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
1/TEffective = 1/TPhysical +1/TBiological
Examples of the half-lives show that biological clearing is sometimes dominant and sometimes physical decay is the dominant influence.
||Half-lives in days||TPhysical||TBiological||TEffective
|3H||4.5 x 103||12||12|
|90Sr||1.1 x 104||1.8 x 104||6.8 x 103|
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.
Nuclear applications to health