The Great Smoky Mountains is the nation’s most polluted national park, with air quality rivaling that of Los Angeles, environmental groups said in a survey released Monday.

Acadia National Park, along the Maine coast, was ranked the fifth most polluted.

The survey was released the same day a National Park Service study found air quality has improved or at least stayed the same in more than half of 32 monitored parks since 1990.

“In most parks, air quality exceeds standards set by the Environmental Protection Agency to protect public health and welfare,” Park Service Director Fran Mainella said in a statement. “Our findings also show that some parks occasionally experience pristine air quality conditions, unaffected by air pollution.”

The National Parks Conservation Association said that wasn’t good enough. “Clean air in our parks shouldn’t be an occasion, it should be a condition,”

said Don Barger, Southeast regional director.

Using park service data, the association and two other environmental groups, Appalachian Voices and Our Children’s Earth, rated the Smokies as the most olluted park in the country.

Shenandoah National Park in Virginia was second, followed by Mammoth Cave ational Park in Kentucky, Sequoia and Kings Canyon National Parks in California nd Acadia.

Acadia National Park superintendent Paul Haertel said Tuesday the Maine park’s high rating does not surprise him. High ozone levels, mercury in some of the park lakes and acid rain all contribute to Acadia’s pollution problem, he said.

Haertel said most visitors don’t even realize the park is polluted, however. Unhealthy ozone levels, for example, occur on the hottest summer days. “They think it’s just a hot, muggy day,” Haertel said. “So they don’t notice it, but it’s insidious.”

He said finding soultions to pollution will take “a great deal of cooperation” among private and public entities in and outside of Maine. “It’s not an easily addressed problem, but one that should be on everyone’s priority list,” he said.

The Smokies, on the Tennessee-North Carolina line, is the nation’s most opular park with more than 9 million visitors a year. Park spokesman Bob Miller oted that efforts by area utilities are being made to cut down on pollution.

“But the fact remains our air quality has not gotten better,” Miller said.

Ozone levels in the Smokies violated federal health standards more than 175 times since 1998, threatening the health of hikers and damaging 30 species of plants, the environmental groups said.

The report used a plant ozone-exposure standard of 60 parts per billion for comparison purposes. Ozone exposure numbers were computedby adding the concentrations for all hours experiencing 60 parts per billion or greater for the months of April through October.

According to the study, the Smokies’ average annual ozone exposure of 133,200 ppb-hours exceeded that found in Atlanta, Knoxville, Tenn., and Charlotte, N.C. Only one city in the study’s analysis, Los Angeles, with more than 180,000 ppb-hours, exceeded the Smokies.

The groups said the Smokies’ pollution threatened the health of hikers and 30 species of plants.