Data / Rankings / Share of days with good air quality (US EPA AQI) / Counties in Massachusetts

Counties in Massachusetts by share of days with good air quality (US EPA AQI)

Middlesex County had the highest share of days with good air quality (US EPA AQI) of 13 counties in 2015, at 95.3%. Suffolk County had the lowest, at 38.6%. The median was 71.2%. Source: EPA.

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Highest95.3Middlesex County
Lowest38.6Suffolk County
Median71.213 counties

Map of counties, 2015

Barnstable County: 91.2 %Berkshire County: 59.7 %Bristol County: 71.2 %Dukes County: 90.4 %Essex County: 69.9 %Franklin County: 63.8 %Hampden County: 63.3 %Hampshire County: 81.7 %Middlesex County: 95.3 %Norfolk County: 78.6 %Plymouth County: 67.8 %Suffolk County: 38.6 %Worcester County: 72.9 %
  • Under 64
  • 64 to 69
  • 69 to 74
  • 74 to 87
  • 87 and over
  • No data
%, 2015. Hatched areas have no value for this period. Source: Invokely calculation with data from EPA. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 2015

Middlesex County: 95.3 %Middlesex County95.3Barnstable County: 91.2 %Barnstable County91.2Dukes County: 90.4 %Dukes County90.4Hampshire County: 81.7 %Hampshire County81.7Norfolk County: 78.6 %Norfolk County78.6Worcester County: 72.9 %Worcester County72.9Bristol County: 71.2 %Bristol County71.2Essex County: 69.9 %Essex County69.9Plymouth County: 67.8 %Plymouth County67.8Franklin County: 63.8 %Franklin County63.8Hampden County: 63.3 %Hampden County63.3Berkshire County: 59.7 %Berkshire County59.7Suffolk County: 38.6 %Suffolk County38.6Middlesex County: 95.3 %Middlesex County95.3Barnstable County: 91.2 %Barnstable County91.2Dukes County: 90.4 %Dukes County90.4Hampshire County: 81.7 %Hampshire County81.7Norfolk County: 78.6 %Norfolk County78.6Worcester County: 72.9 %Worcester County72.9Bristol County: 71.2 %Bristol County71.2Essex County: 69.9 %Essex County69.9Plymouth County: 67.8 %Plymouth County67.8Franklin County: 63.8 %Franklin County63.8Hampden County: 63.3 %Hampden County63.3Berkshire County: 59.7 %Berkshire County59.7Suffolk County: 38.6 %Suffolk County38.6
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2015 (%)Change on 2014
1Middlesex County95.3+0.6 pts
2Barnstable County91.2-4.4 pts
3Dukes County90.4-3.2 pts
4Hampshire County81.7+19.2 pts
5Norfolk County78.6+0.2 pts
6Worcester County72.9+6.6 pts
7Bristol County71.2+4.6 pts
8Essex County69.9+3.1 pts
9Plymouth County67.8+4.7 pts
10Franklin County63.8+5.2 pts
11Hampden County63.3+4.9 pts
12Berkshire County59.7+14.6 pts
13Suffolk County38.6+5.2 pts

Other periods

About this ranking

Days with a daily Air Quality Index of 0 to 50 (good) 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; both counts are stored with each value.

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: good days 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 styles

The publisher is the author. Invokely is named only as the place the figures were read, where a style asks for it.

APA 7

U.S. Environmental Protection Agency. (2026). EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts (Counties in Massachusetts by share of days with good air quality (US EPA AQI), 2015) [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: good days as a share of days with an AQI, calculated with stored counts, Counties in Massachusetts by share of days with good air quality (US EPA AQI), 2015. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/good-air-quality-days-share/massachusetts-counties?period=2015. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. Counties in Massachusetts by share of days with good air quality (US EPA AQI), 2015. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/good-air-quality-days-share/massachusetts-counties?period=2015.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts, Counties in Massachusetts by share of days with good air quality (US EPA AQI), 2015 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/good-air-quality-days-share/massachusetts-counties?period=2015 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_massachusetts_by_2015,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts: Counties in Massachusetts by share of days with good air quality (US EPA AQI), 2015}},
  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/good-air-quality-days-share/massachusetts-counties?period=2015. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days 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.},
}
RIS
TY  - DATA
AU  - U.S. Environmental Protection Agency
TI  - EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts: Counties in Massachusetts by share of days with good air quality (US EPA AQI), 2015
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/good-air-quality-days-share/massachusetts-counties?period=2015
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/massachusetts-counties?period=2015.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days 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.
ER  -
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Questions

Which county has the highest share of days with good air quality (US EPA AQI)?

Middlesex County, with 95.3% in 2015, according to EPA.

Which county has the lowest share of days with good air quality (US EPA AQI)?

Suffolk County, with 38.6% in 2015.

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: good days as a share of days with an AQI, calculated with stored counts. Retrieved 2026-10-12.

Share of days with good air quality (US EPA AQI): other rankings

Counties in Massachusetts: more rankings