The bases of working limits for environmental contamination are the permissible limits for internal emitters. If the internal emitter is a bone-seeker (examples are plutonium, Sr⁹⁰, and the rare earths) the criterion for establishing limits is based on the history of humans who have had radium deposited in their skeletons. To experimentally find the maximum quantity of a radioactive bone-seeker that can deposit in the human skeleton without adverse effect, it is usual to compare the toxicity in experimental animals of the bone-seeker with that of radium.
Source: wiktionary