Median AQI
41
Good
Maryland · County air quality · 2025
Frederick County, Maryland
Median AQI 41 (Good). 85.7% of monitored days rated Good, direct from EPA Air Quality System stations.
- 41
- Median AQI
- 85.7%
- Good-air days
- 0
- Unhealthy days
- 67%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Frederick County's air is good on balance, cleaner than the median US county, a median AQI of 41 with 85.7% of 237 monitored days in the Good band, driven mainly by Ozone.
- 41
- median AQI · Good
- 85.7%
- days rated Good
- Ozone
- main pollutant
- 67%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 93.
Data updated July 2026
- Good · 203 d · 85.7%
- Moderate · 34 d · 14.3%
How did Frederick County's air break down in 2025?
Across 237 monitored days, 203 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 93 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 237 days monitored. Median AQI: 41 (Good).
Insights: Frederick County, Maryland. EPA monitors recorded a median Air Quality Index of 41 across 237 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 203 (86%) fell in the Good band (AQI 0–50), 34 (14%) 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 93 (Moderate). Residents averaged about 25.7 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (237 days). Across the 2020–2025 trend window, good days shifted by -14 and the median AQI moved +1 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 93, 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
85.7%
203 of 237 monitored days
10.7pp vs 75% target
Max AQI Recorded
93
Moderate
Health Grade
Good
90th-pct AQI 58
Frederick County, Good days share 85.7%
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 237 monitored days across EPA AQI categories.
Good
203
86%
Moderate
34
14%
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.
- Ozone · 237 days · 100.0%
Health Impact Assessment
Good
Frederick County's air quality is rated Good based on 85.7% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 85.7% of monitored days in 2025, with a median AQI of 41. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 237 of 237 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Frederick County. Best year: 2020 (217 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 217 | 20 | 0 | 0 | 40 | 90 |
| 2021 | 207 | 36 | 1 | 0 | 41 | 126 |
| 2022 | 212 | 23 | 1 | 0 | 41 | 115 |
| 2023 | 167 | 59 | 6 | 0 | 44 | 136 |
| 2024 | 192 | 51 | 0 | 0 | 43 | 100 |
| 2025 | 203 | 34 | 0 | 0 | 41 | 93 |
How does Frederick County compare?
How Frederick County compares
Good-day share better than US avg
Frederick County 85.7%
Anne Arundel County, Maryland 87.2%
US average 76.5%
Median AQI worse than US avg
Frederick County 41
Anne Arundel County, Maryland 40
US average 38
Unhealthy days / yr about average
Frederick 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 Frederick County.
Albemarle County, Virginia
AQI 41 · 77.9% good
Anderson County, South Carolina
AQI 41 · 90.0% good
Berkshire County, Massachusetts
AQI 41 · 71.6% good
Berrien County, Michigan
AQI 41 · 80.9% good
Blair County, Pennsylvania
AQI 41 · 71.9% good
Bonner County, Idaho
AQI 41 · 71.3% good
What Frederick 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 41 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 Maryland neighbours before drawing a conclusion about the region. All of Maryland
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 Frederick County, Maryland? ▼
Frederick County has a median Air Quality Index (AQI) of 41, rated "Good." 85.7% of monitored days (203 out of 237) had good air quality in 2025.
What is the main pollutant in Frederick County? ▼
The primary pollutant in Frederick County is Ozone (O3), which was the main AQI driver on 237 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
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 93.
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: 2020 with 217 good days. Worst year: 2023 with 167 good days.
What does AQI 41 mean for health? ▼
Frederick County's median AQI of 41 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 Maryland? ▼
Frederick County has 85.7% 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 Frederick County? ▼
The 90th percentile AQI for Frederick County is 58, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 93 (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.