EnviroNearby

Knox County, Maine air quality history

Daily Air Quality Index summaries from EPA monitors in Knox, Maine, 1986–2025.

98%
Good days in 2025
33
Median daily AQI, 2025
0
Days above 100 in 2025
359
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Knox, Maine on 359 days. 351 of those days (98%) were in the Good range, and the median daily AQI was 33. That median is about the same as the typical monitored county in Maine (median 34 across 11 counties), and lower than the typical monitored U.S. county (40). The main pollutant was ozone on 359 days. Across the years with fuller monitoring, the share of Good days rose: an average of 76% in 1987–1989 versus 98% in 2023–2025.

Years with fewer than 120 monitored days (1986) are shown in the table but left out of comparisons and trend statements.

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%1986: Good 92 of 115 days1986: Moderate 14 of 115 days1986: Unhealthy for Sensitive Groups 6 of 115 days1986: Unhealthy 3 of 115 days19861987: Good 128 of 169 days1987: Moderate 26 of 169 days1987: Unhealthy for Sensitive Groups 7 of 169 days1987: Unhealthy 2 of 169 days1987: Very Unhealthy 6 of 169 days1988: Good 151 of 223 days1988: Moderate 39 of 223 days1988: Unhealthy for Sensitive Groups 12 of 223 days1988: Unhealthy 10 of 223 days1988: Very Unhealthy 11 of 223 days1989: Good 210 of 249 days1989: Moderate 25 of 249 days1989: Unhealthy for Sensitive Groups 8 of 249 days1989: Unhealthy 4 of 249 days1989: Very Unhealthy 2 of 249 days1990: Good 187 of 241 days1990: Moderate 35 of 241 days1990: Unhealthy for Sensitive Groups 11 of 241 days1990: Unhealthy 7 of 241 days1990: Very Unhealthy 1 of 241 days1991: Good 196 of 253 days1991: Moderate 27 of 253 days1991: Unhealthy for Sensitive Groups 15 of 253 days1991: Unhealthy 8 of 253 days1991: Very Unhealthy 7 of 253 days19911992: Good 202 of 250 days1992: Moderate 30 of 250 days1992: Unhealthy for Sensitive Groups 9 of 250 days1992: Unhealthy 7 of 250 days1992: Very Unhealthy 2 of 250 days1993: Good 221 of 262 days1993: Moderate 29 of 262 days1993: Unhealthy for Sensitive Groups 8 of 262 days1993: Unhealthy 3 of 262 days1993: Very Unhealthy 1 of 262 days1994: Good 213 of 256 days1994: Moderate 28 of 256 days1994: Unhealthy for Sensitive Groups 11 of 256 days1994: Unhealthy 4 of 256 days1995: Good 209 of 237 days1995: Moderate 16 of 237 days1995: Unhealthy for Sensitive Groups 6 of 237 days1995: Unhealthy 4 of 237 days1995: Very Unhealthy 2 of 237 days1996: Good 204 of 240 days1996: Moderate 27 of 240 days1996: Unhealthy for Sensitive Groups 8 of 240 days1996: Unhealthy 1 of 240 days19961997: Good 208 of 248 days1997: Moderate 30 of 248 days1997: Unhealthy for Sensitive Groups 5 of 248 days1997: Unhealthy 4 of 248 days1997: Very Unhealthy 1 of 248 days1998: Good 218 of 254 days1998: Moderate 27 of 254 days1998: Unhealthy for Sensitive Groups 6 of 254 days1998: Unhealthy 3 of 254 days1999: Good 207 of 240 days1999: Moderate 25 of 240 days1999: Unhealthy for Sensitive Groups 6 of 240 days1999: Unhealthy 1 of 240 days1999: Very Unhealthy 1 of 240 days2000: Good 187 of 212 days2000: Moderate 22 of 212 days2000: Unhealthy for Sensitive Groups 3 of 212 days2001: Good 176 of 212 days2001: Moderate 18 of 212 days2001: Unhealthy for Sensitive Groups 12 of 212 days2001: Unhealthy 5 of 212 days2001: Very Unhealthy 1 of 212 days20012002: Good 159 of 195 days2002: Moderate 24 of 195 days2002: Unhealthy for Sensitive Groups 7 of 195 days2002: Unhealthy 5 of 195 days2003: Good 169 of 203 days2003: Moderate 23 of 203 days2003: Unhealthy for Sensitive Groups 10 of 203 days2003: Very Unhealthy 1 of 203 days2004: Good 159 of 181 days2004: Moderate 18 of 181 days2004: Unhealthy for Sensitive Groups 4 of 181 days2005: Good 140 of 162 days2005: Moderate 15 of 162 days2005: Unhealthy for Sensitive Groups 6 of 162 days2005: Unhealthy 1 of 162 days2006: Good 154 of 182 days2006: Moderate 23 of 182 days2006: Unhealthy for Sensitive Groups 5 of 182 days20062007: Good 151 of 179 days2007: Moderate 22 of 179 days2007: Unhealthy for Sensitive Groups 4 of 179 days2007: Unhealthy 2 of 179 days2008: Good 171 of 183 days2008: Moderate 12 of 183 days2009: Good 169 of 183 days2009: Moderate 11 of 183 days2009: Unhealthy for Sensitive Groups 3 of 183 days2010: Good 191 of 212 days2010: Moderate 18 of 212 days2010: Unhealthy for Sensitive Groups 2 of 212 days2010: Unhealthy 1 of 212 days2011: Good 166 of 181 days2011: Moderate 12 of 181 days2011: Unhealthy for Sensitive Groups 3 of 181 days20112012: Good 165 