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Research Brief 36Superfund Basic Research ProgramImproving the Health Risk Assessment of Chemical MixturesRelease Date: 12/23/1998 Despite the fact that many hazardous waste sites contain complex mixtures of chemicals, much of what we understand about the health risks of these sites is based on bioassays of individual, purified compounds. Unfortunately, the knowledge acquired from these kinds of experiments does little to inform us of the interactions that may be occurring in chemical mixtures to alter their toxic impact. In addition, for some of the components of complex mixtures there is very limited toxicity information; thus, risk assessments must be performed using either estimates based on model compounds or extrapolations from chemicals with similar structures. While there is a need to develop a better understanding of the health risk of chemical mixtures, this task is highly complicated due in part to the fact that there are limited experimental strategies for characterizing the risks of combinations of chemicals. Researchers at Texas A&M University are developing a comprehensive procedure for characterizing the toxic potential of complex mixtures of polycyclic aromatic hydrocarbons (PAHs) and halogenated aromatic hydrocarbons (HAHs), two important classes of compounds found on hazardous waste sites. Consisting of a battery of PAH and HAH-specific bioassays, the methodology utilizes a variety of cultured cell types and measures a range of toxic endpoints including genotoxicity, developmental toxicity, immunotoxicity, and enzyme induction. The battery is designed to provide a comprehensive evaluation of the health risks of HAH and PAH mixtures; as such, it may prove useful not only for prioritizing the health risks of chemical waste sites, but also for monitoring the effectiveness of remediation activities. The key test systems in the battery include the Ames Salmonella/microsome assay, which is a microbial mutagenicity system being used to investigate the genotoxic effects of chemical mixtures, and the chick embryo screening test (CHEST), which is a good model for understanding the effects of mixtures on vertebrate and mammalian development. DNA adduct formation is also being measured in vivo using the 32P-postlabeling assay. Aryl hydrocarbon (Ah) receptor mediated responses are being measured in vitro using several mammalian cell lines. All of the aforementioned test systems are being validated for their ability to investigate interactions among contaminants. Other key considerations for selecting the test systems include ability to process a large number of samples, ease of use, and low cost. These systems were selected to detect both a range of toxic endpoints and a range of chemical classes. Initial studies with the battery of bioassays focused on screening individual compounds, and simple binary and tertiary mixtures of model compounds. Subsequent studies have included testing complex mixtures of compounds obtained from Superfund sites, as well as isolated fractions from these samples. A recent finding of particular interest is data suggesting that the potency of PAH mixtures may be underestimated using current risk assessment methodologies. The testing of mixtures of PAHs obtained from a Manufactured Gas Plant Residue site (MGPR) suggests that the carcinogenic and immunotoxic potential of the mixtures is often much greater than would be predicted based on the additive concentration of individual PAH. Preliminary data with isolated fractions from the MGPR samples suggests that alkyl-PAHs or high molecular weight dibenzopyrenes may be responsible for some of the "unidentified" genotoxicity. Current risk assessment methodologies cannot consider many of these compounds because toxicity values have not yet been established. In addition to the above results, a comparison of three samples (E1, E2, and E3) from an abandoned MGPR site indicated that in vivo DNA adduct formation increased with increasing concentration of benzo(a)pyrene (BP). The absence of DNA adducts in the animal tissue exposed to the lowest concentration of BP suggests, however, that metabolism or repair mechanisms were capable of reducing or eliminating the genotoxic potential of this sample. In the Ames Salmonella assay all three samples were genotoxic; however, the sample with the highest level of BP was cytotoxic to the bacteria and induced only a moderate genotoxic effect. Altogether, the results from these experiments suggest that a risk assessment for a complex PAH mixture based solely on BP or BP-equivalents may not accurately predict the mixture's genotoxic potential. These efforts should provide information that will reduce the uncertainty associated with risk assessment of hazardous waste sites containing complex mixtures. Because there is very little toxicity data available on complex mixtures, the results are also significant for contributing to the knowledge base on chemical mixtures and their health effects. For More Information Contact: Kirby C. DonnellyEnvironmental & Occupational Health The School of Rural Public Health Bldg. College Station, TX 77843-1266 Tel: (979) 845-7956 Email: To learn more about this research, please refer to the following sources:
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38(8):1721-1732. 
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