BILLINGS - A new proposal for snowmobiling in Yellowstone National Park doesn't go far enough in ensuring the protection of air quality, human health, wildlife and quiet spaces, according to the Environmental Protection Agency.

The EPA says the National Park Service should either change its current "preferred alternative" or choose a different option.

Earlier this year, the Park Service released a draft plan that would allow 720 snowmobiles and 78 snowcoaches per day into Yellowstone.

The machines would have to meet noise and pollution standards and snowmobilers would have to be with a commercial guide.

The proposal, expected to be finalized last year, is the latest attempt to resolve the long-running controversy over snowmobiling in Yellowstone and Grand Teton parks.

Al Nash, a Yellowstone spokesman, said the agency has received about 110,000 comments on the draft plan.

Most were form letters but some, including one from the EPA, are "substantive comments" that the agency will use to tweak its proposal if necessary, he said.

"All of the stuff that we get that is substantive is helpful," Nash said.

The EPA has commented on at least four previous studies of snowmobiles in the park. Last week, Kerrigan Clough, the EPA's deputy regional administrator, sent a letter to the Park Service responding to the latest proposal.

Although earlier moves to reduce snowmobile numbers and require "cleaner and quieter" machines have led to significant improvements, there are still concerns with the latest plan, especially when compared to the option of allowing only snowcoaches, Clough said.

That snowcoach-only option has been considered by the Park Service as the best way to preserve Yellowstone's natural resources. Park officials, though, have been looking for ways to still allow a limited number of snowmobiles while minimizing their impact.

Compared to the snowcoach-only option, the park's latest "preferred alternative" would lead to a five-fold increase in the amount of carbon monoxide emissions, a 17-fold increase in hydrocarbons and double the amount of acres in Yellowstone where snow machines would be heard, the EPA said.

The Park Service's "desired conditions" for winter recreation may not be stringent enough to protect Yellowstone's natural resources, including pristine air quality and natural quiet, the EPA said.

The EPA also listed concerns about park employees.

The latest plan could double the amount of benzene and formaldehyde spewed into the air compared to current conditions, raising concerns about health effects of workers in the park, the EPA said.

If the number of vehicles increases in the winter, the Park Service may want to start medically monitoring Yellowstone employees for exposure to those pollutants, the EPA said.

Noise also may still be an issue for park employees who work at Yellowstone entrances. Earlier noise monitoring apparently tested conditions inside the kiosks, the EPA said.

"If some employees work outside the kiosk as they did in the past it is possible that noise could still represent a significant hazard for them," the EPA said.

The agency's letter is simply a response to the latest Draft Environmental Impact Statement and does not make recommendations on what changes the Park Service should make to its proposal, agency officials said.

"It's their decision and it's their park to manage," Phil Strobel, of the EPA's Denver office, said Tuesday.

The EPA's comments will be added to stacks of other comments, including those from state and local governments that strongly support continued snowmobiling in Yellowstone and some that question the proposal to keep snowmobilers in commercially guided groups.