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CHAPTER 2 COMPARISON OF SOME NATURAL RADIATIONS RECEIVED BY SELECTED ORGANISMS* THEODORE R. FOLSOM, Scripps Institution of Oceanography, La Jolia, California and JOHN H. HARLEY, Health and Safety Laboratory, U. S. Atomic Energy Commission IN ATTEMPTING to consider in numerical terms possible consequences to populations from mu- tations caused by very low levels of artificial radioactivity, it is instructive to collect for quick comparison some estimates of the natural doses to which certain organisms have been exposed for geological periods. These data emphasize that doses from natural sources vary widely and depend not only upon the habitat but also upon the physical size of the organism; this natural radiation background varies particularly widely amongst aquatic organisms. A very useful summary of natural and arti- ficial radiation to which human beings are now exposed has been published by Libby (1955) ; it has already been quoted and some of his com- parisons will be repeated here. Nevertheless, additional radiological factors must be included whenever the natural exposures of marine or- ganisms are to be evaluated. Only sources contributing substantially to the average dose to the organisms as a whole will be listed here. The major contributors are (a) cosmic rays, (b) radioactivity in local sur- roundings, and (c) radioactivity spread through the tissue inside the organism itself. Cosmic rays Cosmic ray intensity decreases far more rap- idly from sea level downward than it increases with increasing elevation above the earth. Fig- ure 1 and Table 1 show the trend of the ioniz- ing component of these rays with elevation above sea level, and with depth in water. The absolute dose which is used in Table 3 and Figure 2 is the average of the two values Libby 1 Contribution from the Scripps Institution of Oceanography, New Series, No. 904. (1955) uses for the geomagnetic equator and for 55° geomagnetic north latitude. (See Fig- ure 1 and Table 1.) External activity Most organisms live close to either (a) igne- ous or metamorphic rock, (b) sedimentary rock, or (c) water. Sea water has a characteristic natural radioactivity â much lower than that of terrestrial rocks but quite appreciable when 400 MRAD/YR 40 MRAO/ YR 200 ^ OEPTHIN SEA (METERS) FIGURE 1 28
Chapter 2 29 Natural Radiation of Selected Organisms TABLE 1 TREND OF COSMIC RAYS WITH DISTANCE ABOVE AND BELOW SEA LEVEL Variation with elevation above sea level, values of intensity of ionizing component (in mrads/year) taken from Libby (1955). Mrad/year Elevation in feet Equator 0 33 5,000 40 10,000 80 15,000 160 20,000 300 Latitude 55C (mag) 37 60 120 240 450 N Variation with depth in water, values computed from average attenuation compiled by George (1952) using Libby's average absolute intensity for mean sea level. Percent of surface Depth in meters 0 10 20 50 100 200 300 Mrad/year . 35 . 10.1 . 4.86 . 1.40 . 0.47 . 0.15 . 0.074 500 0.030 1,000 0.009 4,000 0.007 value 100 28.8 13.9 4.0 1.35 0.42 0.21 0.087 0.025 0.002 compared to that of most natural fresh waters. The major activity in sea water comes from radiopotassium (Revelle, Folsom, Goldberg and Isaacs 1955), and only this constituent will be considered here. Of the metamorphic and igne- ous rocks, granite has the highest activity; for our comparisons the same average radioactivities used by Libby (1955) are used here for granite and sedimentary rocks. Internal sources of activity The bodies of large animals contain a much higher concentration of potassium than is found in sea water. A value of 0.2 per cent is used herein for human tissue (Burch and Spiers, 1954) and 0.3 per cent is used for the potas- sium concentration of large fish (Vinogradov, 1953). Since radio-potassium contributes the major portion, aside from cosmic rays, of the radiation contributing to the average dose to the total body of any marine organisms, the character and distribution of this important natural activity has been compiled in Table 2. Geometrical factors influencing dose A man standing above a granite plane surface receives from the granite roughly one half the radiation which might strike him if he were completely surrounded