Counties in Massachusetts by median Air Quality Index (US EPA)
Hampden County had the highest median Air Quality Index (US EPA) of 13 counties in 2021, at 46. Hampshire County had the lowest, at 35. The median was 39. Source: the U.S. Environmental Protection Agency.
Map of counties, 2021
- Under 36
- 36 to 38
- 38 to 40
- 40 to 44
- 44 and over
- No data
All counties, 2021
Full ranking
| Rank | Counties | 2021 | Change on 2020 |
|---|---|---|---|
| 1 | Hampden County | 46 | +2 |
| 2 | Worcester County | 45 | +2 |
| 3 | Berkshire County | 44 | +4 |
| 3 | Franklin County | 44 | +1 |
| 3 | Suffolk County | 44 | -1 |
| 6 | Bristol County | 39 | -3 |
| 6 | Plymouth County | 39 | -3 |
| 8 | Essex County | 38 | -2 |
| 9 | Middlesex County | 37 | -2 |
| 9 | Norfolk County | 37 | +3 |
| 11 | Barnstable County | 36 | 0 |
| 11 | Dukes County | 36 | -1 |
| 13 | Hampshire County | 35 | -4 |
Other periods
About this ranking
Median of the daily Air Quality Index (AQI) values of a US county over the days of the year with an AQI, from the EPA AirData annual summaries.
More on how it is measured
- The daily county AQI is the highest AQI among the monitors and pollutants reported that day (ozone, PM2.5, PM10, carbon monoxide and nitrogen dioxide).
- Lower is cleaner: 0 to 50 is good, 51 to 100 moderate and above 100 unhealthy for sensitive groups.
United States counties with air quality monitors only, about 1,000 of some 3,100 in recent years; a county without monitors has no value. The number of days with an AQI differs between counties because some monitors run only in the ozone season.
Every value in the ranking comes from the same publisher and the same period, so the counties are compared on one definition. Equal values share a rank.
How to cite
U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.Citation styles
APA 7
U.S. Environmental Protection Agency. (2026). EPA AirData annual AQI by county: median daily AQI (Counties in Massachusetts by median Air Quality Index (US EPA), 2021) [Data set]. Retrieved October 12, 2026, from https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual
MLA 9
EPA AirData annual AQI by county: median daily AQI, Counties in Massachusetts by median Air Quality Index (US EPA), 2021. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/median-air-quality-index/massachusetts-counties?period=2021. Accessed 12 Oct. 2026.
Chicago (author-date)
U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: median daily AQI. Counties in Massachusetts by median Air Quality Index (US EPA), 2021. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/median-air-quality-index/massachusetts-counties?period=2021.
Harvard
U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: median daily AQI, Counties in Massachusetts by median Air Quality Index (US EPA), 2021 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/median-air-quality-index/massachusetts-counties?period=2021 (Accessed: 12 October 2026).
BibTeX
@misc{environmental_counties_in_massachusetts_by_2021,
author = {{U.S. Environmental Protection Agency}},
title = {{EPA AirData annual AQI by county: median daily AQI: Counties in Massachusetts by median Air Quality Index (US EPA), 2021}},
year = {2026},
howpublished = {Data set},
url = {https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual},
urldate = {2026-10-12},
note = {Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/massachusetts-counties?period=2021. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download\_files.html\#Annual. Retrieved 2026-10-12.},
}
RIS
TY - DATA
AU - U.S. Environmental Protection Agency
TI - EPA AirData annual AQI by county: median daily AQI: Counties in Massachusetts by median Air Quality Index (US EPA), 2021
PY - 2026
DA - 2026/07/13
PB - U.S. Environmental Protection Agency
DB - Invokely
UR - https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual
L2 - https://invokely.ai/rankings/median-air-quality-index/massachusetts-counties?period=2021
Y2 - 2026/10/12
N1 - Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/massachusetts-counties?period=2021.
N1 - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
ER -
Use this data in your AI agent
Invokely serves this figure to AI assistants through MCP and to code through a REST API, with the source and citation attached. Ask your assistant:
Rank Counties in Massachusetts by median Air Quality Index (US EPA) using official data, and cite the source.
rank_places {"indicator":"aqi_median","level":"province","within":"Massachusetts","limit":10}
GET https://invokely.ai/api/v1/rank?indicator=aqi_median&level=province&within=Massachusetts&limit=10Questions
Which county has the highest median Air Quality Index (US EPA)?
Hampden County, with 46 in 2021, according to the U.S. Environmental Protection Agency.
Which county has the lowest median Air Quality Index (US EPA)?
Hampshire County, with 35 in 2021.
Where does this ranking come from?
Every value comes from one source and one period, so the places are comparable: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12.