DENVER, JUNE 20 -- The winter air in Colorado's Front Range is so dirty that motorists in the urban part of the state could be the first in the nation to be required to use gasohol or other high-oxygenated fuels in their vehicles in cold weather, anti-pollution authorities said.

The Colorado Air Quality Control Commission gave preliminary approval Friday to a plan prohibiting gasoline sales in the 90-mile corridor from Castle Rock to Fort Collins from December to March, when air pollution is worst. Final approval is expected June 29.

About 70 percent of Colorado's population is concentrated in the Front Range of the Rockies. And some surveys rank Denver's air pollution as the worst in the nation, or second to that of Los Angeles.

Conoco Oil Co. officials indicated they may challenge the requirement and sue the commission for restraint of trade.

"We'll have to look at all of our alternatives," said David Myers, general manager of the Conoco refinery in suburban Commerce City.

Myers testified at a two-day hearing that the refinery could not provide high-oxygen fuels before February.

"We've had to weigh the consumer issues, oil industry interests and air quality benefits," said Jerry Gallagher of the state Health Department's air pollution control division.

High-oxygen fuels, which burn cleaner than pure gasoline, include ethanol or gasohol made from corn, and an ether blend called MTBE.

Officials estimate that use of gasohol and MTBE would raise annual fuel costs of Front Range motorists by about $5. Authorities also stress that the fuels could be used without affecting driving performance or damaging engines.