State health officials are warning of bad air quality in southwestern Colorado on Thursday after a wildfire burning in the San Juan National Forest grew to nearly 5,000 acres.

The lightning-sparked Spruce Creek fire ignited on May 14 on U.S. Forest Service land about 21 miles north of Mesa Verde, fire officials said.

As of Wednesday, the fire was burning on 1,867 acres, according to fire records. The burn area more than doubled overnight, flames jumping to reach 4,962 acres of San Juan National Forest land as lower humidity and warmer temperatures worsened fire conditions.

Heavy smoke choked many areas in southwestern Colorado on Thursday morning, especially along Colorado 145c south of Rico to Dolores, according to a Thursday news release from the Colorado Department of Public Health and Environment.

The agency placed San Juan, Ouray, Hinsdale, Mineral, eastern San Miguel, eastern Dolores, northern La Plata and north-central Montezuma counties under an air quality health advisory for wildfire smoke.

Conditions may be the worst in — but are not limited to — Ouray, Telluride, Silverton, Lake City, Creede, Rico and Dolores throughout the day Thursday, officials said in the news release.

In areas with thick or thickening smoke, residents should remain indoors — especially people with heart disease or respiratory illnesses, as well as the very young and older adults, the officials said. People should limit activity outside when smoke is present and consider temporarily relocating.

If the smoke lessens visibility to five miles or less, the air quality has reached unhealthy levels, state officials said.

The smoke is expected to lift throughout the day as daytime mixing of the atmosphere deepens and winds increase, according to the news release. Winds out of the west or southwest will transport smoke to areas to the east or northeast of the fire during the afternoon Thursday.

The greatest impacts from smoke are expected overnight Thursday and early Friday morning as smoke settles throughout the advisory area, especially in lower terrain and sheltered mountain valleys, officials said.

More than 170 firefighters and workers were on-scene at the Spruce Creek fire Wednesday, but no evacuations had been issued.

The crews plan to use aerial ignition teams and hand crews to secure fire lines along natural features and existing roads in order to contain the flames, fire officials said.

In a Wednesday update, fire officials said crews made significant progress securing the northern and western edges of the fire, but it’s unclear how the overnight spread affected these boundaries.