Invisible high-energy rays called ionizing radiation can damage the body's DNA and genes, possibly enabling cancer-related mutations. There are several sources of ionizing radiation, including cosmic rays, radon, fallout from atomic bombs or above-ground nuclear testing, and screening or diagnostic X-rays.

We are all exposed to the ionizing radiation from the cosmic rays that enter earth's atmosphere from outer space. This radiation may account for a very small percentage (about 1 percent) of our total cancer risk.