The first principle of protection is to avert the deterministic effects that appear at high doses. The second principle is to assume, as a precaution, that any exposure, even to a low dose, can lead to the appearance of stochastic effects. In other words, every exposure entails a risk. This risk has to be managed.

The principle of prevention means setting dose limits below the threshold for the appearance of effects. If these limits are observed, deterministic effects will never appear.

As there is no scientific certainty concerning the effects of low doses, and in order to reduce any 'regrets' to a minimum when misjudgements occur, a prudent approach consists in acting 'as if' effects did in fact exist at low doses. A linear dose-effect relationship, without a threshold, has therefore been chosen, extrapolated from available data on high doses.

For low doses, there is an international consensus on risk estimation which stems from the precautionary principle. The linear dose-effect relationship, with no threshold, shows that if each person in a given population has received a total dose of 1 sievert during his lifetime, 5 % of the population are likely to die of radiation-induced cancer.