Median AQI
39
Good
Montana · County air quality · 2025
Custer County, Montana
Median AQI 39 (Good). 91.2% of monitored days rated Good, direct from EPA Air Quality System stations.
- 39
- Median AQI
- 91.2%
- Good-air days
- 0
- Unhealthy days
- 79%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Custer County's air is good on balance, cleaner than most US counties, a median AQI of 39 with 91.2% of 181 monitored days in the Good band, driven mainly by Ozone.
- 39
- median AQI · Good
- 91.2%
- days rated Good
- Ozone
- main pollutant
- 79%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 95.
Data updated July 2026
- Good · 165 d · 91.2%
- Moderate · 16 d · 8.8%
How did Custer County's air break down in 2025?
Across 181 monitored days, 165 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 95 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 181 days monitored. Median AQI: 39 (Good).
Insights: Custer County, Montana. EPA monitors recorded a median Air Quality Index of 39 across 181 measured days in 2025, earning a Good classification and a Excellent health grade. Of those days, 165 (91%) fell in the Good band (AQI 0–50), 16 (9%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 49, while the worst single-day peak hit 95 (Moderate). Residents averaged about 27.3 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (152 days), PM2.5 (28 days), NO2 (1 days). Across the 2022–2025 trend window, good days shifted by -153 and the median AQI moved -1 points, placing the multi-year trajectory at improving. The worst single reading over the 2022–2025 window was AQI 95, moderate but never Unhealthy. Nearby counties show comparable readings: Beaverhead (100.0% good, AQI 12); Dawson (95.0% good, AQI 14); Fergus (92.2% good, AQI 44); Flathead (82.1% good, AQI 40); Gallatin (99.1% good, AQI 14); Glacier (96.4% good, AQI 22).
Air Quality Summary
Good Days
91.2%
165 of 181 monitored days
16.2pp vs 75% target
Max AQI Recorded
95
Moderate
Health Grade
Excellent
90th-pct AQI 49
Custer County, Good days share 91.2%
National avg
Share of monitored days at AQI 0-50 (Good band).
Custer County, Unhealthy-or-worse share 0.0%
National avg
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
AQI Day Breakdown (2025)
Distribution of 181 monitored days across EPA AQI categories.
Good
165
91%
Moderate
16
9%
USG
0
0%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Custer County.
- Ozone · 152 days · 84.0%
- PM2.5 · 28 days · 15.5%
- NO₂ · 1 days · 0.6%
Health Impact Assessment
Excellent
Custer County's air quality is rated Excellent based on 91.2% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 91.2% of monitored days in 2025, with a median AQI of 39. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 152 of 181 monitored days.
5-Year Air Quality Trend
Air quality is improving in Custer County. Best year: 2024 (328 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2022 | 318 | 46 | 0 | 0 | 40 | 80 |
| 2023 | 288 | 66 | 4 | 5 | 39 | 260 |
| 2024 | 328 | 31 | 0 | 2 | 36 | 153 |
| 2025 | 165 | 16 | 0 | 0 | 39 | 95 |
How does Custer County compare?
How Custer County compares
Good-day share better than US avg
Custer County 91.2%
Beaverhead County, Montana 100.0%
US average 76.5%
Median AQI worse than US avg
Custer County 39
Beaverhead County, Montana 12
US average 38
Unhealthy days / yr about average
Custer County 0
Beaverhead County, Montana 0
US average 0
Other Counties in Montana
Air quality comparison for nearby counties in Montana.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Custer County.
Ada County, Idaho
AQI 39 · 80.6% good
Aiken County, South Carolina
AQI 39 · 89.1% good
Albany County, New York
AQI 39 · 67.4% good
Alexander County, North Carolina
AQI 39 · 92.4% good
Ashland County, Wisconsin
AQI 39 · 85.6% good
Bennington County, Vermont
AQI 39 · 82.9% good
What Custer County's air means for you
The air here stays in the breathable range year-round, with Ozone the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-Ozone days, the guide explains what a median of 39 means for exposure. Health guidance
- See how Custer County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Custer County with its Montana neighbours before drawing a conclusion about the region. All of Montana
Based on 181 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
More Air Quality Data
Air Quality Guides
Frequently Asked Questions
What is the air quality in Custer County, Montana? ▼
Custer County has a median Air Quality Index (AQI) of 39, rated "Good." 91.2% of monitored days (165 out of 181) had good air quality in 2025.
What is the main pollutant in Custer County? ▼
The primary pollutant in Custer County is Ozone (O3), which was the main AQI driver on 152 days (84% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Custer County have? ▼
In 2025, Custer County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 95.
Is the air quality in Custer County getting better or worse? ▼
Air quality in Custer County is improving based on 5-year trends (2022-2025). Best year: 2024 with 328 good days. Worst year: 2025 with 165 good days.
What does AQI 39 mean for health? ▼
Custer County's median AQI of 39 falls in the "Good" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Custer County compare to other counties in Montana? ▼
Custer County has 91.2% good air days. Nearby counties in Montana: Beaverhead (100.0% good), Dawson (95.0% good), Fergus (92.2% good), Flathead (82.1% good), Gallatin (99.1% good), Glacier (96.4% good).
What is the 90th percentile AQI in Custer County? ▼
The 90th percentile AQI for Custer County is 49, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 95 (Moderate).
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer.
Every figure on PlainAir is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.