Polonium in the Earth's crust are not harmful. Polonium has been found in tobacco smoke from tobacco leaves grown with phosphate fertilizers. Because it is present in small concentrations, isolation of polonium from natural sources is a tedious process. The largest batch of the element ever extracted, performed in the first half of the 20th century, contained only (9 mg) of polonium-210 and was obtained by processing 37 tonnes of residues from radium production. Polonium is now usually obtained by irradiating bismuth with high-energy neutrons or protons. In 1934, an experiment showed that when natural Bi is bombarded with neutrons, Bi
Polonium The actual toxicity of Po is lower than these estimates because radiation exposure that is spread out over several weeks (the biological half-life of polonium in humans is 30 to 50 days) is somewhat less damaging than an instantaneous dose. It has been estimated that a median lethal dose of Po is , or 0.089 micrograms, still an extremely small amount. For comparison, one grain of table salt is about 0.06 mg = 60 μg. In addition to the acute effects, radiation exposure (both internal and external) carries a long-term risk of death from cancer of 5–10% per Sv. The