Half-life

Half-life means the time required to reduce half a substance's original quantity.

The physical half-time is the time needed to let half the amount of a radioactive substance decay. The resulting decay products can be radioactive, too. Half of the remaining material (a quarter of the original amount) will again decay within the same period, and so on. Usually the half-life of a radioactive material is multiplied by ten to assess the point of time when almost all substance will have decayed.

The biological half-life describes the period it takes for an amount of a substance incorporated by a living being (humans, animals, plants, protozoon) to be reduced to half of the original value by biological processes like metabolism, excretion, etc.