America's national parks are suffering from dirty air. A new study released by the non-governmental National Parks Conservation Association says pollution in some parks rivals or even exceeds that of major U.S. cities, such as Los Angeles or Atlanta.

Great Smoky Mountains National Park is a special place. The vast forested area, straddling the southeastern U.S. states of North Carolina and Tennessee, is home to a wide variety of plants and animals. Ten million people visit the park each year, making it the most popular destination in America's nation-wide park system. Great Smoky Mountains' major attraction is the view: the biggest sweep of undeveloped wilderness in the southeastern United States.

But that view is shrinking, according to a new report by the association, called "Code Red: America's Five Most Polluted National Parks." The report says visibility in the parks is being impaired by haze, ozone and acid rain. And Great Smoky Mountains National Park tops the list in all three categories.

"About thirty species of plant are damaged and suffer premature leaf loss at the levels of ozone that we are seeing here," said Park Spokesman Bob Miller, who adds that pollutants are a threat to the land, the wildlife and public health. "We are also seeing a loss of vitality, especially at the upper elevations, because of acid deposition. Soils are so acid that plants cannot effectively take up the nutrients that they need. So the result is that there is a kind of stunted growth. And, last, but not least, this year we had 43 days when the levels of ozone were high enough here so they present a hazard to human health, according to EPA health protection standards."

Mr. Miller said visitors surprised the park is polluted. "They think that what they are seeing is what caused Great Smokey National Park to get its name," he said. "In fact what they are seeing are not the little wisps of moisture or the natural bluish haze which the park is named for. They are seeing smog, a yellowish kind of brown murk that cuts the view and a lot of people don't know that is what the problem is. They think that they are coming to breathe fresh air, and they find out with some amount of trepidation that they are not."

He said the pollution comes from number of things. "The loss of visibility, about seventy percent comes from sulfates. It comes from fossil fuel power plants, almost entirely coal, which is the fuel of the southeast," he said. "It is cheap. It is available. It is all around us, so, of course that is the preference, to fire our boilers with coal. That accounts for almost all of the visibility loss and a good share of the acid precipitation. The same particles that scatter the light and create haze also are leached out by rainfall and produce sulfuric acid on the way down. The other main component, especially the ozone, is about 50-50 with nitrogen oxides that come from power generation and about the same amount comes from transportation, the exhaust from cars, trucks and buses."

Behind Great Smoky Mountains National Park on the most endangered parks list are Shenandoah National Park in Virginia, Mammoth Cave National Park in Kentucky, Sequoia and Kings Canyon National Parks in California and Acadia National Park in Maine.

Tom Kiernan, President of the National Parks Conservation Association, the watchdog group that produced the study said the pollution signals a greater problem that goes beyond park borders.

"We are not living in a sustainable relationship with the natural world and our national parks are telling us that and are giving us a big warning sign that we are damaging both the lands around us and our own health, and these are the places that are calling us to come together as a country to better protect," he said.

He said U.S. lawmakers can help solve the problem. "Congress and most importantly this administration must fully implement the 1990 Clean Air Act. There also needs to be some further tightening, some additional legislation on top of that 1990 Clean Air Act. Both of those strategies need to occur," he said.

Mr. Kiernan is critical of a recent Bush administration proposal that would allow aging coal-fired power plants to upgrade their facilities without having to install costly anti-pollution equipment. He says the plan would increase emissions.

"We will have more pollution coming out of the park because of the change in the way this administration is going to interpret the Clean Air Act," he said. "It will lead to not better air quality in our National Parks, but worse air quality in our National Parks."

Dan Riedinger, a spokesman for the Edison Electric Institute, a trade association that represents power and utility companies, said regardless of the outcome of the proposed rules on aging power plants, emissions will continue to decrease.

"What some old power plants are not required to do automatically is install the very latest modern pollution control technology under this one program that the administration is considering some changes to," Mr. Riedinger said. "But there are numerous other requirements under the federal Clean Air Act and under state programs that have been requiring the power plants, the older ones we are talking about here, to improve their environmental performance and install the latest technologies."

But the industry's argument does not convince Mr. Kiernan, of the National Parks Conservation Association. In the absence of strong federal or state action to reduce emissions, he argues, the government cannot obey its mandate to keep the air in the national parks the cleanest in America.