What is radon gas?

Updated September 24, 2026 · Rapid City, South Dakota

How radon forms

Uranium is present in many rock types. As it decays through a chain of radioactive elements, one intermediate product is radium, which decays into radon gas. Because radon is a gas, it can leave the mineral grain, travel through pore spaces in soil, and enter the air.

Outdoors, radon dilutes quickly to low concentrations. Indoors—especially in basements, crawl spaces, and tightly sealed modern homes—it can accumulate. You cannot see it, smell it, or taste it. A radon test is the only reliable detector for living spaces.

Why homes pull radon inside

Homes act like mild vacuums relative to the soil. Stack effect (warm air rising), exhaust fans, clothes dryers, and HVAC systems can lower indoor air pressure. That pressure difference draws soil gas—including radon—through:

• Floor–wall joints and slab cracks
• Sumps, floor drains, and utility penetrations
• Crawl-space soil and poorly sealed vapor barriers
• Hollow-block foundation walls
• Loose or unsealed pipe openings

Newer, energy-efficient homes are not immune. Tighter envelopes can trap radon once it enters. Older Black Hills basements with stone or block foundations often have more entry paths. Neither construction style guarantees high or low levels—only a test does.

Why Rapid City and the Black Hills see elevated risk

The Black Hills expose ancient granite, metamorphic rock, and sedimentary layers. Some units contain uranium at levels that increase soil-gas radon potential. The EPA’s radon zone map places nearly all of South Dakota—including Rapid City’s Pennington County—in Zone 1 (predicted average indoor levels above 4 pCi/L).²

Zone maps are county-scale predictions, not house-by-house scores. A home on a ridge west of Canyon Lake can test very differently from a slab-on-grade ranch in Rapid Valley. Neighborhood averages do not substitute for a measurement in your lowest lived-in level.

For local geology context, see Black Hills geology and radon and radon levels in South Dakota.

Health risk in plain language

When you breathe radon, its radioactive decay products can attach to lung tissue and deliver alpha radiation. Over years, that raises lung-cancer risk. The EPA and the Surgeon General identify radon as the second leading cause of lung cancer in the United States—after smoking—and the leading cause among non-smokers.¹

Risk rises with concentration and with time spent indoors. Smokers face substantially higher combined risk. Children are not uniquely “immune”; lifetime exposure starting young simply means more years of cumulative dose if levels stay high.

There is no known safe threshold of zero risk, which is why EPA also encourages mitigation when levels are between 2 and 4 pCi/L if you want extra margin.¹ Read more in radon and lung cancer risk and what is a safe radon level.

What Rapid City homeowners should do

1. Test the lowest level you regularly use—often a finished basement. Use a quality short-term or long-term protocol; see short-term vs long-term tests.
2. Interpret results against EPA’s 4.0 pCi/L action level, and consider retesting if a short-term result is borderline.
3. Mitigate if levels are at or above 4.0 pCi/L—or if you choose to lower results in the 2–4 range. Modern radon mitigation typically uses active soil depressurization.
4. Retest after mitigation and every couple of years thereafter.

Buying or selling? Pair this overview with radon testing when buying a home.

Quick answers

Sources

1. U.S. Environmental Protection Agency. A Citizen’s Guide to Radon and radon health risk information. epa.gov/radon
2. U.S. EPA. EPA Map of Radon Zones (South Dakota — Zone 1). epa.gov/radon/epa-map-radon-zones
3. South Dakota Department of Health radon resources for residents.