- 25 - and in Water for Occupational Exposure." The meaning of a maximum permissible value is explained by Dunham (1959) in the following statement: In a sense a maximum permissible value is similar to a speed limit. A speed equal to the speed limit is not an absolutely safe speed since many serious accidents occur at lower speeds. Nor is it extremely dangerous to drive at speeds somewhat greater than the speed limit. Neither a recom- mended maximum permissible dose nor a speed limit has any particular significance beyond marking the point at which an advisory group has agreed to draw the line in recommending a maximum degree of hazard appropriate under a given set of circumstances. There are two types of maximum permissible values, dose and concentration. The maximum permissible dose is a limit for exposure to external emitters, and the recom- mended value is 0.3 rems per week. The maximum permissible concentration (MPC) is a limit for the concentration of radioisotopes in critical organs of the body. The MPC values for 240 radioisotopes are given in Handbook 69.