On a clear day you can still see almost forever, but clear days are getting harder to come by in the national parks.

The Blue Ridge Mountains are just as often gray; the north rim of the Grand Canyon is occasionally obliterated by smog. Tourists are complaining that the air smells bad in Utah's Arches National Park, and pollution levels in Maine's Acadia National Park sometimes exceed the federal standards for safeguarding human health.

With the Clean Air Act back on the congressional agenda and the tourist season in full swing, air quality in the national parks and wilderness areas is shaping up as a major legislative issue and a frustrating problem for federal park officials.

Despite laws protecting national parks as pristine areas where no degradation of air quality is permitted, conservationists contend that the air is slowly going sour over America's "crown jewels," damaging vegetation and threatening to rob the nation of its most spectacular scenery.

"Air pollution now impairs visibility in almost every national park in the mountainous Southwest at certain times," said Terri Martin, a Denver-based air quality specialist with the National Parks and Conservation Association.

The Interior Department says man-made pollutants obscure views more than 90 percent of the time in the parks it monitors, which include the Great Smoky Mountain and Shenandoah national parks in the East and virtually all the parks in the Colorado Plateau's "Golden Circle."

In Utah, famed for its breathtaking 100-mile vistas of red-rock canyons and teal-blue skies, visibility on some winter days is as little as 40 miles. The canyons disappear in the murk, and the sky is haze-gray. At Arches, smog shrouds the wind-etched pinnacles half the time in the winter months and 25 percent of the time in the summer.

The view from the Skyline Drive in Shenandoah on some summer days is limited to the interpretive signs along the road, and the valley views that were the inspiration for the park are not visible at all.

In short, visiting a national park for the scenery is becoming a chancy proposition.

"On occasion, after a rainstorm, a lot of the parks in the middle of the country can still have very clean air," said Chris Shaver, a National Park Service air quality official in Denver. "Even in some eastern parks there are good number of days when the air is clean, but you never know which ones."

The problem, according to federal regulators, is that cleaning up the air in the parks is not as simple as capping a couple of nearby smokestacks. Most of the park pollution wafts in from miles away or is part of a regional haze that affects the parks as well as nearby urban areas.

In the East, the major culprit is sulfur, emitted by fossil fuel-burning plants. Transformed into sulfate in the atmosphere, the pollutant scatters light rays like a frosted-glass window. The problem worsens in the summer because of high humidity, which also scatters light rays.

Using airport data, Interior officials estimate that sulfate haze has reduced visibility in the East by more than 50 percent in the past 40 years, to an average visual range of less than 15 miles.

In the West, regional haze is less a problem than concentrated plumes of pollution from smelters, mines, power plants and major metropolitan areas, sometimes hundreds of miles away. With little rainfall to wash the pollutants from the air, the plumes move across the deserts and canyonlands to settle in the parks.

"In the wintertime, we have a layered haze in the Colorado Plateau," said the park service's Shaver. "Layered haze is visible from Bryce Canyon {in Utah} 75 or 80 percent of the time."

Aside from the esthetic disadvantages for visitors, park officials worry that the pollution may be damaging the health of the parks themselves. In a report last year, the park service said it found "visible foliar injury" to trees and other plants in most of the park units it surveyed. Some vegetation also had elevated levels of toxic trace elements, including lead, arsenic and cadmium.

In Shenandoah, where ozone levels often approach the maximum allowable for human health, eastern white pine stands have been severely damaged. In California's Sequoia National Park, bathed in pollution from the San Joaquin Valley, ponderosa and jeffrey pines are showing stress. Biologists fear that the park's famed sequoias may be next.

"The suspicion is that once you kill off a genotype you may have a hard time getting it back again," Shaver said.

The impact of pollution on vegetation also is causing concern at the Forest Service, which reported last year that regional haze or "plume blight" was affecting at least 33 and possibly as many as 86 of its most pristine wilderness forests.

Because the pollutants are the same as those identified as causing acid rain, environmentalists fear that U.S. forests may be on the verge of suffering the same fate as West Germany's Black Forest. Scientists there blame acid rain and other pollutants for an epidemic of dead and dying trees that now affects as much as 50 percent of the forest.

But protecting the forests and parks has proved to be a difficult and divisive task. Interior took at stab at the problem more than six years ago, proposing to designate "integral vistas" in 43 national parks. The rule would have covered 173 view angles from 136 observation points.

That was a small percentage of park overlooks, but state officials and industry representatives fought it bitterly, contending that it could put huge portions of some western states off-limits to development.

The proposal, published five days before President Reagan took office, was rejected by Interior Secretary Donald Hodel in October 1985. Hodel argued that the designations would "provide a false sense of security" and "would not be good for the parks."

The rejection came as a disappointment to conservationists and park officials, who believed the designations at least would have helped prevent the pollution from worsening. Critics of Hodel's action concede, however, that the integral vistas would not have solved the underlying problem of pollution wafting in from far away.

Instead, conservationists hope that the plight of the parks will provide added impetus for an overall strengthening of the Clean Air Act.

"Monitoring already shows that no unit of the National Park Service in the lower 48 is 'pristine' with respect to air quality," said Paul Pritchard, president of the National Parks and Conservation Association. "These areas are nationally significant and the Congress is responsible for protecting them for the benefit of all Americans."