Stability of Wertheimer-Leeper wire codes as a measure of exposure to residential magnetic fields over a 9- to 11-year interval.

The Wertheimer-Leeper (W-L) wire code is a construct used as a surrogate indicator of residential exposure to electromagnetic fields. However, little is known about how changes in electrical distribution systems affect wire code assignment. The wire code was determined for 150 homes in the Seattle, WA, area twice, 9-11 years apart. For each home, the authors evaluated whether the electrical configuration around the home and the wire code changed between the two time points. The effect of wire code misclassification on observable odds ratios was evaluated, given hypothetical true control distributions and two different dose-response curves. There was an electrical configuration change for 77 (51.3%) homes, which resulted in a wire code change for 29 (19.3%) homes. Eight (5.3%) other homes had a wire code change due to mapping errors or methodological inconsistencies. Misclassification masked the shape of a threshold (nonlinear) dose-response curve and changed the slope of a linear dose-response curve. Although the wire code detected less than half of electrical configuration changes, misclassification of exposure over time may change odds ratios and mask possible dose-response relationships.