Bad Air Days in Mercer County, New Jersey: 6 a Year
6 bad air days a year
More bad air days than three quarters of monitored counties. Ranked 175 of 996 monitored counties, where 1 is worst.
What makes them bad: mostly ozone: summer smog (10% particle days).
The bad-air season here
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in June and July and May. Averaged over 2016 to 2025.
What to buy, and what not to
- Ozone drives many bad days here. Purifiers and filters do not remove ozone, so do not buy one for that. Ozone peaks on hot afternoons: exercise outside in the morning.
The record for Mercer
In 2000 to 2009 this county averaged 28 bad days a year; in 2016 to 2025, 6. The air has improved. The worst recent year was 2016, with 13 bad days on 366 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2023-06-07 | 244 | PM2.5 |
| 2018-07-02 | 187 | Ozone |
| 2023-06-08 | 183 | PM2.5 |
| 2023-06-02 | 182 | Ozone |
| 2023-06-30 | 159 | Ozone |
The air on a normal day
Across 3,653 monitored days from 2016 to 2025, the air in Mercer County rated Good on 62% of them, less often than the 72% typical of monitored counties, and Moderate on 36%. The median day sat at AQI 45; one day in ten went above 65.
The pollutant setting the AQI on an ordinary day here:
- fine particles (PM2.5)54%1,985 days
- ozone46%1,668 days
Mercer County by year
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 365 | 255 | 104 | 6 | 0 | 43 | 126 |
| 2024 | 366 | 246 | 115 | 5 | 0 | 43 | 133 |
| 2023 | 365 | 194 | 160 | 11 | 5 | 49 | 244 |
| 2022 | 365 | 227 | 135 | 3 | 0 | 46 | 119 |
| 2021 | 365 | 193 | 165 | 7 | 0 | 48 | 130 |
| 2020 | 366 | 262 | 101 | 3 | 0 | 42 | 112 |
| 2019 | 365 | 232 | 131 | 2 | 0 | 45 | 136 |
| 2018 | 365 | 229 | 129 | 7 | 1 | 43 | 187 |
| 2017 | 365 | 225 | 136 | 4 | 0 | 46 | 133 |
| 2016 | 366 | 198 | 155 | 13 | 1 | 48 | 151 |
| 2015 | 365 | 210 | 145 | 10 | 0 | 46 | 147 |
| 2014 | 365 | 179 | 180 | 6 | 0 | 51 | 119 |
| 2013 | 365 | 203 | 159 | 3 | 0 | 47 | 122 |
| 2012 | 366 | 206 | 140 | 20 | 0 | 48 | 143 |
| 2011 | 365 | 180 | 172 | 13 | 3 | 51 | 159 |
| 2010 | 365 | 194 | 138 | 33 | 5 | 48 | 174 |
*2025 may be a partial year in EPA's file.
New Jersey compared
- Bergen County7/yr
- Gloucester County6/yr
- Middlesex County5/yr
- Camden County5/yr
- Ocean County4/yr
- Hunterdon County3/yr
- Hudson County3/yr
- Passaic County3/yr
How this is counted
A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day, and on why a purifier does not touch ozone in smoke vs ozone.