Median AQI
43
Good
New Jersey · County air quality · 2025
Mercer County, New Jersey
Median AQI 43 (Good). 73.6% of monitored days rated Good, direct from EPA Air Quality System stations.
- 43
- Median AQI
- 73.6%
- Good-air days
- 0
- Unhealthy days
- 38%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Mercer County's air is good on balance, more polluted than the median US county, a median AQI of 43 with 73.6% of 235 monitored days in the Good band, driven mainly by Ozone.
- 43
- median AQI · Good
- 73.6%
- days rated Good
- Ozone
- main pollutant
- 38%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 122.
Data updated July 2026
- Good · 173 d · 73.6%
- Moderate · 57 d · 24.3%
- USG · 5 d · 2.1%
How did Mercer County's air break down in 2025?
Across 235 monitored days, 173 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 122 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 235 days monitored. Median AQI: 43 (Good).
Insights: Mercer County, New Jersey. EPA monitors recorded a median Air Quality Index of 43 across 235 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 173 (74%) fell in the Good band (AQI 0–50), 57 (24%) registered Moderate readings, and 5 (2%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 66, while the worst single-day peak hit 122 (USG). Residents averaged about 22.1 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (167 days), PM2.5 (68 days). Across the 2020–2025 trend window, good days shifted by -89 and the median AQI moved +1 points, placing the multi-year trajectory at stable. The worst single reading over the 2020–2025 window was AQI 122, usg but never Unhealthy. Nearby counties show comparable readings: Atlantic (79.6% good, AQI 38); Bergen (71.1% good, AQI 40); Camden (69.4% good, AQI 44); Cumberland (73.0% good, AQI 42); Gloucester (72.4% good, AQI 43); Hudson (74.7% good, AQI 41).
Air Quality Summary
Good Days
73.6%
173 of 235 monitored days
-1.4pp vs 75% target
Max AQI Recorded
122
USG
Health Grade
Moderate
90th-pct AQI 66
Mercer County, Good days share 73.6%
National avg
Share of monitored days at AQI 0-50 (Good band).
Mercer 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 235 monitored days across EPA AQI categories.
Good
173
74%
Moderate
57
24%
USG
5
2%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Mercer County.
- Ozone · 167 days · 71.1%
- PM2.5 · 68 days · 28.9%
Health Impact Assessment
Moderate
Mercer County's air quality is rated Moderate based on 73.6% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 73.6% 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 167 of 235 monitored days.
5-Year Air Quality Trend
Air quality is stable in Mercer County. Best year: 2020 (262 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 262 | 101 | 3 | 0 | 42 | 112 |
| 2021 | 193 | 165 | 7 | 0 | 48 | 130 |
| 2022 | 227 | 135 | 3 | 0 | 46 | 119 |
| 2023 | 194 | 160 | 6 | 4 | 49 | 244 |
| 2024 | 246 | 115 | 5 | 0 | 43 | 133 |
| 2025 | 173 | 57 | 5 | 0 | 43 | 122 |
How does Mercer County compare?
How Mercer County compares
Good-day share worse than US avg
Mercer County 73.6%
Atlantic County, New Jersey 79.6%
US average 76.5%
Median AQI worse than US avg
Mercer County 43
Atlantic County, New Jersey 38
US average 38
Unhealthy days / yr about average
Mercer County 0
Atlantic County, New Jersey 0
US average 0
Other Counties in New Jersey
Air quality comparison for nearby counties in New Jersey.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Mercer 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 Mercer 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 Mercer County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Mercer County with its New Jersey neighbours before drawing a conclusion about the region. All of New Jersey
Based on 235 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 Mercer County, New Jersey? ▼
Mercer County has a median Air Quality Index (AQI) of 43, rated "Good." 73.6% of monitored days (173 out of 235) had good air quality in 2025.
What is the main pollutant in Mercer County? ▼
The primary pollutant in Mercer County is Ozone (O3), which was the main AQI driver on 167 days (71% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Mercer County have? ▼
In 2025, Mercer County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 122.
Is the air quality in Mercer County getting better or worse? ▼
Air quality in Mercer County is stable based on 5-year trends (2020-2025). Best year: 2020 with 262 good days. Worst year: 2025 with 173 good days.
What does AQI 43 mean for health? ▼
Mercer 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 Mercer County compare to other counties in New Jersey? ▼
Mercer County has 73.6% good air days. Nearby counties in New Jersey: Atlantic (79.6% good), Bergen (71.1% good), Camden (69.4% good), Cumberland (73.0% good), Gloucester (72.4% good), Hudson (74.7% good).
What is the 90th percentile AQI in Mercer County? ▼
The 90th percentile AQI for Mercer County is 66, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 122 (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.