Answer to Question #9893 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
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.
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