Median AQI
42
Good
Virginia · County air quality · 2025
Frederick County, Virginia
Median AQI 42 (Good). 70.4% of monitored days rated Good, direct from EPA Air Quality System stations.
- 42
- Median AQI
- 70.4%
- Good-air days
- 0
- Unhealthy days
- 32%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Frederick County's air is good on balance, more polluted than the median US county, a median AQI of 42 with 70.4% of 284 monitored days in the Good band, driven mainly by PM2.5.
- 42
- median AQI · Good
- 70.4%
- days rated Good
- PM2.5
- main pollutant
- 32%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 85.
Data updated July 2026
- Good · 200 d · 70.4%
- Moderate · 84 d · 29.6%
How did Frederick County's air break down in 2025?
Across 284 monitored days, 200 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 85 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 284 days monitored. Median AQI: 42 (Good).
Insights: Frederick County, Virginia. EPA monitors recorded a median Air Quality Index of 42 across 284 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 200 (70%) fell in the Good band (AQI 0–50), 84 (30%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 62, while the worst single-day peak hit 85 (Moderate). Residents averaged about 21.1 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (172 days), Ozone (112 days). Across the 2020–2025 trend window, good days shifted by -49 and the median AQI moved +3 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 85, moderate but never Unhealthy. Nearby counties show comparable readings: Albemarle (77.9% good, AQI 41); Arlington (84.8% good, AQI 40); Bristol City (78.3% good, AQI 33); Caroline (96.2% good, AQI 37); Carroll (100.0% good, AQI 9); Charles (88.6% good, AQI 38).
Air Quality Summary
Good Days
70.4%
200 of 284 monitored days
-4.6pp vs 75% target
Max AQI Recorded
85
Moderate
Health Grade
Moderate
90th-pct AQI 62
Frederick County, Good days share 70.4%
National avg
Share of monitored days at AQI 0-50 (Good band).
Frederick 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 284 monitored days across EPA AQI categories.
Good
200
70%
Moderate
84
30%
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 Frederick County.
- PM2.5 · 172 days · 60.6%
- Ozone · 112 days · 39.4%
Health Impact Assessment
Moderate
Frederick County's air quality is rated Moderate based on 70.4% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 70.4% 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 PM2.5, driving AQI on 172 of 284 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Frederick County. Best year: 2024 (264 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 249 | 55 | 0 | 0 | 39 | 81 |
| 2021 | 222 | 140 | 0 | 0 | 44 | 98 |
| 2022 | 234 | 100 | 0 | 0 | 43 | 89 |
| 2023 | 240 | 114 | 5 | 4 | 44 | 185 |
| 2024 | 264 | 90 | 1 | 0 | 41 | 102 |
| 2025 | 200 | 84 | 0 | 0 | 42 | 85 |
How does Frederick County compare?
How Frederick County compares
Good-day share worse than US avg
Frederick County 70.4%
Albemarle County, Virginia 77.9%
US average 76.5%
Median AQI worse than US avg
Frederick County 42
Albemarle County, Virginia 41
US average 38
Unhealthy days / yr about average
Frederick County 0
Albemarle County, Virginia 0
US average 0
Other Counties in Virginia
Air quality comparison for nearby counties in Virginia.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Frederick 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 Frederick 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 42 means for exposure. Health guidance
- See how Frederick County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Frederick County with its Virginia neighbours before drawing a conclusion about the region. All of Virginia
Based on 284 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 Frederick County, Virginia? ▼
Frederick County has a median Air Quality Index (AQI) of 42, rated "Good." 70.4% of monitored days (200 out of 284) had good air quality in 2025.
What is the main pollutant in Frederick County? ▼
The primary pollutant in Frederick County is Fine Particles (PM2.5), which was the main AQI driver on 172 days (61% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Frederick County have? ▼
In 2025, Frederick County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 85.
Is the air quality in Frederick County getting better or worse? ▼
Air quality in Frederick County is worsening based on 5-year trends (2020-2025). Best year: 2024 with 264 good days. Worst year: 2025 with 200 good days.
What does AQI 42 mean for health? ▼
Frederick 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 Frederick County compare to other counties in Virginia? ▼
Frederick County has 70.4% good air days. Nearby counties in Virginia: Albemarle (77.9% good), Arlington (84.8% good), Bristol City (78.3% good), Caroline (96.2% good), Carroll (100.0% good), Charles (88.6% good).
What is the 90th percentile AQI in Frederick County? ▼
The 90th percentile AQI for Frederick County is 62, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 85 (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.