Organic Pollutants Ignored By Current Regulations

By Britta Voss

A large number of organic chemicals are not currently recognized for their potential to build up in food webs, according to a July 13 report in Science. Researchers led by Frank Gobas at Simon Fraser University's School of Resource and Environmental Management found that the fish-based food web model does not account for differences in how aquatic and terrestrial animals absorb organic compounds. This model is currently the gold standard used to determine how a compound will accumulate in organisms, yet it fails to consider the different chemical properties of air and water.

Regulatory groups around the world such as the U.S. Environmental Protection Agency and Environment Canada establish emissions limits for organic pollutants based on the compound's solubility in water and fat. A compound with high fat solubility and low water solubility will accumulate in an aquatic organism because the compound does not readily pass from the organism's tissue into the surrounding water. Biomagnification refers to this gradual build-up of a substance from prey to predator, all the way up a food chain.

Lead author of the report Barry Kelly is calling for a reevaluation of how organic pollutants are regulated. Recognition of the need for evaluating air-breathing physiology has been slow in coming, both among ecotoxicologists and regulators. As Gobas puts it, "We are reaching a stage where we can convince the regulators that the science is no longer in doubt.”

In the early 2000s, Gobas' lab began collaborating with Environment Canada to evaluate thousands of organic compounds for their biomagnification potential. Early on, they realized that for non-aquatic animals, fat and water solubility alone were insufficient to predict actual accumulation levels. For air-breathing animals like humans, the solubility of the chemical in air must be considered.

According to the report, the omission of air solubility analysis leaves approximately 40 percent of commercially-produced organic chemicals exempt from emissions regulations. Whether or not these compounds can actually accumulate in an organism also depends on the organism's physiological ability to metabolize and excrete the compound. This process is not well understood for most organic chemicals. However, overlooking this whole class of compounds with only moderate fat solubility but high air solubility puts humans at an uncertain risk.

Understanding the true biomagnification potential of household and industrial products is critical to balancing environmental protection with the value of organic chemicals to society. Frank Gobas' lab has newly identified certain chemicals as potential biomagnifiers, including certain agricultural products (hexachlorocyclohexanes) and pesticides (endosulfans). Yet in more recent studies they have found that some compounds commonly used as flame retardants called polybrominated diphenyl ethers (PBDEs) do not biomagnify in air-breathing animals. According to the fish-based model, these PBDEs could pose a serious health risk. Exonerating them as a threat could retain their valuable role in protecting humans.

Research at Simon Fraser University has now turned to terrestrial biomagnification in tropical food webs. Bioaccumulation studies typically focus on Arctic ecosystems because of their high accumulation potential and the link to humans through hunting of top predators like whales. Tropical research aims to analyze what other processes may be at work in different kinds of food webs, because, as Gobas notes, we cannot assume that all food webs behave in the same way.

Once chemicals with the newly defined biomagnification potential are identified, further steps can be taken to determine if they pose a serious environmental threat. The physiology of different organisms must be considered to determine if a compound is effectively metabolized, and thus prevented from bioaccumulating. However, very little is known about the metabolic fate of most organic pollutants. Analysis of these chemicals is an iterative process of identifying potentially harmful compounds, then evaluating a suite of complicating factors to determine which ones actually pose an environmental threat.

Britta Voss is currently studying chemical oceanography at the University of Washington.