When pollutants are released within range of the Atlantic Gulf Stream or get picked up by northerly air currents that also blow east, North American pollutants can also be transported across the Atlantic toward Europe. Similarly, air masses may travel a northeasterly path from Asia across the Pacific to North America. Thanks to the trans-Pacific air currents, pollutants released in China make tracks across the north Pacific and cause local air pollution health problems in Japan and Korea. Dust from China can reach California in as few as four days and makes a regular contribution to formation of Los Angeles smog.
The chemistry of some contaminants—those that are heavier and less volatile—causes them to drop out of the atmosphere into the northern Pacific Ocean where they may move slowly through the water or be taken up by fish and marine mammals. Persistent pollutants that include PCBs, brominated flame retardants, and perfluorinated compounds have been consistently found in fish, seals, whales, and fish eating birds along the Pacific coast over the past decade. “Fish can become their own transports of contaminants and fish-eating birds are known to excrete contaminants,” says Robie Macdonald, a research scientist with the Canadian Department of Fisheries and Oceans. “Migrating animals are not a huge transport mechanism but it’s focused, because they take the contaminants to where they feed and hatch their young.”
Lipophilic literally means “fat loving,” and this term is used to describe chemicals that have an affinity for and are soluble in fat. Materials with this property are also often persistent—that they are fat- rather than water-soluble makes them resist environmental degradation. And they are “bioaccumulative”—when they lodge in fat cells they can accumulate in plant or animal tissue as part of the fat reserves being stored for energy. When an animal burns fat for energy—this happens in people as well as in birds and fish—the fat cells release their contaminants.
There are multiple ways people may absorb a particular lipophilic chemical, however, which is one reason figuring out sources of human exposure to these contaminants is tricky. For example, people are exposed to brominated flame retardants through household dust but also through food they eat that has accumulated these chemicals in its fat. In the Arctic—where contaminants are aggregating and animals that are staples of the traditional Northern diet have large stores of fat—the region’s top predators, polar bears and humans, have some of the world’s highest exposures to these pollutants.
Monica Danon-Schaffer is a chemical engineer at the University of British Columbia who is investigating how and why these kinds of chemicals are ending up in water in the Canadian Arctic.
Danon-Schaffer sketches for me a series of molecules—PCBs, PBDEs, a couple of perfluorinated compounds, and another kind of chemical called a short-chain chlorinated paraffin (used as industrial lubricants and coatings, among other applications). The PCBs and PBDEs are markedly similar: strongly bound carbon ring structures with either chlorine or bromine atoms attached. The chlorinated paraffin and PFCs also bear a striking resemblance: Both are made up of long, branching chains. These molecules are strong and don’t easily give up either their structure or its linked chemical activity. The very structure that makes these substances effective in squelching fire, effectively flexible, or adept at resisting moisture, Danon-Schaffer explains, is what makes them so persistent. And this is also what them makes them incompatible with, or toxic to, some vital biological systems.
While these substances resist degradation in the environment, because they are added to—mixed in—rather than chemically bound to the materials they’re used to modify, eventually they become separated and leave the finished product. This is part of what makes it so difficult to keep track of these chemicals. For one, exactly how much of each substance is produced is not precisely known. Nor is it known exactly how much goes into each product, let alone how much can be expected to separate out and when or where this happens. The mixtures of these chemicals used commercially are typically not 100 percent pure and so may contain other synthetics that finished products may also shed. Then there’s the fact that, in the environment, many of these problematic synthetics break down into smaller molecules that may be more persistent or more toxic than their larger cousins.