Air Quality

To protect our health and the quality of the air we breathe, the US Environmental Protection Agency (EPA) sets limits on the amounts of certain pollutants that can safely be in our air. These limits are called the National Ambient Air Quality Standards. When areas exceed these standards, they must take steps to lower pollutant levels.

Areas that have exceeded any of health-based air quality standards are referred to as “nonattainment areas” (they have not “attained” the standard); areas that were nonattainment areas in the past, but have re-attained the standards, are called “maintenance areas,” because they must maintain compliance with the standards.

Northern Ada County violated the federal standard for two air pollutants – carbon monoxide and coarse particulate matter (airborne dust and other particles; referred to as “PM10”) in the 1980s and early 1990s and was a “maintenance area” for these pollutants until December 2022 (carbon monoxide) and November 2023 (PM10). As of the end of November 2023, Northern Ada County is no longer an air quality maintenance area.

Amendments to the Federal Clean Air Act mandate that any transportation project that uses federal funds and/or is deemed to be “regionally significant” in nonattainment or maintenance areas cannot contribute to the degradation of air quality – that is, they must “conform” to air quality plans to maintain compliance with the standards. The process to show that projects will not degrade air quality is called an air quality conformity demonstration. COMPASS conducted the required air quality conformity demonstration for Northern Ada County through 2023.

As the area is no longer a maintenance area, COMPASS will no longer conduct this analysis. However, previous analyses are still available on COMPASS’ air quality conformity webpage.