Pointing to the National Park Service's own testing as evidence, National Parks Conservation Association officials are criticizing a proposed winter-use plan for Yellowstone National Park, saying testing shows snowmobiles have gotten dirtier and noiser, not cleaner and quieter.

In pointing to the park's Yellowstone Over-snow Vehicle Emission Tests – 2012: Preliminary Report, NPCA officials say the trend to dirtier and noiser snowmobiles the past six years "contradicts the snowmobile industry’s repeated promises to make cleaner snowmobiles and keep unhealthy gasses such as carbon monoxide, benzene and formaldehyde from fouling the air of the country’s oldest national park."

The report explains that scientists tested 2011-model snowmobiles in Yellowstone and compared their emissions with 2006 models made by the same companies, NPCA noted in a release.

"One manufacturer’s newer snowmobile emitted over 20 times more carbon monoxide than its earlier model. Another company’s newer model had higher emissions of every exhaust gas sampled, including 5 times more hydrocarbons," the release said.

The report concludes: “The model change in snowmobiles has not been a positive influence on air quality based on the emission data.”

In releasing the park's Draft Supplement Winter-Use Environmental Impact Statement earlier this month, Superintendent Dan Wenk said his proposal to allow up to 480 snowmobiles a day in Yellowstone, more than twice the average entries of recent winters, would make the park “cleaner and quieter.”

However, the National Park Service’s own studies contradict that assertion, the NPCA release said. "That document shows the proposed plan would increase snowmobile noise and pollution in Yellowstone National Park with significantly greater emissions of carbon monoxide and cancer-causing gasses such as formaldehyde and benzene," the park advocacy group said.

“Rewarding a technology that is going backward and getting dirtier is the very opposite of stewardship that Americans expect and deserve in Yellowstone National Park,” said Tom Kiernan, NPCA president. “After 10 years of pledging to make major improvements to emissions and noise, the snowmobile industry has gone back on its promise to the National Park Service and the public."

The emissions study looked at “recent additions to the snowcoach fleet” and concluded: “emissions are generally lower for newer snowcoaches compared to mean values of the earlier fleet and especially compared to the older carbureted engine snowcoaches.”

Indeed, specific data provided in the report show that current snowcoaches are up to 50 times cleaner than current models of “Best Available Technology” snowmobiles when the vehicles’ carbon monoxide emissions are calculated on a per-visitor basis. In per-visitor emissions of hydrocarbons and nitrogen oxides, the report shows snowcoaches are 2-5 times cleaner than snowmobiles. The report reflects that these air-quality advantages of snowcoaches are expected to become even more significant when Yellowstone requires all snowcoaches to utilize newer engines.

“The National Park Service should make an immediate U-turn on this misguided policy. After all, the growing majority of Yellowstone Park’s visitors prefer multi-passenger snow coaches, which are demonstrably cleaner than snowmobiles, which are getting dirtier. Even park officials have acknowledged that,” said Chuck Clusen, director of the National Park Project at the Natural Resources Defense Council. “The only obvious and responsible path forward is to facilitate the use of snow coaches, not snowmobiles.”