<Version 10/08/2010>
Comparative Risk Assessment
Between Ionising Radiation and Other Stressors
Shoichi Fuma1 and Gennady G. Polikarpov2
1
2
National Institute of Radiological Sciences (NIRS), Japan
The A.O. Kovalevsky Institute of Biology of Southern Seas (IBSS), Ukraine
Both conceptual and quantitative methods have been used for comparative evaluation
of ecological effects between ionising radiation and chemical pollutants. One is a
method based on the radiochemoecological conceptual model and the other is a method
based on the ecological effect index (EEI).
1. Radiochemoecological conceptual model
The radioecological conceptual model was originally proposed for ecological risk
assessment of ionising radiation (Polikarpov, 1977, 1998a). This model categorises
responses of organisms, populations and ecosystems to all the existing and possible
dose rates in the environment as follows (Fig. 1):
(1) the uncertainty zone
(below the lowest natural ionising radiation background level),
(2) the radiation well-being zone (natural ionising radiation background range),
(3) the physiological masking zone (0.005-0.1 Gy/y),
(4) the ecological masking zone (0.1-0.4 Gy/y),
(5) the zone of damage to communities/ecosystems (>>0.4 Gy/y),
(6) the radiation threshold for lethality to the biosphere (>>MGy/y)
This model was afterward extended and transformed into the radiochemoecological
conceptual model, which covers effects of both ionising radiation and chemical
pollutants as shown in Fig. 2 (Polikarpov, 1998b, 1999, 2001). This model was used for
comparative risk assessment between radioactive and chemical pollutants in the Black
Sea (Polikarpov, 1999; Polikarpov et al., 2004). Some severe impacts have been
observed in various organisms such as macroalgae, mussels, fish, mammals in the Black
Sea shelf and estuarine ecosystems. According to this model, these impacts are
quantitatively equivalent to the ecological effects of nuclear pollution in the nearest
zone of the Chernobyl nuclear power plant, the Kyshtym trail, the Karachay Lake and
the other similar nuclear areas in the world. Dose rates of ionising radiation to biota are,
however, less than 7 mGy/y in the Black Sea, which is by three to seven orders of
magnitude lower than chronic lethal doses for those organisms. The results (Polikarpov
et al., 1994, 2004) indicate that non-nuclear pollution has caused the severe impacts in
the Black Sea.
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Fig. 1. Zones of dose rates and their effects in the biosphere (Polikarpov, 1998a).
A value expressed as a 210Po alpha-irradiation dose rate was calculated multiplying an
absorbed dose rate by a radiation weighting factor of 20.
This model was also useful for comparative evaluation of effects of ionising radiation
and other various stressors on the aquatic microbial microcosms, one of which was the
three-species microcosm consisting of populations of the flagellate alga Euglena
gracilis as a producer, the ciliate protozoan Tetrahymena thermophila as a consumer,
and the bacterium Escherichia coli as a decomposer, and the other was the multi-species
microcosm consisting of populations of the green algae Chlorella sp., Scenedesmus sp.
and the blue-green alga Tolypothrix sp. as producers; the ciliate protozoan Cyclidium
glaucoma, the rotifers Lecane sp. and Philodina sp. and the oligochaete Aeolosoma
hemprichi as consumers; and bacteria as decomposers (Fuma et al., 2003b, 2009;
Polikarpov 2001, Polikarpov et al., 2004). The resulting comparative effect doses, i.e.,
2
equi-dosimetric data, are shown in Fig. 3 and Table 1 for the three-species and
multi-species microcosms, respectively. If “species reduction”, i.e., extinction of at least
one species in the microcosm, is adopted as an endpoint, 500-1000 Gy acute
γ-irradiation is considered to be equivalent to 5000 J/m2 UV-C, pH3.5 acidification, 550
mg l-1 Mn, 5.9 mg l-1 Ni, 6.4 mg l-1 Cu, 47 mg l-1 Gd and 49-91 mg l-1 Dy for the
three-species microcosm and 500-5000 Gy acute γ-irradiation is considered to be
equivalent to 0.5-8 mg l-1 benthiocarb, 5-10 mg l-1 LAS and 200 mg l-1 soap for the
multi-species microcosm.
Fig. 2. Long-term reactions of populations/ecosystems to ionising radiation as well as
any chemical or physical factors at the doses in “masking” and “damage” zones
(Polikarpov et al., 2004). X, Y and Z are doses of non-radioactive factors in each zone.
Ecological impacts in the Black Sea are due to non-nuclear factors because of low dose
of ionising radiation.
3
Fig. 3. Effect doses of γ-rays and other stressors for the three-species microcosm
evaluated by the radiochemoecological conceptual model (Polikarpov et al., 2004).
Table 1. Effect doses of γ-rays and chemicals for the multi-species microcosm evaluated
by the radiochemoecological conceptual model (Fuma et al., 2009).
Toxic agents
Physiological Zone of damages to ecosystems
References
or ecological
Number
Species
masking zone
reduction
reduction
γ-rays, Gy
100
500-5000
Fuma et al., 2009
Chemicals, mg l-1
Cu
Benthiocarba
Simazinea
LASb
Soap
a
b
1.5
0.6-1.2
0.08-1.28
2.5
0.5-8
5-10
30
50-100
200
Sugiura et al., 1982
Takagi et al., 1994
Takagi et al., 1994
Takamatsu et al.,
1995
Takamatsu et al.,
1995
Herbicide.
Linear alkylbenzenesulfonate (surfactant).
2. Ecological effect index (EEI)
The ecological effect index (EEI) was proposed for holistic evaluation of effects on
various parameters in ecosystems exposed to ionising radiation and other stressors
(Fuma et al., 2003a). As the EEI, degrees of differences in parameter values between
exposed and control ecosystems were expressed as the Euclidean distance weighted by
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ecological importance of each parameter. That is, the effect index on day t after
exposure (EI(t)) was defined as follows:
2
 Pi, Con( t )  Pi, Exp( t )  [%]
 Wi 

