The move raises questions over whether the Trump administration will defend the rule, which was criticized by several industry and Republican officials as too stringent and many environmental and public health advocates as too lax.

Ground-level ozone, a byproduct of fossil-fuel burning, produces hazy skies and is linked to respiratory and heart ailments.

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The U.S. Court of Appeals for the District of Columbia Circuit was slated to hold oral argument on the case April 19.

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EPA Administrator Scott Pruitt joined with lawyers representing nine other states in challenging the ozone National Ambient Air Quality Standards (NAAQS) when he was Oklahoma’s attorney general, along with representatives from the coal, oil and cement industries, as well as larger trade associations.

How Trump is rolling back Obama’s legacy View Graphic How Trump is rolling back Obama’s legacy

“The Trump administration is taking the first step toward tearing down a crucial protection against dirty air,” said Earthjustice attorney Seth Johnson, whose group represents several of the environmental and public health plaintiffs in the case.

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“President Trump is aiding baseless litigation mounted by Scott Pruitt before he was put in charge of EPA over the consensus of doctors and scientists,” Natural Resources Defense Council clean air director John Walke, whose group is also a party to the case, said in a statement.

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In Friday’s court filing, Justice Department lawyers wrote, “At this time, EPA officials appointed by the new Administration are closely reviewing the 2015 Rule to determine whether the Agency should reconsider the rule or some part of it.”

On Saturday, EPA spokesman J.P. Freire said in an email that the agency would evaluate the ozone standard in the context of President Trump’s push for economic development.

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“Given the broad-reaching economic implications of the 2015 ozone standard, we are carefully reviewing the rule to determine whether it is in line with the pro-growth directives of this Administration,” Freire said. “We simply request the court grant us additional time to ensure we can continue this thorough and deliberative process.”

Ozone regulations have been a contentious issue for years. The Clean Air Act dictates the federal government review the standard every five years, if warranted, and groups from one side or the other regularly challenge the federal government’s conclusions in court.

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EPA set the standard of 75 parts per billion in 2008, which was challenged by a coalition of environmentalist and health activists. Barack Obama’s administration asked to hold the case in abeyance in 2009 while it reviewed the rule. But Obama declined to strengthen the standard in September 2011 after a coalition of industry groups and some of his own economic advisers said doing so would exact an economic cost without generating enough of a public health benefit.

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Advocates sued again, and in October 2015, the EPA’s then head, Gina McCarthy, set a more protective standard of 70 parts per billion, based on more recent scientific research linking ozone exposure to development of asthma, particularly in young children.

The agency’s group of independent science advisers, the Clean Air Science Advisory Committee, found that vulnerable groups including the elderly and children could still suffer adverse consequences at the level at 70 ppb, and that the EPA should consider setting it even lower.

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