decides that it is necessary. If the District elects to require a multipathway analysis, 

the components of the diesel exhaust will need to be speciated since there is not an oral cancer 

potency factor for diesel PM. It is recommended that the District be consulted on the procedures 

for conducting a multipathway analysis for diesel exhaust. The District may wish to use 

speciation data from the ARB. If so, a resource for speciation data is available on the ARB’s 

website at www.arb.ca.gov/emisinv/speciate/speciate.htm. 

If a multipathway analysis is required, the speciated data should be compared with the 

substances in Table 5.1. Any substances in the speciation profile that are listed in Table 5.1 and 

have an oral cancer potency factor in Table 7.1 should be included in the multipathway analysis. 

Potential multipathway cancer risks are estimated following the procedures in Chapters 5 and 8 

of this document. These procedures require summing the potential cancer risk from each 

carcinogen to estimate the total facility cancer risk. 

Noncancer Chronic

To determine noncancer chronic inhalation health impacts from exposure to diesel 

exhaust use the methods described in Chapters 6 and 8. 

In most situations, noncancer health impacts from inhalation exposure to whole diesel 

exhaust will outweigh the noncancer multipathway health impacts to the speciated components 

of diesel exhaust. However, there may be situations when the multipathway impacts need to be 

investigated. 

Therefore, the District may elect to require a multipathway analysis if reliable data is

available and they feel it is necessary. If the District elects to require a multipathway analysis, 

the components of the diesel exhaust will need to be speciated since there is not an oral reference

exposure level for diesel PM. A resource for speciation data at the ARB is identified above. It is 

recommended that the District be consulted on the procedures for conducting a multipathway 

analysis. If a multipathway analysis is required, the speciated data should be compared with the 

substances in Table 5.1. Any substances in the speciation profile that are listed in Table 5.1 and 

have an oral chronic REL in Table 6.3 should be included in the multipathway analysis. 

Potential multipathway chronic risks are estimated following the procedures in Chapters 5 and 8 

of this document. 

Noncancer Acute

As stated above, potential cancer risk is usually the driving health impact for diesel 

exhaust. However, there may be certain unusual situations where an evaluation of the acute 

health effects may be warranted. One possible situation is when a nearby receptor is located 

above the emission release point (e.g. on a hillside or in a multistory apartment building). Since 

there is no acute REL for diesel exhaust, the components of the exhaust will need to be speciated

to determine the potential acute health impacts. It is recommended that the District be consulted 

on the procedures for conducting an acute analysis. If an acute analysis is required, the speciated 

Appendix D - 3 
data should be compared with the substances in Table 6.1. Any substances in the speciation 

profile that are listed in Table 6.1 should be included in the acute analysis. A resource for 

speciation data at the ARB is identified above. Potential acute risks are estimated following the 

procedures in Chapters 6 and 8 of this document. 

References: 

ARB 1998. Air Resources Board, “Proposed Identification of Diesel Exhaust as a Toxic Air 

Contaminant, Appendix III, Part A, Exposure Assessment,” April 1998. 

ARB 2000. Air Resources Board, “Risk Reduction Plan to Reduce Particulate Matter Emissions 

from Diesel-Fueled Engines and Vehicles,” October 2000.

Appendix D - 4 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

Appendix E

Toxicity Equivalency Factors for

Polychlorinated Dibenzo-p-Dioxins Dibenzofurans

And Polychlorinated Biphenyls
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

E - 1

Appendix E

Toxicity Equivalency Factors for

Polychlorinated Dibenzo-p-Dioxins Dibenzofurans and Polychlorinated Biphenyls

Introduction

Dioxins and furans vary considerably in their potency for causing both cancer and noncancer 

health impacts. A facility may choose to speciate dioxin and furan emissions in order to obtain a more 

accurate picture of the risks. A scheme, based on both cancer and noncancer toxicity studies, has been 

developed to relate the potency of various dioxin and furan congeners to the potency of 2,3,7,8-

tetrachlorodibenzo-p-dioxin. A detailed explanation of the World Health Organization’s 1997 Toxicity 

Equivalents Factor (WHO97-TEF) (van den Berg, 1998) scheme, the latest scheme adopted by 

OEHHA, is available in OEHHA (2003). 

The individually calculated inhalation or oral doses of each dioxin or furan congener may be 

multiplied times the oral or inhalation cancer potency for each individual congener listed in Table 7.1. 

In order to determine the inhalation chronic hazard index, the ground level concentration of each 

congener may be divided by the chronic REL for each congener in Table 6.2 and the hazard quotients 

may be summed to give the hazard index for dioxins and furans. The oral chronic hazard quotient may 

be calculated by determining the oral dose of each congener and dividing by the individual chronic oral 

REL for each congener. The oral hazard quotients may be summed to give the hazard quotient for oral 

noncancer dioxin risks and may then be added to the inhalation hazard index to give the combined 

inhalation and oral chronic hazard quotient for dioxins. 

A second equivalent procedure may also be used to calculate the cancer risk of a mixture of 

dioxin and furan congeners. The concentration of each congener listed in Table E-1 is multiplied by the 

WHO97-TEF for that congener. For example, for 1,2,3,4,7,8-hexachlorodibenzodioxin the 

concentration (μg/m3) may be multiplied by 0.1 to give the concentration equivalent to 2,3,7,8-

tetrachlorodibenzo-p-dioxin. 2,4,7,8-tetrachlorodibenzodioxin would be multiplied by zero indicating 

no cancer or noncancer toxicity. The 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalent concentrations 

may be summed and treated as if the total concentration were 2,3,7,8-tetrachlorodibenzo-p-dioxin for 

the purposes of calculating cancer and noncancer risks. Thus, the potency adjusted ground level 

concentration can be multiplied by the breathing rate to give dose (see equation 5.4.1), and then 

multiplied times the cancer potency factor for 2,3,7,8- tetrachlorodibenzo-p-dioxin (Table 7.1) to give 

cancer risk for the entire mixture. If a noncancer chronic hazard index needs to be calculated the 

potency adjusted ground level concentration can be divided by the chronic reference exposure level for 

2,3,7,8-tetrachlorodibenzo-p-dioxin to give a hazard index for the entire mixture. The TEF may be 

multiplied times the individual congener dose calculated for the inhalation and oral cancer risk 

calculation, and the oral chronic hazard index determination. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

E - 2

The most recent TEF scheme adopted by OEHHA includes TEF for individual PCB congeners 

(see Table E-1) (OEHHA, 2003). These are the congeners that have dioxin-like biological effects. 

The same procedures as described above may be used to calculate the concentration or dose of these 

congeners. Where data are available on individual PCB congeners emitted by a facility, then these 

TEFs are to be used. If Table E1 is used to adjust the dose or concentration of the individual PCB 

congeners, the 2,3,7,8-tetrachlorodibenzo-p-dioxin oral and inhalation cancer potency factors should 

be used to determine cancer risk. The chronic inhalation and oral REL for 2,3,7,8-

tetrachlorodibenzo-p-dioxin should be used to determine the noncancer chronic hazard index. If only 

total PCB data are available, then the PCB slope factors provided in Table 7.1 can be used for cancer 

risk determination. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

E - 3

Table E1. WHO/97 Toxic equivalency factors (TEFs)

Congener TEFWHO-97

PCDDs

2,3,7,8-TCDD 1

1,2,3,7,8-PeCDD 1

1,2,3,4,7,8-HxCDD 0.1

1,2,3,7,8,9-HxCDD 0.1

1,2,3,6,7,8-HxCDD 0.1

1,2,3,4,6,7,8-HpCDD 0.01

1,2,3,4,6,7,8,9-OCDD 0.0001

PCDFs

2,3,7,8-TCDF 0.1

1,2,3,7,8-PeCDF 0.05

2,3,4,7,8-PeCDF 0.5

1,2,3,4,7,8-HxCDF 0.1

1,2,3,7,8,9-HxCDF 0.1

1,2,3,6,7,8-HxCDF 0.1

2,3,4,6,7,8-HxCDF 0.1

1,2,3,4,6,7,8-HpCDF 0.01

1,2,3,4,7,8,9-HpCDF 0.01

1,2,3,4,6,7,8,9-OCDF 0.0001

PCBs (IUPAC #, Structure)

77 3,3',4,4'-TCB 0.0001

81 3,4,4',5-TCB 0.0001

105 2,3,3',4,4'-PeCB 0.0001

114 2,3,4,4',5-PeCB 0.0005

118 2,3',4,4',5-PeCB 0.0001

123 2',3,4,4',5-PeCB 0.0001

126 3,3',4,4',5-PeCB 0.1

156 2,3,3',4,4',5-HxCB 0.0005

157 2,3,3',4,4',5'-HxCB 0.0005

167 2,3',4,4',5,5'-HxCB 0.00001

169 3,3',4,4',5,5'-HxCB 0.01

189 2,3,3',4,4',5,5'-HpCB 0.0001
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

E - 4

References

OEHHA, 2003. Technical Support Document for Describing Available Cancer Potency Factors, 

Appendix A (revised August, 2003) : Use of the Revised Toxicity Equivalency Factor (TEFWHO-97) 

Scheme for Estimating Toxicity of Mixtures of Dioxin-Like Chemicals, September 2003. Available at 

www.oehha.ca.gov.

van den Berg, M., Birnbaum, L., Bosveld, A. T. C., Brunstrom, B., Cook, P., Feeley, M., Giesy, J. P., 

Hanberg, A., Hasegawa, R., Kennedy, S. W., Kubiak, T., Larsen, J. C., Van Leeuwen, F. X. R., 

Liem, A. K. D., Nolt, C., Peterson, R. E., Poellinger, L., Safe, S., Schrenk, D., Tillitt, D., Tysklind, M., 

Younes, M., Waern, F., and Zacharewski, T. (1998). Toxic equivalency factors (TEFs) for PCBs, 

PCDDs, PCDFs for humans and wildlife. Environ. Health Perspec. V106, 775-792.
Appendix F 

Overview of the Lead Risk Assessment Procedures 

F - 1 
Appendix F

Overview of the Lead Risk Assessment Procedures 

I. Introduction

The objective of this appendix is to provide a method for estimating potential cancer and 

noncancer health effects due to airborne lead exposure. This appendix should facilitate the use 

of the Risk Management Guidelines for New, Modified, and Existing Sources of Lead (Lead RM 

Guidelines) (ARB, 2001) for analysis of lead exposure. The Lead RM Guidelines were 

developed by the Air Resources Board (ARB) with assistance from Office of Environmental 

Health Hazard Assessment (OEHHA) and Department of Health Services (DHS) in March 2001 

to assist local air districts and sources of lead in making consistent risk management decisions 

for new, modified, and existing sources of lead. 

In April 1997, the ARB identified inorganic lead as a toxic air contaminant (TAC) (ARB, 

1997). Lead is unique among other TACs identified by ARB in several ways. First, infants and 

children are particularly susceptible to the health effects of lead, and the risk assessment is based 

on health effects in children. Second, the chronic noncancer effects are related to blood lead 

levels (BLLs) as opposed to ambient air concentrations. These BLLs reflect current and past 

exposure from a number of sources; air emissions may only be a small part of the total exposure. 

Third, based on recommendations of the OEHHA and the Scientific Review Panel on Toxic Air 

Contaminants (SRP), the ARB did not identify a threshold level for chronic noncancer health 

effects due to lead exposure. Threshold levels are levels below which no adverse health effects 

are expected to occur. Since acute or chronic Reference Exposure Levels (RELs) are based on 

threshold levels, none were developed for lead. Thus, a hazard index approach is not used for 

lead. Instead, air concentrations are compared to defined air lead levels associated with specified

percentages of children with BLL ≥ 10 μg/dL. Acceptable risk is based on minimizing the 

number of children at or above a BLL of 10 μg/dL. 

II. Methods for Estimation of Health Risk Effects 

Methods for estimating site-specific noncancer and cancer potential health impacts from

exposure to lead emissions are given in the Lead RM Guidelines. The noncancer health effects 

pose greater public health significance than the cancer health effects. Minimizing noncancer 

health effects of lead will therefore also minimize cancer health effects. 

Chronic noncancer health risks are estimated based on neurodevelopmental health risks 

to children and would also be protective of adults. These health effects can be evaluated using a 

tiered approach based on blood lead level distribution in the population. 

Potential multipathway cancer risks are estimated following the procedures in Chapters 5 

and 8 of this document. These procedures require summing individual cancer risk from each 

carcinogen to estimate the total facility cancer risk. 

F - 2 
In advance of performing a health risk assessment (HRA), it is recommended that the Air 

Pollution Control or Air Quality Management District (District) and the stationary source of lead 

air emissions reach a consensus on the HRA approach for estimating chronic noncancer and 

cancer health risks. See Chapter 9 for an outline of a modeling protocol. 

A. Tiered Approach for Estimating Noncancer Risks due to Lead Exposure 

The Lead Risk Management Guidelines provide three tiers of analysis to determine 

baseline BLL distributions for estimating risk. Although there is a simple risk management 

option provided in the Lead RM Guidelines, in a risk assessment for the Air Toxics Hot Spots 

program one of the following tiers must be used to report estimates of the percent of children 

estimated to be above 10 μg/dL blood lead. The tiered approach is based on an assessment of 

neurodevelopmental risk, with the BLL distribution in the population as the most significant 

factor. The BLL distribution consists of two components: 1) the baseline BLL distribution due 

to all sources of exposure; and 2) the exposure due to emissions from a facility. 

 

Tier I is a default approach that requires minimal site-specific information on 

concentrations of lead in environmental media other than air. Tier I uses two default BLL 

distributions, one for a high exposure scenario and one for an average exposure scenario. The 

exposure scenario is determined using the median age of the homes in the census tract and the 

ratio of area income to the poverty level. The default baseline BLL distribution for each of the 

exposure scenarios is based on a review of neighborhood and community blood lead studies. 

The assessor determines the 30-day average lead concentration due to the facility averaged over 

the 1 square kilometer area centered on the Maximum Offsite Concentration (MOC). The 

percentage of children with BLLs greater than or equal to 10 micrograms per deciliter (≥ 10 

μg/dL) is determined using Table F-1 (also found on page 17 in the Lead RM Guidelines), the air 

lead concentration, and the determined exposure scenario. The 10 μg/dL threshold level has 

been identified by the Centers for Disease Control and Prevention (CDC) as a level where 

potential health effects may occur. The public health goal of management practices should be to 

implement procedures/practices to prevent BLLs at or above this level. The estimated percentage 

of children with BLLs ≥ 10 μg/dL is then used with risk management levels given in Chapter III, 

Section D of the Lead RM Guidelines to assist in making risk management decisions. 

F - 3 
Table F-1 Percentage of Children with Blood Lead Levels ≥ 10 μg/dl for Various Air 

Lead Concentrations at Two Exposure Scenarios 

Air Lead Concentration in the Percent ≥ 10 μg/dL 

Maximum Exposure Area 

(30-day average) 

[μg/m3

] High Exposure 

Scenario

Average Exposure 

Scenario

Baseline* 5.1 1.2

0.02 5.4 1.4

0.06 6.1 1.7

0.10 6.8 2.2

0.20 8.9 3.4

0.25 9.8 4.1

0.50 15.9 8.9

0.75 22.4 15.4

1.0 29.1 23.0

1.5 42.5 39.0

* The baseline represents BLLs due to lead in soil, dust, water, food, and background air lead concentrations. 

Tier II requires the development of site-specific baseline BLL distributions within the 

impacted population using site-specific estimates of lead levels in environmental media, 

including soil, dust, water, and/or food, using the U.S. EPA Integrated Exposure Uptake 

Biokinetic (IEUBK) model. The IEUBK model calculates the probability of an individual 

exceeding a specific BLL based on site-specific information. The aggregate of the individual 

BLLs is used to estimate the neurodevelopmental risk in the maximum exposure area. A detailed 

discussion of this tier is beyond the scope of this overview; see Appendix D in the Lead RM 

Guidelines for a discussion of the IEUBK model and its use. 

 

Tier III involves actual blood lead sampling of the population impacted by the facility to 

define the baseline BLLs. In Tier III, the facility is responsible for conducting BLL testing to 

establish a site-specific BLL distribution. The Lead RM Guidelines recommend the 

neurodevelopmental risk be calculated as the probability of children in an affected exposure area 

having a BLL ≥ 10 μg/dL using the results of the blood lead sampling. It is highly unlikely that 

this option would be used due to the cost incurred and the fact that the sampled population must 

consent to the sampling and an appropriate sampling strategy must be developed to adequately 

characterize the blood lead levels of the impacted population. 

For further information on the tiered approach using the Tier I, Tier II, or Tier III, please

see Chapter II of the ARB Risk Management Guidelines for New, Modified, and Existing Sources 

of Lead (ARB, 2001). This document can be downloaded from the ARB web site at 

http://www.arb.ca.gov/toxics/lead/lead.htm or can be requested by calling (916) 323-4327. 

F - 4 
B. Methods for Estimating Potential Cancer Risks due to Lead 

While lead has a unique noncancer assessment methodology, the determination of 

potential multipathway cancer risk is the same as other carcinogens. Chapters 5, 7, and 8, and 

Appendices I and L provide all the needed information for calculating potential cancer risk. The 

health risk assessment should report the multipathway cancer risks from lead emissions. 

Chapter III in the Lead RM Guidelines provides methods for determining risk 

management of lead exposure, using the results from the cancer risk calculation, and the local 

District’s defined significance levels. 

III. References

ARB, 1997. Proposed Identification Inorganic Lead as a Toxic Air Contaminant, Parts 

A, B, C. California Air Resources Board. April, 1997. 

ARB, 2001. ARB Risk Management Guidelines for New, Modified, and Existing 

Sources of Lead. California Air Resources Board. March 2001 

F - 5 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

APPENDIX G

PAH Potency Factors and Selection of 

Potency Equivalency Factors (PEF) for PAHs 

Based on Benzo[A]Pyrene Potency
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix G

PAH Potency Factors and Selection of Potency Equivalency Factors (PEF)

for PAHs based on Benzo(a)pyrene Potency

Benzo(a)pyrene (BaP) was chosen as the primary representative of the class of 

polycyclic aromatic hydrocarbons (PAHs) because of (1) the large amount of 

toxicological data available on BaP (versus the relatively incomplete database for other 

PAHs), (2) the availability of monitoring techniques for BaP, and (3) the significant 

exposure expected (and found). The Office of Environmental Health Hazard Assessment 

(OEHHA) has developed a Potency Equivalency Factor (PEF) procedure to assess the 

relative potencies of PAHs and PAH derivatives as a group. This procedure can address 

the impact of carcinogenic PAHs in ambient air since they are usually present together. 

This procedure was approved by the Scientific Review Panel (SRP) on Toxic Air 

Contaminants (TAC) as part of the Health Effects Assessment of Benzo(a)pyrene during 

the TAC identification process (OEHHA, 1993).

Due to the variety of data available on the carcinogenicity and mutagenicity of 

PAHs, an order of preference for the use of available data in assessing relative potency 

was developed. If a health effects evaluation and quantitative risk assessment leading to 

a cancer potency value had been conducted on a specific PAH, then those values were 

given the highest preference. Cancer potency values for PAHs developed by this process 

are shown in Table G-1.

Table G-1: Potencies of PAHs and derivatives1

Chemicals Cancer potency factors

(mg/kg-day)-1

Unit risks

(μg/m3)

-1

benzo[a]pyrene 11.5 1.1 × 10-3

dibenz[a,h]anthracene 4.1 1.2 × 10-4

7,12-dimethylbenzanthracene 250 7.1 × 10-2

3-methylcholanthrene 22 6.3 × 10-3

5-nitroacenaphthene 0.13 3.7 × 10-5

1. Source: (OEHHA 1993; Collins et al., 1998). It is assumed that unit risks for inhalation have the same 

relative activities as cancer potencies for oral intake.

If potency values have not been developed for specific compounds, a carcinogenic 

activity relative to BaP, rather than a true potency, can be developed. These relative 

activity values are referred to as Potency Equivalency Factors or PEFs. For air 

contaminants, the relative potency to BaP based on data from inhalation studies would be 

optimal. Otherwise, intrapulmonary or intratracheal administration studies would be 

most relevant, since such studies are in the target organ of interest. Next in order of 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

G - 1

preference is information on activity by the oral route and skin painting. Intraperitoneal 

and subcutaneous administration rank at the bottom of the in vivo tests considered useful 

for PEF development because of their lack of relevance to environmental exposures. 

Next, in decreasing order of preference, are genotoxicity data, which exist for a large 

number of compounds. In many cases genotoxicity information is restricted to 

mutagenicity data. Finally, there are data on structure-activity relationships among PAH 

compounds. Structure-activity considerations may help identify a PAH as carcinogenic, 

but at this time have not been established as predictors of carcinogenic potency.

Using this order of preference, PEFs were derived for 21 PAHs and are presented in 

Table G-2 (OEHHA, 1993: Collins et al., 1998).

Table G-2. OEHHA PEF weighting scheme for PAHs and their resulting cancer 

potency values.

PAH or derivative PEF Unit Risk

(mg/m3)

-1

Inhalation 

Slope Factor

(mg/kg-day)-1

Oral Slope 

Factor

(mg/kg-day)-1

benzo[a]pyrene

(index compound)

1.0 1.1E-3 3.9E+0 1.2E+1

benz[a]anthracene 0.1 1.1E-4 3.9E-1 1.2E+0

benzo[b]fluoranthene 0.1 1.1E-4 3.9E-1 1.2E+0

benzo[j]fluoranthene 0.1 1.1E-4 3.9E-1 1.2E+0

benzo[k]fluoranthene 0.1 1.1E-4 3.9E-1 1.2E+0

dibenz[a,j]acridine 0.1 1.1E-4 3.9E-1 1.2E+0

dibenz[a,h]acridine 0.1 1.1E-4 3.9E-1 1.2E+0

7H-dibenzo[c,g]carbazole 1.0 1.1E-3 3.9E+0 1.2E+1

dibenzo[a,e]pyrene 1.0 1.1E-3 3.9E+0 1.2E+1

dibenzo[a,h]pyrene 10 1.1E-2 3.9E+1 1.2E+2

dibenzo[a,i]pyrene 10 1.1E-2 3.9E+1 1.2E+2

dibenzo[a,l]pyrene 10 1.1E-2 3.9E+1 1.2E+2

indeno[1,2,3-cd]pyrene 0.1 1.1E-4 3.9E-1 1.2E+0

5-methylchrysene 1.0 1.1E-3 3.9E+0 1.2E+1

1-nitropyrene 0.1 1.1E-4 3.9E-1 1.2E+0

4-nitropyrene 0.1 1.1E-4 3.9E-1 1.2E+0

1,6-dinitropyrene 10 1.1E-2 3.9E+1 1.2E+2

1,8-dinitropyrene 1.0 1.1E-3 3.9E+0 1.2E+1

6-nitrochrysene 10 1.1E-2 3.9E+1 1.2E+2

2-nitrofluorene 0.01 1.1E-5 3.9E-2 1.2E-1

chrysene 0.01 1.1E-5 3.9E-2 1.2E-1

1. Source: OEHHA (1993)

The cancer potency comparisons show that some PAHs are more potent than BaP, 

while other PAHs analyzed were less or much less potent. These comparisons indicated 

that considering all PAHs to be equivalent in potency to BaP would likely overestimate 

the cancer potency of a PAH mixture, but such an assumption would be health protective 

and likely to be helpful in a screening estimate of PAH risks (OEHHA, 1993). If one 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

G - 2

assumes that PAHs are as carcinogenic as they are genotoxic, then their hazard relative to 

BaP would be dependent on their concentration in the environment. In light of the 

limited information available on other PAHs, BaP remains an important representative or 

surrogate for this group of air pollutants.

