Answer to Question #9328 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Personnel Monitoring (PM)

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

Q

Why do we have separate annual dose limits for the eye and skin? Why don't these organs have normal tissue-weighting factors?

A

The individual dose limits for the eye and for the skin are based on recognition by the International Commission on Radiological Protection (ICRP) that there are times when relatively large doses to discrete areas of skin and to the lens of the eye may be possible. In such cases, the effective dose limit of 50 mSv, based on doses to all significantly irradiated tissues of the body that are susceptible to stochastic effects, may not be sufficient to prevent deterministic effects to the skin and/or lens of the eye.

Localized doses to the eye may result from some types of operations, such as certain beam viewing procedures that have been used in some accelerator operations. The current annual dose limit of 150 mSv for the eye is based on dose to the lens of the eye, and the limit has been established in specific consideration of radiation-induced cataracts in the lens. This is a deterministic effect (i.e., the effect is associated with a threshold dose, and the probability of observing the effect below a particular threshold dose is zero). As such, there would be no tissue-weighting factor associated with the lens of the eye. Cancer induction in the eye is not recognized as a significant radiation-induced event. The ICRP has been reviewing data relating to radiation effects to the eye, and it is possible that the present limit may change in the future.

The skin dose annual limit of 500 mSv is considered adequate to protect the skin against possible deterministic effects. This value was implemented by ICRP to provide protection when discrete portions of the skin might be irradiated to relatively high doses, as when radioactive contamination is on a small area of the skin. While radiation-induced skin cancer is possible, the ICRP has judged that, when large portions of the body are irradiated, the effective dose limit of 50 mSv provides sufficient protection for the skin. The ICRP (in ICRP Publication 60, 1990 Recommendations of the International Commission on Radiological Protection, Pergamon, 1991) has defined a tissue-weighting factor of 0.01 for the skin in consideration of potential radiation-induced cancer. The same weighting factor has been reaffirmed in the ICRP's most recent recommendations in Publication 103.

I hope this adequately addresses your question.

George Chabot, PhD, CHP

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