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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:

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

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