Abstract
It is often said that radiological protection is a practical science and I quite agree. There are many cases where decisions must be made even if the scientific knowledge is imperfect. In such cases, the decision should be made using the best scientific knowledge available at the time. Effective dose is a typical example of a useful concept for such decisions in radiological protection as a practical science. The biological effect of exposure to radiation below 100 mSv is, as well known, not thoroughly elucidated; nevertheless, radiological protection needs to be implemented appropriately for the safe use of radiation which provides huge benefits to human lives via various applications such as nuclear energy and medical use.
The concept of effective dose was invented more than 40 years ago by Professor Wolfgang Jacobi (Jacobi, 1975), and since its adoption by the International Commission on Radiological Protection (ICRP, 1991) has played an essential role in radiological protection as the basic protection quantity. I was lucky to get the chance to meet Professor Jacobi while attending his 60th birthday party which was held in 1988 at the Gesellschaft für Strahlenforschung (GSF) (now Helmholtz Zentrum München). At that time, Professor Jacobi was serving as the director of one of the research institutes of GSF. He was so kind to smile and talk to the stranger who joined the party by chance while studying at GSF for a single year. During the party, a scientific journal with articles dedicated to him and his work was presented to him as a gift. It was quite an inspiring moment, and I decided to continue working in the field of radiological protection.
I remember, after returning to Japan, arguments with my colleagues on how effective dose is useful in radiological protection. I was in favour of using effective dose, but could not argue sufficiently about its merits at that time. The fact that effective dose has been used for more than 40 years has proved its usefulness.
Dose coefficients for external exposures in the environment have been studied for a long time. The first comprehensive studies on this subject were those by Dillman (1974), and Poston and Snyder (1974). Since then, many researchers, myself included, have tackled this subject, mainly using computer simulations, and several articles have been published. Before effective dose was established, dose coefficients were calculated for the whole body and specific organs. After the invention of effective dose, dose coefficients were estimated mainly in terms of effective dose, and have been an essential input for dose evaluation in the environment.
Exposures in the environment have some features different from those in workplaces. Radiation fields have specific characteristics (energy spectrum, angular distribution, height dependency, etc.) depending on the source distribution. An important characteristic of exposure in the environment is that the incident fluence is approximated to be symmetrical around the body axis. Extremely biased irradiation such as anterior–posterior geometry does not occur in the environment. This suggests that ambient dose equivalent may not be the best approximation of effective dose, making dose coefficients specifically for exposures in the environment necessary.
Further, people from a wide range of ages are exposed in the environment, including infants and children. In many cases, especially after large-scale accidents, concerns arise regarding exposures of children and their protection within the existing system of radiological protection. Therefore, it is desirable to evaluate dose coefficients for different age groups. Age-dependent effective dose evaluation was performed, for example, by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 2000, 2013) for evaluation of exposure of the public to natural sources as well as to radiation released as a result of the accidents at the nuclear power plants (NPPs) in Chernobyl in 1986 and Fukushima in 2011.
The Fukushima Daiichi NPP accident highlighted the importance of appropriate dose evaluation after a large-scale accident. The standards for radiological protection actions were determined partially on simple assumptions which resulted in not-insignificant overestimation of dose. One of the reasons for the overestimation was the ‘not always correct’ assumption that ambient dose equivalent could approximate effective dose with acceptable uncertainty. It was then recognised that more realistic dose coefficients should be used for environmental dose evaluation. Particularly when the number of exposed inhabitants is large or the contaminated areas are wide, the accuracy of dose evaluation can have a significant effect on the appropriateness of decision-making associated with various social and economic problems.
This is the first time that ICRP has published dose coefficients directly applicable to external exposure of the public in the environment. Based on the knowledge accumulated during ICRP's long history, the most appropriate models and simulation methods were developed and used. In the environment, exposures are too diverse and proper simulation of all of them is not possible; therefore, some typical, idealised exposure conditions were assumed. It is believed that the dose coefficients developed for these conditions could be applied for the most important exposures in normal and emergency conditions. The dose coefficients were compiled for five different ages from newborn to 15 years, in addition to adults. Generally, exposures from photons are important in the environment since sources are distributed over wide areas, and the emitted photons could reach the human body from a distance. Electrons emitted from radionuclides in the environment may give non-trivial exposures to the skin and breast, especially in the early stages of an accident. Therefore, the contribution of electrons was also taken into account in the computation of dose coefficients by considering the electron fields.
For accurate dose evaluation in the environment, there are several important challenges to overcome, especially after a large accident. The essential factor is, of course, accurate information on the radiation levels in the environment. The radionuclide deposition density and the associated ambient dose equivalent rate in the contaminated environment tend to vary significantly with location, even in a small area. The absolute activity concentration and variation of radiation fields need to be characterised accurately. The living habits of the population is another important factor for dose estimation. For example, according to detailed investigations in the Fukushima area, it turned out that the public spent more time indoors than had been assumed. Furthermore, the dose reduction effect by buildings needs to be estimated properly.
Besides these crucial issues, dose coefficients are essential for accurate dose evaluation in the environment. The dose coefficients compiled in this publication are the fruit of efforts by a number of researchers who have worked on dose evaluations in the environment and have contributed directly or indirectly to this report. The members who engaged in the development of this publication would like to express their hearty thanks to all of them, and hope that the coefficients will be utilised in dose evaluation and contribute to proper radiological protection in the environment.