by Xindi (Cindy) Hu
figures by Lillian Horin

In March 2017, the U.S. Environmental Protection Agency (EPA) decided not to ban Chlorpyrifos, a widely used pesticide. One year later, in February 2018, a bill was introduced in Hawaii to ban the manufacturing, distribution, and use of chlorpyrifos across all Hawaiian islands. Hawaii House Rep Richard Creagan said the legislation was prompted by the inaction in DC. How did the EPA come to decide against a ban on chlorpyrifos? What does the science say about the health impacts of chlorpyrifos exposure? Why is it controversial to pass a blanket ban?  While opponents of a ban on chlorpyrifos cite the risk of trade disruption, proponents are more concerned about the public health impact, pointing to mounting evidence that chlorpyrifos may impair brain development of children and damage cognitive function among adults.

Chlorpyrifos, one of the most widely used pesticides

Chlorpyrifos (pronounced ‘klaw-pai-ri-fos’) has a difficult name. You may not be able to say it, but it’s probably inside you. Introduced by Dow Chemical in 1965, chlorpyrifos is the most widely-used pesticide on crops, including corn, soybeans, broccoli, and apples, and is also widely used in non-agricultural settings like golf courses (Figure 1). Chlorpyrifos easily finds its way into Americans’ kitchens, and it was found in 91% of homes among a nationally representative sample of the US population in 2001-2002.

Figure 1: Produce most likely to contain chlorpyrifos. Image from, used with permission.

Chlorpyrifos was invented as an alternative to the pesticide DDT – which itself was a substitute for toxic lead arsenate – and has become part of a pattern known as “regrettable substitution”. Chlorpyrifos works by attacking insects’ nervous systems. At high doses, such as what farmers get exposed to when they spray pesticides,  it can cause people to experience nausea, dizziness, and confusion. Because of the potential health concerns, the EPA negotiated a deal with Dow Chemical to phase out all residential uses of chlorpyrifos in 2000, but this deal left the more profitable agricultural applications intact.

A natural experiment revealed the impact of chlorpyrifos on children’s health

The most disconcerting effect of chronic exposure to chlorpyrifos is its potential to impair children’s developing brains. When the residential use ban went into effect in 2000, it just so happened that a team of researchers at Columbia University was in the middle of recruiting participants for a study on childhood development. The ban allowed the researchers to split the study group in two halves, forming a natural experiment where the two groups of pregnant women were identical in every way except that the earlier group was exposed to household chlorpyrifos during pregnancy, and the latter group was not. The researchers found that when children were exposed in the womb, they tended to be smaller, have poorer reflexes, and show higher risks of having ADHD and other developmental disorders years after being exposed. Another team of researchers in Berkeley made similar findings. Since then, peer-reviewed publications have provided strong evidence for the neurodevelopmental toxicity of chlorpyrifos. Only Dow Chemical, the inventor of chlorpyrifos, disagrees with this well-established scientific evidence, citing its own “40 years of high-quality animal research.”

Most of Dow Chemical’s studies relied on standard toxicity testing recommended in the“OECD Guidelines for the Testing of Chemicals.” However, these methods cannot detect the more subtle effects caused by low doses and long-term exposures. Effects of chlorpyrifos on brain development are the focus of many academic research articles but not included in OECD guidelines. Therefore, these academic studies were not originally considered in regulatory decision making.

How are pesticides regulated in the U.S., and what has happened to chlorpyrifos?

While scientific studies can show the effects of different pesticides, in order to actually regulate pesticides like chlorpyrifos in the US, there are two pieces of relevant legislation: the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) and the Federal Food, Drug and Cosmetic Act (FFDCA). Under the FFDCA, the EPA regulates the amount of pesticide residues permissible on food items by establishing a “safe” level at which there is “a reasonable certainty of no harm” from the exposure. To make these safety findings, the EPA Office of Pesticide Programs had historically relied on laboratory animal studies for toxicity information about pesticides. In fact, 2010 was the first time human data were ever included in the assessment of a pesticide, and the EPA did not have a formal framework for incorporating human epidemiological data until December 2016.

In 2007, the Natural Resource Defense Council and the Pesticide Action Network of North America (two NGOs) filed a petition urging the EPA to ban chlorpyrifos in all food uses. Some environmental laws provide citizens the opportunity to petition the EPA for specific rule-making actions (Figure 2). After nine years, they had yet to hear a final decision on the petition, so they brought the case to the courts. The aforementioned Columbia and Berkeley studies both came out while this petition was being reviewed. Because of the EPA’s requirement to stay up-to-date with the latest scientific literature, they  had to revise their human health risk assessment multiple times over this nine-year period. Moreover, because of the diverse nature of epidemiological studies, the Science Advisory Panel that provides independent scientific advice to the EPA on health and safety issues sometimes had disagreements over how best to incorporate these human studies into their risk assessment, which further prolonged the review process. From 2011 to 2016 the EPA revised its human health risk assessment three times and changed their decision on banning chlorpyrifos three times, as well.

Finally, in Nov 2016, during the waning days of the Obama Administration, the EPA appeared to find enough support for a decision to ban chlorpyrifos, as shown in their revised human health risk assessment. However, two days later, Donald Trump was elected president and the rule-making task was left to the hands of the next administration.

