LONG-TERM REDUCTION OF CESIUM CONCENTRATION IN MILK AFTER NUCLEAR FALLOUT

Time trends in activity concentrations in milk as observed in Austria after the Chernobyl accident are presented. Both the short term decrea se immediately after fallout and the medium term decline in the years following the event are very important for estimating the total exposu re to be expected from a given deposition. In order to avoid artifacts due to local fallout, plant variations, or differences in the metabol ism of single animals, large areas of production were used for the obs ervations. This was achieved by observations of activity concentration s in milk powder, produced in large milk powder plants in Austria. Aft er an initial decay with an approximate half-life of 34 days for the p eriod of May to August 1986, a slower decrease in activity was observe d during the following years. Observed half-lives are in the range of 1.5-2.0 a. Differences in the decrease observed between the different producing areas are discussed. The radiocesium contamination of milk a nd milk products depends directly on its presence in grass or hay and therefore, time trends observed in milk correspond closely to the time trend in these fodders. Other foodstuffs which are also produced on g rass and hay, such as beef or lamb, should therefore display similar d ecay patterns, except for the early period after fallout when the biol ogical half-lives in the animals influences the decrease.