EnviroNearby

Siskiyou County, California air quality history

Daily Air Quality Index summaries from EPA monitors in Siskiyou, California, 1981–2025.

78%
Good days in 2025
37
Median daily AQI, 2025
4
Days above 100 in 2025
363
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Siskiyou, California on 363 days. 284 of those days (78%) were in the Good range, and the median daily AQI was 37. That median is lower than the typical monitored county in California (median 45.5 across 46 counties), and lower than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 197 days. Across the years with fuller monitoring, the share of Good days fell: an average of 97% in 1981–1983 versus 78% in 2023–2025.

Years with fewer than 120 monitored days (1991) 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%1981: Good 138 of 141 days1981: Moderate 3 of 141 days19811982: Good 341 of 358 days1982: Moderate 17 of 358 days1983: Good 272 of 282 days1983: Moderate 10 of 282 days1984: Good 256 of 272 days1984: Moderate 16 of 272 days1985: Good 317 of 337 days1985: Moderate 19 of 337 days1985: Unhealthy for Sensitive Groups 1 of 337 days1986: Good 285 of 322 days1986: Moderate 37 of 322 days19861987: Good 206 of 268 days1987: Moderate 56 of 268 days1987: Unhealthy for Sensitive Groups 5 of 268 days1987: Unhealthy 1 of 268 days1988: Good 272 of 306 days1988: Moderate 33 of 306 days1988: Unhealthy for Sensitive Groups 1 of 306 days1989: Good 292 of 329 days1989: Moderate 35 of 329 days1989: Unhealthy for Sensitive Groups 2 of 329 days1990: Good 173 of 213 days1990: Moderate 37 of 213 days1990: Unhealthy for Sensitive Groups 3 of 213 days1991: Good 113 of 115 days1991: Moderate 2 of 115 days19911992: Good 270 of 294 days1992: Moderate 23 of 294 days1992: Unhealthy for Sensitive Groups 1 of 294 days1993: Good 331 of 335 days1993: Moderate 4 of 335 days1994: Good 326 of 356 days1994: Moderate 30 of 356 days1995: Good 315 of 321 days1995: Moderate 6 of 321 days1996: Good 168 of 181 days1996: Moderate 12 of 181 days1996: Unhealthy for Sensitive Groups 1 of 181 days19961997: Good 257 of 285 days1997: Moderate 27 of 285 days1997: Unhealthy for Sensitive Groups 1 of 285 days1998: Good 320 of 345 days1998: Moderate 24 of 345 days1998: Unhealthy for Sensitive Groups 1 of 345 days1999: Good 337 of 362 days1999: Moderate 25 of 362 days2000: Good 292 of 310 days2000: Moderate 17 of 310 days2000: Unhealthy for Sensitive Groups 1 of 310 days2001: Good 287 of 288 days2001: Moderate 1 of 288 days20012002: Good 299 of 352 days2002: Moderate 52 of 352 days2002: Unhealthy for Sensitive Groups 1 of 352 days2003: Good 234 of 291 days2003: Moderate 54 of 291 days2003: Unhealthy for Sensitive Groups 3 of 291 days2004: Good 327 of 352 days2004: Moderate 24 of 352 days2004: Unhealthy for Sensitive Groups 1 of 352 days2005: Good 330 of 356 days2005: Moderate 26 of 356 days2006: Good 276 of 333 days2006: Moderate 55 of 333 days2006: Unhealthy for Sensitive Groups 2 of 333 days20062007: Good 326 of 348 days2007: Moderate 21 of 348 days2007: Unhealthy for Sensitive Groups 1 of 348 days2008: Good 321 of 360 days2008: Moderate 37 of 360 days2008: Unhealthy for Sensitive Groups 2 of 360 days2009: Good 308 of 323 days2009: Moderate 14 of 323 days2009: Unhealthy for Sensitive Groups 1 of 323 days2010: Good 350 of 361 days2010: Moderate 11 of 361 days2011: Good 334 of 356 days2011: Moderate 22 of 356 days20112012: Good 324 of 362 days2012: Moderate 35 of 362 days2012: Unhealthy for Sensitive Groups 1 of 362 days2012: Unhealthy 2 of 362 days2013: Good 323 of 361 days2013: Moderate 34 of 361 days2013: Unhealthy for Sensitive Groups 4 of 361 days2014: Good 330 of 363 days2014: Moderate 31 of 363 days2014: Unhealthy 2 of 363 days2015: Good 330 of 362 days2015: Moderate 28 of 362 days2015: Unhealthy for Sensitive Groups 4 of 362 days2016: Good 351 of 362 days2016: Moderate 11 of 362 days20162017: Good 198 of 225 days2017: Moderate 23 of 225 days2017: Unhealthy for Sensitive Groups 3 of 225 days2017: Unhealthy 1 of 225 days2018: Good 247 of 365 days2018: Moderate 78 of 365 days2018: Unhealthy for Sensitive Groups 16 of 365 days2018: Unhealthy 21 of 365 days2018: Very Unhealthy 3 of 365 days2019: Good 302 of 365 days2019: Moderate 59 of 365 days2019: Unhealthy for Sensitive Groups 2 of 365 days2019: Unhealthy 2 of 365 days2020: Good 272 of 366 days2020: Moderate 71 of 366 days2020: Unhealthy for Sensitive Groups 10 of 366 days2020: Unhealthy 9 of 366 days2020: Very Unhealthy 2 of 366 days2020: Hazardous 2 of 366 days2021: Good 237 of 364 days2021: Moderate 96 of 364 