Air Quality in Washington

Washington earns an average Air Quality Grade of B, with a 5-year median AQI of 31 across 30 monitored areas — 10 points below the national average of 41. The grade combines four signals — 5-year median AQI, 10-year trend direction, count of unhealthy days per year, and dominant pollutant — into a single A-F score. Washington's 30 monitored areas collectively logged 727 days at "Unhealthy for Sensitive Groups" or worse over the last five years.

Air quality in Washington has held roughly steady over the past decade — 16 areas improving, 11 worsening, and 3 stable. That stability makes the state-average grade a reliable signal of what residents can expect.

The dominant pollutant across 29 of 30 Washington areas is Fine Particulate Matter (PM2.5). PM2.5 (fine particulate matter) is most often driven by combustion sources — vehicle exhaust, industrial emissions, residential wood burning, and increasingly wildfire smoke. It penetrates deep into lung tissue and the bloodstream and is the air pollutant most strongly linked to long-term health impacts. Other monitored areas in the state report Ground-Level Ozone (1) as their dominant pollutant.

Within Washington, the gap between best and worst is meaningful: Garfield, Washington tops the state with a Grade A and 5-year median AQI of 20, while Asotin, Washington sits at the bottom with a Grade D and 5-year median AQI of 43. Local terrain, prevailing winds, and proximity to industrial or wildfire emission sources drive most of that within-state variation.

Garfield, Washington is the fastest-improving area in Washington, with median AQI falling by 1.3 points per year over the EPA reporting period. Steady improvement at that pace usually reflects fleet turnover (older diesels retiring), upwind power-plant retirements, and tighter local emissions controls.