BILLINGS, Mont. - The Environmental Protection Agency is asking the National Park Service to take a closer look at how its new plan for snowmobiles and snowcoaches in Yellowstone Park would affect clean air, visibility and human health.

The EPA was one of several agencies asked to review a draft plan for managing winter visitors in Yellowstone and Grand Teton national parks.

Many of the comments from other agencies, especially counties around the parks, focused on urging the Park Service to ease requirements that snowmobilers be accompanied by guides.

But the EPA said the Park Service should, among other things, look at how the proposal jibes with agencywide management policies updated last year intended to limit impacts to the natural environment, minimize harassment of wildlife, preserve quiet in the parks and perpetuate the best air quality possible.

One part of the plan that lists "desired conditions" in the park for human health, wildlife and visitor experience appears to "fall short of the resource protections" established in the newest management policies.

"We also recommend that NPS consider mitigation measures wherever air quality or noise levels have the potential to adversely affect human health," said the EPA's letter, signed by Larry Svoboda, who works in the agency's Office of Ecosystems Protection and Remediation in Denver.

A preliminary draft of the plan released in November would allow up to 720 snowmobiles a day into Yellowstone as long as the machines meet noise and pollution standards and all riders are with commercial guides.

The study considered six alternatives, ranging from banning snowmobiles to allowing up to 1,025 a day. The best option for protecting the environment was prohibiting all over-snow vehicles, the agency said.

Several local, state and federal agencies received early copies of the draft to analyze technical issues.

A more formal draft is expected to be ready for public comment this spring, with a final plan to come later in the year.

The EPA touched on several topics in its nine-page review of the preliminary draft.

For Yellowstone's air - some of the cleanest in the country - to remain pristine, park officials should re-examine how it defines potential impacts of the plan, the EPA said. The draft says that any perceptible and localized change in visibility would be a "minor" impact.

The EPA recommends that any such change be considered "moderate" or "major."

The agency also suggested that any temporary adverse effects to human health from noise or pollution be considered at least "moderate," not "minor" as the draft plan says.

The letter also voiced concern that in alternatives that increase the number of over-snow vehicles, levels of formaldehyde from engine exhaust might come close to "recommended exposure limits" set by the National Institute for Occupational Safety and Health.