75 posure under extreme conditions, with no correc- tive measures taken to reduce internal absorption of radioisotopes prior to evacuation of the exposed people from thefallout area. It did not involve the contaminated pasture-cattle-milk cycle, which might be an important pathwayof radioiodine to man in other types of accidents (such as the Windscale accident).176 Civil Defense planning can provide for several measures that will reduce the hazard of thyroid exposure due to radioiodine absorption and thus largely preclude the degree of thyroid injury sus- mean latent period for radiation-induced thyroid tumors may be as long as 30 years. Cases have been seen as late as 40 years after exposure. Fur- thermore,on the basis of the present data the risk of developing radiation-induced thyroid neoplasia is probably underestimated,since surgical removal of potentially malignanttissues may have occurred ‘and the hormonetreatment may haveinhibited the developmentof some tumors, althoughthelat- ter is questionable. As has been pointed out,also, the true carcinogenic potential of the exposure, particularly in the children receiving the higher tained by the Marshailese. Since the hazard from dose, may have been masked by excessive cellular tant. These include avoiding inhalation of radio- posed people who have not shown any thyroid abnormalities indicates that the thyroid effects in the radioiodine is acute for a period only of days, early protective measures are extremely imporiodines by shelter protection; consuming food and water only from closed containers; feeding cows in contaminated areas protected fodder; and temporarily withholding contaminated milk supplies and diverting them into processed products with a shelflife longer than the life of the isotope. The addition ofstable iodine to food or water during the first week woulld providea relatively inexpensive method of reducing thyroid uptake of radioiodines by isotope dilution and saturation with non-radioactive iodine; this shouldrarely produce anyserious side effects and would be ofparucular value in children and pregnant women. When exposure of the thyroid has already occurred, prophylactic treatment with thyroid hormone, now being used in the Marshallese, may help prevent developmentof thyroid tumors, and even after tumor development, surgical excision may reduce mortality due to malignancy. The thyroid effects in the Marshallese were not anticipated at the time ofthe accidentor during the early years afterwards. In retrospectthis is not surprising, for several reasons. At that time the thy- roid was thoughtto berelatively radioresistant, particularly with regard to radioiodine exposure (on the basis of animal studies and diagnostic and therapeutic use of 1511 in people), and the calculated thyroid doses in the Marshallese were con- sidered to be below thelevels likely to produce tumors.In addition, neither the importanceof the exposureto short-lived iodine isotopes in fallout nor the thyroid dose differential in children due to the smaller size of their gland was fully appreciated. It is quite likely that thefinalresults of thyroid lesions in the Marshallese are incomplete at this ame since new lesions are still occurring. The 99Co172 destruction. The recent finding that subclinical thyroid deficiency is present in some of the exMarshallese may not yet be completely manifest, and continued careful surveillance of this population is necessary. Acknowledgments Because of the broad scopeof the surveys and the complexity of operations it would be impos- sible to acknowledge all the individuals and crgan- izations that have contributed to the success ofthe surveys over the past 20 years. We would like, however, to express thanks to some whohave con- tributed vitally in more recent years. Our foremost appreciation is to the people of . Rongelap and Utirik who haveparticipated in the examinations, including the unexposed people comprising the control population. We treasure the friendship andtrust of these people. We are grateful to many Rongelap and Utirik people for assisting the examining teams overthe years, in- cluding the Magistrates of both islands - at Ron- gelap: John Anjain, Billiet Edmond, Jabwe Jorju, and Niktimos Antak; at Utirik: Kabto Campus. Maja Lena, Harold, the school principal, and Joata, the health aide. The senior authoris most grateful to the many fine medical and otherspecialists (listed in Appendix 2) whohaveso willingly given of their time and talents, without remuneration,to participate in the surveys. Without their dedicated efforts the surveys could never have succeeded. Heis also greatly indebted to the manytechnical assistants who have contributed much tothe success ofthe surveys, particularly to Mr. W. Scott and Mr. D.