DENVER — What the backyard barbecue is to Southern Californians in summer, a cozy fireplace is to Coloradans fresh off the ski slopes in winter. But both traditional barbecues and fireplaces--and all the warm associations that accompany them--face major alterations because of air pollution, much of which is caused in Colorado by smoke from burning wood.

As Californians well know, environmental officials have ruled that, starting in 1992, backyard barbecuers may not soak charcoal bricks with lighter fluid or use instant-light briquettes to start fires quickly. That's just one of many moves in a major Southern California Air Quality Management District attempt to curb what is still the nation's dirtiest air--a little step, but one coupled with others that the agency hopes will help.

But California is not alone in the West in facing an air-pollution crisis. Denver and its nearby mountain resorts are confronted by unique geographical and ecological problems. Colorado's valleys and the surrounding picturesque peaks produce world-class skiing, but they also invite frequent "thermal inversions" when heavy cold air moves down from the mountains into the valleys. The warm air above acts like a lid, keeping cold air and pollution directly over these cities and resorts.

The city of Denver and ski areas like Aspen, Vail and Telluride are taking separate, often controversial, measures to diminish Colorado's brown cloud. One county adjacent to Denver has amended its building code to ban installation of traditional wood-burning fireplaces and stoves in new construction starting this year. Only natural-gas logs or stove inserts meeting state pollution standards can be built.

Colorado State Sen. Pat Pascoe is drafting a bill to make similar measures applicable in metropolitan Denver. Denver alone counts 250,000 standard fireplaces. Pascoe's proposed legislation aims at reducing wood-burning emissions 50% by 1995. About 25% of Denver's particulates comes from wood burning. Already the city has declared wood-burning bans on days when pollution soars.

Ski areas have devised regulations as complicated as airline flight restrictions to drive away embarrassingly ugly air. Aspen, the resort frequented by many Californians, tried "to deal with a potential problem before it became a problem," says Tom Dunlop, director of that area's environmental health department.