Median AQI
40
Good
Maryland · County air quality · 2025
Washington County, Maryland
Median AQI 40 (Good). 79.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 40
- Median AQI
- 79.0%
- Good-air days
- 0
- Unhealthy days
- 54%
- Cleaner than US counties
Washington County's 2025 annual median AQI is 40 (Good), with 79.0% Good days
According to the U.S. EPA Air Quality System, Washington County, Maryland posted a 2025 median AQI of 40 (Good) across 361 monitored days, with 79.0% 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
- Washington County's 79.0% Good-day share places it cleaner than 54% of US counties in 2025.
- Closest median-AQI peer
- Anderson County (91.3% Good, median AQI 40) is the closest nationwide median-AQI peer to Washington County.
- 2020–2025 trend
- Washington County's floor-qualifying Good-day share is stable from 2020 to 2025 (80.2% → 78.9%).
- Median AQI
- Median AQI: 40
The verdict
According to EPA Air Quality System monitoring data, Washington County's air is good on balance, cleaner than the median US county, a median AQI of 40 with 79.0% of 361 monitored days in the Good band, driven mainly by Ozone.
- 40
- median AQI · Good
- 79.0%
- days rated Good
- Ozone
- main pollutant
- 54%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 90.
Data updated August 2026
- Good · 285 d · 78.9%
- Moderate · 76 d · 21.1%
How did Washington County's air break down in 2025?
Across 361 monitored days, 285 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). 361 days monitored. Median AQI: 40 (Good).
Insights: Washington County, Maryland. EPA monitors recorded a median Air Quality Index of 40 across 361 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 285 (79%) fell in the Good band (AQI 0–50), 76 (21%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 58, while the worst single-day peak hit 90 (Moderate). Residents averaged about 23.7 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (195 days), PM2.5 (166 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by -1.3 points and the median AQI moved +0 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 (87.6% good, AQI 40); Baltimore (64.7% good, AQI 45); Baltimore (City) (68.1% good, AQI 43); Calvert (94.6% good, AQI 38); Carroll (90.3% good, AQI 38); Cecil (79.8% good, AQI 41).
Air Quality Summary
Good Days
79.0%
285 of 361 monitored days
4.0pp vs 75% target
Max AQI Recorded
90
Moderate
Health Grade
Good
90th-pct AQI 58
Washington County, Good days share 79.0%
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 361 monitored days across EPA AQI categories.
Good
285
79%
Moderate
76
21%
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 · 195 days · 54.0%
- PM2.5 · 166 days · 46.0%
Health Impact Assessment
Good
Washington County's air quality is rated Good based on 79.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 79.0% of monitored days in 2025, with a median AQI of 40. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 195 of 361 monitored days.
5-Year Air Quality Trend
Air quality is stable in Washington County. Highest Good-day share: 2022 (82.3%, 283 of 344 monitored 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 | 285 | 76 | 0 | 0 | 40 | 90 |
How does Washington County compare?
How Washington County compares
Good-day share better than typical
Washington County 79.0%
Anderson County, South Carolina 91.3%
Typical US county 76.0%
Median AQI worse than typical
Washington County 40
Anderson County, South Carolina 40
Typical US county 39
Unhealthy days / yr better than typical
Washington County 0
Anderson County, South Carolina 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Washington County (40), not geographic neighbors.
Anderson County, South Carolina
Median AQI 40 · 91.3% Good-day share
Andrew County, Missouri
Median AQI 40 · 86.9% Good-day share
Anne Arundel County, Maryland
Median AQI 40 · 87.6% Good-day share
Bergen County, New Jersey
Median AQI 40 · 71.1% Good-day share
Blair County, Pennsylvania
Median AQI 40 · 76.9% Good-day share
Brazos County, Texas
Median AQI 40 · 68.5% Good-day share
Same primary pollutant: Ozone
Ozone led the AQI on the most monitored days here. These peers share that driver and the closest Good-day share (a second relationship type beyond median-AQI neighbors).
Bossier County, Louisiana
78.9% Good-day share · median AQI 42
Cheshire County, New Hampshire
78.9% Good-day share · median AQI 37
Fayette County, Pennsylvania
78.9% Good-day share · median AQI 40
Sheboygan County, Wisconsin
78.9% Good-day share · median AQI 41
Madison County, Ohio
79.1% Good-day share · median AQI 43
Ocean County, New Jersey
78.8% Good-day share · median AQI 39
Maryland counties on the map
Geographic containment only, other monitored counties in Maryland, independent of AQI or pollutant match.
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 40 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 361 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
Keep exploring the annual AQI record
Air-quality reading list
Frequently Asked Questions
What is the air quality in Washington County, Maryland? ▼
Washington County has a median Air Quality Index (AQI) of 40, rated "Good." 79.0% of monitored days (285 out of 361) 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 195 days (54% 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 across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2022 at 82.3% (283 of 344 days). Lowest qualifying Good-day share: 2021 at 67.8% (244 of 360 days).
What does AQI 40 mean for health? ▼
Washington County's median AQI of 40 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Washington County compare to other counties in Maryland? ▼
Washington County has 79.0% good air days. Nearby counties in Maryland: Anne Arundel (87.6% good), Baltimore (64.7% good), Baltimore (City) (68.1% good), Calvert (94.6% good), Carroll (90.3% good), Cecil (79.8% good).
What is the 90th percentile AQI in Washington County? ▼
The 90th percentile AQI for Washington County is 58, 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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Washington County, Maryland · 2020–2025
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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.