Median AQI
45
Good
Montana · County air quality · 2025
Lewis and Clark County, Montana
Median AQI 45 (Good). 65.4% of monitored days rated Good, direct from EPA Air Quality System stations.
- 45
- Median AQI
- 65.4%
- Good-air days
- 0
- Unhealthy days
- 23%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Lewis and Clark County's air is good on balance, among the more polluted 10% of US counties, a median AQI of 45 with 65.4% of 182 monitored days in the Good band, driven mainly by Ozone.
- 45
- median AQI · Good
- 65.4%
- days rated Good
- Ozone
- main pollutant
- 23%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 93.
Data updated July 2026
- Good · 119 d · 65.4%
- Moderate · 63 d · 34.6%
How did Lewis and Clark County's air break down in 2025?
Across 182 monitored days, 119 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 93 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 182 days monitored. Median AQI: 45 (Good).
Insights: Lewis and Clark County, Montana. EPA monitors recorded a median Air Quality Index of 45 across 182 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 119 (65%) fell in the Good band (AQI 0–50), 63 (35%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 69, while the worst single-day peak hit 93 (Moderate). Residents averaged about 19.6 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (101 days), PM2.5 (80 days), CO (1 days). Across the 2020–2025 trend window, good days shifted by -158 and the median AQI moved +3 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 93, 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); Flathead (82.1% good, AQI 40); Gallatin (99.1% good, AQI 14).
Air Quality Summary
Good Days
65.4%
119 of 182 monitored days
-9.6pp vs 75% target
Max AQI Recorded
93
Moderate
Health Grade
Moderate
90th-pct AQI 69
Lewis and Clark County, Good days share 65.4%
National avg
Share of monitored days at AQI 0-50 (Good band).
Lewis and Clark 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 182 monitored days across EPA AQI categories.
Good
119
65%
Moderate
63
35%
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 Lewis and Clark County.
- Ozone · 101 days · 55.5%
- PM2.5 · 80 days · 44.0%
- CO · 1 days · 0.5%
Health Impact Assessment
Moderate
Lewis and Clark County's air quality is rated Moderate based on 65.4% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 65.4% of monitored days in 2025, with a median AQI of 45. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 101 of 182 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Lewis and Clark County. Best year: 2020 (277 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 277 | 84 | 1 | 4 | 42 | 168 |
| 2021 | 249 | 99 | 14 | 3 | 44 | 156 |
| 2022 | 239 | 119 | 4 | 3 | 45 | 167 |
| 2023 | 240 | 120 | 3 | 2 | 45 | 165 |
| 2024 | 233 | 127 | 5 | 1 | 46 | 160 |
| 2025 | 119 | 63 | 0 | 0 | 45 | 93 |
How does Lewis and Clark County compare?
How Lewis and Clark County compares
Good-day share worse than US avg
Lewis and Clark County 65.4%
Beaverhead County, Montana 100.0%
US average 76.5%
Median AQI worse than US avg
Lewis and Clark County 45
Beaverhead County, Montana 12
US average 38
Unhealthy days / yr about average
Lewis and Clark 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 Lewis and Clark County.
Adair County, Oklahoma
AQI 45 · 59.2% good
Allen County, Indiana
AQI 45 · 58.2% good
Atascosa County, Texas
AQI 45 · 58.0% good
Baker County, Florida
AQI 45 · 69.1% good
Carson City County, Nevada
AQI 45 · 79.3% good
Clinton County, Iowa
AQI 45 · 59.9% good
What Lewis and Clark 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 45 means for exposure. Health guidance
- See how Lewis and Clark County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Lewis and Clark County with its Montana neighbours before drawing a conclusion about the region. All of Montana
Based on 182 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 Lewis and Clark County, Montana? ▼
Lewis and Clark County has a median Air Quality Index (AQI) of 45, rated "Good." 65.4% of monitored days (119 out of 182) had good air quality in 2025.
What is the main pollutant in Lewis and Clark County? ▼
The primary pollutant in Lewis and Clark County is Ozone (O3), which was the main AQI driver on 101 days (55% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Lewis and Clark County have? ▼
In 2025, Lewis and Clark County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 93.
Is the air quality in Lewis and Clark County getting better or worse? ▼
Air quality in Lewis and Clark County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 277 good days. Worst year: 2025 with 119 good days.
What does AQI 45 mean for health? ▼
Lewis and Clark County's median AQI of 45 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 Lewis and Clark County compare to other counties in Montana? ▼
Lewis and Clark County has 65.4% good air days. Nearby counties in Montana: Beaverhead (100.0% good), Custer (91.2% good), Dawson (95.0% good), Fergus (92.2% good), Flathead (82.1% good), Gallatin (99.1% good).
What is the 90th percentile AQI in Lewis and Clark County? ▼
The 90th percentile AQI for Lewis and Clark County is 69, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 93 (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.