Best Cities for Air Quality in Delaware
Ranking of cities with the cleanest air in Delaware based on AQI data.
Cleanest air in Delaware: Wilmington
Of the 1 Delaware metro area EPA published a full year of AQI data for in 2025, Wilmington recorded the highest share of Good-air days: 151 of 365 monitored days (41.4%) at AQI 50 or below, with a median AQI of 53 and 13 days above AQI 100. Its peak reading that year was AQI 168. The pollutant that set the daily AQI most often was fine particulate matter (PM2.5) (218 days).
| # | City (EPA area) | Good days (2025) | Median AQI | Days over 100 | Peak AQI | AQI now |
|---|---|---|---|---|---|---|
| 1 | WilmingtonPhiladelphia-Camden-Wilmington, PA-NJ-DE-MD | 151 / 365 (41.4%) | 53 | 13 | 168 | 23 |
Figures are EPA's published annual AQI summaries by metropolitan area (Annual AQI by CBSA, 2019–2025), retrieved 2026-09-07. The daily AQI EPA summarises is the highest value any monitor in the area reported that day, so these are metro-area readings, not statewide or rural ones.
Wilmington's air quality year by year
One clean year can be luck — a mild summer, no wildfire smoke. This is every year EPA has published for the Philadelphia-Camden-Wilmington, PA-NJ-DE-MD area, so you can see whether Wilmington's ranking is durable.
| Year | Days measured | Good days | Median AQI | Days over 100 | Days over 150 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|---|---|
| 2019 | 365 | 113 | 56 | 16 | 0 | 150 | fine particulate matter (PM2.5) |
| 2020 | 366 | 108 | 55 | 8 | 0 | 143 | fine particulate matter (PM2.5) |
| 2021 | 365 | 98 | 56 | 15 | 2 | 153 | fine particulate matter (PM2.5) |
| 2022 | 365 | 97 | 56 | 6 | 0 | 126 | fine particulate matter (PM2.5) |
| 2023 | 365 | 66 | 59 | 25 | 6 | 331 | fine particulate matter (PM2.5) |
| 2024 | 366 | 139 | 53 | 15 | 0 | 150 | fine particulate matter (PM2.5) |
| 2025 | 365 | 151 | 53 | 13 | 2 | 168 | fine particulate matter (PM2.5) |
Across 2019–2025, Wilmington's air worsened: the area's median AQI held near 56 and days above AQI 100 rose from about 13 to 18 per year (comparing the earlier and later halves of the period).
How Delaware compares
Ranked by the median of its metro areas' median AQI in 2025, Delaware is #40 cleanest of the 51 states EPA publishes metro AQI data for, at a median AQI of 53. The cleanest state on that measure is Alaska; the most polluted is Arizona. See the full state air quality rankings.
| Bordering state | Median metro AQI (2025) | National rank (cleanest first) |
|---|---|---|
| New Jersey | 43 | #16 of 51 |
| Pennsylvania | 44 | #21 of 51 |
| Maryland | 48 | #29 of 51 |
What this ranking does and does not cover
We track 1 Delaware city. 1 has a full 2025 EPA record and is ranked above.
EPA's AQI is reported by metropolitan area, and the daily value is the worst reading from any monitor in that area. Rural Delaware — and any town without a regulatory monitor — is not represented here. A high Good-day share means the monitored metro air was clean on most days; it is not a statement about a specific street, and it does not capture short pollution spikes between the daily summaries.