The biological half-life is rather uncertain compared to the exact value of the physical half-life. It is uncertain because the clearance from the body depends upon sex, age of the individual and the chemical form of the radioactive substance. The biological half-life will vary from one type of animal to another and from one type of plant to another. Cs-137, having a physical half-life of 30 years, is a good example. It was the most prominent of the radioactive isotopes in the fallout following the Chernobyl accident in the Ukraine. Cesium is cleared rather rapidly from the body and the biological half-life for an adult human is approximately three months and somewhat less for children. Cs-137 has a biological half-life of 2 to 3 weeks for sheep, whereas for reindeer it is about one month. Due to the fact that the biological half-life for animals like sheep is rather short, it is possible to “feed down” animals, with too high a content of Cs-137, before slaughtering. The animals can simply be fed non-radioactive food for a short period. Another possibility is to give the animals compounds such as “Berlin blue” which is known to speed up the clearance of cesium from the body. The result is a shorter biological half-life. Some radioactive species like radium and strontium are bone seekers and, consequently, are much more difficult to remove. The biological half-life for radium is long, and if this isotope is ingested, it is retained the rest of one’s life. It is possible to reduce the effects of a radioactive compound by simply preventing its uptake. Consider iodine. If people are to be exposed to radioactive iodine, it is possible to add non-radioactive iodine to their food. All iodine isotopes are chemically identical and the body can not discriminate one isotope from the other. There will be a competition between the different isotopes. If the amount of non-radioactive iodine is larger than the radioactive isotope the uptake of radioactivity is hindered. This kind of strategy can also be used to decrease the biological half-life.