contamination of the biosphere with radioactivity

Contamination of the Biosphere with Radioactivity

R. SCOTT RUSSELL, Ph.D. (Lond.), D.Sc. (Oxon.)

Director, The Letcombe Laboratory (formerly The Radiobiological Laboratory), Agricultural Research Council, Wantage, Berkshire, England

A B S T R A C T

The results o f intensive research on the behaviour o f radioactive substances in the environment, and on the biological effects o f radiation, have dispelled many mis- apprehensions regarding the significance o f environmental contamination with radioactive substances under peace-time conditions. The total radiation doses which have, or will be, received as a result o f the testing o f nuclear weapons hitherto, are on average small in relation to natural background radiation. Problems which might arise as a result o f discharges from industrial installations and methods for con- trolling them are examined.

I N T R O D U C T I O N

It is now a quarter of a century since it first became necessary to consider the effects of radioactive products of nuclear fission which have been released into the environment. Probably no other aspect of the protection of Man's environment was, at first, so difficult to judge objectively. The effects of ionizing radiation had been little considered by the majority of scientists, and were virtually unknown to the public until nuclear weapons were developed, so that, from the outset, the presence of radioactive substances in our environment was closely associated in people's minds with the most destructive of all weapons--a view which received further encouragement in the late 1950s when radioactivity from weapon-testing could be detected in the food supplies and bodies of the human population of the entire world.

In addition, there was clear evidence of the great harm which high doses of radiation can cause. Many years before the first nuclear reactor was constructed, the biological effect of high dose-rates had been apparent from sufferings of the early workers with radium and X-rays. No other noxious agent seems so insidious; for exposure was undetectable by the human senses, the effects could be long-delayed, and by the time the effects became apparent they were

2

Biological Conservation, Vol. 2, No. 1, October 1969- -~ Elsevier Publishing

incurable. The intensive research and discussion--both at scientific meetings and in national and international committees--which these gloomy forebodings en- couraged, now make it possible to assess more objectively the effects which may result from the release of radioactive substances, intentionally or accidentally, and also to consider the need for their control and how this can be achieved.

The main sources of information used in the pre- paration of this review are cited in such publications as United Nations (1962, 1964, 1966), International Commission on Radiological Protection (1966, 1966a, 1966b), Glasstone (1962), Hungate (1966), Russell (1966), and A, b e r g & Hungate (1967). The Inter- national Commission on Radiological Protection (ICRP) and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) are the two most important international organizations concerned with this subject. ICRP is the accepted source of guidance on standards for radiation protection throughout the world. UNSCEAR studies the significance and extent of radiation in the environment. It consists of scientists nominated by fifteen nations: Argentina, Australia, Belgium, Brazil, Canada, Czechoslovakia, France, India, Japan, Mexico, Sweden, USSR, United Arab Republic, the United Kingdom, and the United States of America. There could scarecely be a better guarantee that its con- clusions should be free from bias!

The opening parts of the present survey are concerned with the requirements for an adequate assessment of environmental radioactivity and with the significance of the exposures which have hitherto been experienced from world-wide fallout. Subsequently the types of situation which it may be prudent to envisage under peace-time conditions in the future, and possible control measures, are examined. Civil defence problems in areas devastated by nuclear weapons are not considered. The situations which would then arise differ so fundamentally from those due to fallout, or to peace-time discharges, that their conjoint consideration would encourage misunderstanding. Moreover,

Company Ltd, England--Printed in Great Britain

Russell: Contamination of the Biosphere with Radioactivity

several surveys on the effects of nuclear warfare are available (e.g. Glasstone 1962; Brunner & Pr~tre, 1968).

