For more than a decade, residents of the Mile High City have had to make smog forecasts a part of their daily planning - in ways as simple as whether they stoke up the fireplace or take a car to work.
But as pressure to cut down on Denver's chronic pollution problem mounts, the city is making significant strides toward cleaning up its air. The number of "red" days - when residents are prohibited from burning wood and encouraged to take a bus or carpool - are declining, and for the second year in a row, Denver hasn't violated a single federal standard for air quality.
"It's a tremendous change from 20 years - and even 10 years - ago," says Theresa Donahue, manager of the city's department of Environmental Health. "We're very pleased about that."
Considering this city's pollution history, it's an impressive feat. In the 1970s, Denver air exceeded federal limits for carbon monoxide and particulates virtually every day during the high-pollution winter season. And as recently as a decade ago, there were violations of federal air-quality standards every week.
Denver's strides in air quality in fact underscore a nationwide trend. Thanks largely to tougher federal regulations and cleaner-burning automobiles, air quality across the country is improving dramatically, says Christine Sansavero, an air-quality policy analyst at the Environmental Protection Agency (EPA) in Durham, N.C. "In the United States, we've reduced emissions of air toxins by more than 1 billion pounds since 1990. Overall, the air is much cleaner in all metropolitan areas in the US. Even the areas that are not considered clean are cleaner than they were a decade ago."
Los Angeles is a prime example: While maintaining its notoriety for having the dirtiest air in the country, this past year the City of Angels had the smallest number of ozone violations and health advisories in decades.
Automotive technology deserves its share of credit for reducing exhaust emissions throughout the nation. On average, vehicles today burn 95 percent cleaner than they did in the 1970s, when the EPA started enforcing the Clean Air Act. And 1990 amendments to the act, which tightened emissions standards for industry, notched a deeper dent into the quantity of air pollutants released.
But Colorado officials figured out a decade ago that it would take more than advancements from Detroit and federal regulations to clear the high-altitude smog in Denver.
In addition, combating the notorious "brown cloud" that shrouds the city - and tinges the snow-capped Rockies with a shade resembling cafe latte - was deemed as much a matter of economics as health. If Colorado was to continue to lure businesses and tourism to the state's natural splendor, the mountains had better be visible. And the air had better not be.
To that end, Colorado launched an aggressive air-quality improvement campaign in the mid-1980s, and hasn't let up since. Throughout the winter high-pollution season, which spans from Nov. 1 until March 31, all gasoline sold here must be "oxygenated" - meaning it contains additives to reduce carbon-monoxide emissions. The region also has one of the strictest auto-emissions testing programs in the country.
During the winter season, Denver also issues a daily pollution advisory for the area, declaring whether the day is "red" or "blue." (On blue days, there are no restrictions on resident activities.)
"Denver has a tremendous success story to tell in relation to federal standards," reports Patrick Cummins, deputy director of the Regional Air Quality Council, an agency established by the state in 1989. "We are in compliance with all federal standards, and we anticipate being able to stay in compliance."
Still, Denver's brown cloud remains a nagging problem. Even as residents breathe fewer toxins, they continue to view a city skyline colored to some degree by fine particles of dirt and soot. As often as not, the air in downtown Denver violates the state's own visibility standard.
"The state standard for visibility was violated every other day this winter - 73 times out of 152 days," notes Mr. Cummins. "Clearly, there are air-quality challenges remaining in this region."
And now, with the EPA setting about to regulate the fine particulates that contribute most to apparent smog, it is uncertain whether Denver will meet the new standards, he adds. "We need to get more data before we know where we stand. We're fairly confident we'll be under the line, but we think we'll be close to the line."
Yet another worry is whether Denver will violate the EPA's new regional-haze rule, a regulation targeting the aesthetics of air quality, which could become effective later this year.
And brown air isn't just unsightly, either. "Air that you can see is not healthy. If it's visible, it's a problem," maintains Jim Martin, a lawyer with the Environmental Defense Fund in Boulder, Colo. He says that fine particulates are linked to respiratory problems.
But a bill in the state legislature could lead to a sizable reduction in smog here. Under legislation likely to become law, utility companies in Colorado will voluntarily embark on a $211 million program to reduce emissions from their coal-burning plants. The proposed alterations alone are expected to reduce sulfur-dioxide emissions by 70 percent and noticeably improve visibility.
Yet experts say that, ultimately, any long-term plan for clean air must address residents' ongoing love affair with the automobile.
A particular concern is that light trucks and sport utility vehicles are especially popular here in snow country - and they emit far more pollutants than passenger cars do. Also, mass transit around metropolitan Denver is limited to buses, which the majority of people tend to eschew. And as population growth in the region shows no sign of abating, vehicle trips are expected to rise steadily over the next decade.
"The number of vehicle trips is projected to double by the year 2010," says Mark Komp, an environmental scientist with the EPA air program here in Denver. "We anticipate that at some time, the gains we've made [in air quality] will be overcome by the increase in trips people take."
Mr. Martin agrees. "If you're going to get better, cleaner air, you've got to deal with cars. You have to start looking at getting people out of their cars - which is pretty hard to do."