There is no solid evidence from human studies that internal uranium exposure is associated with an increased cancer risk. Although cancer is a well known effect of ionizing radiation exposure, it has never been associated with exposure to uranium. A small fraction of the uranium taken into the body becomes deposited in the skeleton, but scientific observations have not shown any increase in bone cancer in persons exposed to uranium, including enriched uranium, which is about 100 times as radioactive as DU. People are constantly bombarded by radiation from many sources. Radiation comes from space, from radioactive materials in the soil, including natural uranium, and also radioactive isotopes of other more common metals, such as potassium. There is radiation in the air, and the food that we eat. In addition, people receive radiation from routine x-rays to diagnose disease, or with radiation therapy to treat disease. In some areas of the world, people are exposed to levels of background radiation that is three times higher than in the U.S. These people, exposed only to high levels of background radiation, have some alterations in chromosomes due to the higher radiation exposures, but do not show any increase in rates of cancer or other diseases that may be linked to radiation exposure.