More than 100 years ago, scientists discovered that many elements commonly found on Earth occur in different configurations at the most basic (atom) level. These various configurations (called isotopes) have identical chemical properties, but different physical properties. In particular, some isotopes (known as radioisotopes) are radioactive, meaning that they emit energy in several different forms. This energy emission is what we call radiation.
Over time, we have come to think of radiation in terms of its biological effect on living cells. For low levels of radiation exposure, these biological effects are so small that they may not even be detectable. In addition, the human body has defense mechanisms against many types of damage induced by radiation. Consequently, radiation may have one of three biological effects, with distinct outcomes for living cells: (1) injured or damaged cells repair themselves, resulting in no residual damage; (2) cells die, much like millions of body cells do every day, being replaced through normal biological processes; or (3) cells incorrectly repair themselves, resulting in a biophysical change.
The exact effect depends on the specific type and intensity of the radiation exposure. In general, however, a 3-millirem exposure imposes the same chance of death — 1 in a million — as each of the following common life experiences:
Page Last Reviewed/Updated Friday, March 20, 2020