Modesto Air Quality in 2021
In 2021, Modesto's air quality (Modesto, CA area) had a median daily AQI of 61 (Moderate) across 365 monitored days. 47 days (12.9%) reached Unhealthy for Sensitive Groups or worse, 84 were Good, and the year's peak AQI was 172 (Unhealthy). Fine particulate matter (PM2.5) set the daily AQI on 225 of those days. The cleanest month of 2021 was March (median AQI 51, 0 days above 100) and the worst was August (median 97, 13 days above 100, peak 166).
- Good: 23.0%
- Moderate: 64.1%
- Unhealthy for Sensitive Groups: 11.5%
- Unhealthy or worse: 1.4%
Reduce Indoor Ozone in Modesto
Today's main pollutant in Modesto is ground-level ozone (AQI 34). Important: standard HEPA filters do NOT remove ozone — only a purifier with a substantial activated-carbon bed adsorbs it. The biggest wins are cutting indoor sources (gas appliances, certain "ionizer" purifiers) and ventilating when outdoor ozone is low (early morning). A high-carbon unit helps with the gases and VOCs that linger indoors.
Recommended: Levoit Core 300 — True HEPA + activated carbon · CADR ~141 CFM · rooms up to 219 sq ft
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How was Modesto's air quality in 2021?
Moderate on a typical day: median AQI 61, with 47 days above AQI 100 out of 365 monitored — the 7th cleanest of 7 years on record for the area.
- ✓Peak AQI in 2021: 172 (Unhealthy).
- ✓Fine particulate matter (PM2.5) was the main pollutant on 225 days.
- ✓Cleanest month: March; worst: August.
Frequently asked questions
What was the average AQI in Modesto in 2021?▾
EPA's Air Quality System puts Modesto's (Modesto, CA) median daily AQI for 2021 at 61, which is Moderate. 47 days were Unhealthy for Sensitive Groups or worse and the year's highest AQI was 172.
Which month had the worst air quality in Modesto in 2021?▾
The cleanest month of 2021 was March (median AQI 51, 0 days above 100) and the worst was August (median 97, 13 days above 100, peak 166).
Was 2021 a good or bad year for Modesto's air?▾
Among the 7 years EPA has published for the area (2019–2025), 2021 ranks 7th cleanest by days above AQI 100 — the worst year on record here. 2021's median of 61 is above the area's typical annual median of 54.
Modesto month by month in 2021
Modesto air quality month by month, 2021 (Modesto, CA, EPA AQS daily AQI)
| Month | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| January | 31 | 68 | 3 | 120 | PM2.5 |
| February | 28 | 55 | 1 | 105 | PM2.5 |
| Marchcleanest | 31 | 51 | 0 | 69 | PM2.5 |
| April | 30 | 55 | 0 | 90 | PM2.5 |
| May | 31 | 52 | 2 | 115 | PM2.5 |
| June | 30 | 54 | 5 | 172 | O3 |
| July | 31 | 90 | 9 | 143 | O3 |
| Augustworst | 31 | 97 | 13 | 166 | O3 |
| September | 30 | 68 | 5 | 115 | O3 |
| October | 31 | 57 | 4 | 155 | PM2.5 |
| November | 30 | 77 | 3 | 110 | PM2.5 |
| December | 31 | 65 | 2 | 128 | PM2.5 |
How 2021 compares
Among the 7 years EPA has published for the area (2019–2025), 2021 ranks 7th cleanest by days above AQI 100 — the worst year on record here.
2021's median of 61 is above the area's typical annual median of 54.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Modesto
The latest EPA AirNow observation puts Modesto's AQI at 34 (O3). The figures above are multi-year annual 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.