Detailed descriptions on the criteria used for developing individual PEFs can be 

found in (OEHHA, 1999b) Currently, OEHHA is undertaking a review of all recent 

literature pertaining to the carcinogenicityand mutagenicity of PAHs. New cancer 

potency values for PAHs may be developed if an adequate health effects eva luation and 

quantitative risk assessment can be performed. Also, some current PEFs may be 

modified based on new data. Any changes to the potency values and PEFs for PAHs will 

be reflected in the HARP program when they occur. It is incumbent on the risk assessor 

to access the most recent version of the HARP program to ensure that the most up-to-date 

PAH potency values are used.

References

Collins, J.F., Brown, J.P., Alexeeff, G.V., and Salmon, A.G. 1998. Potency equivalency 

factors for some polycyclic aromatic hydrocarbons and polycyclic aromatic 

hydrocarbon derivatives. Regul. Toxicol. Pharmacol. 28:45-54.

OEHHA, 1993. Benzo[a]pyrene as a Toxic Air Contaminant. Part B. Health Effects of 

Benzo[a]pyrene. Air Toxicology and Epidemiology Section, Berkeley, CA.

OEHHA, 1999b. The Air Toxics Hot Spots Program Risk Assessment Guidelines; Part II. 

Technical Support Document for Describing Available Cancer Potency Factors, 

Office of Environmental Health Hazard Assessment, April 1999. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix H

Recommendations for Estimating Concentrations

of Longer Averaging Periods from the

Maximum One-Hour Concentration for Screening Purposes
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix H - 1

Appendix H

Recommendations for Estimating Concentrations

of Longer Averaging Periods from the

Maximum One-Hour Concentration for Screening Purposes

A. Introduction

The U.S. Environmental Protection Agency (U.S. EPA) SCREEN3 air dispersion model 

is frequently used to estimate the maximum one-hour concentration downwind due to emissions 

from a point source to assess impacts from a source. The SCREEN3 model results (or ISCST3 

with screening meteorological data), in conjunction with the U.S. EPA screening factors, are 

frequently used to estimate concentrations for longer averaging periods, such as the maximum 

annual average concentration. In addition, it is permissible to use the ISCST3 air dispersion 

model in a screening mode with identical meteorological conditions as used in the SCREEN3 

model to superimpose results from multiple sources.

This method to assess short-term and long-term impacts may be used as a first-level 

screening indicator to determine if a more refined analysis is necessary. In the event that 

representative meteorological data are not available, the screening assessment may be the only 

computer modeling method available to assess source impacts.

In California, this standard procedure will generally bias concentrations towards over 

prediction in most cases when the source is a continuous release. However, in the case when a 

source is not continuous, these screening factors may not be biased towards over prediction. In 

this case, we recommend an alternative procedure for estimating screening value concentrations 

for longer averaging periods than one-hour for intermittent releases.

B. Current Procedures

The current screening factors used to estimate longer term averages (i.e., 3-hour, 8-hour, 

24-hour, 30-day, and annual averages) from maximum one-hour concentrations in California are 

shown in Table H.1 and Figure H.1. The factors are U.S. EPA recommended values with the 

exception of the 30-day factor. The 30-day factor is an ARB recommended value (ARB, 1994). 

The maximum and minimum values are recommended limits to which one may diverge from the 

general (Rec.) case, (U.S. EPA, 1992). Diverging from the general case should only be done on 

a case by case basis with prior approval from the reviewing agency.

C. Non-Standard Averaging Periods with a Continuous Release

The following is the ARB recommendation for estimating screening concentrations for 

non-standard averaging periods that are not listed in Table H.1 or Figure H.1. Specifically, the 

recommendation is for estimating screening concentrations for 4-hour, 6-hour, and 7-hour 

averaging periods.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix H - 2

The current U.S. EPA screening factors 

applicable to standard averaging periods 

should be used for non-standard averaging 

periods. Specifically for the 4-hour, 

6-hour, and 7-hour averaging periods, we 

recommend that the 3-hour screening 

factor of (0.9± 0.1) be used. The 

following illustrates the method to 

estimate a 6-hour average concentration 

from a continuous release from a single 

point source: 

1. determine the maximum 1-hour 

concentration according to standard 

screening procedures (Cmax1-hr),

2. scale the maximum 1-hour 

concentration by (0.9±0.1), and 

3. the result is the maximum 6-hour 

concentration 

(Cmax6-hr=Cmax1-hr *(0.9±0.1)).

In the case for the 6-hour and 7-hour 

average concentration estimates, the user 

may wish to take the lower bound of 

(0.9±0.1), or 0.8. For the 4-hour average 

estimate, we recommend the user to use 

the 3-hour factor as is, 0.9.

Table H.1 Recommended Factors to Convert Maximum 1-hour Avg. Concentrations to Other 

Averaging Periods (U.S. EPA, 1992; ARB, 1994).

Averaging Time Range Typical Recommended

3 hours 0.8 - 1.0 0.9

8 hours 0.5 - 0.9 0.7

24 hours 0.2 - 0.6 0.4

30 days 0.2 - 0.3 0.3

Annual 0.06 - 0.1 0.08

Figure H.1

Screening Factors to Convert 

1-hr Max. to Longer Avg. Periods

1 1

0.9

0.6

0.3

0.1

1

0.8

0.5

0.2

0.3

0.06

1

0.9

0.7

0.4

0.3

0.08

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Averaging Period

Screening Factor

Max 1 1 0.9 0.6 0.3 0.1

Min 1 0.8 0.5 0.2 0.3 0.06

Rec 1 0.9 0.7 0.4 0.3 0.08

1-hr 3-hr 8-hr 24-hr 30-day Annual
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix H - 3

Table H.2 summarizes these recommendations for the non-standard averaging periods.

Table H.2 Recomme nded Factors to Convert Maximum 1-hour Avg. Concentrations to

Non-Standard Averaging Periods.

Averaging Time Range Typical Recommended

4 hours 0.8 - 1.0 0.9

6 hours 0.8 – 1.0 0.8

7 hours 0.8 – 1.0 0.8

D. Definitions

It is convenient to define the following terms relating to sources with respect to the duration of 

the release.

ß Continuous Release – this is a release that is continuous over the duration of a year. An 

example of this type of release would be fugitive emissions from a 24-hour per day, 7-day 

per week operation or an operation that is nearly continuous.

ß Intermittent Release – many emissions fall under this category. These are emission types 

that are not continuous over the year. Any operation that has normal business hours 

(e.g., 8 am to 6 pm) would fall into this category.

ß Systematic Release – these are intermittent releases that occur at a specific time of the day. 

As an example, these type of releases can occur when a process requires clean out at the end 

of the work day. Thereby releasing emissions only at the end of the workday systematically. 

Systematic releases are similar to intermittent releases with a shorter duration during the 

normal operating schedule.

ß Random Release – these are intermittent releases that can occur any time during the 

operating schedule. An example of this type of release would be of the type that depends on 

batch processing. For example, a brake shop may emit pollutants only when the brakes are 

cleaned which happens randomly throughout the normal business hours.

E. Screening Factors

The U.S. EPA screening factors, as shown in Table H.1, compensate for the effects of 

varying conditions of wind speed, wind direction, ambient temperature, atmospheric stability, 

and mixing height over longer averaging periods, even though it is not explicitly indicated in the 

U.S. EPA Guidance (U.S. EPA, 1992). Figure H.2 shows the variability in wind direction over a 

24-hour period. The data are averaged for two seven-day periods from data collected at 

Los Angeles International Airport (LAX). Figure H.2 was compiled for data collected in 1989 

for January 1 to January 7 and June1 through June 7, 1989. The ordinate in Figure H.2 shows 

the months of the year. Only two months are plotted. The abscissa shows the hour of the day.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix H - 4

As seen in Figure H.2, the wind direction changes throughout all hours of the day. In 

addition, the wind direction for LAX, in the overnight and early morning hours, can vary from 

January to June. During the afternoon hours of 1400 – 1600, the wind direction is similar in both 

months of January and June.

The standard U.S. EPA screening factor to estimate the maximum 24-hour concentration 

from the maximum 1-hour concentration is 0.4, as seen in Table H.1. Figure H.2 shows that for 

15 of 24 hours the wind blows from the west-northwest during June. A 24-hour screening factor 

could be 0.6 (0.6 ≈ 15hrs/24hrs) based on wind direction alone. This is consistent with the upper 

bound of the adjustment factors shown in Table H.1. Including the variability for wind speed, 

ambient temperature, and atmospheric stability could further reduce the estimated scaling factor 

of 0.6 closer towards the U.S. EPA recommended value of 0.4.

 

F. Intermittent Release

Support for the U.S. EPA screening factor is demonstrated for a continuous release 

(i.e., 24 hours per day) in the description above. It is important to be cautious when applying the 

U.S. EPA screening factors to an intermittent source for the purposes of estimating an annual 

average concentration (e.g., a business that may only emit during normal operating hours of 8 am 

to 6 pm). 

2 4 6 8 10 12 14 16 18 20 22 24

Hour of Day

2

4

6

8

10

12

Month

Figure H.2

Hourly Wind Direction - Los Angeles

January (bottom – 1) and June (top - 6)
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix H - 5

Intermittent emissions, such as those from burning barrels, testing a standby diesel 

generator, or any normal business hour operation (e.g., 8am to 6pm Monday through Friday), 

could have the effect of eliminating some of the annual variability of meteorological conditions. 

For example, emissions only during the daytime could eliminate the variability of a drainage 

flow pattern in mountainous terrain. Guidance for estimating long-term averages for a screening 

approach and intermittent emissions is not available. 

For a source located in the LAX meteorological domain, an emission pattern confined to 

the hours of 1400 to 1600 would eliminate any variability associated with the wind direction. In 

this case, estimating a 24-hour average with the U.S. EPA scaling factor of 0.4 would be 

incorrect. 

In the event the emissions are intermittent but randomly distributed throughout the day, 

the scaling factor of 0.4 may be appropriate because the natural diurnal variability of 

meteorological conditions are concurrent with emissions. An additional pro-rating of the 

concentration, when estimating a 24-hour concentration, would be required to discount due to the 

intermittent nature of the emissions. 

We recommend the following steps to estimate a screening based estimate of annual 

average concentrations from intermittent emissions.

1. Estimate the maximum one-hour concentration (c1-hr) based on the SCREEN3 model 

approach (or similar, e.g., ISCST3 with screening meteorological data) for possible 

meteorological conditions consistent with the operating conditions and the actual hourly 

emission rate. It is acceptable to estimate downwind concentrations using all meteorological 

combinations available to SCREEN3. However, it is possible to be selective for the choices 

of meteorological conditions and still be conservative. For example, daytime only emissions 

need not be evaluated for nighttime stable atmospheric conditions (Pasquill-Gifford classes A 

through D are unstable and neutral atmospheric conditions applicable during the day. 

Classes D through F are neutral and stable atmospheric conditions applicable during the 

night.)

2. Estimate the concentration for the longest averaging period applicable based on the length of 

time of the systematic or randomly distributed emissions and the factors in Table H.1. For 

example, the longest averaging period concentration that may be estimated with the 

U.S. EPA scaling factors is an 8-hour concentration (c8-hr) for emissions that are 

systematically released for 12 hours. Scaling factors between 8-hours and 12-hours are not 

available. In the case of the 8-hour concentration, the U.S. EPA screening factor of 0.7 ± 0.2 

to estimate the maximum 8-hour concentration is appropriate. 

The U.S. EPA Screening Guidance allows for deviation from the suggested conversion factor 

on a case-by-case basis. We recommend the lower end of the range for the conversion factor 

(i.e., 0.5 for the 8-hour average) when estimating an annual average concentration. This is 

because variability associated with seasonal differences in wind speed, wind direction, and 

atmospheric stability would not be addressed otherwise. As seen in Figure H.2, there are 

seasonal differences in the wind direction.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix H - 6

For example, if X is the length of time of systematic or randomly distributed emissions, the 

following scalars can apply.

ß X ≤ 2 hrs; Scalar = 1.0 to estimate a 1-hour average

ß 3 hrs ≤ X ≤ 7 hrs; Scalar = 0.8 to estimate a 3-hour average 

ß 8 hrs ≤ X ≤ 20 hrs; Scalar = 0.5 to estimate an 8-hour average (the selection of 

20 hours is arbitrary)

ß 21 hrs ≤ X ≤ 24 hrs; this may be a continuous release, use standard screening 

procedures.

3. Estimate the annual average concentration (cannual) by assuming the longer averaging period 

estimated above is persistent for the entire year. In the above example the 8-hour 

concentration is assumed to be persistent for an entire year to estimate an annual average 

concentration (i.e., the annual average concentration is assumed to be equal to the 8-hour 

concentration). 

In addition, the annual average concentration should be pro-rated over the final averaging 

period based on the pro-rated emissions (i.e., the calculation should include the fact that for 

some hours over the year, the emission rate is zero). 

For example, if Y is the number of operating hours in the year (e.g., Y = X * 365), the 

following may apply.

(cannual) = (c1-hr) (Scalar) (Y/8760hrs/yr)

4. The hourly emission rate should be calculated based on the assumed operating schedule in 

the steps above. An example for a facility operating Y hours per year follows.

(qhourly ) = (Qyearly )/(Y hrs/yr)

5. The annual average concentration (or ground level concentration GLC) can be estimated as 

follows.

GLC = (cannual) (qhourly )

 = (c1-hr)(Scalar) (Yhrs/8760hrs) (Qyearly )/(Y hrs/yr)

 = (c1-hr)(Scalar) (Qyearly )/(8760 hrs/yr)

Practically speaking, the above five steps condense down to determining three values. The first 

value is the maximum 1-hour concentration. The second value is the Scalar (either 1.0, 0.8, or 

0.5). And the third value is the hourly emission rate estimated by uniformly distributed over the 

entire year (8760 hours). The operating hours per year drops out of the calculations for an 

annual average concentration provided the emissions are based on an annual inventory 

(See step 5).

In the event that the acute averaging period is required and the emissions are based on an annual 

inventory, then the annual operating hours are required. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix H - 7

Below are four examples using the steps as outlined above. In each case, the annual average 

concentration is the desired value for use in risk assessment calculations. A fifth example is also 

included to demonstrate the need for the operating hours per year for an acute analysis when the 

inventory is provided on an annual basis. 

Example 1 - Fugitive Gasoline Station Emissions

Emissions are continuous for 24 hours per day and 365 days per year.

1. Estimate the maximum 1-hour concentration with the Screen3 model (or similar 

screening modeling approach), c1-hr.

2. Estimate the annual average concentration, cannual, with the U.S. EPA screening factor 

of 0.08. 

(cannual) = (c1-hr)(0.08)

3. The hourly emission rate, qhourly , for the annual average concentration is based on 

24 hours per day and 365 days per year (8760 hours per year).

(qhourly ) = (Qyearly )/(8760 hrs/yr)

4. The annual average concentration (or ground level concentration GLC) can be 

estimated as follows.

GLC = (cannual) (qhourly )

GLC = (c1-hr)(0.08) (Qyearly)/(8760 hrs/yr)

Example 2 - Dry Cleaner Emissions

Emissions are intermittent over the year but systematic for 10 hours per day, 5 days per 

week and 50 weeks per year.

1. Estimate the maximum 1-hour concentration with the Screen3 model (or similar 

screening modeling approach), c1-hr.

2. Estimate the maximum 8-hour average concentration, c8-hr, with the U.S. EPA 

screening factor of 0.7 ±0.2 as the longest averaging period of continuous release. 

The averaging period would need to be less than 10 hours. Use the lower range of the 

screening factor, 0.5, because the annual average is the final product and variability 

due to seasonal differences are not accounted for otherwise. 

(c8-hr) = (c1-hr)(0.5) 

3. Assume the worst-case 8-hour concentration is persistent throughout the year and 

pro-rate the concentration based on emissions over the year. For this dry cleaner, 

there are 2500 hours of operating condition emissions. Therefore the annual average 

is calculated as follows.

(cannual) = (c8-hr) (2500hrs/8760hrs) 

= (c1-hr)(0.5) (2500hrs/8760hrs) 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix H - 8

4. The hourly emission rate, qhourly , for the annual average concentration is based on 

2500 hours per year. 

(qhourly ) = (Qyearly )/(2500 hrs/yr)

5. The annual average concentration (or ground level concentration GLC) can be 

estimated as follows.

GLC = (cannual) (qhourly )

 = (c1-hr)(0.5) (2500hrs/8760hrs) (Qyearly)/(2500 hrs/yr)

 = (c1-hr)(0.5) (Qyearly)/(8760 hrs/yr)

Example 3 - Burning Barrel Emissions 

Emissions are intermittent over the year and random during daylight hours for two 

hours per burn, two burns per week, and 52 weeks per year. 

1. Estimate the maximum 1-hour concentration with the Screen3 model (or similar 

screening modeling approach), c1-hr. Meteorological combinations may be restricted 

to daytime conditions for this screening analysis. Pasquill-Gifford stability classes A, 

B, C, and D are unstable and neutral conditions for daytime conditions.

2. Estimate the maximum 8-hour average concentration, c8-hr, with the U.S. EPA 

screening factor of 0.7 ±0.2 as the longest averaging period where the emissions have 

the potential to be randomly distributed. Depending on the day of the year and 

latitude of the emissions, the daylight hours can vary. For this example, we assume 

the daylight hours can be as short as 10 hours per day to as long as 14 hours per day. 

Since the emissions are randomly distributed throughout the daylight hours, the 

longest averaging period we can scale with U.S. EPA scaling factors is a 10 hour 

average. In this case, the averaging period becomes the 8-hour average and the 

scaling factor becomes 0.7±0.2. Again since this is for an annual average, we use the 

lower end of the range, 0.5. 

(c8-hr) = (c1-hr)(0.5)

3. Assume the worst-case 8-hour concentration is persistent throughout the year and 

pro-rate the concentration based on the emissions over the year. For the burning 

barrels there are 208 hours of operating condition emissions (208 hrs = 

(2hrs/burn)(2burns/wk)(52wk/yr)). Therefore the annual average concentration is 

calculated as follows.

(cannual) = (c8-hr) (208hrs/8760hrs) 

= (c1-hr)(0.5) (208hrs/8760hrs) 

4. The hourly emission rate, qhourly , for the annual average concentration is based on 

208 hours per year. 

(qhourly ) = (Qyearly )/(208 hrs/yr)

5. The annual average concentration (or ground level concentration GLC) can be 

estimated as follows.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix H - 9

GLC = (cannual) (qhourly )

 = (c1-hr)(0.5) (208hrs/8760hrs) (Qyearly)/(208 hrs/yr)

 = (c1-hr)(0.5) (Qyearly)/(8760 hrs/yr)

Example 4 - Standby Diesel Engine Testing

Emissions are intermittent over the year and systematic for two hours per week and 

50 weeks per year. The engine testing is conducted at 2 pm on Fridays.

1. Estimate the maximum 1-hour concentration with the Screen3 model (or similar 

screening modeling approach), c1-hr. Meteorological combinations may be restricted 

to daytime conditions in this screening analysis because the engine test is conducted 

at 2 pm. Pasquill-Gifford stability classes A, B, C, and D are unstable and neutral 

conditions for daytime conditions.

2. In this case, the emission schedule is systematically fixed over a two hour period. 

Therefore, the longest averaging period which is applicable for the U.S. EPA 

screening factors is one-hour because a two-hour conversion factor is not available. 

Therefore, we assume the maximum 1-hour concentration is persistent for the entire 

year. We still prorate the concentration based on the emissions. There are 100 hours 

of engine testing per year. Therefore the annual average concentration becomes.

(cannual) = (c1-hr) (100hrs/8760hrs) 

3. The hourly emission rate, qhourly , for the annual average concentration is based on 

100 hours per year. 

(qhourly ) = (Qyearly )/(100 hrs/yr)

4. The annual average concentration (or ground level concentration GLC) can be 

estimated as follows.

GLC = (cannual) (qhourly )

 = (c1-hr) (100hrs/8760hrs) (Qyearly )/(100 hrs/yr)

 = (c1-hr) (Qyearly )/(8760 hrs/yr)

Below is an example using the steps above to estimate an acute concentration longer than a 

1-hour averaging period. This case is similar to Example 3 above with the exception of the 

averaging period.

Example 5 - Burning Barrel Emissions – Acute REL 

Emissions are intermittent over the year and random during daylight hours for two 

continuous hours per burn, two burns per week, and 52 weeks per year. The arsenic 

acute REL is for a 4-hour averaging period. The steps below are used to estimate the 

acute concentration, 4-hour REL, for arsenic.

1. Estimate the maximum 1-hour concentration with the Screen3 model (or similar 

screening modeling approach), c1-hr. Meteorological combinations may be restricted 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix H - 10

to daytime conditions for this screening analysis. Pasquill-Gifford stability classes A, 

B, C, and D are unstable and neutral conditions for daytime conditions.

2. The maximum 1-hour concentration is used as is without screening adjustment factors 

listed in Tables H.1 or H.2. The emissions are continuous through a 2-hour event 

within a 4-hour window. The adjustments in Table H.2 would only be used if the 

emissions were continuous for a 4-hour event or randomly distributed through a 

4-hour event. 

3. Assume the worst-case 1-hour concentration is persistent for the 4-hour averaging 

period and pro-rate the concentration based on the emissions over the 4-hour window. 

For the burning barrels there are 2 hours of operating condition emissions (2hrs/burn). 

