Ionizing radiation has the potential to kill cells or cause somatic or germinal mutations. It has this ability by virtue of its power to penetrate living cells and produce highly reactive charged ions. It is the charged ions which cause cell damage. Radiation accidents and the potential for radiation from nuclear (atomic) bombs create a fear of radiation release due to human activity. Many people are subjected to diagnostic and therapeutic radiation, and older Americans were exposed to radioactive fallout from atmospheric testing of nuclear weapons. There is some environmental contamination from nuclear fuel used in power plants. Accordingly, there is considerable interest in radiation effects on biological systems and the sources of radiation in the environment.

Concern for radiation safety is certainly justified and most individuals seek to minimize their exposure to human-generated radiation. However, for most people, exposure levels to radiation from natural sources...