EnviroNearby

Boulder County, Colorado air quality history

Daily Air Quality Index summaries from EPA monitors in Boulder, Colorado, 1980–2025.

58%
Good days in 2025
48
Median daily AQI, 2025
3
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 Boulder, Colorado on 365 days. 213 of those days (58%) were in the Good range, and the median daily AQI was 48. That median is about the same as the typical monitored county in Colorado (median 46 across 27 counties), and higher than the typical monitored U.S. county (40). The main pollutant was ozone on 256 days. Across the years with fuller monitoring, the share of Good days fell: an average of 81% in 1980–1982 versus 56% 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 324 of 349 days1980: Moderate 21 of 349 days1980: Unhealthy for Sensitive Groups 2 of 349 days1980: Unhealthy 2 of 349 days19801981: Good 286 of 362 days1981: Moderate 60 of 362 days1981: Unhealthy for Sensitive Groups 16 of 362 days1982: Good 235 of 326 days1982: Moderate 76 of 326 days1982: Unhealthy for Sensitive Groups 13 of 326 days1982: Unhealthy 2 of 326 days1983: Good 275 of 364 days1983: Moderate 64 of 364 days1983: Unhealthy for Sensitive Groups 21 of 364 days1983: Unhealthy 4 of 364 days1984: Good 251 of 362 days1984: Moderate 92 of 362 days1984: Unhealthy for Sensitive Groups 15 of 362 days1984: Unhealthy 4 of 362 days1985: Good 245 of 364 days1985: Moderate 105 of 364 days1985: Unhealthy for Sensitive Groups 12 of 364 days1985: Unhealthy 1 of 364 days1985: Very Unhealthy 1 of 364 days19851986: Good 240 of 365 days1986: Moderate 103 of 365 days1986: Unhealthy for Sensitive Groups 18 of 365 days1986: Unhealthy 4 of 365 days1987: Good 226 of 365 days1987: Moderate 108 of 365 days1987: Unhealthy for Sensitive Groups 28 of 365 days1987: Unhealthy 3 of 365 days1988: Good 199 of 366 days1988: Moderate 127 of 366 days1988: Unhealthy for Sensitive Groups 38 of 366 days1988: Unhealthy 2 of 366 days1989: Good 240 of 365 days1989: Moderate 107 of 365 days1989: Unhealthy for Sensitive Groups 16 of 365 days1989: Unhealthy 2 of 365 days1990: Good 280 of 365 days1990: Moderate 76 of 365 days1990: Unhealthy for Sensitive Groups 9 of 365 days19901991: Good 263 of 365 days1991: Moderate 89 of 365 days1991: Unhealthy for Sensitive Groups 12 of 365 days1991: Unhealthy 1 of 365 days1992: Good 285 of 366 days1992: Moderate 79 of 366 days1992: Unhealthy for Sensitive Groups 2 of 366 days1993: Good 265 of 365 days1993: Moderate 84 of 365 days1993: Unhealthy for Sensitive Groups 15 of 365 days1993: Unhealthy 1 of 365 days1994: Good 265 of 365 days1994: Moderate 94 of 365 days1994: Unhealthy for Sensitive Groups 6 of 365 days1995: Good 292 of 365 days1995: Moderate 65 of 365 days1995: Unhealthy for Sensitive Groups 8 of 365 days19951996: Good 278 of 366 days1996: Moderate 80 of 366 days1996: Unhealthy for Sensitive Groups 8 of 366 days1997: Good 283 of 365 days1997: Moderate 73 of 365 days1997: Unhealthy for Sensitive Groups 9 of 365 days1998: Good 247 of 365 days1998: Moderate 96 of 365 days1998: Unhealthy for Sensitive Groups 17 of 365 days1998: Unhealthy 5 of 365 days1999: Good 282 of 365 days1999: Moderate 77 of 365 days1999: Unhealthy for Sensitive Groups 5 of 365 days1999: Unhealthy 1 of 365 days2000: Good 246 of 366 days2000: Moderate 115 of 366 days2000: Unhealthy for Sensitive Groups 5 of 366 days20002001: Good 250 of 365 days2001: Moderate 