Montana · County air quality · 2025
Custer County, Montana
Median AQI 40 (Good). 87.2% of monitored days rated Good, direct from EPA Air Quality System stations.
- 40
- Median AQI
- 87.2%
- Good-air days
- 0
- Unhealthy days
- 75%
- Cleaner than US counties
Custer County's 2025 annual median AQI is 40 (Good), with 87.2% Good days
According to the U.S. EPA Air Quality System, Custer County, Montana posted a 2025 median AQI of 40 (Good) across 273 monitored days, with 87.2% rated Good and 0 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Custer County's 87.2% Good-day share places it cleaner than 75% of US counties in 2025.
- Closest median-AQI peer
- Anderson County (91.3% Good, median AQI 40) is the closest nationwide median-AQI peer to Custer County.
- 2022–2025 trend
- Custer County's floor-qualifying Good-day share is stable from 2022 to 2025 (87.4% → 87.2%).
- Median AQI
- Median AQI: 40
According to the 2025 EPA AQS county release on PlainAirData, Custer County, Montana ranks #462 of 951 by median AQI (40) and #241 by good-day share (87.2%). Ponce County, Puerto Rico is #1 of 951 by median AQI (3, 91.4% Good days) at good-day-share #159 ≠ largest of 7 at 100.0% Good days #1 by good-day share (100.0%, median AQI 9) at median-AQI #7.
Annual release change
What changed in the 2025 EPA annual release
- Custer County's Good-day share fell from 90.9% to 87.2% between the 2024 and 2025 EPA annual releases, a change of -3.7 percentage points.
- Custer County's peak single-day AQI fell from 153 to 127 between the 2024 and 2025 EPA annual releases, a change of -26 points.
- Custer County's Unhealthy-category days fell from 2 to 0 between the 2024 and 2025 EPA annual releases, a change of -2 days.
- Custer County's median AQI rose from 36 to 40 between the 2024 and 2025 EPA annual releases, a change of +4 points.
Data updated August 2026
- Good · 238 d · 87.2%
- Moderate · 34 d · 12.5%
- USG · 1 d · 0.4%
How did Custer County's air break down in 2025?
Across 273 monitored days, 238 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 127 (USG).
EPA AQS →Share of monitored days at AQI 0-50 (Good band).
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
Where Custer County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Custer County's Good-day share is 87.2% - cleaner than 75% of that comparison set.
Custer County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
87.2% Cleaner than 75% higher than 75% of 951 US counties
Each bar is a band; taller bars hold more US counties. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.
Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Custer County.
- Ozone · 206 days · 75.5%
- PM2.5 · 66 days · 24.2%
- NO₂ · 1 days · 0.4%
2024 daily-monitor rhythm in Custer County
Month-by-month median AQI from EPA day-level county monitors (361 reported days in 2024). Jul posted the highest typical readings (median AQI 45, peak 67); Dec was cleanest at median 31. Summer months (Jun–Aug) averaged median AQI 44 vs 32 in Dec–Feb. 2 days crossed Unhealthy (AQI >150) at least once that year.
Five-year seasonal record
EPA day-level county files · 2022–20241,089 daily readings show a recurring local pattern: Aug has the highest matched-month median AQI (48), while Dec is lowest (28). Each cell is that month’s median daily AQI, not a current-air forecast.
Lower AQI is better. The colors summarize EPA AQI bands; gaps mean the county had no day-level record for that month.
Health Impact Assessment
Custer County's air quality is rated Good based on 87.2% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 87.2% of monitored days in 2025, with a median AQI of 40. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 206 of 273 monitored days.
5-Year Air Quality Trend
Air quality is stable in Custer County. Highest Good-day share: 2024 (90.9%, 328 of 361 monitored 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 | 238 | 34 | 1 | 0 | 40 | 127 |
How does Custer County compare?
How Custer County compares
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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Custer County, Montana · 2020–2025
Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.
Every figure on PlainAirData 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.