Pi, Con( t )
i 1


n
EI(t) = 100
(1)
where
n = the number of parameters concerned.
Wi = ecological weighting factors for parameter i. This value depends on importance of
each parameter for the ecosystem itself in the ecocentric principle and for man in
the anthropocentric principle. The more important the parameter i is, the larger
n
the Wi value is.  Wi = 1.
i 1
Pi,Con(t) = values of parameter i in the control ecosystem on day t.
Pi,Exp(t) = values of parameter i in the ecosystem exposed to a toxic agent on day t after
the exposure.
The EI(t) should be averaged for experimental periods, because the time-dependent
changes in parameters are investigated for evaluation of ecological effects in general. As
such an averaged index, the ecological effect index (EEI) was defined as follows:
EEI =
1
T

T
EI( t ) dt [%]
(2)
0
where
T = days from the exposure to the end of the observation.
The EEI was applied to population changes in the three-species and multi-species
microcosms exposed to acute γ-rays and various stressors. That is, the effect index for
microcosm (EIM) was defined as follows:
100
EIM 
T
 W 
T
n
k
0
k 1
Nk , Con(t )  Nk , Exp(t )
Nk , Con(t )

2
dt [%]
(3)
where
Nk,Con(t) = the log-transformed population of taxon k in the control microcosm on day
t.
Nk,Exp(t) = the log-transformed population of taxon k in the exposed microcosm on
day t.
Assuming that all the trophic levels, i.e., producers, consumers and decomposers, had
the same ecological importance and all species or taxonomic groups in each trophic
level had the same ecological importance in the microcosm, the following Wk values
were given to each species or taxonomic group in equation (3):
Three-species microcosm
E. gracilis, T. thermophila and E. coli:
1
3
5
Multi-species microcosm
1 1 1
 
