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The Facts on Chlorpyrifos

Both the EPA and its critics say science is on their side in the debate over whether the agricultural insecticide should be banned.


When the Environmental Protection Agency decided to not ban chlorpyrifos, an insecticide widely used in agriculture, both the EPA and its critics claimed “sound” or “solid” science supported their positions. Research does suggest chlorpyrifos impacts human health, but that research has some limitations. 

The EPA, under the Obama administration, proposed to ban chlorpyrifos in November 2015. But EPA Administrator Scott Pruitt reversed that decision last month, arguing he was relying on “sound science.”

Pruitt, March 29: We need to provide regulatory certainty to the thousands of American farms that rely on chlorpyrifos, while still protecting human health and the environment. By reversing the previous Administration’s steps to ban one of the most widely used pesticides in the world, we are returning to using sound science in decision-making – rather than predetermined results.

Sheryl Kunickis, the director of the Office of Pest Management Policy at the U.S. Department of Agriculture, backed Pruitt’s decision, stating it was “grounded in evidence and science.”

But Jim Jones, assistant administrator at the EPA until January, told the New York Times that Pruitt’s decision ignored “science that is pretty solid.” The Times added that Jones — who held various positions at the agency between 1997 and 2017, including director of the EPA’s Office of Pesticides Programs — said that “he believed the ruling would put farm workers and exposed children at unnecessary risk.”

Some studies do suggest that chlorpyrifos exposure can lead to developmental issues in children, for example, but they’re correlational studies, meaning they don’t provide causal links. However, research in rodents has found causal links between chlorpyrifos and developmental issues.

Other studies in human populations have failed to find correlations between chlorpyrifos and development issues in children, but that research may be suffering from what scientists call “confounding variables,” or unrelated factors that may be affecting the study’s results.

In other words, some research does suggest chlorpyrifos poses a threat to human health, but the question is whether or not it’s enough to warrant a complete ban of the insecticide.

We take no position on the EPA’s decisions. But we can outline the strengths and limitations of the research that has been done on chlorpyrifos.


Some Background on Chlorpyrifos

Chlorpyrifos was first registered as an insecticide in 1965. According to the EPA, it’s the “most used conventional insecticide” in the U.S. with roughly 6 million pounds used on around 10 million acres between 2009 and 2013.

It’s used on nearly 50 different crops with soybeans, corn, alfalfa, oranges and almonds topping the list in terms of pounds of chlorpyrifos applied. Farmers also applied chlorpyrifos to over 30 percent of their apple, asparagus, walnut, onion, grape, broccoli, cherry and cauliflower crops.

People can be exposed to chlorpyrifos by ingesting food containing the insecticide, according to the National Institutes of Health. But chlorpyrifos does degrade over time into a substance that doesn’t pose health risks — about 15 days when exposed to sunlight and “much slower” without light, says the NIH.

People can also be exposed to the insecticide by inhaling it, especially from indoor air, and through the skin. While chlorpyrifos “has rarely been found in drinking water and outdoor air,” it “can be carried long distances” in the air, adds the NIH. The EPA also notes the possibility of exposure from treated golf courses.

Chlorpyrifos is called a “broad-spectrum” insecticide because it can kill a wide variety of insects. But its potential impact goes beyond insects.

The insecticide is “toxic” to birds and “extremely toxic” to fish, according to the National Institutes of Health. It’s also “extremely toxic” to non-target insects such as bees.

Chlorpyrifos affects all of the above animals, along with humans and other mammals, in a similar way — by leading to the overstimulation of the nervous system.

In fact, chlorpyrifos targets the same chemical pathway in the body as nerve agents such as sarin gas. Both chlorpyrifos and sarin belong to a class of chemicals called organophosphates, several of which have already been banned by the EPA.

At high doses, nerve agents, chlorpyrifos and other organophosphates impact humans similarly, namely by causing a whole host of symptoms, including headache, nausea, dizziness and confusion. At very high doses, organophosphates can cause vomiting, abdominal pain and diarrhea. And at ever higher doses, such as from spills or accidents, they can lead to death.

To be clear, even though chlorpyrifos and sarin affect the same chemical pathways in the body, chlorpyrifos is way less toxic than sarin.

Scientists quantify a chemical’s toxicity using a metric called the median lethal dose, or the LD50. This is the amount of the chemical needed to kill 50 percent of a study population.

