The Official U.S. Mitigation Threshold

​Radon risk increases gradually with exposure. There is no biological point where risk suddenly begins, but families, contractors, and public‑health programs still need clear decision points. These thresholds are not danger lines. They are tools that help people make consistent, confident choices about when to fix a home.
The United States uses a single national action level 4.0 pCi/L and it has remained stable for decades because it balances scientific evidence, feasibility, and clarity for the public.
Why Thresholds Exist
​Radon behaves differently from hazards that have a clear “safe” or “unsafe” boundary. Risk rises continuously, and even low levels contribute to lifetime exposure.
​Because of this, public‑health agencies create thresholds to:​
  • ​give families a clear point at which action is recommended
  • support consistent real‑estate and mitigation practices
  • align national programs around a single standard
  • avoid confusion caused by multiple competing numbers
​A threshold is a decision point, not a medical cutoff.
​The U.S. National Action Level: 4.0 pCi/L. 
​EPA established 4.0 pCi/L in 1986 and has reaffirmed it repeatedly. It remains the national action level because it is:
  • ​scientifically grounded, based on lifetime risk models
  • feasible for most homes to reach with standard mitigation
  • consistent across real estate, public health, and federal programs
  • clear and stable, which helps families and contractors make decisions
​ EPA’s language is intentional: 4.0 pCi/L is the level at which mitigation is recommended. It is not described as a danger line or emergency threshold.
Why WHO’s 2.7 pCi/L Is Not a Threshold
​The World Health Organization’s 2.7 pCi/L number is often misunderstood. It is not:
  • a global action level
  • a recommended mitigation threshold
  • a replacement for EPA’s 4.0 pCi/L
  • a danger line
​WHO’s number is a reference level—a scientific point countries may consider when setting their own national policies. It reflects the continuous nature of radon risk, but it is not intended for homeowner decision‑making.
​WHO explicitly states that each country should choose its own action level based on feasibility, housing stock, climate, and mitigation capacity. The U.S. evaluated 2.7 pCi/L and did not adopt it.
Why the U.S. Did Not Adopt 2.7 pCi/L
​EPA kept 4.0 pCi/L because lowering the national action level would:
  • double the number of homes labeled “above threshold”
  • create confusion in real estate and public‑health messaging
  • require mitigation systems to reach levels many homes cannot reliably achieve
  • increase cost without improving clarity
  • undermine decades of consistent national guidance
​EPA’s position is that 4.0 pCi/L remains the appropriate U.S. action level.
How to Use These Numbers
​The simplest way to understand the relationship between 2.7 and 4.0 is this:
  • Radon risk increases continuously—there is no safe level.
  • EPA’s 4.0 pCi/L is the U.S. action level—the point where fixing your home is recommended.
  • WHO’s 2.7 pCi/L is a scientific reference, not a U.S. threshold.
  • Lower is always better, but no one needs to panic at any specific number.
​EPA’s 4.0 pCi/L action level is the national standard because it is achievable, consistent, and scientifically grounded.
WHO’s 2.7 pCi/L reference level reflects the continuous nature of radon risk but is not a U.S. threshold. 
Understanding how these numbers were created helps families make clear, confident decisions without fear or confusion.