Note: This is a sidebar to a feature story about how snowmobilers dominate the small town of West Yellowstone.

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WEST YELLOWSTONE, Montana -- Soon this town, as well as Yellowstone National Park and the national forest trails, will begin to get some relief from one chronic problem - the smoke and associated air pollution from snowmobiles.

Pushed by the Clean Air Act amendments of 1990 and a Sierra Club lawsuit, the Environmental Protection Agency is developing the first emission standards for off-road motors. Due in September, the new rules will apply to snowmobiles, as well as other off-road motors, including motorcycles and ATVs.

The regulations will be welcomed by many residents here. The EPA says an average snowmobile of traditional design churns out 100 times the pollution that an average car does. So every 1,000 snowmobiles driven around this town feels -- and looks and smells -- like 100,000 cars. The manufacturers disagree with that formula, but nobody disagrees that the traditional designs are smoke-producers.

The smoke has become legendary at the traffic-clogged west gate to Yellowstone Park. The weekend I'm in town, the park's gate staff don respirators. Gas masks. During the morning rush hours, the inbound snowmobiles disappear into a thick cloud of exhaust hanging on the road just beyond the gate.

In town, the smoke can be bad where snowmobiles idle in bunches -- around motels, at stoplights and stop signs, and around the morning launch of each rental fleet. Many residents report it's worse on windless days when winter inversions put a lid on the town.

The whole town "can be a blue haze," says Charlotte Smith, who with her husband has run the rustic Sleepy Hollow Motel for 19 years. At first, they were glad to have the snowmobile business, but about eight years ago they began closing the motel in winter and leaving town to escape the smoke and noise.

The EPA came out with proposed emission standards six months ago, and the final standards are expected to be similar to what's been proposed: a 30 percent reduction in emissions by the year 2006, and a total 50 percent reduction by 2010.

Snowmobile manufacturers are fighting the 50 percent reduction, but say they can meet the 30 percent. The standard will be applied on a fleetwide basis: A manufacturer will be in compliance when an average of the emissions put out by all its new models meets the standard.

Almost all existing snowmobile engines are two-stroke designs. They are powerful, but mix oil with the gas, producing the infamous blue smoke that's a combination of burned and unburned oil. The pollution includes hydrocarbons, carbon monoxide and nitrogen compounds.

The main four manufacturers are developing cleaner models. Several came on the market this winter, and by fall, all four manufacturers -- Arctic Cat, Polaris, Yamaha and Bombardier/Ski-Doo - will have cleaner models for sale.

So far, the cleaner models are four-stroke designs, similar to car engines. Their exhaust is 60 to 90 percent cleaner, says Ed Klim of the International Snowmobile Manufacturers Association. But they also tend to be heavier, less powerful and more expensive. Because of that, some of the new models will likely be improved two-strokes, says Linc Wehrly, who heads the EPA's effort.

Once the standards take effect, it'll probably take another decade or longer for all U.S. snowmobilers to convert to cleaner machines; some 1.6 million snowmobiles are registered nationally, and about 140,000 new machines are sold each year. Yellowstone National Park is considering a requirement that only the new four-strokes be allowed in the park.

The EPA standards won't directly address the noise snowmobiles make, but it will likely become a federal crime to tamper with exhaust systems. That will cut down on the increased noise that today's "tuned pipes" put out.