Idaho’s air has been unhealthy during much of the summer and fall due to wildfire smoke.

The 2024 wildfire season, July 1-Sept. 30, brought more days with air quality ratings in the “unhealthy” and “very unhealthy” categories than in the previous five years, said Mary Anderson, Air Quality Planning Bureau chief at the state Department of Environmental Quality.

Days with poor air quality “were due to wildfire smoke, with a few of the poor days being due to ozone in Boise on hot days,” she said.

“Overall, wildfire smoke has the greatest impact on air quality and public health, as other sources of emissions are fairly consistent year to year,” Anderson said.

The daily air quality index is calculated for five of the six major air pollutants that the Clean Air Act regulates: ground-level ozone, particulates, carbon monoxide and sulfur and nitrogen dioxides.

Daily air quality can be rated green for “good” and yellow for “moderate.” An orange rating means the day’s air quality is “unhealthy for sensitive groups.” Red indicates air quality is “unhealthy,” and purple and maroon indicate it is “very unhealthy” and “hazardous,” respectively.

In the wildfire season, 57% of the days had at least one area of the state that experienced unhealthy air compared to 1% in 2019, Anderson said.

DEQ monitors about 35 sites. The state’s southwest region saw the most impacts from smoke this year due to fires in southeast Oregon and west-central Idaho, she said.