EnviroNearby

Worcester County, Massachusetts air quality history

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

82%
Good days in 2025
39
Median daily AQI, 2025
0
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 Worcester, Massachusetts on 365 days. 298 of those days (82%) were in the Good range, and the median daily AQI was 39. 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 265 days. Across the years with fuller monitoring, the share of Good days rose: an average of 71% in 1980–1982 versus 81% 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 239 of 334 days1980: Moderate 58 of 334 days1980: Unhealthy for Sensitive Groups 23 of 334 days1980: Unhealthy 10 of 334 days1980: Very Unhealthy 4 of 334 days19801981: Good 217 of 345 days1981: Moderate 92 of 345 days1981: Unhealthy for Sensitive Groups 21 of 345 days1981: Unhealthy 11 of 345 days1981: Very Unhealthy 4 of 345 days1982: Good 286 of 362 days1982: Moderate 65 of 362 days1982: Unhealthy for Sensitive Groups 6 of 362 days1982: Unhealthy 4 of 362 days1982: Very Unhealthy 1 of 362 days1983: Good 253 of 360 days1983: Moderate 75 of 360 days1983: Unhealthy for Sensitive Groups 18 of 360 days1983: Unhealthy 12 of 360 days1983: Very Unhealthy 2 of 360 days1984: Good 289 of 366 days1984: Moderate 55 of 366 days1984: Unhealthy for Sensitive Groups 15 of 366 days1984: Unhealthy 7 of 366 days1985: Good 295 of 365 days1985: Moderate 51 of 365 days1985: Unhealthy for Sensitive Groups 13 of 365 days1985: Unhealthy 5 of 365 days1985: Very Unhealthy 1 of 365 days19851986: Good 267 of 365 days1986: Moderate 84 of 365 days1986: Unhealthy for Sensitive Groups 12 of 365 days1986: Unhealthy 2 of 365 days1987: Good 205 of 365 days1987: Moderate 123 of 365 days1987: Unhealthy for Sensitive Groups 34 of 365 days1987: Unhealthy 3 of 365 days1988: Good 246 of 366 days1988: Moderate 87 of 366 days1988: Unhealthy for Sensitive Groups 19 of 366 days1988: Unhealthy 10 of 366 days1988: Very Unhealthy 4 of 366 days1989: Good 264 of 365 days1989: Moderate 92 of 365 days1989: Unhealthy for Sensitive Groups 7 of 365 days1989: Unhealthy 2 of 365 days1990: Good 303 of 365 days1990: Moderate 52 of 365 days1990: Unhealthy for Sensitive Groups 7 of 365 days1990: Unhealthy 3 of 365 days19901991: Good 295 of 365 days1991: Moderate 47 of 365 days1991: Unhealthy for Sensitive Groups 14 of 365 days1991: Unhealthy 6 of 365 days1991: Very Unhealthy 3 of 365 days1992: Good 277 of 366 days1992: Moderate 60 of 366 days1992: Unhealthy for Sensitive Groups 19 of 366 days1992: Unhealthy 7 of 366 days1992: Very Unhealthy 3 of 366 days1993: Good 208 of 365 days1993: Moderate 138 of 365 days1993: Unhealthy for Sensitive Groups 14 of 365 days1993: Unhealthy 2 of 365 days1993: Very Unhealthy 3 of 365 days1994: Good 248 of 365 days1994: Moderate 91 of 365 days1994: Unhealthy for Sensitive Groups 16 of 365 days1994: Unhealthy 9 of 365 days1994: Very Unhealthy 1 of 365 days1995: Good 290 of 365 days1995: Moderate 53 of 365 days1995: Unhealthy for Sensitive Groups 16 of 365 days1995: Unhealthy 4 of 365 days1995: Very Unhealthy 2 of 365 days19951996: Good 309 of 366 days1996: Moderate 49 of 366 days1996: Unhealthy for Sensitive Groups 8 of 366 days1997: Good 297 of 365 days1997: Moderate 54 of 365 days1997: Unhealthy for Sensitive Groups 9 of 365 days1997: Unhealthy 5 of 365 days1998: Good 289 of 365 days1998: Moderate 54 of 365 days1998: Unhealthy for Sensitive