by granite; likewise a man in a row boat receives from the sea only one half the dose which the sea gives to any submerged organism. Potassium yields both beta and gamma ac- tivity; roughly three fourths of the total energy comes from the beta rays. Nevertheless, because of its short range, the beta particle from the potassium in the surrounding sea contributes TABLE 2 POTASSIUM RADIATION DATA Distribution and Intensities Material Potassium content 0.038% (1) Beta rays Sea water (35%. salinity) Man 0.2% (2) Fish (large) 0.3% (3) d/m/g 0.66 3.5 5.8 mrad/yr 2.7 15 24 Gamma rays d/m/g 0.068 0.36 0.3 mrad/yr 0.9 2.3 (4) 3.7 (4) Physical Nature of Potassum Activity Beta activity = 29 d/s/gram of total potassium Beta ray energy (average)^ 0.5 mev Gamma activity = 3 d/s/gram of total potassium Gamma ray energy = 1.5 mev Sample Calculations for Potassium Activity Beta d/m/g X 1440 X 365 m/yr X 0.5 mev/d X 1.6 X 10« erg/mev 1000 = m>d/ ^ wh;ch 100 erg/rad to, Beta d/m/g X 4.2 â mrad/yr beta; and correspondingly, Gamma d/m/g/X 12.6 = mrad/yr gamma. (1) Sverdrup, Johnson and Fleming (1942). (2) Sherman (1941). (3) Vinogradov (1953). (4) Assume half of the gamma rays from internal activity are absorbed inside the body.
30 Atomic Radiation and Oceanography and Fisheries very little to the total dose of a large animal. On the other hand the beta rays from the sur- roundings can appreciably affect very small or- ganisms and can in fact become the predom- inant contributor to dose whenever the organ- ism has dimensions much smaller than the range of the beta particles in water and tissue. The effect of beta rays starting from internal sources also depends upon the size of the organ- ism. If the organism is very small the beta bombardment from the outside sources may con- tribute much more than does internal activity even though the source of activity is more con- centrated in the tissue than it is in the surround- ing water. It would appear from the character of beta penetration (Friedlander and Kennedy, RAYS FROM INTERNAL POTASSIUM RAYS FROM LOCAL EXTERNAL SOURCES \\\\\\\ V--'.DEEP\SEA:-SEDIMENT:" :',./': !,:':,â¢: :":;V;;::.v-.:.'.v:v<h,gii»A \\\\v-- TOTAL NATURAL DOSES (mrad/yeor) Man over granite Man over sedimentary rock Man over sea Large fish in sea Micro-organism in sea 10.0001 m. S.I. at surf. 100m. at surf. 100m. 207 I 142 75 52 FIGURE 2 64 30 39 5
Chapter 2 31 Natural Radiation of Selected Organisms 1949) that any potassium beta particle which originates inside a small organism will deposit most of its energy outside the organism; appar- ently less than 10 per cent of the total ioniza- tion can take place inside a sphere having a mean radius of 0.1 mm, and perhaps from the activity concentrated inside a phytoplankter hav- ing a mean radius of 0.01 mm only 1 per cent of the energy would be felt by the organism itself. Thus we see that the constitution of the surrounding medium dominates the life of the marine microorganism in a radiological sense as well as in those other manners more familiar to biologists. Units used For quantitative statements concerning such feeble radiations as these it is logical to use a very small unit and preferably one which is defined in terms of energy absorbed; the milli- rad per year (mrad/yr) is such a unit and is used here. The rad unit is only slightly larger than the more familiar roentgen unit, since 1.0 rad by definition causes 100 ergs to be absorbed per gram of matter, and this is approximately the energy deposited by 1.1 roentgen of gamma rays. For converting beta activity to equivalent rad dosage the average beta energy of potassium has been taken as being 0.5 mev. Comparison of natural doses in several domains Figure 2 attempts to bring into a single pic- ture the magnitudes of the main components making up the radiation in each of several do- mains of interest. The approximate total dose to the organism is listed below the figure so that numerical comparisons can be made. In the sea and in deep lakes the dose to small or- ganisms must be evaluated separately from that experienced by large organisms. Circumstances in each domain are given in more detail in Table 3. (See Figure 2 and Table 3.) Discussion Small organisms must be