Each radioactive element in the radon decay chain has a different “half-life”. Half-life is the time required for half the atoms of a radioactive element to decay. (Figure 2-6) Radon’s half-life is 3.8 days, so in 3.8 days you will have half as much radon in the air, all things considered equal. In another 3.8 days you will have half of that half, or one quarter of the radon left. The process continues forever, since you can continue to divide the amount you have by two and never reach zero.
The half-life is important, because that time interval determines the time available for radon and its decay products to be dispersed into the environment. The 3.8-day half-life is long enough to allow radon gas to move many feet in the soil. On the other hand, the half-lives of the first few radon decay products are sufficiently short; if inhaled, they can cause significant radiation to reach the inner surface of the lungs before the natural cleansing process of the lungs removes the decay products.