Modesto Air Quality in December
Across 2019–2025, Modesto's December air quality had a median AQI of 66 (Moderate) over 217 monitored days in the Modesto, CA area. 8.8% of December days (19 of 217) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 92.2% of days. The cleanest December on record here was 2019 (median AQI 60, 0 days above 100) and the worst was 2024 (median 68, 7 days above 100, peak AQI 141).
- Good: 17.5%
- Moderate: 73.7%
- Unhealthy for Sensitive Groups: 8.8%
- Unhealthy or worse: 0.0%
Extra Protection for Modesto Homes
Air quality in Modesto is moderate (AQI 63), with particle pollution 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.
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How is the air in Modesto in December?
Moderate on a typical day: median AQI 66 across 2019–2025, with 8.8% of December days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓17.5% of December days were Good (AQI 0–50), 73.7% Moderate (51–100).
- ✓19 days of 217 reached AQI 101+; the December peak was 141.
- ✓Best December: 2019; worst December: 2024.
Frequently asked questions
What is the air quality like in Modesto in December?▾
Across 2019–2025, the median daily AQI in Modesto in December was 66 (Moderate). 17.5% of days were Good, 73.7% Moderate, and 8.8% 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 Modesto?▾
A typical December day in Modesto is Moderate (median AQI 66): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 11 of December days (8.8%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
Which pollutant causes Modesto's December air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 92.2% of December days in Modesto. The highest December AQI recorded in 2019–2025 was 141 (Unhealthy for Sensitive Groups), in 2024.
Modesto December air quality, year by year
Modesto December air quality by year (Modesto, CA, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019cleanest | 31 | 60 | 0 | 84 | PM2.5 |
| 2020 | 31 | 80 | 4 | 108 | PM2.5 |
| 2021 | 31 | 65 | 2 | 128 | PM2.5 |
| 2022 | 31 | 60 | 1 | 137 | PM2.5 |
| 2023 | 31 | 66 | 5 | 120 | PM2.5 |
| 2024worst | 31 | 68 | 7 | 141 | PM2.5 |
| 2025 | 31 | 65 | 0 | 98 | PM2.5 |
What December air means in Modesto
A typical December day in Modesto is Moderate (median AQI 66): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 11 of December days (8.8%) 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 66 is above Modesto's whole-year median of 54, 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 Modesto runs above the national December norm; 8.8% of its December days topped AQI 100 versus 0.7% nationally (above average).
December is part of Modesto's winter season (December, January, February); the winter page pools all three months.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Modesto
The latest EPA AirNow observation puts Modesto's AQI at 63 (PM10). The figures above are multi-year month statistics, not today's reading.
View current Modesto air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (Modesto, 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.