Toxic substances in the environment

The present controversies about the biological effects of low levels of radiation also demonstrate the wide divergence that can exist between levels that are thought to be safe and the natural background. Natural radiation gives everyone a dose of about 80 to 150 mrem per year, (average 100 mrem). Of this, about 25 mrem is due to the potassium‐40 in the body and 30 to 40 mrem each arises from the surroundings and from cosmic rays. Medical x‐rays make a serious contribution. Against this background, limits of exposure to the public have been recommended by the International Commission on Radiological Protection and imposed by national Governments. These are 500 mrem to members of the public and 5000 mrem to radiation workers whose dose and health are constantly monitored. Thus the latter could receive, as a maximum, some 50 times the natural radiation background; every effort is of course made to minimise the dose in practise.