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In 2003, after reviewing 19 lab and field studies on atrazine and frogs, the U.S. Environmental Protection Agency (EPA) asked Syngenta to do additional research-research using a tiered approach that would address uncertainties found in all previous studies.

Syngenta went on to conduct the most rigorous study to date with atrazine and frogs.
The specifics:
  • Two simultaneous and identical studies were conducted to address uncertainties in previous research.
    • The study was conducted in two separate labs in two countries — the U.S. and Germany.
    • It involved 3,200 frogs and the examination of 100,000 tissue sections.
    • Five concentrations were tested, from 0.01 ppb to 100 ppb of atrazine.
    • A positive control study using estradiol (an estrogen) demonstrated the sensitivity of Xenopus laevis (test species) and suitability of the test system.
  • Researchers sought to eliminate confounding factors in methodology that posed a problem in previous research.
    • The study was conducted in strict compliance with Good Laboratory Practices (GLP).
    • Flow-through tanks helped keep atrazine levels — as well as other factors like temperature and pH — constant and help prevent disease and other conditions that may develop in standing water. Flow-through tanks also reduced stress to the frogs caused by repeated water exchanges and handling.
    • All raw data was shared with EPA and its Scientific Advisory Panel.
  • Results: Atrazine had no effect on the growth, development, survival or sexual differentiation.
    • Dosing overlapped the sensitive window for sexual development and metamorphosis.
    • Positive controls showed effects; atrazine-treated frogs did not.
    • EPA concurred with the conclusions: In its 2007 White Paper, EPA stated, "Based on the negative results of these studies, the Agency concludes that it is reasonable to reject the hypothesis formulated in the 2003 SAP that atrazine exposure can affect amphibian gonadal development. The Agency believes at this time, there is no compelling reason to pursue additional testing with regard to the potential effects of atrazine on amphibian gonadal development."
    Kloas et al. Does Atrazine Influence Larval Development and Sexual Differentiation in Xenopus laevis? ToxSci Advance Access published on November 13, 2008, DOI 10.1093/toxsci/kfn232.

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