S14 PROTRACTED EXPOSURE TO FALLOUT ala rate greater than radioactive decay from 1954 ta the presen: (Ne7?: Ne?9: Br&2). Activitv concentrations of '*’Cs and *’Sr tn vegetation were observed to decline at a rate greater than that predicted bs. radioactive decay alone (Ne77,. Ne’?9} Body burdens and urine acuvity concentrauons were observed to increase tapidiv and to decline slowiv throughout the residence ume of persons at Rongelap and Ltink Atolls (Co73: Le8Qb). These observations led to the seiection of a dechming conunuous intake mode’. An exponentia, decline in the amount of activity ingested each sources was assumed. equations were derived applied to each nuchde dav from the dietan The foilowing general (LekOb!. They mav be of concern. CUO ; —_ AP = = Or- ‘ H—. (]) yA. Pe (Speyer f mie wm Alt pe kn mere] Using adult average data, two consecutive urine or bodv-burden measurements were used to estimate the unknown value of 4. a rate constant describing the removal of radioactivity in diet items. This vielded n — | estimates of k where x as the number of measured adult averaze data points for body burden or urineacuvity concentration during the residence in- terval, An average value of & was assigned for the enure residence interva] during which activity Was measured. After the average & was obtained. an estimate of the atom ingesuon rate on dayof return was calculated based on a value for adult average body burden or urine-activity concentration and the time since dav of return. Tats generatec n values of the atom ingestion Tate on day of return where m was again the number of adult average data points for body burden or urine-activitv concentranon. As indicated by equations (1) or (2). a single exponentiai relationship was used to model the decline of radioactivity in diet items. Use of these equations led to an estimate of the dietary x removal rate constant, k. over the enure residency inter-al. The average percent decrease in g-9' (Sve) Tv x: tA mei f,a (SAQe e moh } the vearly activity ingested was determined from this dietary rate constant as follows: rs day of return of each atoll population), d; A= instantaneous fraction of atoms decaying per unit time. d~'; P = initial daily atom inges- tion rate on dav of return. atoms s7~'; A = instantaneous fraction of atoms removed from compartment / in the body by biological processes, d~*; y;= compartment 7 deposition fraction; y’ = the numberof radioactive atoms in compartment / relative to the numberin all compartments on the day of return (some persons returned with body burdens); U = 24-hr or I-l. urine-activity concentration at any time post-return, Bq !~—'; U, = subject urine excretion rate, |}d~'; f, = fraction of element transferred from GI tract to blood; f, = fraction of element reaching extracellular fluid that is excreted through the urine pathway: k = instantaneous fraction of atoms removed from the atom inges- tion rate per unit time, d~', due to factors other than radioactive decay; g = body burden at any time post-return, Bq; and g° = body burden on the day of return, Ba. vo = 100(1 ~*~ *), (3) where °, = average percent decline in the atom mgestion rate during the residence interval The definitions of the other quantities in equation (3) were the same as previously given. The value of 1 was taken at 365 days and the percent reflected the average yearly decline averaged throughout the interval during which a nuclide was observed in people. Thus for "Cs, the average was for a period of 24 yr at Ron- gelap and 27 yr at Utink. In the development of the three equations, several assumptions were made. For instance, decay of nuclides during transit through the stomach and small intestine was assumed to be negligible relative to their decay within the systemic organs. This was because of the long half-life of the nuclides relative to the transit time through the upper portion of the gastrointestinal tract. Urine activity and body-burden data were assumed to Fonerney where ¢ = time post-onset of intake (time from