WASHINGTON — The Trump administration took a major step to weaken the regulation of toxic chemicals on Thursday when the Environmental Protection Agency announced that it would not ban a widely used pesticide that its own experts have linked to serious health problems in children.
The decision by Andrew R. Wheeler, the E.P.A. administrator, represents a victory for the chemical industry and for farmers who have lobbied to continue using the substance, chlorpyrifos, arguing it is necessary to protect crops.
It was the administration’s second major move this year to roll back or eliminate chemical safety rules. In April, the agency disregarded the advice of its own experts when officials issued a rule that restricted but did not ban asbestos, a known carcinogen. Agency scientists and lawyers had urged the E.P.A. to ban asbestos outright, as do most other industrialized nations.
In making the chlorpyrifos ruling, the E.P.A. said in a statement that the data supporting objections to the use of the pesticide was “not sufficiently valid, complete or reliable.” The agency added that it would continue to monitor the safety of chlorpyrifos through 2022.
The substance, sold under the commercial name Lorsban, has already been banned for household use but remains in widespread use by farmers for more than 50 fruit, nut, cereal and vegetable crops. In 2016, more than 640,000 acres were treated with chlorpyrifos in California alone.
Representatives of Corteva Agriscience, the maker of chlorpyrifos, did not immediately respond to a request for comment on the decision.
The Obama administration announced in 2015 that it would ban chlorpyrifos after scientific studies produced by the E.P.A. showed the pesticide had the potential to damage brain development in children. That ban had not yet come into force when, in 2017, Scott Pruitt, then the administrator of the E.P.A., reversed that decision, setting off a wave of legal challenges.
Those lawsuits culminated in April when a federal appeals court ordered the E.P.A. to issue a final ruling on whether to ban chlorpyrifos by this month.
Patti Goldman, a lawyer for Earthjustice, an environmental group that brought a legal challenge against the E.P.A.’s 2017 decision on behalf of farmworker organizations and others, criticized the decision. She said groups would sue again and ask the United States Court of Appeals for the Ninth Circuit to expedite the case.
“By allowing chlorpyrifos to stay in our fruits and vegetables, Trump’s E.P.A. is breaking the law and neglecting the overwhelming scientific evidence that this pesticide harms children’s brains,” Ms. Goldman said in a statement.
Representatives of the chemical industry expressed satisfaction with the decision. “The availability of pesticides, like chlorpyrifos, is relied upon by farmers to control a variety of insect pests and by public health officials who work to control deadly and debilitating pests like mosquitoes,” Chris Novak, chief executive of CropLife America, said in a statement.
Hawaii banned chlorpyrifos in 2018. California and New York are considering similar actions. The European Commission is under pressure from consumers and environmental groups to ban the pesticide.
The Trump administration has issued several other decisions in recent months relaxing environmental regulations. This week, the E.P.A. acknowledged a new policy doing away with surprise inspections of chemical and power plants. The “no surprises” policy is aimed at fostering better working relationship between the agency and states, E.P.A. officials wrote.
Last week, the E.P.A. approved broad use of the pesticide sulfoxaflor, which is known to harm bees. And this year the agency announced curbs on a lethal chemical found in paint-stripping products that represented a weakening of a ban that the Obama administration proposed.
Senator Tom Udall, Democrat of New Mexico, who introduced legislation to ban chlorpyrifos nationwide, said there was “no excuse” for keeping chlorpyrifos in use.
“The science on chlorpyrifos is clear and unambiguous,” he said. “It damages the developing brains of children and causes serious health problems in those who have been exposed to it.”
The E.P.A. decision is also one of the first concrete results of a separate Trump administration effort to restrict the use of scientific studies involving human subjects.
Under Mr. Pruitt, the agency proposed a rule saying it could not consider scientific research unless the raw data behind it was made public, saying the issue was a matter of transparency. Scientists argued that studies measuring human exposure to pesticides and other chemicals often rely on confidential health information and argued the E.P.A.’s real motivation was to restrict the ability to develop regulations.
In opting not to ban chlorpyrifos, the E.P.A. rejected a major study conducted by Columbia University on its effects on children in New York City. The E.P.A. said because it was unable to obtain the raw data and replicate that study, which linked the insecticide to developmental delays, it could not independently verify the conclusions.
Angela Logomasini, a senior fellow at the Competitive Enterprise Institute, a free-market think tank that rejects the established science of climate change, expressed strong support for the decision. She called the E.P.A.’s Science Advisory Panel decision on which the original ban was based “junk science.”
Kevin Minoli, a former senior E.P.A. lawyer who is now a partner at the Washington law firm Alston & Bird, predicted the courts would ultimately ban chlorpyrifos. He called Thursday “the beginning of the end” for the chemical.
The E.P.A. may argue that the science showing chlorpyrifos is unsafe is unclear. But Mr. Minoli said under the Food Quality Protection Act, the E.P.A. must prove that there was a reasonable certainty that the pesticide would not cause harm.
“Whatever you think about the science, there’s at least a question about that,” Mr. Minoli said.
For more news on climate and the environment, follow @NYTClimate on Twitter.