2,4-D or not 2,4-D? That is EPA's question.


There was an old lady who swallowed a fly – or so began one of my favorite songs from childhood.  An elderly woman, determined to rid herself of a pesky fly problem, starts out by swallowing a spider to catch the fly.  When the spider doesn’t work, however, she proceeds to swallow a bird, a cat, a dog, a goat, a cow, and a horse.  While the story doesn’t have a pretty ending, it always made me and my fellow classmates laugh. 

Solving one problem with another problem can be funny to a kid, but it is no laughing matter when similar problem-solving techniques are used by Agencies charged with protecting public health.  A good example of “swallowing a spider” logic is how the US Department of Agriculture (USDA) and US Environmental Protection Agency (EPA) have treated a new pesticide mix called Enlist Duo.

For many years, glyphosate (the active ingredient of Roundup) has been used to combat weed growth in corn fields.  Farmers plant corn that has been genetically modified (commonly called “GMO corn”) to be resistant to chemicals like glyphosate, allowing them to spray increasing amounts of herbicides to kill weeds without damaging their crops.  Though the GMO corn is “protected” from the killing-action of the herbicide, some plants like milkweed (a staple of the iconic Monarch butterfly - see NRDC Wildlife Conservation Project Director, Dr. Sylvia Fallon’s, blogpost for more details) are destroyed, while other weeds begin to develop resistance to the herbicide. 

So what does one do when your go-to herbicide stops controlling weeds in your field?

The chemical company solution:  Use even MORE herbicide.

Enlist Duo, the agricultural equivalent of swallowing a spider, is a chemical combination of two herbicides: glyphosate and 2,4-D (or 2,4-dichlorophenoxyacetic acid).  Intended for use on genetically modified corn and soy (the GM crop seed was approved by USDA last week), the Enlist Duo chemical mix is currently being reviewed by EPA for market approval.  The Enlist Duo registration has garnered a great deal of attention, with both media (like yesterday’s segment on the Dr. Oz show) and the public (nearly 400,000 comments were submitted to EPA on the Enlist Duo registration – including comments from NRDC) – overwhelmingly opposing the new pesticide. 

EPA should not approve Enlist Duo for a large number of reasons, but several important human health-based ones include:

  • A lack of updated health information on glyphosate and on the glyphosate/2,4-D combination.
  • Ample evidence suggesting adverse impacts of 2,4-D on the thyroid - an organ involved in proper brain formation in humans and other vertebrates.
  • The high potential for human exposure to Enlist Duo via drinking water, breast milk, and air/dust in the proposed registration states. 

Vulnerable populations (like pregnant women, developing fetuses, infants, and children) are particularly susceptible to developmental impacts that could be caused by chemicals like Enlist Duo, so it is absolutely critical that EPA properly and adequately assess both the harms and exposure routes of this proposed pesticide mixture.

NRDC found multiple flaws in the proposed registration for Enlist Duo (our detailed comments can be found here), including EPA’s inadequate demonstration that Enlist Duo would not pose unreasonable adverse effects on the environment (particularly for Monarch populations). Additionally, for human health outcomes:

NRDC argued that EPA could not accurately determine the health impacts of Enlist Duo without updated health information on glyphosate.

Though Enlist Duo contains both 2,4-D and glyphosate, EPA only considered the health effects of 2,4-D in the registration process.  In the 21 years since the glyphosate health assessment was conducted by EPA, more than 3,000 peer-reviewed articles have been published on the topic.  Emerging evidence suggests possible links between glyphosate exposure and kidney disease, pre-term deliveries, attention deficit hyperactivity disorder, birth defects including neural tube defects, and miscarriages.  Additionally, the health impacts of the two chemicals used together (see Dr. Maricel Maffini’s blogpost for additional explanation on cumulative effects) have never been investigated.

NRDC presented scientific evidence that 2,4-D can harm the human body, particularly when exposures take place during early life development.

