“Indeed E.P.A. itself has acknowledged that ‘lead poisoning is the number one environmental health threat in the U.S. for children ages 6 and younger,’ and that the current standards are insufficient,” the ruling said, adding, “The children exposed to lead poisoning due to the failure of E.P.A. to act are severely prejudiced by E.P.A.’s delay.”
A spokesman for the E.P.A. said the agency was reviewing the court’s decision, and declined to say if the agency planned to appeal or seek review in the Supreme Court.
The ruling is the latest legal setback to efforts by the Trump administration to delay or roll back Obama-era regulations it maintains are overly burdensome. In July, a federal appeals court ruled that the E.P.A. must enforce the implementation of methane emissions rules that the agency had sought to delay. A month later the agency reversed a decision to delay putting into effect a rule requiring more stringent air quality standards.
Activists hailed Wednesday’s court ruling and called it long overdue.
Zakia Rafiqa Shabazz of Richmond, Va., who founded United Parents Against Lead, an advocacy group, called the ruling “a win for children.” But Ms. Shabazz said her 23-year-old son still suffered from the effects of elevated lead levels found in his blood as a young child in 1996, and said the lack of action through two administrations to strengthen lead standards had been frustrating.
“That’s the hurtful thing, how many children could have been prevented from suffering the pains of lead poisoning,” she said.
The E.P.A. set standards in 2001 for lead contamination levels in dust and soil in homes. Environmental and health groups including United Parents Against Lead and Earthjustice petitioned the agency in 2009 to tighten standards on lead in dust and soil as well as paint to “more adequately protect” children.
The E.P.A., then under Mr. Obama, acknowledged the need for stricter rules in 2011 and agreed to take action, but never did so and set no timelines for developing a new rule.
Ms. Gartner said the Obama administration never gave a good reason for its delays.
“They never contested that the standard needed to be updated,” she said. “They just didn’t prioritize protecting kids from lead.”
Petitioners in August 2016 asked the appeals court to find that the E.P.A. had unreasonably delayed a new rule. Then, the Trump administration told the court it expected to take another six years to issue a new regulation.
“They already had six years,” Ms. Gartner said. “It’s taken more than a decade to update the standard for how much lead can be in dust before it’s considered a hazard and needs to be cleaned up.”
The judges on Wednesday issued a writ of mandamus, an unusual court order that requires an official or agency to perform a certain duty, in this case for the E.P.A. to issue a proposed rule within 90 days and finalize it within a year after that. The judges said in doing so they were mindful of the agency’s arguments that officials needed more time to deliberate a complex new standard.
“We must observe, however, that E.P.A. has already taken eight years, wants to delay at least six more, and has disavowed any interest in working with petitioners to develop an appropriate timeline through mediation,” the ruling said.
Meanwhile, the court said, the risks to children from lead poisoning under standards the E.P.A. has already called insufficient are “severe.”
Judge Mary M. Schroeder, appointed by President Jimmy Carter, wrote the opinion for the three-judge panel, and was joined by Judge Lawrence L. Piersol, a Bill Clinton appointee who was filling in on the court.
Judge N. Randy Smith, appointed by George W. Bush, dissented. “I do not understand why the E.P.A. has not acted,” he wrote, but he argued that only Congress, not the courts, could mandate that the agency do so.
The judges noted that the court had issued a similar order in 2015 to force the E.P.A. to take action one way or another on a pesticide, chlorpyrifos, after the agency had taken eight years to consider a petition from environmental groups to ban the substance. Mr. Pruitt wound up denying that petition in March, an action the court said complied with its order.
Something similar could happen in the lead case, environmental activists acknowledged. But they said they hoped the E.P.A. would finally toughen the standards.
“It is time that the E.P.A. stop stalling and taking prolonged amounts of time to do the right thing,” said Ms. Shabazz.
A study published in the Journal of Pediatrics in 2016 found that — despite decades of work to reduce lead in paint, dust and water — about 3 percent of children around the country exhibit high levels of the metal in their blood. The problem is particularly acute in parts of the Northeast: The regions with the largest proportions of blood specimens with the highest lead levels were in New York, Pennsylvania and Ohio, according to the study.
The E.P.A. said in a statement that lead exposure remained a significant health threat to children. “EPA will continue to work diligently on a number of fronts to address issues surrounding childhood lead exposure from multiple sources,” the agency said.