San Diego Air Quality in December
Across 2019–2025, San Diego's December air quality had a median AQI of 73 (Moderate) over 217 monitored days in the San Diego-Carlsbad, CA area. 5.5% of December days (12 of 217) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 88.9% of days. The cleanest December on record here was 2019 (median AQI 60, 0 days above 100) and the worst was 2024 (median 81, 6 days above 100, peak AQI 125).
- Good: 10.1%
- Moderate: 84.3%
- Unhealthy for Sensitive Groups: 5.5%
- Unhealthy or worse: 0.0%
Extra Protection for San Diego Homes
Air quality in San Diego is moderate (AQI 54), with fine particles (PM2.5) the main concern. A True-HEPA purifier gives sensitive groups — kids, seniors and anyone with asthma or allergies — cleaner air indoors on days like today.
Recommended: Coway AP-1512HH Mighty — True HEPA + carbon · CADR ~246 CFM · rooms up to 361 sq ft
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How is the air in San Diego in December?
Moderate on a typical day: median AQI 73 across 2019–2025, with 5.5% of December days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓10.1% of December days were Good (AQI 0–50), 84.3% Moderate (51–100).
- ✓12 days of 217 reached AQI 101+; the December peak was 125.
- ✓Best December: 2019; worst December: 2024.
Frequently asked questions
What is the air quality like in San Diego in December?▾
Across 2019–2025, the median daily AQI in San Diego in December was 73 (Moderate). 10.1% of days were Good, 84.3% Moderate, and 5.5% reached Unhealthy for Sensitive Groups or worse, based on 217 monitored days in EPA's Air Quality System.
Is December a good time for outdoor exercise in San Diego?▾
A typical December day in San Diego is Moderate (median AQI 73): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 18 of December days (5.5%) 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 Diego's December air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 88.9% of December days in San Diego. The highest December AQI recorded in 2019–2025 was 125 (Unhealthy for Sensitive Groups), in 2024.
San Diego December air quality, year by year
San Diego December air quality by year (San Diego-Carlsbad, CA, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019cleanest | 31 | 60 | 0 | 85 | PM2.5 |
| 2020 | 31 | 84 | 1 | 101 | PM2.5 |
| 2021 | 31 | 62 | 0 | 93 | PM2.5 |
| 2022 | 31 | 65 | 0 | 91 | PM2.5 |
| 2023 | 31 | 71 | 0 | 99 | PM2.5 |
| 2024worst | 31 | 81 | 6 | 125 | PM2.5 |
| 2025 | 31 | 79 | 5 | 118 | PM2.5 |
What December air means in San Diego
A typical December day in San Diego is Moderate (median AQI 73): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 18 of December days (5.5%) 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 73 is above San Diego's whole-year median of 67, making it one of the area's dirtier periods. Nationally, the median December AQI across every EPA metro area (95,590 metro-days, 2019–2025) is 39, so San Diego runs above the national December norm; 5.5% of its December days topped AQI 100 versus 0.7% nationally (above average).
December is part of San Diego's winter season (December, January, February); the winter page pools all three months.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in San Diego
The latest EPA AirNow observation puts San Diego's AQI at 54 (PM2.5). The figures above are multi-year month statistics, not today's reading.
View current San Diego air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (San Diego-Carlsbad, 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.