Within-day variability of magnetic fields among electric utility workers: Consequences for measurement strategies (vol 60, pg 713, 1999)

Occupational exposure to 50-Hz magnetic fields was surveyed among electric utility workers to investigate (1) components of exposure variability, (2) patterns of autocorrelation between short-term measurements, and (3) imprec ision and misclassification due to short-term measurements. Spot measuremen ts every 10 seconds during 81 working days were analyzed for 42 electric ut ility workers from 10 occupational subgroups and during 8 working days for 4 office workers from the same company. For the 8-hour time-weighted averag e (TWA) magnetic fields, the variability was partitioned into: its componen ts: within workers, between workers, and between groups. For spot measureme nts of magnetic fields, the within-day variance component also was examined . Autocorrelation functions were determined and numbers of short-term measu rements necessary for reliable estimates of 8-hour TWA magnetic fields were assessed. Spot measurements of magnetic fields, as well as 8-hour TWA magn etic fields, were approximately lognormally distributed among workers. The mean exposure to magnetic fields was 0.47 mu T (n=81 days) in electric util ity workers and 0.12 mu T (n=8 days) in office workers, A large fraction, 7 6% of the spot measurements total variance, could be attributed to variabil ity within days. For the 8-hour TWA magnetic fields, between-group variabil ity was small and of the same magnitude as between-worker variability. Sign ificant autocorrelations between short-term averages of 7.5, 15, and 30 min utes were present, when taken within periods of 30 minutes. One-hour averag es showed no autocorrelation, Simulations showed that, due to high within-d ay variability and autocorrelation, a limited number of short-term measurem ents of magnetic fields in electric utility workers are likely to result in imprecise estimates of 8-hour TWA magnetic fields. Measurement strategies relying on short-term (spot) measurements are therefore likely to result in misclassification of exposure and consequently absent or spurious exposure -response relations.