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Sources of Radiation

In 1972 a detailed survey was made of average annual whole-body doses to the U.S.A. population from various sources. Occupational and miscellaneous artificial exposures averaged about 1-2 mR/y (remember, some people got enough to make up for the vast majority who got none!); global fallout from nuclear testing made up about 6 mR/y; medical exposures (X-rays, radiotherapy, etc.) were good for nearly 100 mR/y; and natural background (see below) averaged about 120 mR/y. The numbers have not changed much in the intervening years. One must conclude that for the average person there are only two significant sources of radiation exposure: medical and natural. Although this begs the question of "extraordinary cases" who receive larger exposures in accidents such as Chernobyl, it still helps to set perspectives for those examples. Some medical and natural radiation sources are listed below. For medical examples I have shown the mean dose per exposure. It is important to note that these are only the easily measured forms of radiation - X-rays and $\gamma$-rays - that penetrate flesh (and detectors!) easily. More insidious and difficult-to-measure types will be discussed in the next Section.

• Medical X-rays: Chest, radiographic: 45 mR. Chest, photofluorographic: 504 mR. Spinal (per film): 1265 mR. Dental (average): 1138 mR.¹¹

• Cosmic Rays: Sea level: 30-40 mR/y. Colorado: 120 mR/y. At 40,000 ft: 0.7 mR/h.¹²

• Natural Terrestrial Radionuclides: $\gamma$-radiation is fairly uniform in the U.S.A., ranging from 30 mR/y in Texas to 115 mR/y in South Dakota. Guess where the uranium deposits are!¹³