Air quality in New Castle County
Delaware · FIPS 10003 · 5 years of monitor data
90th percentile AQI, 2025
Acceptable for most, but there may be a risk for people unusually sensitive to air pollution.
EPA monitors recorded 365 days of air quality data in New Castle during 2025. 4 of them were worse than moderate, meaning the air was at least unhealthy for sensitive groups. The pollutant driving the index on most days was ozone.
The 90th percentile AQI of 67 is the more useful headline than the annual maximum: it describes a day worse than nine out of ten, rather than the single worst reading of the year, which a wildfire or an inversion can set on its own.
2021-2025
Five years of days, sorted by category
Between 2021 and 2025, days above moderate went from 4 to 4. Year-to-year movement in a single county is noisy: a wildfire season, a change in which monitors reported, or simply the weather can move it more than any change in local emissions.
EPA Air Quality System annual summaries by county. Days are classified by the highest AQI recorded across the county’s monitors that day. A county with fewer than 365 monitored days was not measuring the whole year.
The scale
What the AQI categories mean
| Category | Index | What it means |
|---|---|---|
| Good | 0-50 | Air quality is satisfactory and air pollution poses little or no risk. |
| Moderate | 51-100 | Acceptable for most, but there may be a risk for people unusually sensitive to air pollution. |
| Unhealthy for sensitive groups | 101-150 | People with lung disease, older adults and children are at greater risk from ozone; people with lung or heart disease, older adults and children are at greater risk from particles. |
| Unhealthy | 151-200 | Some members of the general public may experience health effects; sensitive groups may experience more serious effects. |
| Very unhealthy | 201-300 | Health alert: the risk of health effects is increased for everyone. |
| Hazardous | 301-500 | Health warning of emergency conditions: everyone is more likely to be affected. |
Indoors
Radon in New Castle
Outdoor air is only part of what lungs in this county deal with. New Castle County sits in EPA Radon Zone 2, and radon is the second leading cause of lung cancer in the United States.
Local context
Respiratory health in this county
Nearby