THE RADIOACTIVITY OF THE HUMAN BODY

A M O N G the elements that are essential constituents of the human body, the one naturally occurring, long- lived radioactive nuclide is, of course, K40. There are two short-lived radioactive nuclides among these ele- ments, however, which are also naturally occurring since they are continually produced by natural trans- mutative processes initiated in the atmosphere by cosmic rays. These are C1* and Ha. All three are beta-emitters, CI4 and H3 entirely so, K40 to the extent of 90 per cent, its remaining breakdowns being by way of K-capture. The per cent occurrence and the half- lives of each of these three nuclides are presented in Table 1, the values presented being taken or calculated from data appearing in the recent literature.'. 2.

The human body is, by weight, 0.15 carbon, 0.12 hy- drogen, and 0.01 potassium. From this and from the figures given in Table 1, it is possible4 to calculate the body's natural radioactivity in terms of the number of atoms disintegrating per second (Table 2). Allowing for the K-capture variant in the breakdown of K40, the total number of beta particles produced per second in a 70-kg. human body is about 20,000. Of these 85 per cent arise from K40 and 15 per cent from Cl4. The con- tribution of Hz is insignificant.

ISAAC ASIMOV Boston University School of Medicine, Boston, Massachusetts

C14, however, can form an integral portion of the or- ganic molecules within the cell including the desoxy- nucleoprotein molecules that are commonly called genes. A disintegrating CI4 atom within a gene is not merely the source of a possibly damaging beta particle. That is the least of the affair. In the first place, beta-particle emission converts the carbon atom to N", altering the chemical nature of the molecule. Secondly, the effect

TABLE 1 Radioactive Nuclides Occurrino in the Human Badv

Fractional occurrence in element

1.19 X lo-' 1.85 X loWL1

lowL8

Half-life (seconds)

4 . 4 X 10" 1.8 X 1011 3 . 9 X loB

of the recoil after beta-particle emission may well break the covalent bonds by which the carbon (now nitrogen) is attached to the remainder of the molecule. Again the chemical nature of the molecule is altered.

Now the content of desoxypentosenucleic acid in in- dividual cells of active tissues is about 6 to 7 X 10-12

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 nei- ther avoid nor escape, is responsible for the develop- TABLE 2 ment of "spontaneous" cancers. Nahmiass has cal- Radioactidty in a 70-Kg. Human Body cuhted, however, that the effect of the body's K4"s to exnose each cell of the human bodv to the formation. on Number of

> -

the average, of one ion per year." This effect is of the Numbe? of qtoms disintegrating same order as that produced by cosmic rays and is well Gram present presat per second

below that produced by tolerated doses of short-wave K'O 8 . 3 x lo-' 1 . 2 x 10%' 1 .9 x l o4 radiation. It would seem then that K40 could be elimi- g8' 1.9 x 10-8 8 . 1 x 10" 3 . 1 X l oa

8 . 4 X loWL' 1.7 X 10' 3 . 0 nated as a significant factor in spontaneous carcino- genesis.

~f ~ 4 0 be so eliminated, it would seem afortioe twice the weight of associated protein, we can put the 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

nated as a factor. H ~ ~ ~ , however, there is an im- X lo-'' of that to be GI4. The weight of Cx4 in the portant new consideration to he taken into account, genes of such a cell is therefore just under 2 X 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 C14 present in the genes of an individual cell. random formation of free radicals or ions. Few of these It Can be calculated from this that the number of could be expected, on the grounds of pure chance, to CL4 disintegrations in the genes of the human body is

affect the nucleoprotein arbiters of cellular chemistry. One Per Year for every 1200 cells. This is 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. ' CUERAN, S. C., &uart. Revs. (London), 8 , l (1953) . not to be disregarded. Whereas the K40 disintegrations

HORTECK, P., AND V. FALTINGS, Nature, 166, 1109 (1950). ' ASIMOV, I., J. CHEM. E D U C . , ~ ~ , 24 (1954). a LEUCHTENBERGER, C., ET AL., Proc. .Wall. Acad. Sci. U. S., 37, ' NAHMIAS, M. E., Cahiers phys., 17,27 (1943). 33 (1951).

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FEBRUARY, 1955 8s

may or may not affect the genes to the extent of induc- whether a diet high in CL4 would increase the mutation ing mutation, with the chances probably considerably rate in an animal such as Drosophila or the rate of tumor more than 1200 to one against mutagenesis, every one formation in cancer-prone strains of mice, and whether of the C'4 breakdowns within the gene must result in any correlation existed between the increase (if any) in its chemical alteration. mutagenesis or carcinogenesis and the increase (if any)

In the light of this, it mould be interesting to note of C14 in the genes.