Bad Air Days in New Castle County, Delaware: 5 a Year
5 bad air days a year
More bad air days than three quarters of monitored counties. Ranked 208 of 996 monitored counties, where 1 is worst.
What makes them bad: mostly ozone: summer smog (25% particle days).
Which months to watch
- 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 actually helps here
- 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 New Castle
In 2000 to 2009 this county averaged 31 bad days a year; in 2016 to 2025, 5. The air has improved. The worst recent year was 2016, with 10 bad days on 366 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2023-06-07 | 282 | PM2.5 |
| 2023-06-08 | 246 | PM2.5 |
| 2023-06-29 | 172 | PM2.5 |
| 2025-06-14 | 161 | PM2.5 |
| 2016-05-25 | 154 | Ozone |
Most days, not the worst ones
Across 3,653 monitored days from 2016 to 2025, the air in New Castle County rated Good on 57% of them, less often than the 72% typical of monitored counties, and Moderate on 42%. The median day sat at AQI 48; one day in ten went above 69.
The pollutant setting the AQI on an ordinary day here:
- fine particles (PM2.5)56%2,030 days
- ozone44%1,600 days
- nitrogen dioxide1%23 days
New Castle County by year
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 365 | 235 | 126 | 4 | 1 | 44 | 161 |
| 2024 | 366 | 246 | 116 | 4 | 0 | 43 | 119 |
| 2023 | 365 | 173 | 184 | 8 | 3 | 51 | 282 |
| 2022 | 365 | 208 | 157 | 0 | 0 | 48 | 93 |
| 2021 | 365 | 195 | 166 | 4 | 0 | 48 | 144 |
| 2020 | 366 | 231 | 133 | 2 | 0 | 46 | 105 |
| 2019 | 365 | 207 | 155 | 3 | 0 | 47 | 136 |
| 2018 | 365 | 212 | 147 | 6 | 0 | 48 | 150 |
| 2017 | 365 | 187 | 168 | 10 | 0 | 50 | 147 |
| 2016 | 366 | 173 | 183 | 10 | 1 | 52 | 154 |
| 2015 | 365 | 170 | 185 | 10 | 1 | 52 | 172 |
| 2014 | 365 | 140 | 214 | 11 | 1 | 54 | 153 |
| 2013 | 365 | 159 | 199 | 7 | 0 | 52 | 133 |
| 2012 | 366 | 125 | 221 | 20 | 2 | 55 | 151 |
| 2011 | 365 | 116 | 231 | 18 | 4 | 56 | 187 |
| 2010 | 365 | 131 | 201 | 33 | 5 | 55 | 158 |
*2025 may be a partial year in EPA's file.
Nearby counties on file
- Kent County2/yr
- Sussex County1/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.