# Counties in Massachusetts by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2025

All 13 values come from U.S. Environmental Protection Agency for 2025, so the places are comparable.

- Indicator id: `aqi_unhealthy_sensitive_or_worse_days_share`
- Unit: %
- Definition: Days with a daily Air Quality Index above 100 (unhealthy for sensitive groups, unhealthy, very unhealthy or hazardous) as a percentage of the days of the year with an AQI in a US county. Calculated by Invokely from the day counts in the EPA AirData annual summaries; the counts are stored with each value.
- Source: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts (https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual)
- Retrieved: 2026-10-12
- Citation: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
- Web page: https://invokely.ai/rankings/unhealthy-air-quality-days-share/massachusetts-counties

| Rank | Place | % |
|---:|---|---:|
| 1 | Hampden County | 1.4 |
| 2 | Berkshire County | 1.1 |
| 2 | Hampshire County | 1.1 |
| 4 | Barnstable County | 0.8 |
| 4 | Bristol County | 0.8 |
| 4 | Essex County | 0.8 |
| 4 | Franklin County | 0.8 |
| 8 | Dukes County | 0.6 |
| 9 | Norfolk County | 0.5 |
| 9 | Suffolk County | 0.5 |
| 11 | Middlesex County | 0.3 |
| 11 | Plymouth County | 0.3 |
| 13 | Worcester County | 0 |