Radon Decay and Alpha Radiation

Radon-222 is a gas with a half-life of about 3.8 days. It is produced continuously by radium decaying in soil and rock, and it decays in turn into a series of short-lived “progeny” — polonium-218, lead-214, bismuth-214 and polonium-214 — most of which emit alpha particles.

Why the progeny matter

Radon itself is a gas you inhale and mostly exhale. The progeny are different: they are solid particles that can attach to dust and to surfaces in the airways. When they decay there, the alpha particles they emit deposit their energy in a very small volume of lung tissue.

That is the mechanism behind radon’s health risk. The WHO and the EPA both describe it as a leading environmental cause of lung cancer, and risk rises with the total exposure over years.

Why alpha particles are a local problem

An alpha particle is heavy and carries a lot of energy, but it travels only a few centimetres in air and cannot pass through skin. That is why radon is not a hazard from outside the body, and why the danger comes from what happens inside the lungs.

What this means for measurement

• Detectors count decays. Most instruments count alpha particles from the progeny over a defined volume and time, then calculate a concentration.
• Time is unavoidable. Because radon and its progeny fluctuate with ventilation and weather, a defensible average needs hours to months of data — not a single instantaneous sample.
• Progeny equilibrium matters. Instruments that measure only radon gas assume an equilibrium factor with the progeny to estimate dose-relevant exposure.