Recommendations on Dose Coefficients for Assessing Doses From Accidental Radionuclide Releases to the Environment

Dosimetric Quantities

In 1990, the International Commission on Radiological Protection issued recommendations on basic radiation protection principles in ICRP Publication 60 (1991), replacing its previous guidance (ICRP 1977). Changes made in the new guidance to the terminology and formulation for assessing the health detriment from radiation exposure have a direct impact on the derivation of dose coefficients used in radiological protection and dose assessment. The recommendations of this report are based on the definitions and terminology of ICRP Publication 60.

The fundamental dosimetric quantity is the absorbed dose, D, averaged over a tissue or organ. The probability of radiation effects depends not only on the absorbed dose, but also on the type and energy of radiation incident on the body, and the susceptibility to harm of the tissues and organs receiving the dose. Previously, the absorbed dose to all defined organs, weighted for the quality of the radiation and the sensitivity of the exposed tissue was referred to as the effective dose equivalent, H_E (ICRP 1977). Weighting factors representing the relative sensitivity of the irradiated tissue or organ were defined for six principal organ systems, and methods were provided to rank organs not explicitly assigned a value. The radiation quality factors and weighting factors for tissues and organs used to define effective dose equivalent characterised the health detriment in terms of the risk of fatal cancers and hereditary defects in the first two generations.

In its latest recommendations, the ICRP has changed the manner in which the health detriment arising from the irradiation of different organs and tissues by different types and energies of radiation is to be determined. Radiation weighting factors and tissue weighting factors have been defined. The product of the radiation weighting factor (for radiation incident on the body) and the absorbed dose in a given organ or tissue gives the equivalent dose for that organ or tissue. Multiplying this equivalent dose by the corresponding tissue weighting factor and summing the products for all organs or tissues gives the effective dose, E. The unit of absorbed dose is the gray (Gy), while for both the equivalent dose and the effective dose, it is the sievert (Sv).

Although the effective dose is, in principle, similar to the effective dose equivalent, the selection of the organs contributing to this risk-weighted quantity and their weighting factors have been changed. The ICRP now defines the human body in terms of 12 designated tissues and organs, and the remainder, consisting of 10 additional tissues and organs. Revised tissue weighting factors characterise the health detriment by a weighting of the risks for both fatal and non-fatal cancers, the risk of hereditary defects over all future generations, and the relative loss of life expectancy, given a fatal cancer or severe inherited disorder. They have been selected by the ICRP for a reference population of equal numbers of both sexes over a wide range of ages. Minor revisions to the specification of the remainder were made more recently in ICRP Publications 69 and 71 (1995a, 1995b).

For intakes of radionuclides, the ICRP has defined the committed effective dose as the sum of the tissue-weighted time integrals of the equivalent dose rates that will be received in the tissues and organs of an individual following intake. The period over which the dose is integrated is taken to be 50 years for workers, and up to age 70 years for members of the public. This quantity is an indirect measure of the potential total health detriment to an individual and offspring resulting from the intake of radioactive material.

The ICRP has published values of committed effective dose per unit intake of a radionuclide that incorporate the new risk formulations for the assessment of health detriment. These values are referred to as dose coefficients by the ICRP. Compendiums of dose coefficients for internal exposures resulting from inhalation and ingestion have been made available in ICRP Publications 67 - 69, 71, and 72 (1994a, 1995, 1995a, 1995b, 1996). Many of these values have been derived using the latest ICRP age-dependent model for the respiratory tract, and the gastro-intestinal tract model from ICRP Publication 30 (1979); those derived using older biokinetic models have taken account of variations in body size. Internal dose coefficients are available for both occupational and public exposure scenarios.

For external exposures, the impact of the new ICRP risk formulation on effective dose due to photon irradiation has been investigated by Zankl et al (1992). They found that, for the irradiation geometries studied, the changes in tissue weighting factors result in effective doses that are lower than the corresponding effective dose equivalents for photon energies above 15 keV. Many tabulations of external dose coefficients based on the previous ICRP (1977) definition of effective dose equivalent are available, such as the U.S. EPA's Federal Guidance Report No. 12 (Eckerman and Ryman 1993); however, these findings show that values based on the new definitions should be used for dose assessment purposes.

The ICRP has recently published conversion coefficients for use in protection against external exposures based on a review of available data (ICRP 1996a). While reflecting the latest ICRP recommendations, they are primarily applicable to radiation fields found in occupational settings, rather than those arising from radionuclides distributed in the environment. As a result, the Working Group concentrated on the review of published compilations of dose coefficients for cloudshine and groundshine exposures that incorporated the current ICRP risk formulations.