Two California-based environmental groups have sued the EPA for failing to make a final determination on whether Philadelphia meets acceptable levels of ozone, a pollutant responsible for smog.

Smog is caused by fossil-fuel combustion, such as car exhaust, mixed with heat and sunlight. It is associated with health problems, such as respiratory irritation and asthma.

The Center for Biological Diversity and Center for Environmental Health said in a federal suit filed this week that the Philadelphia metro area previously failed to attain ozone levels set in 2008 under the Clean Air Act.

According to the suit, the EPA had until Jan. 20 to make a determination whether the area now meets acceptable levels of ozone in the air. The suit also made a similar claim for the Washington region.

The EPA made a preliminary determination on April 18 that Philadelphia had met the standards. But Robert Ukeiley, attorney for the environmental groups, said the EPA used incomplete data.

The environmental groups fear the agency will stall on making a final determination, possibly indefinitely. So the goal of the suit, Ukeiley said, is to have a judge impose a deadline.

An EPA spokeswoman said the agency had no immediate comment.

The groups are suing on behalf of their members, some of whom live in the Philadelphia area.

"Plaintiffs' members are compelled to breathe air less pure than that mandated by the Clean Air Act because of EPA's violation of its mandatory duties," states the lawsuit, which names Scott Pruitt, EPA administrator, as the defendant.

The Philadelphia region has experienced the second-highest number of smoggy days in the Northeastern United States, according to a recent report commissioned by the nonprofit advocacy group PennEnvironment Research and Policy Center.