Despite substantial public opposition and the “grave concerns” of about 50 members of Congress and significant unanswered questions about human and environmental health impacts, the Environmental Protection Agency (EPA) has approved a new herbicide called Enlist Duo for use on genetically engineered corn and soybeans in Illinois, Indiana, Iowa, Ohio, South Dakota and Wisconsin. EPA, which says it has approved Enlist Duo “to manage the problem of resistant weeds” is now considering approving Enlist Duo for use in ten more states: Arkansas, Kansas, Louisiana, Minnesota, Missouri, Mississippi, Nebraska Oklahoma, Tennessee and North Dakota.

Immediately following the EPA announcement, the Natural Resources Defense Council (NRDC) filed suit in the US Court of Appeals for the DC Circuit to block the registration of the new weed killer. A week later, on October 22nd, a coalition of farmers and environmental organizations – the National Family Farm Coalition, Center for Food Safety, Environmental Working Group, Earthjustice, Pesticide Action Network North America, Beyond Pesticides and the Center for Biological Diversity – filed suit in the Ninth Circuit Court of Appeals in San Francisco on behalf of their membership in the six Midwestern states  where the herbicide would be used. They are also seeking to block the EPA’s approval of Enlist Duo. Both lawsuits contend that the EPA failed to adequately assess the herbicide’s impacts on human health and wildlife, including imperiled monarch butterflies.

Manufactured by Dow AgroSciences, Enlist Duo combines glyphosate – the active ingredient in Roundup, the most commonly used herbicide in the US – with what’s called a choline salt of 2,4-D – the country’s third most widely used herbicide. Enlist Duo would be used on corn and soybean seeds genetically engineered to withstand this herbicide combo. The US Department of Agriculture (USDA) approved these seeds last month. The aim of Enlist Duo is to tackle weeds that have become resistant to glyphosate that is now used on about 94 percent of the corn and about 89 percent of the soybeans planted in the US. The new seeds have been engineered to resist 2,4-D as well as glyphosate. A DowAgro Sciences press release calls the EPA’s approval of Enlist Duo, “a true victory for farmers” now grappling with glyphosate-resistant weeds across tens of millions of acres of American farmland.

2,4-D is already widely used on a wide variety of edible crops, landscaping plants, and lawn, pasture and turf grasses. Its use in Enlist Duo on the genetically engineered corn and soy could significantly increase use of 2,4-D, say both the EPA and USDA. In 2011, about 5 percent of US corn was sprayed with 2,4-D, which was used on about 11 percent of soybeans planted. In the Midwest where it’s been used most intensively, 2,4-D has drifted away from fields where it has been applied, contaminating surface and groundwater. Weeds resistant to 2,4-D have already be found in the Midwest. Through Enlist Duo, the Department of Agriculture expects 2,4-D use could grow by as much as 300 to 700 percent by 2020.

Human health concerns and data gaps – no exposure monitoring required

In studies with laboratory rats, 2,4-D has been linked to adverse effects that include reproductive, hormonal and other health impacts. 2,4-D itself is considered a synthetic hormone works to kill plants by disrupting the way certain plant cells grow. A number of human epidemiological studies – of farmers in Kansas and Nebraska and of California farm workers, among others – have found links between 2,4-D use and increased risk for non-Hodgkin’s lymphoma. But EPA’s assessment of Enlist Duo’s toxicity, gives the herbicide a pretty clean bill of health.

In its approval documents, EPA explains that it looked only at environmental and health effects of Enlist Duo’s 2,4-D component as it is that ingredient’s use that the agency considered “new” for regulatory purposes. Because glyphosate is already widely used on corn and soy and that ingredient wasn’t changing, EPA didn’t reassess those environmental health effects and concentrated on looking at impacts of expanding 2,4-D use.

The EPA says that, “When used according to label directions, Enlist Duo is safe for everyone, including infants, the developing fetus, the elderly and more highly exposed groups such as agricultural workers” and “safe for the environment, including endangered species.” The details of EPA’s approval documents, however, raise some questions about that safety. Studies reviewed by EPA show that there is evidence of 2,4-D’s adverse effects to the kidney, liver, thyroid and reproductive system when high doses were given to laboratory rats. Such exposure also produced effects on the adrenal glands, various hormones, metabolism and neurological function. The EPA also reports that in rats, lower levels of exposure produced what might be described as subclinical effects on thyroid function. These lower exposures also produced adverse nervous system and kidney effects, gait abnormalities, birth defects and lower birth weights. Previous EPA assessment of 2,4-D have found links to cancer inconclusive.

Yet in addition to an association with non-Hodgkin’s lymphoma, in studies of farmers who worked with 2,4-D, the herbicide has also been linked to suppressed immune system function, thyroid problems, lower sperm count and increased risk for Parkinson’s disease , as noted in a June 2014 letter from nearly three dozen scientists and physicians to EPA Administrator Gina McCarthy asking EPA not to approve Enlist Duo.

