EnviroNearby

Middlesex County, Connecticut air quality history

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

77%
Good days in 2025
42
Median daily AQI, 2025
6
Days above 100 in 2025
213
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Middlesex, Connecticut on 213 days. 165 of those days (77%) were in the Good range, and the median daily AQI was 42. That median is about the same as the typical monitored county in Connecticut (median 41 across 8 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was ozone on 213 days. Across the years with fuller monitoring, the share of Good days rose: an average of 56% in 1980–1982 versus 81% in 2023–2025.

EPA did not report an AQI for every day of 2025 here (213 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.

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 69 of 164 days1980: Moderate 38 of 164 days1980: Unhealthy for Sensitive Groups 19 of 164 days1980: Unhealthy 17 of 164 days1980: Very Unhealthy 21 of 164 days19801981: Good 155 of 236 days1981: Moderate 45 of 236 days1981: Unhealthy for Sensitive Groups 10 of 236 days1981: Unhealthy 10 of 236 days1981: Very Unhealthy 16 of 236 days1982: Good 164 of 272 days1982: Moderate 50 of 272 days1982: Unhealthy for Sensitive Groups 24 of 272 days1982: Unhealthy 18 of 272 days1982: Very Unhealthy 16 of 272 days1983: Good 119 of 206 days1983: Moderate 41 of 206 days1983: Unhealthy for Sensitive Groups 17 of 206 days1983: Unhealthy 13 of 206 days1983: Very Unhealthy 16 of 206 days1984: Good 102 of 183 days1984: Moderate 41 of 183 days1984: Unhealthy for Sensitive Groups 17 of 183 days1984: Unhealthy 16 of 183 days1984: Very Unhealthy 7 of 183 days1985: Good 140 of 209 days1985: Moderate 34 of 209 days1985: Unhealthy for Sensitive Groups 18 of 209 days1985: Unhealthy 13 of 209 days1985: Very Unhealthy 4 of 209 days19851986: Good 144 of 204 days1986: Moderate 40 of 204 days1986: Unhealthy for Sensitive Groups 12 of 204 days1986: Unhealthy 7 of 204 days1986: Very Unhealthy 1 of 204 days1987: Good 147 of 209 days1987: Moderate 27 of 209 days1987: Unhealthy for Sensitive Groups 18 of 209 days1987: Unhealthy 15 of 209 days1987: Very Unhealthy 2 of 209 days1988: Good 149 of 216 days1988: Moderate 26 of 216 days1988: Unhealthy for Sensitive Groups 12 of 216 days1988: Unhealthy 17 of 216 days1988: Very Unhealthy 12 of 216 days1989: Good 170 of 232 days1989: Moderate 32 of 232 days1989: Unhealthy for Sensitive Groups 15 of 232 days1989: Unhealthy 11 of 232 days1989: Very Unhealthy 4 of 232 days1990: Good 172 of 216 days1990: Moderate 27 of 216 days1990: Unhealthy for Sensitive Groups 11 of 216 days1990: Unhealthy 2 of 216 days1990: Very Unhealthy 4 of 216 days19901991: Good 164 of 235 days1991: Moderate 34 of 235 days1991: Unhealthy for Sensitive Groups 20 of 235 days1991: Unhealthy 9 of 235 days1991: Very Unhealthy 8 of 235 days1992: Good 193 of 239 days1992: Moderate 22 of 239 days1992: Unhealthy for Sensitive Groups 15 of 239 days1992: Unhealthy 7 of 239 days1992: Very Unhealthy 2 of 239 days1993: Good 184 of 234 days1993: Moderate 28 of 234 days1993: Unhealthy for Sensitive Groups 13 of 234 days1993: Unhealthy 6 of 234 days1993: Very Unhealthy 3 of 234 days1994: Good 171 of 232 days1994: Moderate 30 of 232 days1994: Unhealthy for Sensitive Groups 17 of 232 days1994: Unhealthy 12 of 232 days1994: Very Unhealthy 2 of 232 days1995: Good 180 of 233 days1995: Moderate 23 of 233 days1995: Unhealthy for Sensitive Groups 19 of 233 days1995: Unhealthy 8 of 233 days1995: Very Unhealthy 3 of 233 days19951996: Good 161 of 202 days1996: Moderate 27 of 202 days1996: Unhealthy for Sensitive Groups 9 of 202 days1996: Unhealthy 5 of 202 days1997: Good 152 of 208 days1997: Moderate 36 of 208 days1997: Unhealthy for Sensitive Groups 8 of 208 days1997: Unhealthy 9 of 208 days1997: Very Unhealthy 3 of 208 days1998: Good 201 of 266 days1998: Moderate 40 of 266 days1998: