Bioassay Studies of Canadian Uranium Mill Workers

INTRODUCTION In most uranium mines the concentration of uranium in the ore is low enough that inhalation of dust does not present a radiological problem. Conversely in the mills there is a potential inhalation hazard from inhaled uranium in concentrated form, namely "yellowcake" and other compounds intermediate between the ore and the final product of the mill. It was generally assumed that the solubility of yellowcake placed it in the category of Class D materials. This plus the fact that the uranium is non-enriched led health physicists to assume that the potential health hazards were a result of the uranium's chemical properties rather than its radiological properties. Bioassay programs have thus been limited to routine urine sampling at a weekly or monthly frequency. In most cases when a high reading was obtained, subsequent sampling within one or two days showed a rapid return to low levels, therefore tending to confirm the class D nature of the material. Two developments over the last few years have however suggested that modifications to this simple bioassay program may be necessary and practical. Firstly, a number of studies on the solubility of uranium mill products, from ore to yellow cake, in simulated lung fluid (Eidson A.F., Mewhinney J.A., 1980) have shown that some of the material is not class D, but class W or even Y. This raised the possibility that, while the urine levels rapidly decreased, indicating rapid excretion of the class D component, the W and Y components remained in the lung. Thus an unsuspected lung burden could have been accumulated. The second development, the ready availability of [in vivo] counting facilities with which to make direct measurements of uranium lung burdens, offered a means of confirming the hypothesis of unsuspected lung burdens. A study was therefore initiated to correlate lung burdens with urinary excretion. METHOD A total of 79 subjects from 3 uranium mills participated in the study. They were brought to the Ottawa laboratories of the Radiation Protection Bureau in groups of 4 or 6, over a period of several months. In Ottawa, [in vivo] thorax measurements were performed, and a urine sample collected. Each subject also brought with him a urine sample that had been collected the previous day. All samples were split, for analysis by both R.P.B. and the mill laboratory. [Concentrations of Uranium Dust] Results obtained by mills A, B and C for concentrations of uranium in air at various locations are given in table 1. Samples taken at mill B were obtained using continuous sampling with a midget impinger over a six hour period. Samples taken at mills A and C were 5-minute gravimetric samples. [Subject Selection] The subjects were selected to represent a wide range of work history in terms of both length of employment and type and location of job, as shown in table 2.