EnviroNearby

Loudoun County, Virginia air quality history

Daily Air Quality Index summaries from EPA monitors in Loudoun, Virginia, 1990–2025.

81%
Good days in 2025
40
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 Loudoun, Virginia on 365 days. 294 of those days (81%) were in the Good range, and the median daily AQI was 40. That median is higher than the typical monitored county in Virginia (median 37 across 25 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was ozone on 232 days. Across the years with fuller monitoring, the share of Good days rose: an average of 63% in 1998–2000 versus 77% in 2023–2025.

Years with fewer than 120 monitored days (1992, 1993, 1994, 1995, 1996, 1997…) 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%1990: Good 38 of 39 days1990: Moderate 1 of 39 days19901991: Good 57 of 57 days1992: Good 59 of 59 days1993: Good 58 of 60 days1993: Moderate 2 of 60 days1994: Good 59 of 59 days1995: Good 59 of 59 days19951996: Good 56 of 58 days1996: Moderate 2 of 58 days1997: Good 42 of 42 days1998: Good 161 of 272 days1998: Moderate 47 of 272 days1998: Unhealthy for Sensitive Groups 49 of 272 days1998: Unhealthy 14 of 272 days1998: Very Unhealthy 1 of 272 days1999: Good 230 of 364 days1999: Moderate 91 of 364 days1999: Unhealthy for Sensitive Groups 36 of 364 days1999: Unhealthy 6 of 364 days1999: Very Unhealthy 1 of 364 days2000: Good 237 of 362 days2000: Moderate 106 of 362 days2000: Unhealthy for Sensitive Groups 18 of 362 days2000: Unhealthy 1 of 362 days20002001: Good 221 of 365 days2001: Moderate 116 of 365 days2001: Unhealthy for Sensitive Groups 20 of 365 days2001: Unhealthy 8 of 365 days2002: Good 206 of 363 days2002: Moderate 95 of 363 days2002: Unhealthy for Sensitive Groups 39 of 363 days2002: Unhealthy 22 of 363 days2002: Very Unhealthy 1 of 363 days2003: Good 234 of 365 days2003: Moderate 112 of 365 days2003: Unhealthy for Sensitive Groups 16 of 365 days2003: Unhealthy 2 of 365 days2003: Very Unhealthy 1 of 365 days2004: Good 243 of 365 days2004: Moderate 108 of 365 days2004: Unhealthy for Sensitive Groups 12 of 365 days2004: Unhealthy 1 of 365 days2004: Very Unhealthy 1 of 365 days2005: Good 236 of 348 days2005: Moderate 99 of 348 days2005: Unhealthy for Sensitive Groups 13 of 348 days20052006: Good 230 of 355 days2006: Moderate 105 of 355 days2006: Unhealthy for Sensitive Groups 18 of 355 days2006: Unhealthy 2 of 355 days2007: Good 222 of 363 days2007: Moderate 109 of 363 days2007: Unhealthy for Sensitive Groups 27 of 363 days2007: Unhealthy 5 of 363 days2008: Good 248 of 366 days2008: Moderate 102 of 366 days2008: Unhealthy for Sensitive Groups 13 of 366 days2008: Unhealthy 3 of 366 days2009: Good 279 of 362 days2009: Moderate 83 of 362 days2010: Good 245 of 363 days2010: Moderate 107 of 363 days2010: Unhealthy for Sensitive Groups 10 of 363 days2010: Unhealthy 1 of 363 days20102011: Good 265 of 365 days2011: Moderate 93 of 365 days2011: Unhealthy for Sensitive Groups 6 of 365 days2011: Unhealthy 1 of 365 days2012: Good 274 of 363 days2012: Moderate 85 of 363 days2012: Unhealthy for Sensitive Groups 4 of 363 days2013: Good 300 of 365 days2013: Moderate 63 of 365 days2013: Unhealthy for Sensitive Groups 2 of 365 days2014: Good 285 of 355 days2014: Moderate 69 of 355 days2014: Unhealthy for Sensitive Groups 1 of 355 days2015: Good 294 of 361 days2015: Moderate 63 of 361 days2015: Unhealthy for Sensitive Groups 4 of 361 days20152016: Good 302 of 366 days2016: Moderate 63 of 366 days2016: Unhealthy for Sensitive Groups 1 of 366 days2017: Good 297 of 365 days2017: Moderate 68 of 365 days2018: Good 313 of 365 days2018: Moderate 52 of 365 days2019: Good 286 of 337 days2019: Moderate 50 of 337 days2019: Unhealthy for Sensitive Groups 1 of 337 days2020: Good 339 of 366 