Guidance for PAHs:
Methods for Estimating Health Risks from
Carcinogenic Polycyclic Aromatic Hydrocarbons (PAHs)
The following guidance was first developed by the Minnesota Department of Health (MDH) in 2001 and updated in 2013 at the request of the Minnesota Pollution Control Agency (MPCA). The guidance was updated in 2014 as MDH continued to review appropriate environmental and risk analyses for PAHs. For the supporting information used to derive this guidance, please contact the Health Risk Assessment Unit or refer to the document "Guidance for Evaluating the Cancer Potency of Polycyclic Aromatic Hydrocarbon (PAH) Mixtures in Environmental Samples", dated February 8, 2016 (PDF).
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 assessment. The combined or cumulative risks for more than one PAH may be estimated using the assumptions that follow. Because of the limited data regarding PAH toxicities and exposures, risk estimates should be presented in the context of their limitations and uncertainties.
MDH recommends using benzo[a]pryene (B[a]P) as the index chemical for other cPAHs, and provides guidance for an evaluation of the potency of other cPAHs relative to benzo(a)pyrene.
1. B[a]P Carcinogenicity Evaluation
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.
- For evaluating risks from ingestion of B[a]P-contaminated water, MDH recommends a Health Based Value (HBV) of 0.06 μg/L. The HBV was developed in 2012 and is based on age-specific ingestion over a lifetime and age-dependent sensitivity to carcinogens. This HBV is based on the 2010 California EPA B[a]P adult-based oral slope factor of 1.7 (mg/kg-day)-1. [see: Toxicological Summary for Benzo[a]pyrene (PDF: 83KB/8 pages)]
- For evaluating risks from inhalation exposure, MDH recommends a Health Risk Value (HRV) of 0.001 μg/m for 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). [see: Air Values Table]
- For evaluating risks from cPAH exposure from other oral exposures (e.g., soil ingestion) and dermal exposures, MDH recommends the use of the 2010 CA EPA B[a]P oral slope factor of 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.
2. Guidance for the relative potency method
MDH finds that the most accurate assessment of cancer risk from a mixture, one based on site-specific whole mixture toxicity information, will not likely be possible. Therefore, other approaches will be needed. Information about a similar mixture ("surrogate whole mixture potency”) is preferred, but is rarely available. Summing the individual potencies of constituents of the mixture (“relative potency method”) is a reasonable alternative (see the table below for a list of priority cPAHs and the potency of each relative to B[a]P). However, it is likely to be the least accurate site-specific method for evaluating mixture cancer potency. Individual cPAHs frequently occur as part of an environmental mixture. The individual potency factors are not intended to be used as the basis of cancer slope factor calculations, rather their purpose is to be used as part of a cumulative risk assessment to estimate the cancer risk from a mixture of cPAHs.
As a default surrogate whole mixture alternative for the most common types of cPAH mixtures, MDH recommends estimating the cancer potency of an environment sample by multiplying the B[a]P concentration in a mixture by a factor of seven (7) and comparing the result to media-specific criteria for B[a]P. If significant fractionation of the original mixture has occurred in the environment (typically from significant movement through groundwater or air) MDH recommends using a site-specific surrogate whole mixture or the relative potency method rather than the default surrogate whole mixture method. MDH makes specific recommendations on cancer risk evaluation of PAH mixtures, relative potencies of individual cPAHs, important cPAH analytes, and determining appropriate analytical detection limits in the MDH “Guidance for Evaluating the Cancer Potency of Polycyclic Aromatic Hydrocarbon (PAH) Mixtures in Environmental Samples", dated October 31, 2014 (PDF: 263KB/15 pages).
MDH recommends that all cPAH cancer risk evaluations include the use of age-dependent exposure estimates in combination with age-dependent adjustments to potency. [see: Risk Assessment Advice for Incorporating Early-Life Sensitivity into Cancer Risk Assessments for Linear Carcinogens (PDF: 29KB/3 pages)]
Finally, it is important to maintain a state-wide database of site and source-type specific cPAH concentrations so that different source-type default multipliers can be developed. This could significantly decrease analytical and cleanup costs in the future for sites with significant cPAH contamination.
|Priority cPAHs||Relative Potency Factors|
References used by MDH to support these recommendations are found in "Guidance for Evaluating the Cancer Potency of Polycyclic Aromatic Hydrocarbon (PAH) Mixtures in Environmental Samples", dated February 8, 2016 (PDF).