THE ASSESSMENT OF

ENVIRONMENTAL RADIOACTIVITY

The biological effects of radiation are due to the absorption into living tissues of energy from ionizations to which radiation gives rise. Injury depends on the absorbed dose* of radiation, irrespective of its origin. At the moderate or low doses to which contamination of the biosphere could normally give rise, injury caused by radiation becomes apparent only after a latent period which may exceed a decade in length, and it is the total dose absorbed, not the rate at which it is delivered, that is the relevant measure of risk.t Thus, if the significance of a particular source of radioactive contamination in the biosphere is to be correctly assessed, the resultant increase in the total radiation dose received by organisms from internal and external sources of radiation must be known. Measurements of the concentration of radioactive nuclides do not themselves provide this information and, unbacked by other data, can be misleading. For example, the statement that the concentration of a radioactive fission product has lately doubled in an organism may by itself seem alarming; but if the dose from this source is still only a minute fraction of that due to other sources, including natural background, its consequences will be undetectable. Again, even if a radioactive nuclide is shown to become highly con- centrated in small organisms, for example Algae or insects, it cannot be concluded that they are subject to a corresponding radiation dose. The range of energetic beta or gamma radiation is such that only a fraction of i t - -sometimes a very small one--will be absorbed by such organisms, the rest being dissipated in the sunounding medium, thus making no contribution to the radiation dose.

The first requirement in surveys of environmental radioactivity is therefore obvious--measurements must be interpretable in terms of the integrated radiation

* The absorbed dose is measured in rads (1 rad = 100 erg/g). The dose equivalent (measured in rein) is the dose in rads multiplied by a quality factor to take account of differences in the magnitude of effect caused by the same absorbed dose from different types of radiation. However, as the quality factor is 1 for beta and gamma radiation, and these are of dominant concern in the present discussion, all doses are here expressed as rads.

doses received by living tissues. A second requirement follows from the fact that the dose delivered by unit quantity of different nuclides within the body varies widely, depending on both physiological and physical factors; individual nuclides must be measured sepa- rately if the radiation dose is to be determined. Despite the fact that mixed fission products contain a large number of nuclides, this requirement can be satisfied relatively easily in a well-equipped laboratory, because usually a very few nuclides are the dominant source of radiation dose, the contribution of others being trivial by comparison. Attention can thus be confined to these 'critical' nuclides. Similarly, it is usually adequate to consider the exposure to radiation of only critical groups of organisms which are subject to the greatest radiation risk, and their internal radiation dose is often due to the passage of radioactivity through certain critical food-chains; when these are known, surveys of limited scope can reveal the significance of envirortmental situations.

This approach is not a mere theoretical idea--i ts applicability has been widely demonstrated in the assessment of the exposure of Man (International Commission on Radiological Protection, 1966), and there are abundant reasons for regarding Man as the critical organism whenever radioactive materials are widely dispersed in the environment. The main delayed effects of radiat ion--namely, increased inci- dences of cancer in the exposed individuals and of genetic injury in future generations--are for obvious reasons of vastly greater significance in Man than in other organisms. Thus, for example, it has been estimated that the levels of ingested strontium-90 which will cause economically significant injury in cattle are more than 50 times greater than those which make their milk unacceptable by present standards as a food for infants (Garner, 1963).

Apart from the special consideration Man gives to his own species, he is among the most sensitive of organisms to radiation. A detailed comparison of the relative sensitivity of different types of organisms is complicated by many factors, but some indication is provided by estimates of the LD50 (i.e. the dose that is lethal to 50 per cent of the population) in acute exposure to external radiation. For mammals the values range from 200-1,000 R,* that for Man being at the lower end (United Nations, 1962); for insects and microorganisms, LD50s appear to range up to about l0 s and over 106 R, respectively. In continuing exposure to low dose-rates, the sensitivity of Man may well be considerably greater, relative to many other

t In contrast, the effects of very high doses of radiation become manifest rapidly, and in such cases the dose rate can have an important effect on the extent of injury.

* The Rontgen (R) is now used to describe exposure to radiation measured in terms of the ionization produced in air.

