Half_Life
Half_Life : (a) The time in which the concentration of a chemical in the environment is reduced by half. [ED, 2000: Environmental Scorecard Glossary]; (b) The time required for half of a substance introduced to a living system or ecosystem to be eliminated or disintegrated by natural processes. [NCSU, 1997: Watershedds Glossary]; (c) the time required for half the amount of a substance to be eliminated from the body or to be converted to another substance(s). [NIAID, 1997: HIV Vaccine Glossary]; (d) The time required for half of a sample of a given radioactive isotope to decay into another element or isotope. The half-life of an isotope is inversely related to its decay constant. [SHSU, 2000: Atmospheric Chemistry Glossary]; (e) (1) The time in which half the atoms of a given quantity of a particular radioactive substance disintegrate to another nuclear form. Measured half_lives vary from millionths of a second to billions of years. (2) Similarly, the time in which half the molecules of a chemical substance disappear as a result of chemical or biochemical transformation. [SRA, 1999: Glossary of Risk Analysis Terms, attributed to Stephen L. Brown]; (f) The time in which half the atoms of a radioactive substance will have disintegrated, leaving half the original amount. Half of the residue will disintegrate in another equal period of time. [USDOE, 2000: RAIS Glossary]; (g) (1) The time required for a pollutant to lose one_half of its original concentration. For example, the biochemical half_life of DDT in the environment is 15 years. (2) The time required for half of the atoms of a radioactive element to undergo self_transmutation or decay (half_life of radium is 1620 years). (3) The time required for the elimination of half a total dose from the body. [USEPA, 1997a: EPA Terms of Environment] [Cf. biological half-life].
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