considered sep- arately from large ones. Only a small fraction of the energy coming from activity inside a very small organism can be absorbed by the organ- ism, whereas a large organism cannot escape so well from its own radioactivity. Near the sea surface a large fish receives about half its total natural exposure from the rays originating in the radio-potassium in its own tissues. On the other hand near the sea surface cosmic rays appear to outweigh all other radiations received by a microorganism. At depths of the order of 100 meters the attenuated cosmic rays no longer contribute sig- nificantly to marine organisms either large or small. However, the beta and gamma rays from potassium in sea water can give small organisms doses amounting to about ten per cent of the total dose they receive at the sea surface; the small marine organism cannot escape this expos- ure to radioactivity in the surrounding water. It is the deep fresh water which makes pos- sible the most extreme variation in natural ex- posure. In the deeper waters living things can hide from external bombardment; fresh water generally contains such small amounts of radio- activity that this source can be neglected even in comparison with the feeble effect of cosmic rays remaining at depths of several hundred meters or more. In pure fresh water the total dose from strongly ionizing rays depends largely upon the size of the organism and upon its living habits. If the organism is small in the sense already discussed, if it lives in deeper waters, if it stays away from the bottom sediments, if it avoids the neighborhood of large masses of living tis- sue or of detritus, and if it avoids as far as pos- sible accumulating excessive amounts of those elements which can be radio-active â then it can remain remarkably free from the ionizing bombardment received by all other living things. It would be interesting to find out how the phytoplankton that seek the deeper portion of the euphotic zone of clear lakes respond to their extremely low external dose. If morphological or other differences are discovered between sur- face specimens and deep-water specimens, then one of the origins of these differences might possibly be the extremely different amounts of strongly ionizing rays in the two biospheres. Geneticists should not overlook another as- pect of the minute cell in feeble radiation; an individual cell has an extremely small proba- bility of being struck at all during one genera- tion. In a deep lake the radiation intensity can be so low that only one phytoplankter in about five hundred would experience an ionizing ray before it divided; at least this is the probability of a cosmic ray hitting an area 0.1 mm square
32 Atomic Radiation and Oceanography and Fisheries at 100 meters depth. Furthermore, should an ried to offspring for ten or more generations individual plankter accidentally concentrate an before any nuclear energy would be released in excessive amount of radioactive material in its ;Ii1V (j] whatever, tissue there is little probability that this indi- Because of ^ '..patchiness" of the radiation, vidual would ever pass along any effect of it; . , .. ... ., .... r i u i_ iv,.i L t j- , t the use of a unit like the milhrad per year for there would be very little chance of a disinte- . . ... gration occuring before division. Purely physi- feeble doses of *""& lonmn« radiatlon un- cal reasoning therefore indicates that mutations fortunately cannot convey the complete picture leading to a capability for accumulating rek- of the interesting bombardments which must be lively large amounts of activity might be car- experienced by the very small organism. TABLE 3 RADIATIONS IN ELEVEN RADIOLOGICAL DOMAINS Man over granite Total mrads/year 1. At 10,000' elevation Cosmic rays 100 + granite 90 + internal 17 = 207 2. At sea surface Cosmic rays 35 + granite 90 + internal 17 = 142 Man over sedimentary rock 3. At sea level Cosmic rays 35 + rock 23 + internal 17 = 75 Man over sea 4. Cosmic rays 35 + sea 0.5 1 + internal 17 = 52 Large fish in sea 5. Near surface Cosmic rays 35 + sea 0.9 * +internal 28 =64 6. 100 meters deep Cosmic rays i + sea 0.9 l + internal 28 = 30 Micro-organism (mean radius 0.01 mm or less) in water 7. Near sea surface Cosmic rays 35 + sea 3.6 * + internal 8 =39 8. 