Atrazine, the most popular herbicide in the U.S., appears to cause a wide range of sexual abnormalities in frogs, according to a study by biologist Tyrone Hayes published today in the Proceedings of the National Academy of Sciences. Seventy-five million pounds of atrazine are used in the U.S. every year, and it is the most common contaminant in the nation’s waterways. Switzerland, Sweden, Germany, Italy, Norway, France, and Belgium have all banned atrazine, and Hayes’ study suggests why: The chemical, an endocrine disrupter, alters hormone function in amphibians and other critters even when it is present at just 0.1 parts per billion — 30 times less than the amount legally permitted in drinking water. The result: Frogs with unusual combinations of testes and ovaries. (You’d be jumpy, too.) Syngenta, the company that manufactures most of the atrazine sold in the U.S., rejected the results of Hayes’ research, saying, “No conclusions can be drawn.”