Radioactivity in Pure Lead

One of the factors is how "old" the lead is since lead is in the decay path of other radioactive elements:

http://chemistry.stackexchange.com/questions/29196/why-is-lead-radioactive

" Important naturally-occurring radioactive lead nuclides are:

²¹⁴Pb (t_1/2 =26.8 min) from ²³⁸U

²¹⁰Pb (t_1/2 =22.3 a) from ²³⁸U

²¹¹Pb (t_1/2 =36.1 min) from ²³⁵U

²¹²Pb (t _1/2 =10.64 h) from ²³²Th

On a geological time scale, their half-lives are very short. Nevertheless, these lead nuclides are members of the ²³⁸U, ²³⁵U, and ²³²Th decay series. Thus, they are constantly reproduced by decay of their respective mother nuclides. Therefore, all environmental samples (including soil, water, air, plants, and animals) naturally contain significant amounts of radioactive lead nuclides. However, in various materials (e.g. residues from mining or oil and gas production, fertilizers, building materials), the concentration of natural lead nuclides can be strongly enhanced by technology.

When lead is freshly refined (chemically purified), most of the mother nuclides are removed and the short-lived lead nuclides quickly decay. Nevertheless, the lead still contains significant amounts of ²¹⁰Pb (t_1/2 =22.3 years) . The presence of ²¹⁰Pb is a source of background mainly through the lead X-rays and bremsstrahlung caused by high-energy beta radiation emitted by its decay product ²¹⁰Bi. This background is a real problem when lead is used as a shielding material for low-background detectors. Therefore, some shields are made from selected lead with certified low ²¹⁰Pb content. Preferably, very old lead is used. Since ²¹⁰Pb decays with a half-life of 22.3 years, samples of lead that are many decades old are relatively free of this activity. In one extreme example, 2000-year-old lead was salvaged from a sunken Roman ship."

There is an implication here that many people will miss. When these these lead atoms transmute they become a different element. Thus the "pure" lead now becomes lead with impurities.