San Jose Air Quality in September
Across 2019–2025, San Jose's September air quality had a median AQI of 50 (Good) over 210 monitored days in the San Jose-Sunnyvale-Santa Clara, CA area. 6.2% of September days (13 of 210) reached Unhealthy for Sensitive Groups or worse, and ground-level ozone set the daily AQI on 51.9% of days. The cleanest September on record here was 2025 (median AQI 42, 0 days above 100) and the worst was 2020 (median 72, 9 days above 100, peak AQI 198).
- Good: 50.5%
- Moderate: 43.3%
- Unhealthy for Sensitive Groups: 3.8%
- Unhealthy or worse: 2.4%
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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How is the air in San Jose in September?
Good on a typical day: median AQI 50 across 2019–2025, with 6.2% of September days above AQI 100. Ground-level ozone is the main driver.
- ✓50.5% of September days were Good (AQI 0–50), 43.3% Moderate (51–100).
- ✓13 days of 210 reached AQI 101+; the September peak was 198.
- ✓Best September: 2025; worst September: 2020.
Frequently asked questions
What is the air quality like in San Jose in September?▾
Across 2019–2025, the median daily AQI in San Jose in September was 50 (Good). 50.5% of days were Good, 43.3% Moderate, and 6.2% reached Unhealthy for Sensitive Groups or worse, based on 210 monitored days in EPA's Air Quality System.
Is September a good time for outdoor exercise in San Jose?▾
A typical September day in San Jose sits in the Good range (median AQI 50), so running, cycling and children's outdoor play are fine for everyone on most days; about 1 day in 16 of September days (6.2%) reached AQI 101+ in 2019–2025.
Which pollutant causes San Jose's September air quality problems?▾
Ground-level ozone set the daily AQI on 51.9% of September days in San Jose. The highest September AQI recorded in 2019–2025 was 198 (Unhealthy), in 2020.
San Jose September air quality, year by year
San Jose September air quality by year (San Jose-Sunnyvale-Santa Clara, CA, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019 | 30 | 44 | 0 | 90 | O3 |
| 2020worst | 30 | 72 | 9 | 198 | PM2.5 |
| 2021 | 30 | 62 | 2 | 101 | PM2.5 |
| 2022 | 30 | 58 | 1 | 119 | O3 |
| 2023 | 30 | 45 | 1 | 134 | O3 |
| 2024 | 30 | 49 | 0 | 74 | PM2.5 |
| 2025cleanest | 30 | 42 | 0 | 58 | O3 |
What September air means in San Jose
A typical September day in San Jose sits in the Good range (median AQI 50), so running, cycling and children's outdoor play are fine for everyone on most days; about 1 day in 16 of September days (6.2%) reached AQI 101+ in 2019–2025.
September's median AQI of 50 is in line with San Jose's whole-year median of 50, so this period is neither a clean nor a dirty stretch for the area. Nationally, the median September AQI across every EPA metro area (100,699 metro-days, 2019–2025) is 43, so San Jose runs above the national September norm; 6.2% of its September days topped AQI 100 versus 2.7% nationally (above average).
September is part of San Jose's fall season (September, October, November); the fall page pools all three months.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in San Jose
The latest EPA AirNow observation puts San Jose's AQI at 53 (PM2.5). The figures above are multi-year month statistics, 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.