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 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 (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
|
31
|
Terrestrial
|
19
|
Radon
|
229
|
Internal (K-40, C-14, etc.)
|
16
|
Manmade: Medical
|
300
|
Consumer Products
|
13
|
All others: Fallout,
air travel, occupational, etc.
|
12
|
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)
|