Background
Radiation
Michael H. Fox
Why We Need Nuclear Power
The Environmental Case
Read Chapter 8 for more information.

Natural background radiation consists of cosmic rays, external and
internal radiation, radon, and a very small component due to nuclear
power.  The average American gets an annual dose of about 3.2 mSv
(milli Siverts) from background radiation and  3.0 mSv from medical
radiation.  
Figure 8.1  Data from NCRP report 160.
Cosmic ray dose varies exponentially with altitude because of less
atmosphere to block the highly energetic cosmic rays.  It is much
higher in the mountains than at the coast.
Figure 8.2.  Variation in cosmic ray dose rate with altitude.
Terrestrial radiation comes from gamma rays that are emitted by
radium, uranium and thorium.  It varies geographically, being generally
higher in mountainous areas.
Figure 8.4.  Background gamma radiation from primordial isotopes.  
Source:  US Geological Survey.
The largest exposure from natural background radiation comes from radon.  
It also varies widely geographically.  It is given in Becquerels (Bq) per cubic
meter.  One Bq is one disintegration of a nucleus per second.
Figure 8.5.  EPA map of radon in the U.S.  Zone 1 is >150 Bq/cubic meter in
air, Zone 2 is 75-150 Bq/m^3, Zone 3 is <75 Bq/m^3.  
Source:  US Environmental Protection Agency.
Because of the geographical variation of various sources of background
radiation, people are naturally exposed to higher doses in some areas than
in others.  There is no evidence that people living in high dose areas have
more cancer than those living in low dose areas.  In fact, residents of
Colorado have the highest background doses in the US but have the fourth
lowest incidence of cancer.  
Figure 8.6.  Natural background radiation in various locations in the United
States.  
Medical radiation has become a large component of the dose most people
are exposed to.  Of course it varies widely depending on what medical
procedures you have.  The table below gives some average doses for
various medical procedures.  
Diagnostic Procedure
Dose (mSv)
Chest X-ray (1 film)
0.1
Dental oral exam
1.6
Mammogram
2.5
Lumbosacral spine
3.2
PET
3.7
Bone scan (Tc-99m)
4.4
Cardiac scan (Tc=99m)
10
Barium contrast GI fluoroscope (2 min)
85
Spiral CT-full body
30-100
Source:  Data from  DOE Ionizing Radiation Dose Ranges Chart, 2005
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