Hot temperatures and light winds are raising ozone levels along the Front Range.
The Denver area is under an Ozone Alert through 4 p.m. Tuesday.
Ozone pollution increases as fumes from gasoline and other chemicals combine in hot weather, without a breeze to clear the air.
It can be harmful to children and older people and it can be especially dangerous to people with existing breathing problems like asthma.
But Megan Alderton with the Regional Air Quality Council says healthy people can be affected too.
"Particularly for those who are working or exercising outdoors, they’re breathing more air in, and therefore are more likely to feel the effects," Alderton said.
Alderton says there have been fewer problem days this season compared to last year, but current conditions are right for creating ozone.
"When it’s real hot and there’s very little wind, those pollutants have more of an opportunity to combine with heat and sunlight and make ozone," she said.
Officials are asking people to drive less, take mass transit, and wait until cooler evening hours to re-fuel their cars and mow their lawns.