Radon and its first decay products being very short-lived, the seed is left in place. After 11 half-lives (42 days), radon radioactivity is at 1/2000 of its original level. At this stage, the predominant residual activity is due to the radon decay product 210Pb, whose half-life (22.3 years) is 2000 times that of radon, and its descendants 210Bi and 210Po, totaling 0.03% of the initial seed activity.