296As early as the 1920s Lacassagne, using polonium provided by his colleague Marie Curie,
showed that the element has a very specific pattern of uptake in rabbit tissues, with high
concentrations particularly in liver, kidney and testes. More recent evidence suggests that
this behavior results from polonium substituting for sulfur in S-containing amino-acids or
related molecules and that similar patterns of distribution occur in human tissues.
Polonium is indeed an element naturally present in all humans, contributing appreciably
to natural background dose, with wide geographical and cultural variations, and
particularly high levels in arctic residents, for example.