San Francisco Air Quality in December
Across 2019–2025, San Francisco's December air quality had a median AQI of 55 (Moderate) over 186 monitored days in the San Francisco-Oakland-Hayward, CA area. 1.1% of December days (2 of 186) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 91.9% of days. The cleanest December on record here was 2019 (median AQI 46, 0 days above 100) and the worst was 2024 (median 59, 1 day above 100, peak AQI 105).
- Good: 31.2%
- Moderate: 67.7%
- Unhealthy for Sensitive Groups: 1.1%
- Unhealthy or worse: 0.0%
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How is the air in San Francisco in December?
Moderate on a typical day: median AQI 55 across 2019–2025, with 1.1% of December days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓31.2% of December days were Good (AQI 0–50), 67.7% Moderate (51–100).
- ✓2 days of 186 reached AQI 101+; the December peak was 105.
- ✓Best December: 2019; worst December: 2024.
Frequently asked questions
What is the air quality like in San Francisco in December?▾
Across 2019–2025, the median daily AQI in San Francisco in December was 55 (Moderate). 31.2% of days were Good, 67.7% Moderate, and 1.1% reached Unhealthy for Sensitive Groups or worse, based on 186 monitored days in EPA's Air Quality System.
Is December a good time for outdoor exercise in San Francisco?▾
A typical December day in San Francisco is Moderate (median AQI 55): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 91 of December days (1.1%) 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 December air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 91.9% of December days in San Francisco. The highest December AQI recorded in 2019–2025 was 105 (Unhealthy for Sensitive Groups), in 2024.
San Francisco December air quality, year by year
San Francisco December 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 |
|---|---|---|---|---|---|
| 2019cleanest | 31 | 46 | 0 | 82 | PM2.5 |
| 2020 | 31 | 60 | 0 | 100 | PM2.5 |
| 2021 | 31 | 49 | 0 | 92 | PM2.5 |
| 2022 | 31 | 56 | 1 | 105 | PM2.5 |
| 2023 | 31 | 57 | 0 | 83 | PM2.5 |
| 2024worst | 31 | 59 | 1 | 105 | PM2.5 |
What December air means in San Francisco
A typical December day in San Francisco is Moderate (median AQI 55): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 91 of December days (1.1%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
December's median AQI of 55 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 December AQI across every EPA metro area (95,590 metro-days, 2019–2025) is 39, so San Francisco runs above the national December norm; 1.1% of its December days topped AQI 100 versus 0.7% nationally (about the same).
December is part of San Francisco's winter season (December, January, February); the winter 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.