Therefore the 4-hour average concentration is calculated as follows.

(c4-hr) = (c1-hr) (2hrs/4hrs) 

4. The hourly emission rate, qhourly , for the annual average concentration is based on 

208 hours per year (208 hrs = (2hrs/burn)(2burns/wk)(52wk/yr)).

(qhourly ) = (Qyearly )/(208 hrs/yr)

5. The 4-hr average concentration (or ground level concentration GLC4-hr) can be 

estimated as follows.

GLC4-hr = (c4-hr) (qhourly )

= (c1-hr) (2hrs/4hrs) (Qyearly )/(208 hrs/yr)

This step of Example 5 differs from the previous Examples because the number of 

operating hours per year does not drop out of the calculation as seen above.

The above methods were used in a recent modeling evaluation for emissions from a 

burning barrel (example 3 above) (ARB, 2002). Table H.3, below, shows results from the 

modeling evaluation. Shown in Table H.3 are the maximum annual average concentration based 

on the screening approach outlined above as well as a refined approach with site specific 

meteorological data from four locations, Alturas, Bishop, San Benito, and Escondido. As seen in 

Table H.3, the screening evaluation as described in the example overestimates the values 

calculated based on the refined analysis. This is the desired outcome of a screening approach. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix H - 11

G. Implementation

The approach outlined above has been implemented in the HARP program. Appendix J 

provides example output files from the Hot Spot Analysis and Reporting Program (HARP). The 

HARP software has been developed by a contractor through consultation with OEHHA, Air 

Resources Board (ARB), and District representatives. The HARP software is the recommended

model for calculating and presenting HRA results for the Hot Spots Program. Information on 

obtaining the HARP software can be found on the ARB’s web site at www.arb.ca.gov. Note, 

since the HARP software is a tool that uses the methods specified in this document, the software 

will be available after these guidelines have undergone public and peer review, been endorsed by

the state’s Scientific Review Panel (SRP) on Toxic Air Contaminants, and adopted by OEHHA.

References

ARB (1994). ARB memorandum dated 4/11/94 from A. Ranzieri to J. Brooks on the subject, 

“One-hour to Thirty-day Average Screening Factor.”

ARB (2002). Staff Report: Initial Statement of Reasons for the Proposed Airborne Toxic 

Control Measure to Reduce Emissions of Toxic Air Contaminants from Outdoor 

Residential Waste Burning, January 2002. California Air Resources Board.

U.S. EPA (1992). Screening Procedures for Estimating the Air Quality Impact of Stationary 

Sources, Revised, October 1992, EPA-454/R-92-019. U.S. Environmental Protection 

Agency, Research Triangle Park, NC.

Table H.3

Maximum Annual Average Concentration (c/q)

Above Ambient Conditions - Burning Barrel Emissions 

Met. City Alturas Bishop San 

Benito

Escondido SCREENING

D (m) (mg/m3

)/(g/s) (mg/m3)/(g/s) (mg/m3)/(g/s) (mg/m3)/(g/s) (mg/m3

)/(g/s)

20 44. 61. 85. 110. 590.

50 12. 16. 22. 30. 230.

100 4. 5. 7. 9. 85.

Notes: (a) Annual χ/q is based on 208 hours of emissions at 1 g/s.

(b) χ/q is the concentration in μg/m3 based on an hourly emission rate of 1 g/s.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix I

Calculation Examples for

Estimating Potential Health Impacts
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix I - 1

Appendix I

Calculation Examples for Estimating Potential Health Impacts

This appendix provides three example calculations to illustrate the procedures to estimate 

potential health impacts from a facility. The examples provided are intended to assist the risk 

assessor in understanding the steps associated with conducting the final step of risk assessment, 

risk characterization. The three examples provided in this appendix evaluate the inhalation 

cancer risk, the noncancer acute hazard quotient (HQ) and hazard index (HI), and the 

multipathway (inhalation and oral) noncancer chronic HQ and HI for seven compounds. 

Specific requirements for health risk assessment (HRA) under the Hot Spots Program are 

presented in Chapter 8. The HARP software will perform the calculations that are presented here 

and required in Chapters 8 and 9. See the ARB’s website at www.arb.ca.gov for more 

information on HARP.

A. Sample Calculation for Inhalation Cancer Health Risk Assessment

The following example illustrates the steps for calculating cancer risk at the maximum 

exposed individual resident (MEIR) using the high-end point-estimate for the inhalation 

exposure pathway. This example does not cover the steps for completing a noninhalation or 

multipathway HRA. Algorithms to estimate point-estimate and stochastic multipathway 

exposure can be found in Chapter 5. For simplicity, it is recommended that the risk assessor use 

HARP to conduct a multipathway risk assessment or stochastic risk assessment. 

Step one - Determine the annual average concentration at the MEIR and inhalation cancer 

potency factor for each emitted compound. 

The risk assessor would obtain the annual average concentrations from the air dispersion 

modeling results. This step has been completed for this example. Table I.1 presents the annual 

average concentrations at our hypothetical facility. In addition, Table I.1 also presents inhalation 

cancer potency factors for each substance, which also can be found in Chapter 7 and 

Appendix L. Note that where no inhalation cancer potency has been developed for a substance, 

the tables in this example have been annotated with dashes, since it will not be possible to 

conduct a quantitative risk assessment for these compounds. As previously stated, this example 

does not take into account multipathway effects for the compounds listed in Table I.1. It is 

recommended that the risk assessor use HARP for conducting such an analysis. 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix I - 2

Table I.1 Annual Average Concentrations at the MEIR and

Inhalation Cancer Potency Factors

Substance

Annual Average 

Concentrations

(mg/m3)

Inhalation Cancer Potency 

Factor

(mg/kg-d)-1

Ammonia -- --

Arsenic 0.0015 12

Benzene 5 0.10

Chlorine -- --

Chlorobenzene -- --

2,3,7,8-TCDD (dioxin) 0.000004 130,000

Nickel 0.02 0.91

Step two - Determine the inhalation dose for each compound. 

Once you have determined the annual average concentration for the emitted substance, 

the equation below is used to calculate the inhalation dose for each substance. This equation is 

listed in Section 5.4.1 of this document, and is also listed in the Air Toxics Hot Spots Risk 

Assessment Guidelines; Part IV; Exposure Assessment and Stochastic Analysis Technical 

Support Document (OEHHA, 2000b) (Part IV TSD).

Where:

dose-inh = Dose through inhalation (mg/kg/d)

1x10-6= Micrograms to milligrams conversion (10-3 mg/μg), liters to cubic 

meters conversion (10-3 m3/l)

Cair = Concentration in air (μg/m3)

DBR = Daily breathing rate (L/kg body weight-day or L/kg-day)

A = Inhalation absorption factor

EF = Exposure frequency (days/year)

ED = Exposure duration (years)

AT = Averaging time period over which exposure is averaged, in days 

(e.g., 25,550 days for 70 year cancer risk)

A summary of the exposure point-estimates and data distributions for use in risk 

assessment can be found in Chapter 5 of this document. For more detail on point-estimates and 

data distributions see the Part IV TSD. The recommended default values presented in Table I.2 

can be used when site-specific information is not available.

( )( )( )( )( )( )

AT

Cair DBR A EF ED 1 10 dose inh

−6

×

− =
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix I - 3

Table I.2 Recommended Default Values

Variable Recommended Default Value

EF 350 days/year

ED 9; 30; or 70 years

AT 70 years (25,550 days)

DBR (used in this example) 

30 and 70 year-exposure

271 (mean); 393 (95th percentile) L/kg 

body weight – day (For other DBRs see 

Table 5.4, Chapter 5)

A

1 (currently used for all substances 

included in the Hot Spots program)

The following equation shows the calculation for the inhalation dose of arsenic by using 

the annual average concentration for arsenic (Table I.1) and the recommended default values in 

Table I.2. Note that the high-end (95th percentile) 70-year daily breathing rate of 

393 liters/kg - day was used in this example.

This calculation would be repeated for each substance under evaluation using their 

respective annual average concentrations. For our hypothetical facility, we have calculated each 

inhalation dose for each substance. Table I.3 shows the results from our analysis.

Table I.3 Calculated Doses for Substances

Compound Calculated Dose

Ammonia --

Arsenic 5.7 x 10-7

Benzene 1.9 x 10-3

Chlorine --

Chlorobenzene --

2,3,7,8-TCDD (dioxin) 1.5 x 10-9

Nickel 7.5 x 10-6

( )

( ) ( )

days

liters

m

g

mg

years

year

days

kg day

liters

m

g

25,550

1x10

1

1x10 70 350 10.0015 393

arsenic dose inh

3 3 3

3 































































−













− =

− −

m

m

( − ) = × mg kg − day −arsenic dose inh 5.7 10 /

7
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix I - 4

Step three – Determine potential inhalation cancer risk for the MEIR.

Once you have calculated the inhalation dose, multiply the dose by the inhalation cancer 

potency factor as shown below. Use a factor of 1x106 to express cancer risk in chances per 

million.

For our hypothetical facility, the equation below shows the calculation for the inhalation 

cancer risk of arsenic. For this example, the inhalation cancer potency factor for arsenic is 

12 (mg/kg-d)-1 taken from Table I.1.

Use the substance-specific inhalation dose and inhalation cancer potency factor to 

determine the cancer risk for each compound by repeating this step. Finally, sum the individual 

substance cancer risks to give you the total facility (inhalation) cancer risk. Table I.4 shows the 

individual substance and total facility potential (inhalation) cancer risk. In this example, our 

hypothetical facility poses a (inhalation) cancer risk of 399 chances per million at the MEIR. 

Note, although not presented here, a facility emitting arsenic or dioxins should also evaluate 

cancer risk from noninhalation exposure pathways. 

Table I.4 Hypothetical Facility Inhalation Cancer Risk

Compound Cancer risk

(per million)

Ammonia --

Arsenic 6.8

Benzene 190

Chlorine --

Chlorobenzene --

2,3,7,8-TCDD (dioxin) 195

Nickel 6.8

Total Facility Inhalation 

Cancer Risk 399

While this example illustrates the steps used to calculate cancer risk using the inhalation 

dose algorithm, steps one through three can also be used to calculate noninhalation cancer risk 

and ultimately multipathway (inhalation and noninhalation pathway) cancer risk. To determine 

noninhalation cancer risk, an assessor should use the appropriate exposure pathway algorithm 

presented in Chapter 5. For example, equation 5.4.3.1.A (Chapter 5) would be used to determine 

5.7 10 12 (1 10 ) 6.8 chances per million 7 6× = 













 −

















−

×

−

mg

kg day

kg day

mg

Cancer Risk 

(chances per million) ( ) = 













 −

















−

6

Inhalation Dose Cancer Potency 1 x 10

mg

kg day

kg day

mg
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix I - 5

dose for the soil ingestion pathway. Once the assessor has determined the ingestion dose, the 

cancer risk for that pathway is calculated using the substance-specific oral slope factor. Oral 

slope factors can be found in Appendix L and Chapter 7. To calculate multipathway cancer risk, 

the cancer risks for all substances and exposure pathways are summed. See Chapter 8 for further 

discussion.

B. Sample Calculation of Noncancer Acute Hazard Indices

Risk characterization for noncancer health impacts are expressed as a hazard quotient (for 

individual substances) or a hazard index (for multiple substances). In addition, all hazard 

quotients (HQ) and hazard indices (HI) must be determined by target organ system. The 

example below illustrates the approach for calculating a noncancer acute HQ and HI at the 

MEIR. As discussed in Chapter 8, the following example is provided to assist the risk assessor 

in understanding how to calculate an acute HQ and HI. Using HARP, both the acute HQ and HI 

will be automatically calculated at each receptor. No exposure duration adjustment should be 

made for noncancer assessments. Specific requirements for risk assessment under the Hot Spots 

Program can be found in Chapters 8 and 9. 

Step one - Determine the 1-hour maximum concentrations at the MEIR and acute reference 

exposure levels (RELs) for each emitted substance.

The risk assessor would obtain the 1-hour maximum (or 4, 6, or 7-hour, if required) 

concentrations from the air dispersion modeling results. This step has been completed for this 

example. Table I.5 presents the maximum 1, 4, 6, or 7-hour concentrations, target organ 

systems, and acute RELs for seven substances. Note that where an acute REL has not been 

developed for a substance, the tables in this example have been annotated with dashes. In this

Table I.5 Concentrations, Acute RELs, and Target Organ System(s)

 for Substances at the MEIR

Substance

1, 4, 6 or 7-hour 

Maximum 

Concentration

(mg/m3)

Acute 

REL

(mg/m3)

Target Organ System(s)

Ammonia 1900 3200 Respiratory system; Eye

Arsenic 0.03 0.19 Reproductive/developmental

Benzene 20 1300 Reproductive/developmental; 

Immune system; Hematologic system

Chlorine 40 210 Respiratory system; Eye

Chlorobenzene -- -- --

2,3,7,8-TCDD (dioxin) -- -- --

Nickel 1.8 6 Respiratory system; Immune system
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix I - 6

example, chlorobenzene and 2,3,7,8-TCDD (dioxin) do not have acute REL values. The acute 

RELs and their corresponding target organ system(s) can be found in Table 6.1 (Chapter 6) and 

also in Appendix L. 

Step two - Determine the hazard quotient for each compound.

The hazard quotients for each compound are calculated by taking the acute maximum 

1, 4, 6, or 7-hour concentration and dividing by the substance-specific acute REL. The following 

equation shows how to calculate the hazard quotient for ammonia.

Quotient

Hazard

Acute

= 































 −

REL

Acute

Concentration

Maximum1, 4, 6, or 7 hr

 ⇒ 

Quotient (ammonia)

Hazard

Acute

= 

( )

( )

3

3

3200 /

1900 /

g m

g m

m

m

 = 0.6

Step three – Determine the HI for all emitted substances.

The acute HI is calculated by summing each hazard quotient for each substance by target 

organ system(s). For example, add the HQs for all substances that impact the respiratory system, 

then repeat this step for the next target organ system (e.g., reproductive/developmental system). 

This step is repeated until all target organs (for the substances emitted) are individually totaled. 

See Table 6.1 for target organ system information. Note, never add together the HQs or HIs for 

different target organ systems (e.g., do not add the impacts for the respiratory system to the 

reproductive/developmental system). Table I.6 shows individual hazard quotients for each 

substance and total hazard index. {Bob, adding benzene (6-hr) and arsenic (4-hr) below OK?]

Table I.6 Individual Hazard Quotients and Total Hazard Index

Substance Immune 

System

Reproductive/

Developmental

Hematologic 

System

Respiratory 

System Eye

Ammonia 0.6 0.6

Arsenic 0.2

Benzene 0.02 0.02 0.02

Chlorine 0.2 0.2

Chlorobenzene

2,3,7,8-TCDD 

(dioxin)

Nickel 0.3 0.3

Total Hazard 

Index 0.32 0.22 0.02 1.1 0.8
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix I - 7

In this example, an HQ of one was not equaled or exceeded for any individual substance. 

However, an HI (the sum of the hazard quotients for each target organ) of one was exceeded for 

the respiratory system. Exceeding a hazard index of one may indicate that there is the potential 

for adverse acute health impacts at this receptor location. Therefore, there is increased concern 

that exposed individuals may experience respiratory system irritation, particularly among 

sensitive individuals. The District and OEHHA should be consulted when a hazard index 

exceeds one (see Section 8.3).

C. Sample Calculation of Noncancer Chronic Hazard Indices

The example below illustrates the approach for calculating a noncancer chronic HQ and 

HI at the MEIR. An HQ expresses the noncancer health impacts for an individual substance and 

an HI expresses the potentia l impacts for multiple substances. As discussed in Chapter 8, the 

following example is provided to assist the risk assessor in understanding the calculation of a 

chronic HQ and HI. Using the HARP software, both the chronic HQ and HI will be 

automatically calculated at each receptor. No exposure duration adjustment (e.g., 9/70) should 

be made for noncancer assessments. Specific requirements for risk assessment under the Hot 

Spots Program can be found in Chapters 8 and 9.

Step one - Determine the annual average concentrations at the MEIR and inhalation and oral 

chronic RELs for each emitted substance.

The risk assessor would obtain the substance-specific annual average concentrations from 

the air dispersion modeling results. This step has been completed for this example. Table I.7 

presents the annual average concentrations, target organ systems, and chronic RELs for seven 

substances. All of the substances have a chronic REL value associated with them. In addition, 

arsenic, dioxins, and nickel are multipathway substances; therefore, oral and dermal exposure 

must be included as potential exposure pathways. The chronic RELs and their corresponding 

target organ system(s) can be found in Tables 6.2 and 6.3 (Chapter 6) and also in Appendix L.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix I - 8

Table I.7 Annual Average Concentrations, Chronic RELs, 

and Target Organ Systems for Substances at the MEIR.

Substance

Annual 

Average 

Conc.

(mg/m3)

Chronic REL

(inhalation)

(mg/m3)

Target Organ 

System(s)

(inhalation)

Chronic 

Oral REL

(mg/kg-day)

Target Organ 

System(s)

(oral/dermal)

Ammonia 160 200 Respiratory System - -

Arsenic 0.0015 0.03

Development; 

Cardiovascular System; 

Nervous System

0.0003 Cardiovascular system; 

skin

Benzene 5 60

Hematopoietic System; 

Development;

Nervous System

- -

Chlorine 0.08 0.2 Respiratory System - -

Chlorobenzene 20 1000

Alimentary System; 

Kidney; Reproductive 

System

- -

2,3,7,8-TCDD 

(dioxin) 0.000004 0.00004

Alimentary System 

(Liver); Reproductive 

System; Development; 

Endocrine System; 

Respiratory System; 

Hematopoietic System

0.00000001

(10 pg/kg-day)

Alimentary System 

(Liver); Reproductive 

System; Development; 

Endocrine System; 

Respiratory System; 

Hematopoietic System

Nickel 0.02 0.05 Respiratory System; 

Hematopoietic System 0.05 Alimentary System

Step two – Determine the inhalation chronic hazard quotient for each substance.

For inhalation exposure, the individual hazard quotients for each substance are calculated 

by taking the annual average concentration and dividing by its corresponding chronic inhalation 

REL. Using the information contained in Table I.7, the equation below is used to calculate the 

inhalation hazard quotient for arsenic.

Quotient

Hazard

Chronic

= 































 °

REL

Chronic

Concentration

Annual avg.

 ⇒ 

Quotient ( )

Hazard

Chronic

arsenic

=

( )

( )

3

3

0.03 /

0.0015 /

g m

g m

m

m

 = 0.05 

Step three – Determine the noninhalation hazard quotient for each substance.

For the substances that are subject to deposition, noninhalation (i.e., oral and dermal) 

exposure pathways need to be considered in the chronic hazard quotient evaluation. The 

point-estimates and algorithms for calculating the oral dose for all of the applicable exposure 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix I - 9

pathways and receptors (e.g., workers or residents) are explained in Chapter 5. Note, the HARP 

software uses the appropriate information and performs all the steps discussed in these examples.

As discussed in Sections 8.2.5 and 8.3, Tier-1 of the tiered approach to risk assessment 

states that the high-end point-estimates are used for the two dominant noninhalation exposure 

pathways and the non-dominant exposure pathways use the average point-estimates to determine 

the dose and chronic health impacts at a residential receptor. To determine which exposure 

pathways are the two dominant ones, high-end point-estimates are used for all applicable 

exposure pathways to see which two pathways provide the highest impacts for each substance. 

Once the two dominant noninhalation pathways are determined for each substance, the doses for 

the remaining noninhalation exposure pathway for that substance are recalculated using the 

average point-estimates. The 70-year exposure duration point-estimates are used for residential 

receptors and the worker (single) point-estimates are used for the maximum exposed worker (see 

Chapter 5). No exposure duration adjustment (e.g., 9/70) should be made for noncancer 

assessments. 

This example shows how to combine the impacts from multiple exposure pathways to 

obtain an oral (noninhalation) hazard quotient for a single substance. For each substance, the 

impacts for a specific exposure pathway are assessed by dividing the oral dose (derived from the 

annual average concentration) in milligrams per kilogram-day (mg/kg-day) by the oral chronic 

REL, expressed in units of (mg/kg-day) (Table 6.3). The next equation shows the HQ 

calculation for arsenic through the soil ingestion (SI) exposure pathway. 

Note, prior to this point in this calculation, we are assuming several steps have taken 

place. These steps include: 1) the completion of air dispersion modeling to obtain the 

ground-level annual-average air concentration; 2) identification of the existing exposure 

pathways at the receptor location; 3) calculation of the concentration in the exposure media 

(e.g., for soil - Equation 5.3.2.A); 4) determination of the dominant noninhalation exposure 

pathway(s) for the substance; and 5) the calculation of the substance-specific dose for that 

exposure pathway (e.g., Equation 5.4.3.1.A is used to calculate the dose from soil ingestion). 

See Chapter 5 for the algorithms for calculating the oral dose for all of the applicable exposure 

pathways and receptors. For this example, the calculated dose for arsenic from soil ingestion is 

assumed to be 0.000015 mg/kg-day.

Quotient

Oral Hazard

Chronic

=

















Oral REL

Chronic

SI dose

 ⇒ 

Quotient ( )

Oral Hazard

Chronic

SI arsenic

=

( )

( mg kg day )

mg kg day

−

−

0.0003 /

0.000015 /

 = 0.05

For each substance, this step is repeated for each applicable noninhalation exposure 

pathway. As illustrated below, the (total) oral HQ for a substance is calculated by summing the 

HQs for all applicable exposure pathways. In this example, the chronic oral HQ is assumed to 

equal 0.1.
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix I - 10

* Noninhalation pathways:

 SI = soil ingestion FI = fisher-caught fish

 DW = drinking water HV= homegrown vegetables

 D = dermal absorption BM= breast milk (not applicable for arsenic exposure)

 MI = meat, milk & egg

Step four – Determine the chronic HI

The chronic HI is calculated by summing each hazard quotient (inhalation and 

noninhalation) for each substance by the target organ system(s). For example, add the HQs for 

all substances that impact the respiratory system, then repeat this step for the next target organ 

Table I.8 Substance-Specific Inhalation and Noninhalation Hazard 

Quotients and the Hazard Index by Target Organ System

Substance

Respiratory

 System

Hematopoietic

 System

Alimentary

System

Endocrine

System

Development

Reproductive 

SystemKidney

Nervous Sys

tem

Cardiovascular

System

Skin

Ammonia 0.8

Arsenic 0.05(i) 0.05(i) 0.05(i)

0.1(ni)

0.1(ni)

Benzene 0.08 0.08 0.08

Chlorine 0.04

Chlorobenzene 0.02 0.02 0.02

2,3,7,8-TCDD 

(dioxin)

0.1(i)

0.2(ni)

0.1(i)

0.2(ni)

0.1(i)

0.2(ni)

0.1(i)

0.2(ni)

0.1(i)

0.2(ni)

0.1(i)

0.2(ni)

Nickel 0.4(i) 0.4(i) 0.1(ni)

Hazard Index 1.54 0.78 0.32 0.3 0.43 0.32 0.02 0.13 0.15 0.1

i = inhalation pathway contribution

ni = noninhalation pathway contribution

Chronic Oral 

Hazard 

Quotient*(arsenic)

= [HQ(SI) + HQ(D) + HQ(DW) + HQ(MI) + HQ(FI) + HQ(HV)]

Chronic Oral 

Hazard 

Quotient*(arsenic)

= 0.1 
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix I - 11

system (e.g., cardiovascular system). This step is repeated until all target organs (for the 

substances emitted) are individually totaled. See Tables 6.2 and 6.3 for target organ system 

information. Note, never add together the HQs or HIs for different target organ systems 

(e.g., do not add the impacts for the respiratory system to the cardiovascular system). No

exposure duration adjustment (e.g., 9/70) should be made for noncancer assessments. Table I.8 

shows individual hazard quotients (inhalation and noninhalation) for each substance and the 

hazard index by target organ system. In this table, arsenic is highlighted in bold to identify how 

the information calculated above is presented and used.

