After nearly 30 years, the federal government has little to show for its efforts to reduce the haze that obscures the views at many national parks, a problem that was singled out for attention in 1977 under the Clean Air Act.

Saying it was time for results, the Bush administration took aim at the matter last month, issuing a regulation that requires power plants and other polluters to install technology to curb emissions of sulfur dioxide and nitrogen oxide, the two main contributors to the haze that often shrouds spectacular vistas in an impenetrable brown cloud.

But no sooner was it announced than the regulation became mired in controversy, with environmentalists saying the rule was a watered-down provision that would have little effect.

It is an accusation the Environmental Protection Agency rejects.

"What the administration's doing with regard to regional haze, we're charging down the road," said William Wehrum, counsel to the assistant administrator for air and radiation at the EPA.

The picture is complicated, however, by an agreement that EPA administrator Michael Leavitt forged with North Dakota Gov. John Hoeven (R) to allow the state to change how it measures air quality, a deal that environmentalists said would torpedo efforts to reduce the worsening haze at Theodore Roosevelt National Park, known for its rugged view of the North Dakota Badlands.

This pact, signed on Feb. 24, sparked a rare outcry from most of the EPA's own specialists in air quality monitoring, who warned in a memo that the "substantial changes" allowed under the agreement would underestimate the impact of air pollution and could "set a precedent" that would undermine cleanup efforts in other regions.

After taking stock of the administration's recent actions, environmentalists argue the end result will be little or no improvement in the vistas at the Great Smoky Mountains and many other iconic national parks.

"Clearly in North Dakota and in some of its national policy decisions there's an urgent need to redouble our efforts to protect national parks and other vital ecosystems from air pollution," said Vickie Patton, a senior lawyer at Environmental Defense. "They're letting some of these high-polluting sources off the hook."

John Stanton of the National Environmental Trust said: "Twenty-seven years ago, Congress told EPA to prevent the further deterioration in air quality. EPA really hasn't done a thing."

The current dispute -- which has drawn in at least one Democratic senator as well as numerous environmentalists -- highlights the contentious debate over air quality and visibility in national parks that attract millions of visitors a year.

Under ideal conditions, visibility from peaks in parks such as Shenandoah, Acadia and the Great Smoky Mountains should be more than 100 miles. But the year-round average at Shenandoah hovers closer to 25 miles, dipping to 15 miles during the summer. EPA monitoring data show that smog levels have increased and that visibility has worsened in parks including Bryce Canyon in Utah, Big Bend in Texas and the Great Smoky Mountains, along with others in Colorado, Utah, Idaho and Wyoming.

Most haze is caused by fine particles of sulfur dioxide and nitrogen oxide produced by the combustion of fossil fuels such as coal and oil, which become suspended in the atmosphere. Coal-fired plants account for most of the visibility impairment in eastern national parks and wilderness areas; in the West, nitrogen oxide, particulate matter and carbon monoxide -- often from mobile sources such as cars and trucks -- are the primary culprits.

Part of the controversy involves disagreements over which set of clean-air rules should be invoked to tackle the haze problem.

On April 16, the EPA announced that power plants contributing to visibility impairment would have to adopt "best available retrofit technology," or BART, by 2018 or prove that the cost of compliance was too high.

"Regional haze is a national problem caused by multiple sources over a wide area," the agency said, adding that reducing haze would have other benefits: "The same pollution that causes haze also poses serious health risks for people with chronic respiratory illnesses."

But administration critics say the EPA may undermine its own initiative by letting polluters rely on a different Clean Air regulation, the Interstate Air Quality Rule, to address fine-particle pollution. That approach -- which the agency is vetting for public comment -- will not translate into quick cuts in sulfur dioxide and nitrogen oxide, they argue. The rule gives power plants in the East the ability to trade pollution "credits."

The utility industry has fought the BART rule in court, seeking to head it off altogether. The industry scored a partial win in the D.C. circuit courts but is still under federal order to reduce emissions.

Quin Shea, executive director for the environment at the Edison Electric Institute, said the industry would prefer to see the Interstate Air Quality Rule adopted rather than BART, calling the regulation "an appropriate surrogate for the electric power sector." He added that the interstate rule, which calls for a two-thirds reduction in sulfur dioxide and nitrogen oxide emissions, is "excellent at meeting a first milestone for visibility."

Air quality has been diminishing in many national parks, according to a study completed by Colorado State University in May 2000 that found that vistas deteriorated in Badlands National Park, Big Bend National Park and Great Smoky Mountains National Park, among others.

Officials of the National Parks Conservation Association, a leading critic of the EPA's efforts to tackle haze over the years, also challenge the administration's timeline for reducing haze, which sets a final target 60 years from now. Joy Oakes, the group's mid-Atlantic regional director, called that far-off goal "fairly ridiculous."

The EPA's Wehrum said the agency hopes to make "reasonable progress" of 15 percent clearer air every decade. "From a technical standpoint, it's a very complex problem to work your way through," he said.

Critics who question the administration's commitment are pointing to the accord the EPA accepted for Theodore Roosevelt Park in North Dakota, which allows the state to choose what year to use as the baseline for measuring air-quality trends and to count average emissions over the entire year, rather than peak emissions.

The EPA's own specialists in measuring air quality trends wrote that the agreement marked "substantial changes from past air quality modeling guidance . . . and accepted methods."

But it drew the praise of Sen. Kent Conrad (D-N.D.). "The air in North Dakota is exceptionally clear," said Conrad, who says the agreement will promote economic development in his home state. "I do believe what happened in North Dakota was an appropriate outcome."

Wehrum, however, acknowledged the political firestorm the pact had prompted. "It has generated an amount of debate, I won't deny that," he said.

EPA spokeswoman Cynthia Bergman said "conversations are still ongoing" with the North Dakota officials on how to measure air pollution trends.

"The negative impact of pollution in our parks is real," said Stanton of the National Environmental Trust. "Why will people travel to scenic areas if they can't see the scenery?"

But Wehrum said it will take time for regulators to hit on the right formula. "How in the world would you do this?" he asked. "It takes a lot of sophistication and complicated analysis."

Great Smoky Mountains National Park is among the U.S. parks where spectacular vistas have been severely compromised by pollution. On bad days, scenic views there are reduced by 60 to 80 percent.