Figure 2: Timeline for chlorpyrifos regulation.

What has happened since EPA’s decision not to ban chlorpyrifos?

In March 2017, despite mounting evidence for its toxicity, Scott Pruitt, head of the current EPA, denied the petition from the two NGOs and decided not to ban chlorpyrifos.  This decision would leave chlorpyrifos on the market until its next registration review, a program that re-evaluates all pesticides on a 15-year cycle. For chlorpyrifos, the deadline is Oct 1st, 2022. In their press release, the EPA acknowledged that current use of chlorpyrifos leads to its incorporation in food and drinking water above safe levels, but they emphasized that chlorpyrifos was a highly effective and widely used pest-management tool. A unilateral ban in the U.S. would disrupt international trading and hurt American farmers and consumers financially.

It is quite likely that chlorpyrifos will remain on the market for at least another five years, leaving millions of developing children and other vulnerable populations open to unnecessary exposure. On June 5, 2017, the two NGOs, joined by five states and the District of Columbia, filed a petition to the U.S. Court of Appeals to force the EPA to take action on chlorpyrifos. As of now,  the court has denied the petition.

In the meantime, chlorpyrifos exposure remains an issue. Farm workers are particularly vulnerable (Figure 3). Since the EPA’s decision not to implement a ban on chlorpyrifos last year, three cases of chlorpyrifos poisoning involving hundreds of farm workers have been reported in California alone. Farm workers are poorly paid, poorly educated, and, according to the Department of Labor, 46 percent undocumented immigrants. Farm workers are directly exposed to high doses of toxic pesticides through inhalation while spraying pesticide in the field. Furthermore, personal protective equipment is often not provided by employers due to cost concerns, and workers are afraid or unable to report such cases due to a lack of reporting channels or fear of job loss. As such, more unreported cases are likely to exist across the country.

Figure 3: Worker working in a strawberry field in Florida. Image from Dan Charles/NPR, used with permission.


It is not just chlorpyrifos

The controversy around chlorpyrifos is just one example of how pesticides and other toxic chemicals are regulated in this country. More than 34,000 pesticides that are derived from about 600 basic chemicals are registered by the EPA for use in this country. In addition, 85,000 more chemicals are regulated separately under the Toxic Substance Control Act (TSCA), which is criticized by many NGOs and academic researchers for being too lax. The EPA used FIFRA to ban or severely restrict the use of 64 active pesticide ingredients between 1972 and 2007, while only  five chemicals have been banned under the TSCA since its inception in 1976.

The large number of chemicals that need regulatory oversight makes setting chemical policy a challenging problem, especially given the lack of effective control measures. Many regulatory decisions depend on weighing the potential benefits of pesticide use against possible health risks. A ban is often hard to pass, especially when safer alternatives are not readily available. When a ban is too hard to pass, consumers often respond by purchasing organic produce or switching to ecologically friendly products to reduce exposure of themselves and their own family. However, such individualized solutions may only be available to a limited section of the population and cannot protect the most vulnerable populations.

The EPA is currently considering revising the way it uses scientific research in policy making. Under the proposed policy, scientific research that requires personal health information and keeps its raw data confidential cannot be used in the agency’s rulemaking. This applies to a lot of  pesticide research, such as the aforementioned Columbia and Berkeley studies. It also applies to many other EPA rules regulating clean air and addressing climate change. The role of science at EPA will be a topic of continuous discussion and will have broader implications on how we safeguard the environment and human health in this nation as well as across the globe.

Xindi (Cindy) Hu is a doctoral candidate at Harvard T. H. Chan School of Public Health in Environmental Health. The author acknowledges Paheliya Aixilafu, MyDzung Chu, Clifton Dassuncao, Hector Maldonado Perez, Jonathan Moch and Rebecca Stern for contributing to the material in this article.

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30 thoughts on “The Most Widely Used Pesticide, One Year Later

  1. Great overview. HAs anyone perhaps info on what are the 600 basic chemicals that are used for the agricultural toins?

  2. Wow, excellent deep dive into the weeds of the EPA’s approval of a clinically proven child neurologically damaging pesticide,
    and Chlorpyrifos is just one of many organophosates in US agriculture production. I would love to see Xindi (Cindy) Hu’s follow up story today, after it took a judicial ruling from the 9th Circuit Court to ban Chlorpyrifos. This report is only half the story. What about the health impact of neurologically/carcinogenic impairing pesticides on the most vulnerable populous prenatal/infant/children thru their consumption of unregulated tolerance pesticide exposure in their diets.
    US Dept. of Health and Human Services Center for Disease Control (CDC) statistics on Autism
    After accidents, cancer is the second leading cause of death in children ages 1 to 14. About 1,190 children under the age of 15 are expected to die from cancer in 2021.
    The World Health Organization and CDC ATSDR carcinogenic risk assessment of Glyphosate is not recognised by the EPA.
    Most of the EPA registered pesticides are without a 10 fold child tolerance level mandated under federal law FQPA.
    We are in a prenatal/infant/child health crisis from the agrochemical political control of the epidemiological review of the dietary residue health impact of the more than 34,000 pesticides that are derived from about 600 basic chemicals registered by the EPA for use in this country.

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