Dioxin, PCBs, the pesticide DDT–these pollutants are considered among the most dangerous on the planet because they don’t break down easily, are highly toxic, and build up in the food chain. Because these chemicals stay put in our body fat, even tiny amounts in food can add up over time and contribute to health problems such as cancer. So worrisome are the risks that more than 140 countries have endorsed a 2001 international treaty that aims to banish a dozen of these substances from the environment.
Now on p. 236, a Canadian team reports that efforts to crack down on persistent organic pollutants, or POPs, may have missed an entire set of them. The problem is that risk assessment experts now finger potential POPs based on whether they build up in fish food webs. That assumption, the authors argue, based on modeling and field data, could be missing chemicals that fish remove from their bodies but that become concentrated in the tissues of mammals and birds, which have a different respiratory physiology.
One-third of the 12,000 or so organic chemicals on the market in Canada fit this new category, say the study’s authors at Simon Fraser University in Burnaby, British Columbia. This study did not examine whether these chemicals are actually harming wildlife and people, they and others are quick to point out. Still, the work “is really raising a red flag and saying we’ve got to pay attention to this,” says ecotoxicologist Lawrence Burkhard of the U.S. Environmental Protection Agency in Duluth, Minnesota.
Biomagnification means that the level of a toxin in animals’ tissues rises as one moves up the food chain. For instance, as larvae eat algae, fish eat the larvae, and bigger fish eat smaller fish, the toxin present in the algae becomes increasingly concentrated; top predators like swordfish and polar bears end up with the highest doses in their tissues. This can happen with stable, fatsoluble chemicals that aren’t easily excreted in urine or feces. Biomagnification was first studied in the late 1960s in aquatic food webs, explains Frank Gobas, professor at Simon Fraser University and leader of the study. To screen chemicals, scientists began using a property known as Kow, which indicates how readily a chemical dissolves in water compared with fat and thus predicts how easily it will move from a fish’s blood lipids into water through its gills. Low-Kow, or more watersoluble, chemicals don’t build up in the fish food chain and were assumed to be safe.
Environmental chemists realized, however, that this assumption might not hold in food chains involving mammals and birds because their lungs are in contact with air, not water. This means that many chemicals that are relatively soluble in water and therefore don’t accumulate in fish might remain in the tissues of land animals if they aren’t volatile enough to easily move from the lungs into the air (predicted by a property called Koa). Supporting this idea, some organic chemicals that don’t biomagnify in fish appeared to be doing so in other wildlife and humans.
To explore this hypothesis, Gobas and graduate student Barry Kelly and colleagues collected plant and animal tissue samples–from lichens to beluga whales killed in Inuit hunts–in the Arctic, where, because of weather patterns and cold temperatures, organic pollutant levels are high. They tested the samples not only for known POPs but also for several chemicals with a low Kow but high Koa, which suggested they might biomagnify in air-breathing animals.
The measured levels of contaminants for various animals in aquatic and land food webs were similar to those predicted from a bioaccumulation model incorporating Koa and Kow, suggesting the model was correct. Chemicals with low Kow and high Koa stood out as potentially risky. Several of the contaminants studied, such as the insecticide lindane, have been proposed for the POPs treaty already. But many others with similar properties have not been scrutinized, Gobas says. The bottom line: “We’re missing a lot of chemicals” that may be building up in the food web, Gobas says.
Canada and countries in Europe that are working through lists of industrial chemicals to identify new potential POPs will now need to revise their approach, says chemist Derek Muir of Environment Canada. He adds, however, that the model has limitations. For one thing, it assumes the chemicals aren’t metabolized; many of them probably are, which may convert them to a form that is easily excreted. Procter & Gamble senior scientist Annie Weisbrod agrees: the Koa of chemicals “will matter in some cases,” she says, “but the number of chemicals [that bioaccumulate] will not be a third of those in commerce.”