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Radiation & the environment:Assessing effects on plants and animals

An overview of a recent report issued by the United NationsScientific Committee on the Effects of Atomic Radiation

The international body known as the UnitedNations Scientific Committee on the Effects ofAtomic Radiation (UNSCEAR) periodicallyreviews the effects of ionizing radiation on theenvironment. Last year, the Committee, for thefirst time, issued a report that contained a reviewspecifically focused on the effects of ionizingradiation on plants and animals.* While thereview contained no surprising findings, it doesserve to focus attention on the changing nature ofthe scientific community's assessment of radia-tion's potential environmental effects.

Previously, scientific assessments had con-sidered plants, animals, and other living organ-isms as part of the environment in whichradionuclides become dispersed. They were fur-ther seen as resources which, when contaminat-ed, may contribute to human radiation expo-sures since some plants and animals are ele-ments of food chains and represent pathwaysfor the transfer of radionuclides to humans. Inbrief, the assessments reflected the generallyaccepted position that priority should be givento evaluating the potential consequences forhumans — which are among the most radiosen-sitive mammalian species — and to providing asound basis for protecting human health.

This position, however, has been questionedrecently. It has been shown that there is at leastone situation — namely in deep-sea sediments, anenvironment very remote from humans — wherethe above accepted priority could be incorrect.**Detrimental effects on the environment also havebeen observed in localized areas as a consequenceof plants and animals having received short-term,very high radiation doses following major acci-dental releases of radionuclides. This has been thecase, for example, in areas affected by the 1957

Mr. Linsley is Head of the Waste Safety Section in theIAEA Division of Radiation and Waste Safety.

accident in the southeastern Urals and by theChernobyl accident in 1986.

UNSCEAR's latest review was done inresponse to such concerns, and to demonstrateexplicitly that full account can be, and is being,taken of the potential effects of radiation on theenvironment. It recognizes that the world'splants, animals, and organisms are themselvesexposed to internal irradiation from accumulat-ed radionuclides and to external exposure fromcontamination of their respective environments.This article highlights the main conclusions ofUNSCEAR's review.

The context of environmental impactassessments

The presence in our environment of cosmicradiation and natural and artificial radionuclidesimplies a consequential radiation exposure ofthe indigenous populations of all organisms.For humans, it is expected that the probabilityof adverse effects are greater where exposuresare higher than the range of natural backgroundradiation dose rates. This also is to be expectedfor other organisms.

However, there is a fundamental difference inthe viewpoint adopted for the evaluation of therisk. For humans, ethical considerations make theindividual the principle object of protection. Inactual practice, this means that the incrementalrisk to a person arising from increased radiation

United Nations Scientific Committee on the Effects ofAtomic Radiation (UNSCEAR) Sources and Effects ofIonizing Radiation, UNSCEAR 1996 Report to the GeneralAssembly, with Scientific Annex, United Nations sales pub-lication E.96.DC.3 (1996).

Assessing the impact of deep-sea disposal of low-levelradioactive waste on living marine resources. TechnicalReports Series No. 288, IAEA, Vienna (1988).

by Gordon Linsley

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exposure must be constrained below some levelwhich society judges to be acceptable. This levelof risk, although small, is not zero.

In the case of other organisms, the case isless clear. Humans display an enormous range ofattitudes towards the other species that share thisplanet — consider, for example, a population ofmosquitoes at one extreme and an individualgiant panda at the other. For the vast majority oforganisms, we consider the population to beimportant, and we set as an appropriate objectivethe protection of each population from anyincreased risk attributed to radiation. Exceptionsmight be populations of small size (rare species)or those reproducing slowly (long generationtimes and/or low fecundity) for which it mightbe more appropriate to target protective mea-sures at the level of the individual organism.

Whether we are interested in the protectionof one or many, the responses are likely to besignificantly different when it comes to theassessment of environmental impacts. One pointundoubtedly is self-evident — namely, that therecannot be any effect at the population level (or atthe higher levels of community and ecosystem)if there are not effects in the individual organ-isms constituting the different populations. Thisdoes not mean, however, that detectable radia-tion-induced effects in some members of a pop-ulation necessarily would have any significantconsequences for the population as a whole.

