Greenville County Is SC's Only Radon Zone 1 County

Updated October 2026 · 5 min read

Of South Carolina's 46 counties, exactly one sits in the EPA's highest radon category: Greenville County. If you live in Greenville, Greer, Simpsonville, Mauldin, Taylors, Travelers Rest or anywhere else in the county, that label is worth understanding. It is also worth knowing what it does not mean.

What "Radon Zone 1" means

The EPA's radon zone map groups every US county by its potential for elevated indoor radon:

• Zone 1: highest potential. Counties where predicted average indoor screening levels are above 4 pCi/L, the EPA action level.
• Zone 2: moderate potential, with predicted averages between 2 and 4 pCi/L.
• Zone 3: low potential, with predicted averages below 2 pCi/L.

The zones are based on things like geology and soil, along with indoor test data. They describe a county's overall tendency, not a specific house.

In South Carolina, the breakdown is 1 Zone 1 county, 8 Zone 2 counties and 37 Zone 3 counties. Greenville is the Zone 1 county. Its Upstate neighbors, Spartanburg, Anderson, Pickens and Oconee, are all Zone 2.

A short history of the map

The EPA first published its radon zone map in 1993 and updated it in 2019. Greenville County's Zone 1 designation reflects that work, and South Carolina's building code treats Greenville the same way: the radon appendix of the residential code lists Greenville as the state's only high-radon-potential county.

What the test data shows for Greenville County

South Carolina's radon program reports county averages from initial tests. Through June 30, 2022, Greenville County had:

• 5,220 initial tests, far more than any other Upstate county
• An average of 3.4 pCi/L, inside the EPA's "consider fixing" range of 2 to 4
• A highest recorded result of 126.6 pCi/L, more than 30 times the action level

The American Lung Association's look at 2008 to 2017 data found Greenville County had the most radon tests in the state, 12,004, with an estimated mean of 2.9 pCi/L, against a statewide mean of 1.7.

Notice that the measured average, 3.4, is below 4. That is not a contradiction. The zone is a prediction of potential based on several factors, and the test average reflects the homes that happened to be tested. Both point the same way: radon in Greenville County runs well above the national home average of about 1.3 pCi/L. For the other four counties, see our county-by-county radon data.

Greenville County's new-home radon code

Here is where Zone 1 has practical effects. Greenville County is the only county in South Carolina that requires radon control methods in new homes. That requirement comes from the radon appendix of the residential building code. In a February 2026 memo, the county asked to keep that appendix, Appendix BE, in place, and the code's Table BE101.1 lists Greenville as the state's only high-radon-potential county.

That matters because Greenville County builds a lot of homes. It issued 4,947 new-home permits in 2025, more than any other Upstate county. Fast-growing places like Simpsonville, Five Forks, Fountain Inn and Travelers Rest have grown quickly in recent years.

Questions about what applies to a specific permit go to Greenville County Building Safety, the county's building department.

Radon-resistant construction is not the same as a test result

If you bought a newer home in Greenville County, it may have been built with radon control features. That is a real advantage, but it does not tell you your radon level. Construction details vary, and the only way to know how a home performs is to test it after you move in.

If the result is high, a system roughed in during construction can often be finished with a fan rather than built from scratch. Our new construction radon page explains how that works.

Why you should test no matter what zone you're in

The EPA recommends testing every home regardless of zone, and the Upstate data shows why.

• Zone 1 does not mean your house is high. Plenty of Greenville County homes test low.
• Zone 2 does not mean your house is fine. Oconee County is Zone 2 and has the highest average of the five Upstate counties at 5.0 pCi/L. Every Zone 2 county in the Upstate has recorded homes at 37 pCi/L or higher.
• Your county line is not a radon line. Greer, for example, is split between Greenville and Spartanburg counties. Fountain Inn straddles Greenville and Laurens. The soil does not change at the border.

Testing is straightforward. Test the lowest lived-in level of the home. Short-term tests run 2 to 90 days; long-term tests run more than 90 days and give a better year-round picture. For short-term tests, keep windows and exterior doors closed for 12 hours before and during the test. See our radon testing page for how we handle it.

What to do if your Greenville County home tests high

The EPA recommends fixing a home at 4 pCi/L or above, and considering it between 2 and 4. For most houses, the fix is an active soil depressurization system: a sealed pipe and an in-line fan that draw radon from beneath the foundation and vent it above the roofline.

• Slab or basement homes usually get sub-slab depressurization, with a typical range of $900–$2,500.
• Crawl space homes usually get a sub-membrane system, with a typical range of $1,500–$3,500.
• Homes with a mix of foundation types typically run $2,200–$3,800.

These are estimates; the final price comes after an on-site look. Our radon mitigation cost guide breaks down what drives the number. Once a system is running, retest to confirm the level came down.

Frequently asked questions