Biological Half-life

The radioactive half-life for a given radioisotope is physically determined and unaffected by the physical or chemical conditions around it. However, if that radioisotope is in a living organism it may be excreted so that it no longer is a source of radiation exposure to the organism. For a number of radioisotopes of particular medical interest, the rate of excretion has been cast in the form of an effective biological half-life. The rate of decrease of radiation exposure is then affected by both the physical and biological half-life, giving an effective half-life for the isotope in the body. Though the biological half-life cannot be expected to be as precise as the physical half-life, it is useful compute an effective half-life from

Examples of the half-lives show that biological clearing is sometimes dominant and sometimes physical decay is the dominant influence.

Tritium, ³H, has a fairly long physical halflife but clears from the body quickly, lessening the exposure. Phosphorous, ³²P, is used for some kinds of bone scans. The phosphorous tends to be held in the bones, leading to a long biological half-life, but its physical half-life is short enough to minimize exposure. Strontium, ⁹⁰Sr, is very bad news in the environment. It mimics calcium and therefore gets trapped in bone. This gives it a long biological half-life to go with its long physical half-life, making it doubly dangerous. Technetium, ^99mTc, is one of the favorites for diagnostic scans because of short physical and biological half-lives. It clears from the body very quickly after the imaging procedures.

Physical, Biological, and Effective Half-lives for Selected Isotopes

The radioactive half-life and the biological half-life for a given radioisotope act together to reduce the radiation exposure from a given radioisotope. The following data for some radioisotopes of medical or general interest was taken from Tuszynski and Dixon's "Biomedical Applications of Introductory Physics".