The effect of radiation in the human body is a health concern regardless of whether is from internal or external sources. Internal soruces could be from radiopharmaceutucals in tablet forms, or external radiation sources like scanning machines or natural radioactivity sources at homes. Radiations ( alpha, beta. gamma, x-rays, protons, neutrons, and particles like pions) could ionize molecules when in contact with them causing enormous damages.
In the human body, cell ionizations will change its normal operations and damaging it in either a somatic or a genatic damage. Thus, the use of radioisotopes in the body requires a balance between benefits and damages. The radioisotopes we introduced into the body should not cause any damage before the expiration of its activity. The time the body removes half of the isotope is its biological half-life., which it the time the radioisotopes resides in the organ before been removed.
Thus an important parameter during the application of radioisotopes in human patient is the effective half life or the actual half-life determined from the expression
1/(T-effective) = 1/(T-nuclear) + 1/(T-biological)
The table below shows the nuclear, biological and effective hal-lives of some isotopes.