The LD50 of chlorpyrifos via oral exposure is estimated to be 92 to 276 milligrams per kilogram of body weight, while the LD50 for sarin is 0.071 to 0.285 mg per kg.

More generally, there is a “wide range of toxicity” among organophosphates, explains the EPA. However, since all organophosphates target the same chemical pathway, exposure to multiple types of the chemical “may lead to serious additive toxicity,” the agency adds.

Farm workers and other workers who handle the chlorpyrifos and other organophosphates on a day-to-day basis are at the “highest risk of exposure,” says the NIH. When the insecticide is sprayed in homes and gardens for pest control, residents are also at a “higher risk of exposure,” the NIH adds.

For this reason and others, the EPA banned chlorpyrifos’ use for residential purposes in 2000, except in select cases, including when contained in ant and roach bait products.

The EPA continued to restrict the use of chlorpyrifos between 2000 and 2012.

For example, the agency banned its use on some crops, such as tomatoes, and limited its use on other crops, including apples, grapes and citrus. The EPA also curbed the insecticide’s use by reducing the rate at which it can be applied and banned its use in certain areas near residential and public spaces.

In 2016, the U.K. decided to ban all uses of chlorpyrifos, except for on brassica seedlings using a specific method. Brassica is a group of crops, including broccoli, cabbage and rape, the last of which is often used to make oil.

Enough Evidence for a Total Ban?

The debate over whether or not to outright ban chlorpyrifos primarily hinged on the effect that exposure has on developing children rather than farm workers and other adults. Like adults, children can be exposed to chlorpyrifos from food, treated golf courses and other sources. 

In September 2007, the Pesticide Action Network North America and the Natural Resources Defense Council petitioned the EPA to outright ban chlorpyrifos for multiple reasons, a number of which pertained to the impact the insecticide has on the development of children.

For example, the petitioners argued the EPA “failed” to take into consideration data that demonstrates “long-lasting effects” on children “from early life exposure” and “disregarded data” that showed there is “no evidence of a safe level of exposure” for fetuses and children.

In November 2015, the EPA proposed to completely ban chlorpyrifos use because it was “unable to conclude that the risk from aggregate exposure from the use of chlorpyrifos meets the safety standard.”

In other words, in its proposal to ban the insecticide, the EPA under the Obama administration followed the precautionary principle, which can be roughly defined as erring on the side of caution in the face of uncertainty.

In November 2016, the EPA reassessed its proposal to ban the insecticide after taking into consideration recommendations made by the agency’s Science Advisory Panel. The panel rejected the EPA’s methodology in quantifying the risk posed by chlorpyrifos.

However, using a different methodology suggested by the panel, the EPA still concluded that, while “uncertainties” remain, a number of studies provide “sufficient evidence” that children experience neurodevelopment effects even at low levels of chlorpyrifos exposure. For this reason, the EPA retained its decision to completely ban chlorpyrifos.

In its denial of the environmental groups’ petition, the EPA under Pruitt’s direction claimed that the impact of low levels of chlorpyrifos exposure on the neurodevelopment of children was a “novel, highly complex and unresolved scientific issue.” But unlike the EPA under Obama, the Pruitt’s EPA decided that it wouldn’t outright ban chlorpyrifos “without first attempting to come to a clearer scientific resolution” on the matter, a task it’s set to complete by 2022.

So what do studies say about chlorpyrifos and its effect on children and fetuses?

Some of the strongest research suggesting that chlorpyrifos does adversely affect children and fetuses comes from the Columbia Center for Children’s Environmental Health at Columbia University. In its November 2016 report on the insecticide, the EPA paid particular attention to studies headed by Virginia Rauh, the deputy director of the Columbia Center.

All of Rauh and her colleagues’ studies relied on blood samples collected from the umbilical cords of mothers right after giving birth that measured the levels of chlorpyrifos directly.

Rauh and her colleagues’ studies were somewhat unique in this way, as other epidemiological studies, which the EPA also used in its 2016 analysis, measured chlorpyrifos metabolites in the urine of expecting mothers, rather than the insecticide directly.

Why is an indirect measure of chlorpyrifos potentially problematic?

It’s possible for individuals to have high levels of chlorpyrifos metabolites in their urine, but have low or no exposure to chlorpyrifos itself. This is because chlorpyrifos degrades in the environment over time, so individuals might be ingesting the degraded substances — the metabolites — directly, say as residue on fruits. And the metabolites themselves don’t pose health risks.

