4. UPTAKE OF Sr?° AND Cs!97 4.1 MILK The comparison of strontium to calcium uptake led to the study of dietary calcium sources for the population. In the United States and, in fact, in most of the temperate countries, milk is a major source of body calcium. This is particularly true during childhood, which is the period of direct skeletal formation, as contrasted to the situation during adult life when calcium turnover is the principal phenomenon, with little or no increase in total bone. Monitoring of Sr® in milk was begun at HASL early in 1954, although individual samples had been analyzed there and elsewhere somewhat earlier. The entire program in the United States and other countries presently includes analysis of milk from about 100 locations and is the largest monitoring program in the study of uptake. In the case of Sr®, although milk has been selected as a monitoringtool, it is not the only source of this isotope. The levels of Srwith respect to calcium are even higher in many vegetables, and the milk source may be most important only in the case of growing children. HASL began analyses of powdered and liquid milk early in 1954. Analysis of the liquid samples was resumed when it was discovered that the powdered milk was not representative of the New York City milkshed. The liquid milk samples are obtained by daily local purchases of 1-qt samples representing major dairies in rotation. These are pooled to give a monthly sample for analysis. The data are reported in Table 23, and the results are plotted in Fig. 9. Weekly powdered milk samples are collected at Perry, N. Y., and these are composited to give monthly samples. These data are reported in Table 24, and the results are plotted in Fig. 10. Other sources of powdered milk have been tested but only Mandan, N. Dak., and Columbus, Wisc., have been able to supply samples on a continuing basis. The data are shown in Table 25 with the Mandan and Columbus results being plotted in Figs. 11 and 12, respectively. Samples of powdered milk have also been received intermittently from Japan and the United Kingdom. At the present time the samples from the United Kingdom are being analyzed in their laboratories, and HASL receives samples for cross-checking purposes only. Japanese milk is obtained when available. The data on samples from these two countries are reported in Table 26. a. USPHS Survey. The USPHS has conducted a pilot study on the radioactivity in milk in five geographic areas. These were the milksheds serving Sacramento, Salt Lake City, St. Louis, Cincinnati, and New York City. The Sr® data are reported in Table 27. In addition, measurements were made on other isotopes: specifically, I'*', sr®*, Ba’, and Cs!97, The USPHSis continuing the analysis of milk and is expanding its network; additional sampling points are being established in the milksheds serving Atlanta, Ga.; Fargo, N. Dak., and Moorhead, Minn.; Austin, Texas; and Spokane, Wash., and a milkshed in southern Wisconsin. This expansion will provide even wider geographic coverage. In order to make these studies, a monthly 1-gal composite sample is collected at a designated point in each of the five milksheds. The collection is arranged through the cooperation of State and municipal health agencies and the dairy industry. 57 mo