Answer to Question #10107 Submitted to "Ask the Experts"

The following discussions are derived from the Recommendations of the International Commission on Radiological Protection (ICRP), Publication 60, 1990. ICRP's most recent recommendations in Publication 103, 2007, reaffirmed the numeric values recommended for the dose limits. Limits are intended to provide a clearly defined boundary for optimization procedures and prevent excessive individual detriment, which might result from a combination of practices. It was the Commission’s intention to choose the values of dose limits so that any continued exposure just above the dose limits would result in additional risks from the defined practices that could reasonably be described as “unacceptable” in normal circumstances. Thus the definition and choice of dose limits involve social judgments.

With that basis, the selection of the occupational dose limit was a multiattribute, risk-informed judgment regarding the level of accumulated exposure, over a working lifetime. The ICRP's conclusion was that an accumulated dose of 1 Sv, which corresponds to an accumulated risk of approximately 5 percent, represented the boundary of acceptability. Thus, a limit of 20 mSv per year, on average, over a working lifetime, would be an acceptable limit.

For public exposures, the considerations included two different approaches. The first was similar to that employed for occupational exposure, examining the level of dose for which the consequences can reasonably be described as unacceptable for individuals in society as a whole. The second approach is to base the judgment on the variations in the existing level of dose from natural sources. This natural background may not be harmless, but it makes only a small contribution to the health detriment that society experiences.  An analysis presented in Annex C of ICRP Publication 60 looked at a range of annual values. They provide no easy basis for a judgment, but do suggest a value of the annual dose limit not much above 1 mSv. On the other hand, the data in Annex C show that, even at a continued exposure of 5 mSv per year, the change in the age-specific mortality rate is very small. Excluding the very variable exposures to radon, the annual effective dose from natural sources is about 1 mSv, with values at high altitudes above sea level and in some geological areas of at least twice this. On the basis of all these considerations, the ICRP recommended an annual limit on effective dose of 1 mSv.

Donald Cool, PhD