Answer to Question #9893 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

Why is 10 CFR 20.1601, Control of Access to High Radiation Areas, a dose equivalent, i.e., sievert*, and 10 CFR 20.1602, Control of Access to Very High Radiation Areas, an absorbed dose, i.e., gray*? The dose equivalent could be significantly higher than the absorbed dose of 5 Gy.

A

The sievert is used only when stochastic effects are the endpoints of concern. The gray is used when we are considering deterministic effects. The reason is that for stochastic effects, the gray is multiplied by a radiation weighting factor (based on cancer risk) to get the sievert  For deterministic effects, the gray is multiplied by a quality factor (Q) (based on the deterministic endpoint being considered and the characteristics of the radiation) to get the dose equivalent.

High radiation areas are controlled at 1 mSv, well within stochastic range, and the sievert is therefore appropriate. Very high radiation areas are controlled at 5 Gy, which is above the threshold for some of the deterministic effects. The regulations could have used dose equivalent, but the problem is that the Q to get this quantity depends quite a bit on what effect you are looking at, e.g., blood, gastrointestinal, lungs, etc., as well as the rate at which the dose is delivered, and many people are not familiar with dose equivalent.

Sami Sherbini, PhD
Cynthia Jones, PhD

*sievert = Sv, gray = Gy

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 6 January 2012. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.