Answer to Question #435 Submitted to "Ask the Experts"

There are actually three features in the system of dose limitation recommended by ICRP, namely justification, optimization, and dose limitation. Justification means that any proposed activity that may cause exposure to persons should yield a sufficient benefit to society to justify the risks incurred by the radiation exposure. This feature is based on the assumption that any radiation exposure, no matter how small, carries with it a certain level of risk that is proportional to the level of exposure. This hypothesis is known as the linear, non-threshold hypothesis, or LNT. An example of an activity that was considered unjustified was the now-discontinued practice of fitting shoes to people's feet using x rays. The exposure resulting from this activity was considered to be unjustified, and the practice was discontinued. The second feature, or element, is optimization, which is also known as the practice of ALARA (as low as reasonably achievable). This means that the radiation exposures resulting from the practice must be reduced to the lowest level possible considering the cost of such a reduction in dose. Optimization, or ALARA, is required by nearly all licensing agencies, including the Nuclear Regulatory Commission. This feature is also based on the LNT. The third element is dose limitation. This involves setting upper limits on the dose that may be received by any member of the public from all man-made exposures other than medical exposures. These limits are imposed by regulatory agencies.

The system is actually more complicated than that. ICRP recommends that each source of exposure, such as a hospital or reactor site, be constrained to a fraction of the dose limit. For exmple, if the dose limit is 100 mrem/yr for members of the public, then the constraint might be set at 30 mrem/yr. This may be viewed as that site's share of the allotted exposure of 100 mrem/yr to any member of the public, some of whom may be simultaneously exposed to radiations from several facilities. Each site will then optimize its doses as far below 30 mrem/yr as possible, that is, ensure that the doses are ALARA.

In this system of dose limitation, optimization plays the central role, and dose limits play a very secondary role, mainly as guidance for setting action levels and other operating parameters, and also as a guide for allocating dose to various sources, that is, as a guide in setting constraints.

Sami Sherbini Senior Health Physics Advisor, NRC