CHEYENNE, Wyo. (AP) — The U.S. Environmental Protection Agency is moving ahead with a proposed regulatory designation that could increase pressure on the gas industry to limit emissions that have led to severe wintertime ozone pollution in western Wyoming.

The EPA is doing so while it settles a nationwide lawsuit over ozone filed by an environmental group, WildEarth Guardians.

Wyoming Gov. Dave Freudenthal in 2009 recommended that the EPA enact what's known as "nonattainment status" for ozone in the Upper Green River Basin. In a letter Dec. 8, the EPA told Gov. Matt Mead it is now prepared to support that recommendation.

"The EPA will continue to work with state officials regarding the appropriate boundary for the area in Wyoming," EPA Region 8 Administrator James Martin wrote.

Mead is pleased the EPA has agreed with the state's recommendations for ozone nonattainment, said his spokesman, Renny MacKay.

"This effort started in earnest in 2008 and there has been a reduction in emissions, but we all recognize more needs to be done," MacKay said by email Wednesday.

Final designations on the nonattainment designations are due next spring, after which states will need to develop plans to meet the federal ozone standard, EPA spokesman Rich Mylott said.

Ground-level ozone can lead to smog and is a respiratory irritant in sufficient concentrations.

Wyoming's ozone problem is fairly unique. While air emissions can cause ground-level ozone to soar in urban areas during the summer, ozone in the Upper Green River Basin peaks each year between January and March.

A combination of bright sunshine, snow on the ground and temperature inversions creates ideal conditions for atmospheric chemical reactions that create ozone. Certain air emissions from the Pinedale Anticline and Jonah gas fields fuel those reactions.

Last winter was especially bad for ozone, with levels near Pinedale exceeding the worst days in Los Angeles in 2010. State regulators urged children, the elderly and people with respiratory conditions to stay indoors at times of peak ozone.

Wyoming regulators have focused on getting gas operators in the Pinedale area to take steps to voluntarily reduce emissions.

"They clearly recognize there's a problem and I will give them some credit for that. But rising to the challenge of this problem is going to require more," said Jeremy Nichols with WildEarth Guardians. "I hope there is a lesson to be learned there and I hope they strive to clean the area up as much as possible and get it off that list."

The EPA's slow response on state recommendations for ozone nonattainment in mainly urban areas prompted WildEarth Guardians to sue in August. A notice of a proposed consent decree resulting from the lawsuit was published Wednesday in the Federal Register.

The proposed consent decree would give EPA until May 31, 2012, to take action on ozone designations nationwide.