FIGURE 1-1 The radon-decay chain. An arrow pointing downward indicates decay by alpha-particle emission; an arrow pointing to the right indicates decay by beta-particle emission. The historical symbols for the nuclides are in parentheses below the modern symbols. Most decay takes place along the unbranched chain marked with thick arrows. The negligible percentage of decay along the thin arrows is shown at critical points. The end of the chain, lead-206, is stable, not radioactive. Half-lives of each isotope are shown as seconds (s), minutes (m), days (d), or years (y). Modified from NRC (1988).

Rensselear Polytechnic Institute that it was the decay products of radon, and not radon, that delivered the pertinent dose to lung cells (Harley 1952, 1953, 1980). Bale learned of this thesis when he visited the Health and Safety Laboratory of the U.S. Atomic Energy Commission, where Harley had done his work, and he confirmed the calculations of lung dose from radon and thoron. Alpha particles released by 2 radioisotopes in the radon-decay chain, polonium-218 and polo-

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