DOSE LIMITS AND THEIR APPLICATIONS
This committee was charged to review and comment on the applicability of the ICRP recommendations on annual dose limits for general populations as they pertain to anticipated dose commitments after resettlement in contaminated areas of the Marshall Islands.
The question of applicability is somewhat academic, inasmuch as radiological criteria for the decision on resettlement have been agreed on and are presented in the MOU (Article V, paragraph 1). These criteria are that no person resettling on the southern islands of Rongelap Atoll and subsisting on a local-food-only diet would receive a calculated annual effective radiation dose exceeding 100 mrem above natural background or would be incidentally exposed to concentrations of transuranics in the soil in excess of the prescribed action level of 17 pCi/g. It should be noted that the committee had serious reservations regarding the MOU provision relating to a dose limit, or action level, based on the maximally exposed individual. Radiation standards and the techniques used in radiation dosimetry do not lend themselves to the determination of dose to the maximally exposed individual. This is particularly true for a population of limited size where the statistical power for determining doses at the extremes of the dose distributions is limited. The committee was informed by DOE representatives, however, that this limit was to be used only as a guideline for resettlement decisions and that, as an internationally negotiated parameter, we were to consider it as a fixed parameter.
The action level of 17 pCi/g for plutonium soil concentrations is based on a U. S. Environmental Protection Agency action level of 0.2 μCi/m2 for public protection from accidental contamination of the environment. The EPA action level was developed for application to recent contamination, thus all of the radioactive contamination was considered to be concentrated within the top 1 cm of the soil. With this approach, soil concentrations less then the action level would warrant no further action. The MOU action limit of 17 pCi/g was derived by DOE from this 0.2 μCi/m2 EPA guideline using a soil density of 1.2 g/cm3 appropriate to Rongelap and assuming that the radioactivity was in the top 1 cm of soil. Because the contamination on Rongelap took place nearly 40 years ago the initial concentration of radionuclides has leached into the soil column and no longer resides on the surface. In addition the vegetation cycle has produced approximately 1 cm of fresh organic material over the last three decades. Taking these considerations into account, the MOU considers the potential for subsequent radionuclide migration in the soil by providing provisions for averaging of the 17 pCi/g radionuclide concentration over the top 5 cm of soil.
The annual dose limit recommended for members of the public by ICRP is not intended to be directly applicable to decisions on when to return to an area that has been evacuated because of radiological concerns raised by potential or actual radiation contamination. In the latter circumstances, ICRP recommends that the decision to return to a previously evacuated area is justified when being back is more beneficial to the people involved than remaining away. The assessment of which is more beneficial must take into account all the factors that
influence health and well-being. A population might expect to achieve the greatest net benefit by appropriate allocation of whatever resources it has available to it within the context of all the factors that affect its health and well-being. It follows that in any specific situation brought about by intervention, a decision by a displaced population needs to be made on the basis of factors that have the greatest influence on them. There is no reason to expect that the magnitude of any particular factor (for example, residual contamination) on which a decision to return is based will be the same from case to case. Each population's situation will involve different tradeoffs.
ICRP recognizes that numerical criteria—action levels—are often helpful in making decisions; in this instance, such criteria have been defined in the MOU. Implicit in the wording of the MOU is that the parties to it believed that a net benefit would accrue to the displaced population when there could be assurance that, on their return to Rongelap, the maximum annual dose received by any person and the concentration of transuranics everywhere in soil had naturally decreased or had been reduced to stated action levels. With those conditions met, resettlement would be justified.
The MOU is not clear on how the specified numerical dose and concentration criteria are to be applied, i.e., how does one define the maximally exposed person? For the individual dose, the general approach in intervention is to apply the criteria to the group involved as a whole, if it is small and is reasonably homogeneous, or to a definable subset of the group whose circumstances and habits are reasonably similar and typical of individuals who could be regarded as those maximally exposed. The latter is the concept of the "critical group." Various combinations of habits and environmental factors are usually examined in deciding the characteristics of the individuals in the critical group. For the returning Rongelap community, such a subset could be a group that has a particular diet, some defined fraction of which is locally grown food. For applying the criterion on soil concentration, another subset of the population could be a group of children whose time is spent playing in particular areas and whose daily soil ingestion could be defined. Such definition of the group of maximally exposed children provides the basis for interpreting the numerical criterion for the concentration of transuranics in soil. The area and depth of soil over which the concentration of transuranics should be averaged can be determined by considering the habits and, in effect, sampling such a group of children.
Those are examples of the scenario approach discussed in the section on diet models. The alternative is to take the statistical or probabilistic approach and assess the probability that a given fraction of the population might receive a maximum annual dose above a given value. The probability that a given concentration in soil, when averaged over a given area, will be exceeded can also be estimated.