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Answer to Question #8900 Submitted to "Ask the Experts"

Category: Policy, Guidelines, and Regulations — Regulations and Guidelines

The following question was answered by an expert in the appropriate field:

Q

What is the basis for the ICRP limit for a member of the public of 1 mSv y-1 and of 20 mSv y-1 for the occupational radiation worker?

A

The basic framework of radiological protection established by the International Commission on Radiological Protection (ICRP) is based on scientific knowledge and data as well as societal and economic considerations. ICRP Publication 103 states: "the primary aim in the Commission’s Recommendations is to contribute an appropriate level of protection for people and the environment against the detrimental effects of radiation exposure without unduly limiting the desirable human actions that may be associated with such exposure." 

The ICRP dose limits are intended to serve as a boundary condition that will prevent deterministic effects and limit the probability of stochastic effects. Radiation health effects are generally divided into two categories: deterministic effects and stochastic effects.1

Deterministic effects are often of an acute nature, are mostly the result of death or malformation of somatic cells following radiation exposure, and only appear if the radiation dose exceeds a threshold value.

Stochastic effects are cancer or heritable effects involving the development of cancer and may occur in either mature somatic cells or through the mutation of germ (reproductive) cells. Current ICRP recommendations, for radiation protection purposes, are based on the conservative assumption that there is no safe level of exposure. In other words, even the smallest exposure has some probability of causing a stochastic effect, such as cancer. Moreover, based on that assumption, the ICRP endorses the general philosophy of not only keeping exposures below the dose limits but also maintaining all exposure levels "as low as reasonable achievable" (ALARA).

How exactly did the ICRP derive the limit of an average of 20 mSv y-1 over five years for the occupational dose limit and 1 mSv y-1 for the public dose limit? The ICRP first examined several sources of radiological data including reference anatomical and physiological models of humans, studies at the molecular and cellular level, experimental animal studies, and human epidemiolgical studies including experimental, cohort, case-control, and ecologic studies (such as the Japanese A-bomb cohort). For the occupational dose limit, the ICRP sought to select a dose that correlated to a tolerable consequence (i.e., one that falls just short of unacceptable) based on quantifiable attributes of detriment. These attributes included lifetime attributable probability of death, the time lost if the attributable death occurs, the reduction of life expectancy (a combination of the first two attributes), the annual distribution of the attributable probability of death, and the increase in the age-specific mortality rate (for example, the probability of dying in a year at any age, conditional on reaching that age). The ICRP examined several possible values of doses that an individual could receive over a full working lifetime and translated each to an annual effective dose. Finally, the ICRP selected the most tolerable dose as the dose limit. In making this determination, the ICRP considered combinations of attributes and did not merely take into account one single attribute. ICRP determined that for the occupational dose limit, an approximate lifetime dose of 1 Sv is tolerable. This correlates to an annual average dose of 20 mSv. To allow for flexibility, the ICRP also recommended that the dose limit should not to exceed 50 mSv in any single year. In summary, the ICRP limits for occupational workers are 20 mSv per year, averaged over defined periods of five years, with the further provision that the dose should not exceed 50 mSv in any single year.

Likewise, the ICRP examined the correlation between annual effective dose and the attributes of detriment, as listed above, due to exposure for individuals aged 0-75 years. Based on the data, the ICRP estimates that doses rising above 1 mSv per year will justify the introduction of protection actions for members of the public. Deterministic effects were also considered. This became the dose limit from all sources for members of the public. The ICRP also recommends that controls be placed on each source, for example, limits on emission of radionuclides from installations including the emission of naturally occurring radionuclides from installations such as waste disposal sites.

Kimyata Morgan Butler, PhD
Donald Cool, PhD

1The ICRP notes that some radiation-associated health consequences, particularly some noncancer effects, are not yet sufficiently well understood to assign to either of these generic categories.

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