radguide

26 Radiation for Radionuclide Users E. Human Exposure to Ionizing Radiation Sources of Exposure Human exposures to ionizing radiation can be classified broadly according to whether they result from sources within the work environment (i.e., occupational exposures) or from sources outside the work environment (i.e., non-occupational exposures). Occupational Exposure Occupational exposures are those received by individuals as a result of working with or near radiation sources (i.e., radioactive material or radiation-producing devices). Occupational exposures differ from non-occupational exposures in that they are generally received during the course of a 40 hour work week as opposed to a 168 hour week for natural exposures. In addition, whereas the non-occupational exposure received by a given individual is largely unknown, an individual’s occupational exposure is closely monitored and controlled. Non-Occupational Exposures Non-occupational exposures can be divided into one of two categories: those originating from natural sources and those resulting from human-made sources. All individuals are continuously exposed to ionizing radiation from various natural sources. These sources include cosmic radiation and naturally occurring radionuclides within the environment and within the human body. The radiation levels resulting from natural sources are collectively called “natural background.” Natural background (and the associated dose it imparts) varies considerably from one location to another in the United States. Historically, the estimated value for the average whole body dose equivalent from natural background in the United States. has been about 100 mrem/person/year. More recently, this figure has been revised upward to between 250 to 300 mrem/person/year to account for the dose contribution from indoor radon. The primary source of human-made radiation exposures ismedical irradiation, particularly diagnostic x-ray and nuclear medicine procedures. Such procedures contribute an average 55 mrem/ person/year in the United States. All other sources of human- made exposures such as nuclear weapons fallout, nuclear power plant operations, and the use of radiation sources in industry and universities contribute an average of less than one mrem/person/ year in the United States.