The possible hazards from the refining and processing of thorium are of wide interest in atomic energy work. A review of the data available was made at Hanford in the last year to support consultations on the hazards of radioisotopes with other plants which may handle thorium. Since a major discrepancy between the published values of air MPC's and the metabolic data was noted, a review of the calculations on thorium was made in order to obtain the best estimates available. This document tabulates the pertinent data and applies them to the calculation of the MPC of thorium.