Ada County/Treasure Valley Air Quality Plans & Reports

Much of the focus on air quality in the Boise Region has centered on the Treasure Valley, which is the largest and most highly populated urban area in Idaho. The valley is home to about one-third of the state's population and a significant percentage of the state's industry and business.

The majority of the valley's population—and the emission sources—are concentrated in Ada and Canyon Counties. Although air pollution is generated in Elmore County as well, the county's location and distance from other populated areas in the valley prevent significant exchange of air pollutants. Remaining counties in the valley are sparsely populated with few emission sources. Emission sources in eastern Oregon also are believed to contribute to air pollution in the Treasure Valley.

Due to topography and weather patterns, the Treasure Valley is subject to some of the most severe wintertime inversions in the intermountain West. During an inversion, colder, heavier air settles into the valley while warm air sits above the inversion. This causes air stagnation as the cold air and accumulating air pollution is trapped. The pollution builds up under the inversion until a strong weather system moves through and mixes the air. During these events, air pollution monitors in the valley have recorded levels above the national health-based standard for both fine particulate matter (PM2.5) and coarse particulate matter (PM10).

The valley experiences air pollution problems in the summer months as well, as stagnant air conditions, heat, and intense sunlight combine to produce unhealthful accumulations of ozone pollution. Monitoring has shown increased levels of ozone in the valley, sometimes to unhealthful levels, during the past several summers.

Air quality improvement plans have been implemented to address carbon monoxide (CO) and PM10 pollution. Pollutants of concern today are PM2.5 in the winter and ozone (O3) in the summer.

Department of
Environmental Quality

Find it Fast