ESTIMATING EXPOSURE IN STUDIES OF RESIDENTIAL MAGNETIC-FIELDS AND CANCER - IMPORTANCE OF SHORT-TERM VARIABILITY, TIME-INTERVAL BETWEEN DIAGNOSIS AND MEASUREMENT, AND DISTANCE TO POWER-LINE

Validity of exposure assessment methods has been a major concern in ep idemiologic studies of magnetic field exposure and cancer. We conducte d a study to evaluate the relative importance of distance to power lin es and calculated historical magnetic fields when estimating past magn etic field exposure. Another goal was to compare results based on vari ous estimates of magnetic field exposure, to assess the importance of shortterm variability in magnetic fields, time between diagnosis and m easurement, and sources of magnetic field exposure. We used data from a Swedish case-control study of residential exposure to magnetic field s and cancer. Childhood leukemia risk was associated with calculated h istorical annual average magnetic fields regardless of distance, and t he association with distance disappeared when both variables were ente red into the same logistic regression model. Relative risks fur measur ements at the time of the study (contemporary annual average fields, s pot calculations, and spot measurements) were all close to or below un ity. The results support the hypothesis that the difference between re sults using historical calculations and spot measurements is explained by the time interval between diagnosis and contemporary magnetic fiel d estimates.