Unhealthy for Sensitive Groups 20 of 266 days1998: Unhealthy 4 of 266 days1998: Very Unhealthy 1 of 266 days1999: Good 297 of 360 days1999: Moderate 38 of 360 days1999: Unhealthy for Sensitive Groups 11 of 360 days1999: Unhealthy 10 of 360 days1999: Very Unhealthy 4 of 360 days2000: Good 295 of 342 days2000: Moderate 28 of 342 days2000: Unhealthy for Sensitive Groups 15 of 342 days2000: Unhealthy 4 of 342 days20002001: Good 204 of 269 days2001: Moderate 37 of 269 days2001: Unhealthy for Sensitive Groups 17 of 269 days2001: Unhealthy 8 of 269 days2001: Very Unhealthy 3 of 269 days2002: Good 116 of 182 days2002: Moderate 33 of 182 days2002: Unhealthy for Sensitive Groups 16 of 182 days2002: Unhealthy 13 of 182 days2002: Very Unhealthy 4 of 182 days2003: Good 130 of 179 days2003: Moderate 33 of 179 days2003: Unhealthy for Sensitive Groups 9 of 179 days2003: Unhealthy 6 of 179 days2003: Very Unhealthy 1 of 179 days2004: Good 136 of 181 days2004: Moderate 29 of 181 days2004: Unhealthy for Sensitive Groups 15 of 181 days2004: Unhealthy 1 of 181 days2005: Good 132 of 182 days2005: Moderate 29 of 182 days2005: Unhealthy for Sensitive Groups 15 of 182 days2005: Unhealthy 4 of 182 days2005: Very Unhealthy 2 of 182 days20052006: Good 130 of 179 days2006: Moderate 30 of 179 days2006: Unhealthy for Sensitive Groups 14 of 179 days2006: Unhealthy 4 of 179 days2006: Very Unhealthy 1 of 179 days2007: Good 120 of 182 days2007: Moderate 37 of 182 days2007: Unhealthy for Sensitive Groups 19 of 182 days2007: Unhealthy 5 of 182 days2007: Very Unhealthy 1 of 182 days2008: Good 142 of 183 days2008: Moderate 29 of 183 days2008: Unhealthy for Sensitive Groups 11 of 183 days2008: Unhealthy 1 of 183 days2009: Good 161 of 182 days2009: Moderate 18 of 182 days2009: Unhealthy for Sensitive Groups 3 of 182 days2010: Good 124 of 179 days2010: Moderate 46 of 179 days2010: Unhealthy for Sensitive Groups 8 of 179 days2010: Unhealthy 1 of 179 days20102011: Good 145 of 180 days2011: Moderate 29 of 180 days2011: Unhealthy for Sensitive Groups 3 of 180 days2011: Unhealthy 3 of 180 days2012: Good 129 of 179 days2012: Moderate 35 of 179 days2012: Unhealthy for Sensitive Groups 13 of 179 days2012: Unhealthy 2 of 179 days2013: Good 139 of 180 days2013: Moderate 33 of 180 days2013: Unhealthy for Sensitive Groups 6 of 180 days2013: Unhealthy 2 of 180 days2014: Good 140 of 182 days2014: Moderate 31 of 182 days2014: Unhealthy for Sensitive Groups 10 of 182 days2014: Unhealthy 1 of 182 days2015: Good 134 of 183 days2015: Moderate 37 of 183 days2015: Unhealthy for Sensitive Groups 12 of 183 days20152016: Good 125 of 174 days2016: Moderate 36 of 174 days2016: Unhealthy for Sensitive Groups 11 of 174 days2016: Unhealthy 2 of 174 days2017: Good 171 of 207 days2017: Moderate 28 of 207 days2017: Unhealthy for Sensitive Groups 7 of 207 days2017: Unhealthy 1 of 207 days2018: Good 168 of 208 days2018: Moderate 32 of 208 days2018: Unhealthy for Sensitive Groups 8 of 208 days2019: Good 175 of 205 days2019: Moderate 23 of 205 days2019: Unhealthy for Sensitive Groups 6 of 205 days2019: Unhealthy 1 of 205 days2020: Good 182 of 208 days2020: Moderate 24 of 208 days2020: Unhealthy for Sensitive Groups 2 of 208 days20202021: Good 171 of 207 days2021: Moderate 25 of 207 days2021: Unhealthy for Sensitive Groups 10 of 207 days2021: Unhealthy 1 of 207 days2022: Good 173 of 212 days2022: Moderate 33 of 212 days2022: Unhealthy for Sensitive Groups 6 of 212 days2023: Good 170 of 209 days2023: Moderate 35 of 209 days2023: Unhealthy for Sensitive Groups 4 of 209 days2024: Good 178 of 209 days2024: Moderate 25 of 209 days2024: Unhealthy for Sensitive Groups 5 of 209 days2024: Unhealthy 1 of 209 days2025: Good 165 of 213 days2025: Moderate 42 of 213 days2025: Unhealthy for Sensitive Groups 5 of 213 days2025: Unhealthy 1 of 213 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 year0501001502002501980: 90th percentile