Tritium is a byproduct in nuclear reactors. The continued action of the emitted alpha particles can cause significant injury: over many years they deposit all their energy in a tiny volume of tissue, because the range of the alpha particles is very short. Once deposited in bone, they remain there essentially unchanged in amount during the lifetime of the individual. These radionuclides have long biological half-lives and are serious internal hazards. Sr-90, Ra-226 and Pu-239 are radionuclides known as bone-seeking radionuclides. For 131I, ICRP has calculated that if you inhale 1 x 10 6 Bq, you will receive a thyroid dose of H T = 400 mSv (and weighted whole-body dose of 20 mSv). The primary risk from exposure to high levels of 131I is the chance occurrence of radiogenic thyroid cancer in later life. As it decays, it may cause damage to the thyroid. 131I decays with a half-life of 8.02 days with beta particle and gamma emissions. When 131I is present in high levels in the environment from radioactive fallout, it can be absorbed through contaminated food, and will also accumulate in the thyroid. Iodine in food is absorbed by the body and preferentially concentrated in the thyroid where it is needed for the functioning of that gland. Biological Half-life of Iodineįor example, the biological half-life for iodine inside the human body is about 80 days (according to ICRP). For example, tritium has the biological half-life about 10 days, while the radioactive half-life is about 12 years. Moreover, if t 1/2 is large in comparison to t b, the effective half-life is approximately the same as t b. If a radioactive compound with radioactive half-life (t 1/2) is cleared from the body with a biological half-life t b, the “effective’’ half-life (t e) is given by the expression:Īs can be seen, the biological mechanisms always decreases the overall dose from internal contamination. This means that a radioactive substance can be expelled before it has had the chance to decay.Īs a result, the biological half-life significantly influences the overall dose from internal contamination. Radioactive isotopes that were ingested or taken in through other pathways will gradually be removed from the body via bowels, kidneys, respiration and perspiration. The biological half-life depends on the rate at which the body normally uses a particular compound of an element. Most radionuclides will give you much more radiation dose if they can somehow enter your body, than they would if they remained outside. The intake of radioactive material can occur through various pathways such as ingestion of radioactive contamination in food or liquids, inhalation of radioactive gases, or through intact or wounded skin. If the source of radiation is inside our body, we say, it is internal exposure. It is also very important in radiation protection, when considering internal exposure. The biological half-live (t biological) can be defined for metabolites, drugs, and other substances. Nuclear medicine The length of time required for a radioisotope to decay to one-half of the original amount having the same radioactivity a radioisotope’s effective T1/2 is either the time of decay-physical T1/2-or the time to elimination from a biological system.In general, the biological half-life is the time taken for the amount of a particular element in the body to decrease to half of its initial value due to elimination by biological processes alone, when the rate of removal is roughly exponential. Immunology The time an immunoglobulin stays in the circulation: 20–25 days for IgG, 6 days for IgA, 5 days for IgM, 2–8 days for IgD, 1–5 days for IgE
Haematology The time that cells stay in the circulation-e.g., red blood cells, 120 days, which increases after splenectomy platelets, 4–6 days eosinophils, 3–7 hours neutrophils, 7 hours The amount of time required for a substance to be reduced to one-half of its previous level by degradation and/or decay (radioactive half-life), by catabolism (biological half-life), or by elimination from a system (e.g., serum half-life) The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company.