Defined model
A radon-222 atom decays within the next year
- Probability:
- (100 − 1.22 × 10⁻²⁷)%
- Odds:
- (8.22 × 10²⁸) to 1
- dBan
- +289.1 dBan
With a rounded half-life of 3.8 days, the cumulative probability that one radon-222 atom decays during the next 365 days is extremely close to one.
Calculation
p = 1 − 2^(−365/3.8)
Assumptions
The model uses a constant exponential decay rate and the EPA’s rounded half-life. About half of a large group of radon-222 atoms would decay within 3.8 days.
Sources
- US EPA: radionuclide basics — radon updated 22 January 2026