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# Effective half-life

Effective half-life denotes the halving of radioactive material in a living organism by means of radioactive decay and biological excretion. A decay constant is needed to calculate the half-life. It is the sum of the biological and physical decay constants, as in the formula:

${\lambda_e} \, = {\lambda_p} \, + {\lambda_b} \,$

With the decay constant it is possible to calculate the effective half-life using the formula:

$t_{1/2} = \frac{\ln (2)}{\lambda_e}$

The biological decay constant is often approximated as it is more difficult to accurately determine than the physical decay constant.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Effective_half-life". A list of authors is available in Wikipedia.
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