Re: What is the threshold value for radiation induced cancer?

What is the threshold value for radiation induced cancer?
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This is a rather timely question since the answer to it is currently
under debate in the scientific community.  

In the early days of radiation science and nuclear power it was assumed 
that there was no threshold for radiation induced cancer.  The early data 
on the chances of inducing a cancer were collected at high doses. This data 
is from the atomic bombings of Japan, medical exposures, and laboratory 
experiments on animals, other than humans.  It was assumed that this high 
dose data could be related to low dose data in a linear fashion.  For 
example, if 250 rem caused a cancer risk of X, then 125 rem caused a cancer 
risk of X/2, and so on.  This theory, in combination with the assumption 
that there was no threshold, is called the Linear No-Threshold Theory 
(LNT).

There is now a great deal of evidence that this relationship is too 
conservative and in fact a threshold may exist.  There is even a fair 
amount of evidence that low-levels of radiation, such as those we receive 
every day from natural sources, may be beneficial.  This concept is refered 
to as radiation hormesis.  This evidence, although substantial, is under 
debate and whether or not a scientific consensus will develop on a 
threshold for radiation induced cancer is yet to be seen.

If you would like to take a look at a compilation of data that contradicts 
the LNT theory you can try the following web site:

	http://www.angelfire.com/mo/radioadaptive/
or
	http://cnts.wpi.edu/rsh/


Also, what is the threshold value for radiation induced death?
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By radiation induced death, I am going to assume that you mean an acute 
death from an exposure to radiation in a short amount of time.  (Since a 
radiation induced cancer death which may develop over a long period of time 
was covered above.)

I have made a table below which you can use to get a better understanding 
of the acute effects of radiation exposure.  For a reference point, the 
average annual background exposure in the United States is around 300 mrem 
per year.  A mrem is one 1/1000 of a rem.

Radiation biologists use a term called the LD50.  The LD50 is the dose that 
would be lethal to 50% of those exposed.  For humans this dose is 300 to 
400 rem for young adults without medical help.  If medical help is 
available or if the person is old or young this dose would likely be 
different.  Some individuals who would otherwise die, may be saved by 
antibiotics, platelet infusions or bone marrow transplants.  In animal 
experiements, medical treatment has raised the LD50 by as much as a factor 
of two.

So, to answer your question:  There is a threshold for radiation induced 
death but is variable.  It depends on the person exposed, their health, and 
medical intervention.  Death is unlikely for everyone up to doses of around 
100 rem.


Dose		Effect
----		------

10,000 rem	results in death in 30-50 hours,  death is caused by 
		permeability changes in the small blood vessels of the 
		brain.

 1,000 rem	dealth occurs in about 9 days due to depletion of the 
		epithelial lining of the gastrointestinal tract.

   300 - 	may cause death due to changes in the supply and production
   800 rem	of blood cells, symptoms would occur 3 weeks or more after
		exposure.

   100 rem 	would likely cause (temporary) illness such as nausea and 
		decreased white blood cell count, but not death. Above
		this, severity of illness increases with dose. 

    25 rem	it is around this dose when changes in blood cell counts
		can be detected, but it is unlikely to have any other 	
		effects.

     5 rem  	conservatively the lowest dose rate where there is any
                evidence of cancer being caused. It is also the dose rate 
                which arisies from natural background levels in several  
                places. Above this, the probability of cancer occurrence 
                (rather than the severity) increases with dose. 

    300 mrem 	the normal background radiation from natural sources in 
                North America received in one year, including an average of
                almost 200 mrem from radon in air. 

10 - 60 mrem  	typical range of dose rates from artificial sources of
                radiation, mostly medical, such as chest x-ray. 

      1 mrem	typical range of exposure due to nuclear power and nuclear
                weapons testing