Workers pick fruit in an orchard in Arvin, Calif., in 2004. On Friday, California regulators said they're tightening the rules on the pesticide chlorpyrifos over new health concerns.

SAN FRANCISCO — California is tightening the strictest rules in the nation on a pesticide that is popular with farmers over new health concerns, officials said Friday.

Farmers use chlorpyrifos to kill pests that attack a wide variety of crops like grapes, almonds and cotton grown in California, the nation’s agricultural leader, as well as across the country.

State officials are taking steps to put it on a list of chemicals known to be harmful to humans and to also increase the distance from schools and homes in which farmers can apply it.

The moves run contrary to a decision by Scott Pruitt, the administrator of the U.S. Environmental Protection Agency, to end his agency’s effort to ban the pesticide sold by Dow Chemical after federal scientists concluded it can interfere with the brain development of fetuses and infants.

Pruitt told Congress in June his decision was based on “meaningful data and meaningful science.” Pruitt’s staff has thus far declined to provide details of what information Pruitt reviewed before making his decision.

California officials say that researchers are learning more about how the pesticide harms the developing brains of unborn babies and young children. Farmers apply it to 60 different crops, and it is most heavily used in San Joaquin Valley farming communities.

“New information in the scientific community leads us to believe the level of risk it poses is greater than previously known,” California EPA Secretary Matthew Rodriquez said in a statement. “The actions we are taking today reflect our commitment to the health and safety of all Californians, and the environment.”

California already prohibits farmers from applying the pesticide within 150 feet of a school or home, but officials say that could be increased to 450 feet.

A draft of the new rules was released Friday, and after a public comment period, officials could begin to enforce them in September — with more regulations expected in late 2018. Methods of applying the pesticide from helicopters and airplanes may also be restricted.

David Sousa, a spokesman for pesticide manufacturer Dow AgroSciences, called California regulators’ new rules “overly conservative.”

Environmental and farmworker advocates say the rules don’t go far enough. They called for an outright ban.

Paul Towers of the advocacy group Pesticide Action Network said that state officials need to more aggressively protect poor Latino farming communities.