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21 November 2009

Answer to Question #1261 Submitted to "Ask the Experts"

Category: Doses and Dose Calculations — Basic dose information, dose quantities, units

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

Why are the radiation exposures received in accidents and medical exposures expressed in units of gray rather than in sievert?

The dose equivalent, expressed in sievert, is defined only for the purpose of comparing radiation exposures with radiation protection limits. The fundamental quantity is dose, the energy deposited per unit mass, and is expressed in gray. The same dose delivered by different types of radiation may produce different consequences, but this is dealt with directly when managing medical radiation exposures or exposures received in an accident. The dose equivalent was defined so that all occupational exposures could be compared to a single radiation protection limit and involves a "quality factor" which is the result of a subjective averaging over all of the different effects that contribute to health risk. The quality factor is useful for estimating the average risk, but is not good for predicting any one specific effect, as is needed in planning medical treatment or response to an accidental exposure. Leslie A. Braby

Answer posted on 23 October 2001. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of 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 specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.