Median AQI
35
Good
Washington · County air quality · 2025
Asotin County, Washington
Median AQI 35 (Good). 73.8% of monitored days rated Good, direct from EPA Air Quality System stations.
- 35
- Median AQI
- 73.8%
- Good-air days
- 1
- Unhealthy days
- 38%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Asotin County's air is good on balance, more polluted than the median US county, a median AQI of 35 with 73.8% of 240 monitored days in the Good band, driven mainly by PM2.5.
- 35
- median AQI · Good
- 73.8%
- days rated Good
- PM2.5
- main pollutant
- 38%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 151.
Data updated July 2026
- Good · 177 d · 73.8%
- Moderate · 61 d · 25.4%
- USG · 1 d · 0.4%
- Unhealthy · 1 d · 0.4%
How did Asotin County's air break down in 2025?
Across 240 monitored days, 177 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 151 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 240 days monitored. Median AQI: 35 (Good).
Insights: Asotin County, Washington. EPA monitors recorded a median Air Quality Index of 35 across 240 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 177 (74%) fell in the Good band (AQI 0–50), 61 (25%) registered Moderate readings, and 1 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 65, while the worst single-day peak hit 151 (Unhealthy). Residents averaged about 22.1 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (240 days). Across the 2020–2025 trend window, good days shifted by -52 and the median AQI moved -3 points, placing the multi-year trajectory at improving. Residents and visitors experienced 1 unhealthy-or-worse days combined, periods when the EPA recommends sensitive groups limit prolonged outdoor exertion. Nearby counties show comparable readings: Adams (94.7% good, AQI 19); Benton (88.9% good, AQI 35); Chelan (87.2% good, AQI 27); Clallam (88.5% good, AQI 26); Clark (89.3% good, AQI 31); Columbia (93.0% good, AQI 37).
Air Quality Summary
Good Days
73.8%
177 of 240 monitored days
-1.3pp vs 75% target
Max AQI Recorded
151
Unhealthy
Health Grade
Moderate
90th-pct AQI 65
Asotin County, Good days share 73.8%
National avg
Share of monitored days at AQI 0-50 (Good band).
Asotin County, Unhealthy-or-worse share 0.4%
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 240 monitored days across EPA AQI categories.
Good
177
74%
Moderate
61
25%
USG
1
0%
Unhealthy
1
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Asotin County.
- PM2.5 · 240 days · 100.0%
Health Impact Assessment
Moderate
Asotin County's air quality is rated Moderate based on 73.8% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 1 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 151. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is PM2.5, driving AQI on 240 of 240 monitored days.
5-Year Air Quality Trend
Air quality is improving in Asotin County. Best year: 2020 (229 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 229 | 112 | 2 | 1 | 38 | 328 |
| 2021 | 176 | 140 | 14 | 4 | 48 | 233 |
| 2022 | 195 | 156 | 3 | 4 | 46 | 200 |
| 2023 | 184 | 169 | 4 | 1 | 49 | 204 |
| 2024 | 224 | 131 | 2 | 4 | 42 | 196 |
| 2025 | 177 | 61 | 1 | 1 | 35 | 151 |
How does Asotin County compare?
How Asotin County compares
Good-day share worse than US avg
Asotin County 73.8%
Adams County, Washington 94.7%
US average 76.5%
Median AQI better than US avg
Asotin County 35
Adams County, Washington 19
US average 38
Unhealthy days / yr worse than US avg
Asotin County 1
Adams County, Washington 0
US average 0
Other Counties in Washington
Air quality comparison for nearby counties in Washington.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Asotin County.
Androscoggin County, Maine
AQI 35 · 85.5% good
Benton County, Washington
AQI 35 · 88.9% good
Benzie County, Michigan
AQI 35 · 91.1% good
Brooke County, West Virginia
AQI 35 · 79.3% good
Bucks County, Pennsylvania
AQI 35 · 87.5% good
Carlton County, Minnesota
AQI 35 · 92.9% good
What Asotin County's air means for you
On most days the air here is fine, but PM2.5 drives 1 unhealthy-or-worse day a year that sensitive groups should plan around.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 35 means for exposure. Health guidance
- See how Asotin County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Asotin County with its Washington neighbours before drawing a conclusion about the region. All of Washington
Based on 240 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 Asotin County, Washington? ▼
Asotin County has a median Air Quality Index (AQI) of 35, rated "Good." 73.8% of monitored days (177 out of 240) had good air quality in 2025.
What is the main pollutant in Asotin County? ▼
The primary pollutant in Asotin County is Fine Particles (PM2.5), which was the main AQI driver on 240 days (100% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Asotin County have? ▼
In 2025, Asotin County recorded 1 unhealthy air days (1 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 151.
Is the air quality in Asotin County getting better or worse? ▼
Air quality in Asotin County is improving based on 5-year trends (2020-2025). Best year: 2020 with 229 good days. Worst year: 2021 with 176 good days.
What does AQI 35 mean for health? ▼
Asotin County's median AQI of 35 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 Asotin County compare to other counties in Washington? ▼
Asotin County has 73.8% good air days. Nearby counties in Washington: Adams (94.7% good), Benton (88.9% good), Chelan (87.2% good), Clallam (88.5% good), Clark (89.3% good), Columbia (93.0% good).
What is the 90th percentile AQI in Asotin County? ▼
The 90th percentile AQI for Asotin County is 65, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 151 (Unhealthy).
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.