IUPAC definition : For a radioactive substance, the time required for the amount of that substance in
a biological system to be reduced to one half of its value by both radioactive decay
and biological processes, when the rate of removal is approximately exponential.;
For a radioactive substance, the time required for the amount of that substance in
a biological system to be reduced to one half of its value by both radioactive decay
and biological processes, when the rate of removal is approximately exponential.