Radon levels in Pennsylvania

About radon in Pennsylvania

Modern radon awareness in the United States started in Pennsylvania. In December 1984, Stanley Watras, a construction engineer at the Limerick nuclear power plant, set off the plant's radiation alarms on his way into work, before the plant had any nuclear fuel. The source turned out to be radon in his own new home (Virginia Department of Health). The house, in Boyertown, Berks County, sat on a geological formation called the Reading Prong (Journal of the American Statistical Association, via NIH).

Four decades later, Pennsylvania still has some of the most complete radon records in the country. The Department of Environmental Protection publishes individual test results, and our county figures are calculated from 3,356,893 of them, recorded from 1986 to 2026. Across those tests, 34% were at or above the EPA's 4 pCi/L action level. The DEP itself estimates that about 40% of homes tested in the state are above that level (PA DEP Radon Division).

High results are not limited to the Reading Prong. In our data, the share of high tests is greatest in Fulton County (66%) and lowest in Philadelphia County (11%). Most of the central and south-central counties sit above 40%, which the county map above shows at a glance. Because the records are tests that people chose to order, often during a home sale, they describe tested homes rather than every home in a county.

Pennsylvania regulates the radon industry closely. Anyone who tests for radon, installs mitigation systems or analyses samples for pay must hold DEP certification, a requirement in place since 1988 (PA DEP). When a home is sold, the seller's property disclosure statement asks about known hazardous substances, including radon (49 Pa. Code § 35.335a).

If you live in Pennsylvania, the DEP Radon Hotline (800-237-2366) can answer questions about testing and certified contractors. Pick your county below to see its own results, how it compares with its neighbours, and what the numbers mean for your home.

Written and reviewed by Ray Roman · 2026-11-03