New Jersey · County air quality · 2025
Mercer County, New Jersey
Median AQI 43 (Good). 69.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 43
- Median AQI
- 69.9%
- Good-air days
- 0
- Unhealthy days
- 32%
- Cleaner than US counties
Mercer County's 2025 annual median AQI is 43 (Good), with 69.9% Good days
According to the U.S. EPA Air Quality System, Mercer County, New Jersey posted a 2025 median AQI of 43 (Good) across 365 monitored days, with 69.9% 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
- Mercer County's 69.9% Good-day share places it cleaner than 32% of US counties in 2025.
- Closest median-AQI peer
- Ashtabula County (81.2% Good, median AQI 43) is the closest nationwide median-AQI peer to Mercer County.
- 2020–2025 trend
- Mercer County's floor-qualifying Good-day share is stable from 2020 to 2025 (71.6% → 69.9%).
- Median AQI
- Median AQI: 43
According to the 2025 EPA AQS county release on PlainAirData, Mercer County, New Jersey ranks #631 of 951 by median AQI (43) and #640 by good-day share (69.9%). 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
- Mercer County's Good-day share rose from 67.2% to 69.9% between the 2024 and 2025 EPA annual releases, a change of +2.6 percentage points.
Data updated August 2026
- Good · 255 d · 69.9%
- Moderate · 104 d · 28.5%
- USG · 6 d · 1.6%
How did Mercer County's air break down in 2025?
Across 365 monitored days, 255 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 126 (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 Mercer County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Mercer County's Good-day share is 69.9% - cleaner than 32% of that comparison set.
Mercer County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
69.9% 32nd percentile higher than 32% 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 Mercer County.
- Ozone · 218 days · 59.7%
- PM2.5 · 147 days · 40.3%
2024 daily-monitor rhythm in Mercer County
Month-by-month median AQI from EPA day-level county monitors (366 reported days in 2024). Jun posted the highest typical readings (median AQI 57, peak 133); Nov was cleanest at median 35. Summer months (Jun–Aug) averaged median AQI 55 vs 37 in Dec–Feb.
Five-year seasonal record
EPA day-level county files · 2020–20241,827 daily readings show a recurring local pattern: Jul has the highest matched-month median AQI (55), while Jan is lowest (40). 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
Mercer County's air quality is rated Moderate based on 69.9% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 69.9% 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 218 of 365 monitored days.
5-Year Air Quality Trend
Air quality is stable in Mercer County. Highest Good-day share: 2020 (71.6%, 262 of 366 monitored 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 | 255 | 104 | 6 | 0 | 43 | 126 |
How does Mercer County compare?
How Mercer 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 Mercer County, New Jersey · 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.