Boise Air Quality in September
Across 2019–2025, Boise's September air quality had a median AQI of 55 (Moderate) over 210 monitored days in the Boise City, ID area. 17.6% of September days (37 of 210) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 63.8% of days. The cleanest September on record here was 2019 (median AQI 39, 0 days above 100) and the worst was 2020 (median 74, 11 days above 100, peak AQI 223).
- Good: 40.0%
- Moderate: 42.4%
- Unhealthy for Sensitive Groups: 9.5%
- Unhealthy or worse: 8.1%
Reduce Indoor Ozone in Boise
Today's main pollutant in Boise is ground-level ozone (AQI 38). 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
As an Amazon Associate, we earn from qualifying purchases.
How is the air in Boise in September?
Moderate on a typical day: median AQI 55 across 2019–2025, with 17.6% of September days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓40.0% of September days were Good (AQI 0–50), 42.4% Moderate (51–100).
- ✓37 days of 210 reached AQI 101+; the September peak was 265.
- ✓Best September: 2019; worst September: 2020.
Frequently asked questions
What is the air quality like in Boise in September?▾
Across 2019–2025, the median daily AQI in Boise in September was 55 (Moderate). 40.0% of days were Good, 42.4% Moderate, and 17.6% 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 Boise?▾
A typical September day in Boise is Moderate (median AQI 55): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 6 of September days (17.6%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
Which pollutant causes Boise's September air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 63.8% of September days in Boise. The highest September AQI recorded in 2019–2025 was 265 (Very Unhealthy), in 2020.
Boise September air quality, year by year
Boise September air quality by year (Boise City, ID, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019cleanest | 30 | 39 | 0 | 64 | PM2.5 |
| 2020worst | 30 | 74 | 11 | 223 | PM2.5 |
| 2021 | 30 | 58 | 6 | 153 | PM2.5 |
| 2022 | 30 | 74 | 9 | 188 | PM2.5 |
| 2023 | 30 | 47 | 0 | 72 | O3 |
| 2024 | 30 | 60 | 11 | 265 | PM2.5 |
| 2025 | 30 | 52 | 0 | 97 | PM2.5 |
What September air means in Boise
A typical September day in Boise is Moderate (median AQI 55): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 6 of September days (17.6%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
September's median AQI of 55 is in line with Boise's whole-year median of 54, 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 Boise runs above the national September norm; 17.6% of its September days topped AQI 100 versus 2.7% nationally (above average).
September is part of Boise's fall season (September, October, November); the fall page pools all three months.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Boise
The latest EPA AirNow observation puts Boise's AQI at 38 (O3). The figures above are multi-year month statistics, not today's reading.
View current Boise air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (Boise City, ID), 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.