In this example, an HQ of one was not equaled or exceeded for any individual substance. 

However, an HI (the sum of the hazard quotients for each target organ) of one was exceeded for 

the respiratory system. Exceeding a hazard index of one may indicate that there is the potential

for adverse chronic health impacts at this receptor location. Therefore, there is increased concern 

that exposed individuals may experience respiratory system irritation or injury, particularly 

among sensitive individuals. The District and OEHHA should be consulted when a hazard index 

exceeds one (see Section 8.3).
 The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

J- 1

Appendix J

Glossary Of Acronyms and Definition of Selected Terms
 The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

J- 2

Glossary of Acronyms and Definitions of Selected Terms

Acute Exposure: One or a series of short-term exposures generally lasting less than 24 hours.

Acute Health Effects: A health effect that occurs over a relatively short period of time (e.g., minutes 

or hours). The term is used to describe brief exposures and effects which appear promptly after 

exposure.

Adverse Health Effect: A health effect from exposure to air contaminants that may range from 

relatively mild temporary conditions, such as eye or throat irritation, shortness of breath, or headaches, 

to permanent and serious conditions, such as birth defects, cancer or damage to lungs, nerves, liver, 

heart, or other organs.

AERMOD: a proposed (by U.S. EPA) steady-state, plume-based air dispersion model for estimating 

near-field impacts from a variety of industrial source types (designed to provide reasonable 

concentration estimates over a wide range of conditions with minimal discontinuities, to be easily 

implemented with reasonable input requirements and computer resource needs, to be based on up-todate science that captures the essential physical processes while remaining simple, and to be easily 

revised as the science evolves). To the extent practicable, the structure of the input or the control file 

for AERMOD is the same as that for ISCST3.

Air Dispersion Modeling: Algorithms, usually performed with a computer, that relate a mass 

emission rate, source configuration, and meteorological information to calculate ambient air 

concentrations.

Air District: The Air Pollution Control and Air Quality Management Districts are the political bodies 

responsible for managing air quality on a regional or county basis. California is currently divided into 35 

air districts.

Air monitoring: The periodic or continuous sampling and analysis of air pollutants in ambient air or 

from individual pollutant sources.

Air Toxics Hot Spots Act Emission Inventory Reports: Documents that contain information 

regarding emission sources, emitted substances, emission rates and release parameters, prepared under 

the Emission Inventory Criteria and Guidelines (also referred to as “Inventory Reports”).

Air Toxics Hot Spots Information and Assessment Act of 1987 (AB 2588): (Health and Safety 

Code, Section 44300-44394) - A state law which established the “Hot Spots” Program to develop a 

statewide inventory of site-specific air toxic emissions, to assess the risk to public health from exposure 

to these emissions, to notify the public of any significant health risks and to reduce emissions below the 

significant risk levels.

Algorithm: a set of rules for solving a problem in a finite number of steps
 The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

J- 3

California Air Resources Board (ARB): The State’s lead air quality management agency consisting 

of an eleven-member board appointed by the Governor. The ARB is responsible for attainment and 

maintenance of the state and federal air quality standards, and is fully responsible for motor vehicle 

pollution control. It oversees county and regional air pollution management programs.

Asthma: A chronic inflammatory disorder of the lungs characterized by wheezing, breathlessness, 

chest tightness, and cough.

Atmospheric half-life: The time required for the concentration of a pollutant or reactant to fall to onehalf of its initial value.

Benchmark Dose: That dose derived from linear regression of one or more dose-response curves 

associated with a specific response rate (such as 1, 5, or 10%) in the test population. This is the starting 

dose to which uncertainty factors are applied to determine a reference exposure level (REL) using the 

benchmark dose approach.

Urban Block Groups (BGs): A geographical unit smaller than a census tract used for reporting 

census data. BGs contain roughly 1,100 persons.

Bioaccumulation: the concentration of a substance in a body or part of a body or other living tissue in 

a concentration higher than that of the surrounding environment

Bioconcentrate: The process of increasing contaminant concentration in biota up the food chain as 

contaminants are ingested and concentrated in tissues of organisms higher up in the chain.

Cancer burden: The estimated number of theoretical cancer cases in a defined population resulting 

from lifetime exposure to pollutants emitted from a facility.

Cancer potency factor (CPF): The theoretical upper bound probability of extra cancer cases 

occurring in an exposed population assuming a lifetime exposure to the chemical when the chemical 

dose is expressed in exposure units of milligrams/kilogram-day (mg/kg-d).

California Air Pollution Control Officers Association (CAPCOA): A non-profit association of the 

air pollution control officers from all 35 air quality districts throughout California. CAPCOA was 

formed in 1975 to promote clean air and to provide a forum for sharing knowledge, experience, and 

information among the air quality regulatory agencies around the state.

Cal/EPA: In July 1991, the California Environmental Protection Agency (Cal/EPA) was created to 

coordinate the State's environmental quality programs and assure that there is a cabinet level voice for 

environmental protection. 

Chemical Abstract Services Registry Number (CAS): The Chemical Abstracts Service Registry 

Number (CAS) is a numeric designation assigned by the American Chemical Society's Chemical 

Abstracts Service and uniquely identifies a specific chemical compound. This entry allows one to 

conclusively identify a material regardless of the name or naming system used. 
 The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

J- 4

CCR: California Code of Regulations 

CERCLA: Comprehensive Environmental Response, Compensation and Liability Act (Superfund), a 

federal regulation providing direction and financial support for the clean-up of major hazardous waste 

sites

Centroid Locations: The location at which calculated ambient concentration is assumed to represent 

the entire subarea, typically the geometric centroid of an area, but possibly the population-weighted 

centroid of the area.

Census Tract: A physical area used by the U.S. Census Bureau to compile population and other 

statistical data.

Chronic Exposure: Long-term exposure, usually lasting one year to a lifetime.

Chronic Health Effect: An adverse non-cancer health effect that develops and persists (e.g., months 

or years) over time after long-term exposure to a substance

Criteria Air Pollutant: a pollutant or precursor to a pollutant for which the U.S. Environmental 

Protection Agency or the Air Resources Board has established an Ambient Air Quality Standard 

(AAQS). Examples include ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, lead, and PM10

and PM2.5.

Default: A value used when specific information that applies to a specific situation is not available.

Developmental toxicity: Adverse effects on the developing organism that may result from exposure 

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. Major manifestations of developmental toxicity include: death of the developing organism; 

induction of structural birth defects; altered growth; and functional deficiency.

Dilution factor (c/Q): a site-specific quantity defined as a ratio of the ground level concentration in 

μg/m3 to the mass emission rate in g/s and represented by c/Q.

Dose: A calculated amount of a substance estimated to be received by the subject, whether human or 

animal, as a result of exposure. Doses are generally expressed in terms of amount of chemical per unit 

body weight; typical units are mg/kg-day.

Dose-response assessment: The process of characterizing the relationship between the exposure to 

an agent and the incidence of an adverse health effect in exposed populations.

DTSC: California Department of Toxic Substances Control

ED: Rural Enumeration District. A geographical unit smaller than a census tract used to report census 

data. EDs contain roughly 1,100 persons.
 The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

J- 5

Emission Inventory Criteria and Guidelines: Regulation and Report adopted by the California Air 

Resources Board specifying criteria and procedures for the preparation of Air Toxics Hot Spots Act 

Emission Inventory Reports (Title 17, California Code or Regulations, Sections 93300-93300.5)

Endpoint: An observable or measurable biological or biochemical event including cancer used as an 

index of the effect of a chemical on a cell, tissue, organ, organism, etc.

Epidemiology: The study of the occurrence and distribution of a disease or physiological condition in 

human populations and of the factors that influence this distribution.

Exposure: Contact of an organism with a chemical, physical, or biological agent. Exposure is 

quantified as the amount of the agent available at the exchange boundaries of the organism (e.g., skin, 

lungs, digestive tract) and available for absorption.

Exposure Pathway: A route of exposure by which xenobiotics enter the human body, (e.g., inhalation, 

ingestion, dermal absorption).

Fugitive Dust: Dust particles that are introduced into the air through certain activities such as soil 

cultivation, or vehicles operating on open fields or dirt roadways. A subset of fugitive emissions.

Fugitive Emissions: Emissions not caught by a capture system which are often due to equipment 

leaks, evaporative processes, and windblown disturbances.

Gaussian Model: An air dispersion model based on the assumption that the time-averaged 

concentration of a species emitted from a point source has a Gaussian distribution about the mean 

centerline.

Genotoxic: having an adverse effect on the genetic material (DNA) resulting in a mutation or in 

chromosome damage

GLC: Estimated ground level concentration, usually for a specified averaging time (e.g., annual 

average, 1 hour, etc.)

Hot Spots Analysis and Reporting Program (HARP): A single integrated software package 

designed to promote statewide consistency, efficiency, and cost-effective implementation of health risk 

assessments and the Hot Spots Program. The HARP software package consists of three modules that 

include: 1) the Emissions Inventory Database Module, 2) the Air Dispersion Modeling Module, and 3) 

the Risk Analysis and Mapping Module.

Health Risk Assessment (HRA): the name of a computer program developed by the ARB, the 

OEHHA, and the University of California which was designed to aid in the computation of risk in the 

Hot Spots program

HSC: Health and Safety Code of the State of California
 The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

J- 6

Haber’s Law: The product of the concentration (C) and time of exposure (t) required to produce a 

specific physiologic effect is equal to a constant level or severity of response (K), or C * t = K

Hazard identification: The process of determining whether exposure to an agent can cause an 

increase in the incidence of an adverse health effect including cancer

Health Risk Assessment: A health risk assessment (HRA) is an evaluation or report that a risk 

assessor (e.g., Air Resources Board, district, consultant, or facility operator) develops to describe the 

potential a person or population may have of developing adverse health effects from exposure to a 

facility’s emissions. Some health effects that are evaluated could include cancer, developmental effects, 

or respiratory illness. The pathways that can be included in an HRA depend on the toxic air pollutants 

that a person (receptor) may be exposed to, and can include breathing, the ingestion of soil, water, 

crops, fish, meat, milk, and eggs, and dermal exposure.

Health Risk Guidance Value (HRGV): A numerical value with which to compare an exposure level 

in order to determine the probability of occurrence of an adverse health effect. In the Hot Spots 

program the toxicity criteria or toxicity values are known as Reference Exposure Levels (RELs) for 

noncancer effects and as inhalation unit risk factors and cancer potency values for cancer effects. 

Hazard Index (HI): The sum of individual acute or chronic hazard quotients (HQs) for each substance 

affecting a particular toxicological endpoint.

Hazard Quotient (HQ): The estimated ground level concentration divided by the reference exposure 

level for a single substance and a particular endpoint. For an acute HQ the one hour maximum 

concentration is divided by the acute Reference Exposure Level for the substance. For a chronic HQ, 

the annual concentration is divided by the chronic Reference Exposure level.

Hot Spot: A location where emissions from specific sources may expose individuals and population 

groups to elevated risks of adverse health effects, including but not limited to cancer, and contribute to 

the cumulative health risks of emissions from other sources in the area.

Individual Excess Cancer Risk: The theoretical probability of an individual person developing 

cancer as a result of lifetime exposure to carcinogenic substances. The Individual Excess Cancer Risk is 

calculated by summing the potential cancer risks due to both inhalation and noninhalation routes of 

exposure.

Inhalation (Breathing) Rate: The amount of air inhaled in a specified time period (e.g., per minute, 

per hour, per day, etc.); also called breathing rate and ventilation rate. This is an example of a variate.

Inhalation unit risk factor: The theoretical upper bound probability of extra cancer cases occurring 

in the exposed population assuming a lifetime exposure to the chemical when the air concentration is 

expressed in exposure units of per microgram/cubic meter (μg/m3)

-1

. 
 The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

J- 7

Initiator carcinogen: A substance which causes the first stage of carcinogenesis, the conversion of a 

normal cell to a neoplastic cell. Initiation is considered to be a rapid, irreversible change often involving 

a change in the DNA caused by the initiator.

Interspecies: Between different species.

Intraspecies: Within the same species.

Industrial Source Complex Dispersion model (ISC3): Air modeling software that incorporates 

three previous programs into a single program. These are the short-term model (ISCST), the long term 

model (ISCLT), and the complex terrain model (COMPLEX).

Isopleth: A line on a map connecting points of equal value (e.g., risk, concentration, etc.).

Lowest-observed adverse effect level (LOAEL): The lowest dose or exposure level of a chemical 

in a study at which there is a statistically or biologically significant increase in the frequency or severity of 

an adverse effect in the exposed population as compared with an appropriate, unexposed control 

group.

Margin of safety: The ratio of the no-observed-adverse-effect level (NOAEL) to the estimated 

human exposure.

Mean: The arithmetic average.

MEI: Maximum exposed individual (theoretical)

MEIR: Maximum exposed individual resident (actual)

MEIW: Maximum exposed individual worker (actual)

Meteorology: The science that deals with the phenomena of the atmosphere especially weather and 

weather conditions. In the area of air dispersion modeling, meteorology is used to refer to 

climatological data needed to run an air dispersion model including: wind speed, wind direction, stability 

class and ambient temperature.

Milligram: One one-thousandth (10-3) of a gram.

Molecular formula: The formula which identifies the atoms and the number of each kind in the 

molecules of a compound. Elements in the molecular formula are listed according to the Hill convention 

(C, H, then other elements in alphabetical order). 

Molecular weight: The sum of the atomic weights of the atoms in a molecule. For example, methane 

(CH4) is 16.043, the atomic weights being carbon = 12.011, hydrogen = 1.008.

Monte Carlo simulation: Application of random sampling to obtain an approximate value of an 

expression.
 The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

J- 8

Multipathway substance: A substance or chemical that once airborne from an emission source can, 

under environmental conditions, be taken into a human receptor by inhalation and by other exposure 

routes such as after deposition on skin or after ingestion of soil contaminated by the emission

No Observed Adverse Effect Level (NOAEL): The highest experimental dose at which there is no 

statistically or biologically significant increase in frequency or severity of adverse health effects in the 

exposed population compared with an appropriate, unexposed population. Effects may be produced at 

this level, but they are not considered to be adverse. Substances are generally considered to not have a 

NOAEL for the cancer endpoint.

Noncarcinogenic Effects: Noncancer health effects which may include birth defects, organ damage, 

morbidity, and death.

Office of Environmental Health Hazard Assessment (OEHHA): An office within the California 

Environmental Protection Agency that is responsible for evaluating chemicals for adverse health impacts 

and establishing safe exposure levels. OEHHA also assists in performing health risk assessments and 

developing risk assessment procedures for air quality management purposes.

PM10, PM2.5: PM10 is particulate matter less than 10 μm in diameter; PM2.5 is particulate matter less 

than 2.5 μm in diameter.

PMI: Off-site point of maximum impact. A location, with or without people currently present, at which 

the total cancer risk, or the total noncancer risk, has the highest numerical value.

Point Estimate: A single value estimate for a given variate

Potency: Essentially the relative effectiveness, or risk, of a standard amount of a substance to cause a 

toxic response.

Potency Slope: Used to calculate the probability or risk of cancer associated with an estimated 

exposure, based on the assumption in cancer risk assessments that risk is directly proportional to dose 

and that there is no threshold for carcinogenesis. It is the slope of the dose-response curve estimated 

at low exposures.

Proposition 65: Safe Drinking Water and Toxic Enforcement Act of 1986, also known as Proposition 

65. This Act is codified in California Health and Safety Code Section 25249.5, 

et seq. No person in the course of doing business shall knowingly discharge or release a chemical 

known to the state to cause cancer or reproductive toxicity into water or into land where such chemical 

passes or probably will pass into any source of drinking water, without first giving clear and reasonable 

warning to such individual.

Resource Conservation and Recovery Act (RCRA) of 1976: A federal law regulating disposal of 

hazardous waste

Receptor: A location with or without people present at which the ground level concentration of an 

emitted chemical can be estimated
 The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

J- 9

Refined Models: Air dispersion models designed to provide more representative concentration 

estimates than screening models taking into account actual meteorological conditions.

Reference Concentration (RfC): An estimate, derived by the U.S. EPA (with an uncertainty 

spanning perhaps an order of magnitude) of a daily exposure to the human population, (including 

sensitive subgroups) that is likely to be without appreciable risk of deleterious effects during a lifetime of 

exposure. The RfC is derived from a no or lowest observed adverse effect level from human or animal

exposures, to which uncertainty or "safety" factors are applied.

Reference Dose (RfD): An estimate delivered by the U.S. EPA (with uncertainty spanning perhaps 

an order of magnitude) of the daily exposure to the human population (including sensitive 

subpopulations) that is likely to be without deleterious effects during a lifetime. The RfD is reported in 

units of mg of substance/kg body weight/day for oral exposures. 

Reference exposure level (REL): expressed in units of μg/m3 for inhalation exposures and of 

mg/kg-d for noninhalation exposures. The REL is an exposure level at or below which no noncancer 

adverse health effect is anticipated to occur in a human population exposed for a specific duration. An 

REL is virtually the same as the terms Reference Concentration (RfC) for inhalation or Reference Dose 

(RFD) used by U.S. EPA, only it may be for varying amounts of time rather than lifetime only. It has 

been given a different name so that the values estimated by the State Office of Environmental Health 

Hazard Assessment can easily be distinguished from those developed by the U.S. EPA. RELs are used 

to evaluate toxicity endpoints other than cancer.

Reproductive toxicity: Harmful effects on fertility, gestation, or offspring, caused by exposure of 

either parent to a substance.

Risk: The (characterization of the) probability of potentially adverse effects to human health, in this 

instance from the exposure to environmental hazards.

Risk Assessment: The characterization (in the present context) of the probability of potentially 

adverse health effects to people from exposure to environmental chemical hazards.

Risk Management: An evaluation of the need for and feasibility of reducing risk. It includes 

consideration of magnitude of risk, available control technologies, and economic feasibility.

Risk Management and Prevention Program (RMPP): A program administered by the Office of 

Emergency Services (OES) and local agencies to reduce the frequency and severity of accidental 

releases of toxic materials

Scientific Review Panel on Toxic Air Contaminants or SRP: A nine-member panel appointed to 

advise the Air Resources Board and the Department of Pesticide Regulation in their evaluation of the 

adverse health effects toxicity of substances being evaluated as Toxic Air Contaminants.

Screening Models: Dispersion models used to provide a maximum concentration that is likely to 

overestimate public exposure.
 The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

J- 10

Sensitive Receptor: A location such as a hospital or daycare center where the human occupants are 

considered to be more sensitive to pollutants than “average”.

Severity Level: The (acute) discomfort or mild effect level; the concentration of an airborne substance 

at or below which exposure for one hour may result in some odors, tastes, visual cues, and sensations 

but which will cause no adverse health effects in essentially all of the population. Exposure to 

concentrations above this level, depending on the chemical, may result in minor health effects, such as 

mild eye and respiratory irritation, skin irritation, minor histologic effects, and headaches.

Severity Level II: The (acute) disability or serious effect level. Exposure for one hour to an airborne 

substance above this level may lead individuals to seek assistance. Exposures above this level, 

depending on the chemical, may result in serious health effects such as severe eye irritation, severe 

respiratory irritation, bronchospasm, shortness of breath, disorientation, blurred vision, vomiting, cardiac 

arrhythmia and adverse outcomes of an existing or subsequent pregnancy.

Stationary source: A non-mobile source of air pollutants which can be either a point or area source.

Stochastic: A process that involves random variation

Synergism: A pharmacologic or toxicologic interaction in which the combined effect of two or more 

chemicals is greater than the sum of the effects of each chemical alone.

Subcensus Tract: Smaller population unit within a census tract. 

Surrogate: As used in this document refers to a single substance category used to represent a family of 

related chemical compounds, e.g., gasoline vapors or POM (polycyclic organic matter) in place of 

benzo(a)pyrene.

Threshold, Nonthreshold: A threshold dose is the minimally effective dose of any chemical that is 

observed to produce a response (e.g., enzyme change, liver toxicity, death). For most toxic effects, 

except carcinogenesis, there appear to be threshold doses. Nonthreshold substances are those 

substances, including nearly all carcinogens, that are known or assumed to have some risk of response 

at any dose above zero.

Toxic air contaminant (TAC): As defined by California Health and Safety Code, Section 39655 (a): 

an air pollutant which may cause or contribute to an increase in mortality or in serious illness, or which 

may pose a present or potential hazard to human health. Substances, which have been identified by the 

United States Environmental Protection Agency as hazardous air pollutants (e.g. benzene, asbestos), 

shall be identified by the Board as toxic air contaminants.

Toxicology: The multidisciplinary study of toxicants, their harmful effects on biological systems, and 

the conditions under which these harmful effects occur. The mechanisms of action, detection, and 

treatment of the conditions produced by toxicants are studied.

Uncertainty: True uncertainty is that which is not known about a factor that influences its value.
 The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. 

August 2003.

J- 11

URF: See inhalation unit risk factor

UTM Coordinates: Universal Transfer Mercator Coordinates. Coordinates used to define a specific 

location by means of two values (i.e., easting and northing coordinates).

