A study published today in the Proceedings of the National Academy of Sciences links atrazine, the most heavily used herbicide in the U.S., to an array of sexual abnormalities in frogs.
The study, from Tyrone B. Hayes, a specialist in the hormone systems of amphibians at UC Berkeley, comes as the Environmental Protection Agency enters the final phase of a "special review" of the chemical, prompted in 1994 by concerns over its prevalence in drinking water.
Hayes' findings provide the latest evidence that commonly used chemicals may alter hormone function in fish, reptiles and amphibians. Since the mid-1990s, researchers in England and Florida have been documenting sex changes in fish downstream from sewage outfalls and alligators exposed to agricultural chemicals.
Atrazine has not been shown to be dangerous to humans.
Hayes first found that atrazine was a powerful endocrine disruptor in frogs in a study completed last year. In a follow-up--the study being published today--he found that exposure of tadpoles to water with 0.1 part per billion atrazine, or 30 times less than the amount legally permitted in drinking water, produced frogs with mixes of testes and ovaries. "The atrazine turns on an enzyme called aromatase," he said. "This converts testosterone to estrogen."
Recent studies by the U.S. Geological Survey have detected levels as high as 2,300 parts ppb in water near agricultural areas.
Biologist Darcy Kelly of Columbia University said Monday she isn't sure that the studies make a definitive link between the herbicide, the enzyme and the sex changes, but she said that the study should be taken seriously.
"You take the tadpoles, you give them a little bit of this herbicide and you screw them up rather profoundly," she said. "How it works, whether it has anything to do with human beings, I don't think those are solved, but it's a cautionary note."
Syngenta, the company that manufactures most of the atrazine sold in the U.S. and paid for the first study, has rejected the results of Hayes' research.
Speaking from Washington Monday, Syngenta toxicologist Tim Panoor said other studies commissioned by the firm produced contrary results. "No conclusions can be drawn," he said.
Release of the study comes on the day of a public hearing about the safety of atrazine before the EPA makes a final decision about relicensing the pesticide. The EPA is expected to make its relicensing decision in August.
Atrazine, first synthesized in Switzerland in 1955 by scientists for J.R. Geigy (now part of Syngenta), was first licensed in the U.S. in 1959. Of the 75 million pounds now used in the U.S. every year, most are applied in the Midwest on corn, sorghum and sugar cane crops. Corn growers estimate it saves them $36 an acre by suppressing weeds, reducing labor costs and increasing productivity.
In California, 55,000 pounds are used every year, mainly in agriculture. According to the Department of Pesticide Regulation, only professional applicators are supposed to apply it around parks, schools and home gardens. Syngenta sells it only directly to homeowners as a turf-care product in Florida, Louisiana, Mississippi, Texas, Georgia and North Carolina.
Combined use is so heavy that by the mid-1990s, the U.S. Geological Survey found that it was the most frequently detected contaminant in waterways.
Traditionally, toxicity studies for farm chemicals are carried out on lab animals, such as rats. A sudden crash in the global amphibian population in 1989 prompted the U.S. Department of the Interior and a number of federal agencies to set up the Amphibian Research and Monitoring Initiative.
Meanwhile, in Europe, concerns over high levels of atrazine detected in drinking water led to bans in Switzerland, Sweden, Germany, Italy, Norway, France and, most recently, Belgium.
By the mid-1990s, under pressure from the EPA for new safety data, Syngenta commissioned an estimated 100 new atrazine safety studies in North America, including one from Hayes.