Issued February 21, 1980
The National Council on Radiation Protection and Measurements (NCRP) has followed closely the recent developments in the assessment of the risks of induction of leukemia and other cancers from exposure to neutrons (Rossi, 1977a: 1977b; Rossi and Mays, 1978; Bond, 1979; Roberts, 1979; Mole, 1979; Beebe and Land, 1979; Jablon, 1979). The risk estimates have been derived from only a single source, i.e., the data on the Japanese atomic bomb survivors. There seems to be reasonable agreement on the magnitude of the leukemia risk, with less certainty about the risks for other cancers. However, there is considerable diversity of opinion on how to interpret and use this information at the present stage of development.
The NCRP is actively engaged in a detailed examination of the risks associated with exposure to all forms of ionizing radiation, high-LET (linear energy transfer) as well as the more prevalent low-LET radiations. The NCRP committees currently engaged in this work are studying all relevant dosimetric, biological and epidemiological data, together with other important aspects, in order to evaluate the total risk from each kind of radiation and the influence of dose size, dose rate, LET and other variables on the magnitude of the risks. These assessments are expected to form the basis for NCRP recommendations on maximum permissible dose levels for all types of radiation for radiation workers as well as for members of the general public. This effort may result in substantial changes in the current system of radiation protection and thus be more than a simple updating of previous NCRP recommendations (NCRP, 1971). The maximum permissible dose for neutrons is thus only a part of a much larger issue which is complex and has not been finally resolved.
Although the neutron dose limits that will ultimately be recommended cannot be given at this time, they may well be less than the current ones. The NCRP has, therefore, decided to recommend that, as an interim precautionary measure, steps be taken to reduce the potential exposure of individuals who may receive neutron doses that are a substantial fraction of the current maximum permissible dose. Such steps should be based on the tentative assumption that future changes, if any, in the current values of the maximum permissible neutron doses might reduce the values by a factor of as much as three, but are unlikely to reduce them by more than a factor of 10.
In line with established principles that apply to all ionizing radiations, further general reductions in neutron exposure should be made whenever practicable.
When the neutron energy is not known, it has been customary to assume a quality factor of about 10. The current occupational maximum permissible dose equivalent is a long-term average of 5 rem per year. Using a quality factor of 10, the current occupational maximum permissible absorbed dose for neutrons is thus a long-term average of 500 mrad per year.
It should be noted that, in many instances, exposures of personnel are assessed in terms of the absorbed dose (or corresponding dose equivalent) at the body surface. In the case of neutrons in the fission energy range, the absorbed dose at the body surface is substantially larger than the absorbed dose in internal organs. For example, the surface dose is larger by a factor of three or four than the absorbed dose in the bone marrow, the organ at risk for leukemia. The corresponding factor for other cancers of internal organs is also substantial. Hence, if the neutron exposure of personnel is established in terms of the surface dose or a similar quantity, the prudent actions required may be minimal.
Beebe, G.W. and Land, C.E. (1979). Comments on “Leukemia risk from neutrons,” by Rossi, H.H. and Mays, C.W., Letter to the Editor, Health Phys. 36, 465.
Bond, V.P. (1979). “The risk from fast neutron exposure,” Health Phys. 37, 391.
Jablon, S. (1979). Comments on “Leukemia risk from neutrons,” by Rossi, H.H. and Mays, C.W., Letter to the Editor, Health Phys. 36, 205.
Mole, R.G. (1979) “RBE for carcinogenesis by fission neutrons,” Letter to the Editor, Health Phys. 36, 463.
NCRP (1971). National Council on Radiation Protection and Measurements. Basic Radiation Protection Criteria, NCRP Report No. 39 (National Council on Radiation Protection and Measurements, Washington, D.C.).
Roberts, P.B. (1979). Comments on “Leukemia risk from neutrons,” by Rossi, H.H. and Mays, C.W., Letter to the Editor, Health Phys. 37, 601.
Rossi, H.H. (1977a). “The effects of small doses of ionizing radiation: Fundamental biophysical characteristics,” Radiat. Res. 71, 1.
Rossi, H.H. (1977b). “A proposal for revision of the quality factor,” Radiat. Environ. Biophys. 14, 275.
Rossi, H.H. and Mays, C.W. (1978). “Leukemia risk from neutrons,” Health Phys. 34, 353.