Issued September 18, 1984
The National Council on Radiation Protection and Measurements (NCRP) has considered the problems raised by the Congressional requirement that the Environmental Protection Agency (EPA) develop standards for radionuclides as part of the National Emission standards for Hazardous Air Pollutants. The EPA has proposed rules under 40 CFR Part 61 and the NCRP President, with the advice of an ad hoc group of Council members, has commented on these proposals by correspondence and during EPA and Congressional hearings. The Council considers it desirable at this time to present positive recommendations based on published Council reports and current work in progress. The NCRP Scientific Committee 1 on Basic Radiation Protection Criteria has drafted a report defining the relevant recommendations of the Council. While this draft is still unpublished, some of the pertinent numerical values are included in NCRP Report No. 77, Exposures from the Uranium Series with Emphasis on Radon and its Daughters. These are detailed here.
- The limit of 500 mrem whole body dose equivalent in a year, not including medical and natural background radiation, is still recommended for individuals in the population when the exposure is not continuous. As a corollary, the NCRP advises remedial action, where possible, when the external whole body dose equivalent exceeds 500 mrem/year from all environmental sources, including natural background.
- The recommended limit for continuous exposure of an individual in the population to external radiation is 100 mrem/year whole body dose equivalent, not including exposure from natural background and medical procedures. A dose equivalent rate of 100 mrem/year is considered to be associated with a lifetime risk of developing cancer of about one in a thousand.
- These recommendations on limits are only part of a total system of dose limitation which must also include justification and considerations of ALARA (as low as reasonably achievable).
While the NCRP has in the past specifically declined to introduce a sub-set of limits, it is sympathetic to the needs of regulatory bodies who must control individual sources of radiation exposure. In particular, it is necessary to consider the situation where a member of the public may be exposed to radiation from more than one of the controlled sources. In looking at the possibility of multiple exposures, it seems that large installations which could cause exposures that are a significant fraction of the 100 mrem/year limit are unlikely to be geographically located in such a manner that the sum of the exposures from two sources would outweigh the exposures to individuals closer to either of the separate sources. At the other end of the scale, small installations that may be more closely spaced should produce only relatively small exposures, so that even the sum of their exposures would not approach the 100 mrem/year limit for continued exposure. The Council (NCRP) appreciates, however, that a regulatory agency charged with protection of the public may consider it necessary to regulate individual sources in order to assure that no individual receives a continuous radiation dose above the 100 mrem/year recommended limit. Thus, whenever the potential exists for an individual to exceed 25 percent of the limit, for whole-body dose equivalent from any single site, the site operator should be required to assure that the exposure of the maximally exposed individual from all sources would not exceed 100 mrem/year on a continuous basis. This recommendation of the NCRP concerns whole-body irradiation but the Council has also considered the situation for the exposure of individual organs, such as lung or bone. Dose limits for individual organs will necessarily be higher than that for the whole body in the inverse ratio of the risk for a particular organ to the total risk for whole body exposure. Radiation doses at the limits considered are not readily measured for continuous external whole-body exposure and such doses cannot be measured directly for internal emitters. Hence, it has been customary to use mathematical models to relate release quantities and the consequent doses to individual in the public. This will still be necessary, but the NCRP recommends that implementation of standards for air emission use models that are realistic, thoroughly documented and capable of validation. While the internal doses are usually estimated rather than measured, validating measurements can be made at steps in the environmental chain of exposure that are closer to the receptor than the releases. The need for realistic models is obvious; for example, a calculated dose that is in error by a factor of five in either direction can either misjudge the risk from exposure by a comparable factor, or increase the cost of compliance. This subject is treated more fully in the recently released NCRP Report No. 76, Radiological Assessment: Predicting the Transport, Bioaccumulation and Intake by Man of Radionuclides Released to the Environment.