EnviroNearby

Middlesex County, Massachusetts air quality history

Daily Air Quality Index summaries from EPA monitors in Middlesex, Massachusetts, 1980–2025.

82%
Good days in 2025
38
Median daily AQI, 2025
1
Days above 100 in 2025
365
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Middlesex, Massachusetts on 365 days. 299 of those days (82%) were in the Good range, and the median daily AQI was 38. That median is about the same as the typical monitored county in Massachusetts (median 38 across 13 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was ozone on 230 days. Across the years with fuller monitoring, the share of Good days rose: an average of 68% in 1980–1982 versus 82% in 2023–2025.

How the days broke down each year

Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.

Share of monitored days in each AQI category, by year0%25%50%75%100%1980: Good 236 of 340 days1980: Moderate 65 of 340 days1980: Unhealthy for Sensitive Groups 19 of 340 days1980: Unhealthy 15 of 340 days1980: Very Unhealthy 5 of 340 days19801981: Good 260 of 354 days1981: Moderate 79 of 354 days1981: Unhealthy for Sensitive Groups 10 of 354 days1981: Unhealthy 5 of 354 days1982: Good 170 of 280 days1982: Moderate 79 of 280 days1982: Unhealthy for Sensitive Groups 27 of 280 days1982: Unhealthy 3 of 280 days1982: Very Unhealthy 1 of 280 days1983: Good 167 of 332 days1983: Moderate 114 of 332 days1983: Unhealthy for Sensitive Groups 31 of 332 days1983: Unhealthy 9 of 332 days1983: Very Unhealthy 11 of 332 days1984: Good 234 of 366 days1984: Moderate 97 of 366 days1984: Unhealthy for Sensitive Groups 19 of 366 days1984: Unhealthy 13 of 366 days1984: Very Unhealthy 3 of 366 days1985: Good 218 of 339 days1985: Moderate 99 of 339 days1985: Unhealthy for Sensitive Groups 17 of 339 days1985: Unhealthy 3 of 339 days1985: Very Unhealthy 2 of 339 days19851986: Good 203 of 261 days1986: Moderate 53 of 261 days1986: Unhealthy for Sensitive Groups 5 of 261 days1987: Good 167 of 212 days1987: Moderate 27 of 212 days1987: Unhealthy for Sensitive Groups 12 of 212 days1987: Unhealthy 6 of 212 days1988: Good 282 of 354 days1988: Moderate 33 of 354 days1988: Unhealthy for Sensitive Groups 14 of 354 days1988: Unhealthy 12 of 354 days1988: Very Unhealthy 13 of 354 days1989: Good 296 of 363 days1989: Moderate 49 of 363 days1989: Unhealthy for Sensitive Groups 12 of 363 days1989: Unhealthy 4 of 363 days1989: Very Unhealthy 2 of 363 days1990: Good 294 of 355 days1990: Moderate 41 of 355 days1990: Unhealthy for Sensitive Groups 15 of 355 days1990: Unhealthy 4 of 355 days1990: Very Unhealthy 1 of 355 days19901991: Good 299 of 365 days1991: Moderate 41 of 365 days1991: Unhealthy for Sensitive Groups 12 of 365 days1991: Unhealthy 10 of 365 days1991: Very Unhealthy 3 of 365 days1992: Good 305 of 366 days1992: Moderate 39 of 366 days1992: Unhealthy for Sensitive Groups 16 of 366 days1992: Unhealthy 6 of 366 days1993: Good 304 of 365 days1993: Moderate 39 of 365 days1993: Unhealthy for Sensitive Groups 17 of 365 days1993: Unhealthy 4 of 365 days1993: Very Unhealthy 1 of 365 days1994: Good 293 of 365 days1994: Moderate 54 of 365 days1994: Unhealthy for Sensitive Groups 11 of 365 days1994: Unhealthy 6 of 365 days1994: Very Unhealthy 1 of 365 days1995: Good 306 of 365 days1995: Moderate 34 of 365 days1995: Unhealthy for Sensitive Groups 17 of 365 days1995: Unhealthy 7 of 365 days1995: Very Unhealthy 1 of 365 days19951996: Good 292 of 366 days1996: Moderate 