For more terms, see IEER's on-line glossary.

Becquerels (Bq): The standard international (SI) unit of radioactivity equal to one disintegration per second. It is a very small unit equal to about 27 picocuries.

Curie (Ci): The traditional unit of radioactivity equal to the radioactivity of one gram of pure radium. It is equal to 37 billion disintegrations per second (37 billion becquerels).

Rad (radiation absorbed dose): a unit of absorbed dose of radiation defined as deposition of 100 ergs of energy per gram of tissue.

Gray (Gy): A unit of absorbed radiation dose equal to 100 rads.

Roentgen:The old unit of radiation exposure. It is a unit of gamma radiation measured by the amount of ionization in the air. In non-bony biological tissue, a roentgen delivers a dose equal to about 0.93 rad.

Rem (radiation equivalent man): a unit of equivalent absorbed dose of radiation, which takes into account the varying ways in which ionizing radiations transfer their energy to human tissue (known as relative biological effectiveness or RBE). While rads and grays measure deposition of energy in tissue, rem and sieverts measure biological damage. Rems are derived from rads by mutiplying rads by a "quality factor" which approximates the RBE For beta and gamma radiation the quality factor is taken as one-that is rems equals rads. For alpha radiation, the quality factor is taken as 20-that is rems equal 20 times rads.

Sievert (Sv): a unit of equivalent absorbed dose equal to 100 rems.

Person-Sievert: the population dose as distinct from individual dose. The population dose is the sum of individual doses in a defined population.

Working level (WL): Unit of dose used in uranium mining. A working level is a unit of air concentration of potential alpha energy released from radon and its decay products. If the radon has remained in the air for some time, then 1 WL is equal to about 100 picocuries of radon per liter of air.

Working level month (WLM): A working level month is defined as the exposure to an average of 1 WL for a working month of 170 hours.