Quantifying the effect of population mixing on childhood leukaemia risk: the Seascale cluster

A statistical model was developed based on Poisson regression of incidence of childhood leukaemia and non-Hodgkin's lymphoma (NHL) in relation to popu lation mixing among all 119 539 children born 1969-1989 to mothers living i n Cumbria, north-west England, (excluding Seascale), This model was used to predict the number of cases in Seascale (the village adjacent to the Sella field nuclear installation) children, born 1950-1989 and diagnosed before 1 993. After allowing for age, the incidence of acute lymphoblastic leukaemia (ALL) and NHL was significantly higher among children born in areas with t he highest levels of population mixing, relative risk (RR) = 11.7 (95% conf idence interval (CI) 3.2-43) and was highest among children of incomers. Th e model predicted up to 3.0 (95% CI 1.3-6.0) cases of ALL/NHL in children b orn in Seascale compared to six observed and 2.0 (95% CI 1.0-3.4) cases in children resident, but not born, in Seascale compared to two observed, Popu lation mixing is a significant risk factor for ALL/NHL, especially in young children, accounting for over 50% of cases in Cumbria and most cases in Se ascale.