In the industrial refining and application radioactive substances, the gaseous emanations almost invariably escape, in part at least, into the air. The inhalation of these radioactive gases is known to be hazardous (Mo38; Pe39; Pi32; Ec31b; Te37; Wo31). So few quantitative data are available that it is difficult to estimate the minimal concentration that can be tolerated safely. The limited amount of evidence which has been reported (Be27; Ev37b; Lu24; Re39; Sc31a), however, suggests that concentrations in excess of 10−11 Ci per 1. of air may be injurious over extended exposures (Ba34).