deposits in the bones, about 15% goes to the liver, 2% to the kidneys, and the rest leaves the body. The biological half-life of protactinium is about 50 years in the bones, whereas in other organs the kinetics has a fast and slow component. For example, 70% of the protactinium in the liver has a biological half-life of 10 days, and the remaining 30% for 60 days. The corresponding values for kidneys are 20% (10 days) and 80% (60 days). In all these organs, protactinium promotes cancer via its radioactivity. The maximum safe dose of Pa in the human body is , which corresponds to 0.5 micrograms of 231Pa. This isotope is 2.5 times more toxic than hydrocyanic acid. The maximum allowed concentrations of 231Pa in the air in Germany is .