Possible health issues of ELF EMFs include cardiovascular effects since both electrocardiogram and heart-rate changes have been reported in the literature. A non-linear relationship between field strength and biological response has been reported in some studies. In this study, a total of 59 subjects, divided into three independent magnetic field strength groups, were compared. A calculated 12-hour time weighted average (TWA) value of the fields was used as an exposure metric for each of the three locations ("low": 0.067 μT, "medium": 1.18 μT and "high": 5.2 μT) and subsequently used to estimate workers' exposure at these sites. Electrocardiograms were recorded in the resting position. Five parameters were derived from the ECG: heart rate (HR), duration of P wave and QRS wave, and duration of PR and QT intervals. The QT intervals were normalized to a heart rate of 60 (QTc). The obtained data were analyzed first by means of multivariate analysis of covariance and then oneway univariate analyses of covariances (ANCOVA) using exposure duration as a covariate. Only the ANCOVA on the QTc interval was significant. Our results suggest that the relationship between field strength and response is non-linear: the adjusted mean QTc values are similar between the "low" and the "high" group, but significantly lower in the "medium" group. One possible interpretation of our results is that a specific exposure pattern might be responsible for the non-linear effects observed, so that generally, characterizing exposure to electric and magnetic fields using simple metrics such as TWA may be insufficient.

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