What Is "Dose"? What Units Are Used to Express Radiation Dose?

"Dose" is a word that in some general English uses and in medical terminology may mean something different than is meant in radiation protection. We talk about taking a "dose" of whiskey or getting our daily "dose" of news or anything else we may like; similarly, in medical applications we get "doses" of medicine. In radiation protection, "dose" has a more specific meaning—it is the energy of ionizing radiation absorbed per unit mass of any material. Mostly we talk about the dose to people, or to parts of the body, but we can define the dose in air, water, human tissue, or any other material. Energy is most often given in units of ergs (erg), joules (J), electron volts (eV), or multiples thereof (for example, kilojoules [kJ] and megaelectron volts [MeV]). Mass is most often expressed in terms of grams or kilograms (g or kg). Special units exist for dose, including the "rad," which is defined as 100 erg/g, and the "gray" (Gy), which is defined as absorption of 1 J/kg. 1 gray is equal to 100 rad. These units similarly are used with multipliers, for example, millirad (mrad = 0.001 rad, one one-thousandth of a rad), milligray (mGy), and microgray (μGy = 0.000001 Gy, one one-millionth of a gray).

In many situations, the energy of radiation absorbed per unit mass of material can be related directly to radiation effects. If a large population of people is exposed to 5.0 Gy (500 rad) all at once, about half may be expected to die within 60 days of this exposure. If the skin is exposed to more than a few Gy of radiation (a few hundred rad), this may cause some transient or permanent reddening and, at higher doses, permanent and more severe damage may occur. We are all exposed to around 3 mGy (300 mrad) of radiation every year from natural sources coming from space and radioactive sources in the earth, building materials, and other natural sources. In some situations, however, we find that the absorbed dose does not tell the whole story. Some types of radiation in certain experimental conditions cause more observed effects, given the same amount of absorbed dose, than others. Factors, called "radiation weighting factors" (earlier called "quality factors"), are used to convert absorbed doses (in rad or gray) to "equivalent doses." These equivalent doses have different names, the rem and sievert (Sv). As with dose, 1 Sv = 100 rem, and multipliers are employed (for example millirem [mrem] and millisievert [mSv]). Equivalent dose is only defined for human tissue (that is, not for air, water, etc.).

Another quantity sometimes used in radiation protection is the "effective dose." If the whole body is exposed to radiation more or less uniformly, we can define a single number that gives the dose or equivalent dose to any organ and the whole body. If the body is exposed in a nonuniform manner, however, it becomes more difficult to compare different exposures. Different x-ray procedures expose different organs to different doses, and different radioactive materials inside the body tend to concentrate in different organs, giving a different pattern of dose. Using values that approximately represent the likelihood that the different organs may express radiation effects, organ weighting factors were developed. If each organ is multiplied by its weighting factor and the values are added up, we obtain a dose that is "effectively" like a uniform whole-body dose. We can then compare different nonuniform exposures or add them together to express the total risk of a mixed-exposure situation (for example, a uniform whole-body exposure to an external source in addition to inhalation of some radioactive iodine).

How is dose important in radiation protection?

Our system of radiation protection is based on the idea that limiting the equivalent dose received by workers (estimated during a calendar year) will eliminate the possibilities of workers receiving doses that can cause immediately observable effects (radiation sickness, skin effects, etc.) and will maintain their risks of longer-term effects to levels that are similar to risks we accept in other industries and activities of life (longer-term effects include cancer, hereditary effects, and so on—see the section on "Radiation Effects" at http://hps.org/publicinformation/ate/cat25.html). These doses may be received from exposures to sources outside of the body (external dose) or radioactive material that may enter the body by being inhaled or swallowed (internal dose). We set lower limits for radiation dose for minors, members of the public, and pregnant women. Much more detail can be found on our website on Policy, Guidelines, and Regulations (http://hps.org/publicinformation/ate/cat37.html). Radiation workers who may have significant exposures are routinely monitored for their external doses by wearing radiation measuring devices and having these devices evaluated at periodic intervals. If workers could be reasonably expected to take radioactive material into their bodies, they receive special evaluations by direct measurements made on the worker's body or by indirect measures, which involve measuring radioactivity leaving the body in urine or other materials. For the general public, external and internal exposures are usually estimated using simulation models (which are often computer based), supported by spot measurements made at strategic locations around facilities that may represent sources of radiation to the public.