days2021: Unhealthy for Sensitive Groups 7 of 364 days2021: Unhealthy 21 of 364 days2021: Very Unhealthy 3 of 364 days20212022: Good 269 of 362 days2022: Moderate 86 of 362 days2022: Unhealthy for Sensitive Groups 2 of 362 days2022: Unhealthy 4 of 362 days2022: Hazardous 1 of 362 days2023: Good 270 of 365 days2023: Moderate 78 of 365 days2023: Unhealthy for Sensitive Groups 7 of 365 days2023: Unhealthy 6 of 365 days2023: Very Unhealthy 3 of 365 days2023: Hazardous 1 of 365 days2024: Good 294 of 366 days2024: Moderate 72 of 366 days2025: Good 284 of 363 days2025: Moderate 75 of 363 days2025: Unhealthy for Sensitive Groups 2 of 363 days2025: Unhealthy 2 of 363 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 year0501001501981: 90th percentile AQI 431982: 90th percentile AQI 461983: 90th percentile AQI 431984: 90th percentile AQI 471985: 90th percentile AQI 471986: 90th percentile AQI 541987: 90th percentile AQI 711988: 90th percentile AQI 511989: 90th percentile AQI 511990: 90th percentile AQI 671991: 90th percentile AQI 371992: 90th percentile AQI 481993: 90th percentile AQI 421994: 90th percentile AQI 481995: 90th percentile AQI 441996: 90th percentile AQI 471997: 90th percentile AQI 501998: 90th percentile AQI 471999: 90th percentile AQI 472000: 90th percentile AQI 492001: 90th percentile AQI 292002: 90th percentile AQI 612003: 90th percentile AQI 612004: 90th percentile AQI 492005: 90th percentile AQI 482006: 90th percentile AQI 582007: 90th percentile AQI 472008: 90th percentile AQI 522009: 90th percentile AQI 442010: 90th percentile AQI 452011: 90th percentile AQI 482012: 90th percentile AQI 512013: 90th percentile AQI 512014: 90th percentile AQI 502015: 90th percentile AQI 492016: 90th percentile AQI 442017: 90th percentile AQI 572018: 90th percentile AQI 1192019: 90th percentile AQI 542020: 90th percentile AQI 662021: 90th percentile AQI 862022: 90th percentile AQI 622023: 90th percentile AQI 642024: 90th percentile AQI 582025: 90th percentile AQI 571981: Median AQI 281982: Median AQI 311983: Median AQI 281984: Median AQI 301985: Median AQI 281986: Median AQI 371987: Median AQI 421988: Median AQI 351989: Median AQI 351990: Median AQI 321991: Median AQI 191992: Median AQI 331993: Median AQI 301994: Median AQI 351995: Median AQI 301996: Median AQI 291997: Median AQI 371998: Median AQI 351999: Median AQI 332000: Median AQI 362001: Median AQI 132002: Median AQI 332003: Median AQI 392004: Median AQI 332005: Median AQI 352006: Median AQI 362007: Median AQI 322008: Median AQI 352009: Median AQI 312010: Median AQI 332011: Median AQI 372012: Median AQI 372013: Median AQI 372014: Median AQI 372015: Median AQI 362016: Median AQI 272017: Median AQI 282018: Median AQI 422019: Median AQI 382020: Median AQI 392021: Median AQI 432022: Median AQI 412023: Median AQI 432024: Median AQI 402025: Median AQI 371981198619911996200120062011201620212025
  • 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
Fine particles (PM2.5)19754%
Ozone16646%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20253632847522003757166PM2.5
2024366294720000405894Ozone
20233652707876314364320Ozone
20223622698624014162454Ozone
202136423796721304386210PM2.5
202036627271109223966468Ozone
20193653025922003854164Ozone
20183652477816213042119219Ozone
20172251982331002857167Ozone
2016362351110000274493Ozone
20153623302840003649139Ozone
20143633303102003750162Ozone
20133613233440003751121Ozone
20123623243512003751168Ozone
2011356334220000374871Ozone
2010361350110000334590Ozone
20093233081410003144110Ozone
20083603213720003552115Ozone
20073483262110003247126Ozone
20063332765520003658105Ozone
2005356330260000354882Ozone
20043523272410003349101Ozone
20032912345430003961112Ozone
20023522995210003361115Ozone
200128828710000132973Ozone
20003102921710003649101Ozone
1999362337250000334790Ozone
19983453202410003547101Ozone
19972852572710003750112Ozone
19961811681210002947117Ozone
199532131560000304474Ozone
1994356326300000354893Ozone
1993335331400003042100Ozone
19922942702310003348108Ozone
199111511320000193753Ozone
19902131733730003267119Ozone
19893292923520003551119Ozone
19883062723310003551101Ozone
19872682065651004271190Ozone
19863222853700003754100Ozone
19853373171910002847115Ozone
1984272256160000304787Ozone
1983282272100000284374Ozone
1982358341170000314687Ozone
198114113830000284358Ozone

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 California with a similar median AQI in 2025

All monitored counties in California

Source: U.S. EPA AirData, “Annual AQI by County” files for 1981–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem