Mead Gruver, Associated Press
In this Jan. 22, 2010 photo, antelope graze not far from gas drilling rigs in western Wyoming's Upper Green River Basin.

CHEYENNE, Wyo. — Environmentalists said they served formal notice Thursday to the U.S. Environmental Protection Agency in preparation for a lawsuit over severe wintertime ozone pollution linked to gas drilling in western Wyoming.

Several groups said they sent a letter to EPA Administrator Lisa Jackson that meets a requirement to provide at least 60 days' notice before a lawsuit can be filed.

The Upper Green River Basin is home to the Pinedale Anticline and Jonah gas fields, which are among the largest U.S. gas fields. Last winter, ozone in the basin exceeded the worst days in any major U.S. city all last year, according to EPA data.

Ozone pollution can cause respiratory irritation and burning eyes and authorities urged the elderly, children and people with respiratory problems to stay inside.

The Wyoming group Citizens United for Responsible Energy Development, represented by the environmental legal firm EarthJustice, said the EPA should have declared long ago that air in the Pinedale area exceeded the federal limit for ozone pollution. Doing so would put more pressure on the Wyoming Department of Environmental Quality and gas industry to reduce the type of emissions that lead to ozone under certain conditions including bright sunshine, temperature inversions and snow on the ground.

"What they're doing is not working, obviously," said Elaine Crumpley of CURED. "I don't think we have to sell out our health for economic survival here. One shouldn't be a tradeoff for the other."

A message left for an EPA spokesman wasn't immediately returned.

President Barack Obama earlier this month backed off on plans to lower the nationwide EPA limit for ozone pollution, keeping the limit at 75 parts per billion. Average ozone in the basin exceeded that level on 13 days in February and March.

Eight-hour average ozone levels at one monitoring station reached 124 parts per billion, compared to the peak of 114 parts per billion in Los Angeles all last year.

Gas industry officials say the pollution occurred even though they have reduced truck traffic and taken other steps to curtail emissions that cause ozone pollution.

In 2009, Gov. Dave Freudenthal recommended that the EPA designate the Upper Green River Basin an ozone nonattainment area under the Clean Air Act. The EPA has held off on making such designations, however, leading to CURED's threat to sue.

Last month, the group WildEarth Guardians sued to try to force the EPA to decide on several proposed nonattainment areas including the one in Wyoming. The EPA said it will do so by mid-2012.

Other gas-drilling areas in the West, including northeastern Utah, have problems with ozone pollution and WildEarth Guardians will stand by its lawsuit, said Jeremy Nichols with WildEarth Guardians.