AQI 2181981: 90th percentile AQI 1561982: 90th percentile AQI 1661983: 90th percentile AQI 1821984: 90th percentile AQI 1591985: 90th percentile AQI 1401986: 90th percentile AQI 1001987: 90th percentile AQI 1431988: 90th percentile AQI 1741989: 90th percentile AQI 1121990: 90th percentile AQI 801991: 90th percentile AQI 1361992: 90th percentile AQI 1011993: 90th percentile AQI 931994: 90th percentile AQI 1151995: 90th percentile AQI 1191996: 90th percentile AQI 871997: 90th percentile AQI 971998: 90th percentile AQI 971999: 90th percentile AQI 842000: 90th percentile AQI 642001: 90th percentile AQI 1082002: 90th percentile AQI 1402003: 90th percentile AQI 1002004: 90th percentile AQI 902005: 90th percentile AQI 1082006: 90th percentile AQI 1012007: 90th percentile AQI 1152008: 90th percentile AQI 842009: 90th percentile AQI 582010: 90th percentile AQI 842011: 90th percentile AQI 742012: 90th percentile AQI 1002013: 90th percentile AQI 802014: 90th percentile AQI 802015: 90th percentile AQI 902016: 90th percentile AQI 972017: 90th percentile AQI 612018: 90th percentile AQI 672019: 90th percentile AQI 612020: 90th percentile AQI 542021: 90th percentile AQI 802022: 90th percentile AQI 672023: 90th percentile AQI 642024: 90th percentile AQI 612025: 90th percentile AQI 771980: Median AQI 671981: Median AQI 411982: Median AQI 441983: Median AQI 441984: Median AQI 451985: Median AQI 431986: Median AQI 401987: Median AQI 431988: Median AQI 401989: Median AQI 411990: Median AQI 351991: Median AQI 411992: Median AQI 351993: Median AQI 381994: Median AQI 401995: Median AQI 391996: Median AQI 381997: Median AQI 411998: Median AQI 401999: Median AQI 372000: Median AQI 322001: Median AQI 402002: Median AQI 442003: Median AQI 432004: Median AQI 412005: Median AQI 432006: Median AQI 412007: Median AQI 442008: Median AQI 422009: Median AQI 352010: Median AQI 442011: Median AQI 432012: Median AQI 442013: Median AQI 432014: Median AQI 432015: Median AQI 432016: Median AQI 442017: Median AQI 402018: Median AQI 402019: Median AQI 412020: Median AQI 392021: Median AQI 392022: Median AQI 402023: Median AQI 432024: Median AQI 402025: Median AQI 421980198519901995200020052010201520202025
  • 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
Ozone213100%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20252131654251004277151Ozone
20242091782551004061177Ozone
20232091703540004364140Ozone
20222121733360004067147Ozone
202120717125101003980151Ozone
20202081822420003954129Ozone
20192051752361004161154Ozone
20182081683280004067126Ozone
20172071712871004061151Ozone
201617412536112004497187Ozone
201518313437120004390147Ozone
201418214031101004380151Ozone
20131801393362004380179Ozone
2012179129351320044100182Ozone
20111801452933004374161Ozone
20101791244681004484151Ozone
20091821611830003558136Ozone
200818314229111004284164Ozone
2007182120371951044115206Ozone
2006179130301441041101205Ozone
2005182132291542043108205Ozone
200418113629151004190192Ozone
200317913033961043100206Ozone
20021821163316134044140222Ozone
2001269204371783040108206Ozone
200034229528154003264195Ozone
1999360297381110403784224Ozone
199826620140204104097209Ozone
19972081523689304197242Ozone
19962021612795003887177Ozone
1995233180231983039119219Ozone
19942321713017122040115232Ozone
199323418428136303893208Ozone
1992239193221572035101210Ozone
1991235164342098041136239Ozone
199021617227112403580220Ozone
19892321703215114041112229Ozone
198821614926121712040174286Ozone
19872091472718152043143252Ozone
1986204144401271040100226Ozone
19852091403418134043140269Ozone
19841831024117167045159233Ozone
198320611941171316044182258Ozone
198227216450241816044166237Ozone
198123615545101016041156248Ozone
19801646938191721067218267Ozone

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 Connecticut with a similar median AQI in 2025

All monitored counties in Connecticut

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