“How many animals have to die before we do what Europe did 12 years ago and ban atrazine?”

With a few months to go until the end of the Environmental Protection Agency’s public comment period on its risk assessment of the herbicide atrazine (the deadline was recently extended from August 5 to October 4), a battle is raging between environmentalists and farmers. Environmentalists, on the side of EPA, are pointing to evidence that atrazine affects human and animal health, particularly frog reproduction.

Farmers are siding with the agrochemical industry, questioning the science behind the assessment and asserting the herbicide is completely safe.

Seemingly contradictory scientific evidence and claims of significant economic impact are causing confusion. This has resulted in an unclear picture of a vital issue—one that could impact future generations, as well as plants and animals on land and in the water.

This leaves consumers in a difficult position. Who’s right? Are we safe? And is there something we should be doing?

Feminizing frogs

The second most widely used weedkiller in the U.S. after glyphosate (the main ingredient in Monsanto’s popular RoundUp herbicide), atrazine kills weeds that affect crops like maize and sugarcane by disrupting photosynthesis so they can’t make energy from sunlight. But it also affects hormones, making it a potential threat to humans and wildlife.

Atrazine tends to linger in the top six inches of soil, which helps to prevent the growth of new weeds. But this also makes it prone to being washed into groundwater; with around 70 million pounds of the chemical being released into the environment every year, the impact could be significant. Much of it becomes part of the water cycle: In the U.S., over half a million pounds of atrazine are precipitated in rainfall every year.

Amphibians appear to be hardest hit. A number of studies conducted in the early 2000s revealed that when exposed to very low levels of atrazine (0.0001 milligrams per liter (mg/L), far below the EPA’s safe limit of 0.003 mg/L), frogs grow to have several testes and ovaries. Some males even grew eggs in their testes, and some became females; they mated with other males and had the capacity to lay eggs, even though they were genetic males.

One study, published in the journal Proceedings of the National Academy of Sciences showed exposure to low levels of atrazine caused African clawed frogs (Xenopus laevis) to develop as hermaphrodites and led to a ten-fold decrease in testosterone. Another study published in the journal Nature revealed 10-92 percent of male wild leopard frogs (Rana pipiens) had gonadal abnormalities following exposure to the herbicide.

The biologist behind these studies is Tyrone Hayes, a biology professor at UC Berkeley. After publishing his findings, Hayes campaigned to ban atrazine, taking part in a documentary and a TED talk on the topic. The effects, he believes, are much more serious than altering frogs’ development.

He explains that atrazine switches on an enzyme called aromatase, which converts testosterone into estrogen, causing the chemical castration in male frogs. This action also impacts the development of breast cancer in humans, he says, which is controlled by estrogen.

As the evidence for the negative impacts of atrazine on the environment, wildlife and human health have built up over the years, there has been bubbling concern in the U.S., leading to a growing movement of environmentalists calling for a ban on the herbicide.