58 of 366 days1996: Unhealthy for Sensitive Groups 14 of 366 days1996: Unhealthy 2 of 366 days1997: Good 282 of 353 days1997: Moderate 51 of 353 days1997: Unhealthy for Sensitive Groups 13 of 353 days1997: Unhealthy 6 of 353 days1997: Very Unhealthy 1 of 353 days1998: Good 259 of 341 days1998: Moderate 57 of 341 days1998: Unhealthy for Sensitive Groups 19 of 341 days1998: Unhealthy 4 of 341 days1998: Very Unhealthy 2 of 341 days1999: Good 270 of 358 days1999: Moderate 63 of 358 days1999: Unhealthy for Sensitive Groups 17 of 358 days1999: Unhealthy 8 of 358 days2000: Good 297 of 366 days2000: Moderate 61 of 366 days2000: Unhealthy for Sensitive Groups 7 of 366 days2000: Unhealthy 1 of 366 days20002001: Good 265 of 365 days2001: Moderate 76 of 365 days2001: Unhealthy for Sensitive Groups 14 of 365 days2001: Unhealthy 10 of 365 days2002: Good 274 of 365 days2002: Moderate 69 of 365 days2002: Unhealthy for Sensitive Groups 13 of 365 days2002: Unhealthy 6 of 365 days2002: Very Unhealthy 3 of 365 days2003: Good 338 of 365 days2003: Moderate 23 of 365 days2003: Unhealthy for Sensitive Groups 4 of 365 days2004: Good 337 of 366 days2004: Moderate 26 of 366 days2004: Unhealthy for Sensitive Groups 3 of 366 days2005: Good 307 of 365 days2005: Moderate 46 of 365 days2005: Unhealthy for Sensitive Groups 9 of 365 days2005: Unhealthy 3 of 365 days20052006: Good 298 of 365 days2006: Moderate 58 of 365 days2006: Unhealthy for Sensitive Groups 9 of 365 days2007: Good 293 of 365 days2007: Moderate 53 of 365 days2007: Unhealthy for Sensitive Groups 14 of 365 days2007: Unhealthy 5 of 365 days2008: Good 310 of 366 days2008: Moderate 50 of 366 days2008: Unhealthy for Sensitive Groups 5 of 366 days2008: Unhealthy 1 of 366 days2009: Good 323 of 365 days2009: Moderate 38 of 365 days2009: Unhealthy for Sensitive Groups 4 of 365 days2010: Good 313 of 365 days2010: Moderate 50 of 365 days2010: Unhealthy for Sensitive Groups 2 of 365 days20102011: Good 324 of 363 days2011: Moderate 37 of 363 days2011: Unhealthy for Sensitive Groups 1 of 363 days2011: Unhealthy 1 of 363 days2012: Good 324 of 365 days2012: Moderate 37 of 365 days2012: Unhealthy for Sensitive Groups 3 of 365 days2012: Unhealthy 1 of 365 days2013: Good 335 of 358 days2013: Moderate 21 of 358 days2013: Unhealthy for Sensitive Groups 2 of 358 days2014: Good 338 of 357 days2014: Moderate 19 of 357 days2015: Good 345 of 362 days2015: Moderate 16 of 362 days2015: Unhealthy for Sensitive Groups 1 of 362 days20152016: Good 341 of 362 days2016: Moderate 21 of 362 days2017: Good 341 of 356 days2017: Moderate 14 of 356 days2017: Unhealthy for Sensitive Groups 1 of 356 days2018: Good 308 of 365 days2018: Moderate 57 of 365 days2019: Good 289 of 364 days2019: Moderate 75 of 364 days2020: Good 306 of 366 days2020: Moderate 60 of 366 days20202021: Good 299 of 365 days2021: Moderate 64 of 365 days2021: Unhealthy for Sensitive Groups 1 of 365 days2021: Unhealthy 1 of 365 days2022: Good 310 of 365 days2022: Moderate 55 of 365 days2023: Good 281 of 365 days2023: Moderate 83 of 365 days2023: Unhealthy for Sensitive Groups 1 of 365 days2024: Good 314 of 366 days2024: Moderate 52 of 366 days2025: Good 299 of 365 days2025: Moderate 65 of 365 days2025: Unhealthy for Sensitive Groups 1 of 365 days2025
  • Good
  • Moderate
  • Unhealthy for Sensitive Groups
  • Unhealthy
  • Very Unhealthy
  • Hazardous

Typical and high-end AQI over time

The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.

