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Thus, the mixture taken as a whole will not decay by halves.A biological half-life or elimination half-life is the time it takes for a substance (drug, radioactive nuclide, or other) to lose one-half of its pharmacologic, physiologic, or radiological activity.In a medical context, the half-life may also describe the time that it takes for the concentration of a substance in blood plasma to reach one-half of its steady-state value (the "plasma half-life").The relationship between the biological and plasma half-lives of a substance can be complex, due to factors including accumulation in tissues, active metabolites, and receptor interactions.In other words, the probability of a radioactive atom decaying within its half-life is 50%.For example, the image on the right is a simulation of many identical atoms undergoing radioactive decay.

The decay of many physical quantities is not exponential—for example, the evaporation of water from a puddle, or (often) the chemical reaction of a molecule.Note that after one half-life there are not exactly one-half of the atoms remaining, only approximately, because of the random variation in the process.Nevertheless, when there are many identical atoms decaying (right boxes), the law of large numbers suggests that it is a very good approximation to say that half of the atoms remain after one half-life.For example, if there is just one radioactive atom, and its half-life is one second, there will not be "half of an atom" left after one second.Instead, the half-life is defined in terms of probability: "Half-life is the time required for exactly half of the entities to decay on average".

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