write a research investigation using these three articles: FEATURES
Radiation & the environment:
Assessing effects on plants and animals
An overview of a recent report issued by the United Nations
Scientific Committee on the Effects of Atomic Radiation
The international body known as the United
Nations Scientific Committee on the Effects of
Atomic Radiation (UNSCEAR) periodically
reviews the effects of ionizing radiation on the
environment. Last year, the Committee, for the
first time, issued a report that contained a review
specifically focused on the effects of ionizing
radiation on plants and animals.* While the
review contained no surprising findings, it does
serve to focus attention on the changing nature of
the scientific community's assessment of radiation's potential environmental effects.
Previously, scientific assessments had considered plants, animals, and other living organisms as part of the environment in which
radionuclides become dispersed. They were further seen as resources which, when contaminated, may contribute to human radiation exposures since some plants and animals are elements of food chains and represent pathways
for the transfer of radionuclides to humans. In
brief, the assessments reflected the generally
accepted position that priority should be given
to evaluating the potential consequences for
humans — which are among the most radiosensitive mammalian species — and to providing a
sound basis for protecting human health.
This position, however, has been questioned
recently. It has been shown that there is at least
one situation — namely in deep-sea sediments, an
environment very remote from humans — where
the above accepted priority could be incorrect.**
Detrimental effects on the environment also have
been observed in localized areas as a consequence
of plants and animals having received short-term,
very high radiation doses following major accidental releases of radionuclides. This has been the
case, for example, in areas affected by the 1957
Mr. Linsley is Head of the Waste Safety Section in the
IAEA Division of Radiation and Waste Safety.
accident in the southeastern Urals and by the
Chernobyl accident in 1986.
UNSCEAR's latest review was done in
response to such concerns, and to demonstrate
explicitly that full account can be, and is being,
taken of the potential effects of radiation on the
environment. It recognizes that the world's
plants, animals, and organisms are themselves
exposed to internal irradiation from accumulated radionuclides and to external exposure from
contamination of their respective environments.
This article highlights the main conclusions of
UNSCEAR's review.
The context of environmental impact
assessments
The presence in our environment of cosmic
radiation and natural and artificial radionuclides
implies a consequential radiation exposure of
the indigenous populations of all organisms.
For humans, it is expected that the probability
of adverse effects are greater where exposures
are higher than the range of natural background
radiation dose rates. This also is to be expected
for other organisms.
However, there is a fundamental difference in
the viewpoint adopted for the evaluation of the
risk. For humans, ethical considerations make the
individual the principle object of protection. In
actual practice, this means that the incremental
risk to a person arising from increased radiation
United Nations Scientific Committee on the Effects of
Atomic Radiation (UNSCEAR) Sources and Effects of
Ionizing Radiation, UNSCEAR 1996 Report to the General
Assembly, with Scientific Annex, United Nations sales publication E.96.DC.3 (1996).
Assessing the impact of deep-sea disposal of low-level
radioactive waste on living marine resources. Technical
Reports Series No. 288, IAEA, Vienna (1988).
by Gordon Linsley
IAEA BULLETIN, 39/1/1997 17
FEATURES
18
exposure must be constrained below some level
which society judges to be acceptable. This level
of risk, although small, is not zero.
In the case of other organisms, the case is
less clear. Humans display an enormous range of
attitudes towards the other species that share this
planet — consider, for example, a population of
mosquitoes at one extreme and an individual
giant panda at the other. For the vast majority of
organisms, we consider the population to be
important, and we set as an appropriate objective
the protection of each population from any
increased risk attributed to radiation. Exceptions
might be populations of small size (rare species)
or those reproducing slowly (long generation
times and/or low fecundity) for which it might
be more appropriate to target protective measures at the level of the individual organism.
Whether we are interested in the protection
of one or many, the responses are likely to be
significantly different when it comes to the
assessment of environmental impacts. One point
undoubtedly is self-evident — namely, that there
cannot be any effect at the population level (or at
the higher levels of community and ecosystem)
if there are not effects in the individual organisms constituting the different populations. This
does not mean, however, that detectable radiation-induced effects in some members of a population necessarily would have any significant
consequences for the population as a whole.
There are other factors to keep in mind as
well when considering the assessment of environmental impacts. For one, natural populations
of organisms exist in a state of dynamic equilibrium within their communities and environments and ionizing radiation is only one of the
stresses that may influence this equilibrium.
