Reference Dose Rates

The ultimate goal of ecological protection is to ensure that communities and populations oforganisms can thrive and all the component parts will be self-sustaining. This simple principle requires a shift of focus, typical of most chemical criteria, from individual organism protection to community protection. Focus on individuals, usually the most sensitive individual, is a natural product of

Table 2 Radiation weighting factors for internal alpha radiation for deterministic effects in nonhuman biota (relative to low-LET radiation)

Nominal value

Comment

1a

Built-in conservatism in dose model

20b

Keep same as for humans

2-10c

Nonstochastic effect of neutrons and heavy ions

5d

Average for deterministic effects

10e

Deterministic population-relevant endpoints

20f

Likely to be conservative for deterministic effects

40g

Includes studies with high RBEs

5-20 (10)h

5-10 Deterministic effects (cell-killing, reproductive)

10-20 Cancer, chromosome abnormalities

10 Nominal central value

5-50 (10/

10 To illustrate effect of alpha RBE

aUnited States National Council on Radiation Protection and Measurements (1991) Effectys of ionizing radiation on aquatic organisms. NCRP Report No. ICQ.

international Atomic Energy Agency (1992) Effects of ionizing radiation on plants and animals at levels implied by current radiation protection standards. Technical Reports Series No. 332. Vienna: IAEA.

cBarendsen GW (1992) RBE for non-stochastic effects. Advances in Space Research 12(2-3): 385-392.

dUnited Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) (1996) Effects of Radiation on the Environment. Annex to Sources and Effects of Ionizing Radiation (Report to the General Assembly, with one Annex), Scientific Committee on the Effects of Atomic Radiation. New York: United Nations.

eTrivedi A and Gentner NE (2002) Ecodosimetry weighting factor for non-human biota. In: IRPA-10. Proceedings of the International Radiation Protection Association, Japan, 14-19 May 2000 (available on CD-ROM).

fCopplestone D, Bielby S, Jones SR, et al. (2001) R&D Publication 128: Impact Assessment of Ionising Radiation on Wildlife. Bristol: United Kingdom Environment Agency.

Environment Canada (2000) Priority substances list assessment report (PSL2). Releases of Radionuclides from Nuclear Facilities (Impact on NonHuman Biota). Ottawa: Environment Canada and Health Canada.

h Advisory Committee on Radiological Protection (2002) Protection of Non-Human Biota from Ionizing Radiation. Canadian Nuclear Safety Commission (CNSC), INFO-0703, March.

Framework for Assessment of Environment Impact (FASSET) (2003) Deliverable 3: Dosimetric models and data for assessing radiation exposures to biota (Prohl G, ed.).

controlled laboratory testing. While laboratory data have been essential to developing environmental criteria based on the sensitivity of species to single or multiple stressors, they do not ensure ecological protection. On the other hand, the ICRP indicates that effects on ecosystems are usually observed at the population or higher levels of organization but information on dose responses to radiation is usually obtained experimentally at the level of individual organisms.

The effects of ionizing radiation on biota have been reviewed several times by national and international authorities. Critical radiation effects for natural biota are those that directly affect reproductive success, via significant impairment of gametogenesis or embryonic development and survival, for example. Measurable responses to radiation exposure (e.g., biochemical changes, histological changes in kidney tubules) can occur at exposure levels well below those that actually impair reproduction or survival at any life stage. Such changes are usually regarded as 'biomarkers' ofexposure, and in general are considered as poor endpoints for ecological risk assessments.

UNSCEAR has noted that the sensitivity of an organism to radiation depends on the life stage at exposure and that embryos and juvenile forms are more sensitive than adults. Overall, UNSCEAR concluded that ''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.''

Based on a number of literature studies, UNSCEAR concluded that:

• Exposure to chronic dose rates less than 400 pGyh-1 (10mGyd~ ) would have ''effects, although slight, in sensitive plants but would be unlikely to have significant deleterious effects in the wider range of plants present in natural plant community''.

• Chronic dose rates below 400 pGy are unlikely to cause adverse effects in most sensitive animal species.

• Maximum dose rates of400 pGyh~ to a small proportion of the individuals were unlikely to ''have any detrimental effects at the population level'' in aquatic organisms.

With the exception of observations from the Chernobyl accident, few new data on effects of exposure to ionizing radiation are available since 1996. Much of the information on radiation levels and effects on biota observed in the region around the Chernobyl nuclear reactor has been reported by the Chernobyl forum. The main observations from the followup studies at Chernobyl can be summarized as follows:

• Irradiation from radionuclides released from the Chernobyl accident caused numerous acute adverse effects in the biota up to distances of tens of kilometers from the release point.

• The radio-ecotoxicological environmental response to the Chernobyl accident involved a complex interaction among radiation dose, dose rate, and its temporal and spatial variations, as well as the radiosensitivities of the different taxons. Both individual and population effects caused by radiation-induced cell death were observed in plants and animals as follows:

1. increased mortality of coniferous plants, soil invertebrates, and mammals;

2. reproductive losses in plants and animals;

3. chronic radiation sickness of animals (mammals, birds, etc.).

• No adverse radiation-induced effect was reported in plants and animals exposed to a cumulative dose of less than 0.3 Gy during the first month after the accident.

• Following the natural reduction of exposure levels due to radionuclide decay and migration, populations have been recovering as a result of the combined effects of reproduction and immigration.

In general terms, these observations from the Chernobyl accident appear to support the 1996 conclusions of UNSCEAR and suggest that dose rates below about 400 mGyh-1 (i.e., 0.3 Gy/30 days/24 h) are unlikely to effect results in populations of biota.

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