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EPA splits Mid-Atlantic ozone map into three zones
EPA says the split better reflects local air-quality and boundary data for Maryland and Delaware. The revised areas still carry the same classifications: Marginal under the 2008 standard and Serious under the 2015 standard.
For Cecil County, Maryland, and New Castle County, Delaware, the federal ozone map changes on July 6, 2026. The Environmental Protection Agency’s final rule splits the old Philadelphia-Wilmington-Atlantic City nonattainment area into three separate zones for the 2008 and 2015 ozone national ambient air quality standards, or NAAQS.
Comment deadline: February 9, 2026 Effective date: July 6, 2026
The practical effect is mostly administrative, but it matters for how the region is counted and planned. EPA says the available air-quality, emissions, weather, geography and boundary data show Cecil County does not contribute to a violation in the other area, and the same is true for New Castle County. The agency is also issuing clean-data determinations for the revised Maryland and Delaware areas under both ozone standards.
The limits stay in place
The split does not make either county ozone-clean in the legal sense. Cecil County and New Castle County both remain designated nonattainment, with the same classifications: Marginal under the 2008 ozone standard and Serious under the 2015 standard.
The rest of the former region becomes the Philadelphia-Atlantic City, Pennsylvania-New Jersey, nonattainment area. So the federal government is redrawing the boundary without lifting the ozone obligations attached to it.
What the clean-data call does
A clean-data determination means the EPA has found the air-quality record for the revised area meets the standard at issue, even though the nonattainment designation itself remains in place. That leaves the counties with a cleaner legal chart, not a blank slate.
For people living, working or planning projects in the two counties, the result is a narrower map and the same ozone rules that already apply.
Agency: Environmental Protection Agency Docket ID: EPA-R03-OAR-2025-1872 CFR parts: 52, 81 Comment deadline: February 9, 2026 Effective date: July 6, 2026 Contact: Sarah McCabe • Planning & Implementation Branch (3AD30), Air & Radiation Division • (215) 814-5786 • mccabe.sarah@epa.gov • 1600 John F. Kennedy Boulevard, Philadelphia, Pennsylvania 19103