that time, the metrics used in science and medicine have changed to internationally accepted units, and the relevant index in radiology is now ''entrance skin air kerma" (ESK) in units of milligray (mGy). An entrance skin exposure of 100 mR is equivalent to an ESK of 0.869 mGy; both measures quantify the intensity of x-rays in air at the patient's skin.
To calculate a patient's radiation dose for a specific organ for a given ESK, radiologists must consider both the attenuation and the scattering of the x-ray within the patient. To help in this process, the Food and Drug Administration (FDA) of the Department of Health and Human Services and others have published tables and computer codes that give the dose to various organs per unit ESK for many different radiographic procedures. To estimate patient risk or to compare a nonuniform exposure to whole body exposure, the International Commission on Radiological Protection recommends that individual organ doses be converted to "effective dose equivalent" (EDE) in units of millisievert (mSv).1 EDE is the dose that, if delivered uniformly, would have the same biological effect as the actual (nonuniform) dose. It is calculated as the product of individual organ dose and an "organ weighting factor" given by the International Commission on Radiological Protection.
Collective dose equivalent is the product of EDE and the number of persons receiving a radiation dose; it is expressed in units of person-sievert (person-Sv). Collective dose equivalent for diagnostic medical x-rays, for example, can be estimated as the product of the EDE per procedure and the total number of procedures.
Collective dose equivalent is of interest in terms of broad population exposure to radiation overall and to radiation from various medical uses. If medical uses produced a very high exposure compared to the exposure levels encountered in ordinary daily activities, policymakers and others might have reason to direct relatively more attention to the regulation of medical uses of ionizing radiation. Conversely, if it is the case that common daily activities expose the general population to radiation levels significantly far in excess of those from medical applications, policymakers might have less reason or interest in such regulation.
The discussions that follow present information on radiation exposure from various procedures in some depth, with the aim of providing the reader with a sense of the inherent risk (or lack of it) of various techniques that use ionizing radiation. These interventions are by no means the only source of exposure to radiation in this country, however. Normal background radiation from all natural sources (including radon) in the United States exposes each member of the population to an average EDE of about 3 mSv per year.