Objectives The purpose of this study was to estimate exposure to extremely low frequency (ELF) magnetic and electric fields in the Québec electrical utility Hydro-Québec.

Methods Personal exposures to ELF magnetic and electric fields were measured for workers randomly selected from 32 job categories at Hydro-Québec. Weekly arithmetic and geometric means, and other indices of exposure were estimated from 465 worker-weeks of data.

Results By job category, the arithmetic means of the ELF magnetic field exposures ranged from 0.09 to 2.36 µT. Those of the ELF electric field exposures ranged from 2.5 to 400 V m. Within each field, correlations of either the arithmetic or geometric means with alternative indices, including an index of the time rate of change, were generally high (r 0.8). Exceptions were the 20th percentile of the electric means and the proportion of time above 12.4 and 100 µT. The day-to-day variation of exposure was greater than the variation between workers. The median between-day and between-worker components of variance (as geometric standard deviations) by job category were 2.13 and 1.71 for magnetic fields (2.24 and 1.81 for electric fields).

Conclusions Substation workers, hydroelectric generating station operators, and cable splicers showed the highest arithmetic means for 60-Hz magnetic fields above 1 µT. For 60-Hz electric fields, forestry workers, equipment electricians in 735 kV substations, and distribution linemen (contact method) had arithmetic mean exposures greater than 100 V/m. Of the total variance in the logarithms of the weekly magnetic and electric field means, job category explained 49.6% and 59.5%, respectively.

Key terms correlation; exposure index; job-exposure matrix; positron exposure meter; sampling strategy; variability