Maryland · County air quality · 2025
Harford County, Maryland
Median AQI 42 (Good). 75.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 42
- Median AQI
- 75.5%
- Good-air days
- 0
- Unhealthy days
- 45%
- Cleaner than US counties
Harford County's 2025 annual median AQI is 42 (Good), with 75.5% Good days
According to the U.S. EPA Air Quality System, Harford County, Maryland posted a 2025 median AQI of 42 (Good) across 343 monitored days, with 75.5% 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
- Harford County's 75.5% Good-day share places it cleaner than 45% of US counties in 2025.
- Closest median-AQI peer
- Ada County (74.0% Good, median AQI 42) is the closest nationwide median-AQI peer to Harford County.
- 2020–2025 trend
- Harford County's floor-qualifying Good-day share is worsening from 2020 to 2025 (80.1% → 75.5%).
- Median AQI
- Median AQI: 42
According to the 2025 EPA AQS county release on PlainAirData, Harford County, Maryland ranks #581 of 951 by median AQI (42) and #522 by good-day share (75.5%). Ponce County, Puerto Rico is #1 of 951 by median AQI (3, 91.4% Good days) at good-day-share #159 ≠ largest of 7 at 100.0% Good days #1 by good-day share (100.0%, median AQI 9) at median-AQI #7.
Annual release change
What changed in the 2025 EPA annual release
- Harford County's Good-day share rose from 72.2% to 75.5% between the 2024 and 2025 EPA annual releases, a change of +3.3 percentage points.
- Harford County's peak single-day AQI fell from 154 to 143 between the 2024 and 2025 EPA annual releases, a change of -11 points.
- Harford County's Unhealthy-category days fell from 1 to 0 between the 2024 and 2025 EPA annual releases, a change of -1 days.
Data updated August 2026
- Good · 259 d · 75.5%
- Moderate · 81 d · 23.6%
- USG · 3 d · 0.9%
How did Harford County's air break down in 2025?
Across 343 monitored days, 259 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 →Share of monitored days at AQI 0-50 (Good band).
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
Where Harford County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Harford County's Good-day share is 75.5% - cleaner than 45% of that comparison set.
Harford County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
75.5% 45th percentile higher than 45% of 951 US counties
Each bar is a band; taller bars hold more US counties. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.
Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Harford County.
- Ozone · 202 days · 58.9%
- PM2.5 · 141 days · 41.1%
2024 daily-monitor rhythm in Harford County
Month-by-month median AQI from EPA day-level county monitors (363 reported days in 2024). Jun posted the highest typical readings (median AQI 51, peak 122); Jan was cleanest at median 30. Summer months (Jun–Aug) averaged median AQI 48 vs 37 in Dec–Feb. 1 day crossed Unhealthy (AQI >150) at least once that year.
Five-year seasonal record
EPA day-level county files · 2020–20241,796 daily readings show a recurring local pattern: Jul has the highest matched-month median AQI (53), while Jan is lowest (32). Each cell is that month’s median daily AQI, not a current-air forecast.
Lower AQI is better. The colors summarize EPA AQI bands; gaps mean the county had no day-level record for that month.
Health Impact Assessment
Harford County's air quality is rated Good based on 75.5% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 75.5% 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 202 of 343 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Harford County. Highest Good-day share: 2020 (80.1%, 286 of 357 monitored 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 | 259 | 81 | 3 | 0 | 42 | 143 |
How does Harford County compare?
How Harford County compares
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 Harford County, Maryland · 2020–2025
Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.
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.