But some scientists and the US Department of Agriculture argue that chlorpyrifos is not harmful at exposure levels upheld by the EPA.
"Dow AgroSciences remains confident that authorized uses of chlorpyrifos products offer wide margins of protection for human health and safety," spokesman David Sousa said. "This is the right decision for farmers who, in about 100 countries, rely on the effectiveness of chlorpyrifos to protect more than 50 crops."
"We've banned pesticides before, and farmers have turned to safer alternatives. The notion that we should continue to use a pesticide linked to autism because it's needed to feed the world is an outrageous, ridiculous statement," said Scott Faber, senior vice president of government affairs at the Environmental Working Group, a nonprofit environmental research organization.
Faber said Dow Agrosciences itself makes safer alternatives, which Dow confirmed.
After his review of past EPA decisions concerning the pesticide, Pruitt concluded that the science remains unresolved and that further evaluation is warranted to achieve greater certainty about possible neurodevelopmental effects of exposure.
The EPA has also expressed concerns about the research.
What does the science say?
Going by pounds of active ingredient, chlorpyrifos is the most widely used conventional insecticide in the country, according to the EPA. Registered uses include a large variety of food crops, including apples, lettuce, peaches and potatoes, as well as non-food use, such as non-residential turf.
In 2000, after increasing scientific evidence that chlorpyrifos might be harmful to human health, EPA reached an agreement with manufacturers to voluntarily phase out all residential uses beginning in 2001.
Though it's no longer used in homes, the chemical has been used in agriculture as well as in public places, protecting ornamental plants and turf. In 2006, the EPA completed its legally required reassessment for chlorpyrifos; it is required to complete the next re-evaluation of chlorpyrifos by October 1, 2022.
The Federal Insecticide, Fungicide, and Rodenticide Act requires all pesticides be registered and then continuously reviewed to determine appropriate use and possible hazards.
Scientific developments led the agency in 2009 to prioritize its review of chlorpyrifos.
In October 2015, the EPA responded to the petition from the Natural Resources Defense Council and the Pesticide Action Network North America by proposing to revoke all food uses for chlorpyrifos.
This proposal relied primarily on four studies from Columbia Center for Children's Environmental Health
that examined associations with chlorpyrifos and neurodevelopment. The Columbia researchers looked at chlorpyrifos concentrations in umbilical cord blood of babies over a roughly 10-year period -- five years before the residential ban and five years after.
In the first half of the study, researchers could measure concentrations in picograms in some children, though not all. (One picogram equals one trillionth of a gram.) After the residential ban took effect, no measure amount was found in any of the children.
The researchers also compared the children based on level of exposure to the chemical. At age 3 they found increased odds of mental delay, psychomotor delay, attention disorders and pervasive developmental disorders among the high-exposure children. In a follow-up study at age 11, the researchers observed increased odds of mild to moderate tremor.
In April 2016, a scientific advisory panel of the Federal Insecticide, Fungicide, and Rodenticide Act, which is administered by the EPA, expressed concerns regarding data to support this proposal to ban the pesticide.
"I agree very much with the scientific advisory panel," said Dr. William Banner, president of the American Association of Poison Control Centers. He added that what the studies have in common is that "they did not show any causal link. What they showed was an association."
Banner said the study was not conducted in a way to rule out other factors that might have influenced the participants' development, including their gestational age at birth and their exposure to environmental stress.
Banner, who reviewed the studies and presented his findings at the panel hearing on behalf of Dow Agrosciences, did not speak on behalf of the poison control center.
Despite claims of study flaws, the EPA said there was sufficient evidence of neurodevelopmental effects and, in November 2016
, revised its assessment for chlorpyrifos. It said expected residues of chlorpyrifos on food crops exceed the safety standard under the Federal Food, Drug, and Cosmetic Act.
Pushback from both sides
In a public document
issued January 17, Sheryl H. Kunickis, director of the Office of Pest Management Policy at the USDA, expressed "grave concerns about the EPA process that has led to the Agency publishing three wildly different human health risk assessments for chlorpyrifos within two years."
Kunickis also said she had "severe doubts about the validity of the scientific conclusions underpinning EPA's latest chlorpyrifos risk assessment."
Michael Dourson, a professor at the University of Cincinnati, said the scientific advisory panel for the EPA had a difficult task in assessing chlorpyrifos because there were "legions of papers" that needed to be reviewed.
Dourson spoke during the April 2016 scientific panel hearing that preceded the EPA revised assessment on behalf of CropLife America, a national trade association representing the manufacturers of pesticides. He argued that numerous health organizations, including the World Health Organization and HealthCanada, have determined a safe dose of chlorpyrifos.
"It really is a matter of how much," said Dourson, who worked for the EPA for 15 years on assessments of chlorpyrifos and for 21 years at a nonprofit.
What scientists worry about when it comes to this chemical is the fact that it causes a reaction to an enzyme, cholinesterase, that helps your nervous system work well: It essentially protects your nerves.
"Chlorypyrifos disrupts this enzyme in insects and in animals and in people," Dourson said, but not at the levels allowed.
Laura Anderko, a professor at Georgetown University and director of the Mid-Atlantic Center for Children's Health and the Environment, says the science is consistent.
"Three big studies in 2011, and they all came up with the same findings that kids and moms who consume this pesticide are more at risk of giving birth to kids with ADHD and other neurological problems," she said.
"So there's a push and pull, as you know, between cheap alternatives for industry and public health concerns," she said, noting that often in hearings, there's an element of doubt: If we get rid of this, what are the economic implications?
"We're always looking at retrospectively what the data shows," she said. "Unfortunately, for example, take tobacco: It took 50, 60 years of research before policy catches up with what the science is showing," Anderko said.
Faber explained that the EPA needs to show that there's a reasonable certainty of no harm under certain conditions of use, such as the equipment worn by agricultural workers and the amount applied to crops.
"Under the law, if there isn't sufficient science to demonstrate safety, then EPA is obligated to ban the pesticide," Faber said. "That's why a number of similar pesticides, what are called organophosphate pesticides, are no longer in use in the US."
The EPA concluded that chlorpyrifos was not safe for use in residential settings, and the science has only gotten stronger, he said. Ultimately, the agency's risk assessment indicated that "chlorpyrifos posed significant risks to children, even at very low exposures," he added.
"There's simply no way EPA could reach any conclusion other than it should no longer be used on food crops," Faber said. "What's outrageous about Scott Pruitt's decision is that the science is so strong, so overwhelming, that chlorpyrifos causes neurological problems.
"In the short run, there are a list of fruits and vegetables that consumers should avoid
or when possible choose organic again because even low levels of exposure have been linked to neurological problems in children," said Faber.
"If you can buy organic, that's great, but a lot of people can't, and we encourage parents to feed their kids fruits and vegetables, because it's healthy and reduces obesity, all that. But in the end, they're providing toxins to their kids that could hurt them," Anderko said. "And parents shouldn't have to worry about that."
The EPA did not respond to requests for comment.