Groups 16 of 365 days1998: Unhealthy 4 of 365 days1998: Very Unhealthy 2 of 365 days1999: Good 220 of 365 days1999: Moderate 119 of 365 days1999: Unhealthy for Sensitive Groups 19 of 365 days1999: Unhealthy 7 of 365 days2000: Good 229 of 366 days2000: Moderate 128 of 366 days2000: Unhealthy for Sensitive Groups 8 of 366 days2000: Unhealthy 1 of 366 days20002001: Good 138 of 365 days2001: Moderate 211 of 365 days2001: Unhealthy for Sensitive Groups 11 of 365 days2001: Unhealthy 5 of 365 days2002: Good 168 of 365 days2002: Moderate 173 of 365 days2002: Unhealthy for Sensitive Groups 16 of 365 days2002: Unhealthy 7 of 365 days2002: Very Unhealthy 1 of 365 days2003: Good 113 of 301 days2003: Moderate 170 of 301 days2003: Unhealthy for Sensitive Groups 17 of 301 days2003: Unhealthy 1 of 301 days2004: Good 148 of 366 days2004: Moderate 207 of 366 days2004: Unhealthy for Sensitive Groups 11 of 366 days2005: Good 168 of 365 days2005: Moderate 182 of 365 days2005: Unhealthy for Sensitive Groups 12 of 365 days2005: Unhealthy 3 of 365 days20052006: Good 214 of 365 days2006: Moderate 140 of 365 days2006: Unhealthy for Sensitive Groups 9 of 365 days2006: Unhealthy 2 of 365 days2007: Good 203 of 365 days2007: Moderate 134 of 365 days2007: Unhealthy for Sensitive Groups 19 of 365 days2007: Unhealthy 9 of 365 days2008: Good 205 of 366 days2008: Moderate 151 of 366 days2008: Unhealthy for Sensitive Groups 8 of 366 days2008: Unhealthy 2 of 366 days2009: Good 221 of 365 days2009: Moderate 136 of 365 days2009: Unhealthy for Sensitive Groups 7 of 365 days2009: Unhealthy 1 of 365 days2010: Good 190 of 365 days2010: Moderate 170 of 365 days2010: Unhealthy for Sensitive Groups 5 of 365 days20102011: Good 191 of 365 days2011: Moderate 170 of 365 days2011: Unhealthy for Sensitive Groups 3 of 365 days2011: Unhealthy 1 of 365 days2012: Good 228 of 366 days2012: Moderate 135 of 366 days2012: Unhealthy for Sensitive Groups 2 of 366 days2012: Unhealthy 1 of 366 days2013: Good 228 of 365 days2013: Moderate 135 of 365 days2013: Unhealthy for Sensitive Groups 2 of 365 days2014: Good 242 of 365 days2014: Moderate 122 of 365 days2014: Unhealthy for Sensitive Groups 1 of 365 days2015: Good 266 of 365 days2015: Moderate 99 of 365 days20152016: Good 285 of 366 days2016: Moderate 77 of 366 days2016: Unhealthy for Sensitive Groups 4 of 366 days2017: Good 288 of 365 days2017: Moderate 74 of 365 days2017: Unhealthy for Sensitive Groups 3 of 365 days2018: Good 265 of 365 days2018: Moderate 96 of 365 days2018: Unhealthy for Sensitive Groups 4 of 365 days2019: Good 238 of 365 days2019: Moderate 127 of 365 days2020: Good 257 of 366 days2020: Moderate 109 of 366 days20202021: Good 218 of 365 days2021: Moderate 144 of 365 days2021: Unhealthy for Sensitive Groups 3 of 365 days2022: Good 228 of 365 days2022: Moderate 137 of 365 days2023: Good 268 of 365 days2023: Moderate 93 of 365 days2023: Unhealthy for Sensitive Groups 4 of 365 days2024: Good 321 of 366 days2024: Moderate 45 of 366 days2025: Good 298 of 365 days2025: Moderate 67 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 1051981: 90th percentile AQI 1031982: 90th percentile AQI 671983: 90th percentile AQI 971984: 90th percentile AQI 841985: 90th percentile AQI 711986: 90th percentile AQI 771987: 90th percentile AQI 1011988: 90th percentile AQI 931989: 90th percentile AQI 651990: 90th percentile AQI 631991: 90th percentile AQI 741992: 90th percentile AQI 861993: 90th