California · County air quality · 2024
San Mateo County, California
Median AQI 27 (Good). 88.1% of monitored days rated Good, direct from EPA Air Quality System stations.
- 27
- Median AQI
- 88.1%
- Good-air days
- 0
- Unhealthy days
- 67%
- Cleaner than US counties
San Mateo County's 2024 annual median AQI is 27 (Good), with 88.1% Good days
According to the U.S. EPA Air Quality System, San Mateo County, California posted a 2024 median AQI of 27 (Good) across 319 monitored days, with 88.1% rated Good and 0 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- San Mateo County's 88.1% Good-day share places it cleaner than 67% of US counties in 2024.
- Closest median-AQI peer
- San Francisco County (76.0% Good, median AQI 33) is the closest nationwide median-AQI peer to San Mateo County.
- 2020–2024 trend
- San Mateo County's floor-qualifying Good-day share is improving from 2020 to 2024 (70.2% → 88.1%).
- Median AQI
- Median AQI: 27
According to the 2024 EPA AQS county release on PlainAirData, San Mateo County, California is #1 of 15 by median AQI (27, 88.1% Good days) at good-day-share #5 ≠ Chattooga County, Georgia #1 lowest good-day share (96.3%, median AQI 39).
Annual release change
What changed in the 2024 EPA annual release
- San Mateo County's Good-day share fell from 97.8% to 88.1% between the 2023 and 2024 EPA annual releases, a change of -9.7 percentage points.
- San Mateo County's median AQI fell from 31 to 27 between the 2023 and 2024 EPA annual releases, a change of -4 points.
Data updated August 2026
- Good · 281 d · 88.1%
- Moderate · 38 d · 11.9%
How did San Mateo County's air break down in 2024?
Across 319 monitored days, 281 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 73 (Moderate).
EPA AQS →Share of monitored days at AQI 0-50 (Good band).
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
Where San Mateo County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2024. San Mateo County's Good-day share is 88.1% - cleaner than 67% of that comparison set.
San Mateo County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
88.1% Cleaner than 67% higher than 67% of 951 US counties
Each bar is a band; taller bars hold more US counties. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.
Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2024
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in San Mateo County.
- PM2.5 · 258 days · 80.9%
- Ozone · 48 days · 15.0%
- NO₂ · 10 days · 3.1%
- CO · 3 days · 0.9%
2024 daily-monitor rhythm in San Mateo County
Month-by-month median AQI from EPA day-level county monitors (319 reported days in 2024). Dec posted the highest typical readings (median AQI 38, peak 73); Mar was cleanest at median 12. Summer months (Jun–Aug) averaged median AQI 30 vs 32 in Dec–Feb.
Five-year seasonal record
EPA day-level county files · 2020–20241,778 daily readings show a recurring local pattern: Feb has the highest matched-month median AQI (40), while Jul is lowest (27). Each cell is that month’s median daily AQI, not a current-air forecast.
Lower AQI is better. The colors summarize EPA AQI bands; gaps mean the county had no day-level record for that month.
Health Impact Assessment
San Mateo County's air quality is rated Good based on 88.1% good air days. No unhealthy air days were recorded in 2024.
Air quality was acceptable on 88.1% of monitored days in 2024, with a median AQI of 27. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 258 of 319 monitored days.
5-Year Air Quality Trend
Air quality is improving in San Mateo County. Highest Good-day share: 2023 (97.8%, 355 of 363 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 257 | 99 | 5 | 5 | 37 | 199 |
| 2021 | 290 | 75 | 0 | 0 | 36 | 90 |
| 2022 | 267 | 98 | 0 | 0 | 36 | 85 |
| 2023 | 355 | 8 | 0 | 0 | 31 | 71 |
| 2024 | 281 | 38 | 0 | 0 | 27 | 73 |
How does San Mateo County compare?
How San Mateo County compares
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for San Mateo County, California · 2020–2025
Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.
Every figure on PlainAirData is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.