Polycyclic aromatic hydrocarbons are a class of structurally similar chemical compounds characterized by the presence of fused aromatic rings. PAHs are typically formed during the incomplete burning of organic material including coal, oil, gasoline and garbage. PAHs are also found in crude oil, coal tar, creosote and asphalt. PAHs are associated with human activities (the combustion of fossil fuels) and natural occurrences (such as forest fires), and they are considered to be ubiquitous in the environment at some level.
PAHs are often discussed as a group because they are commonly found as mixtures of two or more compounds in the environment. In addition, they are often treated similarly in risk assessments due to their similar structures and toxicities. It should be noted that, while PAHs are often discussed as a group, the individual chemicals are evaluated as separate chemicals in the risk characterization. There are over 100 chemicals in this family of compounds, although a smaller number are routinely reported at disposal sites (Table 7.3). The PAH's which are often present at sites but are unreported may result in the underestimation of potential risks.
Among the polycyclic aromatic hydrocarbons, the USEPA (IRIS, 1993) has classified seven chemicals as probable human carcinogens (identified in Table 7.3 as USEPA Class B2). The classification of PAHs by the International Agency for Research on Cancer (IARC) is fairly consistent with that of the EPA. PAH's which are considered unclassified (either N/A, D or 3 in Table 7.3) may also contribute to carcinogenic risk (Nisbet and LaGoy, 1992) and should not necessarily be assumed to be "noncarcinogens" which would be USEPA Class E.
All PAHs identified as contaminants of concern should be evaluated in terms of potential noncancer risk. Remember that the carcinogenic PAHs may also be associated with noncancer health effects and must be included in this evaluation.
Historically, risk assessments involving PAHs become problematic due to the general lack of toxicity information available for many of the compounds reported at disposal sites. The following paragraphs discuss the MA DEP recommended approaches for the evaluation of cancer and noncancer risk of harm posed by exposure to polycyclic aromatic hydrocarbons.
PAH Cancer Risk: Until recently the only cancer slope factor the USEPA published for PAH's was for the chemical benzo[a]pyrene (B[a]P). In the absence of further chemical-specific information, the EPA and MADEP guidance instructed risk assessors to assign the B[a]P slope factor to all PAHs considered to be carcinogenic. This approach was considered to be protective of public health as benzo[a]pyrene is thought to be one of the most potent carcinogens among the PAH's. In 1993, USEPA formally adopted provisional guidance for estimating cancer risks associated with polycyclic aromatics hydrocarbons (USEPA, 1993). The procedure uses information from the scientific literature to estimate the carcinogenic potency of several PAHs relative to benz[a]pyrene. These relative potencies may be used to modify the CSF developed for benzo[a]pyrene for each PAH, or to calculate B[a]P-equivalent concentrations for each of the PAH's (which would then be used with the B[a]P slope factor). The latter approach is similar to that used for the evaluation of dioxins .
PAH's Commonly Reported at c.21E Disposal Sites and Carcinogenicity Weight-of-Evidence Classifications
|USEPA (1)||IARC (2)|
The relative potency values published by the USEPA and others (Chu and Chen, 1984; Clement, 1988; Nisbet and LaGoy, 1992) are being reviewed and may be adopted (perhaps in a modified form) by MA DEP Office of Research and Standards. A list of the USEPA relative potency values is presented in Table 7.4 for use in c.21E risk characterizations pending publication of MADEP recommended values.
Relative Potency Values for Individual PAH's (USEPA, 1993):
|Compound||Relative Potency Factor|
|NA - Chemical is not currently considered to be carcinogenic by USEPA so no relative potency value is currently applicable.|
PAH Noncancer Risk:
While the USEPA has published (in IRIS and HEAST) threshold effects toxicity information for a number of polycyclic aromatic hydrocarbons, for many other members of this chemical family such information has not yet been developed. In order to adequately characterize the noncancer risks associated with these PAHs, MADEP recommends that the published reference dose, reference concentration, or analogous value for a structurally similar PAH be adopted for each compound for which sufficient chemical-specific toxicological information is unavailable.
Examples of how the potential toxicity of individual PAHs may be evaluated are described in Example 7.2.
Evaluation of Polycyclic Aromatic Hydrocarbons (PAH's)
A polycyclic aromatic hydrocarbon for which a cancer slope factor has not been developed by USEPA may be evaluated using the relative potency values recommended by USEPA (Table 7.4). These values can be used in one of two ways which are mathematically equivalent. To illustrate, let's assume that Indeno[1,2,3-cd]pyrene was reported at a disposal site at a concentration of 2 mg/kg.
- In the first approach, the relative potency factor for indeno[1,2,3-cd]pyrene (0.1, from Table 7.4) is used to estimate a cancer slope factor for this compound by adjusting the slope factor for benzo[a]pyrene (7.3 mg/kg/day, from USEPA IRIS, 1993):
CSFi[1,2,3-cd]p = 0.1 x 7.3 (1/mg/kg/day) = 0.73 (1/mg/kg/day)
- The second approach would be to adjust the concentration of indeno[1,2,3-cd]pyrene (2 mg/kg, in this example) by the relative potency value (0.1, from Table 7.4) to estimate a benzo[a]pyrene equivalent concentration, to which the B[a]P slope factor would be applied:
B[a]Pequiv. conc. = 0.1 x 2 mg/kg = 0.2 mg/kg
A polycyclic aromatic hydrocarbon for which a reference dose (RfD) has not been developed by USEPA may be evaluated using a reference dose from a structurally similar PAH. Using the example above, indeno[1,2,3-cd]pyrene (for which there is currently no RfD) is structurally similar to fluoranthene: both chemicals have a 5-carbon ring structure bound to three aromatic rings, although indeno[1,2,3-cd]pyrene has two additional aromatic rings (see Figure 7.1). The reference dose for fluoranthene is 0.04 mg/kg/day (USEPA IRIS, 1993). This value would be adopted to evaluate potential noncancer risks associated with indeno[1,2,3-cd]pyrene.