The following guidance was developed by the Minnesota Department of Health (MDH) in 2001 at the request of the Minnesota Pollution Control Agency (MPCA). For the supporting information used to derive this guidance, please contact the Health Risk Assessment Unit.
Polycyclic Aromatic Hydrocarbons: Methods for Estimating Health Risks from Carcinogenic PAHs
The Minnesota Department of Health (MDH) prepared this guidance to identify a consistent approach for agencies and programs to assess health risks from exposures to carcinogenic polycyclic aromatic hydrocarbons (cPAHs) in air, water, soil, and other media. Because of uncertainties associated with the toxicities of PAH mixtures, MDH uses conservative assumptions to evaluate potential risks. As more data become available, MDH re-evaluates and revises its risk assessment methods and procedures, as appropriate.
PAHs include hundreds of different chemicals that commonly occur as mixtures in the environment. Limited toxicological data are available on PAH mixtures; therefore, individual PAHs are typically evaluated as separate chemicals for risk characterization. The combined or cumulative risks for more than one PAH may be estimated using assumptions, where appropriate. Because of the limited data regarding PAH toxicities and exposures, risk estimates should be presented in the context of their limitations and uncertainties.
Several agencies have classified a limited number of PAHs as probable or possible carcinogens (see Table 1). These classifications are based on the available toxicological data. MDH recommends the 25 PAHs identified by the California Environmental Protection Agency (CA EPA) be evaluated as probable or possible carcinogens at this time. Naphthalene is not currently listed as a probable or possible cPAH, however, the National Toxicology Program (NTP) has concluded that there is clear evidence of carcinogenic activity in animals (NTP, 2001b). Where possible, MDH evaluates the supporting toxicological data to determine if the classification incorporates significant new findings in current scientific literature.
This guidance describes two general approaches for evaluating cPAH risks: (1) use of benzo[a]pryene (B[a]P) as the index chemical for other cPAHs, and (2) conducting a qualitative assessment for cPAHs when health risk estimates cannot be determined.
1. Use of B[a]P as the Index Chemical for cPAHs
B[a]P is commonly used as the index chemical to estimate the health risks from exposure to cPAHs when no other appropriate toxicity value (oral or inhalation) has been identified for an individual cPAH.
- MDH recommends the use of the 2010 CA EPA B[a]P oral slope factor for estimating health risks from cPAHs absorbed from oral and dermal exposure pathways. The CA EPA B[a]P adult-based oral slope factor is 1.7 (mg/kg-day)-1. Because early life exposures to B[a]P are a concern and B[a]P is a linear carcinogen, it is appropriate to apply MDH’s methodology for addressing age-dependent sensitivity. The resulting age-adjusted slope factor for the oral exposure route is 2.8 (mg/kg-day)-1 for a 70 year exposure period, including early life exposure.
(Updated March, 2012)
- For evaluating risks from inhalation exposure, MDH recommends the use of the Health Risk Value of 0.001 μg/kg-day for benzo[a]pyrene (B[a]P) which is based on a slope factor of 7.3 (mg/kg-day)-1. This slope factor is the geometric mean of the B[a]P slope factor range used by the U.S. Environmental Protection Agency (U.S. EPA, 2001).
The advice that follows on this page is currently under review. Contact MDH for site specific guidance.
Applying Potency Equivalency Factors (PEFs) for cPAHs
If no appropriate toxicity value (oral or inhalation) is identified for a cPAH, CA EPA Potency Equivalency Factors (PEFs) may be used to estimate their potency relative to B[a]P (see Table 2).
- If no appropriate PEF is available on the table, additional oral and inhalation toxicity values for B[a]P and other cPAHs (e.g., dibenz[a,h]anthracene, 7,12-dimethylbenzanthracene, 3-methylcholanthrene, and 5-nitroacenaphthene) have been developed by CA EPA (for a list of toxicity values, see Air Toxics Hot Spots Program Risk Assessment Guidelines, CA 1999b). CA EPA toxicity values may be utilized to screen for B[a]P and other cPAHs. Note that if a value is used for purposes other than screening and there is evidence that the chemical is a risk driver (i.e., has a significant impact on risk estimates requiring a risk management recommendation or action), MDH recommends further evaluation of the supporting toxicological data to determine if the value is appropriate.
