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Open Source Training Modules


Open Source Radiation Safety Training

Module 2: Background Radiation & Other Sources of Exposure

This module provides information about the following topics:

Natural background radiation
Radioactivity in the earth
Cosmic radiation
Natural radioactivity in the body
Radiation doses to the U.S. population
Average doses from some common activities

Natural Background Radiation (top)

We are all exposed to ionizing radiation from natural sources at all times. This earthradiation 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 (top)

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 (top)

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 (top)

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 (top)

Radiation Source
Average Annual Whole Body Dose (millirem/year)
Natural: Cosmic
Internal (K-40, C-14, etc.)
Manmade: Medical
Consumer Products
All others: Fallout, air travel, occupational, etc.
Average annual total
620 millirem/year
Tobacco (if you smoke, add ~280 millirem)

Note that medical exposure contributes more than 50% of the average person's dose. Medical radiation dose comes primarily from CT scans and nuclear medicine procedures.

Average doses from some common activities (top)

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

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

Go to Module 3 (Biological Effects)


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