Apples & Oranges - Understanding Curies & REM in Radiation Sources

by Jeffrey D. Williams

Hazardous materials personnel are faced with a broad range of chemical, biological, and radiological hazards. However, not all hazards are equal, nor are similar quantities. Responders who encounter radiological materials need to know the relationship of quantity and biological impact of specific materials by first understanding the terminology of measurement units.

The world of the hazardous material responder has changed substantially in the past two decades. Past responses tended to be for accidental releases or misplaced sources, whereas recent events have shown a growing capability for intentional releases with political or terrorist motivations. These events – whether involving a vehicle-borne improvised explosive device, the release of hazardous chemicals, or the dispersal of biological agents – have increased the need for all emergency responders and preparedness professionals to expand their skills and scope of actions. This is particularly true for radiological terrorism, which fortunately has not had a real-world event to date.

With the increasing variety of chemical, biological, radiological, nuclear, and high-yield explosive threats comes the need to understand and become comfortable with the specialized terms of each threat scenario. One of the challenges in discussing radiological sources and their impacts is in understanding the size and threat from a source or an event. Two different ways of looking at hazards are: (a) in the quantity of material involved; and (b) in the biological impact of that material. The first involves understanding the term “curies,” whereas the latter involves understanding the term “rem,” which is an acronym for roentgen equivalent man. Although both describe an aspect of the scale of an event, the terms relate to different aspects of a source and are not equivalent to each other.

Measuring Quantities of Radiological Material The unit for measuring the amount, or activity, of radioactive material is the curie (or the becquerel in the International System of Units [SI]). The act of a single atom undergoing decay and changing to another element is one disintegration. Once that particular decay has occurred, an atom that has changed into another isotope or another element will not undergo that same process again, although the new isotope or element may have its own decay process. A curie is defined as 3.7 x 1010 disintegrations per second (dps); the definition of becquerel is 1 dps.

Although curies and becquerels measure the same event, they are obviously significantly different in scale. The practical consequence of this is that small millicurie sources are gigabecquerel sources, whereas curie-sized sources are terabecqueral or petabecquerel sources. This terminology can present problems with the public, and even responders, as these prefixes are not part of traditional experiences in scale. There can even be a psychological impact in these numbers, with a perception that the large prefix represents an inordinate hazard.

A major issue with the measurement of radioactive material is that knowing the quantity does not indicate the level of hazard it represents. Although the term expresses the rate of emissions, it does not factor in the type of radiation (alpha, beta, gamma, or neutron) being emitted – the type of radiation is highly significant in defining the level and scope of hazard of an isotope – nor does it reflect the energetic strength of the radiation. There are critical parameters because a 100-curie (intact) source of alpha-emitting Americium represents a minimal external hazard and can be closely approached with no risk or harm, whereas a 100-curie Cobalt-60 source, which is a high-energy gamma emitter, would require a safe standoff distance of several hundred feet.

Understanding Biological Terms of Radiological Material Curies describe a source’s strength in terms of the rate of decay of a source; describing the strength in terms of the impact of that radiation on a human body is the dose. There are three terms that are frequently, if inaccurately, used interchangeably: (a) the roentgen; (b) the rad; and (c) the rem. The terms are distinct and refer to three noticeably different measurements, a distinction frequently lost in the semantics of the terms.

The roentgen, which is a measure of exposure to radiation, is a measure of the amount of energy deposited in a volume of air that results in the production of a specific rate of ionization. Exposure in this sense is not the same as the common usage of the term (“I’ve been exposed to chlorine gas”). However, it does convey some of the same sense of having been in contact with the radiation, even if not in contact with the source itself. The term “rad” is used to define a dose, and stands for radiation-absorbed dose. It represents the amount of energy deposited and absorbed by a body (the SI unit is a gray [Gy] and 100 rad =1 Gy). The amount of energy deposited does not depend on either the type of the radiation or the energy of the radiation, just on the energy per mass absorbed.

Although rad does not depend on the type of radiation deposited on a body, the impact of that energy on a biological unit is dependent on the type of radiation. The rem is a dose equivalent and accounts for the difference in biological impacts based on particle size (the SI equivalent is sievert [Sv], where 100 rem = 1 Sv). The larger particles of alpha and neutron radiation do greater damage at the point of impact because their energy is deposited in a small space. The lighter beta particle or the energy-only gamma have a more linear energy deposition form, allowing energy to be deposited over a greater volume and, therefore, with less intensity. A quality factor allows the conversion of rad to rem by accounting for the type of radiation and the differing effects. As an example, a 10 rad dose of gamma results in a 10 rem of exposure, and a 10 rad dose of alpha results in 200 rem of exposure.

There is a further complexity in defining the impact of radiation on the human body. Although ad and rem account for the radiation type and the doe equivalent respectively, both assume and external whole body impact from an intact, external sources. Yet, in both medicine and accidental releases, radiation can enter the body by different pathways and can have significantly different impacts on different tissue systems. This is the effective does and accounts for biological uptake, organ selectivity, etc. Regardless of radiation energy or type, internal exposure is always worse than external exposure due to the sensitivity of the tissues involved. 

It is clear that describing source strength in terms of curies or rem provide two very different descriptions looking at very different aspects. It also is clear that, for both terms, that number by itself does not provide a full picture of the hazard presented by the source. The type of radiation and its energy level affect the usefulness of both terms. In understanding radiation measurement, no single term or aspect gives a full picture of the hazard, so the more information available, the better the opportunity to correctly evaluate the hazard for the scene.


Jeffrey Williams has served over the last 20 years as an environmental engineer in the U.S. Department of Defense. He also has served on two different emergency response teams, during which assignments he became an expert on radiological dispersal devices and various related topics. He has been a speaker at a number of public and private forums on topics ranging from environmental regulations to radiological preparedness. Prior to assuming his DoD post, he worked on the design and construction of hazardous-waste disposal sites for industrial facilities. He holds a Bachelor's degree in Nuclear Engineering and a Master's degree in Environmental Engineering from the University of Maryland as well as a Master's degree in Legal Studies from the University of Baltimore. He also has studied at the Massachusetts Institute of Technology’s Center for Advanced Engineering Studies.