Institute for Energy and Environmental Research
Science for Democratic Action Vol. 4 No. 4
The principal basis for radiation protection until now has been to set limits on the maximum allowable exposures to individuals from man-made sources. For example, the overall individual dose limit for the general population from all sources of radiation (other than medical) is 100 millirem per year. The limit for exposure due to emissions from specific facilities is generally in the range of 5 to 25 millirem per year.
Setting limits on general population exposure is much more difficult due to the logistics involved in measuring doses to all individuals. In addition, the large number of sources of radiation, both natural and artificial, make it very difficult to pin down exposure to any particular source, unless it were large relative to all the others. Therefore, radiation protection of the general population, while aimed at limiting doses, is, in practice, often based on two key concepts: limiting the total releases of individual radionuclides (or groups of radionuclides); and limiting the concentrations of radionuclides in air, water or soil. Both of these concepts are incorporated into current standards for high-level waste repositories, codified in federal regulations 40 CFR 191.
The first practice, limiting the total releases of individual radionuclides (or groups of radionuclides), limits the dose received by the total population. The second practice, limiting the concentrations of radionuclides in air, water or soil, limits the dose to the maximally exposed individual. If this individual is not exposed over a certain limit, then it follows that essentially all of the rest of the population would be exposed to health risks lower than those created by the upper limit of exposure. The "maximally exposed individual" is a hypothetical construct, corresponding to a set of "reasonable" assumptions about human needs and activities. People who may be unusually sensitive to radiation or who have unusual habits are not used for standard setting. For example, a British inquiry omitted people who subsisted mainly on clams from its definition of the affected population because this diet was considered unusual. (4)
When the main route of exposure over long time periods is expected to be via the use of water for drinking and subsistence farming, it is the general practice to use the "subsistence farmer scenario" for calculating exposure. This approach assumes that a person would unknowingly use contaminated water for drinking and farming and would grow all their own food.
For the purposes of calculating radiation dose, a small, homogeneous group of individuals is used to define a "critical group." The International Commission on Radiological Protection (ICRP) explicitly states that the critical group "represents an extreme" of radiation exposures within the entire population in the area in order "to ensure that no individual doses are unacceptably high."(5) (emphasis added) It recommends that critical groups be small so that they are homogenous, with the upper limit to size usually being a "up to a few tens of persons" and they could be as small as only one person. (6)
The device of a small critical group is used to represent the maximally exposed individual for regulatory purposes. Once the exposure scenario for the maximally exposed individual is selected, then it is possible to derive secondary standards for limiting concentrations of radionuclides in air, water, and soil. These secondary standards, if adhered to, would result in compliance with the primary dose standard.
Since it is difficult or impossible to measure radiation doses and risks to the general population from particular radiation facilities, secondary standards that limit concentrations of radionuclides are essential to ensuring compliance with dose or risk limits. Setting secondary standards to protect the general population from radioactive contaminants is used throughout the world in radiological protection, including in the United States.
Institute for Energy and Environmental Research
Comments to Outreach Coordinator, Pat Ortmeyer: ieer@ieer.org
Takoma Park, Maryland, USA
Revised March 21, 1996