20 years after exposure would be safe, 5 microcuries would begin to produce radiographically demonstrable skeletal changes, and 50 - 200 microcuries might correspond to the Leo in 20 years.” It is felt that even this statement cannot be depended upon because of the uncertainties discussed in prior sections relating to the state of knowledge of the deleterious effects of both radium and strontium-90 when lodged in the body. At present, it is perhaps fair to state that the MPC as derived may have to be lowered when applied to children and to longer periods of exposure than 20 years. No projec- tion of current knowledge to levels of hazard beyond the MPC and, perhaps, the lowest estimated damage burden should be made. The Iodine-131 Problem. Reports from Operation CASTLE that radioactive iodine was found in the urine of natives and test personnel aave been substantiated by several test programs in this country Wiz43: 46/ In these programs, iodine-131 was detected in appreciable amounts in cattle and humans and found to have a biological half-life such as to indicate that it must have originated in the CASTLE series of nuclear detonations. The quantitative similarity in amount of iodine present in persons in the United States and in Honolulu as compared with the CASTLE test participants exposed to fall-out in the Pacific test area warrants consideration of this element both as a local fallout hazard and as a world-wide contaminant. As a consequence of nuclear fission and subsequent chain decay many isotopes of iodine are formed. Of these only two, iodine-131 and iodine-133, appear to have half-lives of a duration worth considering as potentially hazardous. Iodine-133 has a half-life of 22 hours as compared with that of 8 days for iodine-131, and thus at time of 4h/ Jones, H., Confirmation of Radioactivity in Thyroid of Various 3 ° . 45/ Van Middlesworth; Nucleonics, Vol. 12, Sept 1954. 46/ Hartgering, J-B., et al., unpublished, Army Medical Service Grad“uate School, 1955. 122