Median AQI
40
Good
Montana · County air quality · 2025
Flathead County, Montana
Median AQI 40 (Good). 82.1% of monitored days rated Good, direct from EPA Air Quality System stations.
- 40
- Median AQI
- 82.1%
- Good-air days
- 0
- Unhealthy days
- 58%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Flathead County's air is good on balance, cleaner than the median US county, a median AQI of 40 with 82.1% of 223 monitored days in the Good band, driven mainly by Ozone.
- 40
- median AQI · Good
- 82.1%
- days rated Good
- Ozone
- main pollutant
- 58%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 80.
Data updated July 2026
- Good · 183 d · 82.1%
- Moderate · 40 d · 17.9%
How did Flathead County's air break down in 2025?
Across 223 monitored days, 183 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 80 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 223 days monitored. Median AQI: 40 (Good).
Insights: Flathead County, Montana. EPA monitors recorded a median Air Quality Index of 40 across 223 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 183 (82%) fell in the Good band (AQI 0–50), 40 (18%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 59, while the worst single-day peak hit 80 (Moderate). Residents averaged about 24.6 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (157 days), PM2.5 (59 days), PM10 (7 days). Across the 2020–2025 trend window, good days shifted by -103 and the median AQI moved +3 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 80, moderate but never Unhealthy. Nearby counties show comparable readings: Beaverhead (100.0% good, AQI 12); Custer (91.2% good, AQI 39); Dawson (95.0% good, AQI 14); Fergus (92.2% good, AQI 44); Gallatin (99.1% good, AQI 14); Glacier (96.4% good, AQI 22).
Air Quality Summary
Good Days
82.1%
183 of 223 monitored days
7.1pp vs 75% target
Max AQI Recorded
80
Moderate
Health Grade
Good
90th-pct AQI 59
Flathead County, Good days share 82.1%
National avg
Share of monitored days at AQI 0-50 (Good band).
Flathead 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 223 monitored days across EPA AQI categories.
Good
183
82%
Moderate
40
18%
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 Flathead County.
- Ozone · 157 days · 70.4%
- PM2.5 · 59 days · 26.5%
- PM10 · 7 days · 3.1%
Health Impact Assessment
Good
Flathead County's air quality is rated Good based on 82.1% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 82.1% 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 157 of 223 monitored days.
5-Year Air Quality Trend
Air quality is improving in Flathead County. Best year: 2020 (286 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 286 | 72 | 3 | 4 | 37 | 206 |
| 2021 | 268 | 88 | 8 | 1 | 39 | 155 |
| 2022 | 249 | 113 | 2 | 1 | 41 | 152 |
| 2023 | 237 | 120 | 6 | 2 | 43 | 160 |
| 2024 | 253 | 113 | 0 | 0 | 42 | 95 |
| 2025 | 183 | 40 | 0 | 0 | 40 | 80 |
How does Flathead County compare?
How Flathead County compares
Good-day share better than US avg
Flathead County 82.1%
Beaverhead County, Montana 100.0%
US average 76.5%
Median AQI worse than US avg
Flathead County 40
Beaverhead County, Montana 12
US average 38
Unhealthy days / yr about average
Flathead 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 Flathead County.
Amador County, California
AQI 40 · 84.8% good
Anne Arundel County, Maryland
AQI 40 · 87.2% good
Arlington County, Virginia
AQI 40 · 84.8% good
Bergen County, New Jersey
AQI 40 · 71.1% good
Berkeley County, West Virginia
AQI 40 · 81.1% good
Billings County, North Dakota
AQI 40 · 80.3% good
What Flathead 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 40 means for exposure. Health guidance
- See how Flathead County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Flathead County with its Montana neighbours before drawing a conclusion about the region. All of Montana
Based on 223 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 Flathead County, Montana? ▼
Flathead County has a median Air Quality Index (AQI) of 40, rated "Good." 82.1% of monitored days (183 out of 223) had good air quality in 2025.
What is the main pollutant in Flathead County? ▼
The primary pollutant in Flathead County is Ozone (O3), which was the main AQI driver on 157 days (70% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Flathead County have? ▼
In 2025, Flathead County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 80.
Is the air quality in Flathead County getting better or worse? ▼
Air quality in Flathead County is improving based on 5-year trends (2020-2025). Best year: 2020 with 286 good days. Worst year: 2025 with 183 good days.
What does AQI 40 mean for health? ▼
Flathead County's median AQI of 40 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 Flathead County compare to other counties in Montana? ▼
Flathead County has 82.1% good air days. Nearby counties in Montana: Beaverhead (100.0% good), Custer (91.2% good), Dawson (95.0% good), Fergus (92.2% good), Gallatin (99.1% good), Glacier (96.4% good).
What is the 90th percentile AQI in Flathead County? ▼
The 90th percentile AQI for Flathead County is 59, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 80 (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.