The incremental radiation exposure from
human activities cannot, therefore, be considered in isolation from other sources of stress.
This includes those that are either natural (e.g.
climate, altitude, volcanic activity) or of human
origin (e.g. synthetic chemical toxins, oil discharges, exploitation for food or sport, habitat
destruction). When, as is not uncommon, ionizing radiation and chemicals, both from human
activities, are acting together on a population,
the difficult problem arises of correctly attributing any observed response to a specific cause.
Conclusions of the UNSCEAR Review
All living organisms exist and survive in
environments where they are subject, to a
IAEA BULLETIN.39/1/1997
greater or lesser degree, to radiation from both
natural and anthropogenic sources, including
the contamination from global fallout which
followed atmospheric nuclear weapons tests. At
times, and generally in restricted areas, there are
additional increments of radiation exposures
either from authorized (controlled) discharges
of radioactive wastes to the air, ground, or
aquatic systems or from accidental releases. In
the majority of cases there have been no apparent effects in wild plants and animals from these
additional exposures. Following severe accidents, however, damage has been observed in
individual organisms and populations, and
long-term effects could develop in communities
and ecosystems from the continuing increased
chronic irradiation.
The available data on the exposure of wild
organisms to radiation from the natural background and from contaminant radionuclides are
relatively limited. They relate to a very restricted variety or organisms, although for the marine
environment they do provide a reasonably representative picture of the range of dose-rate
regimes likely to be experienced. Because the
estimates are largely derived either from localized measurements of the concentrations of
radionuclides within the organism and in its
immediate external environment or from models that assume an equilibrium state, there is
very little information on the temporal variation
in dose rates to be expected from short-term
fluctuations in discharge rates, differing stages
in the life cycle, changes in behaviour and
short-term environmental factors such as seasonality. It is thus very difficult to estimate from
the available data the total doses that are likely
to be accumulated over specific stages of the
life cycle, e.g. during embryonic development
or up to reproductive age.
For both terrestrial and aquatic environments, there appears to be a significant contribution to the natural background dose rate from
alpha radiation. For the former the main source
appears to be radon-222 and its short-lived
decay products, and for the latter the main
source is polonium-210. Owing to the short
range of alpha particles, the absorbed dose rates
are tissue-specific, and the results underline the
crucial need for more detailed information on
the distribution of the radionuclides relative to
the biological targets that might be considered
important (e.g. the developing embryo or the
gonads) if accurate estimates of background
radiation exposure are to be made. The usual
range for the background radiation exposure is
FEATURES
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)
I
104
The graphs show the approximate lethal dose ranges for various taxonomic
groups, an indication of the comparative radiosensitivity.
up to a few microgray per hour, but in exceptional cases (e.g. the hepatopancreas of a small
pelagic marine shrimp) the absorbed dose rate
may be as high as 150 microgray per hour.
Radioactive wastes. It is accepted that the
release of radioactive wastes to the environment
is likely to increase the radiation exposure of
wild organisms. For discharges to the atmosphere, to a landfill or to surface waters, the published assessments reviewed indicate that the
radiation exposures to some (but not all) individuals in endemic wild populations could
reach about 100 microgray per hour in general;
in exceptional cases, depending on the quantities of specific radionuclides in the wastes,
absorbed dose rates might reach several thousand micorgray per hour. In a very limited number of instances the dose rates estimated from
measured concentrations of radionuclides in the
contaminated environment have been broadly
confirmed by in situ measurements employing
dosimeters attached to the animals.
Accidental releases. The dose rates in the
environment following an accidental release
clearly depend on the quantities of specific
radionuclides involved, the time-scale of the
release, the initial dispersal and deposition patterns, and their subsequent redistribution by
environmental processes over time. It is equally
clear that these accidental releases have the
potential to generate much higher dose rates and
higher total doses in the environment than do
normal operations. Such was the case following
the accidents in the southeastern Urals and at
Chernobyl, where numerous studies have indicated that trees (and, by reasonable extension,
other organisms) close to the release points
could have accumulated doses up to 2000 gray
and 100 gray at the two accident sites, respectively, over relatively short periods of time. At
both sites, longer-term chronic exposures from
the deposit of longer-lived radionuclides have
continued to be significantly higher than exposures from controlled waste disposal.
