(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