A validity analysis of residential magnetic fields estimated from high-voltage transmission lines

Between November 1994 and May 1995, indoor power frequency magnetic fields of 407 residences in northern Taiwan were assessed by short-term on site measurements and by modeling computerized power lines information from high-voltage transmission lines. The study residences were selected according to the distance from the transmission lines with cutoff points of 50 meters (m), 100 m, and 150 m, which gave rise to four categories of residences. The analysis showed that the measured magnetic fields were higher than the calculated magnetic fields for most residences, especially for those with lower measured magnetic fields. The measured and calculated magnetic fields showed an agreement of 0.93 (intra-class correlation coefficient, [ICC] = 0.93) for the residences within 50 m of the transmission lines. The ICC declined with the distance from the transmission lines with a lowest ICC of 0.42 for the residences located more than 149 m away from power lines. When both measured and calculated magnetic fields were categorized into three levels with cutoff points of 1 milligauss (mG) and 2 mG, the indices of agreement were very similar for the three residential categories within 149 m of the lines with Kappa (K) between 0.51 and 0.55. The K for the residences more than 149 m from the lines was low at 0.29. The ICC and K observed from a reduced sample of 114 residences presumably representative of all residences in northern Taiwan with respect to the distribution of household magnetic fields was 0.90 and 0.64, respectively. Our data show that using exclusively computerized power lines information to assess residential magnetic fields exposure entails a certain degree of exposure misclassification, but in the absence of direct measurement, this way of doing provides reasonable estimates of magnetic fields exposure within 100 m of transmission lines.