Unit 1 Fundamental Concepts 1-9 Natural and Man-made Radiation Sources Before we discuss the biological effects of nuclear radiation, it is important to remember the many sources of natural and man-made radiation that surround us. Individuals are exposed to minute amounts of radiation from the environment daily. This natural background radiation comes from three main sources: O Cosmic radiation: Cosmic radiation reaches the earth primarily from the sun. It is composed of a very wide range of penetrating radiations which undergo many types of reactions with the elements they encounter in the atmosphere. The atmosphere acts as a shield and considerably reduces the amount of cosmic radiation reaching the earth's surface. The average dose rate in the U.S. from cosmic radiation is approximately 0.3 mSv/year or 30 mrem/year. Much higher doses (up to 140 mrem/year or 1.4 mSv/year) are received by individuals living in higher elevation areas such as Denver. O
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