Ionizing Radiation Fact Sheet

Ionizing radiation can cause changes in the chemical balance of cells. Some of those changes can result in cancer. In addition, by damaging the genetic material (DNA) contained in all cells of the body, ionizing radiation can cause harmful genetic mutations that can be passed on to future generations. Exposure to large amounts of radiation, a rare occurrence, can cause sickness in a few hours or days and death within 60 days of exposure. In extreme cases, it can cause death within a few hours of exposure.

Sources of Exposure

The ionizing radiations of primary concern are alpha and beta particles, gamma rays, and x rays. Alpha and beta particles and gamma rays can come from natural sources or can be technologically produced. Most of the x-ray exposure people receive is technologically produced. Natural radiation comes from cosmic rays, naturally occurring radioactive elements found in the earth's crust (uranium, thorium, etc.), and radioactive decay products such as radon and its subsequent decay products. The latter group represents the majority of the radiation exposure of the general public.

In addition to these natural sources, radiation can come from such wide-ranging sources as hospitals, research institutions, nuclear reactors and their support facilities, certain manufacturing processes, and Federal facilities involved in nuclear weapons production. The following figure shows the percentage contribution that various radiation sources make toward the yearly average effective dose received by the U.S. population (NCRP Report No. 93).

  • Radon: 55%
  • Natural Sources (excluding Radon): 26%
  • Medical X-rays: 11%
  • Nuclear Medicine: 4%
  • Consumer Products: 3%
  • Other: <1%