Half-life, effective

[Rennhack]

The time required for a radionuclide contained in a biological system, such as a human or an animal, to reduce its activity by one-half as a combined result of radioactive decay and biological elimination.

[Rennhack]

Incorporates both the radioactive and biological half-lives. It is used in calculating the dose received from an internal radiation source.

[Rennhack]

http://www.epa.gov/radiation/understand/halflife.htm