Median AQI
43
Good
Maryland · County air quality · 2025
Harford County, Maryland
Median AQI 43 (Good). 74.6% of monitored days rated Good, direct from EPA Air Quality System stations.
- 43
- Median AQI
- 74.6%
- Good-air days
- 0
- Unhealthy days
- 40%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Harford County's air is good on balance, more polluted than the median US county, a median AQI of 43 with 74.6% of 303 monitored days in the Good band, driven mainly by Ozone.
- 43
- median AQI · Good
- 74.6%
- days rated Good
- Ozone
- main pollutant
- 40%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 143.
Data updated July 2026
- Good · 226 d · 74.6%
- Moderate · 74 d · 24.4%
- USG · 3 d · 1.0%
How did Harford County's air break down in 2025?
Across 303 monitored days, 226 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 143 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 303 days monitored. Median AQI: 43 (Good).
Insights: Harford County, Maryland. EPA monitors recorded a median Air Quality Index of 43 across 303 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 226 (75%) fell in the Good band (AQI 0–50), 74 (24%) registered Moderate readings, and 3 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 61, while the worst single-day peak hit 143 (USG). Residents averaged about 22.4 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (205 days), PM2.5 (98 days). Across the 2020–2025 trend window, good days shifted by -60 and the median AQI moved +2 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 143, usg 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
74.6%
226 of 303 monitored days
-0.4pp vs 75% target
Max AQI Recorded
143
USG
Health Grade
Moderate
90th-pct AQI 61
Harford County, Good days share 74.6%
National avg
Share of monitored days at AQI 0-50 (Good band).
Harford 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 303 monitored days across EPA AQI categories.
Good
226
75%
Moderate
74
24%
USG
3
1%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Harford County.
- Ozone · 205 days · 67.7%
- PM2.5 · 98 days · 32.3%
Health Impact Assessment
Moderate
Harford County's air quality is rated Moderate based on 74.6% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 74.6% of monitored days in 2025, with a median AQI of 43. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 205 of 303 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Harford County. Best year: 2020 (286 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 286 | 70 | 1 | 0 | 41 | 119 |
| 2021 | 219 | 129 | 9 | 0 | 46 | 122 |
| 2022 | 262 | 98 | 0 | 0 | 43 | 97 |
| 2023 | 238 | 111 | 7 | 3 | 45 | 200 |
| 2024 | 262 | 95 | 5 | 1 | 42 | 154 |
| 2025 | 226 | 74 | 3 | 0 | 43 | 143 |
How does Harford County compare?
How Harford County compares
Good-day share worse than US avg
Harford County 74.6%
Anne Arundel County, Maryland 87.2%
US average 76.5%
Median AQI worse than US avg
Harford County 43
Anne Arundel County, Maryland 40
US average 38
Unhealthy days / yr about average
Harford 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 Harford County.
Allegan County, Michigan
AQI 43 · 71.2% good
Allen County, Ohio
AQI 43 · 65.9% good
Ashtabula County, Ohio
AQI 43 · 81.2% good
Bell County, Kentucky
AQI 43 · 63.5% good
Boise County, Idaho
AQI 43 · 72.9% good
Brazos County, Texas
AQI 43 · 63.4% good
What Harford 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 43 means for exposure. Health guidance
- See how Harford County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Harford County with its Maryland neighbours before drawing a conclusion about the region. All of Maryland
Based on 303 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 Harford County, Maryland? ▼
Harford County has a median Air Quality Index (AQI) of 43, rated "Good." 74.6% of monitored days (226 out of 303) had good air quality in 2025.
What is the main pollutant in Harford County? ▼
The primary pollutant in Harford County is Ozone (O3), which was the main AQI driver on 205 days (68% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Harford County have? ▼
In 2025, Harford County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 143.
Is the air quality in Harford County getting better or worse? ▼
Air quality in Harford County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 286 good days. Worst year: 2021 with 219 good days.
What does AQI 43 mean for health? ▼
Harford County's median AQI of 43 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 Harford County compare to other counties in Maryland? ▼
Harford County has 74.6% 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 Harford County? ▼
The 90th percentile AQI for Harford County is 61, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 143 (USG).
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.