Atrazine, the most commonly used weed killer in the world, disrupts the sexual development of male frogs.

At levels of 0.1 ppb and higher, atrazine causes male tadpoles of African clawed frogs to develop both male and female organs, reports Tyrone B. Hayes of the University of California, Berkeley [Proc. Natl. Acad. Sci. USA, 99, 5476 (2002)].

 "Atrazine-exposed males have ovaries in their testes and much smaller larynges [voice boxes]," Hayes says. The herbicide also lowers the levels of testosterone in sexually mature male frogs by a factor of 10.

There is virtually no place in the U.S. that is not contaminated with atrazine, Hayes says. It is the second most frequently detected water contaminant and, according to U.S. Geological Survey studies, can be found at levels as high as 21 ppb in groundwater and 42 ppb in surface waters during the growing season in the Midwest. So the chances that amphibians are exposed to atrazine levels of 0.1 or more are extremely high, Hayes explains.

The drinking water standard for atrazine is 3 ppb. EPA is currently reevaluating the allowable levels of atrazine in drinking water. When 54,000 rural wells were tested, only 28 exceeded the standard, says Timothy P. Pastoor, head of global risk assessment for Syngenta, a major producer of atrazine.

Belgium, France, Germany, Italy, Norway, Sweden, and Switzerland have banned the use of atrazine. These countries have banned all pesticides that tend to occur in drinking water at levels higher than 0.1 ppb, Pastoor says, primarily on the basis of occurrence in water, not health studies.

Ronald J. Kendall, director of the department of environmental toxicology at Texas Tech University, has been trying to replicate Hayes's work. In an as-yet-unpublished study, he found that only the highest atrazine level tested--25 ppb--caused abnormal sexual development in African clawed frogs, and this occurred in only 3% of the frogs tested.

John P. Giesy, a chemist and ecotoxicologist at Michigan State University, and his student Katherine K. Coady are also investigating the effects of atrazine on frogs exposed in the wild. Even if atrazine is disrupting sexual development, it may not be harming the survival of the species, Giesy says.

Hayes believes, however, that atrazine could be one factor contributing to the worldwide decline of amphibians. As C&EN went to press, EPA was scheduled to release its revised risk assessment for atrazine.

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