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.

Cosmic radiation accounts for five percent of total radiation exposure. This source is always present though it affects some parts of the earth more than others. The north and south pole receive more cosmic radiation than areas near the equator because the earth's magnetic field diverts the radiation.

The atmosphere acts as a shield and considerably reduces the amount of cosmic radiation reaching the earth's surface. The level of exposure to cosmic radiation also increases with altitude since there is less air as altitude increases to act as a shield. Consequently, someone living at sea level will, on average, receive a lesser dose of cosmic radiation every year than an individual living in locations above 2,000 meters.

This means that while the average dose rate in the U.S. from cosmic radiation is approximately 30 mrem/year or 0.3 mSv/year, someone living in higher elevation areas like Denver, CO would receive much higher doses (up to 140 mrem/year or 1.4 mSv/year).