Montana · County air quality · 2025
Lewis and Clark County, Montana
Median AQI 44 (Good). 71.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 44
- Median AQI
- 71.5%
- Good-air days
- 0
- Unhealthy days
- 35%
- Cleaner than US counties
Lewis and Clark County's 2025 annual median AQI is 44 (Good), with 71.5% Good days
According to the U.S. EPA Air Quality System, Lewis and Clark County, Montana posted a 2025 median AQI of 44 (Good) across 365 monitored days, with 71.5% 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
- Lewis and Clark County's 71.5% Good-day share places it cleaner than 35% of US counties in 2025.
- Closest median-AQI peer
- Allegan County (63.9% Good, median AQI 44) is the closest nationwide median-AQI peer to Lewis and Clark County.
- 2020–2025 trend
- Lewis and Clark County's floor-qualifying Good-day share is worsening from 2020 to 2025 (75.7% → 71.5%).
- Median AQI
- Median AQI: 44
According to the 2025 EPA AQS county release on PlainAirData, Lewis and Clark County, Montana ranks #680 of 951 by median AQI (44) and #611 by good-day share (71.5%). 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
- Lewis and Clark County's Good-day share rose from 63.7% to 71.5% between the 2024 and 2025 EPA annual releases, a change of +7.8 percentage points.
- Lewis and Clark County's peak single-day AQI fell from 160 to 93 between the 2024 and 2025 EPA annual releases, a change of -67 points.
- Lewis and Clark County's Unhealthy-category days fell from 1 to 0 between the 2024 and 2025 EPA annual releases, a change of -1 days.
Data updated August 2026
- Good · 261 d · 71.5%
- Moderate · 104 d · 28.5%
How did Lewis and Clark County's air break down in 2025?
Across 365 monitored days, 261 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 →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 Lewis and Clark County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Lewis and Clark County's Good-day share is 71.5% - cleaner than 35% of that comparison set.
Lewis and Clark County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
71.5% 35th percentile higher than 35% 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 Lewis and Clark County.
- Ozone · 194 days · 53.2%
- PM2.5 · 171 days · 46.8%
2024 daily-monitor rhythm in Lewis and Clark County
Month-by-month median AQI from EPA day-level county monitors (366 reported days in 2024). Jul posted the highest typical readings (median AQI 72, peak 113); Nov was cleanest at median 37. Summer months (Jun–Aug) averaged median AQI 56 vs 47 in Dec–Feb. 1 day crossed Unhealthy (AQI >150) at least once that year.
Five-year seasonal record
EPA day-level county files · 2020–20241,827 daily readings show a recurring local pattern: Jul has the highest matched-month median AQI (61), while Feb is lowest (42). 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
Lewis and Clark County's air quality is rated Moderate based on 71.5% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 71.5% of monitored days in 2025, with a median AQI of 44. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 194 of 365 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Lewis and Clark County. Highest Good-day share: 2020 (75.7%, 277 of 366 monitored 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 | 261 | 104 | 0 | 0 | 44 | 93 |
How does Lewis and Clark County compare?
How Lewis and Clark 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 Lewis and Clark 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.