4 Biological

animals, than the above figures suggest, as his longer life-span gives greater opportunity for the delayed effects of radiation to become manifest. In the plant kingdom sensitivity to radiation varies largely, though not entirely, with the interphase chromosome volume; gymnosperms, which have relatively large chromosome volumes, show LD50s in acute exposure that are comparable to those for mammals, but with angiosperms the values range up to over 17,000 R (Sparrow et al., 1968).

Taking all aspects into account, risks to Man are the most appropriate general basis for assessing the significance of the contamination of the biosphere with radioactive substances--except in special cases when the doses received by Man are very much lower than those to which other organisms are exposed. Thus the final step in the assessment of the significance of such contamination of the biosphere is usually the assessment of the risks to Man resulting from the estimated radiation dose he has received.

With other potentially noxious substances, risks are usually assessed from direct observation of dose/response relationships, and levels of exposure which have led to no detectable effect in careful studies are regarded as safe. An entirely different and more scientific basis has been adopted for the risks of radiation: here it is assumed that some effect, however slight or infrequent, may occur down to the lowest levels of exposure, no dose thus being 'safe' in the strict meaning of that word. This does not, however, imply that all radiation doses--however small--are 'dangerous' in the sense in which that term is normally used. This is evident from a report on risks from low doses of radiation, issued by the International Commission on Radiological Protection (1966a), which explains that a major obstacle to estimating the effects of low doses of radiation is the fact that some types of injury, both genetic and somatic, which radiation can cause, also arise frequently from other, unidentified causes--in common parlance, from 'natural causes'. The only practicable way to estimate the effects of low doses of radiation is to assume that the response per unit dose at very low doses is the same as that observed in exposures which are sufficiently high to allow reasonably quantitative observation of their effects. It is explained in the report that this procedure is likely in the majority of cases to indicate an upper limit of risk; but such calculations as are now possible none the less show that the risk of genetic injury and of all malignant diseases, from doses of radiation recommended as the maximum permissible in occupational work, are very small in comparison with the normal incidence of such effects.

An alternative way of considering the significance of environmental contamination is to compare the doses

Conservation

from this source with the average dose from natural background radiation; as the effects of radiation depend on the dose absorbed by tissues, irrespective of its origin, the comparative risk from the two sources is validly shown. This procedure has been used by UNSCEAR to assess world-wide fallout. The estimated average dose from natural background to the world's human population is shown in Table I. Considerable

TABLE I

Radiation Dose-rate due to Natural Sources in 'Normal' Areas (United Nations, 1966)

Source o f irradiation Dose rates (mrad/year)

Bone Gonads Bone marrow

External irradiation Cosmic rays

Ionizing component 28 28 28 Neutrons* 0.7 0"7 0"7

Terrestrial radiation (including air) 50 50 50

Internal irradiation Potassium-40 20 15 15 Rubidium-87 0.3 <0.3 <0.3 Carbon-14 0.7 1.6 1.6 Radium and decay

products* 0"6 3'7 0'6

TOTAL 100 99 96

* If the doses were expressed in rem (see first footnote, page 3), these components of background would be increased because of the higher biological effectiveness of alpha radiation and neutrons (see United Nations, 1962).

variation occurs: doses from cosmic rays increase with altitude above sea-level, and in some limited regions high levels of terrestrial radiation from the underlying rocks causes the background dose to approach ten times the average; variations by 10 to 20 per cent are relatively common.

Investigations in areas of high radiation background have provided no evidence of an enhanced incidence of abnormalities, so that doses in the range of the normal natural background cannot be expected to engender detectable ecological change. Nonetheless it is highly desirable that each potential new source of radiation to which modern technology may give rise should be carefully examined, especially if it is likely to affect large populations over an extended period. Otherwise the cumulative total of numerous small changes might become unacceptable. The manner in which these matters should be judged has been referred to by the International Commission on Radiological Protection (1966b).