2,4-D has been associated with a number of negative health outcomes including decreased fertility, higher rates of birth defects, and other signs of endocrine disruption – especially through heightened impacts on the thyroid system.

Thyroid hormones play an essential role in the maintenance of body temperature, metabolism, cell differentiation (helping cells know what type of tissue or organ to turn into), and fetal and postnatal brain development.  Mounting scientific evidence at the molecular, cellular, animal, and human levels demonstrate the ability of 2,4-D to throw off the delicately balanced thyroid system. 

At the molecular level, new scientific tools developed by EPA, the Food and Drug Administration (FDA), the National Institutes of Health (NIH), and other government partners (Tox21) have shown that 2,4-D can prevent the thyroid receptor from binding to thyroid hormones (a process that can impact the proper functioning of the thyroid).  Studies in zebrafish (an important animal model for developmental, reproductive, and neurological health impacts) have shown that 2,4-D can decrease thyroid hormone levels.  Even EPA’s own health assessment of 2,4-D  demonstrated that it can impair thyroid functioning in rats and dogs (see NRDC comments at pages 20-25 for more specific details), although EPA disregarded these effects. Finally, epidemiologic studies in people have shown that farm workers exposed to 2,4-D are at increased risk of developing hypothyroidism (an underactive thyroid) compared to farm workers that were not exposed to 2,4-D.

NRDC argued that the potential for Enlist Duo to impact a large portion of the U.S. population requies EPA to adequately evaluate all of the potential ways that people may be exposed.

Humans can be exposed to 2,4-D by absorbing it through our skin, breathing it into our lungs, or ingesting it from our food and beverages.  To adequately protect vulnerable populations, EPA must use health protective values in its assessment of potential exposure to 2,4-D via all routes, including (but not limited to) drinking water, breast milk and air/dust.

2,4-D has been detected in domestic finished (tap water) ground and surface waters (see pages 41 through 45 of this EPA report for more details), indicating that U.S. populations could be exposed to 2,4-D through their drinking water. 

Infants could be exposed to 2,4-D via both breast milk and formula.  Studies in pregnant rats have demonstrated that 2,4-D can be transferred through maternal milk to pups (see page 10), acting as a source of exposure for the developing pup.  2,4-D has also been shown, in rodent studies, to alter the nutritional content of maternal milk by decreasing the levels of important fats. Additionally, infant formula prepared with 2,4-D contaminated drinking water could lead to early childhood exposures.

Human populations can also be exposed to 2,4-D via inhalation or ingestion (by placing hands contaminated with 2,4-D directly into our mouths or onto the food we are about to eat) of 2,4-D particles in air and dust.  Pesticides can be breathed in when the particles enter the air and travel far from their source via processes known as spray drift and volatilization.  Spray drift is the movement of pesticide droplets in the air, typically as a function of the pesticide application.  Volatilization is when a liquid or solid form of the pesticide converts into a gas form, allowing it to travel long distances.  2,4-D can travel by both spray drift and volatilization to contaminate distant sites including homes, school yards, gardens, and nearby crops that are not genetically modified to withstand the herbicide.

Enlist Duo has been proposed to be used over a large geographic area and over a longer period of time (into the hot summer months) than the registration for 2,4-D alone allowed, leading to potential increases in exposure to vulnerable populations from spray drift and volatilization.  In the proposed application states, nearly 8 million children (ages 0 -19) and women of child-bearing age (15-49) live in the 2,4-D current use counties, and an additional 12 million children and women of childbearing age in the expanded use areas (2011 USGS and 2010 U.S. Census data).



While NRDC’s comments to EPA on Enlist Duo contained over 50 pages demonstrating why the agency should not approve the herbicide, it is not hard to see from the points highlighted above that more chemicals are not the solution to our herbicide resistance problem.  Regulators should consider public health protective strategies like integrated pest management before using potentially harmful solutions to our agricultural problems, and realize that there are gentler ways to catch a fly than to swallow a horse.