Module 2 - Basic Radiation Exposure

Natural Background Radiation

We are all exposed to ionizing radiation from natural sources at all times. This radiation is called natural background radiation, and its main sources are the following:

  •  radioactive substances in the earth's crust;
  • emanation of radioactive gas from the earth;
  • cosmic rays from outer space which bombard the earth;
    trace amounts of radioactivity in the body.


Radioactivity in the Earth

When the earth was formed four billion years ago, it contained many radioactive isotopes.  Since then, all the shorter lived isotopes have decayed.  Only those isotopes with very long half lives (100 million years or more) remain, along with the isotopes formed from the decay of the long lived isotopes.

These naturally-occurring isotopes include uranium and thorium and their decay products, such as radon. The presence of these radionuclides in the ground leads to both external gamma ray exposure and internal exposure from radon and its progeny.


Cosmic Radiation

Cosmic rays are extremely energetic particles, primarily protons, which originate in the sun, other stars and from violent cataclysms in the far reaches of space.  Cosmic ray particles interact with the upper atmosphere of the earth and produce showers of lower energy particles. Many of these lower energy particles are absorbed by the earth's atmosphere.  At sea level, cosmic radiation  is composed mainly of muons, with some gamma-rays, neutrons and electrons.

Because the earth's atmosphere acts as a shield, the exposure of an individual to cosmic rays is greater at higher elevations than at sea level.  For example, the annual dose from cosmic radiation in Denver is 50 millirem while the annual dose at sea level is 26 millirem


Natural Radioactivity in the Body

Small traces of many naturally occurring radioactive materials are present in the human body. These come mainly from naturally radioactive isotopes present in the food we eat and in the air we breathe.

These isotopes include tritium (H-3), carbon-14 (C-14), and potassium-40 (K-40).


Radiation Doses to the U.S. Population

Radiation Source Average Annual Whole Body Dose  (millirem/year)
Terrestrial 29
Radon 200
Internal (K-40, C-14, etc.) 40
Diagnostic x-ray
Nuclear Medicine 14
Consumer Products 11
All others (fallout, air
travel, occupational, etc.)
Average Annual Total
360 millirem/year
(if you smoke, add ~280 millirem)


Average doses from some common activities

Activity Typical Dose
Smoking 280 millirem/year
Using radioactive materials 
 in an Authorized lab
< 5 millirem/year
Dental x-ray 10 millirem per x-ray
Chest x-ray  8 millirem per x-ray
Drinking water  5 millirem/year
Cross country round trip by air  5 millirem per trip 
Coal Burning power plant 0.165 millirem/year 


This is the end of the Natural Background Radiation Module, which is the second of the seven Sealed Source Radiation Basics modules.  The next module is the Biological Effects Module.

Biological Effects (Module 3)

Go back to the Sealed Source Training Introduction Page