Air Quality Rankings for Delaware (2026)
Delaware has 3 cities tracked by EPA air-quality monitors, with a state-wide 5-year median AQI of 45 — 4 points more polluted than the national average of AQI 41. Kent, Delaware ranks #1 with the cleanest air (AQI 43, Grade B), while New Castle, Delaware sits at the bottom (AQI 48, Grade C).
How Delaware Compares
Delaware has 3 cities tracked by EPA air-quality monitors, with a state-wide 5-year median AQI of 45 — 4 points more polluted than the national average of AQI 41. Kent, Delaware ranks #1 with the cleanest air (AQI 43, Grade B), while New Castle, Delaware sits at the bottom (AQI 48, Grade C). The rankings below are computed from the EPA Air Quality System (AQS), which aggregates daily AQI readings from federally certified monitors into annual averages. Cities are sorted by 5-year median AQI (lowest = cleanest = #1). The 5-year window smooths out year-to-year volatility from weather and wildfire events.
Delaware is on an improving trajectory: 3 of 3 monitored cities show measurably cleaner air over the past decade, against just 0 that are getting worse. That mirrors the broader national pattern of falling particulate and ozone pollution as cleaner vehicles, cleaner power generation, and tighter industrial standards take effect.
The dominant pollutant across 2 of 3 Delaware cities is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle and industrial emissions. It is worst on hot, sunny, stagnant summer days and is the leading air quality concern across much of the Sun Belt and California. Other Delaware cities report Fine Particulate Matter (PM2.5) (1) as their dominant concern.
The fastest-improving city in Delaware is New Castle, Delaware, with median AQI falling by 0.5 points per year. Steady improvement at that pace usually reflects fleet turnover (older diesels retiring), upwind power-plant retirements, or tighter regional emissions controls.
Full Delaware Ranking
| # | City | 5yr Avg AQI | Current AQI | Worst Pollutant | Trend | Grade |
|---|---|---|---|---|---|---|
| 1 | Kent, Delaware | 43 | 46 | Ozone | Stable | B |
| 2 | Sussex, Delaware | 43 | 47 | Ozone | Improving | B |
| 3 | New Castle, Delaware | 48 | 51 | PM2.5 | Improving | C |
Air quality data for Delaware is sourced from the EPA Air Quality System (AQS), which monitors outdoor air quality at thousands of stations nationwide.
Frequently Asked Questions
Kent, Delaware has the best air quality in Delaware with a 5-year average AQI of 43 and a Grade B (65/100). Its dominant pollutant is Ground-Level Ozone and the long-run trend is stable.
New Castle, Delaware has the worst air quality in Delaware with a 5-year average AQI of 48 and a Grade C (64/100). Its dominant pollutant is Fine Particulate Matter (PM2.5).
Delaware has 3 cities with EPA air quality monitoring data, covering 2014-2023 of daily AQI measurements aggregated into annual averages.
Delaware's state-wide 5-year median AQI is 45, 4 points more polluted than the national average of AQI 41. Delaware is on an improving trajectory: 3 of 3 monitored cities show measurably cleaner air over the past decade, against just 0 that are getting worse. That mirrors the broader national pattern of falling particulate and ozone pollution as cleaner vehicles, cleaner power generation, and tighter industrial standards take effect.
Ground-Level Ozone is the dominant pollutant in 2 of 3 monitored Delaware cities. Ground-level ozone forms when sunlight reacts with vehicle and industrial emissions. It is worst on hot, sunny, stagnant summer days and is the leading air quality concern across much of the Sun Belt and California.
Delaware cities log an average of 2 days per year at "Unhealthy for Sensitive Groups" or worse, based on EPA monitor data over the last five years. Across all 3 Delaware cities tracked, that totals 33 unhealthy days over the period.
Cities ranked by 5-year average AQI (lower is better). Grades factor in average AQI, trend direction, unhealthy days, and dominant pollutant.
The this entity category groups every U.S. air quality and pollution monitoring entity sharing this attribute. The list above is the data; the paragraphs below explain what the grouping means against the broader the EPA Air Quality System (AQS) distribution and how to read the relative rankings within the category.
For readers using this category as a starting point, the per-entity detail pages linked from the table above carry the underlying the EPA Air Quality System (AQS) data in full. The category-level view is the filter; the per-entity pages are the actual answer.
Source: EPA Outdoor Air Quality Data, 2026.