Russell: Contamination of the

RADIATION DOSES RECEIVED HITHERTO FROM ENVIRONMENTAL CONTAMINATION

Judged on the global scale, world-wide fallout is the predominant source of radiation, additional to natural background, to which the biosphere has been exposed. The results of extensive surveys in many countries have been assembled by UNSCEAR, which has computed the 'dose commitment ' to the world's population from this cause: the dose commitment can be approximately defined as the integrated average dose to different types of living tissue which have been or will be received in consequence of the release of fission products.

UNSCEAR' s latest estimate of the effects of all weapons-testing before 1965 is shown in Table II. An assessment for the United Kingdom is comparable

TABLE II

Dose Commitments* to the Worm Population from Nuclear Explosions up to 1965 (United Nations, 1966)

Bone Bone Source of radiation Gonads mrads marrow

External : short-lived 23 23 23 Caesium-137 25 25 25

Internal: Strontium-90 - - 156 78 Caesium-137 15 15 15 Carbon-14 13 20 13 Strontium-89 - - 0.3 0.15

TOTAL 76 240 150 Period in which equivalent Months dose is delivered by 8 30 18 natural background

* Dose commitments are calculated from the commence- ment of weapons-testing until the year 2000. After that date carbon-14 will continue to deliver some dose because of its very long half-life (5,760 years), but at a much- reduced rate.

(Medical Research Council, 1966), and it is known that the more recent weapons-tests caused only a very small addition to the dose commitment. For reasons which will be explained later, one significant component of world-wide fallout, iodine-131, is excluded f rom Table II. In contrast with strontium-90 and caesium-137, which are formed by nuclear fission, carbon-14 is an 'induced activity' created by the cap- ture of neutrons by nitrogen. Moreover, unlike the other nuclides shown in Table 1I, carbon-14 has always been present in the environment-- i t arises from the interaction of cosmic rays with nitrogen in the atmosphere and thus contributes to the normal background radiation dose (see Table I).

The dose commitment to the gonads, on which genetic effects depend, is equivalent to about 8 months '

Biosphere with Radioactivity 5

background; that to the bone marrow, which is of particular interest in relation to the induction of leukaemia, represents about 18 months ' background, while the corresponding period for bone itself is about two-and-a-half years. When is it borne in mind that the dose commitment is the total exposure from fallout during half-a-century, it is evident that, on the world average, the components of fallout listed in Table II have made but a very small increase in the total radiation dose.

This conclusion is unchanged when account is taken of iodine-131. Because of its short half-life (8 days), this nuclide is deposited for only short periods after weapons have been detonated, and for reasons which are considered later, considerably higher doses are received by infants in the first year of life than by other members of the human community. It is estimated that in 1961 and 1962, when the rate of deposition of iodine-131 was highest, infants in many parts of the northern hemisphere may have received about 170 millirads per year if they were fed on fresh mi lk - - the foodstuff from which iodine-131 mainly enters the body (United Nations, 1964; International Commission on Radiological Protection, 1966a). These doses were not recurrent, those received by the older age-groups being considerably smaller.

Considerable local variations in doses from world- wide fallout have been identified. Of these perhaps the most interesting are the much-enhanced doses from caesium-137 which occur in the far north of Europe and North America (~berg & Hungate, 1967). The highest body-burdens of this nuclide, representing 10-100 times that in temperate latitudes, are received by persons who consume relatively large quantities of the flesh of reindeer and caribou; this is due to the very efficient entrapment of the deposit in the slowly-growing cryptogamic and other vege- tation on which these animals graze. In some of these northern areas, levels in freshwater fish have also been considerably higher than elsewhere, largely on account of the low mineral content of the water (United Nations, 1966). In more temperate regions, the entrapment of strontium-90 and caesium-137 in the surface mat which develops under slowly-growing pastures in cool, moist areas, often on hills, has sometimes caused dietary contamination to approach 10 times the normal average.

