Answer to Question #1269 Submitted to "Ask the Experts"

Q

I've read that the dose equivalent limit for the whole body of a radiation worker is 50 mSv, yet I came across another question that asked, What is the dose equivalent limit a radiation worker can receive in any one quarter (13 weeks) of year? The answer was 30 mSv. Can you please explain why? I would think it would be 1/4 of 50.

A

Current regulations and guidance specify dose limits in terms of annual doses rather in the previously used quarterly system. Therefore, the limit of 50 mSv may be delivered in a quarter or spread out over the year, depending on the details of the licensee's operations. There are no quarterly limits. It may be that the information you have dated back to the period when quarterly limits were still in use, which would be roughly pre-1977, if we use publication of ICRP-26 as the relevant date, or 1990 if we use implementation of the revised pertinent Nuclear Regulatory Commission (NRC) regulations as the dividing line.

The old NRC regulations permitted a whole body dose of up to 3 rem in a quarter provided that the person's lifetime dose did not exceed D=5(N-18) rem, where N is the person's age in years. The annual limit in this system was implicit in the lifetime equation and came into force only when the exposed worker's lifetime dose reached the limit given by the equation.

Sami Sherbini
Nuclear Regulatory Commission