"The rule is if you can see or smell smoke, stay inside," Boulder County health specialist Chana Goussetis told the Daily Camera.
Yet many Denverites went about business as usual, bicycling to work and jogging along the many paths through the city.
"How bad could it really get?" Ben Paolillo asked as he sat at a downtown cafe, having coffee with his wife, Julie.
The ground-hugging smoke temporarily grounded the air attack on the High Park Fire, centered some 15 miles northwest of Fort Collins. Some helicopters took to the smoky skies by midday.
"We're going to get those aircraft up as soon as we can," said incident commander Bill Hahnenberg.
The fire killed a 62-year-old woman inside her mountain cabin and destroyed more than 100 structures. Larimer County authorities allowed some residents evacuated since Saturday to return home but warned dozens more to be prepared to leave their homes if fire lines approach.
In southern New Mexico, firefighters made progress on a 56-square-mile fire that forced hundreds of residents to evacuate near the mountain town of Ruidoso. Gov. Susana Martinez declared a state of emergency to free funding and firefighting resources. Ruidoso remains on alert for possible evacuation.
Wildfires in the drought-stricken West have tested federal resources.
U.S. Forest Service Chief Tom Tidwell announced late Monday the agency was contracting eight heavy air tankers to increase the aging national fleet to 17.
Still, Colo. U.S. Sen. Michael Bennet urged President Barack Obama to sign a bill that would allow the Forest Service to buy as many as seven large air tankers outright. The U.S. House and Senate passed the bill last week.
In Colorado's Larimer County, authorities and family said Linda Steadman, 62, perished inside her mountain cabin. Her home received two evacuation warnings that weren't answered, and a firefighter tried to reach the cabin before fire overtook the site, Sheriff Justin Smith said.
Across the West:
California • A wildfire that briefly threatened homes in Kern County is nearing containment.
Colorado • The 68-square-mile High Park Fire is 5 percent contained. As many as 800 firefighters are expected on the lines by Wednesday. Ten air tankers and 14 water dropping helicopters are attacking the blaze.
New Mexico • Nearly 1,000 firefighters and more than 200 National Guardsmen are battling the 56-square-mile Little Bear fire. Containment is 30 percent.
Utah •Two wildfires blackened 2,000 acres in Fishlake National Forest in southern Utah. A third fire believed to have been sparked by target shooting near Centerville, 15 miles north of Salt Lake City, was quickly contained late Monday.
Wyoming • A 4-square-mile blaze at Guernsey State Park is 80 percent contained. Six helicopters and 600 firefighters are deployed. Firefighters contained 95 percent of a 13-square-mile fire in Medicine Bow National Forest and completely contained a 1,700-acre fire in Weston County.