Data / Median Air Quality Index (US EPA) / Middlesex County

Median Air Quality Index (US EPA) in Middlesex County

Median Air Quality Index (US EPA) in Middlesex County was 38 in 2025, according to the U.S. Environmental Protection Agency. That is up 1 from 2024. That is the 5th highest of 13 counties in 2025.

2025, index38
Change on 2024+1
Rank5th of 13Counties in Massachusetts ranked
Series high501983
Series low142004

Next release: Tuesday, 13 July 2027, estimated by Invokely from past publication dates (low confidence), not announced by the publisher. Release calendar

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Median Air Quality Index (US EPA) in Middlesex County, 1980 to 2025

102030405019801987199420012008201520251980: 41 index1981: 41 index1982: 44 index1983: 50 index1984: 44 index1985: 45 index1986: 42 index1987: 36 index1988: 35 index1989: 33 index1990: 32 index1991: 34 index1992: 32 index1993: 33 index1994: 35 index1995: 31 index1996: 35 index1997: 31 index1998: 37 index1999: 35 index2000: 26 index2001: 37 index2002: 32 index2003: 19 index2004: 14 index2005: 31 index2006: 33 index2007: 34 index2008: 35 index2009: 33 index2010: 35 index2011: 34 index2012: 34 index2013: 34 index2014: 32 index2015: 33 index2016: 33 index2017: 33 index2018: 38 index2019: 41 index2020: 39 index2021: 37 index2022: 38 index2023: 39 index2024: 37 index2025: 38 index102030405019801996201220251980: 41 index1981: 41 index1982: 44 index1983: 50 index1984: 44 index1985: 45 index1986: 42 index1987: 36 index1988: 35 index1989: 33 index1990: 32 index1991: 34 index1992: 32 index1993: 33 index1994: 35 index1995: 31 index1996: 35 index1997: 31 index1998: 37 index1999: 35 index2000: 26 index2001: 37 index2002: 32 index2003: 19 index2004: 14 index2005: 31 index2006: 33 index2007: 34 index2008: 35 index2009: 33 index2010: 35 index2011: 34 index2012: 34 index2013: 34 index2014: 32 index2015: 33 index2016: 33 index2017: 33 index2018: 38 index2019: 41 index2020: 39 index2021: 37 index2022: 38 index2023: 39 index2024: 37 index2025: 38 index
index. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12, published 2026-07-13.
All 46 values
Periodindex
202538
202437
202339
202238
202137
202039
201941
201838
201733
201633
201533
201432
201334
201234
201134
201035
200933
200835
200734
200633
200531
200414
200319
200232
200137
200026
199935
199837
199731
199635
199531
199435
199333
199232
199134
199032
198933
198835
198736
198642
198545
198444
198350
198244
198141
198041

Middlesex County compared with nearby places

Barnstable County: 36 indexBerkshire County: 38 indexBristol County: 38 indexDukes County: 35 indexEssex County: 40 indexFranklin County: 38 indexHampden County: 39 indexHampshire County: 36 indexMiddlesex County: 38 indexNorfolk County: 39 indexPlymouth County: 37 indexSuffolk County: 37 indexWorcester County: 39 index
  • Under 36
  • 36 to 38
  • 38 to 39
  • 39 and over
  • No data
Median Air Quality Index (US EPA), 2025, index. Middlesex County is outlined. Same source and period for every area shown. Source: U.S. Environmental Protection Agency. Boundaries: U.S. Census Bureau cartographic boundary files.

Rankings

Definition

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.

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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

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: median daily AQI (Median Air Quality Index (US EPA), Middlesex County, 1980 to 2025) [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, Median Air Quality Index (US EPA), Middlesex County, 1980 to 2025. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/en/median-air-quality-index/middlesex-county-massachusetts. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: median daily AQI. Median Air Quality Index (US EPA), Middlesex County, 1980 to 2025. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/en/median-air-quality-index/middlesex-county-massachusetts.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: median daily AQI, Median Air Quality Index (US EPA), Middlesex County, 1980 to 2025 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/en/median-air-quality-index/middlesex-county-massachusetts (Accessed: 12 October 2026).

BibTeX
@misc{environmental_median_air_quality_index_2025,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: median daily AQI: Median Air Quality Index (US EPA), Middlesex County, 1980 to 2025}},
  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/en/median-air-quality-index/middlesex-county-massachusetts. 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: Median Air Quality Index (US EPA), Middlesex County, 1980 to 2025
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/en/median-air-quality-index/middlesex-county-massachusetts
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/en/median-air-quality-index/middlesex-county-massachusetts.
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  -
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Questions

What is the median Air Quality Index (US EPA) in Middlesex County?

Median Air Quality Index (US EPA) in Middlesex County was 38 in 2025, according to the U.S. Environmental Protection Agency. That is up 1 from 2024. That is the 5th highest of 13 counties in 2025.

Where does this figure come from?

U.S. Environmental Protection Agency. EPA AirData annual AQI by county: median daily AQI. Retrieved by Invokely on 2026-10-12.

What was the highest median Air Quality Index (US EPA) in Middlesex County?

50 in 1983, the highest value since 1980 in this series.

Related statistics for Middlesex County

All statistics for Middlesex County | Middlesex County in figures