In one study, published in the journal Pediatrics in December 2006, Rauh’s group looked at what effect prenatal exposure to chlorpyrifos has on the cognitive and motor development of children.

The researchers found that 3-year-olds “highly exposed” to chlorpyrifos prenatally scored, on average, 6.5 points lower on a motor development test compared with children who had “low exposure” to the insecticide. They also found that the highly exposed children scored 3.3 points lower on a cognitive development test, on average, though they had less confidence in this second finding. These tests have “moderate predictive power for subsequent intelligence and school performance,” the researchers write.

Also, 3-year-olds exposed to high levels of chlorpyrifos prenatally were “significantly more likely to score in the clinical range” for attention deficit hyperactivity disorder problems and pervasive developmental disorder problems, a category of disorders under which autism falls, the researchers found.

The group does point out, however, that its study cannot account for chlorpyrifos exposure that children may have encountered after birth, since they only sampled umbilical cord blood at delivery.

In an April 2011 study published in the journal Environmental Health Perspectives, Rauh and others also looked at the effect prenatal chlorpyrifos exposure may have had on the intelligence and memory of 7-year-olds.

They found that the more exposure to the insecticide the umbilical cord blood showed at delivery, the lower the child’s IQ and working memory. Working memory “assesses children’s ability to memorize new information, hold it in short-term memory, concentrate, and manipulate information,” the researchers explain.

In another study, Rauh and colleagues evaluated whether or not children with high exposure to chlorpyrifos experienced tremor in their arms, which is a neurological issue.

The researchers first asked children age 9 to 13 to draw a spiral on a piece of paper. A neurologist specializing in movement disorders, who was blind to the children’s prenatal chlorpyrifos levels, then evaluated these drawings for signs of tremor.

Published in NeuroToxicology in December 2015, the study found that up to 39.5 percent of the high exposure group showed signs of mild to moderate tremor, compared with up to 22.8 percent in the low exposure group.

In one more study, Rauh and others looked at what effect prenatal exposure to chlorpyrifos might have on brain structure.

Published in Proceedings of the National Academies of Sciences in May 2012, the study found “significant abnormalities” in the morphology of brains of children who were exposed to higher levels of chlorpyrifos prenatally. The researchers add that their “findings are consistent with the effects of early developmental exposure to [chlorpyrifos] in animal models.”

In fact, the “effects of low-level organophosphate exposures on brain development in animal models” is what “triggered” the study of these chemicals’ effects on children in the first place, they write.

While suggestive, Rauh and her groups’ studies all had sample sizes below 300 children, which means that the studies may not apply to the broader public. The larger the sample size of a study, the more confident scientists are in concluding that their results correspond to the population as a whole.

However, an additional study, which the EPA also took into account in its 2016 review, included 970 participants and looked at children’s prenatal pesticide exposure more generally.

Conducted by Irva Hertz-Picciotto, an environmental epidemiologist at the University of California, Davis, and others, the study found that children age 2 to 5 were 60 percent more likely to have autism spectrum disorder, also a developmental disorder, compared with normally developing children, when organophosphates were applied within 1.25 kilometers of their homes while their mothers were pregnant.

For chlorpyrifos in particular, the researchers found that every 100 pounds of the insecticide applied within 1.5 kilometers of pregnant mothers’ homes increased the chance of their child developing autism spectrum disorder by 14 percent.

In its 2016 review, the EPA did note that some studies didn’t find an association between organophosphates and developmental issues in children, specifically one that took place in China and another that took place in Canada. Both of these studies measured children’s chlorpyrifos metabolites (that is, not chlorpyrifos levels directly) at one time only, which both sets of authors admit limits their findings.

In fact, the Chinese study, conducted by Shen Xiaoming, a doctor at Shanghai Jiao Tong University, and others, concludes that their “results should be interpreted with caution, and more studies of children living in China are warranted.”

There are still other studies that have looked at the relationship between chlorpyrifos, and organophosphates generally, and developmental issues in children, but we chose to concentrate on the research that the EPA paid particular attention to in proposing to ban the insecticide.

Based on the available research, there is evidence to suggest that chlorpyrifos negatively impacts the development of children. But that research does have some limitations, and whether it is sufficient evidence is debatable.

Editor’s Note: SciCheck is made possible by a grant from the Stanton Foundation.


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