On a few occasions, unexpected weather conditions have caused small populations near weapon-testing grounds to receive, f rom local fallout, radiation doses considerably above the world-wide average, and so remedial action has been taken (Cohn et al., 1960; Bostrom, 1962). None the less, the fears once widely voiced that world-wide fallout has created a serious new risk on a global scale appears unfounded.

6 Biological Conservation

Indeed, the average extra radiation due to this source was less than the change in background radiation which is not infrequently experienced when people move their homes from one locality to another. The most lasting value of the investigations of past world- wide fallout, as opposed to surveillance near weapon- proving grounds, is perhaps the information they have provided for the more efficient and economical investi- gation of situations which may arise in the future.

POSSIBLE FUTURE SOURCES OF CONTAMINATION

IN PEACE-TIME

The careful control which has always been exercised over the operation of nuclear installations ensures satisfactory conditions in their normal operation. The main causes of environmental contamination which it is necessary to envisage for the future, other than fallout from nuclear detonations, are industrial accidents. Any release of fission products would almost always occur predominantly into the atmosphere, so that they would be deposited both on the land surface and on water. The deposit on dry land could be directly entrapped on plants or animals, whereas material entering water would be subject to dilution with stable elements before entering food- chains. Thus considerably higher radiation doses would usually be experienced by terrestrial organisms. This is illustrated by results of surveys of world-wide fallout; even in those countries where marine foods are most extensively consumed, they have contributed but small fractions of the total radioactivity ingested by the population, and the highest levels of radioactivity in fish have been considerably below those found in the flesh of terrestrial animals (United Nations, 1966).

Discharges Into the Atmosphere The contrasting half-lives of different fission pro-

ducts cause their relative abundance to vary greatly with time, but irrespective of the circumstances in which mixed fission products enter the atmosphere, or the interval of time since they were produced, isotopes of iodine, strontium, and caesium, will always deserve the greatest consideration as sources of internal radiation (Russell, 1966; Food and Agricultural Organization, in press); for they are the critical nuclides.* No other fission products of appreciable half-life enter so freely into biological systems-- iodine is accumulated in the thyroid glands of animals, strontium like calcium is deposited in bone, and

* Induced activities, for example of carbon-14, could accompany the fission products but are unlikely to be of major concern.

caesium like potassium circulates throughout the body. Moreover, these three elements are transferred rapidly from the diet of cattle to their milk. This fact, combined with the efficient retention of finely-divided airborne debris on the herbage of pastures, and the wide areas on which cattle graze, is likely to cause milk to be the most contaminated food-source after fission products have been deposited.

Iodine-131 would be of particular concern if children were fed on fresh milk. This nuclide is considerably more abundant in fresh fission products than are the isotopes of strontium or caesium, and despite the small size of the thyroid glands of an infant (2 g or less at six months), one quarter or more of the dietary intake may be deposited in it; no comparable concentration occurs with other nuclides. Adults are likely to receive much lower doses from iodine-131 than infants who are fed on fresh milk--both because milk is less important in the adult diet and because, although the thyroid glands of adults are about ten times the size of those of infants, they absorb only about the same fraction of the ingested iodine, so that the radiation dose from the same intake of iodine-131 is correspondingly smaller. After accidents to nuclear reactors, the significance of iodine-131 relative to other fission products can be further enhanced because its volatility is likely to cause it to be preferentially released (Farmer, 1967); under these circumstances the ingestion of all other nuclides is by comparison insig- nificant. Similarly, if fallout descends rapidly near sites where nuclear detonations have taken place, iodine-131 is again of dominant concern from the viewpoint of ingestion, being indeed the only nuclide that is likely to deliver internal radiation doses which are appreciable relative to the external dose from short-lived nuclides (Russell, 1968).

The short half-life of iodine-131 causes its effects to abate rapidly in contrast with those of the longer-lived isotopes of strontium and caesium. Soon after fission has occurred, strontium-89 (half-life 5 l days), which is then much more abundant than strontium-90 (half- life 28 years), can deliver higher doses; but after five or six months the longer-lived nuclide becomes dominant. The half-life of strontium-90 is a little shorter than that of caesium-137 (30 years), but its effects are likely to be more prolonged, as it is not only retained for longer periods in the body but it usually enters plants, and hence animals, more freely from the cumulative deposit in the soil.