While the types of crops on which Enlist Duo will be used involve less labor of the type that would expose agricultural workers and its chemical ingredients are less toxic than some other widely used pesticides, enforcement of use requirements will be critical to minimizing drift and human exposure, says Migrant Clinicians Network Director of Environmental Health, Amy Liebman. Further, she says, “It is our long-held contention that chemicals should not be put on the market without a way to monitor exposure, particularly for applicators.” Approval of Enlist Duo does not include any requirements for human exposure monitoring.

Despite the lab study results, the EPA did not find any exposure risks of concern for those mixing, applying or otherwise handling Enlist Duo. And because of what EPA describes as the “low acute inhalation toxicity” for 2,4-D choline salt, it did not perform “a quantitative occupational post-application inhalation exposure assessment.” EPA also found no concerns posed by skin contact.

The agency assumes that because there is no concern for health risks to those handling and applying the herbicide, there would also be no health risks for those inhaling Enlist Duo in the field after it is applied. Using the same logic, EPA says there is no risk from such inhalation to bystanders.

Despite these conclusions, labeling requirements for Enlist Duo tell a different story. These say that “only protected handlers may be in the area during application” and no one should enter treated fields for 48 hours after the herbicide has been applied without personal protective equipment. Specified protective gear includes chemical-resistant gloves and protective eyewear, such as goggles, safety glasses or a face-shield.  A Dow AgroSciences’ Material Safety Data Sheet (MSDS) also says: “After using this product, remove clothing and launder separately” and to promptly wash any exposed skin and to remove all clothing and shower after work and to launder any protective clothing separately from other household laundry.

Lack of proper training in pesticide use worries Jeannie Economos, Pesticide Safety and Environmental Health Project Coordinator with the Farmworker Association of Florida. “Farm workers that are not properly trained are likely to risk very dangerous take-home exposures,” she says and unknowingly expose their families.

Drift and other exposures

A big concern for human health and environmental exposure to Enlist Duo is drift, explains Bill Freese, Science Policy Advisor with the Center for Food Safety, a Washington, DC-based non-profit. In its approval of Enlist Duo, the EPA explains that the herbicide has been formulated to reduce volatility and hence drift. EPA also explains that the amounts of 2,4-D that would be used via Enlist Duo present no risks of concern from drift. Yet to minimize potential drift , EPA specificies that Enlist Duo can only be applied when wind is 15 miles per hour or less and with 30 foot buffer zones.EPA’s use instructions also caution against applying the herbicide during temperature inversions as that could increase risk of drift. Dow AgroSciences’ MSDS further says to prevent runoff, application should be avoided when heavy rain is forecast. Freese points out that pesticide application during high winds is common and that historically, 2,4-D has been frequently implicated in herbicide drift.

While the new formulation is supposed to minimize drift, other changes in how 2,4-D can be used in Enlist Duo have the potential to increase exposure. NRDC notes that Enlist Duo can be used during more of the growing season than was allowed for 2,4-D and thus “could mean wider human exposure.” On its own, 2,4-D was approved for use only on very short plants and could be applied “over-the-top” only on corn plants up to 8 inches tall and as a “pre-plant” application on soybeans. In its Enlist Duo formulation, 2,4-D can be used “over-the-top” on corn pants up to 48 inches tall and “over-the-top” on the soybean plants. A question to be asked is what the exposure potential would be for corn detasseling crews – that in the Midwest are often made up of teen-agers as young as 13 ­– that work during hot summer months, if adjacent fields are in the process of being treated.

Left unanswered in EPA’s Enlist Duo toxicity assessment, is that of effects of cumulative exposure to Enlist Duo and other pesticide chemicals. The EPA explains that it did not do such an assessment because it does not think 2,4-D behaves biologically in the same way as other herbicides and pesticides. But without exposure monitoring for Enlist Duo, it will be almost impossible to know if farm workers have been exposed, whether to this chemical formulation alone or in combination with others. And as Economos points out, “farm workers do travel,” so could be exposed to a variety of pesticides in multiple locations.

Unless the NGOs’ lawsuits prevail, Enlist Duo is poised to be used across great swaths of the American landscape, with uncertain health effects and even less information about where and to what extent people are being exposed. The EPA is accepting comments on its approval of Enlist Duo in the ten additional states until November 14, 2014.

Elizabeth Grossman is the author of Chasing Molecules: Poisonous Products, Human Health, and the Promise of Green Chemistry, High Tech Trash: Digital Devices, Hidden Toxics, and Human Health, and other books. Her work has appeared in a variety of publications includingScientific American, Yale e360, Environmental Health Perspectives, Mother Jones, Ensia, The Washington Post, Salon and The Nation.