Radon (Rn-222) is a naturally occurring radioactive gas. It is released during the radioactive decay of uranium. Natural radioactivity is the process where unstable nuclei form more stable atoms by ejecting mass or energy or both at a fixed rate. Each time the atom loses protons, as in the release of an alpha particle, it becomes a different element. Uranium-238 will eventually become lead-206, which is stable (not radioactive). This is known as the uranium-238 decay series.
Uranium, on its way to lead, will decay and become radium which becomes radon which becomes polonium and on and on until it becomes lead. Uranium changes from a solid to a gas (radon) back to a solid (polonium). Because radon is a gas, it can move through the soil and potentially into your home. If it stayed a solid, like uranium, radium, polonium and the rest of the decay products, radon would be of little concern.
The rate of radioactive decay is called "half life" and is different for each element. Uranium has a half life of 4.4 billion years, while radon has a half life of 3.8 days. The half life is the amount of time it takes for one half of a given amount of the element to decay. This means that radon is emitting alpha particles at a high rate and delivers a higher dose of radiation per unit time. Since it takes 4.4 billion years for half of the uranium in the ground under your house to decay there is no chance of that uranium going away soon, it is going to remain essentially constant for our life time producing a constant amount of radon.
Radon, on the other hand, has a half life of 3.8 days, so some of the radon moving up through the soil decays to polonium and stays in the soil. The amount of radon moving up through the soil and into your house changes with season, weather conditions, soil structure and conditions, and even time of day. Even though a given amount of uranium will produce essentially a constant supply of radon, the amount of radon moving up through the soil is not constant. Because there are many variables with complex interactions the amount of radon that will be present in a home cannot be predicted. That is why everyone should test their home for radon.
Because of the following characteristics, radon is generally the greatest source of natural radiation exposure.
- It is a gas and can move easily through the soil and into your house.
- It emits alpha particles (the radiation that presents the greatest hazard to lung tissue).
- It has a short half life (emits alpha particles at a high rate).