of 177 days2012: Moderate 12 of 177 days2013: Good 188 of 208 days2013: Moderate 15 of 208 days2013: Unhealthy for Sensitive Groups 5 of 208 days2014: Good 194 of 205 days2014: Moderate 11 of 205 days2015: Good 164 of 178 days2015: Moderate 13 of 178 days2015: Unhealthy for Sensitive Groups 1 of 178 days2016: Good 171 of 181 days2016: Moderate 9 of 181 days2016: Unhealthy for Sensitive Groups 1 of 181 days20162017: Good 205 of 215 days2017: Moderate 7 of 215 days2017: Unhealthy for Sensitive Groups 3 of 215 days2018: Good 209 of 219 days2018: Moderate 9 of 219 days2018: Unhealthy for Sensitive Groups 1 of 219 days2019: Good 204 of 213 days2019: Moderate 9 of 213 days2020: Good 142 of 148 days2020: Moderate 6 of 148 days2021: Good 192 of 211 days2021: Moderate 17 of 211 days2021: Unhealthy for Sensitive Groups 2 of 211 days20212022: Good 206 of 212 days2022: Moderate 6 of 212 days2023: Good 296 of 301 days2023: Moderate 5 of 301 days2024: Good 343 of 355 days2024: Moderate 12 of 355 days2025: Good 351 of 359 days2025: Moderate 8 of 359 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 year0501001501986: 90th percentile AQI 901987: 90th percentile AQI 971988: 90th percentile AQI 1471989: 90th percentile AQI 741990: 90th percentile AQI 931991: 90th percentile AQI 1081992: 90th percentile AQI 871993: 90th percentile AQI 641994: 90th percentile AQI 711995: 90th percentile AQI 541996: 90th percentile AQI 611997: 90th percentile AQI 641998: 90th percentile AQI 581999: 90th percentile AQI 672000: 90th percentile AQI 542001: 90th percentile AQI 872002: 90th percentile AQI 842003: 90th percentile AQI 742004: 90th percentile AQI 582005: 90th percentile AQI 642006: 90th percentile AQI 612007: 90th percentile AQI 712008: 90th percentile AQI 472009: 90th percentile AQI 482010: 90th percentile AQI 502011: 90th percentile AQI 492012: 90th percentile AQI 482013: 90th percentile AQI 502014: 90th percentile AQI 472015: 90th percentile AQI 502016: 90th percentile AQI 482017: 90th percentile AQI 452018: 90th percentile AQI 452019: 90th percentile AQI 472020: 90th percentile AQI 452021: 90th percentile AQI 502022: 90th percentile AQI 452023: 90th percentile AQI 442024: 90th percentile AQI 432025: 90th percentile AQI 441986: Median AQI 321987: Median AQI 411988: Median AQI 441989: Median AQI 341990: Median AQI 371991: Median AQI 381992: Median AQI 361993: Median AQI 331994: Median AQI 341995: Median AQI 311996: Median AQI 341997: Median AQI 331998: Median AQI 381999: Median AQI 352000: Median AQI 332001: Median AQI 382002: Median AQI 372003: Median AQI 382004: Median AQI 362005: Median AQI 372006: Median AQI 382007: Median AQI 342008: Median AQI 352009: Median AQI 352010: Median AQI 402011: Median AQI 342012: Median AQI 342013: Median AQI 382014: Median AQI 342015: Median AQI 362016: Median AQI 332017: Median AQI 352018: Median AQI 362019: Median AQI 362020: Median AQI 312021: Median AQI 352022: Median AQI 352023: Median AQI 332024: Median AQI 332025: Median AQI 33198619911996200120062011201620212025
  • 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
Ozone359100%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
202535935180000334477Ozone
2024355343120000334384Ozone
202330129650000334487Ozone
202221220660000354580Ozone
20212111921720003550119Ozone
202014814260000314571Ozone
201921320490000364784Ozone
2018219209910003645112Ozone
2017215205730003545143Ozone
2016181171910003348122Ozone
20151781641310003650119Ozone
2014205194110000344787Ozone
20132081881550003850140Ozone
2012177165120000344897Ozone
20111811661230003449115Ozone
20102121911821004050166Ozone
20091831691130003548150Ozone
2008183171120000354793Ozone
20071791512242003471177Ozone
20061821542350003861133Ozone
20051621401561003764192Ozone
20041811591840003658115Ozone
200320316923100103874202Ozone
20021951592475003784179Ozone
200121217618125103887206Ozone
20002121872230003354147Ozone
19992402072561103567205Ozone
19982542182763003858192Ozone
19972482083054103364204Ozone
19962402042781003461185Ozone
19952372091664203154210Ozone
199425621328114003471177Ozone
19932622212983103364201Ozone
19922502023097203687206Ozone
1991253196271587038108218Ozone
199024118735117103793226Ozone
19892492102584203474210Ozone
198822315139121011044147261Ozone
19871691282672604197223Ozone
1986115921463003290159Ozone

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 Maine with a similar median AQI in 2025

All monitored counties in Maine

Source: U.S. EPA AirData, “Annual AQI by County” files for 1986–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem