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Nothing, therefore, is assumed.Along with laboratory experiments, regulatory agencies usually require proof of harm from both studies of wildlife and epidemiological research on people—where exposure to a contaminant is correlated to health problems—before banning or restricting a chemical.

“Our society demands a lot of evidence before we take policy actions,” says Karasov, “and that goes for protecting human life and wildlife.”

But others worry that the level of certainty required to ban a chemical creates a wedge for manufacturers, who can argue that the clear economic and social benefits of using chemicals outweigh the potential threats. “It’s precautionary to assume that if a chemical causes harm to other animals then it could be harmful to people,” says Langston. “But each time there’s political pressure, that caution gets eroded.”

One chemical whose toxic effects are undisputed is mercury. In its silvery, elemental form, mercury is relatively harmless. But the metal can also take an organic form, called methylmercury, that can accumulate in tiny organisms and the larger animals who eat them, sometimes with tragic results.

That is what happened in the 1950s in the Japanese town of Minimata, where a factory had begun dumping mercury-laden wastewater into a nearby bay. Unbeknownst to the company or townspeople, the waste mercury wasn’t washing out to sea, but was instead accumulating as methylmercury inside bay fish, a chief source of food in the local diet. The result was a public health disaster.

“Babies were born with a terrible cerebral palsy-type condition where they were virtually helpless, and they had all kinds of neurological problems,” says Kanarek, who has studied mercury exposure in Wisconsin communities that depend on fishing. Children and adults in Minimata lost the ability to walk or speak. Some shook violently. The strange ailment afflicted entire families, and it soon was dubbed Minimata disease.

Thinking the condition was contagious, the community isolated the victims and disinfected their homes. But people eventually suspected a different cause. Cats were behaving strangely after eating fish tossed from boats returning from the bay. “The cats would do this dance in the air and then drop dead,” says Kanarek. “That’s when people first began realizing that maybe fish were causing the problem.”

Although concentrated sources of mercury pollution are virtually nonexistent today, methylmercury in fish remains a public health threat in many parts of the world, including Wisconsin. Released into the atmosphere primarily through coal burning and small gold-mining operations, mercury can travel anywhere from a few miles to halfway around the globe before falling to earth. When it reaches lakes and oceans, microorganisms convert it into methyl form, which then gets stuck in small organisms like plankton. Plankton is eaten by fish, which in turn are eaten by larger fish, passing mercury contamination up the food chain. As a result, “tiny quantities in water end up being hundreds of thousands of times more concentrated in fish,” says Henry Anderson, the State of Wisconsin’s chief medical officer for environmental and occupational health. “Then we’re at the top of the chain, so it accumulates in people.” The same mechanism explains how toxins such as PCBs and toxaphene reach unsafe levels.

It’s the job of Anderson’s department to help people reduce their exposure, which it does first by monitoring a host of contaminants in fish—everything from older banned chemicals, like DDT, to newer ones, such as flame retardants. The group then issues consumption guidelines for fish caught in Wisconsin lakes and sold in grocery stores.

But the advice grows quickly complicated. For one, species from the same lake often contain different amounts of toxins; a walleye, for example, typically has four times the methylmercury of a bluegill. This means that warnings to merely stay away from certain lakes are too simple.

There are other complexities, as well. Methylmercury does slowly leave the body, for instance. So if a woman wants to get pregnant, she can reduce her mercury level by half if she stops eating contaminated fish for two months. PCBs are another story. “You just accumulate them over your lifetime,” Anderson says. At the same time, since PCBs build up exclusively in fat, a diner can cut exposure to PCBs by as much as half simply by trimming away a fish’s skin and belly fat. The same trick does nothing to lower methylmercury, however, which settles in muscle tissue throughout the body.