days2020: Moderate 26 of 366 days2020: Unhealthy for Sensitive Groups 1 of 366 days20202021: Good 308 of 364 days2021: Moderate 55 of 364 days2021: Unhealthy for Sensitive Groups 1 of 364 days2022: Good 328 of 364 days2022: Moderate 34 of 364 days2022: Unhealthy for Sensitive Groups 2 of 364 days2023: Good 306 of 364 days2023: Moderate 54 of 364 days2023: Unhealthy for Sensitive Groups 2 of 364 days2023: Unhealthy 2 of 364 days2024: Good 245 of 366 days2024: Moderate 121 of 366 days2025: Good 294 of 365 days2025: Moderate 70 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 year0501001501990: 90th percentile AQI 341991: 90th percentile AQI 361992: 90th percentile AQI 291993: 90th percentile AQI 341994: 90th percentile AQI 311995: 90th percentile AQI 311996: 90th percentile AQI 281997: 90th percentile AQI 241998: 90th percentile AQI 1331999: 90th percentile AQI 1122000: 90th percentile AQI 772001: 90th percentile AQI 972002: 90th percentile AQI 1262003: 90th percentile AQI 772004: 90th percentile AQI 802005: 90th percentile AQI 742006: 90th percentile AQI 862007: 90th percentile AQI 972008: 90th percentile AQI 742009: 90th percentile AQI 642010: 90th percentile AQI 742011: 90th percentile AQI 672012: 90th percentile AQI 712013: 90th percentile AQI 552014: 90th percentile AQI 582015: 90th percentile AQI 602016: 90th percentile AQI 582017: 90th percentile AQI 572018: 90th percentile AQI 542019: 90th percentile AQI 542020: 90th percentile AQI 482021: 90th percentile AQI 542022: 90th percentile AQI 502023: 90th percentile AQI 582024: 90th percentile AQI 612025: 90th percentile AQI 581990: Median AQI 191991: Median AQI 161992: Median AQI 171993: Median AQI 141994: Median AQI 131995: Median AQI 141996: Median AQI 151997: Median AQI 141998: Median AQI 461999: Median AQI 442000: Median AQI 402001: Median AQI 452002: Median AQI 462003: Median AQI 432004: Median AQI 422005: Median AQI 332006: Median AQI 442007: Median AQI 442008: Median AQI 402009: Median AQI 362010: Median AQI 422011: Median AQI 382012: Median AQI 382013: Median AQI 352014: Median AQI 402015: Median AQI 382016: Median AQI 362017: Median AQI 382018: Median AQI 352019: Median AQI 382020: Median AQI 342021: Median AQI 372022: Median AQI 372023: Median AQI 392024: Median AQI 432025: Median AQI 4019901995200020052010201520202025
  • 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
Ozone23264%
Fine particles (PM2.5)12434%
Nitrogen dioxide (NO₂)92%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20253652947010004058120Ozone
20243662451210000436187Ozone
20233643065422003958167Ozone
20223643283420003750112Ozone
20213643085510003754122Ozone
20203663392610003448108Ozone
20193372865010003854112Ozone
2018365313520000355493Ozone
20173652976800003857100Ozone
20163663026310003658112Ozone
20153612946340003860115Ozone
20143552856910004058105Ozone
20133653006320003555115Ozone
20123632748540003871129Ozone
20113652659361003867151Ozone
2010363245107101004274166Ozone
2009362279830000366497Ozone
2008366248102133004074187Ozone
2007363222109275004497164Ozone
2006355230105182004486192Ozone
200534823699130003374140NO2
2004365243108121104280202Ozone
2003365234112162104377212Ozone
20023632069539221046126215Ozone
2001365221116208004597197Ozone
2000362237106181004077169Ozone
1999364230913661044112201Ozone
19982721614749141046133202Ozone
1997424200000142433PM10
1996585620000152854PM10
1995595900000143143PM10
1994595900000133148PM10
1993605820000143455PM10
1992595900000172934PM10
1991575700000163649PM10
1990393810000193457PM10

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 Virginia with a similar median AQI in 2025

All monitored counties in Virginia

Source: U.S. EPA AirData, “Annual AQI by County” files for 1990–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem