BILLINGS (LEE) - You can see farther in Yellowstone National Park these days, but the amount of invisible smog continues to grow.

Air quality in four of six categories is worsening in Yellowstone even though visibility is improving, according to the latest study of Yellowstone and other parks by the National Park Service.

Among the pollutants that are increasing is ground-level ozone - a gaseous mix of machinery exhaust cooked in the sun's rays. The smoggy mixture is increasing at several parks in the West. At high enough levels, ozone can cause respiratory problems and other ailments and threaten plant health.

At Yellowstone, the ozone isn't a threat to people and doesn't exceed any national standards, according to the Park Service.

Still, Yellowstone had more categories in which air quality was worsening 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."

The study compiled air-quality trends dating back to 1995.

and measured several categories across the country.

Visibility throughout national parks has improved and, for about two-thirds of the parks, ozone levels are dropping.

But 10 national parks in the West, including Yellowstone, Glacier and Rocky Mountain, continue to see ozone increase.

What's tricky is understanding why.

Ozone is typically separated into two categories: the protective layer of gases 10 to 30 miles above the Earth that helps shield the sun's rays, and the layer closer to the ground that forms when chemicals produced by vehicles, industrial operations and other sources react with sunlight and create smog.

That smog can be swept up and find its way to pristine places like Yellowstone.

There's no telling whether the ozone in the park comes from pollution from around the globe or from more local sources, such as vehicles in the park or nearby power plants.

The question "is kind of a tough nut to crack," Bunyak said.

Because the Park Service doesn't provide pollution permits for sources such as power plants, the agency is working with states and the Environmental Protection Agency to talk about how some operations may affect air quality in the parks, he said.

Fourteen Western states are also working on plans to reduce pollution that creates haze and other visibility problems.

A 35 percent reduction in sulfur dioxide emissions from coal-fired power plants in the West since 1998 has certainly improved air quality in recent years, according to Patrick Cummins, co-director of the Western Regional Air Partnership, which represents Western governors and tribes.

Those improvements will continue along with others, such as an estimated two-thirds reduction in nitrogen oxides over the next 15 years from recent rules curbing pollution in certain vehicles, Cummins said.

Some areas still need attention, though, including oil and gas development activities and emissions from Mexico, Canada and elsewhere, he said.

"Clearly there's a lot of concern about the growth in energy development in general," Cummins said. "Right now, the biggest concern with that is the increased oil and gas."

The air in Yellowstone is still better than most places in the United States, but that doesn't mean it's time to let the guard down, said Mark Wenzler, clean-air program director for the National Parks Conservation Association.

Coupled with progress in improving emissions at old power plants is the need to be cautious with new power plants in the Yellowstone region that could further compromise the park's air quality, he said.

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

It's up to state and federal governments to keep that pollution in check and, in many cases, simply to adhere to federal laws already in place, Wenzler said.

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

The air-quality categories that worsened in Yellowstone are ozone, ammonium, sulfates and nitrates. Two categories measuring visibility improved.