Combining perfluoroalkane acid exposure levels for risk assessment.

Perfluoroalkane acids are present in biologic samples from >90% of people in the developed world. Because people may be exposed to multiple perfluoroalkane acids, it is reasonable to consider whether the exposure levels of these agents can be combined for risk assessment purposes. To investigate this possibility, we considered whether the literature on perfluoroalkane acids could be used to justify a scaling system analogous to the Toxic Equivalency Factor (TEF) system used for polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins, and polychlorinated dibenzofurans. We evaluated pairs of studies performed with different perfluoroalkane acids in the same species using the same design and found that endpoints for perfluorooctanesulfonate (PFOS), perfluorooctanoic acid (PFOA), perfluorobutanesulfonate (PFBS), and perfluorodecanoic acid (PFDA) could be discordant. We evaluated pairs of rat studies of PFOS, PFOA, and PFBS performed with the same design for which dose-response curves could be modeled for the concordant endpoints, but we were unable to identify a scaling system that gave values consistently within an order of magnitude for the same compounds. Currently available data do not support the combining of exposure levels of perfluoroalkane acids for risk assessment, although re-evaluation after additional data are available is recommended.