Abstract
From an analysis of data on persons exposed in the United States to R it is estimated that 100 [mu] Ci introduced into the bloodstream over a period of a few months or years would lead to an incidence of 1 bone tumor/20 persons exposed. The tumors would be expected to appear at around 15-30 years after exposure. The main source of uncertainty in this risk estimate is the possibility of bias in the United States cases. The risk estimate is consistent with the absence of tumors in the British luminisers, a group which is known to be free from bias. For full occupational exposure to R during a 50-year working life the risk of induction of a tumor is estimated to be about 1/1,200 persons exposed. This value was derived on the assumption that the tumor incidence would be the same as for a single intake of the same total amount of R.