Intensity of electric and magnetic fields from power lines within the business district of 60 Ontario communities.

Electric and magnetic fields were measured during the summer of 1998 in south-central Ontario within the business district of 60 communities, ranging in size from 1000 to 2.3 million people. The mean magnetic flux density for the 60 communities was 5.8 mG. Communities with larger populations generally had higher magnetic flux densities than those with smaller populations. Communities with populations above 100,000, between 50,000 and 100,000, between 10,000 and 50,000, and less than 10,000 had mean magnetic flux densities of 14, 7, 4 and 2.4 mG, respectively. The city of Kingston, population 123,000, had the highest mean magnetic flux density (47 mG) while Burks Falls, population 1000, had the lowest (0.8 mG). More than 90% of the sites measured in Kingston, Toronto, Oshawa, London, Pickering Village and Bellville were above 2 mG, the lower limit associated with childhood cancers. In only one community (Burks Falls) were all of the measurements in the business district below 2 mG. Diurnal variations were detected in the magnetic field (but not in the electric field) with highest fields measured during business hours. For electric fields, the mean for the 60 communities was 3.2 V/m. Electric fields were generally low. Eight communities had maximum field strengths above 30 V/m and all of these were associated with overhead wires. In larger communities with underground distribution lines the electric fields were low or undetectable (<0.1 V/m) but the magnetic fields were often high. High electric fields were generally associated with low magnetic fields but the relationship was not sufficiently robust to enable prediction of one from the other. Data for the business district measured during business hours appear to be relatively consistent for both electric field and magnetic flux density over a two-year period. Two classification schemes that can be used independently or in combination are proposed to facilitate community comparisons. One is based on the average intensity of the fields (FI) and the other on the percentage of measurements that exceed a critical limit (CL) that has biological significance. The critical value of 5 V/m is proposed for electric fields and 2 mG for magnetic fields. Both classification schemes use the traffic light analogy for exposure (green-low, amber-medium, red-high exposure) with an additional category (black) for very high exposure. This classification system facilitates information transfer and can easily be understood and used by the public, public utilities, policy makers, and those wanting to practice prudent avoidance.