CORRELATIONS AMONG INDEXES OF ELECTRIC AND MAGNETIC-FIELD EXPOSURE INELECTRIC UTILITY WORKERS

Power-frequency electric and magnetic fields are known to exhibit mark ed temporal variation, yet in the absence of clear biological indicati ons, the most appropriate summary indices for use in epidemiologic stu dies are unknown. In order to assess the statistical patterns among ca ndidate indices, data on 4383 worker-days for magnetic fields and 2082 worker-days for electric fields collected for the Electric and Magnet ic Field Project for Electric Utilities using the EMDEX meter [Bracken (1990): Pale Alto, CA: Electric Power Research institute] were analyz ed. We examined correlations at the individual and job title group lev els among indices of exposure to both electric and magnetic fields, in cluding the arithmetic mean, geometric mean, median, 20th and 90th per centiles, time above lower cutoffs of 20 V/m and 0.2 mu T, and time ab ove higher cutoffs of 100 V/m and 2.0 mu T. For both electric and magn etic fields, the arithmetic mean was highly correlated with the 90th p ercentile; moderately correlated with the geometric mean, median, and lower and higher cutoff scores; and weakly correlated with the 20th pe rcentile. Electric and magnetic field indices were generally weakly co rrelated with one another. Rank-order correlation coefficients were co nsistently greater than product-moment correlation coefficients. Job t itle group summary scores showed higher correlations among electric fi eld indices and magnetic field indices and between electric and magnet ic field indices than was found for individual worker-days, with only the 20th percentile clearly independent of the others. These results s uggest that individuals' exposures are adequately characterized by a m easure of central tendency for electric and magnetic fields, such as t he arithmetic or geometric mean, and an indicator of a lower threshold or cutoff for each field type, such as the 20th percentile or proport ion of time above 20 V/m or 0.2 mu T. A single measure of central tend ency for each type of field appears to be adequate when exposures are assessed at the job title level. (C) 1994 Wiley-Liss, Inc.