Short answer
A half-life is the time it takes for half of the radioactive atoms in a sample to decay. Radon-222, radon's most stable form, has a half-life of about 3.8 days.
What half-life means
EPA's definition is the plain one: radioactive half-life is the time required for half of the radioactive atoms present to decay. After one half-life half of the atoms are left, after two a quarter, and so on.
Radon does not run out, because the ground keeps making it. Radium decays to produce radon gas, and radium-226 has a half-life of about 1,600 years.
How much radon-222 is left after each half-life
Each row halves the row above it and adds 3.8 days.
| Half-lives | Days | Radon-222 left |
|---|---|---|
| 0 | 0 | 100% |
| 1 | 3.8 | 50% |
| 2 | 7.6 | 25% |
| 3 | 11.4 | 12.5% |
| 4 | 15.2 | 6.25% |
| 5 | 19 | 3.125% |
| 10 | 38 | 0.098% |
What it means for your home
A house is not a sealed sample. Radon that comes in decays within days, and the soil sends more, so a home does not air itself clean for good.
EPA describes what happens after a house is aired out by opening windows, doors and vents on the lower floors: once they are closed again, radon most often returns to its earlier level within about 12 hours.
Where to go next
- Radon level interpreter. Read the level your test found against EPA's guidance.
- Radon the element: symbol, atomic number and uses. Radon's place in the periodic table, its isotopes and its half-life.