The fuel is cheap compared with other fossil fuels, but emits more pollutants, such as mercury and smog components sulfur dioxide and nitrogen oxides, as well as the greenhouse gas carbon dioxide.

"President George Bush's administration is responding to this growing threat to our national parks by seeking to weaken and rewrite the laws that protect national park air quality," Mark Wexler, the clean air director for the National Parks Conservation Association, a Washington-based advocacy group, said by telephone.

The Environmental Protection Agency has proposed refinements in so-called New Source Review rules that would change the way air pollution is calculated, allowing manipulation by industries seeking pollution permits, the NPCA said.

The changes would enable electricity generators, for example, to hide pollution spikes on hot days when their units typically run hardest.

Complaints that the revisions would harm park air quality have been underscored by US Rep. Henry Waxman, a California Democrat, who wrote to the EPA this year urging Administrator Stephen Johnson to abandon the proposed rule change.

The EPA did not immediately respond to an inquiry about the report. In the past, it has said that the rules would provide greater regulatory certainty.

28 PLANTS PROPOSED NEAR PARKS

Dirty air in a national park can range from "merely inconvenient," such as when visitors can't see more than a few miles due to sooty air, to dangerous, when a child has an asthma attack because of excessive ozone pollution, the report said.

Over the long term, the report added, pollution can harm or even kill wildlife in the parks.

Power companies have proposed more than 110 new US coal-fired plants, with 47 under construction or in the process of getting permits, the National Energy Technology Laboratory said in February.

In particular, plans for 28 coal-fired plants threaten the air quality in 10 national parks, including the Great Smoky Mountains in Tennessee and North Carolina, Zion in Utah, and the Badlands in South Dakota, the NPCA said in its report, called "Dark Horizons."

Near Virginia's Shenandoah National Park, eight coal plants are under development, while pollution already has cut average views to a quarter of what they used to be, the report said.

The 28 plants would emit annually a combined 122 million tons of carbon dioxide, 79,000 tons of SO2, 52,000 tons of NOx, and 4,000 pounds of mercury into the air near the 10 national parks for at least 50 years, it said.

Coal plant developers argue that many of the new plants will be the cleanest ever built.

Frank Maisano, a spokesman for the Four Corners coal plant proposed to be built in New Mexico, which NPCA said would threaten Colorado's Mesa Verde, a park featuring native American cliff dwellings, said the plant could even replace power generation from old power plants, which may find it too expensive to cut emissions.

But Wexler said old power plants should be held accountable to current air laws, before new ones are built.