The U.S. Environmental Protection Agency has reversed course on delaying an ozone rule, a day after being challenged in federal court by Pennsylvania and 14 other states.

The EPA, under Administrator Scott Pruitt, notified states in June that it was delaying for a year new ozone pollution rules.

Gov. Wolf and state Attorney General Josh Shapiro announced Tuesday that they were joining the suit challenging the delay. The suit was filed in federal appeals court in Washington.

On Wednesday, Pruitt changed his mind and said the EPA was withdrawing action to delay implementation of Obama-era rules that required reduction in ground-level ozone, also known as smog.

The EPA's notice of its reversal came Wednesday evening.

"While we welcome this corrective action, we are deeply concerned about the threat that Administrator Pruitt's actions present to the fundamental right to clean, healthy air guaranteed by our nation's bipartisan and time-tested clean air laws," said Peter Zalzal, an attorney for the Environmental Defense Fund.

At issue was an Oct. 1 deadline for states to begin meeting 2015 standards for ground-level ozone. Pruitt announced in June he would delay compliance by one year to give his agency more time to study the plan and avoid "interfering with local decisions or impeding economic growth."

The Obama-era rule lowered allowable ground-level ozone levels from 75 parts per billion to 70 parts per billion under National Ambient Air Quality Standards.

Governors had to submit a list of areas that were not in compliance, and then the EPA had up to two years to declare which areas did not meet standards, and could seek an extension if it needed more information.

New York first filed suit challenging Pruitt's seeking to delay the lower ozone levels.  Wolf said Pennsylvania was joining, in part, because he believed lower ozone levels will reduce health-care costs.  He said past EPA findings showed 230,000 asthma attacks a year could be prevented in American children if pollution were reduced.

Pruitt, the EPA administrator, invoked that extension in June, saying the EPA needed "time to better understand some lingering, complicated issues so that air attainment decisions can be based on the latest and greatest information."  He also established an Ozone Cooperative Compliance Task Force "to develop additional flexibilities for states to comply with the ozone standard."

Pruitt presented the change of heart Wednesday as his agency being more responsive than past administrations to the needs of state environmental regulators. He made no mention of the legal challenge.

Pruitt, who was Oklahoma's state attorney general prior to his appointment by President Trump, has long been a reliable opponent of stricter environmental regulations.

The sudden reversal is the latest legal setback for Pruitt's regulatory rollback agenda. Last month, a federal appeals court in Washington ruled the EPA administrator overstepped his authority in trying to delay implementation of an Obama administration rule requiring oil and gas companies to monitor and reduce methane leaks.

But Republicans in Congress are pushing for a broader rewrite of the ozone rules. A House bill approved last month seeks to delay implementation of the 2015 rules at least eight years. The measure has not yet been brought to a vote in the Senate.

Ground-level ozone, or smog, is a result of pollution.  Nitrogen oxides and volatile organic compounds combine under sunlight and heat to help produce ozone.

Philadelphia has long had difficulty with smog because it is surrounded by emissions given off by motor vehicles, industry, power plants, and refineries.

Pennsylvania named the Philadelphia and Pittsburgh metropolitan areas in its report to the EPA in 2016 as exceeding ozone limits.

In May, environmental groups sued the EPA over failing to make a final determination on whether Philadelphia meets acceptable levels of ozone.  And, in April, an advocacy group, using EPA data, issued a study saying Philadelphia was among the smoggiest Northeastern cities.

 The Associated Press contributed to this article.