That study was one of many. Decades of research into the effects of chlorpyrifos strongly suggests that exposure at even low levels may threaten children. A few years ago, scientists at the Environmental Protection Agency concluded that it should be banned altogether.
Yet chlorpyrifos is still widely used in agriculture and routinely sprayed on crops like apples, oranges, strawberries and broccoli. Whether it remains available may become an early test of the Trump administration’s determination to pare back environmental regulations frowned on by the industry and to retreat from food-safety laws, possibly provoking another clash with the courts.
In March, the new chief of the E.P.A., Scott Pruitt, denied a 10-year-old petition brought by environmental groups seeking a complete ban on chlorpyrifos. In a statement accompanying his decision, Mr. Pruitt said there “continue to be considerable areas of uncertainty” about the neurodevelopmental effects of early life exposure to the pesticide.
Even though a court last year denied the agency’s request for more time to review the scientific evidence, Mr. Pruitt said the agency would postpone a final determination on the pesticide until 2022. The agency was “returning to using sound science in decision-making — rather than predetermined results,” he added.
Agency officials have declined repeated requests for information detailing the scientific rationale for Mr. Pruitt’s decision.
Lawyers representing Dow and other pesticide manufacturers have also been pressing federal agencies to ignore E.P.A. studies that have found chlorpyrifos and other pesticides are harmful to endangered plants and animals.
A statement issued by Dow Chemical, which manufactures the pesticide, said: “No pest control product has been more thoroughly evaluated, with more than 4,000 studies and reports examining chlorpyrifos in terms of health, safety and environment.”
A Baffling Order