Median AQI
27
Good
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
The verdict
According to EPA Air Quality System monitoring data, San Mateo County's air is good on balance, cleaner than the median US county, a median AQI of 27 with 88.1% of 319 monitored days in the Good band, driven mainly by PM2.5.
- 27
- median AQI · Good
- 88.1%
- days rated Good
- PM2.5
- main pollutant
- 67%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 73.
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 →Air quality data from EPA Air Quality System (2024). 319 days monitored. Median AQI: 27 (Good).
Insights: San Mateo County, California. EPA monitors recorded a median Air Quality Index of 27 across 319 measured days in 2024, earning a Good classification and a Good health grade. Of those days, 281 (88%) fell in the Good band (AQI 0–50), 38 (12%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 51, while the worst single-day peak hit 73 (Moderate). Residents averaged about 26.4 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (258 days), Ozone (48 days), NO2 (10 days), CO (3 days). Across the 2020–2024 trend window, the share of monitored days rated Good shifted by +17.9 points (measured against 366 monitored days in 2020 and 319 in 2024, so raw day counts are not comparable) and the median AQI moved -10 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2024 window was AQI 73, moderate but never Unhealthy. Nearby counties show comparable readings: Alameda (53.0% good, AQI 49); Amador (87.4% good, AQI 38); Butte (65.2% good, AQI 46); Calaveras (82.2% good, AQI 40); Colusa (79.2% good, AQI 40); Contra Costa (53.5% good, AQI 48).
Air Quality Summary
Good Days
88.1%
281 of 319 monitored days
13.1pp vs 75% target
Max AQI Recorded
73
Moderate
Health Grade
Good
90th-pct AQI 51
San Mateo County, Good days share 88.1%
National avg
Share of monitored days at AQI 0-50 (Good band).
San Mateo County, Unhealthy-or-worse share 0.0%
National avg
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
AQI Day Breakdown (2024)
Distribution of 319 monitored days across EPA AQI categories.
Good
281
88%
Moderate
38
12%
USG
0
0%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
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%
Health Impact Assessment
Good
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
Good-day share better than US avg
San Mateo County 88.1%
San Francisco County, California 76.0%
US average 76.0%
Median AQI better than US avg
San Mateo County 27
San Francisco County, California 33
US average 39
Unhealthy days / yr better than US avg
San Mateo County 0
San Francisco County, California 0
US average 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as San Mateo County (27), not geographic neighbors.
San Francisco County, California
Median AQI 33 · 76.0% Good-day share
Columbia County, Georgia
Median AQI 36 · 94.2% Good-day share
Sumter County, Georgia
Median AQI 38 · 93.4% Good-day share
Big Horn County, Wyoming
Median AQI 39 · 89.2% Good-day share
Chattooga County, Georgia
Median AQI 39 · 96.3% Good-day share
Weston County, Wyoming
Median AQI 39 · 87.7% Good-day share
Same primary pollutant: PM2.5
PM2.5 led the AQI on the most monitored days here. These peers share that driver and the closest Good-day share (a second relationship type beyond median-AQI neighbors).
California counties on the map
Geographic containment only, other monitored counties in California, independent of AQI or pollutant match.
What San Mateo County's air means for you
The air here stays in the breathable range year-round, with PM2.5 the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 27 means for exposure. Health guidance
- See how San Mateo County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare San Mateo County with its California neighbours before drawing a conclusion about the region. All of California
Based on 319 EPA-monitored days in 2024; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
Keep exploring the annual AQI record
Air-quality reading list
Frequently Asked Questions
What is the air quality in San Mateo County, California? ▼
San Mateo County has a median Air Quality Index (AQI) of 27, rated "Good." 88.1% of monitored days (281 out of 319) had good air quality in 2024.
What is the main pollutant in San Mateo County? ▼
The primary pollutant in San Mateo County is Fine Particles (PM2.5), which was the main AQI driver on 258 days (81% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does San Mateo County have? ▼
In 2024, San Mateo County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 73.
Is the air quality in San Mateo County getting better or worse? ▼
Air quality in San Mateo County is improving across floor-qualifying years (2020-2024); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2023 at 97.8% (355 of 363 days). Lowest qualifying Good-day share: 2020 at 70.2% (257 of 366 days).
What does AQI 27 mean for health? ▼
San Mateo County's median AQI of 27 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does San Mateo County compare to other counties in California? ▼
San Mateo County has 88.1% good air days. Nearby counties in California: Alameda (53.0% good), Amador (87.4% good), Butte (65.2% good), Calaveras (82.2% good), Colusa (79.2% good), Contra Costa (53.5% good).
What is the 90th percentile AQI in San Mateo County? ▼
The 90th percentile AQI for San Mateo County is 51, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 73 (Moderate).
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
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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.