Re: VARIATIONS IN BACKGROUND

[JGinniver@AOL.COM]

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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