San Francisco Air Quality in Fall
Across 2019–2025, San Francisco's fall (September–November) air quality had a median AQI of 54 (Moderate) over 565 monitored days in the San Francisco-Oakland-Hayward, CA area. 4.4% of fall days (25 of 565) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 85.3% of days. The cleanest fall on record here was 2025 (median AQI 20, 0 days above 100) and the worst was 2020 (median 59, 14 days above 100, peak AQI 243).
- Good: 34.2%
- Moderate: 61.4%
- Unhealthy for Sensitive Groups: 3.0%
- Unhealthy or worse: 1.4%
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How is the air in San Francisco in fall?
Moderate on a typical day: median AQI 54 across 2019–2025, with 4.4% of fall days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓34.2% of fall days were Good (AQI 0–50), 61.4% Moderate (51–100).
- ✓25 days of 565 reached AQI 101+; the fall peak was 243.
- ✓Cleanest month: November; worst month: September.
Frequently asked questions
What is the air quality like in San Francisco in fall?▾
Across 2019–2025, the median daily AQI in San Francisco in fall was 54 (Moderate). 34.2% of days were Good, 61.4% Moderate, and 4.4% reached Unhealthy for Sensitive Groups or worse, based on 565 monitored days in EPA's Air Quality System.
Is fall a good time for outdoor exercise in San Francisco?▾
A typical fall day in San Francisco is Moderate (median AQI 54): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 23 of fall days (4.4%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
Which pollutant causes San Francisco's fall air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 85.3% of fall days in San Francisco. The highest fall AQI recorded in 2019–2025 was 243 (Very Unhealthy), in 2020.
San Francisco fall air quality, year by year
San Francisco fall air quality by year (San Francisco-Oakland-Hayward, CA, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019 | 91 | 55 | 1 | 102 | PM2.5 |
| 2020worst | 91 | 59 | 14 | 243 | PM2.5 |
| 2021 | 91 | 54 | 5 | 129 | PM2.5 |
| 2022 | 91 | 55 | 2 | 129 | PM2.5 |
| 2023 | 91 | 54 | 3 | 130 | PM2.5 |
| 2024 | 91 | 52 | 0 | 82 | PM2.5 |
| 2025cleanest | 19 | 20 | 0 | 66 | PM2.5 |
What fall air means in San Francisco
A typical fall day in San Francisco is Moderate (median AQI 54): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 23 of fall days (4.4%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
Fall's median AQI of 54 is in line with San Francisco's whole-year median of 53, so this period is neither a clean nor a dirty stretch for the area. Nationally, the median Fall AQI across every EPA metro area (296,275 metro-days, 2019–2025) is 40, so San Francisco runs above the national Fall norm; 4.4% of its Fall days topped AQI 100 versus 1.5% nationally (above average).
Within the season, November is the cleanest month (median AQI 55, 0.0% of days above 100) and September the worst (median 54, 10.0% above 100).
Activity-specific thresholds: running, cycling, kids' sports.
Right now in San Francisco
The latest EPA AirNow observation puts San Francisco's AQI at 36 (PM2.5). The figures above are multi-year season statistics, not today's reading.
View current San Francisco air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (San Francisco-Oakland-Hayward, CA), 2019–2025. Daily AQI per CBSA is the highest AQI reported by any monitor in the area that day; the defining parameter is the pollutant that set it. Month and season statistics pool every such day in the period across all years listed. Retrieved 2026-09-09.