Dose constraints and reference levels

The dose limits suggested by the ICRP are intended to be applied to the protection of individuals from all regulated sources of radiation. For the purposes of optimizing protection for individual sources of radiation, dose constraints and reference levels are applied. The increased emphasis on dose constraints and reference levels in the new ICRP recommendations probably represents the most significant change as compared with the previous recommendations (Wrixon, 2008).

Dose constraints are applied to occupational and public exposure in planned exposure situations. Dose constraints are set separately for each controlled source and serve as a boundary in defining the range of options for optimization. While the objectives of the use of dose constraints for controlling occupational and public exposure are similar, they are applied in different ways. For occupational exposure, the dose constraint is a tool to be established and used by the person or organization responsible for any facility or activity in the optimization of protection and safety. For public exposure in planned exposure situations, the government or regulatory body establishes or approves dose constraints, taking into account the characteristics of the site and the facility, the exposure scenarios and the views of interested parties. After the exposures have occurred, the dose constraint may be used as a benchmark when assessing the suitability of the optimized protection strategy that has been implemented and for making adjustments as judged necessary.

Reference levels are used for optimization in emergency exposure situations and existing exposure situations. These are established or approved by the government, regulatory body or other relevant authority. For occupational and public exposure in emergency exposure situations, a reference level serves as the boundary in defining the range of options in optimization for implementing protection actions. The reference level represents the level of dose or risk above which it is judged to be inappropriate to plan to allow exposures to occur, and below which optimization of protection is implemented. Once an emergency exposure situation has occurred, actual exposures may be above or below the reference level, which would then be used as a benchmark to judge whether further protective measures are necessary and to assist in prioritizing their application.

The ICRP recommends a range of doses spanning two orders of magnitude within which the value of a dose constraint or reference level usually should be chosen. At the lower end of this range, the dose constraint or reference level represents an increase, up to about 1 mSv, above the dose received in a year from natural background radiation (the worldwide annual average radiation dose from radiation sources of natural origin, including radon, is 2.4 mSv (UNSCEAR, 2000)) and should be used when individuals are exposed to a source that gives them little or no individual benefit, but for which there may be benefits to society in general. This is the case, for instance, when establishing dose constraints for public exposure in planned exposure situations. Dose constraints or reference levels of 1-20 mSv should be used when individuals usually receive benefit from the exposure situation, but not necessarily from the exposure itself. This is the case, for instance, when establishing dose constraints for occupational exposure in planned exposure situations. Reference levels of 20-100 mSv should be used when individuals are exposed to sources that are not under control or where actions to reduce doses would be disproportionately disruptive. This is the case, for instance, when establishing reference levels for the residual dose from a radiological emergency. Any situation resulting in a dose above 100 mSv incurred acutely or in a year should be considered unacceptable except under circumstances addressed specifically to the exposure of emergency workers (ICRP, 2007).