Geology and Soil Mechanics, UW-Stout

Quantifying Risk and Radiation

    To understand how radiation damages biological organisms, we must understand what factors go into the measurement of a Sievert, Sv, (or rem - roentgen-man-equivalent).  Either of these measurements can be used to quantify amounts of biological damage received by radiation.   In essence,

Sievert (Sv) = (energy deposited per mass of organism) x (relative biological effectiveness or RBE)

more appropriately written as

Sievert (Sv) = (Gray) x (Quality Factor).

1 Sv = 100 rem, 1 Gray = 1 J/kg, and the quality factors

which depend on the type of radiation.  This indicates that alpha particles are 20 times more damaging than beta particles given the same amount of energy deposited per mass.  The measurement of the Gray can be further broken down into the following factors:

(activity of source) x (energy of each particle in source) x (energy transferred to organism at this particle energy) x (shielding material)^-1 x (distance from source)^-2.

So what does all this mean and how can we put it into useful terms?

General Effects:
    2.5 mSv/yr is the natural background exposure (average).  Note:   1000 mSv = 1 Sv
    5 mSv/yr is the maximum permissible dose-equivalent
    ~ 1 or 2 Sv causes acute symptoms (loss of hair, bleeding, etc.)
    ~ 4 Sv  results in death with a probability of 50%
    > 10 Sv is lethal

Assuming a linear proportionality between risk and dose, 1 mSv increases the risk of lethal leukemia and cancer by about 50 microrisks.  In other words, out of 1 million people exposed to this radiation, 50 of those people will die from this exposure sometime during their life.

1 mSv (exposure) = 50 microrisks

Additional Information about Radiation

Breakdown of the total radiation exposure for the average U.S. citizen
    ~ 0.45 mSv (18%) Non-natural sources - nuclear medicine, medical x-rays, fallout, nuclear power, etc.   
    ~ 2.05 mSv (82%) Natural sources - Radon from the ground, Cosmic rays, rocks and soil, internal consumption, etc.  It is important to note that radon accounts for 55% of the average total exposure to radiation from all sources.

Comparing "Risky" Activity
(information from R. Wilson, Comparing Risks, Physics and Society, October 1990, p. 3-5)

Activities Carrying an Average Risk of Death of One Part per Million (1 microrisk)

Analysis of Linear, No-Threshold (LNT)

    It should be noted that quantifying the health risks of low level exposures to radon gas has been difficult. Public policy has traditionally followed the linear, no threshold model. This model simply states that the more exposure (no matter what level) the higher your health risks. Scientific studies on laboratory rats and atomic bomb survivors (Hiroshima and Nagasaki) suggest that a very small exposure may reduce your health risks!

"This article presents arguments and data from rodents and humans to support the thesis that low level radiation (<0.2 Gy) may be beneficial to the health of the exposed individual."¹    

"...low dose irradiation has been shown to ... trigger ...adaptive responses ... to protect ... biologically important molecules including DNA in the exposed tissue, irrespective of the cause of such damage."¹

"Several recent epidemiologic studies with high statistical significance have reported that exposure to low or intermediate levels of radiation are associated with decreased mortality and/or decreased incidence of cancer..."²

Are you radioactive?

Consider an average 75 kg person who has 75x10²⁶ total atoms
    10% (by weight) is H, ³H/¹H=10^-17, or 45 billion ³H atoms, Activity = 100 decays/sec
    60% is O
    20% is C, ¹⁴C/¹²C=10^-12, Activity = 3000 decays/sec
Carbon 14 is continously generated by cosmic rays hitting the atmosphere
    5.4% is N
    1.3% is P
    1.3% is Ca
    0.5% is S
    0.3% is K, ⁴⁰K/K=10^-4, or about 30x10²⁰ radioactive ⁴⁰K, Activity = 5500 decays/sec
⁴⁰K was generated in supernova explosions before the solar system was formed
    0.02% Rb, ⁸⁷Rb/Rb=0.28, or about 25x10²⁰ radioactive ⁸⁷Rb, Activity = 100 decays/sec

⁴⁰K are the most abundant and energetic.  2/3 of all the decay energy escapes the body, 1/3 gets absorbed into the body giving us a dose-equivalent of 0.15 mSv/year.  Adding ¹⁴C we get 0.18 mSv/year.  The striking conclusion is that 1 person in every 2,000 will be killed by their own radiation sometime during their life (assuming LNT).

The average man irradiates his wife with about 5 nanosieverts every night.  "...A thousand and one happy nights can give her 0.005 mSv.  In this happy way she takes about 1/4 microrisk equivalent to the risk from five pulls on a cigarette!  Is it worth taking?  (Let me inform you that a pull on a cigarette would shorten your life expectancy by 25 seconds, but a virgin lifestyle would shorten your life expectancy by about eight years according to the statistics.)"³

Related Web Links
Radiation Related Rules, Regulations and Laws
Radiation Effects Research Foundation
International Atomic Energy Agency
National Council on Radiation Protection and Measurements

Sources:

¹L.E. Feinendegen, V.P. Bond, C.A. Sondhaus, Can Low Level Radiation Protect Against Cancer?, Physics and Society, v27 (April 1998)

²Myron Pollycove, M.D., The Rise and Fall of the Linear No-Threshold (LNT) Theory of Radiation Carcinogenesis, Physics and Society, v27 (April 1998)

Art Hobson, Physics: Concepts and Connections, Prentice Hall (1995)

W.R. Leo, Techniques for Nuclear and Particle Physics Experiments, Springer-Verlag (1987, p. 65)

³G. Marx, Risks of Radioactivity, Physics Education, v28 (March 1993), p 121-125

For questions or comments regarding these pages contact Dr. Alan Scott / scotta@uwstout.edu / this page was last updated April 05, 2000