Median AQI
19
Good
Washington · County air quality · 2025
Jefferson County, Washington
Median AQI 19 (Good). 96.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 19
- Median AQI
- 96.5%
- Good-air days
- 0
- Unhealthy days
- 93%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Jefferson County's air is good on balance, among the cleanest 10% of US counties, a median AQI of 19 with 96.5% of 230 monitored days in the Good band, driven mainly by PM2.5.
- 19
- median AQI · Good
- 96.5%
- days rated Good
- PM2.5
- main pollutant
- 93%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 59.
Data updated July 2026
- Good · 222 d · 96.5%
- Moderate · 8 d · 3.5%
How did Jefferson County's air break down in 2025?
Across 230 monitored days, 222 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 59 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 230 days monitored. Median AQI: 19 (Good).
Insights: Jefferson County, Washington. EPA monitors recorded a median Air Quality Index of 19 across 230 measured days in 2025, earning a Good classification and a Excellent health grade. Of those days, 222 (97%) fell in the Good band (AQI 0–50), 8 (3%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 36, while the worst single-day peak hit 59 (Moderate). Residents averaged about 29.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (230 days). Across the 2020–2025 trend window, good days shifted by -99 and the median AQI moved -4 points, placing the multi-year trajectory at stable. The worst single reading over the 2020–2025 window was AQI 59, moderate but never Unhealthy. Nearby counties show comparable readings: Adams (94.7% good, AQI 19); Asotin (73.8% good, AQI 35); Benton (88.9% good, AQI 35); Chelan (87.2% good, AQI 27); Clallam (88.5% good, AQI 26); Clark (89.3% good, AQI 31).
Air Quality Summary
Good Days
96.5%
222 of 230 monitored days
21.5pp vs 75% target
Max AQI Recorded
59
Moderate
Health Grade
Excellent
90th-pct AQI 36
Jefferson County, Good days share 96.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
Jefferson 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 230 monitored days across EPA AQI categories.
Good
222
97%
Moderate
8
3%
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 Jefferson County.
- PM2.5 · 230 days · 100.0%
Health Impact Assessment
Excellent
Jefferson County's air quality is rated Excellent based on 96.5% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 96.5% of monitored days in 2025, with a median AQI of 19. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 230 of 230 monitored days.
5-Year Air Quality Trend
Air quality is stable in Jefferson County. Best year: 2021 (352 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 321 | 11 | 2 | 2 | 23 | 232 |
| 2021 | 352 | 6 | 0 | 0 | 20 | 79 |
| 2022 | 327 | 22 | 0 | 0 | 21 | 90 |
| 2023 | 336 | 14 | 0 | 0 | 21 | 88 |
| 2024 | 300 | 11 | 0 | 0 | 21 | 57 |
| 2025 | 222 | 8 | 0 | 0 | 19 | 59 |
How does Jefferson County compare?
How Jefferson County compares
Good-day share better than US avg
Jefferson County 96.5%
Adams County, Washington 94.7%
US average 76.5%
Median AQI better than US avg
Jefferson County 19
Adams County, Washington 19
US average 38
Unhealthy days / yr about average
Jefferson County 0
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 Jefferson County.
Adams County, Washington
AQI 19 · 94.7% good
Bonneville County, Idaho
AQI 19 · 94.9% good
Elko County, Nevada
AQI 19 · 90.8% good
Grays Harbor County, Washington
AQI 19 · 97.9% good
Luna County, New Mexico
AQI 19 · 79.5% good
Mohave County, Arizona
AQI 19 · 93.4% good
What Jefferson 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 19 means for exposure. Health guidance
- See how Jefferson County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Jefferson County with its Washington neighbours before drawing a conclusion about the region. All of Washington
Based on 230 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 Jefferson County, Washington? ▼
Jefferson County has a median Air Quality Index (AQI) of 19, rated "Good." 96.5% of monitored days (222 out of 230) had good air quality in 2025.
What is the main pollutant in Jefferson County? ▼
The primary pollutant in Jefferson County is Fine Particles (PM2.5), which was the main AQI driver on 230 days (100% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Jefferson County have? ▼
In 2025, Jefferson County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 59.
Is the air quality in Jefferson County getting better or worse? ▼
Air quality in Jefferson County is stable based on 5-year trends (2020-2025). Best year: 2021 with 352 good days. Worst year: 2025 with 222 good days.
What does AQI 19 mean for health? ▼
Jefferson County's median AQI of 19 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 Jefferson County compare to other counties in Washington? ▼
Jefferson County has 96.5% good air days. Nearby counties in Washington: Adams (94.7% good), Asotin (73.8% good), Benton (88.9% good), Chelan (87.2% good), Clallam (88.5% good), Clark (89.3% good).
What is the 90th percentile AQI in Jefferson County? ▼
The 90th percentile AQI for Jefferson County is 36, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 59 (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.