Median AQI
23
Good
Idaho · County air quality · 2025
Lemhi County, Idaho
Median AQI 23 (Good). 69.1% of monitored days rated Good, direct from EPA Air Quality System stations.
- 23
- Median AQI
- 69.1%
- Good-air days
- 0
- Unhealthy days
- 29%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Lemhi County's air is good on balance, more polluted than the median US county, a median AQI of 23 with 69.1% of 181 monitored days in the Good band, driven mainly by PM2.5.
- 23
- median AQI · Good
- 69.1%
- days rated Good
- PM2.5
- main pollutant
- 29%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 85.
Data updated July 2026
- Good · 125 d · 69.1%
- Moderate · 56 d · 30.9%
How did Lemhi County's air break down in 2025?
Across 181 monitored days, 125 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 85 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 181 days monitored. Median AQI: 23 (Good).
Insights: Lemhi County, Idaho. EPA monitors recorded a median Air Quality Index of 23 across 181 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 125 (69%) fell in the Good band (AQI 0–50), 56 (31%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 65, while the worst single-day peak hit 85 (Moderate). Residents averaged about 20.7 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (181 days). Across the 2020–2025 trend window, good days shifted by -69 and the median AQI moved -23 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 85, moderate but never Unhealthy. Nearby counties show comparable readings: Ada (80.6% good, AQI 39); Bannock (91.5% good, AQI 31); Benewah (74.6% good, AQI 37); Blaine (99.4% good, AQI 22); Boise (72.9% good, AQI 43); Bonner (71.3% good, AQI 41).
Air Quality Summary
Good Days
69.1%
125 of 181 monitored days
-5.9pp vs 75% target
Max AQI Recorded
85
Moderate
Health Grade
Moderate
90th-pct AQI 65
Lemhi County, Good days share 69.1%
National avg
Share of monitored days at AQI 0-50 (Good band).
Lemhi 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 181 monitored days across EPA AQI categories.
Good
125
69%
Moderate
56
31%
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 Lemhi County.
- PM2.5 · 181 days · 100.0%
Health Impact Assessment
Moderate
Lemhi County's air quality is rated Moderate based on 69.1% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 69.1% of monitored days in 2025, with a median AQI of 23. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 181 of 181 monitored days.
5-Year Air Quality Trend
Air quality is improving in Lemhi County. Best year: 2023 (251 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 194 | 160 | 8 | 3 | 46 | 168 |
| 2021 | 188 | 157 | 16 | 4 | 47 | 172 |
| 2022 | 199 | 136 | 18 | 10 | 44 | 212 |
| 2023 | 251 | 110 | 2 | 0 | 32 | 119 |
| 2024 | 207 | 143 | 12 | 4 | 37 | 198 |
| 2025 | 125 | 56 | 0 | 0 | 23 | 85 |
How does Lemhi County compare?
How Lemhi County compares
Good-day share worse than US avg
Lemhi County 69.1%
Ada County, Idaho 80.6%
US average 76.5%
Median AQI better than US avg
Lemhi County 23
Ada County, Idaho 39
US average 38
Unhealthy days / yr about average
Lemhi County 0
Ada County, Idaho 0
US average 0
Other Counties in Idaho
Air quality comparison for nearby counties in Idaho.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Lemhi County.
Lawrence County, Alabama
AQI 23 · 100.0% good
Routt County, Colorado
AQI 23 · 91.6% good
Wallowa County, Oregon
AQI 23 · 91.2% good
Wasco County, Oregon
AQI 23 · 92.8% good
Alamosa County, Colorado
AQI 24 · 90.8% good
Anchorage County, Alaska
AQI 24 · 80.8% good
What Lemhi 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 23 means for exposure. Health guidance
- See how Lemhi County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Lemhi County with its Idaho neighbours before drawing a conclusion about the region. All of Idaho
Based on 181 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 Lemhi County, Idaho? ▼
Lemhi County has a median Air Quality Index (AQI) of 23, rated "Good." 69.1% of monitored days (125 out of 181) had good air quality in 2025.
What is the main pollutant in Lemhi County? ▼
The primary pollutant in Lemhi County is Fine Particles (PM2.5), which was the main AQI driver on 181 days (100% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Lemhi County have? ▼
In 2025, Lemhi County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 85.
Is the air quality in Lemhi County getting better or worse? ▼
Air quality in Lemhi County is improving based on 5-year trends (2020-2025). Best year: 2023 with 251 good days. Worst year: 2025 with 125 good days.
What does AQI 23 mean for health? ▼
Lemhi County's median AQI of 23 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 Lemhi County compare to other counties in Idaho? ▼
Lemhi County has 69.1% good air days. Nearby counties in Idaho: Ada (80.6% good), Bannock (91.5% good), Benewah (74.6% good), Blaine (99.4% good), Boise (72.9% good), Bonner (71.3% good).
What is the 90th percentile AQI in Lemhi County? ▼
The 90th percentile AQI for Lemhi County is 65, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 85 (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.