clusters and nuclear facilities :

by Joel I. Cehn R«Leports of clusters of cases of disease, allegedly resulting from exposure to ionizing radiation, frequently appear in the press. To the laymen, these reports are difficult to interpret and are often frightening. How do we evaluate such reports? A cluster of disease cases refers to what appears to be an unusual increase in the number of cases. Such clusters can occur either in space or time, or both. Clusters in time would occur, for example, during a winter flu epidemic. Clusters in space are represented by so-called "leukaemia houses" in which several families, successively occupying the same residence, have experienced leukaemia among one or more children. More commonly, clusters refer to groupings over space and time: disease rates in a small geographic area are increased for a period of time. Study of such clustering is important; clusters of cases in a population may provide important clues to agents later recognized as harmful. Much of our knowledge of dangerous exposures has resulted as a consequence of the observations of an astute physician, who has noted an unusual occurrence of disease. Some examples: • The relationship between deafness in children and the existence of measles during the mother's pregnancy was first recognized by an alert Australian ear specialist. He saw an unusual number of cases of deafness in children, which he traced back to a recent epidemic of measles. • The role of vinyl chloride in the genesis of a variety of liver cancer was first uncovered by an industrial physician. He observed several cases of this rare disease among employees of one manufacturer — a far higher incidence than could reasonably be expected.