Median AQI
42
Good
Maryland · County air quality · 2025
Washington County, Maryland
Median AQI 42 (Good). 77.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 42
- Median AQI
- 77.3%
- Good-air days
- 0
- Unhealthy days
- 45%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Washington County's air is good on balance, more polluted than the median US county, a median AQI of 42 with 77.3% of 304 monitored days in the Good band, driven mainly by Ozone.
- 42
- median AQI · Good
- 77.3%
- days rated Good
- Ozone
- main pollutant
- 45%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 90.
Data updated July 2026
- Good · 235 d · 77.3%
- Moderate · 69 d · 22.7%
How did Washington County's air break down in 2025?
Across 304 monitored days, 235 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 90 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 304 days monitored. Median AQI: 42 (Good).
Insights: Washington County, Maryland. EPA monitors recorded a median Air Quality Index of 42 across 304 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 235 (77%) fell in the Good band (AQI 0–50), 69 (23%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 59, while the worst single-day peak hit 90 (Moderate). Residents averaged about 23.2 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (197 days), PM2.5 (107 days). Across the 2020–2025 trend window, good days shifted by -53 and the median AQI moved +2 points, placing the multi-year trajectory at stable. The worst single reading over the 2020–2025 window was AQI 90, moderate but never Unhealthy. Nearby counties show comparable readings: Anne Arundel (87.2% good, AQI 40); Baltimore (70.4% good, AQI 44); Baltimore (City) (68.7% good, AQI 44); Calvert (94.6% good, AQI 38); Carroll (90.3% good, AQI 38); Cecil (80.9% good, AQI 41).
Air Quality Summary
Good Days
77.3%
235 of 304 monitored days
2.3pp vs 75% target
Max AQI Recorded
90
Moderate
Health Grade
Good
90th-pct AQI 59
Washington County, Good days share 77.3%
National avg
Share of monitored days at AQI 0-50 (Good band).
Washington 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 304 monitored days across EPA AQI categories.
Good
235
77%
Moderate
69
23%
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 Washington County.
- Ozone · 197 days · 64.8%
- PM2.5 · 107 days · 35.2%
Health Impact Assessment
Good
Washington County's air quality is rated Good based on 77.3% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 77.3% of monitored days in 2025, with a median AQI of 42. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 197 of 304 monitored days.
5-Year Air Quality Trend
Air quality is stable in Washington County. Best year: 2020 (288 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 288 | 71 | 0 | 0 | 40 | 83 |
| 2021 | 244 | 115 | 1 | 0 | 44 | 115 |
| 2022 | 283 | 61 | 0 | 0 | 40 | 87 |
| 2023 | 267 | 82 | 4 | 2 | 42 | 172 |
| 2024 | 287 | 75 | 0 | 0 | 40 | 90 |
| 2025 | 235 | 69 | 0 | 0 | 42 | 90 |
How does Washington County compare?
How Washington County compares
Good-day share better than US avg
Washington County 77.3%
Anne Arundel County, Maryland 87.2%
US average 76.5%
Median AQI worse than US avg
Washington County 42
Anne Arundel County, Maryland 40
US average 38
Unhealthy days / yr about average
Washington County 0
Anne Arundel County, Maryland 0
US average 0
Other Counties in Maryland
Air quality comparison for nearby counties in Maryland.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Washington County.
Adams County, Pennsylvania
AQI 42 · 73.8% good
Anderson County, Tennessee
AQI 42 · 91.0% good
Andrew County, Missouri
AQI 42 · 84.3% good
Armstrong County, Pennsylvania
AQI 42 · 68.1% good
Baldwin County, Alabama
AQI 42 · 72.2% good
Belmont County, Ohio
AQI 42 · 63.0% good
What Washington County's air means for you
The air here stays in the breathable range year-round, with Ozone the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-Ozone days, the guide explains what a median of 42 means for exposure. Health guidance
- See how Washington County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Washington County with its Maryland neighbours before drawing a conclusion about the region. All of Maryland
Based on 304 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 Washington County, Maryland? ▼
Washington County has a median Air Quality Index (AQI) of 42, rated "Good." 77.3% of monitored days (235 out of 304) had good air quality in 2025.
What is the main pollutant in Washington County? ▼
The primary pollutant in Washington County is Ozone (O3), which was the main AQI driver on 197 days (65% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Washington County have? ▼
In 2025, Washington County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 90.
Is the air quality in Washington County getting better or worse? ▼
Air quality in Washington County is stable based on 5-year trends (2020-2025). Best year: 2020 with 288 good days. Worst year: 2025 with 235 good days.
What does AQI 42 mean for health? ▼
Washington County's median AQI of 42 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 Washington County compare to other counties in Maryland? ▼
Washington County has 77.3% good air days. Nearby counties in Maryland: Anne Arundel (87.2% good), Baltimore (70.4% good), Baltimore (City) (68.7% good), Calvert (94.6% good), Carroll (90.3% good), Cecil (80.9% good).
What is the 90th percentile AQI in Washington County? ▼
The 90th percentile AQI for Washington County is 59, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 90 (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.