Median and 90th-percentile daily AQI, by year0501001501980: 90th percentile AQI 1161981: 90th percentile AQI 801982: 90th percentile AQI 1031983: 90th percentile AQI 1221984: 90th percentile AQI 1001985: 90th percentile AQI 901986: 90th percentile AQI 591987: 90th percentile AQI 901988: 90th percentile AQI 1261989: 90th percentile AQI 801990: 90th percentile AQI 771991: 90th percentile AQI 801992: 90th percentile AQI 801993: 90th percentile AQI 711994: 90th percentile AQI 801995: 90th percentile AQI 801996: 90th percentile AQI 741997: 90th percentile AQI 771998: 90th percentile AQI 901999: 90th percentile AQI 802000: 90th percentile AQI 622001: 90th percentile AQI 802002: 90th percentile AQI 802003: 90th percentile AQI 462004: 90th percentile AQI 462005: 90th percentile AQI 672006: 90th percentile AQI 642007: 90th percentile AQI 742008: 90th percentile AQI 582009: 90th percentile AQI 522010: 90th percentile AQI 612011: 90th percentile AQI 512012: 90th percentile AQI 512013: 90th percentile AQI 482014: 90th percentile AQI 462015: 90th percentile AQI 462016: 90th percentile AQI 462017: 90th percentile AQI 442018: 90th percentile AQI 542019: 90th percentile AQI 542020: 90th percentile AQI 542021: 90th percentile AQI 552022: 90th percentile AQI 542023: 90th percentile AQI 572024: 90th percentile AQI 542025: 90th percentile AQI 561980: Median AQI 411981: Median AQI 411982: Median AQI 441983: Median AQI 501984: Median AQI 441985: Median AQI 451986: Median AQI 421987: Median AQI 361988: Median AQI 351989: Median AQI 331990: Median AQI 321991: Median AQI 341992: Median AQI 321993: Median AQI 331994: Median AQI 351995: Median AQI 311996: Median AQI 351997: Median AQI 311998: Median AQI 371999: Median AQI 352000: Median AQI 262001: Median AQI 372002: Median AQI 322003: Median AQI 192004: Median AQI 142005: Median AQI 312006: Median AQI 332007: Median AQI 342008: Median AQI 352009: Median AQI 332010: Median AQI 352011: Median AQI 342012: Median AQI 342013: Median AQI 342014: Median AQI 322015: Median AQI 332016: Median AQI 332017: Median AQI 332018: Median AQI 382019: Median AQI 412020: Median AQI 392021: Median AQI 372022: Median AQI 382023: Median AQI 392024: Median AQI 372025: Median AQI 381980198519901995200020052010201520202025
  • Median daily AQI
  • 90th-percentile daily AQI

Main pollutant by day, 2025

On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.

PollutantDays as main pollutantShare of monitored days
Ozone23063%
Fine particles (PM2.5)13437%
Nitrogen dioxide (NO₂)1<1%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20253652996510003856105Ozone
2024366314520000375493Ozone
20233652818310003957101Ozone
2022365310550000385490Ozone
20213652996411003755153PM2.5
2020366306600000395493PM2.5
2019364289750000415480PM2.5
2018365308570000385497Ozone
20173563411410003344105Ozone
20163623412100003346100Ozone
20153623451610003346119Ozone
2014357338190000324697Ozone
20133583352120003448101Ozone
20123653243731003451159Ozone
20113633243711003451151Ozone
20103653135020003561129Ozone
20093653233840003352140Ozone
20083663105051003558161Ozone
200736529353145003474174Ozone
20063652985890003364147Ozone
20053653074693003167174Ozone
20043663372630001446150CO
20033653382340001946143CO
200236527469136303280205Ozone
2001365265761410003780200Ozone
20003662976171002662185CO
199935827063178003580197Ozone
199834125957194203790212Ozone
199735328251136103177201Ozone
199636629258142003574161Ozone
199536530634177103180206Ozone
199436529354116103580206Ozone
199336530439174103371219Ozone
199236630539166003280185Ozone
1991365299411210303480214Ozone
199035529441154103277208Ozone
198936329649124203380203Ozone
198835428233141213035126235Ozone
198721216727126003690200Ozone
19862612035350004259140CO
198533921899173204590221CO
19843662349719133044100224CO
198333216711431911050122254CO
1982280170792731044103224Ozone
198135426079105004180182NO2
19803402366519155041116232CO

What this data can and can’t tell you

It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.

It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.

Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.

Counties in Massachusetts with a similar median AQI in 2025

All monitored counties in Massachusetts

Source: U.S. EPA AirData, “Annual AQI by County” files for 1980–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem