Air Quality in Massachusetts

Massachusetts earns an average Air Quality Grade of B, with a 5-year median AQI of 41 across 13 monitored areas — right around 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. Massachusetts's 13 monitored areas collectively logged 103 days at "Unhealthy for Sensitive Groups" or worse over the last five years.

Massachusetts is bucking the national trend of broad improvement: 5 of 13 monitored areas are showing measurably worse air over the past decade, more than the 3 that are improving. Across the western U.S. that pattern usually traces back to expanding wildfire smoke exposure; elsewhere it can reflect rising local emissions from population or freight growth.

The dominant pollutant across 8 of 13 Massachusetts areas is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle and industrial emissions. It is worst on hot, sunny, stagnant summer days. Ozone irritates the lungs and triggers asthma — even healthy adults can feel it after exercising on high-ozone days. Other monitored areas in the state report Fine Particulate Matter (PM2.5) (5) as their dominant pollutant.

Within Massachusetts, the gap between best and worst is meaningful: Suffolk, Massachusetts tops the state with a Grade B and 5-year median AQI of 44, while Middlesex, Massachusetts sits at the bottom with a Grade C and 5-year median AQI of 39. Local terrain, prevailing winds, and proximity to industrial or wildfire emission sources drive most of that within-state variation.

Suffolk, Massachusetts is the fastest-improving area in Massachusetts, with median AQI falling by 1.4 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.