'
SeENCLOSURE "A"
STATEMENT
on
20407
OMNIPRESENCE OF BACKGROUND RADIATION
Prepared by the
Division of Biology & Medicine
Not all of the radiation which one ordinarily observes with a Geiger
counter comes from atomic bombs. There is a natural background of
radiation everywhere. It varies from place to place, and even from
time to time, resulting in 30~300 counts per minute on the counter.
Everywhere on the earth's surface, in the air, and even in human
and animal bodies and in plants, there is natural radioactivity due
to the presence of uraniua, radium, radiopotassium and other active
elements in the earth, radon and thoron in the air, and cosmic ray
particles from outer space. Altogether, these radiations result in
doses to human beings of 80 to 800 milliroentgens per year. Depend-
ing somewhat upon the particular counter used, these doses correspond
to 30-300 counts per minute. Dose levels of this amount are harmless
to human beings. In fact, dose rates as high as 300 milliroentgens
per week, corresponding to a rate of about 5500 counts per minute,
have been agreed by international authorities to be safe for human
beings.
All measurements of radiation intensities or contaninations should be
expressed as over and above the background intensity of 30-50 counts
per minute,
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