Applicable Radiation Exposure Standards and Guides: Past and Present
According to the ''Statement of Task," the committee is "to provide information that will enable the DHHS to educate and inform members of the public, especially those likely to have been most heavily exposed at the most vulnerable ages, as to what the NCI estimates and their attendant uncertainties mean for the individual, what the risks are relative to other environmental risks, and what types of appropriate actions they should take."
In responding to this request, it would appear beneficial to review the radiation protection standards, guides, and regulations that applied during the period from 1950 to 1970 (as well as in the subsequent years), and to review and evaluate the thyroid doses that occurred during the I-131 fallout in comparison to these limits and recommendations.
II. Relevant Standards and Guides—National Council on Radiation Protection and Measurements
1. Handbook 52
In 1953, the National Council on Radiation Protection (NCRP) published Handbook 52, "Maximum Permissible Amounts of Radioisotopes in the Human Body and Maximum Permissible Concentrations in Air and Water" (NCRP 1953). Although the accompanying standards were developed primarily for application to radiation workers, because few members of the public were anticipated to be exposed to artificial sources of radiation exposure (beyond the use of radiation in the healing arts) the NCRP also stated that the permissible concentrations were "for use beyond the control area." (Table 2, page 11).
The recommended limit for I-131 in drinking water was 3 × 10-5 μCi per milliliter (Table 3, page 15). This corresponds to a daily intake limit of 6.6 × 10-2 μCi which, if consumed over an entire year, would result in an estimated dose rate to the thyroid of 0.3 rem per week, or 15 rem per year (Section E, page 12).
2. Handbook 59
In a report issued in 1954, the NCRP (1954) recommended that
… in the course of their normal activities, protective measures be taken to make sure that no minor actually receives radiation at a weekly rate higher than one-tenth the respective permissible weekly doses for the critical organs (page 78).
3. Addendum to Handbook 59
In 1957, the NCRP issued an addendum to Handbook 59. This addendum repeated the NCRP recommendation for an occupational dose rate limit of 15 rem per year, but the maximum accumulated dose was limited to 5 times the number of years beyond age 18 (NCRP, 1957, paragraph 1, page 2). This addendum also included new recommendations for limiting the genetic dose to the population (paragraph 5, page 3), and stated, in terms of internal emitters, that:
For individuals outside of the controlled area, the maximum permissible concentrations should be one-tenth of those for occupational exposures (paragraph 6, page 3).
The NCRP also recommended:
An average per capita dose limit of 0.5 rem per year for "persons outside of controlled areas, but exposed to radiation from a controlled area" (paragraph 12, page 4).
It should be noted, however, that this dose rate limit was based on the assumption that:
… the total integrated RBE dose received by radiation workers will be small in comparison with the integrated RBE dose of the whole population (paragraph 12, page 4).
In addition, for purposes of controlling potential genetic effects, it was further assumed that:
… persons outside of controlled areas, but exposed to radiation from a controlled area, constitute only a small portion of the whole population.
4. Handbook 69
In 1959, the NCRP issued Handbook 69 (NCRP 1959) which incorporated new data and methods for estimating the dose from internally deposited radionuclides. In the Handbook the NCRP stated its recommendations as follows:
The radiation or radioactive materials outside a controlled area, attributable to normal operations within the controlled area, shall be such that it is improbable that any individual will receive a dose of more than 0.5 rem in any 1 year from external radiation (Section 2.4, page 6).
The maximum permissible average body burden of radionuclides in persons outside the controlled area and attributable to the operations within the controlled area shall not exceed one-tenth that for radiation workers (Section 2.4, page 6).
At the same time, the NCRP repeated the limit on cumulative dose to radiation workers, namely, that the cumulative dose "shall not exceed 5 rems multiplied by the number of years beyond age 18."
At the same time, however, the NCRP increased the permissible dose rate to the thyroid of radiation workers to 30 rem per year (Section 2.1, pages 4-5).
With the application of the one-tenth factor, the limiting concentration of I-131 in drinking water was reduced to 2 × 10-6μCi per milliliter, as contrasted to the 3 × 10-5μCi per milliliter, as given in Handbook 52.
In support of these recommendations, the NCRP stated:
The maximum permissible dose and the maximum permissible concentrations of radionuclides as recommended … are primarily for the purpose of keeping the average dose to the whole population as low as reasonably achievable, and not because of the likelihood of specific injury to the individual.
5. Report of Ad Hoc Committee
In 1960, the NCRP published a report of an Ad Hoc Committee established "to re-examine the problem of exposure of the population to man-made radiations from the point of view of somatic effects as distinct from genetic effects" (page 482). This review was undertaken in specific response to "the widespread public concern over the possible effect of radiation from fallout on the population …" (NCRP 1960).
