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.
The accompanying table shows the reduction of a quantity as a function of the number of half-lives elapsed.
A half-life usually describes the decay of discrete entities, such as radioactive atoms.
In that case, it does not work to use the definition that states "half-life is the time required for exactly half of the entities to decay".
Consider a mixture of a rapidly decaying element A, with a half-life of 1 second, and a slowly decaying element B, with a half-life of 1 year.
In a couple of minutes, almost all atoms of element A will have decayed after repeated halving of the initial number of atoms, but very few of the atoms of element B will have done so as only a tiny fraction of its half-life has elapsed.
As an example, the radioactive decay of carbon-14 is exponential with a half-life of 5,730 years.
A quantity of carbon-14 will decay to half of its original amount (on average) after 5,730 years, regardless of how big or small the original quantity was.
Mathematically, the sum of two exponential functions is not a single exponential function.
A common example of such a situation is the waste of nuclear power stations, which is a mix of substances with vastly different half-lives.
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.
Thus, the mixture taken as a whole will not decay by halves.