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706 LEON F. CURTISS tion of the more important of these and their principles of operation will be found in Volume III of this book. E. BLOOD COUNTS AS INDEX OF INJURY In spite of all due caution in manipulations and proper measurements to insure that the tolerance dose is not exceeded, it is conceivable that some indi viduals may be more than ordinarily sensitive to radiation, or may accidentally receive excessive exposures not detected by metering badges supposedly worn by all personnel in radiation areas. The best check on such possibilities known at present is the use of regular blood counts. Although such observations are not satisfactory for revealing moderate overexposures, it is well established that strong irradiation by penetrating radiation causes characteristic changes in the blood picture. The real difficulty is that these changes frequently become definitely recognizable only after the damage has been done. Furthermore, the results of the counts, which always should be made by the same technician for each person under observation, and under similar conditions, should be interpreted by an experienced hematologist. These counts should include: hemoglobin test, red, white, and differential counts, including the percentages of polymorphonuclear cells, small and large lymphocytes (separately), eosinophiles and basophiles. Blood platelet, sedimentation, and coagulation tests also are of value. A guide in determining departures from a normal blood count is provided by Table 3 as given by Goodfellow,4 who has investigated the effects on the blood count of exposures ..to gamma rays. He finds that the only sign .of .overexposure... common to all workers is a leucopenia, due to a reduction of circulating neutrophiles. There is evidence that different individuals vary in susceptibility to the effects of irradiation. Those who are more sensitive exhibit an absolute lympho cytosis with an absolute leucopenia as the first sign of overexposure. Others, less sensitive, under the same conditions show a lymphocythemia with a monocytosis. Eosinophilia is frequently seen as a result of overexposure,' and abnormal or embryonic leucocytes have been observed. Goodfellow states that vacations of less than four weeks do not appear to be of value in restoring leucocyte counts to normal. F. RADIOISOTOPES In the last ten, years the use of artificially produced radioisotopes of many common elements has given rise to new industrial problems of protection, which may be expected to grow in magnitude and complexity. Although the principles of_health .proteetion remain the same as in the days when radium and its decay products were about the only sources of radioactivity, they must now be applied to a greater variety of situations. There are available now, also, sources of many times the intensity of former preparations of radium. At the same time many types `D. R. Goodfellow, Brit. J. Radiol., 8, 669, 752 (1936). RADIANT ENERGY 707 TABLE 3 Typical Blood Counts as Reported by Various Authorities Type of cell Total W.B.C.s Max. Min. Mean NeutrophilpB Max. Min. Mean Lymphocytes . Max. Min. Mean Monocytes Max. Min. --Mean ' Eosinophiles Max. Min. Mean Basophiles Max. Min. Mean Schil Mottrara ling Dyke Shaw Emer son Simp son Taylor, Practice of Medicine: Accepted Simpson: normal Widest limits variations (in morn observed ing) 8,000 8,500 9,650 8,000 11,200 18,100 -- 6,000 5,500 3,200 6,000 5,100 2,800 -- -- -- 5,987 -- -- -- 12,000 3,000 -- -- 75% 69.8% 70% 8,920 13,100 60% 37.0% 63% 2,920 1,600 -- 67% -- 53.2% -- -- -- _6,000 40% 51.2% 30% 2,440 4,040 1,500 -- 25% 22.6% 20% 1,010 820 -- 23% -- 36.8% -- -- -- -- -- '-- -- 7% 10.0% 10% -- 2% 3.0% -- 6% -- - 6-7% -- -- -- -- -- -- -- _ _ 2% 6.4% 2% _ -- -- 0% 0.0% -! -- -- 3% -- 2.6% -- -- _ _ 1% 2.0% 0.5% _ -- -- 0% 0.0% -- -- " i% -- 0.7% -- i_ -- --- _ -- 9,000 4,000 -- 5,000 2,600 -- 3,000 1,600 -- 600 20 -- 100 20 -- 10 0 -- of work involving radioactivity, for example, investigations with radioactive tracers, conveniently use relatively weak sources. ^ In a general survey of, eonyegient,,.to.divide, the Sfarious"operations mto'seyeraTranges^ofmfehSty7AT presentTIie'ci6mm6nly''used gfuhit for measurement of the quantity of radioactivity is the curie, defined as that L, amount of any radioisotope which is undergoing 3.7 X 1010 disintegrations per ||. second. This automatically defines a different,number of atoms for'each radio- riTpllrlp nq a- unit-, qimntit.y- fr.Virr'mirh thfv r Snumber of atoms present at the moment of measurement, dN/dt is the rate of disintegration, and A is the disintegration constant. N for one curie..of.a nuclide pis.then defined by 3.7 X 1010 = --XN; and, since A is different for each nuclide, N [Hwill also be different. However, for a given nuclide the radiation emitted is