criterion of exccssive exzgosure of a group to radiation is a significcnt decrease in the grou, average Leukocyte count below the pre-exposure level or below the ¢ of the controi croup. As scon as any of the above criteria have been wn dononstrated, tha individusls must be rcmoved from any possible contact with : adiation and the following studics should be performed in order to dotermine if radiation injuzy is the cause of the abnormality. In principle, the use of a group as its ou pre-exposure control sounds secure, but admittedly the observa- ticas of Wald ct al. indicate the potential unreliability of this appzoach. Accordingly, whenever possible, a comoarable population of unexposed people stould also be followed. 2. Endeavor to eliminate ail other causes of temporary hematologic abnormatities, such es infectious mononucleosis, infectious lyapho- cytosis, infectious hepatitis, virus diseases, benzol and heavy metal poisonins. 2. Study the excreta and expired ais for the possible presence of ion- sadiation to help determine the type and degree of ternal exe posure to radioactive isotopes. 3. Study the blood at weehly intervals to detect further changes and to search for the presence of refractive, neutral red bodies in the lynaphocytes. In this discussion of the sole of hematology in the detection of chronic radiation injury, 3 is eppropriste to state the changes discussed aliy cftes the domase has beon sustained. appear gener= The main protection against ionizing radictica injury must be on accurate physical control of the radiation intensities to which personnel asc exposed. In concluding this section, tno seader is referred to the work of Knowlton and Cartes in which a croup of ton individuals were carefully followed soguii over a long

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