Average Natural Background: 300 Millirems
The average exposure in the United States, from natural sources of radiation (mostly cosmic radiation and radon), is 300 millirems per year at sea level. Radiation exposure is slightly higher at higher elevations-thus the exposure in Denver averages 400 millirems per year.
(A milliRem is 1/1000th of a Rem. According to McGraw-Hill's Dictionary of Scientific and Technical Terms, a Rem is a unit of ionizing radiation equal to the amount that produces the same damage to humans as one roentgen of high-voltage x-rays. The name is derived from "Roentgen equivalent man." Wilhelm Roentgen discovered ionizing radiation in 1895 at about the same time that Pierre and Marie Curie discovered radium.)
All of these limits are for the amount of radiation exposure in addition to background radiation and medical radiation.
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