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Air quality in Massachusetts

EPA monitors report from 13 of Massachusetts’s 14 counties. That is the honest limit of this page: the other 1 are not measured, not clean. In 2025 the most affected monitored county was Hampden County, with 5 days above the moderate category.

13
Counties with monitors
1
Counties without
33
Days above moderate, 2025
Summed across monitored counties
11.6%
Adults with asthma
Population-weighted county estimate

County by county

Monitored Massachusetts counties

GoodModerateUnhealthy for sensitive groupsUnhealthyVery unhealthyHazardous
County90th %ile AQIAbove moderateMax AQI2025 days by category
Hampden County615124
Berkshire County594124
Hampshire County544117
Barnstable County463129
Bristol County563119
Essex County563136
Franklin County593114
Dukes County452151
Norfolk County552126
Suffolk County602106
Middlesex County561105
Plymouth County541108
Worcester County560100

EPA Air Quality System annual summaries by county, 2025. The 90th percentile AQI describes a day worse than nine out of ten, which is a steadier measure than the annual maximum a single wildfire can set.

Not measured

1 counties without a monitor

No EPA monitor reported from these counties, so there is no air quality record to publish. Their radon zone is known, and radon is measured in the building rather than the air outside it.

Other states

Air quality elsewhere