Washington · County air quality · 2025
Lewis County, Washington
Median AQI 20 (Good). 91.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 20
- Median AQI
- 91.3%
- Good-air days
- 0
- Unhealthy days
- 83%
- Cleaner than US counties
Lewis County's 2025 annual median AQI is 20 (Good), with 91.3% Good days
According to the U.S. EPA Air Quality System, Lewis County, Washington posted a 2025 median AQI of 20 (Good) across 323 monitored days, with 91.3% 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 County's 91.3% Good-day share places it cleaner than 83% of US counties in 2025.
- Closest median-AQI peer
- Jefferson County (97.3% Good, median AQI 20) is the closest nationwide median-AQI peer to Lewis County.
- 2020–2025 trend
- Lewis County's floor-qualifying Good-day share is stable from 2020 to 2025 (94.0% → 91.3%).
- Median AQI
- Median AQI: 20
According to the 2025 EPA AQS county release on PlainAirData, Lewis County, Washington ranks #36 of 951 by median AQI (20) and #164 by good-day share (91.3%). 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 County's peak single-day AQI fell from 129 to 74 between the 2024 and 2025 EPA annual releases, a change of -55 points.
- Lewis County's Good-day share fell from 93.3% to 91.3% between the 2024 and 2025 EPA annual releases, a change of -2.0 percentage points.
Data updated August 2026
- Good · 295 d · 91.3%
- Moderate · 28 d · 8.7%
How did Lewis County's air break down in 2025?
Across 323 monitored days, 295 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 74 (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 County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Lewis County's Good-day share is 91.3% - cleaner than 83% of that comparison set.
Lewis County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
91.3% Cleaner than 83% higher than 83% 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 County.
- PM2.5 · 323 days · 100.0%
2024 daily-monitor rhythm in Lewis County
Month-by-month median AQI from EPA day-level county monitors (360 reported days in 2024). Dec posted the highest typical readings (median AQI 33, peak 66); Jun was cleanest at median 16. Summer months (Jun–Aug) averaged median AQI 22 vs 29 in Dec–Feb.
Five-year seasonal record
EPA day-level county files · 2020–20241,772 daily readings show a recurring local pattern: Oct has the highest matched-month median AQI (41), while Jun is lowest (17). 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 County's air quality is rated Excellent based on 91.3% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 91.3% of monitored days in 2025, with a median AQI of 20. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 323 of 323 monitored days.
5-Year Air Quality Trend
Air quality is stable in Lewis County. Highest Good-day share: 2020 (94.0%, 314 of 335 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 314 | 20 | 0 | 0 | 25 | 258 |
| 2021 | 328 | 22 | 0 | 0 | 23 | 71 |
| 2022 | 285 | 70 | 7 | 3 | 26 | 198 |
| 2023 | 314 | 46 | 1 | 1 | 25 | 159 |
| 2024 | 336 | 23 | 1 | 0 | 22 | 129 |
| 2025 | 295 | 28 | 0 | 0 | 20 | 74 |
How does Lewis County compare?
How Lewis 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 County, Washington · 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.