- 29 Publication 658 of the National Academy of Sciences National Research Council (1959). The calculation is essentially as given above with the exception that it has been refined by taking into account the fraction of the maximun permissible dose alloted to nuclear-powered ships for various zone of the sea. If the ratio of water intake to food intake in the fish is the same as the concentration factor for the isotope from sea water the the PSC equals the (MPC)y as given in Handpook 69. The error in estimating PSC oy this method is often great because of the limited information about the concentration of elements in the sea by fish. The MPC values are not always an adequate estimate of hazard because the uptake of some radioisotopes is influenced by the presence of chemically similar isotopes. Strontium and calcium are chemically similar as are cesium and potassium. able form, When calcium is abundant and in an avail- less gsr® is taken up, and therefore a measure of only Sr9° is not a reliable estimate of hazard. For Sr90 a better estimate than the MPC value is the ratio of sr? to calcium. The ratio of one micro-microcurie of Sr?° tp one gram of calcium is called a strontium unit, and for