There are other factors to keep in mind aswell when considering the assessment of envi-ronmental impacts. For one, natural populationsof organisms exist in a state of dynamic equi-librium within their communities and environ-ments and ionizing radiation is only one of thestresses that may influence this equilibrium.The incremental radiation exposure fromhuman activities cannot, therefore, be consid-ered in isolation from other sources of stress.This includes those that are either natural (e.g.climate, altitude, volcanic activity) or of humanorigin (e.g. synthetic chemical toxins, oil dis-charges, exploitation for food or sport, habitatdestruction). When, as is not uncommon, ioniz-ing radiation and chemicals, both from humanactivities, are acting together on a population,the difficult problem arises of correctly attribut-ing any observed response to a specific cause.

Conclusions of the UNSCEAR Review

All living organisms exist and survive inenvironments where they are subject, to a

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greater or lesser degree, to radiation from bothnatural and anthropogenic sources, includingthe contamination from global fallout whichfollowed atmospheric nuclear weapons tests. Attimes, and generally in restricted areas, there areadditional increments of radiation exposureseither from authorized (controlled) dischargesof radioactive wastes to the air, ground, oraquatic systems or from accidental releases. Inthe majority of cases there have been no appar-ent effects in wild plants and animals from theseadditional exposures. Following severe acci-dents, however, damage has been observed inindividual organisms and populations, andlong-term effects could develop in communitiesand ecosystems from the continuing increasedchronic irradiation.

The available data on the exposure of wildorganisms to radiation from the natural back-ground and from contaminant radionuclides arerelatively limited. They relate to a very restrict-ed variety or organisms, although for the marineenvironment they do provide a reasonably rep-resentative picture of the range of dose-rateregimes likely to be experienced. Because theestimates are largely derived either from local-ized measurements of the concentrations ofradionuclides within the organism and in itsimmediate external environment or from mod-els that assume an equilibrium state, there isvery little information on the temporal variationin dose rates to be expected from short-termfluctuations in discharge rates, differing stagesin the life cycle, changes in behaviour andshort-term environmental factors such as sea-sonality. It is thus very difficult to estimate fromthe available data the total doses that are likelyto be accumulated over specific stages of thelife cycle, e.g. during embryonic developmentor up to reproductive age.

For both terrestrial and aquatic environ-ments, there appears to be a significant contri-bution to the natural background dose rate fromalpha radiation. For the former the main sourceappears to be radon-222 and its short-liveddecay products, and for the latter the mainsource is polonium-210. Owing to the shortrange of alpha particles, the absorbed dose ratesare tissue-specific, and the results underline thecrucial need for more detailed information onthe distribution of the radionuclides relative tothe biological targets that might be consideredimportant (e.g. the developing embryo or thegonads) if accurate estimates of backgroundradiation exposure are to be made. The usualrange for the background radiation exposure is

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Viruses

Molluscs

Protozoa

Bacteria

Moss, Lichen, Algae

Insects

Crustaceans

Reptiles

Amphibians]

Fish

Higher Plants

Mammals

10°I I I

101 102 103

Acute lethal dose (Gy)

I104

The graphs show the approximate lethal dose ranges for various taxonomicgroups, an indication of the comparative radiosensitivity.

up to a few microgray per hour, but in excep-tional cases (e.g. the hepatopancreas of a smallpelagic marine shrimp) the absorbed dose ratemay be as high as 150 microgray per hour.

Radioactive wastes. It is accepted that therelease of radioactive wastes to the environmentis likely to increase the radiation exposure ofwild organisms. For discharges to the atmos-phere, to a landfill or to surface waters, the pub-lished assessments reviewed indicate that theradiation exposures to some (but not all) indi-viduals in endemic wild populations couldreach about 100 microgray per hour in general;in exceptional cases, depending on the quanti-ties of specific radionuclides in the wastes,absorbed dose rates might reach several thou-sand micorgray per hour. In a very limited num-ber of instances the dose rates estimated frommeasured concentrations of radionuclides in thecontaminated environment have been broadlyconfirmed by in situ measurements employingdosimeters attached to the animals.

Accidental releases. The dose rates in theenvironment following an accidental releaseclearly depend on the quantities of specificradionuclides involved, the time-scale of therelease, the initial dispersal and deposition pat-

terns, and their subsequent redistribution byenvironmental processes over time. It is equallyclear that these accidental releases have thepotential to generate much higher dose rates andhigher total doses in the environment than donormal operations. Such was the case followingthe accidents in the southeastern Urals and atChernobyl, where numerous studies have indi-cated that trees (and, by reasonable extension,other organisms) close to the release pointscould have accumulated doses up to 2000 grayand 100 gray at the two accident sites, respec-tively, over relatively short periods of time. Atboth sites, longer-term chronic exposures fromthe deposit of longer-lived radionuclides havecontinued to be significantly higher than expo-sures from controlled waste disposal.

