Physical half-life time (Tp)
The time interval required for an amount of certain radioactive nuclei to decay to its half of original value.
Biological half-life time (Tb)
The time interval required for the body to eliminate 50% of any substance by normal routes of elimination: metabolic turnover and excretion.
Effective half-life time (Te)
The time interval required for the radioactivity of a certain amount of radioactive substance distributed in tissues and organs to decrease to half its original value due to radioactive decay and biological elimination.
The physical half-life time > biological half-life time > effective half-life time.
1/Te = 1/Tp+1/Tb