Perfluoroalkyl Acids: What Is the Evidence Telling Us?

Environmental Health Perspectives, Kellyn S. Betts
Published April 30, 2007

It was 2000 when the scientific community first became widely aware that perfluorooctanyl sulfonate (PFOS), then the key ingredient in 3M Company's popular Scotchgard stain repellent, was being found at extremely low levels throughout the environment and the human population. Since that time, environmental scientists and toxicologists have begun paying much more attention to PFOS, its sister compound perfluorooctanoic acid (PFOA; known for its use in DuPont's Teflon products), and other members of the family of perfluoroalkyl acids (PFAAs). As more tests have been conducted, the research has revealed that laboratory animals respond in vastly different ways to PFAAs and related compounds, which can make it difficult to pinpoint the mechanisms underlying the responses. However, toxicologists are making headway in their understanding of these compounds, an important fact in light of new research suggesting that the levels being found in both people and animals may have an impact on their health. The tremendous variation in the speed with which humans and laboratory animals can eliminate PFOA is one example of why understanding how the compounds are processed in the body poses such a formidable challenge. "You go from hours for the female rat, to days for the male rat, to months for the monkey, to almost four years in humans," explains Jennifer Seed, a branch chief with the EPA Office of Pollution Prevention and Toxics. "We truly don't understand what are the biological events that drive this difference," says Christopher Lau, a lead research biologist with the EPA National Health and Environmental Effects Research Laboratory (NHEERL). "Are there binding protein differences? Do humans have a different set of transporters that is not the same as in animals?" Lau terms these gaps in understanding "a black hole." These gaps render the toxicologist's goal of extrapolating from one species to another "a very complex state of affairs," as Seed puts it. For this reason, deciphering the human risk posed by exposure to PFAAs is a major challenge, Lau says. "We need to go to the next level to identify the underlying events that drive the adverse effects," he says. Anatomy of a PFAA The compounds used in commercial perfluorinated formulations are sometimes identified by the number of carbon atoms they contain. In general, the longer the carbon chain length, the more the PFAA persists in the body, according to Naomi Kudo, an associate professor of toxicology and applied pharmacology at Josai University in Japan. For example, perfluorobutane sulfonate (PFBS), which has 4 carbons, is eliminated in a little over 1 month in humans, on average, while PFOA and PFOS (so-called C8 compounds with 8 carbons each) are eliminated in 3.8 and 5.4 years, respectively. Perfluorohexane sulfonate (PFHxS), with 6 carbons, is an exception to the rule; it is eliminated in 8.5 years. 3M no longer manufactures PFOS, and the compound is now used only in relatively small quantities for applications for which there is no acceptable substitute, such as in semiconductor manufacturing. All eight of the companies currently using PFOA