Persistent Organic Pollutants

SOME PERSISTENT organic pollutants (POPs) can reach high concentrations in humans and other air-breathing animals even though they don't bioaccumulate in fish, according to a study by Canadian researchers (Science 2007, 317, 236). The observation suggests that the regulatory criteria now used to flag potential POPs may need revision.

Bioaccumulative compounds are usually assumed to be hydrophobic and fat-soluble if they have an octanol-water partition coefficient (K_OW) greater than 100,000. Screening of commercial chemicals to identify potentially bioaccumulative compounds is usually based on the K_OW or laboratory tests with fish. But research by environmental chemist Frank A. P. C. Gobas, grad student Barry C. Kelly, and coworkers at Simon Fraser University, in Burnaby, British Columbia, shows that such an approach may overlook a significant fraction of pollutants that pose health risks to air-breathing animals.

The Canadian researchers show that even moderately hydrophobic compounds with K_OW between 100 and 100,000 can increase in concentration at each step in the food chain, a process known as biomagnification. Biomagnification of such a compound can occur in food webs that include humans and other air-breathing animals, even when it doesn't happen in food webs that are limited to fish and aquatic invertebrates. To biomagnify in air-breathing animals, a compound must have a high octanol-air partition coefficient (K_OA), and it must be metabolized slowly.

Gobas and his coworkers first hypothesized in 2001 that compounds with high K_OA would biomagnify in air-breathing animals. At that time, they observed that certain substances with a relatively low K_OW biomagnified significantly in the lichen-caribou-wolf food chain. They have now looked at a variety of food webs, including one with only water-respiring organisms, one with only air-breathing organisms, and one with both (including humans). All the food webs they studied are found in northern Canada.

The polychlorinated biphenyl congener 153 (PCB153) is an example of a compound with high K_OW and high K_OA. As expected, it biomagnifies in all three food webs. In contrast, β-hexachlorocyclohexane (the insecticide lindane) has a low K_OW and a high K_OA. It does not biomagnify in the food web that includes only water-respiring organisms, but it does biomagnify in both webs that include air-breathing animals.

Gobas hastens to point out that their model assumes that the chemicals are not metabolized. "The degree to which chemicals are transformed in organisms is difficult to predict at this point," he says.

Gobas hopes that environmental regulations will change as a result of this work. "For regulatory agencies and chemical manufacturers, it is important to recognize that chemicals with a high octanol-air partition coefficient have the potential to bioaccumulate in terrestrial and human food webs," he says.

Lynn R. Goldman, a professor of environmental health sciences at the Johns Hopkins University Bloomberg School of Public Health, says that the paper suggests "a broader range of chemicals, with high K_OA but lower K_OW, should perhaps be considered to be persistent organic pollutants." Goldman served as assistant administrator for the EPA's Office of Prevention, Pesticides & Toxic Substances during the Clinton Administration.

"Current risk assessments that classify a chemical as a persistent organic pollutant (POP) have based their conclusions primarily on science drawn from aquatic toxicology," says Lawrence Burkhard, an EPA research chemist. "This paper strongly suggests risk assessment methodologies need to be changed to include data on bioaccumulation in birds and mammals. If this is done, more chemicals may be classified as POPs than have been in the past."