Re: VARIATIONS IN BACKGROUND

[BERNARD L COHEN <blc+@PITT.EDU>]

	NCRP 45 was published in 1975, long before anything was known

about radon exposures in U.S. Information on that should be disregarded.



Bernard L. Cohen

Physics Dept.

University of Pittsburgh

Pittsburgh, PA 15260

Tel: (412)624-9245

Fax: (412)624-9163

e-mail: blc@pitt.edu





On Wed, 11 Sep 2002 JGinniver@AOL.COM wrote:



> While looking into the original question a bit further I "discovered" a copy

> of NCRP report No. 45 Natural Background Radiation in the United States

> gathering dust on one of my bookshelves.  Section 8.2 covers the variability

> across the US.  To summarise it states:

>

> 8.2.1  Cosmic radiation - the variations of cosmic radiation with latitude,

> solar cycles and the amount of structural shielding within the US are of the

> order of 10%.  Altitude is a significant factor, with a doubling of sea-level

> dose equivalent rate at about 2000 meters.

>

> 8.2.2 Cosmogenic Radionuclides - the average dose equivalent rate is <1

> mrem/y, so that variations are not significant.

>

> 8.2.3  External Terrestrial Radiation - The variability in external

> terrestrial radiation is larger than that for other natural sources of human

> exposure.  The dose is largely determined by the concentrations of K-40 and

> the members of the uranium and thorium series in the soil.  The report

> describes 3 general areas in section 5.4 (Atlantic and Gulf Coastal Plains,

> for the majority of the US and for an indeterminate area along the Rocky

> Mountains).  The external terrestrial dose equivalent rates to the whole body

> of 15, 30 and 55 mrem/y.

>

> 8.2.4  Inhaled Radionuclides - Variability is dependant on local

> concentrations of Rn-222.  There is some increase in areas with elevated soil

> radium levels and a decrease in coastal regions during periods of onshore

> winds.

>

> 8.2.5  Radionuclides in the Body - This mode of exposure is dominated by the

> 20 mrem/y whole body dose equivalent rate from K-40 which is under

> homeostatic control in the body.  Variation in age and sex are given in

> section 7 of the report.  Variations in inhalation and ingestion do not

> produce large changes in dose equivalent for the air and diet levels existing

> in the US for all radionuclides.  The greatest variation would occur in the

> Ra-226 contribution from drinking water and in the lunbg dose from the long

> lived daughter products of Rn-222.

>

> 8.2.6 Overall variability - In looking at the possible exposure variability,

> it must be kept in mind that many differences become blurred in an urbanized

> society.  Most city dwellers have little exposure to bedrock or soil,

> building materials are rarely of local origin and diets are frequently based

> on foods with nationwide distribution.  Thus the exposure of the total US

> population is probably more uniform than would be indicated by comparison of

> terrestrial gamma radiation levels on a geographic basis.

>

> Please note the above is just a summary of section 8.2 of the NCRP report.

> there is considerably more detail in each section of the report which deals

> with each exposure pathway in turn.

>

> Warmest regards

>                    Julian Ginniver

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