Mold in Pennsylvania: what the numbers actually say

Pennsylvania is not a Gulf Coast state, so its mold risk surprises people. The climate is humid continental (Koppen Dfa), which means warm humid summers and cold snowy winters with no dry season at any point in the year. According to NOAA-derived station data and published climatological summaries, every major Pennsylvania metro posts an annual average relative humidity in the mid to high 60s percent, and the northeast of the state runs higher. That is close to the U.S. national average on paper, but two local factors push real indoor risk well above what the headline number suggests.

The first factor is the housing stock. Pennsylvania has one of the oldest housing inventories in the United States, with a large share of homes built before 1970. Many of those homes sit on stone or concrete-block foundations with unconditioned basements and cellars. A below-grade wall stays close to the year-round ground temperature, which in Pennsylvania is roughly the low to mid 50s Fahrenheit. When warm humid summer air at a 60 F to 65 F dew point reaches a 55 F wall or floor, water condenses on the surface. EPA guidance says porous materials kept damp longer than 24 to 48 hours are at risk of mold colonization. A Pennsylvania basement can hold those conditions for months.

The second factor is flood history. Pennsylvania drains two major river systems, the Susquehanna and the Delaware, plus the Ohio and Allegheny in the west. Hurricane Agnes in June 1972 remains the wettest tropical cyclone on record for the state and crested the Susquehanna at Wilkes-Barre more than 20 feet above flood stage, damaging tens of thousands of homes. Tropical Storm Lee in 2011 and the remnants of Hurricane Ida in 2021 both triggered federal disaster declarations for Pennsylvania counties. Each major flood resets the bulk-water clock on porous materials in basements and first floors across the affected river valleys.

Pennsylvania vs U.S. average

Humidity and dew point figures are approximate climatological normals drawn from NOAA National Weather Service station data and published summaries, not live or exact readings. Treat them as typical-year ranges.

Why Scranton and Wilkes-Barre top the ranking. Three factors compound in northeast Pennsylvania. First, humidity: the Avoca reporting station that serves the Scranton and Wilkes-Barre metro posts the highest annual average RH of the five metros, with winter months near 84 percent. Second, flood history: the Wyoming Valley sits in the Susquehanna and Lackawanna floodplains, and Hurricane Agnes crested the river here more than 20 feet above flood stage in 1972, an event the region still references when assessing basement risk. Third, housing stock: the anthracite coal era left a dense inventory of late-1800s and early-1900s homes with stone foundations and chronically damp cellars that hold condensation through the humid months.

Why Harrisburg ranks lowest but is still a real risk. Harrisburg posts the lowest annual average RH of the five metros, which pulls its score down. The trade-off is geography: the city sits directly on the lower Susquehanna, the same river that delivered the Agnes and Lee floods upstream, so flood exposure is direct rather than theoretical. Combine that with February humidity near 80 percent and the same old-housing basement pattern as the rest of the state, and Harrisburg still clears the threshold for ongoing attention.

Every EPA indoor air quality guide lists the same core species for cool, damp, basement-driven climates. Pennsylvania homes concentrate five of them, with the basement leading the way.

Species information is general and informational. Health responses vary. Consult a physician for symptom evaluation. Species sources: EPA Mold Remediation in Schools and Commercial Buildings, EPA A Brief Guide to Mold Moisture and Your Home, CDC National Center for Environmental Health guidance.

Three actions account for most of the mold risk reduction available to a Pennsylvania homeowner. Each maps to an existing free Mold Scanner AI tool.

A. Control the basement first

The basement is where most Pennsylvania mold starts. Run a basement-rated dehumidifier from late spring through early fall and keep it draining to a sump or floor drain so you do not have to empty a tank. Grade soil away from the foundation, extend downspouts at least four feet from the wall, and clear any French drain or interior perimeter drain that has silted up. Never vent a clothes dryer indoors. After any heavy rain, check the basement within 48 hours: EPA guidance says porous materials wet longer than 24 to 48 hours are at risk of colonization. Materials that still read above 16 percent moisture content after 72 hours of drying should be removed.

B. Keep indoor RH below 50 percent in the humid months

Use a hygrometer in the basement and on the main living level. Add portable dehumidification in any room above 55 percent. EPA guidance: 30 to 50 percent indoor relative humidity. For Pennsylvania, target 40 to 50 percent in summer, and watch the basement most closely because it runs wettest.

C. Scan quarterly, especially in spring and after storms

Pennsylvania’s four highest-risk windows are May (spring rain plus snowmelt runoff), August (peak humidity and dew point), September (tail of tropical-remnant rain), and the spring thaw when frozen ground releases moisture into foundations.

Monthly average RH across the five reporting stations, expressed as percent of the year the indoor growth threshold is most often approached. Based on approximate NOAA NWS climatological normals 1991-2020 and published station summaries.

Relative humidity stays elevated every month in Pennsylvania, but the real growth driver is the gap between humid air and cool basement surfaces. Summer dew points in the 60s F over 55 F basement walls produce condensation even when the upstairs feels comfortable. There is no true dry season for a basement here.

Pennsylvania has no state-level mold disclosure or remediation licensing statute, and the Department of Environmental Protection does not regulate residential mold. The agencies below are the authoritative starting points.

Every source chip on this page points here. Agency and statute pages first, then the federal technical references.