Kathleen Newhouse, Assessment Manager

IRIS Draft Toxicological 

Review for Benzo[a]pyrene 

(BaP)

Office of Research and Development

National Center for Environmental Assessment December 13, 2013
Office of Research and Development

National Center for Environmental Assessment

General Information

Five-ring polycyclic aromatic hydrocarbon 

• Relatively insoluble in water

• Low volatility 

Ubiquitous in the environment primarily as a result of incomplete combustion 

emissions

• Natural sources include forest fires and volcanoes

• Anthropogenic sources include stoves/furnaces burning fossil fuels (especially 

wood and coal), motor vehicle exhaust, and various industrial combustion 

processes

Exposure

Major sources of occupational exposure involve

• Production of aluminum, coke, graphite, and silicon carbide

• Coal tar distillation

Major sources of non-occupational exposure

• Tobacco products

• Diet (e.g. barbequed or charred meats)

1
Office of Research and Development

National Center for Environmental Assessment

Agency Interest in BaP

Hazardous substance under the Comprehensive Environmental Response, 

Compensation, and Liability Act of 1980 (CERCLA)

• Found at 524 hazardous waste sites on the National Priorities List (NPL)

Drinking water contaminant under the Safe Drinking Water Act (SDWA)

• Maximum Contaminant Level Goal (MCLG) of zero and enforceable Maximum 

Contaminant Level (MCL) of 0.0002 mg/L have been established

Component in a class of chemicals referred to as Polycyclic Organic Matter, 

defined as a Hazardous Air Pollutant in the 1990 amendments to the Clean Air 

Act

Index chemical for the EPA Relative Potency Factor approach for PAH mixtures

2
Office of Research and Development

National Center for Environmental Assessment 3
Office of Research and Development

National Center for Environmental Assessment 3

Literature Search Strategy

•The literature search identified 

more than 21,000 studies for BaP.

• Approximately 700 references 

were cited in the August 2013 

Toxicological Review (Public 

Comment Draft).

4
Office of Research and Development

National Center for Environmental Assessment

Health Hazards Identified in the Public Comment Draft

EFFECTS OTHER THAN CANCER

• Animal studies indicate that the strongest evidence for potential hazard 

following BaP exposure is for developmental and reproductive toxicity and 

immunotoxicity.

• Human studies report effects that are generally analogous to the effects 

observed in animal toxicological studies, and provide qualitative, supportive 

evidence.

CANCER

• Under EPA’s Guidelines for Carcinogen Risk Assessment (2005) BaP is 

“carcinogenic to humans” based on strong and consistent evidence in 

humans and animals, as well as mechanistic data.

• The weight of evidence supports mutagenicity as the primary mode of action 

for BaP-induced carcinogenicity.

5
Office of Research and Development

National Center for Environmental Assessment

Specific Issues Identified in the Public Comment Draft Charge

The new document structure

• The Preamble

• Executive Summary

• Literature Search Strategy/Study Selection

The synthesis, integration, and weight of the evidence for the identified hazards

The development of candidate values and organ/system-specific reference values

The development of cancer risk estimates for oral, inhalation, and dermal exposure

• Including the method used for interspecies scaling of the dermal slope 

factor

6
Office of Research and Development

National Center for Environmental Assessment

Innovative Features of the BaP Public Comment Draft

• New document structure, responsive to NRC recommendations

• First IRIS derivation of non-cancer reference values for BaP

• First assessment to calculate multiple organ/system specific reference values 

(facilitates subsequent risk assessments of multiple chemicals)

• First IRIS inhalation unit risk for BaP; updated oral slope factor

• Presentation of the first dermal cancer risk estimate for any chemical on IRIS

• First characterization of a chemical as “carcinogenic to humans” based in part 

on mechanistic data

• First systematic analysis of transcriptomics data in an IRIS assessment

7