THE RADIOACTIVITY OF THE HUMAN BODY ISAAC ASIMOV Boston University School of Medicine, Boston, Massachusetts
A M O N G the elements that are essential constituents of
C14,however, can form an integral portion of the orthe human body, the one naturally occurring, long- ganic molecules within the cell including the desoxylived radioactive nuclide is, of course, K40. There are nucleoprotein molecules that are commonly called genes. two short-lived radioactive nuclides among these ele- A disintegrating CI4 atom within a gene is not merely ments, however, which are also naturally occurring the source of a possibly damaging beta particle. That since they are continually produced by natural trans- is the least of the affair. In the first place, beta-particle mutative processes initiated in the atmosphere by emission converts the carbon atom to N", altering the cosmic rays. These are C1* and Ha. All three are chemical nature of the molecule. Secondly, the effect beta-emitters, CI4and H3 entirely so, K40to the extent of 90 per cent, its remaining breakdowns being by way TABLE 1 of K-capture. The per cent occurrence and the halfRadioactive Nuclides Occurrino in the Human Badv lives of each of these three nuclides are presented in Fractional Table 1, the values presented being taken or calculated occurrence Half-life in element from data appearing in the recent literature.'. 2. (seconds) The human body is, by weight, 0.15 carbon, 0.12 hy1.19 X lo-' 4 . 4 X 10" 1.85 X loWL1 drogen, and 0.01 potassium. From this and from the 1.8 X 1011 lowL8 3 . 9 X loB figures given in Table 1, it is possible4to calculate the body's natural radioactivity in terms of the number of atoms disintegrating per second (Table 2). Allowing of the recoil after beta-particle emission may well break for the K-capture variant in the breakdown of K40,the the covalent bonds by which the carbon (now nitrogen) total number of beta particles produced per second in a is attached to the remainder of the molecule. Again 70-kg. human body is about 20,000. Of these 85 per the chemical nature of the molecule is altered. Now the content of desoxypentosenucleic acid in incent arise from K40and 15 per cent from Cl4. The condividual cells of active tissues is about 6 to 7 X 10-12 tribution of Hzis insignificant. There has been speculation in the past to the effect grams.' Supposing the genes to consist of this plus that beta-radiation from K4", which the body can neither avoid nor escape, is responsible for the developTABLE 2 ment of "spontaneous" cancers. Nahmiass has calRadioactidty i n a 70-Kg. Human Body cuhted, however, that the effect of the body's K4"s to Number of exnose each cell of the human bodv to the formation.> onNumbe? of qtoms disintegrating the average, of one ion per year." This effect is of the Gram present presat per second same order as that produced by cosmic rays and is well below that produced by tolerated doses of short-wave KO ' 8 . 3 x lo-' 1 . 2 x 10%' 1.9 x lo4 1 . 9 x 10-8 8 . 1 x 10" 3.1 X loa radiation. It would seem then that K40could be elimi8 . 4 X loWL' 1.7 X 10' 3.0 nated as a significant factor in spontaneous carcinogenesis. be so eliminated, it would seem afortioe twice the weight of associated protein, we can put the ~f ~ 4 0 that Ci4, which is responsible for less than a fifth of the weight of the genes in a cell a t 2 X lo-" grams. About beta-particle production that K40is,should also be elimi- half of this weight may be taken to be carbon and 1.85 ~ ~ there , is an im- X lo-'' of that t o be GI4. The weight of Cx4in the nated as a factor. H ~ however, genes of such a cell is therefore just under 2 X portant new consideration to he taken into account, the case of ~ 4 0 ,beta-particle production results in a grams. This means that there is just about one atom random bombardment of surrounding molecules, with of C14present in the genes of an individual cell. It Can be calculated from this that the number of random formation of free radicals or ions. Few of these CL4 disintegrations in the genes of the human body is could be expected, on the grounds of pure chance, to One Per Year for every 1200 cells. This is affect the nucleoprotein arbiters of cellular chemistry. only one twelve-hundredth the number of breakdowns ' "Handbook of Chemistry and Physics," 35th ed., Chemical hi^^ calculates for ~ 4 0 yet , it is figure which ought Rubber Publishing Co., Clevelxnd, Ohio, 1953. not to be disregarded. Whereas the K40disintegrations ' CUERAN,S. C., &uart. Revs. (London), 8 , l ( 1 9 5 3 ) .
g8'
HORTECK, P., AND V. FALTINGS, Nature, 166, 1109 (1950).
' ASIMOV,I., J. CHEM.E D U C . , 24 ~ ~(1954). , ' NAHMIAS, M. E., Cahiers phys., 17,27 (1943).
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LEUCHTENBERGER, C., ET AL., Proc. .Wall. Acad. Sci. U.S., 37,
33 (1951). 84
FEBRUARY, 1955
may or may not affect the genes to the extent of inducing mutation, with the chances probably considerably more than 1200 to one against mutagenesis, every one of the C'4 breakdowns within the gene must result in its chemical alteration. I n the light of this, it mould be interesting to note
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whether a diet high in CL4would increase the mutation rate in an animal such as Drosophila or the rate of tumor formation in cancer-prone strains of mice, and whether any correlation existed between the increase (if any) in mutagenesis or carcinogenesis and the increase (if any) of C14in the genes.
