into the body where the nuclide concentrates in the thyroid gland.°* 1°77 13° About 5 to 10 per cent of the I’%! ingested by cows goes into their milk. It is noted that, for the same milk consumption. the thyroid gland of children concentrates much more iodine than that of adults. Fetal thyroids contain more iodine-131 than inaternal thyroid glands on a per gram weight basis.°7 14 The most critical age in postnatal life in regard to dosage has been estimated to be between 6 months and 2 years of age.*” 3! Thyroid doses from fallout iodine-!31 in infants calculated on the basis of 0.7 liter of fresh animal milk consumed daily were (for the United States) 250, 440, and 21 millirads for 1961, 1962, and 1963, respectively.** Evaluations of the relationship of radiation dose to thyroid carcinogenesis have been made onclinical and epidemiological data from populations with known external irradiation by doses in the therapeutic range (50 r to 5,000 r),13?135 Crude risk estimates for thyroid carcinogenesis, based on average follow-up time of 16 years and thought to be valid for acute irradiation of children only in the exposure range of 100 to 300 roentgens,*” have been calculated to be 0.3 to 1.6 cases per 10° exposed population per year per roentgen.*? A risk estimate of high magnitude (35 per 10° per rad) also has been derived.13+ Reports concerning association of iodine-131 and malignant tumors of the thyroid are scarce.***137 [The risk of thyroid cancer from internal exposure to radioiodine has been estimated to be one-tenth that from external x-irradiation.1°*7 The substitution of radio- iodine-free milk (canned or shipped milk) and the feeding of stored feed to cattle have been recommended when critically high levels of iodine-131 are detected.** 15° The use of organic and inorganic thyroid-blocking agents (such as iodides, perchlorides, and thiouracil) has been studied as a counter- measure for sudden high radioiodine contamination events.?°9 Carbon-14. The long-term risk is primarily that from carbon-14, the doses from which are delivered at extremely low rates over a very long period of time.1!° Carbon-14 is present almost exclusively as CO,.*" Testing in 1961 and 1962 has caused a threefold increase in carbon-14 in the different carbon reservoirs (stratosphere, troposphere, sur- face oceans, deep oceans).°? A peak excess of carbon-14 activity at about 70 per cent has been predicted in 1964 or 1965, “The level will then fall to some 60 per cent in seven or eight years’ time. Between 1970 and 2040, the level will gradually fall to some 3 per cent and will remain below this level while the excess C'* will decay radioactively.���5? Carbon-14 contributes verylittle to the total body dose to any single individual during his lifetime. The cumulative genetic effect over a numberof generations, however, may be significant.1°° Relative risks. The long-range risk from fallout has been expressed in terms of dose commitment, which is defined as “the total dose that will be delivered as an average for the world population, to the relevant tissues during the complete decay of radio-active material introduced into the environment.”** The estimates of the dose commitments from all tests before January, 1963, are shown in Table I. The relative risks from fallout radiation from tests carried out before 1963 have been expressed in terms of the period of time dur- Rel”EIAE: tamination.'**"'°9 Food, chiefly milk and its fresh products in children, provides entry ¢ October 1965 °* ing which natural radiation would have to be doubled to give a dose equal to the dose commitment.” Calculated in this way, the periods equal approximately 9 months for the gonads, 32 months for cells lining the bone surfaces, and 20 months for the bone marrow.°*" The likelihood of developing cancer as the result of chronic fallout radiation from the tests of weapons through 1961 has been estimated by the Federal Radiation Council.‘ 196 During the next 70 years it is expected that 840,000 persons will develop leukemia and 140,000 bone cancer. Of these, the esti- mated number of cases caused by natural radiation are 0 to 84,000 for leukemia and 0 to 14,000 for bone cancer. The estimated ehreea Sutow and Conard * deRE mean oegeie 666