Updated · Rapid City, South Dakota

Numbers to remember: EPA action level 4.0 pCi/L. EPA also says consider fixing between 2 and 4. WHO reference level ≈ 2.7 pCi/L (100 Bq/m³). Outdoor air is often around 0.4 pCi/L.12

Why “safe” is the wrong word

Radiation risk models treat radon as a carcinogen without a bright line of absolute safety. “Safe” in homeowner language usually means “within a widely accepted action framework.” EPA chose 4.0 pCi/L balancing risk reduction and the practicality of measuring and fixing homes nationwide.1

WHO 2.7 pCi/L Reference Standard vs. EPA 4.0 pCi/L Guidance

The World Health Organization (WHO) recommends an indoor radon reference level of 100 Bq/m³, which converts directly to approximately 2.7 pCi/L. This international guideline is grounded in pooled residential epidemiological studies across North America and Europe demonstrating measurable increases in lung cancer rates even at concentrations well below 4.0 pCi/L.

The WHO recommends that national authorities establish an action level of 100 Bq/m³, but notes that if local construction styles or economic constraints make 100 Bq/m³ unfeasible, the level should not exceed 300 Bq/m³ (approx 8.0 pCi/L). Because the EPA set the 4.0 pCi/L benchmark in the 1980s based on technological feasibility of mitigation systems at the time, many modern health advocates and medical professionals advise treating 2.0 to 2.7 pCi/L as an actionable threshold for long-term health protection, particularly when finishing living quarters for young children.

How to decide in the 2–4 pCi/L band

Ask:

  • How many hours per day does your family spend in the basement or lower level?
  • Does anyone smoke or formerly smoke?
  • Are children or long-term occupants in the home?
  • Is mitigation priced reasonably for your foundation type?

In Zone 1 markets like Rapid City, many owners mitigate in the 2–4 range—especially before finishing a basement—because systems are proven and costs are predictable. See mitigation cost.

Units: pCi/L vs Bq/m³

U.S. reports use picocuries per liter. Multiply pCi/L by 37 to approximate Bq/m³. A 4.0 pCi/L result is about 148 Bq/m³.

Sources

  1. U.S. EPA. A Citizen’s Guide to Radon (EPA 402/K-12/002). epa.gov/radon/citizens-guide-radon
  2. World Health Organization (WHO). WHO Handbook on Indoor Radon: A Public Health Perspective. who.gov/publications/i/item/9789241547673
  3. Agency for Toxic Substances and Disease Registry (ATSDR) & CDC. Toxicological Profile for Radon. atsdr.cdc.gov/toxprofiles/tp145.pdf
  4. South Dakota Department of Health. Understanding Your Radon Test Results. doh.sd.gov/programs/radon

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