The pesticides found in the Missouri men are often found in drinking water sources. Water treatment plants are not designed to remove these chemicals, so Swan says it is likely that drinking water is the main source of exposure.
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The popular weed killer atrazine, widely used by farmers in the U.S. Midwest, may also be the cause of declining populations of the northern leopard frog in the U.S. heartland.
Research by Tyrone Hayes, a biologist at the University of California, Berkeley, has shown that atrazine causes the frog's male hormone testosterone to turn into the female hormone estrogen. As a result, male leopard frogs not only grow eggs but develop ovaries, turning them into hermaphroditesmale-female hybrids.
What's happening in the frogs could have implications for humans. Hormonal functions are similar across most species of animals, including humans. As Hayes warns in the documentary, "Frogs tell us something about us."
(Hayes has received grants from the National Geographic Society's Committee for Research and Exploration and was recently named an Emerging Explorer by the Society.)
The biologist's research also suggests that very little atrazine is needed to induce the deformities in frogs. Larger doses actually appeared less harmful. These findings raise doubts about chemical safety laws and regulations that assume that bigger doses of chemicals are always more harmful.
Elsewhere, chemical runoff is suspected of killing beluga whales in the northern waters of Canada's St. Lawrence River. The beluga whales there have been found to have some of the highest cancer rates of any wild animals studied.
Some dead belugas have been so full of waterborne toxins that their corpses technically qualify as hazardous waste.
Biologists suspect that a mixture of chemicalsnot a single toxinis causing the belugas to develop such high cancer rates.
Sylvain DeGuise, a University of Connecticut pathobiologist, tested whales' immune cells to a variety of chemicals and found that the chemicals may not be toxic individually but could turn poisonous when mixed.
Testing the toxicity of all chemical mixtures is practically impossible. The U.S. government alone registers an average of 2,000 newly synthesized chemicals each year.
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