100 meters deep in sea or more Cosmic rays 0.5 + sea 3.6 * + internal 5 = < 5 9. Buried in deep sea sediments Cosmic rays 0.000 + day 40-620 + internal 4 = 40-620 10. Near fresh water surface Cosmic rays 35 + water activity 2 + internal 2 =35 11. 100 meters deep in a fresh lake Cosmic rays < 0.5 + water activity 2 + internal 2 = < 0.5 1 For every radiopotassium disintegration there are 10 betas having average energy 0.5 mev and also one gamma ray having 1.5 mev. The man receives half the gammas from activity in the sea; the large fish, substantially all the gammas; while the micro-organism receives gammas and betas together. 2 In fresh water natural activity is extremely low and little of this energy stays in the cell. For example (Robeck et al., 1954) in the Columbia River the beta background of the water is at or below 1 X 10-* micro- curie per ml (2 X 10-* d/m/g) while the activity of aquatic organisms is at or below 1 X 10-* microcuries per gram (2 X 10-* d/m/g). For comparison, the beta activity in normal sea water is 0.66 d/m/g. 3 The marine microplankton probably carries more internal activity than does the lake plankton, never- theless effect can be neglected unless activity is concentrated more than 100 fold. 4 All deep-water organisms have not escaped radiations. Micro-organisms buried in true deep-sea sedi- ments have exceptionally high exposure to radium (Love, 1951); they receive 40-620 mrads/year de- pending upon the type of sediment. CONCLUSIONS 1. Some humans actually live under exposure 2. A man may experience 207 mrad/year on levels surprisingly near the magnitude, 10 roent- high mountains, or 142 on a sandy shore; he gen during 40 years, which has been suggested may reduce this further by half, say, by staying as a genetic tolerance level, i.e., see Figure 2 aboard a ship. and Table 2 (domain 1, high elevation over 3. A large fish experiences a 50 per cent reduc- granite). tion in dose when going to a depth of 100
Chapter 2 33 Natural Radiation of Selected Organisms meters; it carries along its own source of in- ternal radiation, however. 4. A marine microorganism, having a mean radius of 0.01 mm, receives only about 10 per cent of the surface dose at a depth of 100 meters in the sea; most of the dose comes from sea water activity unless exceptionally high in- ternal activities are accumulated. 5. In a deep fresh water lake those microor- ganisms living in deep water (but not right at the bottom) receive from their surroundings what is probably the lowest natural ionizing dose within the biospheres of the earth. It would ap- pear that geneticists should consider seeking evidence of abnormal mutation rates amongst microorganisms which live in deep waters of clear lakes, particularly amongst those which have low affinity for radioactive elements. REFERENCES BURCH, P. R. J., and F. W. SPIERS. 1954. Ra- dioactivity of the human being. Science 120:719-720. FRIEDLANDER, G., and J. W. KENNEDY. 1949. Introduction to radiochemistry. J. Wiley and Sons, New York: xiii+412. GEORGE, E. P. 1952. Progress in cosmic rays. J. C. Wilson, ed. '52 Interscience, North- Holland Publ. Co.: xviii+557. LIBBY, W. F. 1955. Dosages from natural ra- dioactivity and cosmic rays. Science 112 (3158) : 57-58. LOVE, S. K. 1951. Natural radioactivity of water. Ind. Eng. Chem.4$:l54l. REVELLE, R. R., T. R. FOLSOM, E. D. GOLD- BERG, and J. D. ISAACS. 1955. Nuclear science and oceanography. International conference on the peaceful uses of atomic energy, Geneva. Paper no. 277:22. ROBECK, G. G., C. HENDERSON, and R. C. PALANGE. 1954. Water quality studies on the Columbia River. U. S. Dept. of Health, Education, and Welfare. Robert A. Taft Sanitary Engineering Center; Cin- cinnati, Ohio: viii+294. SHERMAN, H. C. 1941. Chemistry of food and nutrition. 6th Ed., McMillan, New York: x+611. SVERDRUP, H. U., M. W. JOHNSON, and R. H. FLEMING. 1942. The oceans, their physics, chemistry and general biology. Prentice- Hall, Inc.: x+1087. VINOGRADOV, A. P. 1953. The elementary chemical composition of marine organisms. Trans. Julia Efron and Jane K. Setlow, Sears Foundation for marine research, Yale Univ., New Haven: xiv+647.
