Re: Ra, 25 mrem/y

["Otto G. Raabe" <ograabe@ucdavis.edu>]

At 03:43 PM 12/09/1999 -0600, Paul Frame wrote:
>> The NRC DCGL for Ra-226 is given as:
>> Ra-226............................  7.0 E-01 pCi/g
>> 
>I don't know what the known dose from 0.7 pCi/g of Ra-226 is supposed to be,
>but let me argue that the real value might be much higher or, at the very
>least, pretty close to 25 mrem.
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December 10, 1999
Davis, CA

Paul makes some very good points about exposure world-wide from the natural
presence of radium-226 in all soil and rock at an average concentration of
0.7 pCi/g. Most of the exposure from this natural radioactivity comes from
the radon-222 that may be released to the air, so that all air on the
surface of the earth contains radon and its decay products. He estimates 15
mrem/year to soft tissue of the body in addition to the EDE of 200
mrem/year from alpha irradiation to lung tissue. In addition, he estimates
8 mrem from external exposure to gamma radiation from radon decay products
from radium-226 in soil to a depth of 15 cm. So, the total is about 25
mrem/year from 0.7 pCi/g of radium-226 in soil. 

However, it may not be reasonable to expect this same level of exposure to
be associated with geographically tiny sites having contamination above
background that results in an additional equal concentration of radium-226
but in a limited area or volume of soil. Radon effluent from small outdoor
sources will be quickly diluted to negligible levels by mixing in ambient
air. It has been estimated (NCRP, BEIR V) that the average external
exposure from all of the numerous natural radionuclides in soil and rock
from the uranium and thorium decay series averages about 28 mrem/year. This
includes the radium-226 and its decay products. Even if 8 mrem/year comes
from the radium-226, a limited area would have to be quite extensively
contaminated to approach the exposure that occurs from the world-wide
natural concentration of radium-226. Even with all of the radium naturally
found in soil and water, the internal dose from world-wide exposure to
radium-226 is only a very small fraction of the 39 mrem/year that has been
estimated (mostly from K-40). Most radium ends up in the skeleton where the
target organ (bone surfaces) has a very small tissue weighting factor
(0.01). Hence, the only way to get near 25 mrem per year from 0.7 pCi/g
above background levels would be to have a vary large area and quantity of
soil contaminated to this level so that a person might be able to live
totally in this contaminated micro-environment. [Alternatively, we could
move to Colorado.]

Otto
 

		*****************************************************
		Prof. Otto G. Raabe, Ph.D., CHP
              Institute of Toxicology & Environmental Health (ITEH)
		   (Street address: Building 3792, Old Davis Road)
		University of California, Davis, CA 95616
		Phone: 530-752-7754  FAX: 530-758-6140
		E-mail ograabe@ucdavis.edu
              *****************************************************
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