Median AQI
41
Good
Montana · County air quality · 2025
Flathead County, Montana
Median AQI 41 (Good). 73.7% of monitored days rated Good, direct from EPA Air Quality System stations.
- 41
- Median AQI
- 73.7%
- Good-air days
- 0
- Unhealthy days
- 41%
- Cleaner than US counties
Flathead County's 2025 annual median AQI is 41 (Good), with 73.7% Good days
According to the U.S. EPA Air Quality System, Flathead County, Montana posted a 2025 median AQI of 41 (Good) across 365 monitored days, with 73.7% 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
- Flathead County's 73.7% Good-day share places it cleaner than 41% of US counties in 2025.
- Closest median-AQI peer
- Armstrong County (70.5% Good, median AQI 41) is the closest nationwide median-AQI peer to Flathead County.
- 2020–2025 trend
- Flathead County's floor-qualifying Good-day share is worsening from 2020 to 2025 (78.4% → 73.7%).
- Median AQI
- Median AQI: 41
The verdict
According to EPA Air Quality System monitoring data, Flathead County's air is good on balance, more polluted than the median US county, a median AQI of 41 with 73.7% of 365 monitored days in the Good band, driven mainly by PM2.5.
- 41
- median AQI · Good
- 73.7%
- days rated Good
- PM2.5
- main pollutant
- 41%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 103.
Annual release change
What changed in the 2025 EPA annual release
- Flathead County's Good-day share rose from 69.1% to 73.7% between the 2024 and 2025 EPA annual releases, a change of +4.6 percentage points.
Data updated August 2026
- Good · 269 d · 73.7%
- Moderate · 95 d · 26.0%
- USG · 1 d · 0.3%
How did Flathead County's air break down in 2025?
Across 365 monitored days, 269 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 103 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 365 days monitored. Median AQI: 41 (Good).
Insights: Flathead County, Montana. EPA monitors recorded a median Air Quality Index of 41 across 365 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 269 (74%) fell in the Good band (AQI 0–50), 95 (26%) registered Moderate readings, and 1 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 62, while the worst single-day peak hit 103 (USG). Residents averaged about 22.1 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (177 days), Ozone (175 days), PM10 (13 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by -4.4 points and the median AQI moved +4 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 103, usg but never Unhealthy. Nearby counties show comparable readings: Beaverhead (94.7% good, AQI 13); Cascade (59.8% good, AQI 46); Custer (87.2% good, AQI 40); Dawson (90.3% good, AQI 18); Fergus (90.7% good, AQI 41); Gallatin (98.1% good, AQI 11).
Air Quality Summary
Good Days
73.7%
269 of 365 monitored days
-1.3pp vs 75% target
Max AQI Recorded
103
USG
Health Grade
Moderate
90th-pct AQI 62
Flathead County, Good days share 73.7%
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 365 monitored days across EPA AQI categories.
Good
269
74%
Moderate
95
26%
USG
1
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.
- PM2.5 · 177 days · 48.5%
- Ozone · 175 days · 47.9%
- PM10 · 13 days · 3.6%
Health Impact Assessment
Moderate
Flathead County's air quality is rated Moderate based on 73.7% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 73.7% of monitored days in 2025, with a median AQI of 41. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 177 of 365 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Flathead County. Highest Good-day share: 2020 (78.4%, 286 of 366 monitored 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 | 269 | 95 | 1 | 0 | 41 | 103 |
How does Flathead County compare?
How Flathead County compares
Good-day share worse than US avg
Flathead County 73.7%
Armstrong County, Pennsylvania 70.5%
US average 76.0%
Median AQI worse than US avg
Flathead County 41
Armstrong County, Pennsylvania 41
US average 39
Unhealthy days / yr better than US avg
Flathead County 0
Armstrong County, Pennsylvania 0
US average 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Flathead County (41), not geographic neighbors.
Armstrong County, Pennsylvania
Median AQI 41 · 70.5% Good-day share
Benewah County, Idaho
Median AQI 41 · 63.4% Good-day share
Berrien County, Michigan
Median AQI 41 · 80.9% Good-day share
Callaway County, Missouri
Median AQI 41 · 89.6% Good-day share
Cambria County, Pennsylvania
Median AQI 41 · 74.5% Good-day share
Cass County, Missouri
Median AQI 41 · 77.6% Good-day share
Same primary pollutant: PM2.5
PM2.5 led the AQI on the most monitored days here. These peers share that driver and the closest Good-day share (a second relationship type beyond median-AQI neighbors).
Eau Claire County, Wisconsin
73.7% Good-day share · median AQI 38
Escambia County, Florida
73.7% Good-day share · median AQI 43
Mendocino County, California
73.7% Good-day share · median AQI 38
Rapides County, Louisiana
73.7% Good-day share · median AQI 37
Monroe County, Indiana
73.6% Good-day share · median AQI 36
Tuscaloosa County, Alabama
73.6% Good-day share · median AQI 41
Montana counties on the map
Geographic containment only, other monitored counties in Montana, independent of AQI or pollutant match.
What Flathead County's air means for you
The air here stays in the breathable range year-round, with PM2.5 the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 41 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 365 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
Keep exploring the annual AQI record
Air-quality reading list
Frequently Asked Questions
What is the air quality in Flathead County, Montana? ▼
Flathead County has a median Air Quality Index (AQI) of 41, rated "Good." 73.7% of monitored days (269 out of 365) had good air quality in 2025.
What is the main pollutant in Flathead County? ▼
The primary pollutant in Flathead County is Fine Particles (PM2.5), which was the main AQI driver on 177 days (48% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
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 103.
Is the air quality in Flathead County getting better or worse? ▼
Air quality in Flathead County is worsening across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2020 at 78.4% (286 of 366 days). Lowest qualifying Good-day share: 2023 at 64.9% (237 of 365 days).
What does AQI 41 mean for health? ▼
Flathead County's median AQI of 41 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Flathead County compare to other counties in Montana? ▼
Flathead County has 73.7% good air days. Nearby counties in Montana: Beaverhead (94.7% good), Cascade (59.8% good), Custer (87.2% good), Dawson (90.3% good), Fergus (90.7% good), Gallatin (98.1% good).
What is the 90th percentile AQI in Flathead County? ▼
The 90th percentile AQI for Flathead County is 62, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 103 (USG).
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 Flathead County, Montana · 2020–2025
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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.