Different units are used in measuring radiation depending on whether discussing radiation being emitted from a radioactive source, the radiation dose received by a person, or the risk of suffering health effects due to radiation exposure.
Similar to the differences in the way we in the United States measure distances (feet, yards) and much of the rest of the world uses the metric system (meters, kilometers), the United States measures radiation using the conventional system while most of the international community uses the System Internationale (SI) that evolved from the metric system.
An unstable radioactive atom emits radioactivity as its nucleus disintegrates in an attempt to become stable and nonradioactive. The number of disintegrations of radioactive atoms is measured over a period of time in units known as conventional unit curie (Ci) or the SI unit becquerel (Bq).
Exposure to radiation causes a person’s body tissues to absorb energy. The absorbed dose is the amount of energy absorbed per unit of weight of human tissue, which is measured by using conventional unit rad (radiation absorbed dose) or the SI unit Gy (one Gy equals 100 rad).
The health risk to a person is measured by using the conventional unit rem or the SI unit Sv (one Sv equals 100 rem). Because different types of ionizing radiation have differing abilities to transfer energy to cells of the body, a number has been assigned to each type of ionizing radiation known as the Quality Factor (Q). The risk to a person is then calculated by multiplying Q times rad, which equals rem. Common sources of radiation exposure include naturally occurring radioactive materials in soil (such as radon) and cosmic rays from outer space that reach the ground (the earth’s atmosphere blocks most radiation from reaching the ground).
Examples of radiation exposure risk to people due to common sources of radiation would include a chest x-ray (10 millirem ( a millirem is 1/1000 of a rem)) or a round trip coast-to-coast flight (3 millirem due to cosmic rays). The average person’s one year exposure to radiation is 300 millirem.
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