- 26 For fallout radioisotopes of biological interest, the MPC values are given in Table I. The difficulty in estimating the maximum permis- sible concentration (MPC) is indicated by the number of factors which must be considered in calculating MPC values. There are ten factors which include, "quantities available," "4nitial body retention," "fraction going from blood to critical body tissue," "radiosensitivity of tissue," “size of critical organ," "essentiality of the critical organ to the proper function of the body," "biological half life," "padioactive half life," "energy of the radiation," and the "specific tonization and attenuation of energy in tissue. For lack of better information, some factors have been es- timated with a large degree of uncertainty and, MPC therefore, .values will need to be revised from time to time as more information becomes available. One of the most likely sources of error is in the extrapolation of the effects upon laboratory animals to the effects upon man, which is necessary though because the data on the biological effects of ijonizing radiations on man are meager. The radioisotopes that present the greatest potential hazard are those for which there is a physiological need, which are abundant ina