Lesson 11c - Understanding the Basics of Radiation

Natural Sources of Radiation

Four major sources continuously expose humans to natural radiation--radiation that is emitted as part of the normal processes that support the planet. While the distinction must be made between natural sources and man-made sources of radiation, the critical difference between the two kinds of radiation is simply their origin: the radiation itself is not different. Radiation emitted from man-made sources is identical to the radiation that results from the following natural sources:

Cosmic radiation comes to the earth from the sun, the stars, and outer space, either in positively charged particles or electromagnetic waves. People living at sea level are more protected from some of this cosmic radiation by the atmosphere, so the average cosmic radiation dose in these areas is about 26 mrem per year. At higher elevations, the thinner atmosphere allows a somewhat larger amount of cosmic rays to pass through, so the dose increases in accordance with an increase in elevation. As an average, the total annual dose to the general population from cosmic radiation is roughly 30 mrem.

Terrestrial radiation comes from the earth's crust, from the very elements that compose the ground, rocks. Natural radioactive elements such as radium, uranium, and thorium are found in various concentrations around the globe, due to the geological features of different regions. The average total dose to humans from terrestrial sources measures approximately 28 mrem a year.

Radon is naturally present in soil, emitting from the radioactive decay of radium, already mentioned above as one of the terrestrial sources. It may seem that a radioactive property coming from soil would belong to the terrestrial category, but because radon is a gas, it exhibits different chemical behavior than the other terrestrial sources.

Both odorless and colorless, radon gas can travel up through the soil and seep into basements or other enclosed areas of a home, where, undetected, it emits a type of radiation named alpha. Alpha radiation cannot penetrate the outer layer of skin on the human body, but it can be inhaled. Radon and its decay products are always present in the air and account for about 200 mrem of the total background radiation dose.

Previous Page Next Page          IEMP Workbook Home