Risk assessment of complex mixtures: development of toxic equivalency factors (TEFs) for polycyclic aromatic hydrocarbons (PAHs)

Abstract

Risk assessment is a very valuable tool with which to frame complex environmental problems. Although risk assessment is a very valuable tool it does have significant limitations. There are many aspects of the process that introduce considerable amounts of uncertainty to the outcome. The majority of the uncertainty in a risk assessment is due to the toxicity and exposure assessments. Extrapolations of available data, generalizations of characteristics, "upper-bound" estimates, and assumptions about the exposed population are just a few assumptions that contribute to the overall uncertainty in a risk assessment. The focus of this research is on chemical mixtures of polycyclic aromatic hydrocarbons (PAHs). This group of chemicals represents a class of man-made environmental pollutants to which humans are regularly exposed. The primary objective of this research is to reduce the uncertainty in the toxicity assessment of a complex mixture of polycyclic aromatic hydrocarbons (PAHs). This is accomplished by developing a "user-friendly" criterion for the development of toxic equivalency factors (TEFs) for polycyclic aromatic hydrocarbons (PAHs) that have little or no toxicity data. This criterion will also be used to develop toxic equivalency factors for the alkylated versions of the parent polycyclic aromatic hydrocarbons (PAHs). A secondary objective was to identify the sensitive parameters in the assessment. This will help to identify modifications that will be necessary in current software/model equations and to provide a means to better design sampling regimes. It is the overall goal of this research to provide improvements in the science of risk assessment of complex mixtures and lead to a more focused research area for the future. This research shows the importance of structure and substitutions of chemicals in the overall toxicity of that compound. Secondly, the proposed toxic equivalency factors (TEFs) increased the risk estimates two to four orders or magnitude from the intial assessments that did not include the toxic equivalency factors (TEFs). Finally, this research provides a focus for future research in the science of risk assessment of complex mixtures.