(a) that it deposits upon the ground adsorbed to a solid or liquid
particulate and is subliming, continuing to contaminate the environmen-
tal air; or (b) that it deposits upon crops, is ingested by cattle and
reaches the human through milk and milk products.
Although this latter
pathway has been demonstrated possible, it should not, on a basis of
present information, be considered the only route after initial exposure
has occurred.
Despite large individual variations in iodine uptake, data to
date indicate: (a) that iodine-131 becomes available for wide distribution over the earth's surface from air, land surface and water surface
bursts; and (b) fractionation appears to occur to the extent that urine
samples indicate that iodine-13l tends to a global equilibrium state and,
except for the factor of time of arrival, the doses to local and to distant personnel are comparable.
The maximum sefely permissible amount of I-131 in the human
body on a continuing exposure basis has been stated to be 0.3 microcur-
ies B/ While this implies that for short periods of exposure a higher
dose rate may be permissible, such as medical tracer doses of 5-50 microcuries, it should not be overlooked that the degree of thyroid tissue
damage is a function of dose rate as well as dose and that consequently
the tissue effect from acute and chronic doses of the same total amount
are not comparable.
Thus, for purposes of setting meaningful fall-out exposure tol-
erances, this MFC for iodine-131 is misleading and inadequate.
Although
an exposure to fall-out is somewhat longer than the exposure from the
ingestion of the medically used isotope, the similarity is sufficient to
suggest employment of the "safe" single dose of 5-50 microcuries established by the medical profession.
It is of interest to point out that no
evidenceoftissuedamagehas been reported for amounts under 100 micro-
sugies
If the MPC for iodine is exceeded, the significance would be
of a lower order of magnitude than in the case of strontium-90 due to
48/ National Bureau of Standards Handbook 52.
12h