Ionizing radiation results from the release of atomic material or energy due to instability of the atomic structure. Some radiation comes from natural sources, such as that from cosmic rays and radioactive substances in the earth's crust. The gray (Gy) is the current official unit of measure for radiation exposure, but references to rads and rems are still used; a gray is equal to 100 rad. People are exposed to this "background" radiation at a rate of about 1 to 2 mGy per year. There are three types of radiation particulates: alpha particles, which consist of a hydrogen nucleus, a beta particle which consists of an electron or positron, and a neutron. The additional types of radiation are x-rays and gamma rays. The alpha particle is a relatively large mass unit which has, due to its size and lower energy, a low ability to penetrate tissue. External alpha sources cannot penetrate the dead keratin epithelial layer of the skin but are extremely toxic to the susceptible tissue such as occurs when the radioactive substance is taken internally, via ingestion or inhalation. Beta radiation has greater penetration but generate relatively less damage per particle. Likewise, gamma and x-rays have very high penetration, and a high energy level. When the relative biological effect of the individual types of radiation is considered, the biological radiation effective dose is measured as rems (Roentgen Equivalent Man). Since DNA is one of the most sensitive targets of radiation exposure, cancer is an important effect. Cancers of the lung, breast, bone, brain, hematopoietic system (leukemia and lymphoma), skin, and thyroid have all been associated with radiation exposure. Important epidemiologic associations between exposure to radiation and cancer have been identified in Japanese atomic bomb survivors for leukemia and breast cancer, uranium miners and lung cancer, and patients treated with radiation to the thymus and thyroid cancer. In addition to cancers and other conditions due to chronic exposures, acute radiation illnesses may be seen in occupational disasters. Acute radiation illness may begin after an exposure in excess of 1Gy (100 rad) and is always expected with exposures in excess of 4Gy. The three main organ systems affected are the gastrointestinal, hematopoietic and neurological system.