United States Environmental Protection Agency (U.S. EPA): The Federal agency charged with 

setting policy and guidelines, carrying out legal mandates, for the protection, and national interests in 

environmental resources.

Vapor: The gaseous phase of liquids or solids at atmospheric temperature and pressure.

Vapor Pressure: The pressure exerted by a chemical vapor in equilibrium with its liquid or solid phase 

at any given temperature, used to calculate the rate of evaporation of a substance.

Variability: Ability to have different numerical values of a parameter, such as height or weight 

Variate: A variable quantity associated with a probability distribution (e.g. inhalation rate)

Volatile: Chemicals that rapidly pass off from the liquid state in the form of vapors.

Xenobiotic: A toxic agent; a relatively small (MW<1000), non-nutritive chemical that is foreign to the 

species being studied

Zone of impact: The area in the vicinity of the facility in which an individual is exposed to a specified 

cancer risk, usually one in a million or greater. 
Technical Support Document for Exposure Assessment and Stochastic Analysis 

August 2000 

Appendix K 

HRA Forms and Maps Used With Air Dispersion Modeling

- Example of Census Tract Map 

- Example of 7.5 minute Series Map 

- Examples of Tables for Emissions Reporting 

K-1 
Technical Support Document for Exposure Assessment and Stochastic Analysis 

August 2000 

K-2 
Technical Support Document for Exposure Assessment and Stochastic Analysis 

August 2000 

Technical Support Document for Exposure Assessment and Stochastic Analysis 

August 2000 

K-3 
Technical Support Document for Exposure Assessment and Stochastic Analysis 

August 2000 

K-4 
Technical Support Document for Exposure Assessment and Stochastic Analysis 

August 2000 

HEALTH RISK ASSESSMENT

EMISSION RATE BY SUBSTANCE AND SOURCE RAG-001 

 FACILITY NAME / FACILITY ADDRESS / SITE ID#: 

SOURCE 

ID No. SOURCE NAME SUBSTANCE NAME CAS No. 

1-HOUR 

MAXIMUM 

(lb/hr) 

1-HOUR 

MAXIMUM 

(g/s) 

ANN

AVE

(lb

K-5 
Technical Support Document for Exposure Assessment and Stochastic Analysis 

August 2000 

HEALTH RISK ASSESSMENT

EMISSION RATE BY SUBSTANCE – TOTALS – RAG-002 

FACILITY NAME / FACILITY ADDRESS / SITE ID# 

SUBSTANCE NAME CAS No. 

1-HOUR 

MAXIMUM 

(lb/hr) 

1-HOUR 

MAXIMUM 

(g/s) 

ANNUAL 

AVERAGE 

(lb/yr) 

A

A

K-6 
Technical Support Document for Exposure Assessment and Stochastic Analysis 

August 2000 

HEALTH RISK ASSESSMENT

SOURCE PARAMETERS – STACKS – RAG-003 

 FACILITY NAME / FACILITY ADDRESS / SITE ID# 

SOURCE 

ID No. STACK NAME 

UTM 

Easting 

UTM 

Northing 

HEIGHT 

(m) 

DIAMETER 

(m) 

TEMP. 

(F) (K) 

FLOW RA

(ACFM

K-7 
Technical Support Document for Exposure Assessment and Stochastic Analysis 

August 2000 

HEALTH RISK ASSESSMENT

SOURCE OPERATING HOURS – RAG-004

 FACILITY NAME / FACILITY ADDRESS / SITE ID# 

SOURCE STACK

AVERAGE OPERATING 

HOURS 

MAXIMUM OPERATING 

HOURS 

ID No. NAME (hr/day) (days/year) (hr/day) (days/year)

K-8 
Appendix L

OEHHA/ARB Approved Health Values 

for Use in Hot Spot Facility Risk Assessments
The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk 

Assessments. August 2003.

Appendix L-2

Purpose of the Appendix L Tables:

The purpose of the following reference tables is to provide a quick list of all health values 

that have been approved by the Office of Environmental Health Hazard Assessment (OEHHA) 

and the Air Resources Board (ARB) for use in facility health risk assessments conducted for the 

AB 2588 Air Toxics Hot Spots Program. The OEHHA has developed and adopted new risk 

assessment guidelines that update and replace the California Air Pollution Control Officers 

Association’s (CAPCOA) Air Toxics “Hot Spots” Program Revised 1992 Risk Assessment 

Guidelines, October 1993. 

The following tables list the OEHHA adopted inhalation and oral cancer slope factors, 

noncancer acute Reference Exposure Levels (RELs), and inhalation and oral noncancer chronic 

RELs. In addition, these tables list the substances in Appendix A-I (Substances For Which 

Emissions Must Be Quantified) and Appendix F (Criteria For Inputs For Risk Assessment Using 

Screening Air Dispersion Modeling) of the ARB’s Hot Spots Emission Inventory Criteria and 

Guidelines (EICG). OEHHA is still in the process of adopting new noncancer chronic RELs. 

Therefore, new health values will periodically be added to, or deleted from, these tables. Users 

of these tables are advised to monitor the OEHHA website (www.oehha.ca.gov) for any updates 

to the health values.

Substances written in italics do not have explicit OEHHA approved health values, but are 

included in this table to clarify applicability of OEHHA adopted heath effects values to 

individual or grouped substances listed in the Hot Spots Emission Inventory Criteria and 

Guidelines, Appendix A-I list of “Substances For Which Emissions Must Be Quantified.”

The “Date Value Reviewed” column lists the date that the health value was last reviewed 

by OEHHA and the Scientific Review Panel, and/or approved for use in the AB 2588 Air Toxics 

Hot Spots Program. This information is useful to tell where the number came from. If the health 

value is unchanged since it was first approved for use in the Hot Spots Program, then the date 

that the value was first approved for use by CAPCOA is listed within the brackets [ ]. 

• April 1999 is listed for the cancer potency values and noncancer acute RELs, which 

have been adopted by the OEHHA as part of the AB 2588 “Hot Spots” Risk 

Assessment Guidelines. 

• February 2000, April 2000, January 2001, and December 2001 are listed for the first 

set of 22, the second set of 16, the third set of 22, and the fourth set of 12 noncancer 

chronic RELs, respectively. 

• October 2000 is listed for the oral chronic RELs and oral cancer slope factors. 1996 is 

listed for the U.S. EPA Reference Concentrations. Dates of 1990-1992 and 1996 are 

listed for CAPCOA chronic RELs that may eventually be dropped or replaced. 

• For the substances identified as Toxic Air Contaminants, the Air Resources Board 

hearing date is listed. The dates for acetaldehyde, benzo[a]pyrene, and methyl tertiarybutyl ether represent the dates the values were approved by the Scientific Review 

Panel.
Noncancer Effects Cancer Risk

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Acute 

Inhalation

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Inhalation 

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Oral 

REL

(mg/kg/d)

Date u

Value 

Reviewed

[Added]

Inhalation

Cancer 

Potency 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

Oral Slope 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

M®

W

A

F

ACETALDEHYDE 75-07-0 9.0E+00 5/93 1.0E-02 4/99

[5/93] 1

ACETAMIDE 60-35-5 7.0E-02 4/99 1

ACROLEIN 107-02-8 1.9E-01 4/99 6.0E-02 1/01 --

ACRYLAMIDE 79-06-1 4.5E+00 4/99

[7/90] 1

ACRYLIC ACID 79-10-7 6.0E+03 4/99 --

ACRYLONITRILE 107-13-1 5.0E+00 12/01 1.0E+00 4/99

[1/91] 1

ALLYL CHLORIDE 107-05-1 2.1E-02 4/99 1

2-AMINOANTHRAQUINONE 117-79-3 3.3E-02 4/99 1

AMMONIA 7664-41-7 3.2E+03 4/99 2.0E+02 2/00 --

ANILINE 62-53-3 5.7E-03 4/99 1

Antimony Compounds 7440-36-0 --

ANTIMONY TRIOXIDE 1309-64-4 --

ARSENIC AND COMPOUNDS 

(INORGANIC) TAC ®

7440-38-2

1016

[1015]

1.9E-01

AveP

4/99 3.0E-02 1/01 3.0E-04 10/00 1.2E+01 

TAC

7/90 1.5E+00 10/00 1

ARSINE 7784-42-1 1.6E+02 4/99 --

ASBESTOSTAC 3 1332-21-4

1.9E-04

TAC3

3/86 333.33

3

BENZENETAC 71-43-2

1.3E+03

AveP

4/99 6.0E+01 2/00 1.0E-01 

TAC

1/85 1

BENZIDINE (AND ITS SALTS) 

values also apply to: 92-87-5 5.0E+02 4/99[1/91] 1

Benzidine based dyes 1020 5.0E+02 4/99

[1/91] 1

Direct Black 38 1937-37-7 5.0E+02 4/99

[1/91] 1

Direct Blue 6 2602-46-2 5.0E+02 4/99

[1/91] 1

Direct Brown 95 (technical grade) 16071-86-6 5.0E+02 4/99

[1/91] 1

BENZYL CHLORIDE 100-44-7 4E+02 4/99 1.7E-01 4/99 1

BERYLLIUM AND COMPOUNDS ® 7440-41-7

[1021] 7.0E

-03 12/01 2.0E-03 12/01 8.4E+00 4/99

[7/90] 1

BIS(2-CHLOROETHYL)ETHER 

(Dichloroethyl ether) 111-44-4 2.5E+00 4/99 1

BIS(CHLOROMETHYL)ETHER 542-88-1 4.6E+01 4/99

[1/91] 1

1,3-BUTADIENETAC 106-99-0 2.0E+01 1/01 6.0E-01 

TAC

7/92 1
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-4

Noncancer Effects Cancer Risk

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Acute 

Inhalation

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Inhalation 

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Oral 

REL

(mg/kg/d)

Date u

Value 

Reviewed

[Added]

Inhalation

Cancer 

Potency 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

Oral Slope 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

M®

W

A

F

CADMIUM AND COMPOUNDSTAC ® 7440-43-9

[1045] 2.0E

-02 1/01 5.0E-04 10/00 1.5E+01 

TAC

1/87 1

CARBON DISULFIDE 75-15-0

6.2E+03

AveP

4/99 8.0E+02

RfC --

CARBON MONOXIDE 630-08-0 2.3E+04 4/99 --

CARBON TETRACHLORIDETAC

(Tetrachloromethane) 56-23-5

1.9E+03

AveP

4/99 4.0E+01 1/01 1.5E-01 

TAC

9/87 1

CHLORINATED PARAFFINS 108171-26-2 8.9E-02 4/99 1

CHLORINE 7782-50-5 2.1E+02 4/99 2.0E-01 2/00 --

CHLORINE DIOXIDE 10049-04-4 6.0E-01 1/01 --

4-CHLORO-O-PHENYLENEDIAMINE 95-83-0 1.6E-02 4/99 1

CHLOROBENZENE 108-90-7 1.0E+03 1/01 --

CHLORODIFLUOROMETHANE 

 ... (see Fluorocarbons)

CHLOROFORM TAC 67-66-3

1.5E+02

AveP

4/99 3.0E+02 4/00 1.9E-02 

TAC

12/90 1

Chlorophenols 1060 --

PENTACHLOROPHENOL 87-86-5 1.8E-02 4/99 1

2,4,6-TRICHLOROPHENOL 88-06-2 7.0E-02 4/99

[1/91] 1

CHLOROPICRIN 76-06-2 2.9E+01 4/99 4.0E-01 12/01 --

CHLOROPRENE 126-99-8 --

p-CHLORO-o-TOLUIDINE 95-69-2 2.7E-01 4/99 1

CHROMIUM 6+TAC ®values also apply to: 18540-29-9 2.0E-01 1/01 2.0E-02 10/00 5.1E+02 

TAC

1/86 1

Barium chromate® 10294-40-3 2.0E-01 1/01 2.0E-02 10/00 5.1E+02

TAC

1/86 0.2053

Calcium chromate® 13765-19-0 2.0E-01 1/01 2.0E-02 10/00 5.1E+02

TAC 1/86 0.3332

Lead chromate® 7758-97-6 2.0E-01 1/01 2.0E-02 10/00 5.1E+02

TAC

1/86 0.1609

Sodium dichromate® 10588-01-9 2.0E-01 1/01 2.0E-02 10/00 5.1E+02

TAC 1/86 0.397

Strontium chromate® 7789-06-2 2.0E-01 1/01 2.0E-02 10/00 5.1E+02

TAC 1/86 0.2554

CHROMIUM TRIOXIDE ®

(as chromic acid mist) 1333-82-0 2.0E-03 1/01 2.0E-02 10/00 5.1E+02 TAC

1/86 0.52

COPPER AND COMPOUNDS 7440-50-8

[1067] 1.0E+02 4/99 --

p-CRESIDINE 120-71-8 1.5E-01 4/99 1

CRESOLS (mixtures of) 1319-77-3 6.0E+02 1/01 --

m-CRESOL 108-39-4 6.0E+02 1/01 --
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-5

Noncancer Effects Cancer Risk

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Acute 

Inhalation

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Inhalation 

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Oral 

REL

(mg/kg/d)

Date u

Value 

Reviewed

[Added]

Inhalation

Cancer 

Potency 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

Oral Slope 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

M®

W

A

F

o-CRESOL 95-48-7 6.0E+02 1/01 --

p-CRESOL 106-44-5 6.0E+02 1/01 --

CUPFERRON 135-20-6 2.2E-01 4/99 1

Cyanide Compounds (inorganic) 57-12-5

1073 3.4E+02 4/99 9.0E+00 4/00 --

HYDROGEN CYANIDE 

(Hydrocyanic acid) 74-90-8 3.4E+02 4/99 9.0E+00 4/00 --

2,4-DIAMINOANISOLE 615-05-4 2.3E-02 4/99 1

2,4-DIAMINOTOLUENE 95-80-7 4.0E+00 4/99 1

1,2-DIBROMO-3-CHLOROPROPANE 

(DBCP) 96-12-8 7.0E+00 4/99[1/92] 1

p-DICHLOROBENZENE 106-46-7 8.0E+02 1/01 4.0E-02 4/99

[1/91] 1

3,3-DICHLOROBENZIDINE 91-94-1 1.2E+00 4/99

[1/91] 1

1,1,-DICHLOROETHANE 

(Ethylidene dichloride) 75-34-3 5.7E-03 4/99 1

1,1-DICHLOROETHYLENE 

 ... (see Vinylidene Chloride)

DI(2-ETHYLHEXYL)PHTHALATE (DEHP) 117-81-7 8.4E-03 4/99

[1/92] 8.4E-03 10/00 1

DIESEL EXHAUST 

 ... (see Particulate Emissions 

 from Diesel-Fueled Engines)

DIETHANOLAMINE 111-42-2 3.0E+00 12/01 --

DIMETHYLAMINE 124-40-3 --

p-DIMETHYLAMINOAZOBENZENE 60-11-7 4.6E+00 4/99 1

N,N-DIMETHYL FORMAMIDE 68-12-2 8.0E+01 1/01 --

2,4-DINITROTOLUENE 121-14-2 3.1E-01 4/99 1

1,4-DIOXANE 

(1,4-Diethylene dioxide) 123-91-1 3.0E+03 4/99 3.0E+03 4/00 2.7E-02 4/99[1/91] 1

EPICHLOROHYDRIN 

(1-Chloro-2,3-epoxypropane) 106-89-8 1.3E+03 4/99 3.0E+00 1/01 8.0E-02 4/99[1/92] 1

1,2-EPOXYBUTANE 106-88-7 2.0E+01 1/01 --

ETHYL ACRYLATE 140-88-5 --

ETHYL BENZENE 100-41-4 2.0E+03 2/00 --

ETHYL CHLORIDE (Chloroethane) 75-00-3 3.0E+04 4/00 --

ETHYLENE DIBROMIDETAC 

(1,2-Dibromoethane) 106-93-4 8.0E-01 12/01 2.5E-01 TAC

7/85 1
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-6

Noncancer Effects Cancer Risk

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Acute 

Inhalation

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Inhalation 

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Oral 

REL

(mg/kg/d)

Date u

Value 

Reviewed

[Added]

Inhalation

Cancer 

Potency 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

Oral Slope 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

M®

W

A

F

ETHYLENE DICHLORIDETAC 

(1,2-Dichloroethane) 107-06-2 4.0E+02 1/01 7.2E-02 TAC

9/85 1

ETHYLENE GLYCOL 107-21-1 4.0E+02 4/00 --

ETHYLENE GLYCOL BUTYL ETHER

 ... (see Glycol ethers)

ETHYLENE OXIDETAC 

(1,2-Epoxyethane) 75-21-8 3.0E+01 1/01 3.1E-01 TAC

11/87 1

ETHYLENE THIOUREA 96-45-7 4.5E-02 4/99 1

Fluorides 1101 2.4E+02 4/99 1.3E+01 8/03 4.0E-2 8/03 --

HYDROGEN FLUORIDE 

(Hydrofluoric acid) 7664-39-3 2.4E+02 4/99 1.4E+01 8/031 4.0E-2

--

FORMALDEHYDETAC 50-00-0 9.4E+01 4/99 3.0E+00 2/00 2.1E-02 

TAC

3/92 1

GASOLINE VAPORS 1110 --

GLUTARALDEHYDE 111-30-8 8.0E-02 1/01 --

GLYCOL ETHERS 1115

ETHYLENE GLYCOL MONOBUTYL 

ETHER – EGBE 111-76-2 1.4E+04 4/99 --

ETHYLENE GLYCOL MONOETHYL 

ETHER – EGEE 110-80-5

3.7E+02

AveP

4/99[1/92] 7.0E+01 2/00 --

ETHYLENE GLYCOL MONOETHYL 

ETHER ACETATE – EGEEA 111-15-9

1.4E+02

AveP

4/99 3.0E+02 2/00 --

ETHYLENE GLYCOL 

MONOMETHYL ETHER – EGME 109-86-4

9.3E+01

AveP

4/99 6.0E+01 2/00 --

ETHYLENE GLYCOL 

MONOMETHYL ETHER ACETATE –

EGMEA

110-49-6 9.0E+01 2/00 --

HEXACHLOROBENZENE 118-74-1 1.8E+00 4/99

[1/91] 1

HEXACHLOROCYCLOHEXANES 

(mixed or technical grade)

608-73-1

1120 4.0E+00 4/99[1/91] 4.0E+00 10/00[1/92] 1

AlphaHEXACHLOROCYCLOHEXANE 319-84-6 4.0E+00 4/99

[1/91] 4.0E+00 10/00[1/92] 1

beta- HEXACHLOROCYCLOHEXANE 319-85-7 4.0E+00 4/99

[1/91] 4.0E+00 10/00[1/92] 1

GammaHEXACHLOROCYCLOHEXANE 

(Lindane)

58-89-9 1.1E+00 4/99 1.1E+00 10/00 1
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-7

Noncancer Effects Cancer Risk

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Acute 

Inhalation

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Inhalation 

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Oral 

REL

(mg/kg/d)

Date u

Value 

Reviewed

[Added]

Inhalation

Cancer 

Potency 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

Oral Slope 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

M®

W

A

F

n-HEXANE 110-54-3 7.0E+03 4/00 --

HYDRAZINE 302-01-2 2.0E-01 1/01 1.7E+01 4/99

[7/90] 1

HYDROCHLORIC ACID 

(Hydrogen chloride) 7647-01-0 2.1E+03 4/99 9.0E+00 2/00--

HYDROGEN BROMIDE 

... (see Bromine & Compounds)

HYDROGEN CYANIDE 

... (see Cyanide & Compounds)

HYDROGEN FLUORIDE 

... (see Fluorides)

HYDROGEN SELENIDE 

... (see Selenium & Compounds)

HYDROGEN SULFIDE 7783-06-4 4.2E+01 4/99[7/90] 1.0E+01 4/00 --

ISOPHORONE 78-59-1 2.0E+03 12/01 --

ISOPROPYL ALCOHOL (Isopropanol) 67-63-0 3.2E+03 4/99 7.0E+03 2/00 --

LEAD AND COMPOUNDSTAC W ®

(inorganic) values also apply to:

7439-92-1

1128

[1130]

4.2E-02 

TAC 4/97 8.5E-03 10/00 1

Lead acetate® 301-04-2

4.2E-02

TAC 4/97 8.5E-03 10/00 0.637

Lead phosphate® 7446-27-7

4.2E-02

TAC 4/97 8.5E-03 10/00 0.7659

Lead subacetate® 1335-32-6

4.2E-02

TAC 4/97 8.5E-03 10/00 0.7696

LINDANE 

... (see gamma-Hexachlorocyclohexane)

MALEIC ANHYDRIDE 108-31-6 7.0E-01 12/01 --

MANGANESE AND COMPOUNDS 7439-96-5

[1132] 2.0E-01 4/00 --

MERCURY AND COMPOUNDS 

(INORGANIC)

7439-97-6

[1133] 1.8E+00 4/99 9.0E

-02 2/00 3.0E-04 10/00

[1/92] --

Mercuric chloride 7487-94-7 1.8E+00 4/99 9.0E-02 2/00 3.0E-04 10/00

[1/92] --

MERCURY AND COMPOUNDS 

(ORGANIC) values also apply to: N/A

METHYL MERCURY 593-74-8 --

METHANOL 67-56-1 2.8E+04 4/99 4.0E+03 4/00 --

METHYL BROMIDE (Bromomethane) 74-83-9 3.9E+03 4/99 5.0E+00 2/00 --

METHYL tertiary-BUTYL ETHER 1634-04-4 8.0E+03 2/00 9.1E-04 11/99 1

METHYL CHLOROFORM 

(1,1,1-Trichloroethane) 71-55-6 6.8E+04 4/99 1.0E+03 2/00 --
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-8

Noncancer Effects Cancer Risk

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Acute 

Inhalation

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Inhalation 

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Oral 

REL

(mg/kg/d)

Date u

Value 

Reviewed

[Added]

Inhalation

Cancer 

Potency 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

Oral Slope 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

M®

W

A

F

METHYL ETHYL KETONE (2-Butanone) 78-93-3 1.3E+04 4/99 --

METHYL ISOCYANATE 624-83-9 1.0E+00 12/01 --

METHYL MERCURY 

... (see Mercury & Compounds)

