San Jose Best & Worst AQI Days: Air Quality Guide
The worst air quality day on record for San Jose (San Jose-Sunnyvale-Santa Clara, CA area) was September 11, 2020, when the daily AQI reached 198 (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; 9 of the 10 were driven by fine particulate matter (PM2.5) and 1 by ground-level ozone, and 10 of them fell in 2020. The cleanest day was August 16, 2025 at AQI 9 (Good); the 10 best days span 2 different years.
Extra Protection for San Jose Homes
Air quality in San Jose is moderate (AQI 53), 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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When were San Jose's worst and best air quality days?
Worst: September 11, 2020, AQI 198 (Unhealthy, PM2.5). Best: August 16, 2025, AQI 9 (Good). Based on 2,557 EPA-monitored days in the San Jose-Sunnyvale-Santa Clara, CA area, 2019–2025.
- ✓10 of the 10 worst days were Unhealthy (AQI 151+) or worse; the 10th-worst was AQI 154 (Unhealthy).
- ✓Fine particulate matter (PM2.5) drove 9 of the 10 worst days.
- ✓2020 accounts for 10 of the 10 worst days.
Frequently asked questions
What was the worst air quality day in San Jose?▾
September 11, 2020: the San Jose-Sunnyvale-Santa Clara, CA area's daily AQI hit 198 (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 San Jose?▾
August 16, 2025, with a daily AQI of 9 (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 San Jose's worst air quality day in 2025?▾
October 21, 2025, AQI 71 (Moderate), set by ground-level ozone.
Which pollutant causes San Jose's worst air days?▾
Fine particulate matter (PM2.5) set the AQI on 9 of the 10 worst days; ground-level ozone on 1. The tables above list the defining pollutant for every day.
San Jose's 10 worst air quality days, 2019–2025
Highest daily AQI recorded for the San Jose-Sunnyvale-Santa Clara, 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) | 198 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 2 | (2020 page) | 192 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 3 | (2020 page) | 186 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 4 | (2020 page) | 184 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 5 | (2020 page) | 172 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 6 | (2020 page) | 168 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 7 | (2020 page) | 162 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 8 | (2020 page) | 158 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 9 | (2020 page) | 154 | Unhealthy | O3 — ground-level ozone |
| 10 | (2020 page) | 154 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
Worst day of each year in San Jose
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 | 136 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season | |
| 2020 | 198 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2021 | 147 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season | |
| 2022 | 119 | Unhealthy for Sensitive Groups | O3 — ground-level ozone | |
| 2023 | 134 | Unhealthy for Sensitive Groups | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2024 | 111 | Unhealthy for Sensitive Groups | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2025 | 71 | Moderate | O3 — ground-level ozone |
San Jose's 10 best air quality days, 2019–2025
Lowest daily AQI recorded for the San Jose-Sunnyvale-Santa Clara, CA area. Ties are listed most recent first.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2025 page) | 9 | Good | O3 — ground-level ozone, in peak ozone season |
| 2 | (2025 page) | 19 | Good | O3 — ground-level ozone |
| 3 | (2025 page) | 20 | Good | O3 — ground-level ozone |
| 4 | (2022 page) | 20 | Good | O3 — ground-level ozone, in peak ozone season |
| 5 | (2025 page) | 21 | Good | O3 — ground-level ozone |
| 6 | (2025 page) | 22 | Good | O3 — ground-level ozone |
| 7 | (2025 page) | 23 | Good | O3 — ground-level ozone |
| 8 | (2025 page) | 23 | Good | O3 — ground-level ozone |
| 9 | (2025 page) | 24 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 10 | (2025 page) | 25 | Good | O3 — ground-level ozone, in peak ozone season |
Right now in San Jose
The latest EPA AirNow observation puts San Jose's AQI at 53 (PM2.5). The days above are historical records, not today's reading.
View current San Jose air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (San Jose-Sunnyvale-Santa Clara, 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.