On the basis of its review, the NCRP stated that:
"… we recommend that the population permissible dose for man-made radiation be based on the average natural background level. Although it is not our responsibility to determine the exact level, we believe that the population permissible somatic dose for man-made radiations, excluding medical and dental sources, should not be larger than that due to natural background radiation, without a careful examination of the reasons for, and the expected benefits to society from a larger dose" (NCRP, 1960, page 485). Since the NCRP, in its report, assumed that the background dose rate was 100 mrem per year (NCRP, 1960, page 485),
this statement implied that the dose rate limit that they were recommending for the general population was 100 mrem (0.1 rem) per year.
III. Relevant Standards and Guides—Federal Radiation Council
In 1959, the Congress (Public Law 86-373) established the Federal Radiation Council (FRC) to "… advise the President with respect to radiation matters, directly or indirectly affecting health, including guidance for all Federal agencies in the formulation of radiation standards and in the establishment and execution of programs of cooperation with States…". The formation of the FRC was an outgrowth of public hearings held in 1957 by the Joint Committee on Atomic Energy, on the "The Nature of Radioactive Fallout and Its Effects on Man." In all of these hearings, questions on the biological effects of radiation and protection against excessive exposure to radiation received attention.
1. FRC Report No. 1
In 1960, the FRC issued Report No. 1 (FRC 1960) in which they established the concept of a Radiation Protection Guide (RPG) which was defined as:
… it is our basic recommendation that the yearly radiation exposure to the whole body of individuals in the general population (exclusive of natural background and the deliberate exposure of patients by practitioners of the healing arts) should not exceed 0.5 rem (paragraph 5.3, page 26).
Because of continuing concern about the potential genetic effects, the FRC went on to state:
When the size of the population group under consideration is sufficiently large, consideration should be given to the genetically significant population dose. The Federal Radiation Council endorses in principle the recommendations of such groups as the NAS-NRC, the NCRP, and the ICRP concerning population genetic dose, and recommends the use of the Radiation Protection Guide of 5 rem in 30 years (exclusive of natural background and the purposeful exposure of patients by practitioners of the healing arts) for limiting the average genetically significant exposure of the total U. S. population. The use of 0.17 rem per year … is likely in the immediate future to assure that the gonadal exposure Guide is not exceeded (paragraph 5.5, page 27).
2. FRC Report No. 2
In 1961, the FRC issued Report No. 2 (FRC 1961) in which they recommended:
… RPG's for the thyroid gland of 1.5 rem per year for individuals and 0.5 rem per year to be applied to the average of suitable samples of an exposed group in the general population as representing a reasonable balance between biological risk and benefit to be derived from useful applications of radiation and atomic energy (paragraph 2.9, page 11).
3. FRC Report No. 5
Confirming that their previous recommendations applied to "normal peacetime operations," in 1964 the FRC issued Report No. 5 (FRC 1964) in which they specifically addressed the concept of Protective Action Guides (PAGs) for application in the case of unusual concentrations of radionuclides in various foods due to the atmospheric testing of nuclear weapons (Introduction, page 1).
At that time, the FRC noted that a report of a panel of experts of the NAS-NRC Committees stated that:
… although therapeutic doses from iodine-131 to the thyroid have been in the range of a few thousands rads upwards, iodine-131 has not been identified in a causative way with the development of thyroid cancer in humans, except in one doubtful case. X-ray doses to the thyroid appear to be from 5 to 15 times as effective in producing biological changes as iodine-131 (Section IV, page 12).
On the basis of this and other information, the FRC stated:
Considering existing information on the biological risks associated with doses from iodine-131 and the kinds of protective actions available to avert the dose from iodine-131 that has been deposited on pastures by dairy cows, the Council has concluded that such protective action as the diversion of milk or the substitution of stored feed for pasturage to avert individual doses less than 30 rads would not usually be justifiable under the conditions considered most likely to occur. This dose is recommended as the Protective Action Guide for iodine-131 (Section IV, page 12).
The FRC went on to say:
… it is assumed that the majority of the individuals do not vary from the average by a factor greater than three. As an operational technique, it is considered that the PAG will not be exceeded if the average projected doses to the thyroids of a suitable sample of the population do not exceed 10 rads. A suitable sample is considered to consist of children of approximately one year of age using milk from a reasonably homogeneous supply" (Section IV, page 12). "The PAG is stated in terms of the projected dose; i.e., the dose that might otherwise be received if the protective action were not initiated (Section IV, page 13).