METHYL METHACRYLATE 80-62-6 --

4,4'-METHYLENE 

BIS (2-CHLOROANILINE) (MOCA) 101-14-4 1.5E+00 4/99 1

METHYLENE CHLORIDE TAC 

(Dichloromethane) 75-09-2 1.4E+04 4/99 4.0E

+02 2/00 3.5E-03 

TAC

7/89 1

4,4'-METHYLENE DIANILINE 

(AND ITS DICHLORIDE) 101-77-9 2.0E

+01 12/01 1.6E+00 4/99 1.6E+00 10/00 1

METHYLENE DIPHENYL ISOCYANATE 101-68-8 7.0E-01 1/01 --

MICHLER'S KETONE 

(4,4’-Bis(dimethylamino)benzophenone) 90-94-8 8.6E-01 4/99 1

N-NITROSO-n-BUTYLAMINE 924-16-3 1.1E+01 4/99

[1/92] 1

N-NITROSODI-n-PROPYLAMINE 621-64-7 7.0E+00 4/99

[1/91] 1

N-NITROSODIETHYLAMINE 55-18-5 3.6E+01 4/99

[1/91] 1

N-NITROSODIMETHYLAMINE 62-75-9 1.6E+01 4/99

[1/91] 1

N-NITROSODIPHENYLAMINE 86-30-6 9.0E-03 4/99 1

N-NITROSO-N-METHYLETHYLAMINE 10595-95-6 2.2E+01 4/99

[7/90] 1

N-NITROSOMORPHOLINE 59-89-2 6.7E+00 4/99

[7/92] 1

N-NITROSOPIPERIDINE 100-75-4 9.4E+00 4/99

[7/92] 1

N-NITROSOPYRROLIDINE 930-55-2 2.1E+00 4/99

[7/90] 1

NAPHTHALENE 

... (see Polycyclic aromatic hydrocarbons)

NICKEL AND COMPOUNDSTAC 

®

values also apply to:

7440-02-0

[1145] 6.0E+00 4/99 5.0E-02 2/00 5.0E-02 10/00 9.1E-01 

TAC 8/91 1

Nickel acetate® 373-02-4 6.0E+00 4/99 5.0E-02 2/00 5.0E-02 10/00 9.1E-01

TAC 8/91 0.3321

Nickel carbonate® 3333-39-3 6.0E+00 4/99 5.0E-02 2/00 5.0E-02 10/00 9.1E-01

TAC

8/91 0.4945

Nickel carbonyl® 13463-39-3 6.0E+00 4/99 5.0E-02 2/00 5.0E-02 10/00 9.1E-01

TAC 8/91 0.3438

Nickel hydroxide® 12054-48-7 6.0E+00 4/99 5.0E-02 2/00 5.0E-02 10/00 9.1E-01

TAC

8/91 0.6332
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-9

Noncancer Effects Cancer Risk

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Acute 

Inhalation

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Inhalation 

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Oral 

REL

(mg/kg/d)

Date u

Value 

Reviewed

[Added]

Inhalation

Cancer 

Potency 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

Oral Slope 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

M®

W

A

F

Nickelocene® 1271-28-9 6.0E+00 4/99 5.0E-02 2/00 5.0E-02 10/00 9.1E-01

TAC 8/91 0.4937

NICKEL OXIDE® 1313-99-1 6.0E+00 4/99 1.0E-01 2/00 5.0E-02 10/00 9.1E-01 

TAC 8/91 0.7859

Nickel refinery dust from the 

pyrometallurgical process 1146 6.0E+00 4/99 5.0E-02 2/00 5.0E-02 10/00 9.1E-01

TAC 8/91 1

Nickel subsulfide® 12035-72-2 6.0E+00 4/99 5.0E-02 2/00 5.0E-02 10/00 9.1E-01

TAC

8/91 0.2443

NITRIC ACID 7697-37-2 8.6E+01 4/99 --

NITROGEN DIOXIDE 10102-44-0 4.7E+02 4/99[1/92] --

2-NITROPROPANE 79-46-9 --

p-NITROSODIPHENYLAMINE 156-10-5 2.2E-02 4/99 1

OZONE 10028-15-6 1.8E+02 4/99[1/92] --

PARTICULATE EMISSIONS FROM 

DIESEL-FUELED ENGINESTAC n

9901 5.0E+00

TAC 8/98 1.1E+00 TAC 8/98 1

PENTACHLOROPHENOL 

... (see Chlorophenols)

PERCHLOROETHYLENETAC 

(Tetrachloroethylene) 127-18-4 2.0E+04 4/99 3.5E+01TAC

10/91 2.1E-02 

TAC

10/91 1

PHENOL 108-95-2 5.8E+03 4/99 2.0E+02 4/00 --

PHOSGENE 75-44-5 4.0E+00 4/99 --

PHOSPHINE 7803-51-2

8.0E-01 9/02 --

PHOSPHORIC ACID 7664-38-2 7.0E+00 2/00 --

PHTHALIC ANHYDRIDE 85-44-9 2.0E+01 1/01 --

PCB (POLYCHLORINATED BIPHENYLSunspeciated mixture) [lowest risk] H 1336-36-3 7.0E-02 2/02 7.0E-02 2/02 1

PCB (POLYCHLORINATED BIPHENYLSunspeciated mixture) [low risk]H 1336-36-3 4.0E-01 2/02 4.0E-01 2/02 1

PCB (POLYCHLORINATED BIPHENYLS -

unspeciated mixture) [high risk] H 1336-36-3 2.0E+00 2/02 2.0E+00 2/02 1

PCB (POLYCHLORINATED BIPHENYLS 

(speciated)∇

3,3’,4,4’-TETRACHLOROBIPHENYL (77) 35298-13-3 4.0E-01 8/03 1.0E –04 8/03 1.3E +01 8/03 1.3E +01 8/03

3,4,4’,5-TETRACHLOROBIPHENYL (81) 70362-50-4 4.0E-01 8/03 1.0E –04 8/03 1.3E +01 8/03 1.3E +01 8/03

2,3,3’,4,4’- PENTACHLOROBIPHENYL 

(105)

32598-14-4

4.0E-01 8/03 1.0E –04 8/03 1.3E +01 8/03 1.3E +01 8/03

2,3,4,4’5- PENTACHLOROBIPHENYL (114) 74472-37-0 8.0E-02 8/03 2.0E –05 8/03 6.5E +01 8/03 6.5E +01 8/03

2,3’4,4’,5- PENTACHLOROBIPHENYL 

(118)

31508-00-6

4.0E-01 8/03 1.0E –04 8/03 1.3E +01 8/03 1.3E +01 8/03
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-10

Noncancer Effects Cancer Risk

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Acute 

Inhalation

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Inhalation 

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Oral 

REL

(mg/kg/d)

Date u

Value 

Reviewed

[Added]

Inhalation

Cancer 

Potency 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

Oral Slope 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

M®

W

A

F

2’,3,4,4’,5- PENTACHLOROBIPHENYL 

(123)

65510-44-3

4.0E-01 8/03 1.0E –04 8/03 1.3E +01 8/03 1.3E +01 8/03

3,3’,4,4’,5- PENTACHLOROBIPHENYL 

(126)

57465-28-8

4.0E-04 8/03 1.0E –07 8/03 1.3E +04 8/03 1.3E +04 8/03

2,3,3’,4,4’,5-HEXACHLOROBIPHENYL 

(156)

38380-08-4

8.0E-02 8/03 2.0E –05 8/03 6.5E +01 8/03 6.5E +01 8/03

2,3,3’,4,4’,5’-HEXACHLOROBIPHENYL 

(157)

69782-90-7

8.0E-02 8/03 2.0E –05 8/03 6.5E +01 8/03 6.5E +01 8/03

2,3’,4,4’,5,5’-HEXACHLOROBIPHENYL 

(167)

52663-72-6

4.0E-00 8/03 1.0E –03 8/03 1.3E +00 8/03 1.3E +00 8/03

3,3’,4,4’5,5’- HEXACHLOROBIPHENYL 

(169)

32774-16-6

4.0E-03 8/03 1.0E –06 8/03 1.3E +03 8/03 1.3E +03 8/03

2,3,3’4,4’,5,5’- HEPTACHLOROBIPHENYL 

(189) 

39635-31-9

4.0E-01 8/03 1.0 E-04 8/03 1.3E +01 8/03 1.3E +01 8/03

POLYCHLORINATED DIBENZO-PDIOXINS (PCDD)

(AS 2,3,7,8-PCDD EQUIVALENT)TAC •

1085

1086

2,3,7,8-TETRACHLORODIBENZO-PDIOXINTAC 1746-01-6 4.0E-05 2/00 1.0E-08 10/00 1.3E+05 

TAC

8/86 1.3E+05

TAC

8/86 1

1,2,3,7,8-PENTACHLORODIBENZOP-DIOXIN 40321-76-4 8.0E-05 2/00 2.0E-08 10/00 1.3E+05 4/99 1.3E+05 10/00 1

1,2,3,4,7,8-HEXACHLORODIBENZOP-DIOXIN 39227-28-6 4.0E-04 2/00 1.0E-07 10/00 1.3E+04 4/99 1.3E+04 10/00 1

1,2,3,6,7,8-HEXACHLORODIBENZOP-DIOXIN 57653-85-7 4.0E-04 2/00 1.0E-07 10/00 1.3E+04 4/99 1.3E+04 10/00 1

1,2,3,7,8,9-HEXACHLORODIBENZOP-DIOXIN 19408-74-3 4.0E-04 2/00 1.0E-07 10/00 1.3E+04 4/99 1.3E+04 10/00 1

1,2,3,4,6,7,8-

HEPTACHLORODIBENZO-P-DIOXIN 35822-46-9 4.0E-03 2/00 1.0E-06 10/00 1.3E+03 4/99 1.3E+03 10/00 1

1,2,3,4,6,7,8,9-

OCTACHLORODIBENZO-P-DIOXIN 3268-87-9 4.0E-02 2/00 1.0E-05 10/00 1.3E+01 4/99 1.3E+01 10/00 1

POLYCHLORINATED DIBENZOFURANS 

(AS 2,3,7,8-PCDD EQUIVALENT) 

(PCDF) TAC •

1080

2,3,7,8-

TETRACHLORODIBENZOFURAN 5120-73-19 4.0E-04 2/00 1.0E-07 10/00 1.3E+04 4/99 1.3E+04 10/00 1

1,2,3,7,8-

PENTACHLORODIBENZOFURAN 57117-41-6 8.0E-04 2/00 2.0E-07 10/00 6.5E+03 4/99 6.5E+03 10/00 1

2,3,4,7,8-

PENTACHLORODIBENZOFURAN 57117-31-4 8.0E-05 2/00 2.0E-08 10/00 6.5E+04 4/99 6.5E+04 10/00 1

1,2,3,4,7,8-

HEXACHLORODIBENZOFURAN 70648-26-9 4.0E-04 2/00 1.0E-07 10/00 1.3E+04 4/99 1.3E+04 10/00 1
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-11

Noncancer Effects Cancer Risk

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Acute 

Inhalation

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Inhalation 

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Oral 

REL

(mg/kg/d)

Date u

Value 

Reviewed

[Added]

Inhalation

Cancer 

Potency 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

Oral Slope 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

M®

W

A

F

1,2,3,6,7,8-

HEXACHLORODIBENZOFURAN 57117-44-9 4.0E-04 2/00 1.0E-07 10/00 1.3E+04 4/99 1.3E+04 10/00 1

1,2,3,7,8,9-

HEXACHLORODIBENZOFURAN 72918-21-9 4.0E-04 2/00 1.0E-07 10/00 1.3E+04 4/99 1.3E+04 10/00 1

2,3,4,6,7,8-

HEXACHLORODIBENZOFURAN 60851-34-5 4.0E-04 2/00 1.0E-07 10/00 1.3E+04 4/99 1.3E+04 10/00 1

1,2,3,4,6,7,8-

HEPTACHLORODIBENZOFURAN 67562-39-4 4.0E-03 2/00 1.0E-06 10/00 1.3E+03 4/99 1.3E+03 10/00 1

1,2,3,4,7,8,9-

HEPTACHLORODIBENZOFURAN 55673-89-7 4.0E-03 2/00 1.0E-06 10/00 1.3E+03 4/99 1.3E+03 10/00 1

1,2,3,4,6,7,8,9-

OCTACHLORODIBENZOFURAN 39001-02-0 4.0E-02 2/00 1.0E-05 10/00 1.3E+01 4/99 1.3E+01 10/00 1

POLYCYCLIC AROMATIC 

HYDROCARBON (PAH)

1150

1151

BENZ(A)ANTHRACENEv 56-55-3 3.9E-01 4/99

[4/94] 1.2E+00 10/00[4/94] 1

BENZO(A)PYRENEv 50-32-8 3.9E+00 4/99

[4/94] 1.2E+01 10/00[4/94] 1

BENZO(B)FLUORANTHENEv 205-99-2 3.9E-01 4/99

[4/94] 1.2E+00 10/00[4/94] 1

BENZO(J)FLUORANTHENEv 205-82-3 3.9E-01 4/99

[4/94] 1.2E+00 10/00[4/94] 1

BENZO(K)FLUORANTHENEv 207-08-9 3.9E-01 4/99

[4/94] 1.2E+00 10/00[4/94] 1

CHRYSENEv 218-01-9 3.9E-02 4/99

[4/94] 1.2E-01 10/00[4/94] 1

DIBENZ(A,H)ACRIDINEv 226-36-8 3.9E-01 4/99

[4/94] 1.2E+00 10/00[4/94] 1

DIBENZ(A,H)ANTHRACENEv 53-70-3 4.1E+00 4/99

[4/94] 4.1E+00 10/00[4/94] 1

DIBENZ(A,J)ACRIDINEv 224-42-0 3.9E-01 4/99

[4/94] 1.2E+00 10/00[4/94] 1

DIBENZO(A,E)PYRENEv 192-65-4 3.9E+00 4/99

[4/94] 1.2E+01 10/00[4/94] 1

DIBENZO(A,H)PYRENEv 189-64-0 3.9E+01 4/99

[4/94] 1.2E+02 10/00[4/94] 1
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-12

Noncancer Effects Cancer Risk

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Acute 

Inhalation

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Inhalation 

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Oral 

REL

(mg/kg/d)

Date u

Value 

Reviewed

[Added]

Inhalation

Cancer 

Potency 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

Oral Slope 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

M®

W

A

F

DIBENZO(A,I)PYRENEv 189-55-9 3.9E+01 4/99

[4/94] 1.2E+02 10/00[4/94] 1

DIBENZO(A,L)PYRENEv 191-30-0 3.9E+01 4/99

[4/94] 1.2E+02 10/00[4/94]

1

7H-DIBENZO(C,G)CARBAZOLEv 194-59-2 3.9E+00 4/99

[4/94]

1.2E+01 10/00

[4/94] 1

7,12-

DIMETHYLBENZ(A)ANTHRACENEv 57-97-6 2.5E+02 4/99[4/94] 2.5E+02 10/00

[4/94] 1

1,6-DINITROPYRENEv 42397-64-8 3.9E+01 4/99

[4/94] 1.2E+02 10/00[4/94] 1

1,8-DINITROPYRENEv 42397-65-9 3.9E+00 4/99

[4/94] 1.2E+01 10/00[4/94] 1

INDENO(1,2,3-C,D)PYRENEv 193-39-5 3.9E-01 4/99

[4/94] 1.2E+00 10/00[4/94] 1

3-METHYLCHOLANTHRENEv 56-49-5 2.2E+01 4/99

[4/94] 2.2E+01 10/00[4/94] 1

5-METHYLCHRYSENEv 3697-24-3 3.9E+00 4/99

[4/94] 1.2E+01 10/00[4/94] 1

NAPHTHALENE 91-20-3 9.0E+00 4/00 --

5-NITROACENAPHTHENEv 602-87-9 1.3E-01 4/99

[4/94] 1.3E-01 10/00[4/94] 1

6-NITROCHRYSENEv 7496-02-8 3.9E+01 4/99

[4/94]

1.2E+02 10/00

[4/94] 1

2-NITROFLUORENEv 607-57-8 3.9E-02 4/99

[4/94] 1.2E-01 10/00[4/94] 1

1-NITROPYRENEv 5522-43-0 3.9E-01 4/99

[4/94] 1.2E+00 10/00[4/94] 1

4-NITROPYRENEv 57835-92-4 3.9E-01 4/99

[4/94] 1.2E+00 10/00[4/94] 1

POTASSIUM BROMATE....

... (see Bromine & Compounds)

1,3-PROPANE SULTONE 1120-71-4 2.4E+00 4/99 1

PROPYLENE (PROPENE) 115-07-1 3.0E+03 4/00 --

PROPYLENE GLYCOL MONOMETHYL 

ETHER 107-98-2 7.0E+03 2/00

--

PROPYLENE OXIDE 75-56-9 3.1E+03 4/99 3.0E+01 2/00 1.3E-02 4/99

[7/90] 1

SELENIUM AND COMPOUNDS 7782-49-2

[1170] 2.0E+01 12/01 --

HYDROGEN SELENIDE 7783-07-5 5.0E+00 4/99 --

Selenium sulfide 7446-34-6 2.0E+01 12/01 --

SODIUM HYDROXIDE 1310-73-2 8.0E+00 4/99 4.8E+00 7/90 --
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-13

Noncancer Effects Cancer Risk

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Acute 

Inhalation

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Inhalation 

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Oral 

REL

(mg/kg/d)

Date u

Value 

Reviewed

[Added]

Inhalation

Cancer 

Potency 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

Oral Slope 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

M®

W

A

F

STYRENE 100-42-5 2.1E+04 4/99 9.0E+02 4/00 --

SULFATES 9960 1.2E+02 4/99 2.5E+01 1/92 --

SULFUR DIOXIDE 7446-09-5 6.6E+02 4/99[1/92] 6.6E+02 1/92 --

SULFURIC ACID AND OLEUM 7664-93-9 1.2E+02 4/99 1.0E+00 12/01 --

SULFURIC ACID 7664-93-9 1.2E+02 4/99 1.0E+00 12/01 --

SULFUR TRIOXIDE 7446-71-9 1.2E+02 4/99 --

OLEUM 8014-95-7 1.2E+02 4/99 1.0E+00 12/01 --

1,1,2,2-TETRACHLOROETHANE 79-34-5 2.0E-01 4/99 1

TETRACHLOROPHENOLS 

 ... (see Chlorophenols)

2,4,5-TRICHLOROPHENOL 

 ... (see Chlorophenols)

2,4,6-TRICHLOROPHENOL 

... (see Chlorophenols)

THIOACETAMIDE 62-55-5 6.1E+00 4/99 1

TOLUENE 108-88-3 3.7E+04 4/99 3.0E+02 4/00 --

Toluene diisocyantates 26471-62-5

1204 7.0E-02 1/01 3.9E-02 4/99 1

TOLUENE-2,4-DIISOCYANATE 584-84-9 7.0E-02 1/01 3.9E-02 4/99 1

TOLUENE-2,6-DIISOCYANATE 91-08-7 7.0E-02 1/01 3.9E-02 4/99 1

1,1,2-TRICHLOROETHANE 

(Vinyl trichloride) 79-00-5 5.7E-02 4/99 1

TRICHLOROETHYLENETAC 79-01-6 6.0E+02 4/00 7.0E-03 

TAC

10/90 1

TRIETHYLAMINE 121-44-8 2.8E+03 4/99 2.0E

+02 9/02 --

URETHANE (Ethyl carbamate) 51-79-6 1.0E+00 4/99

[7/90] 1

Vanadium Compounds N/A

Vanadium (fume or dust) 7440-62-2 3.0E+01 4/99 --

VANADIUM PENTOXIDE 1314-62-1 3.0E+01 4/99 --

VINYL ACETATE 108-05-4 2.0E+02 12/01 --

VINYL CHLORIDETAC (Chloroethylene) 75-01-4 1.8E+05 4/99 2.7E-01 

TAC

12/90 1

VINYLIDENE CHLORIDE 

(1,1-Dichloroethylene)

75-35-4 7.0E+01 1/01 --
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-14

Noncancer Effects Cancer Risk

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Acute 

Inhalation

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Inhalation 

REL

(μg/m3)

Date u

Value 

Reviewed

[Added]

Chronic 

Oral 

REL

(mg/kg/d)

Date u

Value 

Reviewed

[Added]

Inhalation

Cancer 

Potency 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

Oral Slope 

Factor

(mg/kg-d)-1

Date u

Value 

Reviewed

[Added]

M®

W

A

F

XYLENES (mixed isomers) 1330-20-7

1210 2.2E+04 4/99 7.0E+02 4/00 --

m-XYLENE 108-38-3 2.2E+04 4/99 7.0E+02 4/00 --

o-XYLENE 95-47-6 2.2E+04 4/99 7.0E+02 4/00 --

p-XYLENE 106-42-3 2.2E+04 4/99 7.0E+02 4/00 --
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-15

Purpose: The purpose of this reference table is to provide a quick list of all health values that have been approved by the Office of Environmental Health Hazard Assessment (OEHHA) and the Air Resources Board (ARB) 

for use in facility health risk assessments conducted for the AB 2588 Air Toxics Hot Spots Program. The OEHHA has developed and adopted new risk assessment guidelines that update and replace the California 

Air Pollution Control Officers Association’s (CAPCOA) Air Toxics “Hot Spots” Program Revised 1992 Risk Assessment Guidelines, October 1993. The OEHHA has adopted five technical support documents for 

these guidelines. 

This table lists the OEHHA adopted inhalation and oral cancer slope factors, noncancer acute Reference Exposure Levels (RELs), and inhalation and oral noncancer chronic RELs. In addition, it lists the substances 

in Appendix A-I (Substances For Which Emissions Must Be Quantified) and Appendix F (Criteria For Inputs For Risk Assessment Using Screening Air Dispersion Modeling) of the ARB’s Air Toxics “Hot Spots” 

Emission Inventory Criteria and Guidelines (EICG). OEHHA is still in the process of adopting new noncancer chronic RELs. Therefore, new health values will periodically be added to, or deleted from, this table. 

Users of this table are advised to monitor the OEHHA website (www.oehha.ca.gov) for any updates to the health values.

b Substances written in italics do not have explicit OEHHA approved health values, but are included in this table to clarify applicability of OEHHA adopted heath effects values to individual or grouped substances 

listed in the Air Toxics “Hot Spots” Emission Inventory Criteria and Guidelines, Appendix A-I list of “Substances For Which Emissions Must Be Quantified”.

t Chemical Abstract Service Number (CAS): For chemical groupings and mixtures where a CAS number is not applicable, the 4-digit code used in the Air Toxics “Hot Spots” Emission Inventory Criteria and 

Guidelines (EICG) Report is listed. The 4-digit codes enclosed in brackets [ ] are codes that have been phased out, but may still appear on previously reported Hot Spots emissions. For information on the origin 

and use of the 4-digit code, see the EICG report.

u Date Value Reviewed [Added]: These columns list the date that the health value was last reviewed by OEHHA and the Scientific Review Panel, and/or approved for use in the AB 2588 Air Toxics Hot Spots 

Program. If the health value is unchanged since it was first approved for use in the Hot Spots Program, then the date that the value was first approved for use by CAPCOA is listed within the brackets [ ]. 