- If no appropriate PEF or other factor is available for a cPAH, potency may be assumed to be equivalent to B[a]P (i.e., a PEF of 1). Note that if a value is used for purposes other than screening and there is evidence that the chemical is a risk driver, MDH recommends further evaluation of the supporting toxicological data to determine if the value is appropriate.
- MDH will be evaluating the PEF approach recommended in this guidance and may issue updated guidance in the summer of 2012 to evaluate health risks from cPAHs to include PEFs for additional PAHs.
Evaluating Cumulative Risks to cPAHs
For screening purposes, risk estimates for individual cPAHs may be summed to calculate total (or cumulative) cancer risk. Due to limited toxicological data regarding the pharmacokinetics, mechanisms of action, and patterns of exposure for many PAHs, this cumulative risk approach is based on an additional set of assumptions. Note that if this screening estimate becomes a risk driver, MDH recommends further evaluation of the supporting data to determine if the cumulative approach is appropriate.
2. Qualitative Assessment
Where possible, MDH recommends collecting data on individual PAHs in mixtures, regardless of whether they have toxicity values, to reduce uncertainty and better understand potential risks. If adequate toxicological data are not available to estimate risks for PAHs, MDH recommends a qualitative assessment. This guidance is based on an evaluation of the available data regarding exposure, toxicity, pharmacokinetics, and carcinogenicity.
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|Table 1: Listed Probable or Possible cPAHs|
** U.S. EPA, 2001
*** IARC, 2001
**** CA EPA, 1999b
Note: Naphthalene is not currently listed as a probable or possible cPAH; however, NTP has concluded that there is clear evidence of carcinogenic activity in animals (NTP, 2001b). Where possible, MDH evaluates the supporting toxicological data to determine if the classification incorporates significant findings in current scientific literature.
Go to > topTable 2: Potency Equivalency Factors*
|PAH (or PAH Derivative)||Potency Equivalency Factors||PAH (or PAH Derivative)||Potency Equivalency Factors|
Agency for Toxic Substances and Disease Registry, Minimal Risk Levels for Hazardous Substances, (Accessed January 22, 2001)*
California Environmental Protection Agency, (April 1999a; June 1999b) Air Toxics Hot Spots Program Risk Assessment Guidelines, (Accessed: January 2001).*
Collins, J.F., Brown, J.P., Alexeeff, G.V., Salmon, A.G. (1998) Potency Equivalency Factors for Some Polycyclic Aromatic Hydrocarbons and Polycyclic Aromatic Hydrocarbon Derivatives, Regulatory Toxicology and Pharmacology, 28, 45-54.
International Agency for Research on Cancer, Overall Evaluations of Carcinogenicity to Humans, (Accessed: February 14, 2001).
Minnesota Pollution Control Agency, Air Emissions Risk Analysis, (Accessed: July 2, 2004).
National Toxicology Program, 10th Report on Carcinogens, (Accessed: July 2, 2004).
National Toxicology Program, Toxicology and Carcinogenesis Studies of Naphthalene (CAS No 91-20-3) in F344/N Rats (Inhalation Studies), (Accessed: July 2, 2004).
US Environmental Protection Agency (August 1997) Exposure Factors Handbook, Volumes 1-3, EPA/600/P-95/002Fa.
US Environmental Protection Agency, Integrated Risk Information System, Benzo[a]pyrene, (Accessed: January 2001).*
US Environmental Protection Agency (1993) Provisional Guidance for Quantitative Risk Assessment of Polycyclic Aromatic Hydrocarbons, EPA/600/R-93/089.
*The above references are sources of PAH toxicity values. This is not a comprehensive list of all possible sources. Additional values may be found by searching the primary literature, agency web sites, and other references. If a toxicity value is used for purposes other than screening and there is evidence that the chemical is a risk driver, MDH conducts further review of the supporting toxicological data. For consultation regarding a specific toxicity value, consult the Health Risk Assessment Unit.