There is no escape from smog: Warnings will go up at Shenandoah National Park beginning tomorrow to tell visitors when the increasingly dirty air obscures the view from scenic Skyline Drive or violates standards for human health.

Park visibility has dropped 50 percent over the last 40 years, mainly because of sulfur dioxide fumes from power plants and pulp mills. Half a century ago, visitors reputedly could see the Washington Monument, 70 miles away, on a clear day. Now, the longest view is 40 miles away, to less inspiring Warrenton.

Park officials will issue daily advisories indicating whether visibility is less than 10 miles, from 10 to 30 miles, or more than 30 miles. Already this summer, Superintendent J.W. Wade said yesterday, there have been about a dozen days of low visibility, when people could not even see the Massanutten Range to the west. The park stretches along the Blue Ridge from Front Royal to Waynesboro.

"If somebody leaves the city and expects to have wonderful views, we feel like we need to let them know," Wade said. "People are disappointed about the fact that they get up here and can't see very far."

The park also will publish daily advisories on ozone, which first reached unhealthy levels for two days in a row in 1988. Ozone is formed when nitrogen oxides and volatile organic compounds, mainly emitted by cars, drift in on air currents from other areas and cook in the sunlight to form smog.

Unlike its stratospheric counterpart, which protects the planet from the sunlight's damaging rays, low-level ozone can cause eye irritation and lung damage, especially in the old, the young, exercisers and the unusually sensitive. The park will tell visitors whether levels are good, medium or unhealthy.

"They deserve to be warned of the fact that the ozone level is relatively high and have the option of adjusting their level of activity," Wade said. Ozone has not reached unhealthy levels this year, he said, but only because of weather patterns.

Signs will be posted at park entrances and the park's two visitor centers.

The advisories are a pilot program at five National Park Service sites, designed to help the government write a policy on the growing issue of health and air pollution at national parks. The others, all of which have had at least one ozone pollution episode, are Acadia National Park in Maine, Cuyahoga National Recreation Area in Ohio, Pinnacles National Monument in California, and Sequoia National Park in California.