110 of 365 days2001: Unhealthy for Sensitive Groups 5 of 365 days2002: Good 256 of 365 days2002: Moderate 97 of 365 days2002: Unhealthy for Sensitive Groups 11 of 365 days2002: Unhealthy 1 of 365 days2003: Good 240 of 365 days2003: Moderate 102 of 365 days2003: Unhealthy for Sensitive Groups 22 of 365 days2003: Unhealthy 1 of 365 days2004: Good 283 of 366 days2004: Moderate 83 of 366 days2005: Good 242 of 365 days2005: Moderate 115 of 365 days2005: Unhealthy for Sensitive Groups 8 of 365 days20052006: Good 207 of 365 days2006: Moderate 139 of 365 days2006: Unhealthy for Sensitive Groups 16 of 365 days2006: Unhealthy 3 of 365 days2007: Good 204 of 365 days2007: Moderate 139 of 365 days2007: Unhealthy for Sensitive Groups 19 of 365 days2007: Unhealthy 3 of 365 days2008: Good 208 of 366 days2008: Moderate 142 of 366 days2008: Unhealthy for Sensitive Groups 16 of 366 days2009: Good 243 of 365 days2009: Moderate 112 of 365 days2009: Unhealthy for Sensitive Groups 10 of 365 days2010: Good 226 of 365 days2010: Moderate 127 of 365 days2010: Unhealthy for Sensitive Groups 12 of 365 days20102011: Good 212 of 365 days2011: Moderate 137 of 365 days2011: Unhealthy for Sensitive Groups 16 of 365 days2012: Good 185 of 359 days2012: Moderate 139 of 359 days2012: Unhealthy for Sensitive Groups 31 of 359 days2012: Unhealthy 4 of 359 days2013: Good 228 of 360 days2013: Moderate 116 of 360 days2013: Unhealthy for Sensitive Groups 15 of 360 days2013: Unhealthy 1 of 360 days2014: Good 254 of 360 days2014: Moderate 101 of 360 days2014: Unhealthy for Sensitive Groups 5 of 360 days2015: Good 253 of 365 days2015: Moderate 105 of 365 days2015: Unhealthy for Sensitive Groups 7 of 365 days20152016: Good 178 of 204 days2016: Moderate 26 of 204 days2017: Good 233 of 357 days2017: Moderate 113 of 357 days2017: Unhealthy for Sensitive Groups 11 of 357 days2018: Good 198 of 362 days2018: Moderate 135 of 362 days2018: Unhealthy for Sensitive Groups 28 of 362 days2018: Unhealthy 1 of 362 days2019: Good 192 of 365 days2019: Moderate 168 of 365 days2019: Unhealthy for Sensitive Groups 5 of 365 days2020: Good 173 of 366 days2020: Moderate 172 of 366 days2020: Unhealthy for Sensitive Groups 19 of 366 days2020: Unhealthy 2 of 366 days20202021: Good 154 of 365 days2021: Moderate 176 of 365 days2021: Unhealthy for Sensitive Groups 32 of 365 days2021: Unhealthy 3 of 365 days2022: Good 224 of 365 days2022: Moderate 134 of 365 days2022: Unhealthy for Sensitive Groups 7 of 365 days2023: Good 192 of 365 days2023: Moderate 167 of 365 days2023: Unhealthy for Sensitive Groups 4 of 365 days2023: Unhealthy 2 of 365 days2024: Good 212 of 366 days2024: Moderate 144 of 366 days2024: Unhealthy for Sensitive Groups 9 of 366 days2024: Unhealthy 1 of 366 days2025: Good 213 of 365 days2025: Moderate 149 of 365 days2025: Unhealthy for Sensitive Groups 3 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 471981: 90th percentile AQI 741982: 90th percentile AQI 771983: 90th percentile AQI 771984: 90th percentile AQI 871985: 90th percentile AQI 821986: 90th percentile AQI 801987: 90th percentile AQI 931988: 90th percentile AQI 1041989: 90th percentile AQI 871990: 90th percentile AQI 681991: 90th percentile AQI 761992: 90th percentile AQI 671993: 90th percentile AQI 741994: 90th percentile AQI 771995: 90th percentile AQI 641996: 90th percentile AQI 671997: 90th percentile AQI 671998: 90th percentile AQI 841999: 90th