3 3 9
1 1 1
<Consumers> C. glaucoma, Lecane sp., Philodina sp. and A. hemprichi:  
3 4 12
1
<Decomposers> Bacteria:
3
<Producers> Chlorella sp., Scenedesmus sp. and Tolypothrix sp.:
In general, the EIM was positively correlated with log-transformed doses of each
stressor, and the relationship between them could be fitted by a sigmoid curve (e.g., Fig.
4). A 50 % effect dose for the microcosm (EDM50), at which the EIM became 50 %,
could be calculated in a similar manner to a 50 % effect concentration (EC50), which has
been regarded as one of important toxicity data in conventional single-species tests for
chemicals. The resulting EDM50 values are shown in Table 2. Comparison of Table 2
with Table 1 indicates that the EDM50 corresponds to “species reduction zone” in the
radiochemoecological conceptual model, suggesting that these two methods for
comparative evaluation of ecological effects are related each other. The equi-dosimetric
data obtained by these methods will be useful for comparative risk assessment between
ionising radiation and the other stressors in aquatic microbial communities.
(B) Multi-species
microcosm
(A) g-rays
EIM, %
(A) Three-species microcosm
100
90
80
70
60
50
40
30
20
10
0
0.5624
Y=100-100/{1+(X/5609)
100
500 1000
}
5000
Doses, Gy
Fig. 4. Dose-EIM relationship in the γ-irradiated three-species and multi-species
microcosms (Fuma et al., 2003a, 2009). EIM: Effect index for microcosm.
Table 2. EDM50a of γ-rays and other stressors for the three-species and multi-species
microcosm (Fuma et al., 2003a, 2005, 2009).
Microcosm γ-rays, UV-C, Mn, Ni, Cu, Gd, Dy, Benthiocarbb, LASc,
Gy
J/m2 mg/l mg/l mg/l mg/l mg/l
mg/l
mg/l
Three-species 530
2100 230 2.6 7.0
39
49
Multi-species 5600
6.7
6.1
a
50 % effect doses for microcosm
b
Herbicide
c
Liner alkylbenzenesulfonate (surfactant)
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3. References
Fuma, S., Doi, M., Takeda, H., Kawabata, Z., Saito, M., 2003a. Index for comparative
evaluation of ecological effects between ionizing radiation and other toxic agents:
application to the model ecosystem data, in: International Conference on the Protection
of the Environment from the Effects of Ionizing Radiation: Contributed Papers
(IAEA-CN-109), International Atomic Energy Agency, Vienna, pp. 55-58.
Fuma, S., Ishii, N., Takeda, H., Miyamoto, K., Yanagisawa, K., Doi, K., Kawaguchi, I.,
Tanaka, N., Inamori, Y., Polikarpov, G.G., 2009, Effects of acute γ-irradiation on the
aquatic microbial microcosm in comparison with chemicals. J. Environ. Radioact. 100,
1027-1033.
Fuma, S., Ishii, N., Takeda, H., Miyamoto, K., Yanagisawa, K., Ichimasa, Y., Saito, M.,
Kawabata, Z., Polikarpov, G.G., 2003b. Ecological effects of various toxic agents on the
aquatic microcosm in comparison with acute ionizing radiation. J. Environ. Radioact. 67,
1-14.
Fuma, S., Miyamoto, K., Takeda, H., Yanagisawa, K., Inoue, Y., Sato, N. et al. (1998a).
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Fuma, S., Takeda, H., Miyamoto, K., Yanagisawa, K., Inoue, Y., Ishii, N. et al. (2000).
Simple aquatic microcosm for ecotoxicity screening at the community level. Bulletin of
Environmental Contamination and Toxicology, 65, 699-706.
Fuma, S., Takeda, H., Miyamoto, K., Yanagisawa, K., Inoue, Y., Ishii, N. et al. (2001).
Ecological evaluation of gadolinium toxicity compared with other heavy metals using
an aquatic microcosm. Bulletin of Environmental Contamination and Toxicology, 66,
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Miyamoto, K., Fuma, S., Takeda, H., Yanagisawa, K., Inoue, Y., Sato, N. et al. (1998)
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