The radiation may come from nuclear explosions and the resulting fallout, from medical and industrial uses of radioisotopes, or from particle accelerators or X-ray machines. Exposures to high levels of radiation may cause lasting injury or even death. Some types of cells are more easily injured by radiation than others. The most sensitive cells are those of the blood-forming bone marrow and lymphoid tissues and those of a human embryo. Adult muscle and brain cells are the least sensitive to radiation.
Scientists use a unit called the rem as a measure of radiation exposure.
Over a lifetime, a person typically receives 7 to 14 rems from natural sources of radiation, such as cosmic rays. A single exposure of 5 to 75 rems produces few observable symptoms. Vomiting, fatigue, and loss of appetite accompany exposures of 75 to 200 rems, and recovery takes a few weeks. Severe changes in blood cells and hemorrhage occur with exposures of more than 300 rems. Above 600 rems, additional symptoms include loss of hair and loss of the body's ability to fight infection, usually resulting in death.