EnviroNearby

San Mateo County, California air quality history

Daily Air Quality Index summaries from EPA monitors in San Mateo, California, 1980–2024.

88%
Good days in 2024
27
Median daily AQI, 2024
0
Days above 100 in 2024
319
Days with an AQI in 2024 (of 366)

In 2024, EPA reported a daily Air Quality Index (AQI) for San Mateo, California on 319 days. 281 of those days (88%) were in the Good range, and the median daily AQI was 27. That median is lower than the typical monitored county in California (median 46.5 across 52 counties), and lower than the typical monitored U.S. county (41). The main pollutant was fine particles (PM2.5) on 258 days. Across the years with fuller monitoring, the share of Good days fell: an average of 90% in 1980–1982 versus 86% in 2022–2024.

The most recent year with data for this county is 2024; EPA's files for 2025–2025 contain no row for it, which usually means its monitors stopped reporting.

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 294 of 365 days1980: Moderate 58 of 365 days1980: Unhealthy for Sensitive Groups 11 of 365 days1980: Unhealthy 2 of 365 days19801981: Good 346 of 365 days1981: Moderate 18 of 365 days1981: Unhealthy 1 of 365 days1982: Good 344 of 365 days1982: Moderate 21 of 365 days1983: Good 327 of 365 days1983: Moderate 34 of 365 days1983: Unhealthy for Sensitive Groups 2 of 365 days1983: Unhealthy 2 of 365 days1984: Good 316 of 366 days1984: Moderate 47 of 366 days1984: Unhealthy for Sensitive Groups 3 of 366 days1985: Good 300 of 365 days1985: Moderate 57 of 365 days1985: Unhealthy for Sensitive Groups 8 of 365 days19851986: Good 284 of 365 days1986: Moderate 73 of 365 days1986: Unhealthy for Sensitive Groups 8 of 365 days1987: Good 315 of 365 days1987: Moderate 44 of 365 days1987: Unhealthy for Sensitive Groups 6 of 365 days1988: Good 299 of 366 days1988: Moderate 57 of 366 days1988: Unhealthy for Sensitive Groups 10 of 366 days1989: Good 282 of 365 days1989: Moderate 75 of 365 days1989: Unhealthy for Sensitive Groups 8 of 365 days1990: Good 305 of 365 days1990: Moderate 58 of 365 days1990: Unhealthy for Sensitive Groups 2 of 365 days19901991: Good 290 of 365 days1991: Moderate 72 of 365 days1991: Unhealthy for Sensitive Groups 3 of 365 days1992: Good 321 of 366 days1992: Moderate 45 of 366 days1993: Good 307 of 365 days1993: Moderate 57 of 365 days1993: Unhealthy for Sensitive Groups 1 of 365 days1994: Good 323 of 365 days1994: Moderate 41 of 365 days1994: Unhealthy for Sensitive Groups 1 of 365 days1995: Good 331 of 365 days1995: Moderate 29 of 365 days1995: Unhealthy for Sensitive Groups 3 of 365 days1995: Unhealthy 2 of 365 days19951996: Good 341 of 366 days1996: Moderate 25 of 366 days1997: Good 347 of 365 days1997: Moderate 17 of 365 days1997: Unhealthy for Sensitive Groups 1 of 365 days1998: Good 357 of 365 days1998: Moderate 8 of 365 days1999: Good 303 of 365 days1999: Moderate 59 of 365 days1999: Unhealthy for Sensitive Groups 2 of 365 days1999: Unhealthy 1 of 365 days2000: Good 311 of 366 days2000: Moderate 50 of 366 days2000: Unhealthy for Sensitive Groups 5 of 366 days20002001: Good 311 of 365 days2001: Moderate 49 of 365 days2001: Unhealthy for Sensitive Groups 4 of 365 days2001: Unhealthy 1 of 365 days2002: Good 301 of 365 days2002: Moderate 61 of 365 days2002: Unhealthy for Sensitive Groups 3 of 365 days2003: Good 315 of 365 days2003: Moderate 47 of 365 days2003: Unhealthy for Sensitive Groups 3 of 365 days2004: Good 186 of 364 days2004: Moderate 172 of 364 days2004: Unhealthy for Sensitive Groups 6 of 364 days2005: Good 167 of 365 days2005: Moderate 191 of 365 days2005: Unhealthy for Sensitive Groups 7 of 365 days20052006: Good 202 of 365 days2006: Moderate 161 of 365 days2006: Unhealthy for Sensitive Groups 1 of 365 days2006: Unhealthy 1 of 365 days2007: Good 205 of 365 days2007: Moderate 155 of 365 days2007: Unhealthy for Sensitive Groups 5 of 365 days2008: Good 187 of 366 days2008: Moderate 178 of 366 days2008: Unhealthy for Sensitive Groups 1 of 366 days2009: Good 186 of 365 days2009: Moderate 179 of 365 days2010: Good 250 of 362 days2010: Moderate 110 of 362 days2010: Unhealthy for Sensitive Groups 2 of 362 days20102011: Good 236 of 365 days2011: Moderate 128 of 365 days2011: Unhealthy for Sensitive Groups 1 of 365 days2012: Good 233 of 366 days2012: Moderate 133 of 366 days2013: Good 193 of 365 days2013: Moderate 168 of 365 days2013: Unhealthy for Sensitive Groups 4 of 365 days2014: Good 278 of 365 days2014: Moderate 87 of 365 days2015: Good 297 of 365 days2015: Moderate 67 of 365 days2015: Unhealthy for Sensitive Groups 1 of 365 days20152016: Good 214 of 366 days2016: Moderate 152 of 366 days2017: Good 238 of 365 days2017: Moderate 120 of 365 days2017: Unhealthy for Sensitive Groups 4 of 365 days2017: Unhealthy 3 of 365 days2018: Good 236 of 365 days2018: Moderate 116 of 365 days2018: Unhealthy for Sensitive Groups 3 of 365 days2018: Unhealthy 10 of 365 days2019: Good 276 of 365 days2019: Moderate 87 of 365 days2019: Unhealthy for Sensitive Groups 2 of 365 days2020: Good 257 of 366 days2020: Moderate 99 of 366 days2020: Unhealthy for Sensitive Groups 5 of 366 days2020: Unhealthy 5 of 366 days20202021: Good 290 of 365 days2021: Moderate 75 of 365 days2022: Good 267 of 365 days2022: Moderate 98 of 365 days2023: Good 355 of 363 days2023: Moderate 8 of 363 days2024: Good 281 of 319 days2024: Moderate 38 of 319 days2024
  • 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 year0501001980: 90th percentile AQI 681981: 90th percentile AQI 471982: 90th percentile AQI 461983: 90th percentile AQI 511984: 90th percentile AQI 571985: 90th percentile AQI 681986: 90th percentile AQI 681987: 90th percentile AQI 571988: 90th percentile AQI 681989: 90th percentile AQI 791990: 90th percentile AQI 571991: 90th percentile AQI 681992: 90th percentile AQI 571993: 90th percentile AQI 581994: 90th percentile AQI 551995: 90th percentile AQI 491996: 90th percentile AQI 471997: 90th percentile AQI 431998: 90th percentile AQI 431999: 90th percentile AQI 582000: 90th percentile AQI 542001: 90th percentile AQI 552002: 90th percentile AQI 582003: 90th percentile AQI 532004: 90th percentile AQI 722005: 90th percentile AQI 742006: 90th percentile AQI 662007: 90th percentile AQI 672008: 90th percentile AQI 722009: 90th percentile AQI 682010: 90th percentile AQI 612011: 90th percentile AQI 642012: 90th percentile AQI 612013: 90th percentile AQI 702014: 90th percentile AQI 592015: 90th percentile AQI 562016: 90th percentile AQI 592017: 90th percentile AQI 622018: 90th percentile AQI 612019: 90th percentile AQI 572020: 90th percentile AQI 642021: 90th percentile AQI 562022: 90th percentile AQI 602023: 90th percentile AQI 412024: 90th percentile AQI 511980: Median AQI 311981: Median AQI 281982: Median AQI 281983: Median AQI 301984: Median AQI 331985: Median AQI 341986: Median AQI 381987: Median AQI 371988: Median AQI 381989: Median AQI 381990: Median AQI 381991: Median AQI 381992: Median AQI 331993: Median AQI 321994: Median AQI 331995: Median AQI 351996: Median AQI 341997: Median AQI 311998: Median AQI 331999: Median AQI 332000: Median AQI 332001: Median AQI 332002: Median AQI 352003: Median AQI 342004: Median AQI 502005: Median AQI 522006: Median AQI 472007: Median AQI 462008: Median AQI 492009: Median AQI 502010: Median AQI 422011: Median AQI 422012: Median AQI 422013: Median AQI 492014: Median AQI 382015: Median AQI 352016: Median AQI 472017: Median AQI 422018: Median AQI 422019: Median AQI 392020: Median AQI 372021: Median AQI 362022: Median AQI 362023: Median AQI 312024: Median AQI 271980198519901995200020052010201520202024
  • Median daily AQI
  • 90th-percentile daily AQI

