The insecticide DDT is infamous for traveling up the food chain and, in addition to its other ills, making the shells of bald eagle eggs paper thin. The carcinogen entered the food chain in small concentrations, but because of its ability to hide in fat molecules, it reached larger and larger concentrations (a process called biomagnification) as it traveled from algae to larvae to fish to eagle—leading to its 1972 ban in the U.S. To avoid similar perils in the future, governments and chemists have begun evaluating thousands of everyday chemicals to assess their safety by measuring how easily they dissolve in water versus fat. Those that dissolve easily in water usually do not build up in the food chain.

But this approach entirely overlooks a large portion of Earth's food web: air-breathing animals, from rodents to humans. A new study has measured how easily a chemical moves from the lungs into air versus how easily it dissolves in fats and water. The research, reported in Science, reveals that thousands of chemicals may be capable of building up in air-breathing animals, if not water-breathing ones.

A large number of chemicals that dissolve relatively easily in water do not break down as readily in air, building up "specifically in nonaquatic food webs: mammals, birds, human beings," explains Frank Gobas, an environmental toxicologist at Simon Fraser University in British Columbia, who led the research. "In mammals and humans, we don't breathe water, we breathe air."

Gobas and colleagues studied the roughly 12,000 chemicals under review by the Canadian government to determine their environmental and health effects; they found that as many as one third may be in danger of accumulating in air-breathing animals. For example, the pesticide lindane, used in agriculture as well as to treat head lice, did not accumulate in fish but did build up in Canadian wolves that had fed on caribou, which in turn had been eating lichen. "About one third of all the commercial chemicals that are in use right now belong in this group of chemicals that are potentially biomagnifying," Gobas says. "In Canada, it will be three to four thousand. And our list of chemicals is small compared to the list of chemicals in the U.S. and E.U."

The total chemical count could reach as high as 10,000 worldwide, Gobas says, though he stresses that most, if not all, will prove benign because many may be broken down by cellular processes. Unfortunately, such metabolism is largely a mystery. "We are not very good at predicting or measuring how quickly chemicals are being broken down by organisms," Gobas admits. "[Metabolism] information is missing for more than 90 percent of the chemicals in commerce, which is a big problem."

The potentially dangerous chemicals range from pesticides to those used in perfumes and fabrics. Understanding how cellular processes break down these chemicals is the critical next step. "Many years down the road, we hope to relate [metabolic transformation rates] to the physical, chemical nature of substances," Gobas says. "Based on structure, we would like to have a better way of estimating metabolic rates and therefore bioaccumulation capacity." In the meantime, he argues, it might make sense to test the airborne as well as the waterborne bioaccumulation potential of chemicals.