From these data it may be concluded that it
is the responses of plants and animals to chronic radiation exposures up to a maximum
absorbed dose rate of 1000 microgray per hour
that are of interest from the viewpoint of providing a basis for assessing the environmental
impact of controlled radioactive waste releases;
in practice, information at lower dose rates, up
to 100 microgray per hour would probably be
sufficient in the great majority of cases.
Comparative
radiosensitivity
among organisms
IAEA BULLETIN, 39/1/1997 19
FEATURES
For accident situations, experience has
clearly demonstrated that initial dose rates can
be high enough to allow accumulating lethal
doses in relatively short periods (days). In light
of this, data are needed to provide the basis for
predicting the progress of environmental recovery at generally lower, long-term chronic dose
rates, down to the upper end (1000 microgray
per hour) of the range of interest for assessing
waste disposal practices.
Radiosensitivity. There is a wide range over
which organisms are sensitive to the lethal
effects of radiation. A general classification has
been devised based on the interphase chromosome volume of sensitive cells. These and other
results of experimental irradiations show mammals to be most sensitive, followed by birds,
fish, reptiles, and insects. Plants show a wide
range of sensitivity that generally overlaps that
of animals. Least sensitive to acute radiation
exposures are mosses, lichens, algae and
micro-organisms, such as bacteria and viruses.
(See figure, previous page.)
Sensitivity of the organism to radiation
depends on the life stage at exposure. Embryos
and juvenile forms are more sensitive than
adults. Fish embryos, for example, have been
shown to be quite sensitive. The various developmental stages of insects are quite remarkable
for the range of sensitivities they present.
Overall, the available data indicate that the production of viable offspring through gametogenesis and reproduction is a more radiosensitive
population attribute than the induction of individual mortality.
In the most sensitive plant species, the
effects of chronic irradiation were noted at
dose rates of 1000 to 3000 microgray per hour.
It was suggested that chronic dose rates less
than 400 microgray per hour (10 milligray per
day) would have effects, although slight, in
sensitive plants. They would be unlikely, however, to have significant deleterious effects in
the wider range of plants present in natural
plant communities.
Effects of ionizing radiation on plants and animals at levels implied by current radiation protection standards,
Technical Reports Series No. 332, IAEA, Vienna (1992).
International Commission on Radiological Protection,
1990. Recommendations of the International Commission
on Radiological Protection, ICRP Publication 60, Annals
of 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, mammals, there is little indication that dose rates of
400 microgray per hour to the most exposed
individual would seriously affect mortality in
the population. For dose rates up to an order of
magnitude less (40-100 microgray per hour),
the same statement could be made with respect
to reproductive effects. For aquatic organisms,
the general conclusion was that maximum dose
rates of 400 microgray per hour to a small proportion of the individuals and, therefore, a lower
average rate to the remaining organisms would
not have any detrimental effects at the population level. The radiation doses necessary to produce a significant deleterious effect are very difficult to estimate because of long-term recovery
(including natural regeneration and the migration of individuals from surrounding areas that
are less affected), compensatory behaviour, and
the many confounding factors present in natural
plant and animal communities in both terrestrial and aquatic environments.
IAEA activities and plans related to
environmental protection
The results of the UNSCEAR review of the
effects of radiation on the environment generally confirm the conclusions reached in an IAEA
study issued in 1992. * They further support the
general view of the International Commission
on Radiological Protection (ICRP) that the
"standard of environmental control needed to
protect man to the degree currently thought
desirable will ensure that other species are not
. . „ **
put at risk .
However, it is recognized in both the
UNSCEAR and IAEA reviews that there are circumstances where this general conclusion may
not be valid. Moreover, there is a view that the
ICRP statement could be misinterpreted as indicating a lack of concern for the environment. For
these and other reasons, there is a movement in
some countries towards the establishment of
specific standards for the protection of the environment. There were discussions on this theme
at an IAEA symposium in 1996.*** In recognition of this ongoing debate, the Agency will hold
a series of expert consultations during 1997 and
1998 with a view to determining the prevailing
view in its Member States on these issues.
Depending upon the outcome of these discussions, one possible objective is the development
of a Safety Standard that incorporates international consensus on this important subject. O
20 IAEA BULLETIN, 39/1/1997