In terms of the need for special consideration for children, the FRC stated that for I-131:
… a given intake would result in a ten times larger dose to the thyroid of a one year old child (thyroid weight 2 grams) than to an adult (thyroid weight 20 grams) (Section IV, page 11).
In terms of operating criteria, the FRC stated:
A total intake of iodine-131 of 600 nanocuries would result in a dose to about 10 rads to a 2 gram thyroid (Section IV, page 14).
IV. Current Standards and Guides—International Commission on Radiological Protection, National Council on Radiation Protection and Measurements, and the U. S. Nuclear Regulatory Commission
Although they would not have applied during the period from 1950 to 1970, it is useful also to consider the current recommendations and/or regulations of the International Commission on Radiological Protection (ICRP), the NCRP, and the U. S. Nuclear Regulatory Commission (U. S. NRC) relative to dose rate limits for members of the public.
1. ICRP Publication 60
In Publication 60, the ICRP (ICRP 1991) recommends that:
… the limit for public exposure should be expressed as an effective dose of 1 mSv in a year. However, in special circumstances, a higher effective dose could be allowed in a single year, provided that the average over 5 years does not exceed 1 mSv per year (paragraph S40, page 75).
2. NCRP Report No. 116
In Report No. 116, the NCRP (NCRP 1993) recommends:
For continuous (or frequent) exposure, it is recommended that the annual effective dose not exceed 1 mSv." "Furthermore, a maximum annual effective dose limit of 5 mSv is recommended to provide for infrequent annual exposures. An annual effective dose limit recommendation of 5 mSv is made because annual exposures in excess of the 1 mSv recommendation, usually to a small group of people, need not be regarded as especially hazardous, provided it does not occur often to the same groups and that the average exposure to individuals in these groups does not exceed an average annual effective dose of about 1 mSv (Section 15, page 46).
3. U. S. Nuclear Regulatory Commission
In its "Standards for Protection Against Radiation" (Title 10, Code of Federal Regulations, Part 20), issued in 1991, the U. S. NRC has stipulated:
Each licensee shall conduct operations so that — (1) The total effective dose equivalent to individual members of the public from the licensed operation does not exceed 0.1 rem (1 millisievert) in a year, exclusive of the dose contributions from background radiation, any medical administration the individual has received, voluntary participation in medical research programs, and the licensee's disposal of radioactive materials into sanitary sewerage … (paragraph 20.1301).
In summary, all three organizations recommend the restriction of average long-term effective (whole body) dose rates to the public to no more than 1 mSv (0.1 rem) per year.
4. U. S. Environmental Protection Agency and Food and Drug Administration
The U. S. Environmental Protection Agency and the Food and Drug Administration in their guides to emergency and protective action insofar as dose to the thyroid is concerned set the standards for action as 5-25 rad and 1.5-15 rad, respectively. Their ingestion protective action guides are set at 1.5 rad and 15 rad, the former being the preventive action guide and the latter the emergency action guide. At thyroid doses of 25 rad or greater administration of stable iodine is recommended.
FRC (Federal Radiation Council). 1960. Background Material for the Development of Radiation Protection Standards. Report No. 1. Washington, DC.
FRC (Federal Radiation Council). 1961. Background Material for the Development of Radiation Protection Standards. Report No. 2. Washington, DC.
FRC (Federal Radiation Council). 1964. Background Material for the Development of Radiation Protection Standards. Report No. 5. Washington DC.
ICRP (International Commission on Radiological Protection). 1991. 1990 recommendations of the International Commission on Radiological Protection. ICRP Publication 60, Annals of the ICRP 21(1-3). Oxford: Pergamon Press.
NCRP (National Council on Radiation Protection and Measurements). 1953. Maximum Permissible Amounts of Radioisotopes in the Human Body and Maximum Permissible Concentrations in Air and Water. Handbook 52. Washington, DC: National Bureau of Standards.
NCRP (National Council on Radiation Protection and Measurements). 1954. Permissible Dose from External Sources of Ionizing Radiation. Handbook 59. Washington, DC: National Bureau of Standards.
NCRP (National Council on Radiation Protection and Measurements). 1959. Maximum Permissible Body Burdens and Maximum Permissible Concentrations of Radionuclides in Air and Water for Occupational Exposure. Handbook 69. Washington, DC: National Bureau of Standards.
NCRP 1960. Somatic dose for the general population. Report of an Ad Hoc Committee. Science 131:482-486.
NCRP (National Council on Radiation Protection and Measurements). 1993. Limitation of Exposure to Ionizing Radiation. NCRP Report No. 116. Bethesda, MD: National Council on Radiation Protection and Measurements.