• April 1999 is listed for the cancer potency values and noncancer acute RELs, which have been adopted by the OEHHA as part of the AB 2588 “Hot Spot” Risk Assessment Guidelines. 

• February 2000, April 2000, January 2001, and December 2001 are listed for the first set of 22, the second set of 16, the third set of 22, and the fourth set of 12 noncancer chronic RELs, respectively. 

• October 2000 is listed for the oral chronic RELs and oral cancer slope factors. 1996 is listed for the U.S. EPA Reference Concentrations. Dates of 1990-1992 and 1996 are listed for CAPCOA chronic RELs, 

which may eventually be dropped or replaced. 

• For the substances identified as Toxic Air Contaminants, the Air Resources Board hearing date is listed. The dates for acetaldehyde, benzo[a]pyrene, and methyl tertiary-butyl ether represent the dates the values 

were approved by the Scientific Review Panel.

♣ Molecular Weight Adjustment Factor: Molecular weight adjustment factors (MWAF) are only to be used when a toxic metal has a cancer potency factor. For most of the Hot Spots toxic metals, the OEHHA cancer 

potency factor applies to the weight of the toxic metal atom contained in the overall compound. Some of the Hot Spots compounds contain various elements along with the toxic metal atom 

(e.g.,“Nickel hydroxide”, CAS number 12054-48-7, has a formula of H2NiO2). Therefore, an adjustment to the reported pounds of the overall compound is needed before applying the OEHHA cancer potency 

factor for “Nickel and compounds” to such a compound. This ensures that the cancer potency factor is applied only to the fraction of the overall weight of the emissions that are associated with health effects of the 

metal. In other cases, the Hot Spots metals are already reported as the metal atom equivalent (e.g., CAS 7440-02-0, “Nickel”), and these cases do not use any further molecular weight adjustment. (Refer to 

Note [7] in Appendix A, List of Substances in the EICG Report for further information on how the emissions of various Hot Spots metal compounds are reported.) The appropriate molecular weight adjustment 

factors (MWAF) to be used along with the OEHHA cancer potency factors for Hot Spots metals can be found in the MWAF column of this table. A double dash (--) was entered into the column if the substance 

does not currently have a cancer potency factor.

So, for example, assume 100 pounds of “Nickel hydroxide” emissions are reported under CAS number 12054-48-7. To get the Nickel atom equivalent of these emissions, multiply by the listed MWAF (0.6332) for 

Nickel hydroxide: 

• 100 pounds x 0.6332 = 63.32 pounds of Nickel atom equivalent

This step should be completed prior to applying the OEHHA cancer potency factor for “Nickel and compounds” in a calculation for a prioritization score or risk assessment calculation. (For more information see 

Chapter 4 and Appendix H of OEHHA’s The Air Toxics Hot Spots Program Risk Assessment Guidelines; Part V; Technical Support Document; Guidance Manual for Preparation of Health Risk Assessments.)

Note: The value listed in the MWAF column for Asbestos is not a molecular weight adjustment. This is a conversion factor for adjusting mass to fibers or structures. See Appendix C of OEHHA’s The Air Toxics 

Hot Spots Program Risk Assessment Guidelines; Part V; Technical Support Document; Guidance Manual for Preparation of Health Risk Assessments for more information on Asbestos, or see the EICG report for 

reporting guidance. Also see the Asbestos footnote (designated by the symbol 3)

N/A Not Applicable

 Ñ Values calculated using WHO TEF procedure in OEHHA, 2003 

TAC Toxic Air Contaminant: The Air Resources Board has identified this substance as a Toxic Air Contaminant.
APPENDIX L - TABLE 1

OEHHA/ARB APPROVED HEALTH VALUES FOR USE IN HOT SPOT FACILITY RISK ASSESSMENTS b

Appendix L-16

AveP The averaging period of noncancer acute RELs is generally a one-hour exposure. However, some are based on several hour exposure for reproduct ive/developmental endpoints (see section 1.6 of OEHHA’s 

technical support document for The Determination of Acute Reference Exposure Levels for Airborne Toxicants, March 1999). Typically the RELs for the following substances are compared to modeled emission 

concentrations of the same duration rather than maximum one-hour concentrations (e.g., a 4-hour REL should be compared to the maximum 4-hour average concentration from the air dispersion model).

4-Hour: Arsenic and Inorganic Arsenic Compounds

6-Hour: Benzene, Carbon disulfide, Ethylene glycol monoethyl ether, Ethylene glycol monoethyl ether acetate, Ethylene glycol monomethyl ether

7-Hour: Carbon tetrachloride, Chloroform

3 Asbestos: The units for the Inhalation Cancer Potency factor for asbestos are (100 PCM fibers/m3)

-1

. A conversion factor of 100 fibers/0.003 μg can be multiplied by a receptor concentration of asbestos expressed 

in μg/m3. Unless other information necessary to estimate the concentration (fibers/m 3) of asbestos at receptors of interest is available. A unit risk factor of 2.7E 10-6 (μg/m3)

-1

 and an inhalation cancer potency 

factor of 2.2E 10+2 (mg/kg BW * day)-1 are available. For more information on asbestos quantity conversion factors, see Appendix C of OEHHA’s The Air Toxics Hot Spots Program Risk Assessment Guidelines; 

Part II; Technical Support Document for Describing Available Cancer Potency Factors, and Appendix C of OEHHA’s The Air Toxics Hot Spots Program Risk Assessment Guidelines; Part V; Technical Support 

Document; Guidance Manual for Preparation of Health Risk Assessments. 

W Inorganic Lead: Inorganic Lead was identified by the Air Resources Board as a Toxic Air Contaminant in April 1997. Since information on noncancer health effects show no identified threshold, no Reference 

Exposure Level has been developed. The document, Risk Management Guidelines for New, Modified, and Existing Sources of Lead, March 2001, has been developed by ARB and OEHHA staff for assessing 

noncancer health impacts from sources of lead. See Appendix F of OEHHA’s The Air Toxics Hot Spots Program Risk Assessment Guidelines; Part V; Technical Support Document; Guidance Manual for 

Preparation of Health Risk Assessments for an overview of how to evaluate noncancer impacts from exposure to lead using these risk management guidelines.

v Polycyclic Aromatic Hydrocarbons (PAHs): These substances are PAH or PAH-derivatives that have OEHHA-developed Potency Equivalency Factors (PEFs) which were approved by the Scientific Review Panel 

in April 1994 (see ARB document entitled Benzo [a]pyrene as a Toxic Air Contaminant). PAH inhalation slope factors listed here have been adjusted by the PEFs. See Appendix G of OEHHA’s The Air Toxics 

Hot Spots Program Risk Assessment Guidelines; Part V; Technical Support Document; Guidance Manual for Preparation of Health Risk Assessments for more information.

H Polychlorinated Biphenyls: (unspeciated mixtures)

Lowest Risk: For use in cases where congeners with more than four chlorines comprise less than one-half percent of total polychlorinated biphenyls.

High Risk: For use in cases where congeners with more than four chlorines do not comprise less than one-half percent of total polychlorinated biphenyls. 

The Low Risk: This number would not ordinarly be used in the Hot Spots program. 

• Polychlorinated Dibenzo-p-dioxins and Polychlorinated Dibenzofurans (also referred to as chlorinated dioxins and dibenzofurans): The OEHHA has adopted the World Health Organization 1997 (WHO-97) 

Toxicity Equivalency Factor ) scheme for evaluating the cancer and noncancer risk due to exposure to samples containing speciated mixtures of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated 

dibenzofurans (PCDF). See Appendix A of OEHHA’s Technical Support Document For Describing Available Cancer Potency Factors for more information about the scheme. See Appendix E of OEHHA’s The 

Air Toxics Hot Spots Program Risk Assessment Guidelines; Part V; Technical Support Document; Guidance Manual for Preparation of Health Risk Assessments for the methodology for calculatin g 2,3,7,8-

equivalents for PCDD and PCDFs. 

n Particulate Emissions from Diesel-Fueled Engines: The inhalation cancer potency factor and chronic REL were derived from whole diesel exhaust and should be used only for impacts from the inhalation pathway. 

The inhalation impacts from speciated emissions from diesel-fueled engines are already accounted for in the inhalation cancer potency factor and REL. However, at the discretion of the risk assessor, speciated 

emissions from diesel-fueled engines may be used to estimate acute noncancer health impacts or the contribution to cancer risk or chronic noncancer health impacts for the non-inhalation exposure pathway. See 

Appendix D of OEHHA’s The Air Toxics Hot Spots Program Risk Assessment Guidelines; Part V; Technical Support Document; Guidance Manual for Preparation of Health Risk Assessments for more 

information.

Table last updated: August, 2003
APPENDIX L - TABLE 2 OEHHA/ARB ACUTE REFERENCE EXPOSURE LEVELS AND TARGET ORGANS b

Appendix L-17

Target Organs

Substance b

Chemicalt

Abstract 

Service

Number

(CAS)

Acute REL

(μg/m3)

Date u

Value 

Reviewed

Alimentary Tract

Cardiovascular

Developmental

Eye

Hematologic

ImmuneNervous

Reproductive

Respiratory

Skin

ACROLEIN 107-02-8 1.9E-01 4/99 X X

ACRYLIC ACID 79-10-7 6.0E+03 4/99 X X

AMMONIA 7664-41-7 3.2E+03 4/99 X X

ARSENIC AND COMPOUNDS (INORGANIC) TAC

7440-38-2

1016

[1015]

1.9E-01AveP 4/99 X X

ARSINE 7784-42-1 1.6E+02 4/99 X

BENZENETAC 71-43-2 1.3E+03 AveP 4/99 X X X X

BENZYL CHLORIDE 100-44-7 2.4E+02 4/99 X X

CARBON DISULFIDE 75-15-0 6.2E+03 AveP 4/99 X X X

CARBON MONOXIDE 630-08-0 2.3E+04 4/99 X

CARBON TETRACHLORIDETAC (Tetrachloromethane) 56-23-5 1.9E+03 AveP 4/99 X X X X

CHLORINE 7782-50-5 2.1E+02 4/99 X X

CHLOROFORM TAC 67-66-3 1.5E+02 AveP 4/99 X X X

CHLOROPICRIN 76-06-2 2.9E+01 4/99 X X

COPPER AND COMPOUNDS 7440-50-8

[1067] 1.0E+02 4/99 X

Cyanide Compounds (inorganic) 57-12-5

1073 3.4E+02 4/99  ̧

HYDROGEN CYANIDE (Hydrocyanic acid) 74-90-8 3.4E+02 4/99 X

1,4-DIOXANE; (1,4-Diethylene dioxide) 123-91-1 3.0E+03 4/99 X X

EPICHLOROHYDRIN (1-Chloro-2,3-epoxypropane) 106-89-8 1.3E+03 4/99 X X

Fluorides and Compounds 1101 2.4E+02 4/99  ̧  ̧

HYDROGEN FLUORIDE (Hydrofluoric acid) 7664-39-3 2.4E+02 4/99 X X

FORMALDEHYDETAC 50-00-0 9.4E+01 4/99 X X X

GLYCOL ETHERS 1115

ETHYLENE GLYCOL BUTYL ETHER – EGBE 111-76-2 1.4E+04 4/99 X X

ETHYLENE GLYCOL ETHYL ETHER – EGEE 110-80-5 3.7E+02 AveP 4/99

[1/92] X X

ETHYLENE GLYCOL ETHYL ETHER ACETATE - EGEEA 111-15-9 1.4E+02 AveP 4/99 X X X

ETHYLENE GLYCOL METHYL ETHER – EGME 109-86-4 9.3E+01 AveP 4/99 X X
APPENDIX L - TABLE 2 OEHHA/ARB ACUTE REFERENCE EXPOSURE LEVELS AND TARGET ORGANS b

Appendix L-18

Target Organs

Substance b

Chemicalt

Abstract 

Service

Number

(CAS)

Acute REL

(μg/m3)

Date u

Value 

Reviewed

Alimentary Tract

Cardiovascular

Developmental

Eye

Hematologic

ImmuneNervous

Reproductive

Respiratory

Skin

HYDROCHLORIC ACID (Hydrogen chloride) 7647-01-0 2.1E+03 4/99 X X

HYDROGEN CYANIDE (Hydrocyanic acid) ... (see Cyanide Compounds)

HYDROGEN FLUORIDE (Hydrofluoric acid) ... (see Fluorides & Compounds)

HYDROGEN SELENIDE ... (see Selenium & Compounds)

HYDROGEN SULFIDE 7783-06-4 4.2E+01 4/99

[7/90] X

ISOPROPYL ALCOHOL (Isopropanol) 67-63-0 3.2E+03 4/99 X X

MERCURY AND COMPOUNDS (INORGANIC) 7439-97-6

[1133] 1.8E+00 4/99 X X

Mercuric chloride 7487-94-7 1.8E+00 4/99  ̧  ̧

METHANOL 67-56-1 2.8E+04 4/99 X

METHYL BROMIDE (Bromomethane) 74-83-9 3.9E+03 4/99 X X X X

METHYL CHLOROFORM (1,1,1-Trichloroethane) 71-55-6 6.8E+04 4/99 X

METHYL ETHYL KETONE (2-Butanone) 78-93-3 1.3E+04 4/99 X X

METHYLENE CHLORIDETAC (Dichloromethane) 75-09-2 1.4E+04 4/99 X

NICKEL AND COMPOUNDSTAC 7440-02-0

[1145] 6.0E+00 4/99 X X

Nickel acetate, 373-02-4 6.0E+00 4/99  ̧  ̧

Nickel carbonate 3333-39-3 6.0E+00 4/99  ̧  ̧

Nickel carbonyl 13463-39-3 6.0E+00 4/99  ̧  ̧

Nickel hydroxide 12054-48-7 6.0E+00 4/99  ̧  ̧

Nickelocene 1271-28-9 6.0E+00 4/99  ̧  ̧

NICKEL OXIDE 1313-99-1 6.0E+00 4/99 X X

Nickel refinery dust from the pyrometallurgical process 1146 6.0E+00 4/99  ̧  ̧

Nickel subsulfide 12035-72-2 6.0E+00 4/99  ̧  ̧

NITRIC ACID 7697-37-2 8.6E+01 4/99 X

NITROGEN DIOXIDE 10102-44-0 4.7E+02 4/99

[1/92] X

OZONE 10028-15-6 1.8E+02 4/99

[1/92] X X

PERCHLOROETHYLENETAC (Tetrachloroethylene) 127-18-4 2.0E+04 4/99 X X X

PHENOL 108-95-2 5.8E+03 4/99 X X
APPENDIX L - TABLE 2 OEHHA/ARB ACUTE REFERENCE EXPOSURE LEVELS AND TARGET ORGANS b

Appendix L-19

Target Organs

Substance b

Chemicalt

Abstract 

Service

Number

(CAS)

Acute REL

(μg/m3)

Date u

Value 

Reviewed

Alimentary Tract

Cardiovascular

Developmental

Eye

Hematologic

ImmuneNervous

Reproductive

Respiratory

Skin

PHOSGENE 75-44-5 4.0E+00 4/99 X

PROPYLENE OXIDE 75-56-9 3.1E+03 4/99 X X X X

Selenium and Compounds 7782-49-2

[1170]

HYDROGEN SELENIDE 7783-07-5 5.0E+00 4/99 X X

SODIUM HYDROXIDE 1310-73-2 8.0E+00 4/99 X X X

STYRENE 100-42-5 2.1E+04 4/99 X X

SULFATES 9960 1.2E+02 4/99 X

SULFUR DIOXIDE 7446-09-5 6.6E+02 4/99

[1/92] X

SULFURIC ACID AND OLEUM 7664-93-9 1.2E+02 4/99 X

SULFURIC ACID 7664-93-9 1.2E+02 4/99  ̧

SULFUR TRIOXIDE 7446-71-9 1.2E+02 4/99  ̧

OLEUM 8014-95-7 1.2E+02 4/99  ̧

TOLUENE 108-88-3 3.7E+04 4/99 X X X X X

TRIETHYLAMINE 121-44-8 2.8E+03 4/99 X X

Vanadium Compounds N/A

Vanadium (fume or dust) 7440-62-2 3.0E+01 4/99  ̧  ̧

VANADIUM PENTOXIDE 1314-62-1 3.0E+01 4/99 X X

VINYL CHLORIDETAC (Chloroethylene) 75-01-4 1.8E+05 4/99 X X X

XYLENES (mixed isomers) 1330-20-7

1210 2.2E+04 4/99 X X

m-Xylene 108-38-3 2.2E+04 4/99 X X

o-Xylene 95-47-6 2.2E+04 4/99 X X

p-Xylene 106-42-3 2.2E+04 4/99 X X
APPENDIX L - TABLE 2 OEHHA/ARB ACUTE REFERENCE EXPOSURE LEVELS AND TARGET ORGANS b

Appendix L-20

Purpose: The purpose of this reference table is to provide a quick list of all health values that have been approved by the Office of Environmental Health Hazard Assessment (OEHHA) and 

the Air Resources Board (ARB) for use in facility health risk assessments conducted for the AB 2588 Air Toxics Hot Spots Program. The OEHHA has developed and adopted 

new risk assessment guidelines that update and replace the California Air Pollution Control Officers Association’s (CAPCOA) Air Toxics “Hot Spots” Program Revised 1992 

Risk Assessment Guidelines, October 1993. The OEHHA has adopted five technical support documents for these guidelines. 

This table lists the OEHHA adopted noncancer acute Reference Exposure Levels (RELs). In addition, it lists the substances in Appendix A-I (Substances For Which Emissions 

Must Be Quantified) and Appendix F (Criteria For Inputs For Risk Assessment Using Screening Air Dispersion Modeling) of the ARB’s Air Toxics “Hot Spots” Emission 

Inventory Criteria and Guidelines (EICG). Users of this table are advised to monitor the OEHHA website (www.oehha.ca.gov) for any updates to the health values.

b Substances written in italics and with a  ̧ do not have explicit OEHHA approved health values, but are included in this table to clarify applicability of OEHHA adopted health 

effects values to individual or grouped substances listed in the Air Toxics “Hot Spots” Emission Inventory Criteria and Guidelines, Appendix A-I list of “Substances For Which 

Emissions Must Be Quantified”.

t Chemical Abstract Service Number (CAS): For chemical groupings and mixtures where a CAS number is not applicable, the 4-digit code used in the Air Toxics “Hot Spots” 

Emission Inventory Criteria and Guidelines (EICG) Report is listed. The 4-digit codes enclosed in brackets [ ] are codes that have been phased out, but may still appear on 

previously reported Hot Spots emissions. For information on the origin and use of the 4-digit code, see the EICG report.

u Date Value Reviewed [Added]: This column lists the date that the health value was last reviewed by OEHHA and the Scientific Review Panel, and/or approved for use in the 

AB 2588 Air Toxics “Hot Spots” Program. If the health value is unchanged since it was first approved for use in the “Hot Spots” Program, then the date that the value was first 

approved for use by CAPCOA is listed within the brackets [ ]. 

• April 1999 is listed for the noncancer acute RELs which have been adopted by the OEHHA as part of the AB 2588 “Hot Spot” Risk Assessment Guidelines. 

TAC Toxic Air Contaminant: The Air Resources Board has identified this substance as a Toxic Air Contaminant.

AveP The averaging period of noncancer acute RELs is generally a one-hour exposure. However, some are based on several hour exposure for reproductive/developmental endpoints 

(see sect ion 1.6 of OEHHA’s technical support document for The Determination of Acute Reference Exposure Levels for Airborne Toxicants, March 1999). Typically the RELs 

for the following substances are compared to modeled emission concentrations of the same duration rather than maximum one-hour concentrations (e.g., a 4-hour REL should be 

compared to the maximum 4-hour average concentration from the air dispersion model).

