Custer County, Montana air quality history
Daily Air Quality Index summaries from EPA monitors in Custer, Montana, 2022–2025.
In 2025, EPA reported a daily Air Quality Index (AQI) for Custer, Montana on 273 days. 238 of those days (87%) were in the Good range, and the median daily AQI was 40. That median is higher than the typical monitored county in Montana (median 25.5 across 18 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was ozone on 206 days.
EPA did not report an AQI for every day of 2025 here (273 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.
How the days broke down each year
Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.
- Good
- Moderate
- Unhealthy for Sensitive Groups
- Unhealthy
- Very Unhealthy
- Hazardous
Typical and high-end AQI over time
The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.
- Median daily AQI
- 90th-percentile daily AQI
Main pollutant by day, 2025
On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.
| Pollutant | Days as main pollutant | Share of monitored days |
|---|---|---|
| Ozone | 206 | 75% |
| Fine particles (PM2.5) | 66 | 24% |
| Nitrogen dioxide (NO₂) | 1 | <1% |
Year-by-year data
| Year | Days with AQI | Good | Moderate | Unhealthy for Sensitive Groups | Unhealthy | Very Unhealthy | Hazardous | Median AQI | 90th pct. | Max AQI | Main pollutant |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 2025 | 273 | 238 | 34 | 1 | 0 | 0 | 0 | 40 | 53 | 127 | Ozone |
| 2024 | 361 | 328 | 31 | 0 | 2 | 0 | 0 | 36 | 50 | 153 | Ozone |
| 2023 | 364 | 288 | 66 | 4 | 5 | 1 | 0 | 39 | 61 | 260 | Ozone |
| 2022 | 364 | 318 | 46 | 0 | 0 | 0 | 0 | 40 | 52 | 80 | Ozone |
What this data can and can’t tell you
It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.
It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.
Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.
Counties in Montana with a similar median AQI in 2025
All monitored counties in Montana
Source: U.S. EPA AirData, “Annual AQI by County” files for 2022–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem