NEWS AND INFORMATION
Year : 2017 | Volume
: 40 | Issue : 3 | Page : 159--160
A brief summary of ICRP publication 136: Dose coefficients for nonhuman biota environmentally exposed to radiation
Editor, RPE, Ex. Head, IDS, RSSD, BARC, Mumbai, Maharashtra, India
D D Rao
Editor, RPE, Ex. Head, IDS, RSSD, BARC, Mumbai, Maharashtra
|How to cite this article:|
Rao D D. A brief summary of ICRP publication 136: Dose coefficients for nonhuman biota environmentally exposed to radiation.Radiat Prot Environ 2017;40:159-160
|How to cite this URL:|
Rao D D. A brief summary of ICRP publication 136: Dose coefficients for nonhuman biota environmentally exposed to radiation. Radiat Prot Environ [serial online] 2017 [cited 2020 Feb 21 ];40:159-160
Available from: http://www.rpe.org.in/text.asp?2017/40/3/159/225581
The International Commission on Radiological Protection (ICRP) has released a publication for dealing with the radiation exposure of nonhuman biota. Earlier on, the philosophy was that the protection of radiation exposure to humans will adequately ensure the protection of nonhuman biota. Although it may still be applicable, over the years, protection philosophy specifically for nonhuman biota is evolved and ICRP has thus brought out this publication in continuation of their earlier publications. Even as this publication is released, ICRP still is of the opinion that the data and models on the radiation exposure to nonhuman biota are to be further strengthened. A brief summary of ICRP-136 is outlined below.
ICRP Publication 108 provided dose coefficients (DCs) for a group of reference entities (Reference Animals and Plants [RAPs]). The DCs can be used to evaluate doses and dose rates, and to compare the latter with derived consideration reference levels (DCRLs), which are bands of dose rate where some sort of detrimental effect in a particular RAP may be expected to occur following chronic, long-term radiation exposure, as outlined in Publication 124. These dosimetric models assume simple body shapes with uniform composition and density, homogeneous internal contamination, limited sets of idealized sources of external exposure to ionizing radiation for aquatic and terrestrial animals and plants, and truncated radioactive decay chains. This methodology is further developed and systematically extended in this publication, which supersedes the DC values of Publication 108. Significant methodological changes since Publication 108 include implementation of a new approach for external exposure of terrestrial animals with an extended set of environmental radioactive sources in soil and in air; considering an extended range of organisms and locations in contaminated terrain; transition to the contemporary radionuclide database of Publication 107; assessment-specific consideration of the contribution of radioactive progeny to DCs of parent radionuclides; and use of generalized allometric relationships in the estimation of biokinetic or metabolic parameter values. These methodological developments resulted in changes to previously published tables of DCs for RAPs, and revised values are provided in this publication. This publication is complemented by a new software tool called “BiotaDC,” which enables the calculation of DCs for internal and external exposures of organisms with user-defined masses, shapes, and locations in the environment and for all radionuclides in Publication 107.
The DCs are applicable to organisms with body masses in the range from 1 mg to 1000 kg, at heights above the ground surface from 0.1 to 500 m, for five types of environmental sources in soil and in ambient air. The absorbed fractions (AFs) and DCs for photons and electrons have been extended to maximum energy of 10 MeV to address radionuclide properties in the new database. This publication is supplemented by tables of DCs for RAPs.
The main purpose of the DCs is to enable evaluation of doses and dose rates so that they can be compared with the DCRLs established for the purposes of environmental protection. The DCRLs for the 12 RAPs are given (0.1–1 mGy/d for deer, rat, duck, and pine tree; 1–10 mGy/d for frog, flatfish, trout, grass, and seaweed; 10–100 mGy/d for bee, crab, and earthworm).
Radiological protection of nonhuman biota is formulated for biological endpoints at the level of populations, and thus be regarded as focusing on tissue reactions of radiation on populations, whereas human radiological protection is for the prevention of tissue reactions and also minimization of stochastic effects. Some of the important differences in dosimetric quantities between human exposure and nonhuman exposure to radiation are as follows: (a) absorbed dose is averaged over tissue/organ for human, and it is averaged over whole body for nonhuman biota; (b) radiological protection end points are stochastic for humans and deterministic/tissue reactions for nonhuman biota; (c) relevance of RBE is at low dose and dose rates for humans and at moderate and high doses for nonhuman biota; (d) radiation weighting factors are used for human exposure and no WR are recommended for nonhuman biota exposure to radiation; and (e) control level for radiation protection is based on effective dose for human and DCRLs for nonhuman biota.
For internal exposure assessment, AFs for penetrating radiation (photons and electrons) have been assessed based on systematically calculated energy-dependent AFs for simple spherical and ellipsoidal bodies surrounded by infinite water medium. The calculated photon and electron AFs were found to be smooth functions of particle energy and the mass of the organism's body; thus, AFs for other masses and energies could be easily derived by interpolation from the data provided. To facilitate interpolation of the AFs between different shapes, an analytical approximation was suggested, for arbitrary ellipsoidal bodies by means of a scaling factor.
DCs for external exposure of terrestrial organisms are based on results obtained by Monte Carlo simulation of radiation transport in terrestrial environments, where only external exposure to photons was considered. Any contributions from alpha particles and electrons to external doses to terrestrial organisms were neglected because of their short range and due to the fact that radiosensitive tissues are usually covered by inert layers (e.g., dead skin, fur, feather, shell, or bark). Several conditions of exposures in soil for RAPs are considered. They are: (a) like in the middle of a uniformly contaminated volume radionuclide source with a thickness of 50 cm, (b) on and above the ground surface due to a planar radionuclide source at a depth of 0.5 g/cm 2 in the soil, (c) on and above the ground surface due to a uniformly contaminated volume radionuclide source within soil with a thickness of 10 cm, (d) on and above the ground surface due to an infinitely deep radioactive source in soil, and (e) on and above the ground surface due to immersion in air contaminated with radioactive materials.
For example, the DCs of Cs isotope for frog are: 1.3 × 10−4 μGy/h/Bq/kg for internal exposure; 8.6 × 10−4 μGy/h/Bq/L for aquatic exposure; 8.2 × 10−4 μGy/h/Bq/kg for in soil exposure; 3.5 × 10−4 μGy/h/Bq/kg for on-ground exposure; and 4.2 × 10−4 μGy/h/Bq m −3 for immersion in air exposure.