Air Quality in Maine

Maine earns an average Air Quality Grade of B, with a 5-year median AQI of 35 across 10 monitored areas — 6 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. Maine's 10 monitored areas collectively logged 33 days at "Unhealthy for Sensitive Groups" or worse over the last five years.

Maine is on a clear improving trajectory: 6 of 10 monitored areas are showing measurably cleaner air over the past decade, versus only 2 that are getting worse. That mirrors the broader national pattern of falling particulate and ozone pollution as cleaner vehicles, cleaner power generation, and tighter industrial standards take effect.

The dominant pollutant across 5 of 10 Maine 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 (5) as their dominant pollutant.

Within Maine, the gap between best and worst is meaningful: Androscoggin, Maine tops the state with a Grade B and 5-year median AQI of 34, while Washington, Maine sits at the bottom with a Grade B and 5-year median AQI of 34. Local terrain, prevailing winds, and proximity to industrial or wildfire emission sources drive most of that within-state variation.

Aroostook, Maine is the fastest-improving area in Maine, with median AQI falling by 1.0 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.