Health officials in northern Arizona are urging people who smell or see smoke from the fire burning in Oak Creek Canyon south of Flagstaff to curtail their outdoor activity.
The Slide Fire has been burning since Tuesday, and its smoke is present in a wide area, including Flagstaff.
Health officials say small particles are the primary health concern from wildfire smoke.
Guidance provided by health officials includes general advice to curtail outdoor activity, but older people, children and people with heart disease, lung disease or asthma should avoid the outdoors entirely.
Closing windows and turning off evaporative coolers when smoke is present helps to reduce exposure to the smoke.
Health officials say those steps should be adequate to protect people without respiratory problems.