BILLINGS, Mont. — Air quality in four of six categories is worsening at Yellowstone National Park, a new study by the National Park Service shows.

The study compiled air-quality trends dating back to 1995 at national parks across the country.

One pollutant on the rise in Yellowstone is ground-level ozone, which can cause respiratory problems and threaten plant health. But the levels aren't high enough to pose a risk and don't exceed any national standards, the Park Service said.

Still, Yellowstone had more categories in which air quality was getting worse than any other park in the country.

"Even though the levels are increasing, they're still at a relatively low level," said John Bunyak, with the Park Service's air resources division in Denver. "It's still not at alarming levels, but it's not a good thing."

Visibility has improved at most national parks, and ozone levels have dropped at about two-thirds of them. However, 10 national parks in the West -- including Yellowstone, Glacier and Rocky Mountain -- continue to see an increase in ozone.

Experts say there's no way of telling whether the ozone at Yellowstone comes from pollution from around the globe or from nearby sources, such as vehicles in the park or area power plants.

The air in Yellowstone is still better than most places in the United States, said Mark Wenzler, clean-air program director for the National Parks Conservation Association.

But it's important to continue improving emissions at old power plants, he said, and to be cautious with new power plants in the region.

"Let's not blow all that progress by failing to properly control the new plants," Wenzler said.

Yellowstone "isn't in trouble at this point," he said. "But you can see the trends heading there."

Other air-quality categories that worsened at Yellowstone were ammonium, sulfates and nitrates. Two categories measuring visibility showed improvements.

Source: Associated Press