Half-life
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Half-life: time needed to decrease the amount of chemical to one-half (see also PCBs - half-life, PCDD/F - elimination). Most chemicals are eliminated out of the body by the so-called first-order elimination. This means that a fixed proportion (e.g. one per cent) of the chemical remaining in the body is eliminated per unit of time (e.g. per hour). Therefore when the concentration in the body is high, more is eliminated in absolute terms (e.g. milligrams of chemical) than later when the concentration has already decreased.
A convenient measure for the rate of elimination is the half-life: this is the time during which the amount of chemical in the body will decrease to 50% of the amount in the beginning of the observation. The half-life is a constant for a chemical, so during the first half-life the amount decreases to 50%, during the next to 25%, during the next to 12.5% and so on. As a practical rule it is considered that a substance is totally eliminated within 5 half-lives (in fact about 3% of it still remains). Elimination half-life is equal to cumulation half-time. This means that with a constant intake, body burden increases practically 5 half-lives, after which a steady state is achieved.[1]