Detailed discussions of the food-chains which these nuclides traverse are available (e.g. United Nations, 1962; Russell, 1966). Recent research has shown that the continuing risk from strontium-90 was formerly overestimated. Because considerably higher concentrations of strontium-90 are found in the bones of


infants than of adults soon after fallout has been deposited, it was at one time imagined that they were undergoing correspondingly greater risk than were adults; this is now disproved by evidence that strontium is eliminated much more rapidly from the bones of the young (Fletcher et al., 1966). Moreover, strontium-90 enters plants from the soil to a smaller extent than it was prudent to suspect when information was scarce (Russell & Bruce, in press). Taking account not only of these facts but also of the composition and physical form of the fission products which may be released under different circumstances, it seems most unlikely that strontium-90 will be the major source of internal radiation after any release of mixed fission products which causes widespread problems; the same is true of caesium-137.

For obvious reasons it is impossible to predict the possible scale on which contamination with mixed fission products may occur in the future. Massive detonation of nuclear weapons could cause alarming depositions of iodine-131 over a wide area, as well as world-wide fallout on a vastly greater scale than has hitherto been observed. However, the worst and most improbable accidents arising from the peace-time uses of nuclear energy (Farmer, 1967) would create only local problems--not more than a small fraction of a country's milk supply would be affected.

Attention has sometimes been given to the possible effects of the release of fissile materials such as plutonium. Mishaps to aircraft carrying unarmed nuclear weapons could give rise to such situations. The available reports (Brunner & Pr~tre, 1968), however, encourage the view that their biological effects would be small--if indeed detectable--even if no remedial action were taken. Accidents are conceivable at installations where fission products are being pro- cessed or when isotopes are being used in industry, medicine, or scientific research; however, in the worst such circumstances it is difficult to imagine effects comparable to those which could follow reactor accidents.

Discharges Directly into Water With the exception of accidents to nuclear-powered

vessels, discharges of radioactivity directly into water are likely to arise solely from the planned discharge of dilute radioactive effluence from nuclear installations. At first sight the intentional releases of such material into water, whether fresh or marine, may seem a negation of the prudent principle that all doses from radiation should be kept as low as is practicable. That, however, is an unrealistic attitude. All natural water contains radioactive substances in amounts which are readily measurable by modern procedures. The

average concentrations of a number of naturally- occuring nuclides in the oceans are given in Table III. Fresh water usually has a lower burden of radioactivity--except for thermal springs in some areas. The rational approach is clear: if the discharge of radioactivity into water is proved not to cause an

TABLE Iil

Naturally Occurring Radioactivity in the Oceans, Based on Data Assembled by Chipman (1966).

Primordial isotope Picocuries per litre

Uranium-238 1.0 Uranium-235 0.045 Thorium-232 < 0.002 Thorium-230 <0'05 Radium-226 0.1 Rubidium-87 2.6 Potassium-40 280

Produced by cosmic rays

Carbon- 14 0' 10-0" 14 Tritium 0-70-5

unacceptable increase in the doses received by organisms, it is an acceptable practice. Procedures whereby wastes can be safely discharged have been examined in considerable detail (International Atomic Energy Agency, 1966).

Both the nature of the nuclides which may be released in dilute effluent and the critical food-chains that they traverse, may vary widely--depending on the type of nuclear installation. Thus in the Columbia River, USA, the discharge of phorphorus-32 is given special consideration--especially as the water may be used downstream for the spray-irrigation of crops (Kornberg & Davis, 1966). In contrast at Windscale, where discharges are made into the Irish Sea, the adsorption of ruthenium-106 into a seaweed which is eaten by a limited population-group, constitutes the critical food- chain (Preston & Jefferies, 1967).

