Half Lives Explained

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

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

Biological Half Life is the time for 1/2 of the amount of a radionuclide to be expelled from the body (CS-137 = 70 to 100 days)

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 and Biological Half Lives of Select Isotopes
Isotope 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