Fluorinated compounds in the environment: More than PFOA
Data presented at the Fluoros meeting bolster theories about atmospheric transport and oxidation for fluorochemicals.
The poly- and perfluorinated chemicals used in stain repellents, paper coatings, and polishes owe many of their unique properties to the remarkable strength of the carbon–fluorine bond. But these handy compounds and their precursors also include an emerging class of widely spread persistent organic pollutants. At the Fluoros meeting in Toronto in August, researchers unveiled new data on the atmospheric transport theory that explain their presence in remote environments. But many questions about human exposure, toxicity, and ecotoxicology remain for these emerging contaminants.
Arctic transport
University of Toronto chemist and meeting organizer Scott Mabury has theorized that volatile fluorocarbon precursors, fluorotelomer alcohols, are being transported to remote locations where atmospheric oxidation, microbial action, or animal metabolism convert them into nonvolatile, longer-chain PFCAs (perfluorocarboxylic acids). New data presented at the meeting that support this theory include finding PFCAs in snow from remote inland Arctic locations by Mabury’s colleague Cora Young and in rain from North America by Environment Canada scientist Brian Scott. Air monitoring in and around Toronto also shows an increase in concentrations of polyfluororinated telomer alcohols and a decrease in likely volatile precursors of PFOS (perfluorooctane sulfonate) since the 3M withdrawal of Scotchgard. Atmospheric modeling suggests a flux of about 0.1–1.0 tonnes/year, which is broadly consistent with monitoring results, according to Ford Motor Co. atmospheric chemist Tim Wallington.
On the other hand, DuPont scientist Stephen Korzeniowski, Stockholm University modeler Ian Cousins, and their colleagues have compiled past production data for PFCAs and argue that direct sources are more important than air transport and precursor degradation.
Meanwhile, other scientists raised concerns about some of the data being cited. The first interlaboratory study looking at the measurement of perfluorinated chemicals in environmental and human samples found that analyses differed between labs by about twofold. This means that trends based on data from one lab are meaningful, but that data from different labs can’t be compared or lumped into a meta-analysis, Environment Canada scientist Derek Muir told the meeting.
Human blood
PFCAs like PFOA (perfluorooctanoic acid) come in two isomers: linear forms produced by the telomer process currently used by DuPont and other manufacturers and branched isomers from 3M’s now-mothballed process. University of Toronto chemist Amila DiSilva reported that most PFCAs in human blood are in the linear form. No one knows why. 3M biologist John Buttenhoff told the meeting that humans might excrete branched PFOA more efficiently than the linear isomers.
PFOA is often described as being a member of the class of carcinogens known as peroxisome proliferators because they cause an increase in bodies called peroxisomes in the liver, but this is an incomplete description according to Stockholm University biochemist Joseph DePierre. Recent studies have lessened concerns about the cancer risk of peroxisome proliferators because classic proliferators induce liver cancer in rats but not humans (Crit. Rev. Toxicol. 2003, 33, 655–780). Classic peroxisome proliferators also fail to cause effects in knockout mice that lack the appropriate receptor. But DePierre has fed PFOA to such knockout mice, and they still show enlarged livers and immune-system effects. This means that PFOA probably exerts toxic effects through other mechanisms, and this leaves many unanswered questions about the toxicology of these chemicals, he told the meeting.
Ecotoxicology
Research focused on PFOS and PFOA finds that these chemicals are not particularly potent toxins. But University of Guelph toxicologist Michelle MacDonald reports that intermediate breakdown products are much more toxic to some forms of aquatic life than the degradation products. The intermediate fluorotelomer carboxylic acids, which partition into water, are four orders of magnitude more toxic to the water flea (Daphnia magna) than PFCAs such as PFOA. “This does not mean that they present a risk, but it does beg the question of their concentrations in the environment and the mechanism of action,” says University of Guelph toxicologist Keith Solomon.
In research that adds another new dimension to the toxicology of perfluorinated chemicals, Stanford University biochemist David Epel and colleagues report that a number of PFCAs inhibit the activity of the efflux transporters that serve as a first line of cellular defense against xenobiotics in marine mussels. Since these transporters also exist in mammals, the study raises questions about the long-term consequences of exposure to these PFCAs.