percentile AQI 672000: 90th percentile AQI 712001: 90th percentile AQI 692002: 90th percentile AQI 792003: 90th percentile AQI 902004: 90th percentile AQI 612005: 90th percentile AQI 742006: 90th percentile AQI 902007: 90th percentile AQI 932008: 90th percentile AQI 872009: 90th percentile AQI 742010: 90th percentile AQI 772011: 90th percentile AQI 842012: 90th percentile AQI 1002013: 90th percentile AQI 872014: 90th percentile AQI 712015: 90th percentile AQI 712016: 90th percentile AQI 522017: 90th percentile AQI 802018: 90th percentile AQI 962019: 90th percentile AQI 742020: 90th percentile AQI 842021: 90th percentile AQI 1002022: 90th percentile AQI 772023: 90th percentile AQI 742024: 90th percentile AQI 772025: 90th percentile AQI 711980: Median AQI 331981: Median AQI 381982: Median AQI 431983: Median AQI 391984: Median AQI 441985: Median AQI 451986: Median AQI 441987: Median AQI 421988: Median AQI 481989: Median AQI 441990: Median AQI 401991: Median AQI 441992: Median AQI 401993: Median AQI 431994: Median AQI 431995: Median AQI 381996: Median AQI 401997: Median AQI 431998: Median AQI 441999: Median AQI 382000: Median AQI 442001: Median AQI 422002: Median AQI 412003: Median AQI 442004: Median AQI 422005: Median AQI 452006: Median AQI 472007: Median AQI 472008: Median AQI 492009: Median AQI 452010: Median AQI 462011: Median AQI 482012: Median AQI 502013: Median AQI 462014: Median AQI 452015: Median AQI 442016: Median AQI 392017: Median AQI 462018: Median AQI 492019: Median AQI 502020: Median AQI 512021: Median AQI 522022: Median AQI 472023: Median AQI 502024: Median AQI 492025: Median AQI 481980198519901995200020052010201520202025
  • 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
Ozone25670%
Fine particles (PM2.5)10228%
Coarse particles (PM10)72%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
202536521314930004871119Ozone
202436621214491004977154Ozone
202336519216742005074181Ozone
202236522413470004777129Ozone
20213651541763230052100160Ozone
2020366173172192005184181Ozone
201936519216850005074119Ozone
2018362198135281004996159Ozone
2017357233113110004680149Ozone
2016204178260000395297Ozone
201536525310570004471129Ozone
201436025410150004571126Ozone
2013360228116151004687151Ozone
20123591851393140050100172Ozone
2011365212137160004884140Ozone
2010365226127120004677143Ozone
2009365243112100004574147Ozone
2008366208142160004987143Ozone
2007365204139193004793156Ozone
2006365207139163004790154Ozone
200536524211580004574147Ozone
2004366283830000426197Ozone
2003365240102221004490151Ozone
200236525697111004179151Ozone
200136525011050004269119Ozone
200036624611550004471126Ozone
19993652827751003867151Ozone
199836524796175004484177Ozone
19973652837390004367129Ozone
19963662788080004067140Ozone
19953652926580003864119Ozone
19943652659460004377136Ozone
199336526584151004374192Ozone
19923662857920004067122Ozone
199136526389121004476164Ozone
19903652807690004068147Ozone
1989365240107162004487174Ozone
19883661991273820048104172Ozone
1987365226108283004293192Ozone
1986365240103184004480169Ozone
1985364245105121104582201Ozone
198436225192154004487172Ozone
198336427564214003977185Ozone
198232623576132004377187Ozone
198136228660160003874150Ozone
19803493242122003347195Ozone

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 Colorado with a similar median AQI in 2025

All monitored counties in Colorado

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