This article was researched and written by a student at Mount Holyoke College participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.
Since its introduction in 1958, atrazine has become the most widely used herbicide in the United States. It is the top-selling product for Syngenta, the largest chemical corporation in the world. Atrazine has become the most frequently detected contaminant of ground, surface, and drinking water in the midwestern United States (U.S.), and is one of the most controversial “crop protectors” on the market.
Used initially to control the growth of annual broadleaf weeds on such row crops as corn and sugarcane, atrazine is now applied to commercial golf courses, industrial centers, lawns, and along roadsides. According to the Environmental Protection Agency (EPA), over 76 million pounds of atrazine are applied to crops each year. Despite its widespread use in the U.S., the European Union banned atrazine in 2003, citing it as a major contributor to water contamination.
Atrazine in the environment
Atrazine is a chlorotriazine and its chemical properties make it prone to leaching and runoff. According to the Agency for Toxic Substance and Disease Registry (ATSDR), once atrazine is applied to the soil, it can remain there for days or months. It is usually broken down, however, within one growing season. Nonetheless, any atrazine that enters the groundwater or surface waterways through run-off or leaching can remain for a much longer time because the chemical is slow to breakdown in water. This is one reason why people living near agricultural fields oftentimes find atrazine in their drinking water. If atrazine enters the air, other reactive chemicals in the air can break it down—unless atrazine attaches to dust particles. In this case, breakdown is not expected. When this happens, the wind can carry atrazine-laden dust particles long distances from the original application area. Atrazine has been detected in rainwater more than 180 miles (300 kilometers) from where it was applied.
Adverse health effects of atrazine
Field and laboratory studies
Recent studies indicate that atrazine is an endocrine disruptor that interferes with hormone synthesis, secretion, receptor binding, activity or degradation. One of the first researchers to investigate the effects of environmental exposures to atrazine in wildlife was Tyrone B. Hayes, a biology professor at the University of California, Berkeley. He reported that just 0.1 parts per billion (ppb) of atrazine in male leopard frogs (Rana pipiens) and in African clawed frogs (Xenopus laevis), caused gonadal deformities, multiple testes, nonpigmented ovaries, hermaphrodites, and chemical castration.
Hayes' research revealed that atrazine reduces androgens in adult male frogs and reduces the growth of the androgen-dependent larynx in developing male larvae. One mechanism of action leading to these effects is the induction of aromatase, an enzyme responsible for the conversion of androgens to estrogens. Although researchers have published several studies concluding that atrazine is an endocrine disruptor—which can chemically castrate and feminize male amphibians—Syngenta, a major producer of atrazine has refuted these findings. As yet, Syngenta's results have not been published in peer-reviewed literature.
Several other studies indicate that atrazine causes reproductive, developmental, and behavioral effects in other species, including salmon, turtles, and rats. One study on late stage pregnancy of Long-Evans rats concluded that exposure to atrazine at levels as low as 0.09 mg/kg (parts per million or ppm) body weight caused alterations in mammary gland development.
Few studies have confirmed a definite link between atrazine and adverse human health effects, however, studies performed on rodents resulted in the EPA classifying atrazine as a possible human carcinogen.
Atrazine and chemical combinations
Since herbicides are used primarily on agricultural land, the presence of other pesticides and fertilizers in the same area where atrazine has been applied is not uncommon. Although few studies have examined the combined effects of triazine pesticides, the combination of simazine and atrazine have been reported to impact olfactory endocrine function in salmon in an additive manner. Other studies, evaluating combinations of atrazine with other pesticides, have shown a range of additive, synergistic, and antagonistic behavior, but additional studies are warranted.
Human Exposure to Atrazine
The Agency for Toxic Substance and Disease Registry has published a Public Health Statement for Atrazine that reviews important routes of exposure. Briefly, the ATSDR notes that people who live near agricultural areas, golf courses, roadsides, or other areas subject to herbicide applications may be exposed to atrazine. Farm workers and other individuals who apply atrazine are highly susceptible to exposure and although atrazine is not frequently detected in the air, air concentrations may be higher near production and disposal facilities, or areas where it is heavily applied. People may be exposed to atrazine if they come in direct contact with dirt that has atrazine in it. For this reason, children are susceptible to this route of exposure. Additionally, atrazine can contaminate drinking water supplies—the main route of exposure for humans. While it is applied to many crops, atrazine has not been detected readily in food samples. In those cases where it was detected, levels were low. According to the ATSDR, exposure to atrazine via food consumption is unlikely.
Atrazine in drinking water
The Environmental Protection Agency, through the Safe Drinking Water Act, monitors and enforces a maximum contaminant level (MCL) of 3.0 parts per billion (ppb) of atrazine in public drinking water. A recent United States Department of Agriculture (USDA) study reported, however, annual mean concentrations of atrazine that exceeded the MCL in public drinking sources. They found rivers (the Missouri, Mississippi, Wabash, Maumee, and Sandusky), streams (in Illinois, Ohio and Indiana), and water supply reservoirs (in Kansas, Missouri, Iowa, Nebraska, Ohio, and Illinois) with atrazine concentrations above the maximum contaminant level.
Consumer Factsheet on Atrazine 2006. Environmental Protection Agency (EPA).
Environmental Characteristics of Atrazine 1994. United States Department of Agriculture (USDA).
Hazard Ranking System 2005. EPA.
Public Health Statement for Atrazine. 2008. Encyclopedia of Earth.
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