The international units of absorbed dose are sieverts and grays. We Receive Most Americans receive about 360 mrem per year from all sources of radiation, including radon and medical exposure. About 40 mrem per year comes from the natural radioactivity in our own bodies. Most of our natural background radiation comes from cosmic radiation from outer space and from radioactive materials in the earth's rocks and soil. Radon is a naturally occurring radioactive gas that results from the decay of radium.
Radon is the single largest source of radiation exposure.
On average, radon accounts for 200 mrem of our exposure each year. The actual amount of background radiation depends on the location, elevation, rock and soil content, and weather conditions. For example, a person living on the Atlantic coast receives about 65 mrem of natural background radiation per year, while a person in Denver, Colorado, receives about 125 mrem, excluding radon. The difference in the natural background radiation is due in large part to Denver's higher elevation. The higher the elevation, the thinner the atmosphere, meaning that the atmosphere filters out less cosmic radiation.