The pesticide known as chlorpyrifos is both clearly dangerous and in very wide use. It is known to pass easily from mother to fetus and has been linked to a wide range of serious medical problems, including impaired development, Parkinson’s disease and some forms of cancer. That’s not entirely surprising. The chemical was originally developed by Nazis during World War II for use as a nerve gas.

Here’s what is surprising: Tons of the pesticide are still being sprayed across millions of acres of United States farmland every year, nearly five years after the Environmental Protection Agency determined that it should be banned.

The E.P.A. proposed a full federal ban after its scientists concluded that there was substantial risk for children and developing fetuses. But federal officials backed off the proposal soon after President Trump took office, and then reversed it completely this past July. The agency now says the data on chlorpyrifos is insufficient and unreliable, but that argument cuts against well-established scientific opinion and years of careful study by E.P.A. experts.

With the federal government abdicating its responsibility, individual states have been left to fill the void. Hawaii and California have issued statewide bans, and several other states, including New Jersey, are taking steps to do the same. The New York attorney general is spearheading a lawsuit against the E.P.A., contending the agency broke the law when it reversed course on the federal ban of chlorpyrifos. And New York’s State Legislature passed, with broad bipartisan support, a bill to ban the chemical. Nearly 100 medical, consumer and environmental organizations, including the American Academy of Pediatrics, supported the measure.