RESIDENTIAL WIRE CODES - REPRODUCIBILITY AND RELATION WITH MEASURED MAGNETIC-FIELDS

Objectives-To investigate the reproducibility of wire codes to charact erise residential power line configurations and to determine the exten t to which wire codes provide a proxy measure of residential magnetic field strength in a case-control study of childhood leukaemia conducte d in nine states within the United States. Methods-Misclassification o f wire codes was assessed with independent measurements by two technic ians for 187 residences. The association between categories of wire co de and measured level of magnetic field was evaluated in 858 residence s with both a wire code measurement and a 24 hour measurement of the m agnetic field in the bedroom. The strength of the association between category of wire code and risk of leukaemia was examined in two region s with different average levels of magnetic field in homes with high c ategories of wire code. Results-The reproducibility of any of three di fferent classifications of wire codes was excellent (kappa greater tha n or equal to 0.89). Mean and median magnetic fields, and the percenta ge of homes with high magnetic fields increased with increasing catego ry for each of the wire code classification schemes. The size of the o dds ratios for risk of leukaemia and high categories of wire code did not reflect the mean levels of the magnetic field in those categories in two study regions. Conclusion-Misclassification of categories of wi re code is not a major source of bias in the study. Wire codes provide a proxy measure of exposure to residential magnetic fields. If magnet ic fields were a risk factor for leukaemia, however, there would be so me attenuation of risk estimates based on wire codes because of miscla ssification of exposure to magnetic fields at both extremes of the wir e code range. The lack of an association between high categories of wi re code and risk of leukaemia cannot be explained by a failure of the wire code classification schemes to estimate exposure to magnetic fiel ds in the study area.