Modesto Best & Worst AQI Days: Air Quality Guide
The worst air quality day on record for Modesto (Modesto, CA area) was September 13, 2020, when the daily AQI reached 195 (Unhealthy), set by fine particulate matter (PM2.5), in wildfire smoke season. Across 2,557 monitored days in 2019–2025, 10 of the 10 worst days were Unhealthy (AQI 151+) or worse; 8 of the 10 were driven by fine particulate matter (PM2.5) and 2 by coarse dust (PM10), and 9 of them fell in 2020. The cleanest day was April 9, 2019 at AQI 18 (Good); the 10 best days span 4 different years.
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.
Recommended: Coway AP-1512HH Mighty — True HEPA + carbon · CADR ~246 CFM · rooms up to 361 sq ft
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When were Modesto's worst and best air quality days?
Worst: September 13, 2020, AQI 195 (Unhealthy, PM2.5). Best: April 9, 2019, AQI 18 (Good). Based on 2,557 EPA-monitored days in the Modesto, CA area, 2019–2025.
- ✓10 of the 10 worst days were Unhealthy (AQI 151+) or worse; the 10th-worst was AQI 177 (Unhealthy).
- ✓Fine particulate matter (PM2.5) drove 8 of the 10 worst days.
- ✓2020 accounts for 9 of the 10 worst days.
Frequently asked questions
What was the worst air quality day in Modesto?▾
September 13, 2020: the Modesto, CA area's daily AQI hit 195 (Unhealthy), driven by fine particulate matter (PM2.5), in wildfire smoke season. That is the highest of 2,557 monitored days in 2019–2025.
What was the best air quality day in Modesto?▾
April 9, 2019, with a daily AQI of 18 (Good). The daily AQI is the highest reading from any monitor in the area, so even the cleanest day reflects the area's worst monitor.
What was Modesto's worst air quality day in 2025?▾
July 11, 2025, AQI 115 (Unhealthy for Sensitive Groups), set by ground-level ozone, in peak ozone season.
Which pollutant causes Modesto's worst air days?▾
Fine particulate matter (PM2.5) set the AQI on 8 of the 10 worst days; coarse dust (PM10) on 2. The tables above list the defining pollutant for every day.
Modesto's 10 worst air quality days, 2019–2025
Highest daily AQI recorded for the Modesto, CA area (EPA AQS). The daily AQI is the highest reading from any monitor in the area that day.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2020 page) | 195 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 2 | (2020 page) | 191 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 3 | (2020 page) | 189 | Unhealthy | PM10 — coarse dust (PM10) |
| 4 | (2020 page) | 188 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 5 | (2020 page) | 188 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 6 | (2020 page) | 182 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 7 | (2020 page) | 182 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 8 | (2020 page) | 180 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 9 | (2020 page) | 180 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 10 | (2019 page) | 177 | Unhealthy | PM10 — coarse dust (PM10) |
Worst day of each year in Modesto
The single highest-AQI day in each year on record. Click a year for its full month-by-month statistics.
| Year | Worst day | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 2019 | 177 | Unhealthy | PM10 — coarse dust (PM10) | |
| 2020 | 195 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2021 | 172 | Unhealthy | O3 — ground-level ozone, in peak ozone season | |
| 2022 | 166 | Unhealthy | O3 — ground-level ozone | |
| 2023 | 150 | Unhealthy for Sensitive Groups | O3 — ground-level ozone | |
| 2024 | 150 | Unhealthy for Sensitive Groups | O3 — ground-level ozone | |
| 2025 | 115 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
Modesto's 10 best air quality days, 2019–2025
Lowest daily AQI recorded for the Modesto, CA area. Ties are listed most recent first.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2019 page) | 18 | Good | PM10 — coarse dust (PM10) |
| 2 | (2020 page) | 24 | Good | O3 — ground-level ozone |
| 3 | (2024 page) | 25 | Good | O3 — ground-level ozone, in peak ozone season |
| 4 | (2019 page) | 25 | Good | O3 — ground-level ozone |
| 5 | (2025 page) | 26 | Good | O3 — ground-level ozone, in peak ozone season |
| 6 | (2025 page) | 27 | Good | O3 — ground-level ozone |
| 7 | (2024 page) | 27 | Good | O3 — ground-level ozone |
| 8 | (2024 page) | 27 | Good | O3 — ground-level ozone, in peak ozone season |
| 9 | (2020 page) | 27 | Good | O3 — ground-level ozone |
| 10 | (2024 page) | 28 | Good | PM2.5 — fine particulate matter (PM2.5) |
Right now in Modesto
The latest EPA AirNow observation puts Modesto's AQI at 63 (PM10). The days above are historical records, 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.