Optimization is aimed at achieving the best level of protection under the prevailing circumstances through an ongoing iterative process that involves:

• Evaluation of the exposure situation, including any potential exposures, that is, exposures not certain to occur but with a finite probability of occurring;

• Selection of an appropriate value for the constraint or reference level;

• Identification of the possible protection options;

• Selection of the best option under the prevailing circumstances; and

• Implementation of the selected option.

In all situations the process of optimization with the use of constraints or reference levels should be applied in planning protective actions and in establishing the appropriate level of protection (ICRP, 2006).

Dose limits

While the optimization of planned exposures from individual radiation sources is constrained by source related dose constraints, the overall dose to an individual must be kept within dose limits (ICRP, 2007).

For occupational exposure in planned exposure situations, the ICRP recommends that the limit should be an effective dose of 20 mSv per year, averaged over defined 5 year periods (100 mSv in 5 years) with the further provision that the effective dose should not exceed 50 mSv in any single year.

For public exposure in planned exposure situations, the ICRP recommends that the limit should be an effective dose of 1 mSv in a year. However, in special circumstances a higher value of effective dose could be allowed in a single year, provided that the average over the defined 5-year period does not exceed 1mSv per year.

The limits on effective dose apply to the sum of doses due to external exposures and committed doses from internal exposures due to intakes of radionuclides. In addition to the limits on effective dose, separate limits, defined in terms of annual equivalent dose, are set by the ICRP for the lens of the eye and localized areas of skin because these tissues will not necessarily be protected by the limit on effective dose.

Dose limits do not apply in emergency exposure situations where an informed, exposed individual is engaged in volunteered life-saving actions or is attempting to prevent a catastrophic situation. For informed volunteers undertaking urgent rescue operations, the normal dose restriction may be relaxed. However, later phases of recovery and restoration operations should be considered as normal exposure situations and the relevant limits applied.