The results of various epidemiological studies have been incorporated into a rationalised approach pub­lished by the International Commission on Radiolo­gical Protection as ICRP Publication 26 [6]. In this the Commission concludes that the mortality risk factor for radiation-induced cancers is about 102 per sievert for uniform whole-body exposure and on this basis the annual acceptable whole-body dose is 50 mSv. For non-uniform exposure the total risk to the organs irradiated should not exceed the whole-body risk.

Paradoxically, despite this being a rationalised ap­proach, it would be possible, using risk calculations, for a single irradiated organ to receive a relatively large annual dose. For example, the thyroid of a ra­diation worker could receive a dose of 1.7 Sv/year, but at such doses it is thought that non-stochastic effects would become important so a blanket of 0.5 Sv. for all organs except the eye (0.15 Sv) is suggested.

ICRP’s criterion for occupational exposure is that the average risk to those employed in radiation work should be no greater than the average risk to those employed in a moderately safe ‘conventional’ industry. By considering average risk, while setting a maximum annual dose equivalent limit of 50 mSv, ICRP are able _to assume an annual average dose equivalent received of orie-tenth of the limit (5 mSv). The best estimate of total mortality risk from all stochastic effects is taken to be about 1 in 700 per mSv re­ceived per year — of exposure. So that the risk to a radiation worker receiving the average annual expo­sure of 5 mSv would be about 1 in 14 000, which is of the same order of magnitude as the average risk in a moderately safe ‘conventional’ industry, taken to be less than 1 in 10 000 per annum. The average CEGB nuclear power worker in fact receives about

1.5 mSv per annum, equivalent to a annual risk of about 1 in 30 000.

It is salutary to compare this with other risks we run in coming to work each day:

Combination of the conventional accident and radia­tion risks gives an overall annual risk of death for power station work of 1 in 12 000 which is within the 1 in 10 000 figure for safe industries. This com­bined risk is still well below those which we volun­tarily accept as part of our everyday lives.