If the US Environmental Protection Agency (EPA) and the Department of Agriculture (USDA) give their approval to a new herbicide called Enlist Duo and to corn and soybean seeds genetically engineered (GE) to resist that chemical, the United States could see a significant increase in what is already one of the country’s most widely used herbicides. Yet while the EPA seems poised to approve Enlist Duo and USDA, the GE seeds, about 50 members of Congress have written to EPA Administrator Gina McCarthy and Secretary of Agriculture Tom Vilsack expressing their “grave concerns” about “the multiple adverse human health, environmental, agronomic and socioeconomic harms that approval of 2,4D crops will likely cause.”

The EPA says that “on the basis of protective and conservative human health and ecological risk assessments,” it has “confirmed” the new herbicide’s safety “for the public, agricultural worker and non-target species,” but many questions remain about effects of cumulative and long-term exposure, particularly for farm workers and others living near where it’s used. What makes these questions particularly pressing is that the new product combines two of the country’s three most widely used herbicides.

Enlist Duo and the corn and soybean seed engineered to tolerate it – both produced by Dow AgroSciences – were developed in response to the explosion of herbicide resistant weeds that has resulted from the past 15 to 20 years’ rapid expansion in use of glyphosate (the active ingredient of the herbicide sold as Roundup), now the country’s most commonly used herbicide. Enlist Duo would combine glyphosate with what’s called a choline salt of the herbicide 2,4 D – the United States’ third most widely used herbicide. The seeds engineered to resist this combination would enable farmers to use 2,4D on weeds without also destroying the corn and soy plants. The new product, writes the EPA, would “provide an additional tool to reduce the spread of glyphosate resistant weeds.”

2,4-D use could increase by 300 to 700 percent

Yet at the top of the list of the Congressional Representatives’ concerns, and most of the more than 300,000 public comments submitted to EPA as part of the agency’s pesticide approval process, are the human and environmental health effects of the expected increase in 2,4D use that will result from planting 2,4D-resistant corn and soy. (And just to clarify, the term “pesticides, as the agency explains on its “About Pesticides” page, is used to include herbicides as well as fungicides and other substances used to control pests.)

The USDA’s Animal and Plant Health Inspection Service (APHIS) says that expected planting of new 2,4D-resistant seeds could prompt a nearly 300 to 700 percent increase in use of 2,4D by 2020. This is not a hyperbolic projection. According to the USDA, Between 1997 and 2014, the acreage planted in glyphosate-resistant soybeans rose from about 10 to 94 percent; glyphosate-resistant corn from about 10 to 89 percent; and cotton from about 10 to 91 percent of all such crops planted in the US. At the same time, glyphosate use climbed at a similarly steep rate.

The bottom line is essentially that as weeds develop resistance, the more herbicide is needed to control them. As Washington State University research professor and agricultural scientist Charles Benbrook pointed out in his comments to EPA, “There are now more than two-dozen glyphosate-resistant weeds covering perhaps 70+ million acres.” 2,4D resistant weeds have already been reported in the Midwest.

Human exposure concerns

When it comes to human exposure, pesticide drift is a major concern. USGS surveys have found glyphosate and 2,4D to be prevalent in streams throughout the Midwest which has some of the country’s highest concentration of use. These chemicals have also been found in groundwater.

The EPA explains that Enlist Duo is designed to “reduce volatility and off-site movement of the herbicide compared to other forms of 2,4-D.” Proposed conditions of the registration would require 30-foot buffers, and the product’s label instructions would specify that the herbicide not be used when winds are over 15 miles per hour. Aerial applications would be prohibited. The EPA also explains that because it has already approved glyphosate and 2,4 D and no new uses of glyphosate are expected to occur with use of Enlist Duo, no new health assessment is needed for that ingredient. But because use patterns of 2,4D are expected to change if Enlist Duo is approved, health impacts of 2,4D have been assessed as part of the EPA’s registration process.

Occupational and long-term exposure data gaps

While the EPA’s assessment makes 2,4-D sound generally safe, the agency cautions that there is potential for “post-application” exposure for those working or otherwise present where 2,4D has been used. (This includes accidental ingestion by young children playing on “turf” where 2,4D has been applied.) As for occupational exposure, EPA says, “occupational risk handler estimates are not of concern…for all scenarios for use of 2,4D choline salt on GE corn and soybean.” Yet the agency did not investigate inhalation or dermal exposure, explaining that given the low acute toxicity via inhalation and “no potential hazard via the dermal route,” no assessment was done for either as part of EPA’s Enlist Duo review.

