ResourcesGlossary: the science
Decay chain
Short answer
A decay chain is the series of radioactive atoms that one unstable atom passes through on its way to a stable one. Radon is a step in the chain that starts with uranium-238 in rock and soil.
What decay chain means
EPA traces the chain: it begins with uranium-238 and ends at lead-206, after forming uranium-234, thorium-230, radium-226 and radon-222 along the way. The products within the chain are always radioactive, and the atoms keep transforming until they reach a stable state.
Uranium is present naturally in virtually all soil, rock and water, which is why radon turns up everywhere. Radium decays to produce radon gas, and EPA names radon from soil gas as the main cause of radon problems in homes.
The chain in mass numbers
Each row subtracts the mass number from the one above it.
| Step | Mass number | Drop from the step above |
|---|---|---|
| Uranium-238 | 238 | Start |
| Uranium-234 | 234 | 4 |
| Thorium-230 | 230 | 4 |
| Radium-226 | 226 | 4 |
| Radon-222 | 222 | 4 |
| Lead-206, the stable end | 206 | 16 |
What it means for your home
For a home this means radon is not a one-time leak. The radium under a house lasts far longer than the house, so the soil keeps producing radon.
A fix therefore has to keep working. EPA says the fan of an active system should never be turned off: it must run continuously for the system to work correctly.
Where to go next
- Radon map of every U.S. county. See EPA's zone and the reported tests for the county a home is in.
- Where radon comes from. From uranium in the ground to the air in a basement.