Takeaway: South Dakota’s Zone 1 designation means predicted indoor averages above 4 pCi/L for the county scale. Many Black Hills homes test high; many do not. Treat the map as a reason to test, not as your result.1
What EPA Zone 1 actually means
The EPA Map of Radon Zones ranks counties into three predicted potential bands:
| Zone | Predicted indoor average | South Dakota reality |
|---|---|---|
| Zone 1 | > 4 pCi/L | Most of the state, including Pennington County |
| Zone 2 | 2–4 pCi/L | Limited counties |
| Zone 3 | < 2 pCi/L | Not the Black Hills pattern |
Zones are built from geology, soil permeability, and indoor survey data available when the map was developed. They are screening tools for public health planning—not appraisals of your foundation.1
Pennington County and Rapid City
Rapid City sits where plains meet the uplifted Black Hills. That transition includes fractured rock, variable soil depth, and a mix of basement, crawl-space, and slab foundations. Statewide and regional surveys have repeatedly found a high share of South Dakota homes above the EPA action level compared with many lower-risk states.
Practically, that means:
- If you have never tested a Rapid City home you occupy, you should.
- Real estate transactions here commonly include radon contingencies for good reason.
- “My neighbor tested low” does not clear your address.
Why two houses on the same street differ
Indoor radon is a function of source strength under the slab, soil pathways, foundation detailing, and how the house moves air. Small differences—a sealed sump vs. an open pit, a continuous vapor barrier vs. gaps, a powerful bath fan vs. none—change results. Weather and season also move short-term numbers; see winter vs summer testing.
State and national context
The South Dakota Department of Health promotes radon awareness and testing. Nationally, EPA and public-health agencies continue to emphasize Zone 1 states for outreach because population risk is higher where elevated homes are more common.2
World Health Organization guidance uses a reference level of 100 Bq/m³ (about 2.7 pCi/L)—lower than EPA’s action level—which underscores that “below 4” is not the same as “zero concern.”3
How to use this information
- Assume Rapid City–area homes deserve testing until proven otherwise.
- Use professional or carefully executed protocols—especially for real estate. See radon testing.
- If results are ≥ 4.0 pCi/L, plan mitigation. If 2–4 pCi/L, decide based on risk tolerance and how much time you spend in lower levels.
Sources
- U.S. EPA. Map of Radon Zones. epa.gov/radon/epa-map-radon-zones
- South Dakota Department of Health — radon education pages.
- World Health Organization. WHO Handbook on Indoor Radon.
Need a local next step? Call (605) 600-6282 to talk through testing, real estate timing, or mitigation for your Rapid City–area home.