A. Sources of Radiation
We live in a radioactive world and always have. In fact, the majority of us will be exposed to
more ionizing radiation from natural background radiation than from our jobs.
There are several sources of radiation that occur naturally. The radiation emitted from these
sources is identical to the radiation that results from manmade sources.
The four major sources of naturally occurring radiation exposures are:
Sources in the earth's crust, also referred to as terrestrial radiation
Sources in the human body, also referred to as internal sources
Cosmic radiation (total average dose ~ 28 mrem/yr)
Cosmic radiation comes from the sun and outer space. It consists of positively
charged particles and gamma radiation.
At sea level, the average annual cosmic radiation dose is about 26 mrem.
At higher elevations, the amount of atmosphere shielding cosmic rays decreases;
therefore, the dose increases.
Sources in earth's crust (terrestrial) (total average dose ~ 28 mrem/yr)
There are natural sources of radiation in the ground (i.e., rocks and soil).
Some of the contributors to terrestrial sources are the natural radioactive elements
radium, uranium, and thorium.
Many areas have elevated levels of terrestrial radiation due to increased
concentrations of uranium or thorium in the soil.
Internal (total average dose ~40 mrem/yr)
The food we eat and the water we drink contain trace amounts of natural
These naturally occurring radioactive materials deposit in our bodies and cause
internal exposure to radiation.