- 27 biologically available form, are taken up by a critical tissue or organ of the body, are highly ionizing and have both a long physical and a long biological half life. ~ The MPC values for radioisotopes in drinking water, including water in foods, given in Handbook 69 assumes that there is no other source of contamination and that the daily water consumption is 2.2 liters (or kilograms) per day for a 50-year exposure period during which the level of radioactivity remains constant. The MPC values for drinking water can be used for fish or other foods when compensation is made for the difference between the calculated water intake and the actual food intake. If fish are the only source of radioisotopes in the diet and 0.22 kilogram (1/2 pound) of fish which is 1/10 of the water consumption upon which the MPC values were based is eaten every day for 50 years, the MPC values for fish would be obtained by multiplying the values given in Handbook 69 by a factor of 10. The assumption of one-half a pound of fish per day in the diet is nearly twenty times the average consumption per capita in the United States (Taylor 1951) but would be approximately the amount in the diet if fish were the only source of protein.