More than 75 million pounds of the herbicide atrazine are applied annually to corn and other crops in the US and around the world. It is the most commonly detected pesticide contaminant in groundwater, surface water and rain, raising concerns about its safety.

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Tyrone Hayes (University of California, Berkeley) and an international team of researchers reviewed the evidence linking exposure to atrazine to reproductive problems in animals, particularly amphibians and other creatures that encounter it in the water (J. Steroid Biochem. 127, 64–73; 2011). They found that when animals were exposed to even low ecological doses of the herbicide (e.g., 2.5 ppb or below) during development, alterations of male reproductive tissues resulted, including decreases in testicular size, sperm production and androgen production. The 'feminization' of male gonads in many animals, sometimes to the point of complete sex reversal in genetic males, has caused shifts in sex ratios toward females in these populations. The extent of the shifts increases in proportion with increasing doses of atrazine.

Epidemiological and laboratory evidence suggests that atrazine disrupts reproductive health and hormone secretion in female animals as well. To study whether similar effects are present in humans exposed to atrazine, Lori A. Cragin and colleagues at Colorado State University (Fort Collins), together with their collaborators, compared women in Illinois, where atrazine was detected in the water supply at concentrations of 0.7 ppb, to women in Vermont, where atrazine is not commonly used as a pesticide (Environ. Res. 111, 1293–1301; 2011).

Women who drank water containing atrazine had significantly lower levels of estrogen, which were inversely related to the estimated dose of atrazine that they ingested. They also were five times more likely to report irregular periods than women in Vermont, with the likelihood even greater for women who said they drank more than two cups of local tap water daily. The authors of the study considered the effects of lifestyle factors, such as physical activity, weight and diet, but these were not significant. These results are consistent with some studies that suggest that atrazine may block the production of estrogen in the female body. These hormonal changes have been associated with increased risk of osteoporosis, diabetes, heart disease and some cancers.

More than 150 new studies since 2003 have uncovered potential health effects of atrazine. In light of the safety concerns raised by these studies, scientists are calling for increased regulation of herbicides and a discontinuation of atrazine's use.