Main pollutant by day, 2024

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)25881%
Ozone4815%
Nitrogen dioxide (NO₂)103%
Carbon monoxide (CO)31%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
2024319281380000275173PM2.5
202336335580000314171Ozone
2022365267980000366085PM2.5
2021365290750000365690Ozone
20203662579955003764199PM2.5
20193652768720003957122PM2.5
2018365236116310004261197PM2.5
201736523812043004262155PM2.5
20163662141520000475970PM2.5
20153652976710003556101Ozone
2014365278870000385999PM2.5
201336519316840004970115PM2.5
20123662331330000426198PM2.5
201136523612810004264111PM2.5
201036225011020004261122PM2.5
20093651861790000506898PM2.5
200836618717810004972102PM2.5
200736520515550004667126PM2.5
200636520216111004766165PM2.5
200536516719170005274133PM2.5
200436418617260005072120PM2.5
20033653154730003453126Ozone
20023653016130003558119Ozone
20013653114941003355162NO2
20003663115050003354129NO2
19993653035921003358154NO2
199836535780000334361NO2
19973653471710003143108NO2
1996366341250000344790NO2
19953653312932003549185NO2
19943653234110003355102NO2
19933653075710003258119NO2
19923663214500003357100NO2
19913652907230003868105NO2
19903653055820003857105NO2
19893652827580003879125NO2
198836629957100003868119NO2
19873653154460003757112NO2
19863652847380003868106NO2
19853653005780003468122NO2
19843663164730003357133NO2
19833653273422003051157Ozone
1982365344210000284679Ozone
19813653461801002847154Ozone
198036529458112003168192Ozone

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 2024

All monitored counties in California

Source: U.S. EPA AirData, “Annual AQI by County” files for 1980–2024 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem