Notes
| The Commission now considered
that the genetic risks of occupational exposure under normal working conditions
(hereafter referred to as practices), although small compared to other
hazards of life, were nevertheless not negligible. Implicit in the
application of permissible doses had been that unacceptable bodily injury
would not occur from radiation during the lifetime of the individual, even
though that person might be exposed at the annual limit for a working life-time.
However,
it was stressed that the recommended doses were intended to be maximum
and that all doses should be kept as low as practicable and that any unnecessary
exposure should be avoided. The objective was to limit the radiation
dose to that which involved a risk that was acceptable.
In recognising the radiosensitivity of the fetus, the Commission reconsidered past advice by recommending that female workers in medical radiology and industry of reproductive age should be exposed only under conditions where the exposure of the abdomen was limited to 1.3 rems in a 13-week period. Under these circumstances, the dose to the embryo during the 8 weeks post-conception period of organ development would not be expected to be more than 1 rem. The Commission considered such a dose to be associated with an acceptable risk to the embryo. However, because of the biological evidence of early embryonic radiosensitivity, all radiological examinations involving the lower abdomen and pelvis of women of reproductive age that were not essential should be limited to the 10 days following the onset of menstruation, (suite)
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that is the period before ovulation when pregnancy
was considered to be improbable (the "ten-day rule"). It was assumed that
maldevelopment of the embryo could occur soon in early embryogenesis.
This was subsequently considered to be over-cautious since maldevelopment
does not begin to occur in the presence of teratogens until about 4 weeks
after the last menstruation, and that it continues into the fetal period
for the next 4 to 5 months. Nevertheless, the risk of the early embryonic
cells becoming mutated and carrying a potential risk of cancer in the child
could not be entirely ruled out (and this is still the current position).
With regard to incorporated radionuclides, one or more short exposures to radioactive materials within a period of 13 consecutive weeks was considered acceptable, provided the tissue or organ receiving the highest dose resulting from the intake during the following 50 years did not exceed 3 rems for the whole body, 8 rems for the skin, thyroid and the gonads and 4 rems for other organs, allocated to the year of intake. Here was the concept of the annual limit on intake. The general principles regarding working conditions were updated with particular reference to managerial responsibility, controlled areas, personnel and installation monitoring, and categories of exposure. With regard to health surveillance of radiation workers, pre-employment and routine medical examinations were recommended to continue, but the use of frequent blood counting as an index of exposure were considered of less priority than in previous recommendations. p.35
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| The committee structure had by
1962 been reorganised to provide expert reviews on radiation effects, internal
and external exposure, and application of the recommendations.
The biological principles underlying protection were re-stated in the 1965 recommendations, namely that acute effects of radiation should be avoided and the incidence of radiation-induced cancer and severe hereditary diseases in the offspring of irradiated parents should be limited to an acceptable level. Late somatic damage was classified with more confidence, supported by clinical, epidemiological (the Life Span Study), and experimental data. The dose-effect relationships for leukaemia and cancer at doses below 100 rads remained equivocal, but the Commission saw no practical alternative to assuming that for purposes of radiological protection, a linear relationship between dose and effect without a threshold existed at low doses. There was some experimental evidence of life-lengthening in animals exposed to low doses but the overwhelming experimental evidence pointed to life-shortening as a result of radiation-induced tumours. The Commission debated the concept of acceptable dose which depended on what was considered to be an acceptable risk. While unhappy with the term "maximum permissible dose" for exposure of workers, they retained it, but recommended that limitation of exposures from controllable sources should be set at a sufficiently low level such that any further reduction in risk would not be considered to justify the effort required to accomplish it. As a guideline for occupational exposure, the hazards from radiation should not exceed those that were accepted in most other industrial and scientific occupations with a high standard of safety. The Commission now believed that a period of 1 year was the most reasonable length of time over which to assess accumulated exposures. (suite)
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On this basis, the maximum permissible doses
for individual workers were 5 rems in a year for uniform irradiation of
the body, gonads or red bone marrow; 30 rems in a year for skin, thyroid
and bone; 75 rems in a year for the hands, forearms, feet and ankles; and
15 rems in a year for all other organs. In special circumstances
involving irradiation of the gonads and red bone marrow, quarterly assessments
of dose could be justified, provided that the total dose accumulated at
any age over 18 years did not exceed 5 (N -18) rems. Special
cases to be considered were workers under 18 years of age, women of reproductive
age and exposure of pregnant women.
For exposures of individual members of the public, it was proposed that the dose limit for members of the public should be one-tenth of that the corresponding maximum permissible dose recommended for workers. This was an arbitrary decision without sound radiobiological data, but it was conjectured that children could be at greater risk than adults and that they would be exposed during the whole of their lifetime. For members of the public, the risks should be less than or equal to other risks regularly accepted in everyday life. The mean dose to a critical groups - that is, a small identifiable group where individuals are expected to receive the highest dose and who realistically represent the age, size, metabolism and customs of other individuals in the population - was considered to be applicable to recommending the appropriate dose limit for members of the public where actual doses are received. When considering whole populations, the number of persons exposed as well as the doses to individuals becomes important, so that the collective population burden was the parameter to consider at low doses. p.36
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| For workers, it was convenient
to define two conditions of exposure in practices, namely conditions such
that the resulting doses might exceed three-tenths of the maximum permissible
dose, and conditions where this three-tenths level was most unlikely to
be exceeded. Three-tenths of the dose was considered to be
a reference level to be used in the organisation of protection.
For those exceeding the three-tenths level, it was recommended that individual
workers should be subject to special health supervision and personnel monitoring.
Below this level, monitoring of the working environment was considered
sufficient. It was recognised that in certain circumstances, radiation
workers could be exposed beyond the maximum permissible doses recommended
for practices, for example, in the case of malpractice or in emergencies
such as the rescuing of accidentally exposed workers..
Exposure of the population from uncontrolled sources was considered a more complex problem, because social as well as the radiological consequences needed to be taken into account. Basic actions required an attempt to reduce radiations from the original source of exposure, with remedial actions where practicable to reduce environmental levels of contamination. (suite)
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The Commission felt unable at the time to recommend
action levels, but considered that an acute exposure of individuals to
a dose in excess of 100 rems would require action as mandatory.
Numerous reports from the Committees were issued between 1959 and 1977 relating to the scientific basis of protection in industry and medicine and the application of recommendations in specific circumstances [4].They included reviews on protection against different energies of low and high linear energy transfer (LET) radiations; protection of the patient during diagnostic radiological examinations and after the administration of radiopharmaceuticals in nuclear medicine procedures; the radiosensitivity of tissues and organs to different qualities of radiation and the effect of spatial distribution; the evaluation of risk; the metabolism of alkaline earth elements and actinides; evaluation of doses and the derivation of permissible doses from numerous radionuclides; and general principles of monitoring for radiation protection of workers. p.37
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