Montana · County air quality · 2025
Yellowstone County, Montana
Median AQI 38 (Good). 86.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 38
- Median AQI
- 86.3%
- Good-air days
- 0
- Unhealthy days
- 73%
- Cleaner than US counties
Yellowstone County's 2025 annual median AQI is 38 (Good), with 86.3% Good days
According to the U.S. EPA Air Quality System, Yellowstone County, Montana posted a 2025 median AQI of 38 (Good) across 365 monitored days, with 86.3% 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
- Yellowstone County's 86.3% Good-day share places it cleaner than 73% of US counties in 2025.
- Closest median-AQI peer
- Adams County (94.7% Good, median AQI 38) is the closest nationwide median-AQI peer to Yellowstone County.
- 2020–2025 trend
- Yellowstone County's floor-qualifying Good-day share is worsening from 2020 to 2025 (83.5% → 86.3%).
- Median AQI
- Median AQI: 38
According to the 2025 EPA AQS county release on PlainAirData, Yellowstone County, Montana ranks #334 of 951 by median AQI (38) and #256 by good-day share (86.3%). 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
- Yellowstone County's Good-day share rose from 76.2% to 86.3% between the 2024 and 2025 EPA annual releases, a change of +10.1 percentage points.
- Yellowstone County's peak single-day AQI fell from 105 to 91 between the 2024 and 2025 EPA annual releases, a change of -14 points.
Data updated August 2026
- Good · 315 d · 86.3%
- Moderate · 50 d · 13.7%
How did Yellowstone County's air break down in 2025?
Across 365 monitored days, 315 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 91 (Moderate).
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 Yellowstone County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Yellowstone County's Good-day share is 86.3% - cleaner than 73% of that comparison set.
Yellowstone County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
86.3% Cleaner than 73% higher than 73% 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 Yellowstone County.
- Ozone · 220 days · 60.3%
- PM2.5 · 135 days · 37.0%
- NO₂ · 10 days · 2.7%
2024 daily-monitor rhythm in Yellowstone County
Month-by-month median AQI from EPA day-level county monitors (357 reported days in 2024). Jul posted the highest typical readings (median AQI 61, peak 105); Nov was cleanest at median 31. Summer months (Jun–Aug) averaged median AQI 52 vs 34 in Dec–Feb.
Five-year seasonal record
EPA day-level county files · 2020–20241,783 daily readings show a recurring local pattern: Sep has the highest matched-month median AQI (47), while May is lowest (24). 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
Yellowstone County's air quality is rated Good based on 86.3% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 86.3% of monitored days in 2025, with a median AQI of 38. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 220 of 365 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Yellowstone County. Highest Good-day share: 2025 (86.3%, 315 of 365 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 304 | 57 | 2 | 1 | 31 | 163 |
| 2021 | 212 | 118 | 9 | 1 | 40 | 167 |
| 2022 | 307 | 56 | 1 | 1 | 32 | 155 |
| 2023 | 306 | 47 | 3 | 1 | 29 | 191 |
| 2024 | 272 | 84 | 1 | 0 | 39 | 105 |
| 2025 | 315 | 50 | 0 | 0 | 38 | 91 |
How does Yellowstone County compare?
How Yellowstone 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 Yellowstone 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.