Although under careful supervision it is reasonable to dispose of appreciable quantities of dilute radioactivity into water, it must be recognized that, despite their large content of natural radioactivity, the oceans are not 'sumps' of infinite capacity into which radioactive debris could indefinitely be poured or deposited in solid form.

REMEDIAL MEASURES

The necessity of remedial measures against environmental radioactivity could arise in nuclear war, locally after major accidents to nuclear reactors, or sometimes

8

close to weapons-proving grounds, through unpredicted patterns of fallout.

When radioactive substances have escaped into the environment, the mitigation of the radiation doses to which they may give rise can be achieved only by action which interferes to a greater or lesser extent with the normal habits of the population. All such action--whether it be the change of one source of diet for another, evacuation from an area of high external exposure, or the ingestion of substances which reduce the radiation dose--may directly or indirectly create some new risks to the population. The logical basis for deciding whether counter-measures are advisable is thus to compare the risk from radiation which can be avoided with the possible risk of the proposed counter-measures (International Commission on Radiological Protection, 1966b); attention is directed to this obvious principle because when problems of environmental radioactivity were less familiar than they are today, it was not always borne in mind.

For reasons already explained, the presence o f iodine-131 in fresh milk consumed by children would be the most probable situation which would call for remedial action; this need could arise in nuclear war, locally after major accidents to nuclear reactors, or sometimes close to weapons-proving grounds after unpredicted patterns of fallout. Provided that alternative sources of milk for infants, for example dried milk, are available, protection could be readily arranged. The relatively short half-life of iodine-131 causes it to decay by a factor of over 100 in six weeks. Thus milk which was unacceptable in the initial period could be safely used for manufactured products which are stored for some months before use--for example dried milk, butter, cheese, or chocolate. The alternative procedure of feeding cattle on stored food might sometimes be appropriate, and it may be noted that during the winter in the cool-temperate regions, when cattle are normally fed in this manner, the contamination of milk would create no problem. The deposition of iodine-131 in the thyroid can also be much reduced by the administration of tablets containing stable iodide. However, a large measure of protection is provided only if this is done within a few hours of exposure (Pochin & Barnaby, 1962); this type of action might therefore be more appropriate for dealing with the inhalation of iodine-131 (which might be confined to a few hours) than against dietary contamination which would proceed, although at a decreasing rate, for some weeks.

The reduction by a large factor of doses due to strontium-90 would present considerably greater problems, and it is therefore fortunate that this nuclide

Biological Conservation

is unlikely ever to be the major source of radiation on a wide scale. The removal of strontium-90 from milk by ion exchange has been discussed, but it could provide only a small degree of protection even to children, as they would ingest appreciable quantities of strontium-90 in other foods. The application of lime to land, and other agricultural measures, would also be of little effect in the majority of circumstances (Russell, 1966). MoreoveT, as in all conceivable environmental situations strontium-90 would be but one, and frequently not the major, source of radiation dose, its complete removal from diet, even if it were possible, would mitigate the total radiation dose by only a relatively small factor.

CONCLUSION

The aim of this survey has been to encourage per- spective in the discussion of risks which may occur through the contamination of the biosphere with radioactive substances. Three circumstances have assisted greatly in the study of this difficult subject. Firstly, the great sensitivity with which radioactive substances can be detected has made it possible for their behaviour in the environment to be examined with much greater facility than is possible with many other toxic agents. Secondly, natural background radiation provides a valid basis against which the possible effects of low doses of radiation can be assessed. Thirdly, public interest has stimulated the detailed study of this subject whereas many other potential environmental pollutants have been ignored until their ill-effects have become widely manifest.

The future cannot be foretold but it is evident that, provided the holocaust of nuclear warfare is avoided, risks due to the entry of radioactive substances into the biosphere are considerably smaller than it was prudent to suspect when the subject first attracted wide attention.

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contamination of the biosphere with radioactivity

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