But 2,4 D is not only used on corn and soybeans. It is also used on wheat, rice, barley, oats and other grains, on sugar cane, on nut and fruit tree crops (almonds, apples, apricots, cherries, oranges, grapefruit, tangerines, hazelnuts, nectarines, peaches, pears, pecans, peanuts, pistachios, plums, and walnuts) as well as asparagus and sweet corn. It is also used on pasturelands, on turf used for lawns, on landscaping plants and to control weeds along roads, railroads, in managed forestlands and with other non-crop plants. Most of these uses involve greater worker presence in treated orchards and fields than do corn and soybean cultivation.

A DowAgro Sciences material safety data sheet for 2,4D says people should not reenter fields for at least 12 hours after the herbicide has been applied. It also says “do not permit lactating dairy animals to graze fields within 7 days after application,” to withdraw meat animals from treated fields at least 3 days before slaughter, not to harvest “forage within 30 days of application” or harvest “treated mature crop” within 60 days of application. Incidental damage to vegetable crops is also a potential problem.

The National Pesticide Information Center at Oregon State University explains on its 2,4D fact sheet that “Breathing 2,4-D vapors can cause coughing, a burning feeling in the airway, and dizziness” and that “Sunscreen, insect repellents, and drinking alcohol may increase how much 2,4-D is absorbed through the skin.” Epidemiological studies of farm workers exposed to 2,4D and other pesticides found an increase in rates of non-Hodgkin’s lymphoma, but it has not yet been determined whether or how 2,4D may have contributed to this outcome, so EPA does not consider 2,4D a human carcinogen. Studies done in the 1980s and ‘90s of 2,4D used in combination with another pesticide found links to adverse reproductive effects that included birth defects and miscarriages.

While acute effects of 2,4D appear to be few, Science and Environmental Health Network Science Director* Ted Schettler points out that the compound’s non-acute effects on thyroid hormone function may contribute to health effects that would take time to become apparent but that can lead to significant health problems. “Adequate maternal, fetal, and infant thyroid levels are necessary for normal brain development,” says Schettler. Even slightly abnormal thyroid hormone levels during pregnancy “are associated with adverse impacts on brain development and function in children measures several years later,” he explains. Further, according to the US Centers for Disease Control and Prevention (CDC) about one-third of US women of reproductive age aren’t getting enough iodine for healthy thyroid hormone production and “most of us,” says Schettler, are exposed to “environmental chemicals that can disrupt thyroid hormone function in a variety of ways.”

“Thus, as I see it,” says Schettler, “within the general population, there is a significant subset with marginal or frankly inadequate thyroid status at baseline. That’s a very different population than a bunch of healthy rats eating a healthy diet at baseline.”

“We are…concerned that EPA failed to thoroughly examine all of the significant health and environmental risks of 2,4-D including that of inhalation and aggregate exposure… The risks of approving 2,4-D crops are simply too great and the benefits too few to jeopardize public health, the environment and the long-term sustainability of our food supply. We therefore request EPA not register Enlist Duo for use on 2,4-D crops and USDA the regulated status for 2,4-D resistant crops,” wrote Representatives Chellie Pingree (D-ME), Peter DeFazio (D-OR) and colleagues in their July 31st letter to EPA and USDA.

USDA’s final environmental impact statement on the Enlist Duo-resistant corn and soybean seeds is open for public comment until September 8th. The EPA’s comment period closed June 30th. As of this writing neither EPA nor USDA had responded to the Congressional representatives’ letter. Unless the agencies have a dramatic reversal, we’re likely to see a dramatic increase in use of an herbicide with largely unexamined cumulative and long-term environmental health impacts.

*Ted Schettler also serves as Science Director for the Collaborative on Health and the Environment but SEHN is, he points out, his home institution; an earlier version of this piece omitted Schettler's affiliation with SEHN.

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 including Scientific American, Yale e360, Environmental Health Perspectives, Ensia, The Washington Post, Salon and The Nation.