Update (2/19/2016): As part of the settlement of a lawsuit brought by the Center for Biological Diversity, the U.S. Fish & Wildlife Service (USFWS) will analyze the impacts of atrazine and glyphosate—two of the most commonly used pesticides in the U.S.—on 1,500 endangered species. Under the agreement, announced today, the USFWS will also develop conservation measures for these and two other pesticides, which together represent almost 40 percent of annual U.S. pesticide use.

The state of California has been working to list atrazine—the United States’ second most-widely used herbicide—as toxic to the reproductive system under the state’s Proposition 65, which requires public warnings to be posted when and where such chemicals are used. Now, a court decision coming as soon as January will determine how the state will move forward with the listing.

This is a big deal. And it comes at a time when several top herbicides are under fire. In March, the World Health Organization’s International Agency for Research on Cancer (IARC) classified glyphosate—the active ingredient in the pesticide Roundup—as a probable human carcinogen. In August, IARC declared that 2,4-D, is probably carcinogenic to humans. And earlier this month, the U.S. Environmental Protection Agency (EPA) withdrew its approval of a new herbicide called Enlist Duo, which combines glyphosate and 2,4-D.

These are the nation’s most used agricultural chemicals. If you buy conventional groceries, the odds are very good that you’ve eaten food from fields where they have been sprayed. Between them, glyphosate, atrazine, and 2,4-D are used on nearly all of the corn and soy grown in the U.S. as well as on many other crops, including fruit, vegetables, nuts, and wheat.

What sets atrazine apart is its environmental mobility and ability to contaminate water—it has been found in rainwater nearly 200 miles from where it has been sprayed. For these reasons it is no longer approved for use in the European Union. Nonetheless, about 70 million pounds of atrazine are used annually in the U.S. It is applied most intensively in the Midwest and is the herbicide the U.S. Geological Survey has found most frequently in surface water.

Atrazine as been used since the 1950s and was designed to kill weeds by inhibiting photosynthesis. Because atrazine is applied to crops used as livestock feed, its residues are found not only in crops, but also in milk and meat. While not considered acutely toxic to people, atrazine’s long term human health concerns include reproductive, developmental, and possible carcinogenic effects. Atrazine gained notoriety for its potential hormonal effects when, exposure was shown to feminize male frogs in laboratory studies conducted by University of California Berkeley biology professor Tyrone Hayes. These effects have also been shown in experiments with other animals.

New research continues to show adverse reproductive effects, says Hayes, whose atrazine research, has come under fire from Syngenta. And these effects “don’t stop at [a single] generation’s exposure,” Hayes told Civil Eats. There are “long term” and “multi-generational effects” resulting from atrazine’s “altering of modes of reproduction,” he explained.

California Putting Atrazine on Stage

Using science available through the EPA, California’s Office of Environmental Health Hazard Assessment (OEHHA) has decided to list atrazine as a Proposition 65 reproductive toxicant. But the process of completing this listing—which includes setting drinking water limits for the chemical—is being delayed by a lawsuit against the state from atrazine manufacturer Syngenta. In the suit, Syngenta claims that there is no authoritative basis for California’s listing. The company also contends that a Proposition 65 designation would “cause irreparable harm to Syngenta and its agricultural customers” in and outside of California.

The lawsuit also seeks to derail California from listing two other pesticides that resemble atrazine chemically under Proposition 65. In legal documents, Syngenta has said that the listings could cost the company and farmers billions of dollars.

Syngenta argues that the evidence showing that these pesticides can cause reproductive harm to people is not sufficient to warrant a Proposition 65 listing. But University of Texas at Austin professor and Vacek Chair of Pharmacology and editor-in-chief of the journal Endocrinology Andrea C. Gore told Civil Eats the science suggests otherwise.

Atrazine, explained Gore, upsets the balance in production of the enzyme that controls key sex hormones, causing too much of a hormone associated with females to be released, leading to the feminization of males. Not only has this been shown in many species, she explained, but the way these hormones work in other animals is similar to what happens in people. Chemicals like atrazine, which disrupt how these reproductive hormones work, “have a high probability of doing this across the board,” said Gore.

“The clearest example,” of this is that atrazine exposure can lead to overproduction of estrogen and “the production of too much estrogen causes all sorts of problems,” both reproductive system effects and increasing the risk of certain cancers, Hayes further explained.

But, Hayes noted, “You can’t do experiments on people. That’s the Catch-22. And the effects don’t occur right away.”

There is, however, evidence linking pregnant women’s exposure to atrazine through drinking and genital deformities in male babies, an association Syngenta disputes.

Pesticide Safety Based on Manufacturer’s Science

While Syngenta is suing California, some environmental advocates—including the Center for Biological Diversity (CBD) and Center for Environmental Health—have filed comments with the state calling for more stringent exposure limits than California is currently proposing on these herbicides. But a close look at the lawsuit and the comments filed with the state by both Syngenta and the environmental advocates suggest that government regulators are relying heavily on Syngenta’s own science to make decisions about atrazine’s safety.

“More than 7,000 scientific studies conducted over the past 50 years have clearly established its safety for humans and the environment,” says Syngenta of atrazine on its website. But the advocates disagree.

“We’re super happy that California has decided to list this chemical, but we believe the [the exposure limit] is not protective of human safety,” CBD scientist Nathan Donley told Civil Eats. “It’s based on a nearly 20-year-old unpublished industry funded study,” he explained, adding that there are newer published studies showing adverse health effects at lower exposure levels.

“Most people think that the government tests pesticides for safety. But, in fact, it’s the pesticide companies that conduct almost all of the safety testing,” said Center for Environmental Health research director Caroline Cox in a statement. “So it’s no surprise that they almost always find that their own products are safe.”

According to Hayes, Syngenta scientists have reported in company documents that atrazine—and related herbicides—have caused mammary and testicular tumors in rats, i.e., the very same health effects that the company is discounting in the lawsuit.

What Happens Next

According to Sam Delson, OEHHA deputy director for external and legislative affairs, the agency expects a decision in Syngenta’s lawsuit some time in January. That case has to be fully resolved before California can finalize its Proposition 65 listing of atrazine and related chemicals, a process that could extend a year or more if Syngenta decides to appeal the decision.

Syngenta declined to provide a comment for this story, but Dr. Janet E. Collins, senior vice president of science and regulatory affairs at the agricultural chemical trade association CropLife America, calls OEHHA’s decision to list atrazine under Prop 65 “unfortunate and unwarranted.” Collins added that, “CropLife America will continue to gather information, engage with stakeholders, and act as a resource on the safe and effective use of crop protection products.”

Meanwhile, when it comes to glyphosate and 2,4-D, those chemicals’ major manufacturers, Monsanto and Dow, maintain that they are safe to use as directed. As for Enlist Duo, Dow AgroSciences has expressed confidence that EPA’s approval will be reinstated.

These companies, said Hayes, are making economic decisions. The people who are “paying the cost are the ones working in the fields, working in the factories where these chemicals are made, and consumers exposed through drinking water.”

Photos, from top: Crop duster courtesy of Shutterstock/B. Brown; Two frogs from Tyrone Hayes’ laboratory. Both are male, but one shows female traits after being exposed to atrazine.