By Alex Dobuzinskis
LOS ANGELES (Reuters) - A conservation group found 36 U.S. national parks had moderate or worse levels of ozone pollution in a report card released on Tuesday, with four parks in California receiving the worst grades for health effects.
The National Parks Conservation Association study singled out Sequoia, Kings Canyon, Joshua Tree and Yosemite national parks with "F" grades in the report's "healthy air" category for ozone pollution.
In part, that is because those four parks in California, like other parts of the state, suffer from geography that traps air pollution in vast basins or valleys, said Ulla Reeves, who manages the nonprofit group's clean air campaign.
"Honestly, this is a national problem and needs a national solution," Reeves said. "There are certain parks that are suffering more than others. None of the national parks that we looked at are doing very well at all."
The report highlights different forms of air pollution visitors encounter at a national park, where many people go to escape environmental degradation.
Of the 48 national parks that under the 1970 federal Clean Air Act are supposed to receive the most protection from air pollution, 36 parks at some time had ozone levels categorized as moderate or worse, the report said.
The group based its ratings on air quality measurements by the federal government.
"At two parks — Mammoth Cave in Kentucky and Saguaro in Arizona — a natural view is almost impossible to come by, even on the clearest days," the report said.
That is because haze obscures views from high vantage points, according to the report, which gave Mammoth Cave an "F" for visibility and Saguaro a "D."
There were areas of improvement at some parks, as levels of air pollution were reduced.
Tennessee's Great Smoky Mountains National Park, which was so named because of mist that naturally hangs over its mountains, was once so covered in haze that visitors in 1990 could only see 25 miles (40 km) away, the report said.
But because nearby power plants were closed or retrofitted, visibility improved and visitors can now set their sights on land features 46 miles (74 km) away, researchers found.
Several national parks also suffer from climate change, including Glacier National Park in Montana where glaciers are rapidly melting, the report said.
"Generally the NPS (National Park Service) cares about air quality in the parks and is supportive of efforts to improve air quality," NPS spokeswoman Linda Friar said in an email.
(Reporting by Alex Dobuzinskis; Editing by Sandra Maler)