2018-11-20
The draft Radiation Weighting for Reference Animals and Plants is now available for public consultation on our website. We welcome comments from individuals and organisations. The draft document can be downloaded from the ICRP website. Comments must be submitted through the ICRP website no later than March 1, 2019.
Questions and inquiries may be directed to Kelsey Cloutier, Development and communications Manager for ICRP.
It has long been recognised that the degree of biological 40 impact on an organism resulting from a given absorbed dose (in gray, Gy) of ionising radiation can vary depending upon the type of radiation involved. This difference has been experimentally quantified and reported as Relative Biological Effectiveness (RBE) of specific radiation types. RBE values are experimentally determined and are the ratio of doses of a test radiation and a low-LET reference radiation that produce the same level of observed effect. RBE values have been measured for a variety of end points in in vitro experiments that include human and animal cell lines, as well as in in vivo experiments with animals. Such studies have shown that the magnitude of a biological effect depends not only on dose and the type and energy of the radiation delivering the dose, but also on the rate at which the dose is delivered and, most importantly, the endpoint under study. The need to apply this knowledge to radiological protection of humans has led to an aggregation and analysis of RBE data to provide ‘radiation weighting factors’, and to the radiation protection quantity ‘equivalent dose’ (in sievert, Sv) where the absorbed dose is multiplied with the radiation weighting factor appropriate for the type of radiation considered. Whereas protection of humans has focused on avoiding tissue reactions (deterministic) and limiting stochastic (cancer/heritable) effects, protection of biota has largely focused on endpoints relevant to population viability. The present report reviews RBE data relevant to biota for one low energy beta emitter (tritium) and for alpha-emitting radionuclides. For tritium, values obtained centre around 1.5 – 2 compared with x-rays and 2 – 2.5 compared with gamma rays; values for alpha particles are generally substantially higher. It is proposed that for protection purposes, that an RBE weighted absorbed dose be used, with a RBE weighting for biota of 1 for all low-LET radiations and 10 for alpha particles, should be used for relevant RAPs, Use of a single value of 1 for all low-LET radiations is consistent with the approach taken to protection of humans. However, if exposures to tritium beta particles or other low-energy, low-LET radiations are estimated to be within or close to the Derived Consideration Reference Level (DCRL), assessment of the use of higher RBE values may be warranted.