San Francisco Air Quality in May
Across 2019–2025, San Francisco's May air quality had a median AQI of 53 (Moderate) over 197 monitored days in the San Francisco-Oakland-Hayward, CA area. 0.5% of May days (1 of 197) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 82.2% of days. The cleanest May on record here was 2025 (median AQI 42, 0 days above 100) and the worst was 2023 (median 58, 1 day above 100, peak AQI 101).
- Good: 39.1%
- Moderate: 60.4%
- Unhealthy for Sensitive Groups: 0.5%
- Unhealthy or worse: 0.0%
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How is the air in San Francisco in May?
Moderate on a typical day: median AQI 53 across 2019–2025, with 0.5% of May days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓39.1% of May days were Good (AQI 0–50), 60.4% Moderate (51–100).
- ✓1 day of 197 reached AQI 101+; the May peak was 101.
- ✓Best May: 2025; worst May: 2023.
Frequently asked questions
What is the air quality like in San Francisco in May?▾
Across 2019–2025, the median daily AQI in San Francisco in May was 53 (Moderate). 39.1% of days were Good, 60.4% Moderate, and 0.5% reached Unhealthy for Sensitive Groups or worse, based on 197 monitored days in EPA's Air Quality System.
Is May a good time for outdoor exercise in San Francisco?▾
A typical May day in San Francisco is Moderate (median AQI 53): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and fewer than 1% of May days 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 May air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 82.2% of May days in San Francisco. The highest May AQI recorded in 2019–2025 was 101 (Unhealthy for Sensitive Groups), in 2023.
San Francisco May air quality, year by year
San Francisco May 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 | 31 | 44 | 0 | 63 | PM2.5 |
| 2020 | 31 | 48 | 0 | 93 | PM2.5 |
| 2021 | 31 | 61 | 0 | 73 | PM2.5 |
| 2022 | 31 | 53 | 0 | 80 | PM2.5 |
| 2023worst | 31 | 58 | 1 | 101 | PM2.5 |
| 2024 | 31 | 54 | 0 | 73 | PM2.5 |
| 2025cleanest | 11 | 42 | 0 | 62 | PM2.5 |
What May air means in San Francisco
A typical May day in San Francisco is Moderate (median AQI 53): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and fewer than 1% of May days reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
May's median AQI of 53 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 May AQI across every EPA metro area (104,064 metro-days, 2019–2025) is 44, so San Francisco runs above the national May norm; 0.5% of its May days topped AQI 100 versus 1.3% nationally (about the same).
May is part of San Francisco's spring season (March, April, May); the spring page pools all three months.
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 month 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.