- 70 although damage to the thyroid did occur in some fish from a reef close to a test site. Subtle genetic effects that would not be obvious from field observations also may have occurred, The two fallout isotopes of greatest public concern, sr?0 and csl57, are not taken up by marine organisms to any great extent. Practically all radioiso. topes found in fish are isotopes of the non-fission product elements -- zinc, iron, cobalt, and manganese. At the present time the man-made radioisotopes can be detected with sensitive instruments in many of the waters of the world, but their concentration in aquatic organisms is well below the MPC values. Exposure to all ionizing radiations is to be avoided. The amount of risk, above that from exposure to natural sources of radiation, which will be acceptable in the development and use of atomic energy is a decision that society must make, based upon the combined judgment of all able and well-informed persons.