For more than twenty years collective dose has been a useful quantity in radiation protection. However, in recent times it has been criticised on two accounts. Collective doses composed of very small individual dose contributions would give a fair estimate of radiation detriment only if the dose response is linear without a dose threshold (LNT), and this assumption is being challenged. Optimisation of protection by means of cost-benefit assessment based on collective dose is also said to lead to unreasonable protection costs. The critics argue that since small radiation doses give negligible individual risks, the corresponding collective risk must also be negligible. Therefore, all that counts, they say, are doses that are not much lower than those which are known to bring a risk of individual harm.

Some of the aversion to collective dose is undoubtedly caused by obvious misuse of the quantity. The ICRP recommendation on protection optimisation calls for protection costs that are reasonable. It follows that the appropriate cost-benefit assessment based on collective dose should never lead to unreasonable expenses; in those cases where unreasonable expenses have been ordered, the application of the collective dose has not been consistent with the ICRP recommendations. Collective doses have also been calculated with integration of future dose contributions over absurd time periods. However, such misuses do not necessarily disqualify the concept.

It is often said that the individual risk is more relevant than the collective societal risk. But this collocation of individual versus collective is misleading. It is a truism to say that the collective is safe enough if each of its individuals are safe. The distinction should rather be between individual-related and source-related assessments. The fact that all individuals are safe does not always mean that the source is justified or that the source-related protection is optimised. And the fact that each individual is satisfied with his own protection does not necessarily mean that he or she is satisfied with the situation; they may dislike the idea that the odds are that, after all, someone might, unnecessarily, be harmed.

The potential of risk accumulation is an important factor. At the time when the individual dose limit was the operative quantity it was tempting to apply the principle of dilution and dispersion to releases of radioactive material into the environment. `My own small contribution is certainly negligible'. We have seen the consequence of this principle with regard to chemical pollution, e.g. sulphur from domestic fireplaces and industrial power stations. We should be grateful to ICRP for having introduced the principle of protection optimisation of radiation practices.

What is not always recognised is that there are uses of the collective dose which do not depend on the LNT assumption. For example, the LNT assumption is not necessary for costing a unit collective dose. We have been accustomed to say that any cost-benefit analysis implies the LNT assumption and a corresponding decision on how much we can afford to save a human life in a statistical sense. But we could as well directly attach a monetary value to the unit collective dose, so dimensioned that the temptation to dilute and disperse is properly balanced. Also, in this case we would have to include minute dose contributions, because, irrespective of biological assumptions, it is the addition of all contributions that makes up the total.

The collective dose commitment per year of a practice may be used as a means to estimate the highest future per caput dose if the practice continues at constant rate. If the practice is assumed to have a limited lifetime, the truncated collective dose commitment may be used. It can be shown that, provided the population size is constant, the highest future annual collective dose equals the collective dose commitment from one year of the practice. This relation is also purely mathematical and does not require any biological assumption. The addition of doses from expanding practices can be assessed by means of the collective dose per unit practice, also without any biological assumption.

The fact that the LNT assumption is challenged does not immediately make it invalid. However, we must recognise that the detriment coefficients which are used for radiation protection purposes are nominal values which are strictly only representative of the population for which they have been estimated by epidemiological observations at relatively high doses. They may not apply to other populations and certainly not to any single individual. If the collective dose is used to predict the detriment of a practice or an event (like the Chernobyl catastrophe), we must always underline the uncertainties. However, as long as large groups of competent scientists feel that, in spite of all uncertainties, the LNT-assumption and the derived detriment coefficients are more likely than other options, then estimates based on this assumption and calculated by means of the collective dose should not be considered unlikely.

Bo Lindell
Swedish Radiation Protection Institute
SE-17116 Stockholm
Sweden