percentile AQI 871994: 90th percentile AQI 841995: 90th percentile AQI 801996: 90th percentile AQI 591997: 90th percentile AQI 711998: 90th percentile AQI 841999: 90th percentile AQI 882000: 90th percentile AQI 712001: 90th percentile AQI 852002: 90th percentile AQI 872003: 90th percentile AQI 872004: 90th percentile AQI 782005: 90th percentile AQI 852006: 90th percentile AQI 822007: 90th percentile AQI 862008: 90th percentile AQI 742009: 90th percentile AQI 692010: 90th percentile AQI 722011: 90th percentile AQI 672012: 90th percentile AQI 662013: 90th percentile AQI 632014: 90th percentile AQI 622015: 90th percentile AQI 612016: 90th percentile AQI 602017: 90th percentile AQI 562018: 90th percentile AQI 612019: 90th percentile AQI 592020: 90th percentile AQI 582021: 90th percentile AQI 592022: 90th percentile AQI 592023: 90th percentile AQI 582024: 90th percentile AQI 542025: 90th percentile AQI 561980: Median AQI 391981: Median AQI 441982: Median AQI 381983: Median AQI 401984: Median AQI 351985: Median AQI 331986: Median AQI 361987: Median AQI 471988: Median AQI 431989: Median AQI 421990: Median AQI 381991: Median AQI 391992: Median AQI 421993: Median AQI 461994: Median AQI 441995: Median AQI 401996: Median AQI 371997: Median AQI 381998: Median AQI 391999: Median AQI 452000: Median AQI 422001: Median AQI 542002: Median AQI 532003: Median AQI 542004: Median AQI 532005: Median AQI 522006: Median AQI 442007: Median AQI 462008: Median AQI 472009: Median AQI 432010: Median AQI 492011: Median AQI 482012: Median AQI 452013: Median AQI 452014: Median AQI 432015: Median AQI 402016: Median AQI 412017: Median AQI 402018: Median AQI 402019: Median AQI 452020: Median AQI 432021: Median AQI 452022: Median AQI 452023: Median AQI 392024: Median AQI 372025: Median AQI 391980198519901995200020052010201520202025
  • 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
Ozone26573%
Fine particles (PM2.5)9727%
Nitrogen dioxide (NO₂)21%
Coarse particles (PM10)1<1%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20253652986700003956100Ozone
2024366321450000375490Ozone
20233652689340003958148Ozone
202236522813700004559100PM2.5
202136521814430004559125PM2.5
20203662571090000435890PM2.5
20193652381270000455993PM2.5
20183652659640004061129Ozone
20173652887430004056143Ozone
20163662857740004160143Ozone
2015365266990000406193Ozone
201436524212210004362115Ozone
201336522813520004563112PM2.5
201236622813521004566166PM2.5
201136519117031004867159PM2.5
201036519017050004972143PM2.5
200936522113671004369151PM2.5
200836620515182004774197PM2.5
2007365203134199004686177PM2.5
200636521414092004482164PM2.5
2005365168182123005285166PM2.5
2004366148207110005378140PM2.5
2003301113170171005487159PM2.5
2002365168173167105387214PM2.5
2001365138211115005485192PM2.5
200036622912881004271174Ozone
1999365220119197004588190Ozone
199836528954164203984219Ozone
19973652975495003871179Ozone
19963663094980003759133Ozone
199536529053164204080203Ozone
199436524891169104484202NO2
1993365208138142304687216NO2
199236627760197304286207NO2
199136529547146303974204NO2
19903653035273003863192NO2
19893652649272004265177NO2
1988366246871910404393220NO2
19873652051233430047101161NO2
198636526784122003677185Ozone
198536529551135103371228Ozone
198436628955157003584192Ozone
1983360253751812204097209Ozone
19823622866564103867202NO2
19813452179221114044103210Ozone
19803342395823104039105243Ozone

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