Polonium initiator was classified until the 1960s. The Atomic Energy Commission and the Manhattan Project funded human experiments using polonium on five people at the University of Rochester between 1943 and 1947. The people were administered between of polonium to study its excretion. Polonium is a very rare element in nature because of the short half-life of all its isotopes. Po, Po, and Po appear in the decay chain of U; thus polonium can be found in uranium ores at about 0.1 mg per metric ton (1 part in 10), which is approximately 0.2% of the abundance of radium. The amounts
Polonium general population is exposed to small amounts of polonium as a radon daughter in indoor air; the isotopes Po and Po are thought to cause the majority of the estimated 15,000–22,000 lung cancer deaths in the US every year that have been attributed to indoor radon. Tobacco smoking causes additional exposure to polonium. The maximum allowable body burden for ingested Po is only , which is equivalent to a particle massing only 6.8 picograms. The maximum permissible workplace concentration of airborne Po is about 10 Bq/m ( µCi/cm). The target organs for polonium in humans are the spleen and liver.