Half Lives of Some Radionuclides

Time is one of the three basic principles of radiation exposure. Hearing that cesium has a half-life of 30 years can be concerning. However, there are different types of half-lives that describe different things. The 30 years is the physical half-life. Here are the different half-life measures and what they mean.


Half Lives

o   Physical Half Life is the time for a quantity of radioisotope to decay by half (Cs-137 = 30 years)

o   Biological Half Life is the time for of the amount of a radionuclide to be expelled from the body (CS-137 = 70 to100 days)

o   Effective Half Life takes into account both physical & biological half lives

1/T effective= 1/T physical+ 1/T biological



This table presents some of the radioactive isotopes associated with a nuclear power plant with their physical and biological half-lives.


Physical & Biological Half Lives of Select Isotopes


Physical half-life

Biological half-life

Cesium (Cs-137)

30 years

70 days


2 years

70 days

Iodine-131 (I-131)

8.1 days

138 days


28 years

18,000 days