4-Hour: Arsenic and Inorganic Arsenic Compounds

6-Hour: Benzene, Carbon disulfide, Ethylene glycol ethyl ether, Ethylene glycol ethyl ether acetate, Ethylene glycol methyl ether

7-Hour: Carbon tetrachloride, Chloroform

Table last updated: August 2003
APPENDIX L - TABLE 3 OEHHA/ARB CHRONIC INHALATION AND ORAL REFERENCE EXPOSURE LEVELS AND TARGET ORGANS b

Appendix L-21

Target Organs

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Chronic 

Inhalation 

REL

(μg/m3)

Chronic 

Oral REL

Date u

Value 

Reviewed 

[Added]

Alimentary

Bone

Cardiovascular

Developmental

Endocrine

Eye

Hematologic

Immune

Kidney

Nervous

Reproductive

Respiratory

Skin

ACETALDEHYDE 75-07-0 9.0E+00 5/93 X

ACROLEIN 107-02-8 6.0E-02 1/01 X X

ACRYLONITRILE 107-13-1 5.0E+00 12/01 X

AMMONIA 7664-41-7 2.0E+02 2/00 X

3.0E-02 1/01 X X X ARSENIC AND COMPOUNDS (INORGANIC) TAC

7440-38-2

1016

[1015] 3.0E

-04 10/00 X X

BENZENETAC 71-43-2 6.0E+01 2/00 X X X

7.0E-03 12/01 X X BERYLLIUM AND COMPOUNDS 7440-41-7

[1021] 2.0E

-03 12/01 X

1,3-BUTADIENETAC 106-99-0 2.0E+01 1/01 X

2.0E-02 1/01 X X CADMIUM AND COMPOUNDST7AC 7440-43-9

[1045] 5.0E

-04 10/00 X

CARBON DISULFIDE 75-15-0 8.0E+02 11/01 X X

CARBON TETRACHLORIDETAC (Tetrachloromethane) 56-23-5 4.0E+01 1/01 X X X

CHLORINE 7782-50-5 2.0E-01 2/00 X

CHLORINE DIOXIDE 10049-04-4 6.0E-01 1/01 X

CHLOROBENZENE 108-90-7 1.0E+03 1/01 X X X

CHLOROFORM TAC 67-66-3 3.0E+02 4/00 X X X

CHLOROPICRIN 76-06-2 4.0E-01 12/01 X

CHROMIUM 6+TAC 18540-29-9

2.0E-01 1/01 X

2.0E-02 10/00 X

2.0E-01 1/01  ̧

Barium chromate 10294-40-3

2.0E-02 10/00  ̧

2.0E-01 1/01  ̧

Calcium chromate 13765-19-0

2.0E-02 10/00  ̧

2.0E-01 1/01  ̧

Lead chromate 7758-97-6

2.0E-02 10/00  ̧
APPENDIX L - TABLE 3 OEHHA/ARB CHRONIC INHALATION AND ORAL REFERENCE EXPOSURE LEVELS AND TARGET ORGANS b

Appendix L-22

Target Organs

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Chronic 

Inhalation 

REL

(μg/m3)

Chronic 

Oral REL

Date u

Value 

Reviewed 

[Added]

Alimentary

Bone

Cardiovascular

Developmental

Endocrine

Eye

Hematologic

Immune

Kidney

Nervous

Reproductive

Respiratory

Skin

2.0E-01

Sodium dichromate 10588-01-9

1/01  ̧

2.0E-02 10/00  ̧

2.0E-01 1/01  ̧

Strontium chromate 7789-06-2

2.0E-02 10/00  ̧

2.0E-03 1/01 X CHROMIUM TRIOXIDE (as chromic acid mist) 1333-82-0

2.0E-02 10/00  ̧

CRESOLS (mixtures of) 1319-77-3 6.0E+02 1/01 X

m-CRESOL 108-39-4 6.0E+02 1/01 X

o-CRESOL 95-48-7 6.0E+02 1/01 X

p-CRESOL 106-44-5 6.0E+02 1/01 X

Cyanide Compounds (inorganic) 57-12-5

1073 9.0E+00 4/00  ̧  ̧  ̧

HYDROGEN CYANIDE (Hydrocyanic acid) 74-90-8 9.0E+00 4/00 X X X

p-DICHLOROBENZENE 106-46-7 8.0E+02 1/01 X X X X

1,1,-DICHLOROETHYLENE ... (see Vinylidene Chloride)

DIESEL EXHAUST 

 ... (see Particulate Emissions from Diesel-Fueled Engines)

DIETHANOLAMINE 111-42-2 3.0E+00 12/01 X X

N,N-DIMETHYL FORMAMIDE 68-12-2 8.0E+01 1/01 X X

1,4-DIOXANE; (1,4-Diethylene dioxide) 123-91-1 3.0E+03 4/00 X X X

EPICHLOROHYDRIN (1-Chloro-2,3-epoxypropane) 106-89-8 3.0E+00 1/01 X X

1,2-EPOXYBUTANE 106-88-7 2.0E+01 1/01 X X

ETHYL BENZENE 100-41-4 2.0E+03 2/00 X X X X

ETHYL CHLORIDE (Chlorethane) 75-00-3 3.0E+04 4/00 X X

ETHYLENE DIBROMIDETAC (1,2-Dibromoethane) 106-93-4 8.0E-01 12/01 X

ETHYLENE DICHLORIDETAC (1,2-Dichloroethane) 107-06-2 4.0E+02 1/01 X

ETHYLENE GLYCOL 107-21-1 4.0E+02 4/00 X X X

ETHYLENE OXIDETAC (1,2-Epoxyethane) 75-21-8 3.0E+01 1/01 X
APPENDIX L - TABLE 3 OEHHA/ARB CHRONIC INHALATION AND ORAL REFERENCE EXPOSURE LEVELS AND TARGET ORGANS b

Appendix L-23

Target Organs

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Chronic 

Inhalation 

REL

(μg/m3)

Chronic 

Oral REL

Date u

Value 

Reviewed 

[Added]

Alimentary

Bone

Cardiovascular

Developmental

Endocrine

Eye

Hematologic

Immune

Kidney

Nervous

Reproductive

Respiratory

Skin

Fluorides 1101 1.3E+1 4.0E-2 8/03 X  ̧*  ̧  ̧*

HYDROGEN FLUORIDE (Hydrofluoric acid) 7664-39-3 1.4E+1 4.0E-2 8/03 X X* X X*

FORMALDEHYDETAC 50-00-0 3.0E+00 2/00 X X

GLUTARALDEHYDE 111-30-8 8.0E-02 1/01 X

GLYCOL ETHERS 1115

ETHYLENE GLYCOL ETHYL ETHER – EGEE 110-80-5 7.0E+01 2/00 X X

ETHYLENE GLYCOL ETHYL ETHER ACETATE - EGEEA 111-15-9 3.0E+02 2/00 X

ETHYLENE GLYCOL METHYL ETHER – EGME 109-86-4 6.0E+01 2/00 X

ETHYLENE GLYCOL METHYL ETHER ACETATE –

EGMEA 110-49-6 9.0E+01 2/00 X

n-HEXANE 110-54-3 7.0E+03 4/00 X

HYDRAZINE 302-01-2 2.0E-01 1/01 X X

HYDROCHLORIC ACID (Hydrogen chloride) 7647-01-0 9.0E+00 2/00 X

HYDROGEN CYANIDE (Hydrocyanic acid) 

(see Cyanide Compounds)

HYDROGEN FLUORIDE (Hydrofluoric acid) 

(see Fluorides & Compounds)

HYDROGEN SULFIDE 7783-06-4 1.0E+01 4/00 X

ISOPHORONE 78-59-1 2.0E+03 12/01 X X

ISOPROPYL ALCOHOL (Isopropanol) 67-63-0 7.0E+03 2/00 X X

MALEIC ANHYDRIDE 108-31-6 7.0E-01 12/01 X

MANGANESE AND COMPOUNDS 7439-96-5

[1132] 2.0E-01 4/00 X

9.0E-02 2/00 X MERCURY AND COMPOUNDS (INORGANIC) 7439-97-6

[1133] 3.0E

-04 10/00

[1/92] X X

9.0E-02 2/00  ̧

Mercuric chloride 7487-94-7

3.0E-04 10/00

[1/92]  ̧  ̧

MERCURY AND COMPOUNDS (ORGANIC) N/A

METHANOL 67-56-1 4.0E+03 4/00 X

METHYL BROMIDE (Bromomethane) 74-83-9 5.0E+00 2/00 X X X

METHYL tertiary-BUTYL ETHER 1634-04-4 8.0E+03 2/00 X X X
APPENDIX L - TABLE 3 OEHHA/ARB CHRONIC INHALATION AND ORAL REFERENCE EXPOSURE LEVELS AND TARGET ORGANS b

Appendix L-24

Target Organs

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Chronic 

Inhalation 

REL

(μg/m3)

Chronic 

Oral REL

Date u

Value 

Reviewed 

[Added]

Alimentary

Bone

Cardiovascular

Developmental

Endocrine

Eye

Hematologic

Immune

Kidney

Nervous

Reproductive

Respiratory

Skin

METHYL CHLOROFORM (1,1,1-Trichloroethane) 71-55-6 1.0E+03 2/00 X

METHYL ISOCYANATE 624-83-9 1.0E+00 12/01 X X

METHYLENE CHLORIDETAC (Dichloromethane) 75-09-2 4.0E+02 2/00 X X

4,4'-METHYLENE DIANILINE (AND ITS DICHLORIDE) 101-77-9 2.0E+01 12/01 X X

METHYLENE DIPHENYL ISOCYANATE 101-68-8 7.0E-01 1/01 X

NAPHTHALENE 91-20-3 9.0E+00 4/00 X

5.0E-02 2/00 X X

NICKEL AND COMPOUNDSTAC 

7440-02-0

[1145] 5.0E-02 10/00 X

Nickel acetate 373-02-4

5.0E-02 2/00  ̧  ̧

5.0E-02 10/00  ̧

Nickel carbonate 3333-39-3

5.0E-02 2/00  ̧  ̧

5.0E-02 10/00  ̧

Nickel carbonyl 13463-39-3

5.0E-02 2/00  ̧  ̧

5.0E-02 10/00  ̧

Nickel hydroxide 12054-48-7

5.0E-02 2/00  ̧  ̧

5.0E-02 10/00  ̧

Nickelocene 1271-28-9

5.0E-02 2/00  ̧  ̧

5.0E-02 10/00  ̧

1.0E-01 2/00 X X NICKEL OXIDE 1313-99-1

5.0E-02 10/00 X

Nickel refinery dust from pyrometallurgical process 1146

5.0E-02 2/00  ̧  ̧

5.0E-02 10/00  ̧

Nickel subsulfide 12035-72-2

5.0E-02 2/00  ̧  ̧

5.0E-02 10/00  ̧

PCB (POLYCHLORINATED BIPHENYLS (speciated)∇

35298-13-3 4.0E-01 8/03 X X X X X X 3,3’,4,4’-TETRACHLOROBIPHENYL (77)

1.0E –04 8/03 X X X X X X

70362-50-4 4.0E-01 8/03 X X X X X X

3,4,4’,5-TETRACHLOROBIPHENYL (81)

1.0E –04 8/03 X X X X X X
APPENDIX L - TABLE 3 OEHHA/ARB CHRONIC INHALATION AND ORAL REFERENCE EXPOSURE LEVELS AND TARGET ORGANS b

Appendix L-25

Target Organs

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Chronic 

Inhalation 

REL

(μg/m3)

Chronic 

Oral REL

Date u

Value 

Reviewed 

[Added]

Alimentary

Bone

Cardiovascular

Developmental

Endocrine

Eye

Hematologic

Immune

Kidney

Nervous

Reproductive

Respiratory

Skin

32598-14-4 4.0E-01 8/03 X X X X X X 2,3,3’,4,4’- PENTACHLOROBIPHENYL (105)

1.0E –04 8/03 X X X X X X

74472-37-0 8.0E-02 8/03 X X X X X X 2,3,4,4’5- PENTACHLOROBIPHENYL (114)

2.0E –05 8/03 X X X X X X

31508-00-6 4.0E-01 8/03 X X X X X X

2,3’4,4’,5- PENTACHLOROBIPHENYL (118)

1.0E –04 8/03 X X X X X X

65510-44-3 4.0E-01 8/03 X X X X X X 2’,3,4,4’,5- PENTACHLOROBIPHENYL (123)

1.0E –04 8/03 X X X X X X

57465-28-8 4.0E-04 8/03 X X X X X X

3,3’,4,4’,5- PENTACHLOROBIPHENYL (126)

1.0E –07 8/03 X X X X X X

38380-08-4 8.0E-02 8/03 X X X X X X 2,3,3’,4,4’,5-HEXACHLOROBIPHENYL (156)

2.0E –05 8/03 X X X X X X

69782-90-7 8.0E-02 8/03 X X X X X X 2,3,3’,4,4’,5’-HEXACHLOROBIPHENYL (157)

2.0E –05 8/03 X X X X X X

52663-72-6 4.0E-00 8/03 X X X X X X 2,3’,4,4’,5,5’-HEXACHLOROBIPHENYL (167)

1.0E –03 8/03 X X X X X X

32774-16-6 4.0E-03 8/03 X X X X X X 3,3’,4,4’5,5’- HEXACHLOROBIPHENYL (169)

1.0E –06 8/03 X X X X X X

39635-31-9 4.0E-01 8/03 X X X X X X

2,3,3’4,4’,5,5’- HEPTACHLOROBIPHENYL (189) 

1.0 E-04 8/03 X X X X X X

PARTICULATE EMISSIONS FROM 

DIESEL-FUELED ENGINESTACn

9901 5.0E+00TAC 8/98 X

PERCHLOROETHYLENETAC (Tetrachloroethylene) 127-18-4 3.5E+01TAC 10/91 X X

PHENOL 108-95-2 2.0E+02 4/00 X X X X

PHOSPHINE 7803-51-2 8.0E-1 9/02 X X X

PHOSPHORIC ACID 7664-38-2 7.0E+00 2/00 X

PHTHALIC ANHYDRIDE 85-44-9 2.0E+01 1/01 X

POLYCHLORINATED DIBENZO-P-DIOXINS (PCDD) 

(AS 2,3,7,8-EQUIV)TAC •

1085

1086
APPENDIX L - TABLE 3 OEHHA/ARB CHRONIC INHALATION AND ORAL REFERENCE EXPOSURE LEVELS AND TARGET ORGANS b

Appendix L-26

Target Organs

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Chronic 

Inhalation 

REL

(μg/m3)

Chronic 

Oral REL

Date u

Value 

Reviewed 

[Added]

Alimentary

Bone

Cardiovascular

Developmental

Endocrine

Eye

Hematologic

Immune

Kidney

Nervous

Reproductive

Respiratory

Skin

2,3,7,8-TETRACHLORODIBENZO-P-DIOXINTAC 1746-01-6

4.0E-05 2/00 X X X X X X

1.0E-08 10/00 X X X X X X

1,2,3,7,8-PENTACHLORODIBENZO-P-DIOXIN 40321-76-4

4.0E-05 2/00 X X X X X X

1.0E-08 10/00 X X X X X X

1,2,3,4,7,8-HEXACHLORODIBENZO-P-DIOXIN 39227-28-6

4.0E-04 2/00 X X X X X X

1.0E-07 10/00 X X X X X X

4.0E-04 2/00 X X X X X X 1,2,3,6,7,8-HEXACHLORODIBENZO-P-DIOXIN 57653-85-7

1.0E-07 10/00 X X X X X X

4.0E-04 2/00 X X X X X X 1,2,3,7,8,9-HEXACHLORODIBENZO-P-DIOXIN 19408-74-3

1.0E-07 10/00 X X X X X X

4.0E-03 2/00 X X X X X X 1,2,3,4,6,7,8-HEPTACHLORODIBENZO-P-DIOXIN 35822-46-9

1.0E-06 10/00 X X X X X X

4.0E-01 2/00 X X X X X X 1,2,3,4,6,7,8,9-OCTACHLORODIBENZO-P-DIOXIN 3268-87-9

1.0E-04 10/00 X X X X X X

POLYCHLORINATED DIBENZOFURANS (PCDF) 

(AS 2,3,7,8-EQUIV) TAC • 1080

4.0E-04 2/00 X X X X X X 2,3,7,8-TETRACHLORODIBENZOFURAN 5120-73-19

1.0E-07 10/00 X X X X X X

8.0E-04 2/00 X X X X X X 1,2,3,7,8-PENTACHLORODIBENZOFURAN 57117-41-6

2.0E-07 10/00 X X X X X X

8.0E-05 2/00 X X X X X X 2,3,4,7,8-PENTACHLORODIBENZOFURN 57117-31-4

2.0E-08 10/00 X X X X X X

4.0E-04 2/00 X X X X X X 1,2,3,4,7,8-HEXACHLORODIBENZOFURAN 70648-26-9

1.0E-07 10/00 X X X X X X

4.0E-04 2/00 X X X X X X 1,2,3,6,7,8-HEXACHLORODIBENZOFURAN 57117-44-9

1.0E-07 10/00 X X X X X X

4.0E-04 2/00 X X X X X X 1,2,3,7,8,9-HEXACHLORODIBENZOFURAN 72918-21-9

1.0E-07 10/00 X X X X X X

4.0E-04 2/00 X X X X X X 2,3,4,6,7,8-HEXACHLORODIBENZOFURAN 60851-34-5

1.0E-07 10/00 X X X X X X
APPENDIX L - TABLE 3 OEHHA/ARB CHRONIC INHALATION AND ORAL REFERENCE EXPOSURE LEVELS AND TARGET ORGANS b

Appendix L-27

Target Organs

Substance b

Chemicalt

Abstract 

Service 

Number

(CAS)

Chronic 

Inhalation 

REL

(μg/m3)

Chronic 

Oral REL

Date u

Value 

Reviewed 

[Added]

Alimentary

Bone

Cardiovascular

Developmental

Endocrine

Eye

Hematologic

Immune

Kidney

Nervous

Reproductive

Respiratory

Skin

1,2,3,4,6,7,8-HEPTACHLORODIBENZOFURAN 67562-39-4

4.0E-03 2/00 X X X X X X

1.0E-06 10/00 X X X X X X

4.0E-03 2/00 X X X X X X 1,2,3,4,7,8,9-HEPTACHLORODIBENZOFURAN 55673-89-7

1.0E-06 10/00 X X X X X X

4.0E-01 2/00 X X X X X X 1,2,3,4,6,7,8,9-OCTACHLORODIBENZOFURAN 39001-02-0

1.0E-04 10/00 X X X X X X

PROPYLENE (PROPENE) 115-07-1 3.0E+03 4/00 X

PROPYLENE GLYCOL MONOMETHYL ETHER 107-98-2 7.0E+03 2/00 X

PROPYLENE OXIDE 75-56-9 3.0E+01 2/00 X

SELENIUM AND COMPOUNDS (other than hydrogen selenide) 7782-49-2

[1170] 2.0E+01 12/01 X X X

STYRENE 100-42-5 9.0E+02 4/00 X

SULFURIC ACID 7664-93-9 1.0E+00 12/01 X

Sulfuric Acid and Oleum 7664-93-9 1.0E+00 12/01  ̧

Sulfuric Trioxide 7446-71-9 1.0E+00 12/01  ̧

Oleum 8014-95-7 1.0E+00 12/01  ̧

TOLUENE 108-88-3 3.0E+02 4/00 X X X

Toluene diisocyanates 26471-62-5

1204 7.0E-02 1/01  ̧

TOLUENE-2,4-DIISOCYANATE 584-84-9 7.0E-02 1/01 X

TOLUENE-2,6-DIISOCYANATE 91-08-7 7.0E-02 1/01 X

TRICHLOROETHYLENETAC 79-01-6 6.0E+02 4/00 X X

TRIETHYLAMINE 121-44-8 2.0E+02 9/02 X X X

VINYL ACETATE 108-05-4 2.0E+02 12/01 X

VINYLIDENE CHLORIDE (1,1,-Dichloroethylene) 75-35-4 7.0E+01 1/01 X

XYLENES (mixed isomers) 1330-20-7

1210 7.0E+02 4/00 X X

m-XYLENE 108-38-3 7.0E+0 2 4/00 X X

o-XYLENE 95-47-6 7.0E+02 4/00 X X

p-XYLENE 106-42-3 7.0E+02 4/00 X X
Appendix L-1

Purpose: The purpose of this reference table is to provide a quick list of all health values that have been approved by the Office of Environmental Health Hazard Assessment (OEHHA) and the Air Resources Board 

(ARB) for use in facility health risk assessments conducted for the AB 2588 Air Toxics Hot Spots Program. The OEHHA has developed and adopted new risk assessment guidelines that update and replace 

the California Air Pollution Control Officers Association’s (CAPCOA) Air Toxics “Hot Spots” Program Revised 1992 Risk Assessment Guidelines, October 1993. The OEHHA has adopted five technical 

support documents for these guidelines. 

This table lists the OEHHA adopted inhalation and oral noncancer chronic RELs. In addition, it lists the substances in Appendix A-I (Substances For Which Emissions Must Be Quantified) and Appendix F 

(Criteria For Inputs For Risk Assessment Using Screening Air Dispersion Modeling) of the ARB’s Air Toxics “Hot Spots” Emission Inventory Criteria and Guidelines (EICG). OEHHA is still in the 

process of adopting new noncancer chronic RELs. Therefore, new health values will periodically be added to, or deleted from, this t able. Users of this table are advised to monitor the OEHHA website 

(www.oehha.ca.gov) for any updates to the health values.

b Substances written in italics and with a  ̧ do not have explicit OEHHA approved health values, but are included in this table to clarify applicability of OEHHA adopted heath effects values to individual or 

grouped substances listed in the Air Toxics “Hot Spots” Emission Inventory Criteria and Guidelines, Appendix A-I list of “Substances For Which Emissions Must Be Quantified”.

t Chemical Abstract Service Number (CAS): For chemical groupings and mixtures where a CAS number is not applicable, the 4-digit code used in the Air Toxics “Hot Spots” Emission Inventory Criteria 

and Guidelines (EICG) Report is listed. The 4-digit codes enclosed in brackets [ ] are codes that have been phased out, but may still appear on previously reported Hot Spots emissions. For information on 

the origin and use of the 4-digit code, see the EICG report.

u Date Value Reviewed [Added]: This column lists the date that the health value was last reviewed by OEHHA and the Scientific Review Panel, and/or approved for use in the AB 2588 Air Toxics “Hot 

Spots” Program. If the health value is unchanged since it was first approved for use in the “Hot Spots” Program, then the date that the value was first approved for use by CAPCOA is listed within the 

brackets [ ]. 

• February 2000, April 2000, January 2001, and December 2001 are listed for the first set of 22, the second set of 16, the third set of 22, and the fourth set of 12 noncancer chronic RELs, respectively. 

• October 2000 is listed for the oral chronic RELs. The chronic REL for carbon disulfide was adopted in May 2002. Chronic RELS for phosphine and triethylamine were adopted in September 2002. 

Chronic RELs for fluorides including hydrogen fluoride were adopted August 2003.

• For the substances identified as Toxic Air Contaminants, the Air Resources Board hearing date is listed. The date for acetaldehyde represents the date the value was approved by the Scientific Review 

Panel. 

TAC Toxic Air Contaminant: The Air Resources Board has identified this substance as a Toxic Air Contaminant.

H Polychlorinated Biphenyls:

Chronic Oral: The chronic oral value is U.S. EPA’s 1996 oral Reference Dose for Aroclor-1254.

• Polychlorinated Dibenzo-p-dioxins and Polychlorinated Dibenzofurans (also referred to as chlorinated dioxins and dibenzofurans): The OEHHA has adopted the World Health Organization 1997 

(WHO-97)Toxicity Equivalency Factor scheme for evaluating the cancer risk due to exposure to samples containing mixtures of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated 

dibenzofurans (PCDF) and determining cancer risks for a number of specific PCB congeners. See Appendix A of OEHHA’s Technical Support Document For Describing Available Cancer Potency 

Factors for more information about the scheme. See Appendix E of OEHHA’s The Air Toxics Hot Spots Program Risk Assessment Guidelines; Part V; Technical Support Document; Guidance Manual for 

Preparation of Health Risk Assessments for the methodology for calculating 2,3,7,8-equivalents for PCDD, PCDFs and a number of specific PCB congeners. 

n Particulate Emissions from Diesel-Fueled Engines: The unit risk factor and chronic REL were derived from whole diesel exhaust and should be used only for impacts from the inhalation pathway. The 

inhalation impacts from speciated emissions from diesel-fueled engines are already accounted for in the unit risk factor and REL. However, at the discretion of the risk assessor, speciated emissions from 

diesel-fueled engines may be used to estimate acute noncancer health impacts or the contribution to cancer risk or chronic noncancer health impacts for the non-inhalation exposure pathway. See Appendix 

D of OEHHA’s The Air Toxics Hot Spots Program Risk Assessment Guidelines; Part V; Technical Support Document; Guidance Manual for Preparation of Health Risk Assessments for more information.

Table last updated: August 2003