From these data it may be concluded that itis the responses of plants and animals to chron-ic radiation exposures up to a maximumabsorbed dose rate of 1000 microgray per hourthat are of interest from the viewpoint of pro-viding a basis for assessing the environmentalimpact of controlled radioactive waste releases;in practice, information at lower dose rates, upto 100 microgray per hour would probably besufficient in the great majority of cases.

Comparativeradiosensitivityamong organisms

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For accident situations, experience hasclearly demonstrated that initial dose rates canbe high enough to allow accumulating lethaldoses in relatively short periods (days). In lightof this, data are needed to provide the basis forpredicting the progress of environmental recov-ery at generally lower, long-term chronic doserates, down to the upper end (1000 micrograyper hour) of the range of interest for assessingwaste disposal practices.

Radiosensitivity. There is a wide range overwhich organisms are sensitive to the lethaleffects of radiation. A general classification hasbeen devised based on the interphase chromo-some volume of sensitive cells. These and otherresults of experimental irradiations show mam-mals to be most sensitive, followed by birds,fish, reptiles, and insects. Plants show a widerange of sensitivity that generally overlaps thatof animals. Least sensitive to acute radiationexposures are mosses, lichens, algae andmicro-organisms, such as bacteria and viruses.(See figure, previous page.)

Sensitivity of the organism to radiationdepends on the life stage at exposure. Embryosand juvenile forms are more sensitive thanadults. Fish embryos, for example, have beenshown to be quite sensitive. The various devel-opmental stages of insects are quite remarkablefor the range of sensitivities they present.Overall, the available data indicate that the pro-duction of viable offspring through gametogen-esis and reproduction is a more radiosensitivepopulation attribute than the induction of indi-vidual mortality.

In the most sensitive plant species, theeffects of chronic irradiation were noted atdose rates of 1000 to 3000 microgray per hour.It was suggested that chronic dose rates lessthan 400 microgray per hour (10 milligray perday) would have effects, although slight, insensitive plants. They would be unlikely, how-ever, to have significant deleterious effects inthe wider range of plants present in naturalplant communities.

Effects of ionizing radiation on plants and animals at lev-els implied by current radiation protection standards,Technical Reports Series No. 332, IAEA, Vienna (1992).

International Commission on Radiological Protection,1990. Recommendations of the International Commissionon Radiological Protection, ICRP Publication 60, Annalsof the ICRP 21 (1-3) Pergamon Press, Oxford (1991).

See "Environmental impact of radioactive releases:Addressing global issues", IAEA Bulletin, Vol. 38, No. 1(1996).

For the most sensitive animal species, mam-mals, there is little indication that dose rates of400 microgray per hour to the most exposedindividual would seriously affect mortality inthe population. For dose rates up to an order ofmagnitude less (40-100 microgray per hour),the same statement could be made with respectto reproductive effects. For aquatic organisms,the general conclusion was that maximum doserates of 400 microgray per hour to a small pro-portion of the individuals and, therefore, a loweraverage rate to the remaining organisms wouldnot have any detrimental effects at the popula-tion level. The radiation doses necessary to pro-duce a significant deleterious effect are very dif-ficult to estimate because of long-term recovery(including natural regeneration and the migra-tion of individuals from surrounding areas thatare less affected), compensatory behaviour, andthe many confounding factors present in naturalplant and animal communities in both terrestri-al and aquatic environments.

IAEA activities and plans related toenvironmental protection

The results of the UNSCEAR review of theeffects of radiation on the environment general-ly confirm the conclusions reached in an IAEAstudy issued in 1992. * They further support thegeneral view of the International Commissionon Radiological Protection (ICRP) that the"standard of environmental control needed toprotect man to the degree currently thoughtdesirable will ensure that other species are not

. . „ **put at risk .

However, it is recognized in both theUNSCEAR and IAEA reviews that there are cir-cumstances where this general conclusion maynot be valid. Moreover, there is a view that theICRP statement could be misinterpreted as indi-cating a lack of concern for the environment. Forthese and other reasons, there is a movement insome countries towards the establishment ofspecific standards for the protection of the envi-ronment. There were discussions on this themeat an IAEA symposium in 1996.*** In recogni-tion of this ongoing debate, the Agency will holda series of expert consultations during 1997 and1998 with a view to determining the prevailingview in its Member States on these issues.Depending upon the outcome of these discus-sions, one possible objective is the developmentof a Safety Standard that incorporates interna-tional consensus on this important subject. O

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