Why are radon levels high in parts of Southwest Virginia?

The short answer

Southwest Virginia's radon comes from the ground under it. Radon is a naturally occurring, radioactive gas that you cannot see, taste or smell, produced by the breakdown of uranium in soil, rock and water.^[4] Where the rock and soil hold more uranium and radium, and let gas move easily, more radon reaches the foundation. Much of the region lies in the Valley and Ridge, the province of parallel valleys and ridges west of the Blue Ridge, with a small part of the Appalachian Plateaus in the far southwest.^[1] That geology is the main reason.

Geology tells you where radon is more likely, not what is in your house. Even houses next door to each other can test very differently.^[4]

The rocks under the Valley and Ridge

The U.S. Geological Survey's assessment of Virginia, prepared with EPA in 1993, is direct: "The Valley and Ridge has been ranked high in geologic radon potential, with local areas having low to moderate radon potential."^[1] It names the likely sources: "Soils derived from carbonate rocks and black shales, and black shale bedrock may be sources of the moderate to high levels of indoor radon in this province."^[1]

Limestone country explains much of it. Carbonate rocks themselves are usually low in radioactive elements, but the soils that form from them are often elevated in uranium and radium.^[1] When the rock's calcium carbonate dissolves away, the soil is left enriched in the remaining impurities, including radioactive elements.^[1] Karst and cave features are also thought to help radon flow and collect.^[1]

Then the gas has to move. Soil permeability in the Valley and Ridge is moderate to high,^[1] which lets soil gas travel toward basements and slabs instead of staying put.

Farther west, in the coalfield counties of the Appalachian Plateaus, the USGS ranked the geologic radon potential moderate.^[1] There, "Black shales, especially those associated with coal seams, are generally elevated in uranium and may be sources of moderate to high radon levels."^[1] Indoor radon in the plateaus varies from low to high, but indoor radon data there are limited.^[1]

What the house tests showed

The same report looked at indoor results from a 1991-92 state and EPA survey. The clearest pattern was the abundance of counties with high (over 4 pCi/L) indoor radon averages in the Valley and Ridge.^[1] The statewide average was 2.7 pCi/L, and 17.6 percent of the homes tested were over 4 pCi/L.^[1] The authors also noted that data were lacking for much of the western Valley and Ridge and the Appalachian Plateaus,^[1] so a quiet record in a far-western county does not mean low radon.

How the gas gets into a house

Radon follows the easiest path from the soil into the house. VDH lists the common entry points as cracks in foundations, openings around sump pumps and drains, construction joints, cracks in walls, crawl spaces and, in some cases, well water.^[4] It builds up most on the lowest level.

Mountain winters help it along. During winter, the larger difference between outdoor and indoor pressure is likely to draw more radon into a house, and the house is usually less ventilated.^[4] The way a house sits matters too. VDH notes high results in homes built over small uranium deposits or non-native fill, or with foundations cut into valleys or hillsides where they reach deeper geologic layers high in radon source material.^[3] If your house is built into a slope, test the lowest level that is or could be lived in.

What the zone map does and does not tell you

EPA sorts counties into three zones. Zone 1 has the highest potential, with average indoor radon levels that may be greater than 4 pCi/L.^[7] According to VDH, Virginia has 46 counties and 15 cities classified as Zone 1 (high risk) and 24 counties and 8 cities classified as Zone 2 (moderate risk).^[2]

Use the map for context only. The map should not be used to decide whether an individual home needs testing,^[8] and VDH warns that the map was based on a very limited number of radon test results and was never meant to predict indoor levels.^[3] High radon test results have been found in every Virginia county, including those designated low risk.^[3] VDH also publishes a radon map built from test results reported to its radon program from 2016 to 2024.^[3]

Local building rules reflect the risk

Some localities in and near Southwest Virginia require radon-resistant new construction. VDH's list of jurisdictions known to require it includes Montgomery, Rockbridge, Tazewell and Wythe counties, plus Blacksburg, Buena Vista, Christiansburg, Lexington, Radford, Roanoke and Salem.^[6] Homes over ventilated crawl spaces and multi-family buildings are exempt for now.^[6] A newer house built under those rules still needs a test after move-in.

Stop and call a pro if

• A follow-up test confirms 4 pCi/L or higher. See what radon mitigation costs in Virginia.
• Your house has several foundation sections, a walk-out basement into a hillside, or a crawl space, and you want a design that covers all of them.
• You have a private well and the air result is high.
• You are buying a home in a high-potential area and need a test the seller cannot disturb.

Start with how to test your home for radon, browse more inspection answers, or find inspection businesses in Blacksburg.

Ready for a radon test or a mitigation quote? Describe the job once and licensed businesses near you can quote.