PANNA: Atrazine: Possible Cause of Global Decline in Frogs?


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Atrazine: Possible Cause of Global Decline in Frogs?
May 10, 2002

Atrazine, the most commonly used herbicide in the U.S. and possibly the world, causes an array of sexual abnormalities including hermaphrodism (the development of both male and female sex organs) according to a new study published in the Proceedings of the National Academy of Sciences. The results may provide the key to a global mystery. The U.S. Environmental Protection Agency (EPA) is now in the process of evaluating the ecological impacts of atrazine, and we are encouraging the public to send in comments (see below).

For the last decade, scientists have documented a worldwide collapse in frog populations, and some believe that as many as 20 species are now extinct. Perhaps most surprising, frog populations have collapsed even in very remote, pristine areas. While the declines are well documented, the cause is a mystery; suggested culprits have included global climate change, habitat destruction, toxics, predation from introduced species and diseases. Now University of California at Berkeley researcher Tyrone Hayes may have found a key cause that would explain much of the decline.

Atrazine, is used in over 80 countries, and where it is used it is almost invariably found in streams, ponds and lakes. In the U.S., it is found in virtually all waterways. “[It] can be found in rain water, snow runoff, and ground water. There seems to be no atrazine-free environment,” says author Hayes. The reason for this is simple: in addition to being widely used, it is also highly mobile and persistent in the environment. The EPA estimates that the average half-life of atrazine in aquatic environments is 167 days, and in the cold waters of Lake Michigan, it is 31 years. Atrazine flows downstream from farms where it is applied and is also picked up by winds and carried to remote areas. The EPA notes that atrazine “was detected in more than 60% of weekly rainfall samples taken in 1995 from agricultural and urban sites in Mississippi, Iowa and Minnesota.”

While widespread atrazine pollution in the U.S. is well documented, U.S. pesticide manufacturers have long claimed that it is of little concern because the amounts normally found in the environment produce few obvious effects in laboratory studies. However, traditional toxicological studies use very high concentrations of atrazine and look for gross abnormalities. Hayes’s low-dose study, documented subtle sexual abnormalities missed by traditional high-dose atrazine studies. The results of the study, if confirmed, may pave the way to a major rethinking of how toxicological assessments are done in the United States.

Atrazine is a known endocrine disruptor. Endocrine disruptors cause developmental harm in extremely low doses by interfering with hormonal triggers at key points in the development of an organism. Hayes’ study shows significant sexual abnormalities at just 0.1 parts per billion (ppb)–30 times lower than levels allowed by the EPA for drinking water and 120 times lower than the 12 ppb EPA guideline for the protection of aquatic life.

The ubiquity of atrazine in the environment combined with an explanation of how very low concentrations might cause harm to frog populations could provide a key piece of information to unravel the mystery surrounding the decline of frog populations worldwide.

The EPA periodically re-assesses chemicals and is currently finalizing the ecological risk assessment for atrazine. Though this document is supposed to consider all the major ecological impacts, developmental impacts on frogs like those shown by Hayes’ paper are not considered in their risk assessment model. In fact, impacts on amphibians are entirely ignored in their model, which only looks at mammals, birds, fish, aquatic invertebrates and plants. The EPA’s conclusions, based on this flawed assessment are that “potential effects [are] likely to be greatest where concentrations recurrently or consistently exceed 10 to 20 ppb”–100 to 200 times the concentrations where significant sexual abnormalities were observed in Hayes’ study. Though Hayes’ results are mentioned elsewhere in the assessment, these risk assessment models are expected to form the basis of any EPA regulatory action.

* Write the EPA and urge them to include the developmental impacts of atrazine on amphibians in their risk assessment models. The EPA’s “Environmental Fate and Effects Revised Risk Assessment” for atrazine states that: “One of the most important steps in problem formulation is the selection of the endpoints upon which the ecological risk assessment is to be based.” By excluding developmental impacts on frogs, this document fails to accurately assess the likely impacts of continued atrazine use.

Comments should reference the docket number (OPP-34237C) in the subject and must be received by EPA on or before July 5, 2002. Comments can be sent via email or mail.

Email: [email protected]

Public Information and Records Integrity Branch,
Information Resources and Services Division (7502C)
Office of Pesticide Programs
Environmental Protection Agency
1200 Pennsylvania Ave., NW
Washington, DC 20460

Document Number in Subject Line: OPP-34237C

Background information on atrazine can be found on the EPA’s atrazine re-registration Web page at:

For further information on the EPA assessment of atrazine see:

For further chemical information on atrazine see:

For further information on frog declines see:

Sources: “Feminized Frogs: Herbicide disrupts sexual groups,” Science News Online, April 20, 2002, Vol. 161, No. 16,; Hayes, T.B., et al. 2002. “Hermaphroditic, demasculinized frogs after exposure to the herbicide atrazine at low ecologically relevant doses.” Proceedings of the National Academy of Sciences, Vol. 99:5476-5480, April 16, 2002,; “Popular weed killer demasculinizes frogs, disrupts their sexual development, UC Berkeley study shows,” UC Berkeley press release, April 15, 2002; “Amphibian Declines: An Issue Overview” jointly published by the Federal Taskforce on Amphibian Declines and Deformities (TADD), Partners in Amphibian and Reptile Conservation (PARC), the Declining Amphibian Populations Task Force (DAPTF), and the Amphibian Conservation Alliance (ACA),; and “Reregistration Eligibility Science Chapter for Atrazine, Environmental Fate and Effects Chapter,” April 22, 2002,

Contact: PANNA.

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