Improved air quality in Yellowstone National Park is due primarily to low numbers of snowmobiles entering the park, according to two new scientific reports posted on the park's Web site, say leaders of the Coalition of National Park Service Retirees.

The first report from the National Park Service Air Resources Division states: "Most of the gain in air quality (at Yellowstone) can be attributed to the smaller number of snowmobiles."

The second Park Service report cautions that despite improved emissions, new four-stroke snowmobiles remain "much dirtier than light-duty cars and trucks." That report states that peak concentrations of carbon monoxide in Yellowstone's air are greater in winter with an average of 250 snowmobiles per day than they are during summer months even though "summer traffic is 60 times the amount of winter traffic."

The Park Service is gearing up to release a draft environmental impact statement next month, in which park officials will decide a "preferred alternative" for winter use in Yellowstone.

Snowmobile use in the past three years of a temporary winter use plan shows that an average number of 250 snowmobiles per day generates about 114 tons of carbon dioxide per year. Among the alternatives under review, a snowcoach-only alternative would generate 37 tons of CO2, while the maximized snowmobile alternative would generate 267 tons of CO2 per year.

One Park Service report notes that, "The largest reductions in pollutant concentrations and emissions are seen under alternatives that allow only snowcoaches..."

Bill Wade, a former superintendent of Shenandoah National Park and chairman of the retiree group, said: "It is time for the Bush administration to be accountable to its own scientific studies and put clean air in Yellowstone National Park first instead of catering to the snowmobile industry.

"The administration has ordered up endless studies, costing millions of tax dollars, all aimed at justifying a continuation of snowmobile use in Yellowstone, but the science doesn't support such a policy. Snowmobiles are perfectly acceptable in some places, but this report underscores yet again that they are not consistent with the high standards that Americans want maintained in their oldest national park."

Ed Klim, director of the International Snowmobile Manufacturers Association, said that his take on the scientific reports is that worker exposure to harmful emissions in the park is "well below recommended limits" and is well below risk limits set by the Agency for Toxic Substances and Disease Registry.

"Snowmobiles aren't impairing anything in Yellowstone," Klim said. He also said it was only the park retirees' opinion that lower snowmobile numbers are to be credited with air quality improvements.

Yet the reports prepared by the Park Service's Air Resources Division conclude that the sharp reduction in the number of snowmobiles -- which has averaged 250 per day over the past three seasons, instead of the over 700 per day that once dominated the park -- has helped clean up Yellowstone's air far more than the new generation of technology highly touted by the snowmobile industry.

"Few actions in the national park system would be more popular at the moment, and few would do more to restore the morale of the Park Service and the trust of the American people, than for the administration to heed what science has been telling it, repeatedly, for six years about the harmful effects of snowmobile use in Yellowstone," said Rick Smith, a former acting superintendent at Yellowstone. "It's high time to protect Yellowstone's unique winter environment by providing visitors with access using the modern, environmentally friendly snowcoaches that are becoming increasingly popular."

Brodie Farquhar is a freelance correspondent living in Casper. He can be reached via e-mail at brodiefarquhar@hotmail.com.