Median AQI
30
Good
Washington · County air quality · 2025
Whatcom County, Washington
Median AQI 30 (Good). 96.6% of monitored days rated Good, direct from EPA Air Quality System stations.
- 30
- Median AQI
- 96.6%
- Good-air days
- 0
- Unhealthy days
- 93%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Whatcom County's air is good on balance, among the cleanest 10% of US counties, a median AQI of 30 with 96.6% of 237 monitored days in the Good band, driven mainly by PM2.5.
- 30
- median AQI · Good
- 96.6%
- 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 58.
Data updated July 2026
- Good · 229 d · 96.6%
- Moderate · 8 d · 3.4%
How did Whatcom County's air break down in 2025?
Across 237 monitored days, 229 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 58 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 237 days monitored. Median AQI: 30 (Good).
Insights: Whatcom County, Washington. EPA monitors recorded a median Air Quality Index of 30 across 237 measured days in 2025, earning a Good classification and a Excellent health grade. Of those days, 229 (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 43, while the worst single-day peak hit 58 (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 (143 days), Ozone (94 days). Across the 2020–2025 trend window, good days shifted by -112 and the median AQI moved +6 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 58, 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
96.6%
229 of 237 monitored days
21.6pp vs 75% target
Max AQI Recorded
58
Moderate
Health Grade
Excellent
90th-pct AQI 43
Whatcom County, Good days share 96.6%
National avg
Share of monitored days at AQI 0-50 (Good band).
Whatcom 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 237 monitored days across EPA AQI categories.
Good
229
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 Whatcom County.
- PM2.5 · 143 days · 60.3%
- Ozone · 94 days · 39.7%
Health Impact Assessment
Excellent
Whatcom County's air quality is rated Excellent based on 96.6% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 96.6% of monitored days in 2025, with a median AQI of 30. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 143 of 237 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Whatcom County. Best year: 2020 (341 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 341 | 16 | 2 | 4 | 24 | 237 |
| 2021 | 323 | 7 | 2 | 0 | 28 | 129 |
| 2022 | 294 | 40 | 3 | 5 | 32 | 243 |
| 2023 | 327 | 33 | 0 | 0 | 31 | 94 |
| 2024 | 335 | 26 | 0 | 0 | 33 | 82 |
| 2025 | 229 | 8 | 0 | 0 | 30 | 58 |
How does Whatcom County compare?
How Whatcom County compares
Good-day share better than US avg
Whatcom County 96.6%
Adams County, Washington 94.7%
US average 76.5%
Median AQI better than US avg
Whatcom County 30
Adams County, Washington 19
US average 38
Unhealthy days / yr about average
Whatcom 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 Whatcom County.
Canyon County, Idaho
AQI 30 · 87.2% good
Guaynabo County, Puerto Rico
AQI 30 · 86.0% good
Hawaii County, Hawaii
AQI 30 · 81.1% good
Lynchburg City County, Virginia
AQI 30 · 84.2% good
Putnam County, Tennessee
AQI 30 · 88.6% good
Salem City County, Virginia
AQI 30 · 79.6% good
What Whatcom 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 30 means for exposure. Health guidance
- See how Whatcom County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Whatcom County with its Washington neighbours before drawing a conclusion about the region. All of Washington
Based on 237 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 Whatcom County, Washington? ▼
Whatcom County has a median Air Quality Index (AQI) of 30, rated "Good." 96.6% of monitored days (229 out of 237) had good air quality in 2025.
What is the main pollutant in Whatcom County? ▼
The primary pollutant in Whatcom County is Fine Particles (PM2.5), which was the main AQI driver on 143 days (60% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Whatcom County have? ▼
In 2025, Whatcom County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 58.
Is the air quality in Whatcom County getting better or worse? ▼
Air quality in Whatcom County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 341 good days. Worst year: 2025 with 229 good days.
What does AQI 30 mean for health? ▼
Whatcom County's median AQI of 30 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 Whatcom County compare to other counties in Washington? ▼
Whatcom County has 96.6% 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 Whatcom County? ▼
The 90th percentile AQI for Whatcom County is 43, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 58 (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.