| The basic recommendations were
next revised in 1977. They continued to reflect the evolution
of ideas expressed in early recommendations, which were to limit the incidence
of radiation-induced fatal cancers in exposed individuals, and severe hereditary
diseases in the children and grandchildren of irradiated parents to a level
accepted by society. These diseases were collectively called stochastic
effects on the basis that they have a statistical dose-related probability
of occurring at any level of dose.
The risks of stochastic effects were derived from reviews of epidemiological studies UNSCEAR, and the, BEIR committee. The epidemiological data was by now considered sufficiently reliable to provide named tissue and organ risk values for the gonads, breast, red bone marrow, lung, thyroid and bone surfaces. It was assumed that each tissue or organ contributed a fraction of the total risk that could be represented by tissue weighting factors. The total risk was estimated to be 1.65 x 10-4 per rem, (or 1.65 x 10 2 Sv-1 in the SI system of units).
Other harmful effects associated with higher doses should be avoided. These diseases were called non-stochastic effects, the severity and frequency of which increase with dose and for which there is a variable threshold dose for different tissues, below which they did not occur.
The Commission considered that limiting stochastic effects to an acceptable level could be achieved by adopting a system of dose limitation, the main features of which were that all practices needed to be justified;
|that all exposures should be optimised, the doses
being as low as reasonably achievable; and that a dose limit should be
applied to individuals on the basis that the risk should be equal whether
the whole body was irradiated uniformly, or whether there was non-uniform
tissue or organ irradiation. An annual dose limit
- called the effective dose equivalent was proposed
-, and defined as the dose equivalent for each tissue or organ weighted
for the tissue radiosensitivity, thereby reflecting the contribution of
each tissue or organ dose as if the whole body was uniformly irradiated.
For a radiation worker, the recommended limit on annual effective dose equivalent for uniform irradiation was 5 rem (50 millisievert). For individual members of the public, a limit of 0.5 rem (5 millisievert), as applied to critical groups, was considered to provide an adequate degree of safety, in that the application of this limit was likely to result in an average annual effective dose to individuals of less than 0.05 rem (0.5 millisievert).
The committee structure changed after 1977. Committee 1 was made responsible for reviewing radiation effects; Committee 2 on secondary limits applicable to both external and internal irradiation; Committee 3 dealing with the special problem of protection in medicine; and Committee 4 on the application of the Commissionís recommendations.