DENVER - Colorado is suffering through a summer of smog.

With temperatures topping 100 degrees this month in Denver and elsewhere along the populous Front Range, routine activities like filling up at the gas station or mowing the lawn are releasing fumes into a perfect cauldron for creating ozone, a major component of smog.

Activists are sounding the alarm. Government officials are keeping watch. Nobody's breathing easy.

Denver is on a pace to eclipse the ozone-choked summer of 2003, the worst in 20 years, when the state issued 42 ozone alerts warning of unhealthy air. As of Wednesday, this summer has had 34.

Christopher Dann, a spokesman for the Colorado health department, said it's hard to point to one factor causing this summer's increase.

"There's no magic bullet out there that anyone can see," Dann said. "We're not out there generating ozone in great amounts, we're emitting precursor pollutants that are chemically changed in the atmosphere into ozone .... As long as human beings burn stuff, we're going to have pollution problems."

Ozone is created when the sun bakes common pollutants such as engine exhaust, wildfire smoke and vapors from everything from paint cans to oil and gas wells.

It's a particularly vexing problem for Denver, which vanquished its "brown cloud" in 2002 only to get tagged by the federal government two years later for missing new ozone standards.

While rebounding with acceptable ozone levels in 2004 and last year, this year's spike puts the region in jeopardy of coming under federal restrictions if it can't rein in the problem.

For most people, heavy ozone is only irritating, but for people with respiratory difficulties - such as the more than 250,000 Coloradans with asthma - it can lead to intense breathing problems, said Arthur McFarlane, who works with the state health department's asthma program.

The federal Environmental Protection Agency measures ozone at monitoring stations across the Front Range, using a complicated formula that allows for the effects of unusually hot weather.

If all-day smog ratings at any monitoring station, averaged over a three-year period, exceed the federal limit, the EPA can impose restrictions on the state, including limits on federal highway spending.

This summer, some Denver-area monitoring stations are hovering around the maximum. One near Boulder is at the limit; one south of Littleton and another north of Denver are slightly above. Those high numbers may still come down, averaged with last year's ratings, but it leaves the state on shaky footing if next summer's numbers continue the trend.

A station installed this summer west of Fort Collins has far exceeded the standard. Those readings should "set off alarm bells" about human risk and the need for policy makers to take action, said Vickie Patton, an attorney with Environmental Defense.

"The big picture for the Colorado Front Range is a paradox where you have millions of people who are raising families here because they value clean air," she said.

Denver has until the end of 2007 to show ozone levels are in compliance. Other regions around the country, including Roanoke, Va., Greenville-Spartanburg S.C., Johnson City-Bristol, Tenn., Nashville, Tenn., and Frederick County, Md. are also facing the deadline.

In Denver's case, it is a big job, but achievable, experts say.

State health officials ask people to do little things, such as not spilling gasoline at the pump, waiting to refuel until after dusk, when the vapors have less time to cook into ozone, and trading in old gasoline-powered mowers for newer models.

Clean air advocates say there should also be changes in the growing oil and gas fields north of Denver.

"Oil and gas is kind of the last virtually unregulated source of air pollution in the state," said Jeremy Nichols, a volunteer for Rocky Mountain Clear Air Action.

Greg Schnacke, executive vice president of the Colorado Oil and Gas Association, said the industry spent $20 million in Colorado over the past few years to curb escaped gases. Old clunkers on the road are a bigger problem, he said, and forcing any industry to spend millions more on questionable solutions doesn't make sense.

Patton said regardless of the cause or the cures, something must be done.

"Smog pollution problems are no longer confined to urban Denver, but reach far across the Colorado Front Range," she said. "The reality is that we need a comprehensive solution."