The concepts of dose constraints and reference levels are used in conjunction with the optimisation of protection to restrict individual doses. A level of individual dose, either as a dose constraint or a reference level, always needs to be defined. Thus the initial intention would be to not exceed, or remain at, these levels; and the ambition would be to reduce all doses to levels that are as low as reasonably achievable, economic and societal factors being taken into account. In planned exposure situations (with the exception of medical exposure of patients), the term used is ‘‘dose constraint’’, but for emergency and existing exposure situations the term ‘‘reference level’’ is used to describe this level of dose. The difference in terminology is because that, in planned situations, the restriction on individual doses can be applied at the planning stage, and the doses can be forecast so as to ensure that the constraint will not be exceeded. In the other situations, however, a wider range of exposures may exist, and the optimisation process may apply to initial levels of individual dose above the reference level. [Diagnostic reference levels are used in medical diagnosis (i. e., planned exposure situations) to indicate whether, in routine conditions, the levels of patient dose or administered activity from a specified imaging procedure are unusually high or low for that procedure].

A dose constraint is thus a prospective and source-related restriction on the individual dose from a source in planned exposure situations, serving as an upper bound on the predicted dose in the optimisation of protection for that particular source. It is a level of dose above which it is unlikely that protection is optimised for a given source of exposure, and will always be lower than the pertinent dose limit. For potential exposures, the corresponding source-related restriction is called a ‘‘risk constraint’’.

For occupational exposures, the dose constraint is a value of individual dose used to limit the range of options, such that only options expected to cause doses below the constraint are considered in the process of optimisation. For public exposures, the dose constraint is an upper bound on the annual doses that members of the public could receive from the planned operation of a specified controlled source. If a dose constraint is exceeded, then it is necessary to determine whether protection had been optimised, the appropriate dose constraint had been selected, and further steps to reduce doses to acceptable levels would have been appropriate.

Emergency or existing controllable exposure situations are somewhat different. Here the reference levels represent the level of dose or risk above which it is judged to be inappropriate to plan to allow exposures to occur, and for which therefore protective actions should be planned and optimised. The chosen value for a reference level will depend upon the prevailing circumstances of the exposure situation. Quite obviously, when an emergency exposure situation has occurred, or an existing exposure situation has been identified, and protective actions have been implemented, doses to workers and members of the public can be measured or assessed. The reference level may then assume a different function, and serve essentially as a benchmark against which protection options can be judged ret­rospectively. One has to bear in mind that the distribution of doses that result from the implementation of a planned protective strategy may or may not include exposures above the reference level, depending on the success of the strategy.

Because at doses higher than 100 mSv there is an increased likelihood of deterministic effects, and a significant risk of cancer, the maximum value for a reference level should be 100 mSv incurred either acutely or in a year. Higher exposures would only be justified under extreme circumstances, and thus either because the exposure is unavoidable or because the situation was exceptional — such as the saving of life or the prevention of a serious disaster. No other individual or societal benefit would compensate for such high exposures.

Below this, the values fall into three defined bands. They apply across all three exposure situations and refer to the projected dose over a time period that is appropriate for the situation under consideration. Constraints for planned exposures and reference levels in existing situations are conventionally expressed as an annual effective dose (mSv in a year). In emergency situations, the reference level will be expressed as the total residual dose to an individual as a result of the emergency that the regulator would plan not to exceed, either acute (and not expected to be repeated) or, in case of protracted exposure, on an annual basis.

The first band, of 1 mSv or less, applies to exposure situations where individuals receive exposures — usually planned — that may be of no direct benefit to them, but the exposure situation may be of benefit to society, the exposure of members of the public from the planned operation of nuclear power being a prime example. Constraints and reference levels in this band would be selected for situations where there is general information and environmental surveillance, or monitor­ing, or assessment, and where individuals may receive information but no training. The corresponding doses would represent a marginal increase above the natural background, and at least two orders of magnitude lower than the maximum value for a reference level, thus providing a rigorous level of protection.

The second band, greater than 1 mSv but not more than 20 mSv, applies in circumstances where individuals receive direct benefits from an exposure situation. Constraints and reference levels in this band will often be set in circumstances where there is individual surveillance or dose monitoring or assessment, and where individuals benefit from training or information — as is the case in occupational exposure from planned exposure situations. Abnor­mally high levels of natural background radiation, or stages in post-accident rehabilitation, may also be in this band.

The third band, greater than 20 mSv but not more than 100 mSv, applies in unusual, and often extreme, situations where actions taken to reduce expo­sures would be disruptive. Reference levels and, occasionally for “one-off” exposures below 50 mSv, constraints could also be set in this range in cir­cumstances where benefits from the exposure situation are commensurately high. Action taken to reduce exposures in a radiological emergency is the main example of this type of situation. Any dose rising towards 100 mSv will almost always justify protective action. In addition, situations in which the dose threshold for deterministic effects in relevant organs or tissues could be exceeded should always require action.