1 Section of Social Medicine, Department of Public Health, Faculty of Health and Medical Sciences, Københavns Universitet2 Danish Cancer Society Research Center, Copenhagen Ø, Denmark.3 Risø National Laboratory for Sustainable Energy, Radiation Research Division, Technical University of Denmark, Roskilde, Denmark.4 National Institute of Radiation Protection, Herlev, Denmark.5 Department of Environmental Science, Aarhus University, Roskilde, Denmark; Department for Environmental, Social and Spatial Change (ENSPAC), Roskilde University, Roskilde, Denmark.6 International Agency for Research on Cancer (IARC), Section of Environment and Radiation, Lyon, France.7 Section of Social Medicine, Department of Public Health, Faculty of Health and Medical Sciences, Københavns Universitet
An analysis of confounding by and interaction with other potential risk factors
We investigated whether there is an interaction between distance from residence at birth to nearest power line and domestic radon and traffic-related air pollution, respectively, in relation to childhood leukemia risk. Further, we investigated whether adjusting for potential confounders alters the association between distance to nearest power line and childhood leukemia. We included 1024 cases aged <15, diagnosed with leukemia during 1968-1991, from the Danish Cancer Registry and 2048 controls randomly selected from the Danish childhood population and individually matched by gender and year of birth. We used geographical information systems to determine the distance between residence at birth and the nearest 132-400 kV overhead power line. Concentrations of domestic radon and traffic-related air pollution (NOx at the front door) were estimated using validated models. We found a statistically significant interaction between distance to nearest power line and domestic radon regarding risk of childhood leukemia (p = 0.01) when using the median radon level as cut-off point but not when using the 75th percentile (p = 0.90). We found no evidence of an interaction between distance to nearest power line and traffic-related air pollution (p = 0.73). We found almost no change in the estimated association between distance to power line and risk of childhood leukemia when adjusting for socioeconomic status of the municipality, urbanization, maternal age, birth order, domestic radon and traffic-related air pollution. The statistically significant interaction between distance to nearest power line and domestic radon was based on few exposed cases and controls and sensitive to the choice of exposure categorization and might, therefore, be due to chance.