Answer to Question #11309 Submitted to "Ask the Experts"

Category: Radiation Basics — Radiation Quantities and Units

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


In my daily job, I write professional letters, registrations, licenses, and regulatory guides. While reviewing a "Sealed Source and Device Registry Sheet" prepared by one of my colleagues, I noted that he insists on capitalizing the term "REM" (Roentgen equivalent man). In this particular instance, it was actually a reference to "mREM." I have been working as a regulator for nearly 15 years and have found that there is a large consensus in the industry that the term "rem" is treated more as a unit and not as an acronym. I have to concede that it is an acronym though, and as such it would seem to be appropriate to capitalize it. Which is most correct, to write it as "mREM" or as "mrem"? Note, I would love to hear that my past use of the term "mrem" was not incorrect.


According to recommendations developed, adopted, and promulgated by the International Committee on Radiation Units and Measurements and other groups and agencies such as the National Council on Radiation Protection and Measurements, the old (not International System [SI]) special unit of dose equivalent was the rem with no capitalization, the designation you favor. It is true that the rem unit originally came from the acronym for "Roentgen equivalent man," but this did not affect the decision to use rem as the accepted unit. Similarly, the mrem and microrem units did not employ capital letters.

Along a related vein, the proper special SI unit for equivalent dose is the sievert, symbolized as Sv; the spelled unit is not capitalized, despite being named for an individual, Rolf Sievert. A similar situation applies to the old activity unit of curie, not capitalized despite its name in memory of Pierre Curie. There are also uncapitalized radiation quantities that represent acronyms; an example is the quantity kerma, which derives from its verbal specification "kinetic energy released per unit mass."

The bottom line is that you may revel in the fact that your past assumptions and usage have been correct, a rem is not a REM, despite your colleague's desire for language logic.

George Chabot, PhD

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.
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