Results obtained among workers with more than 1.0 or 1.5 hours of substation/3-phase work (Table 4) were very similar to those obtained using the 2-hour cut point (Table 3). Differences between the upper and lower tertiles were progressively greater as the duration of time spent in substation/3-phase environments increased. There were no statistically significant differences in mean 6-OHMS excretion among subjects below the chosen cut points for substation/3-phase activities or among those with 1-phase work activities above or below the cut points (results not shown).
Decreased nocturnal or post-work urinary 6-OHMS excretion have been associated with magnetic field exposures in studies of electric railway workers 20 and in our earlier studies of electric utility workers. 17–18 In the present study, another population of male electric utility workers had decreased overnight 6-OHMS levels as well as lower nocturnal and post-work 6-OHMS/cr concentrations with increasing exposure to 60-Hz magnetic fields in substations or near energized 3-phase conductors. Differences in mean 6-OHMS excretion between the upper and lower exposure tertiles became progressively greater as the cut point for the amount of time spent in substations and in 3-phase environments increased from 0.5 to 2 hours. These findings are consistent with the hypothesis that magnetic fields with circular or elliptical polarization are more effective at suppressing melatonin production than linearly polarized fields. 24–27 The lack of effects observed in those with 2 hours or less of substation/3-phase work or among those with 1-phase exposures further supports the hypothesis. Alternatively, this classification scheme may have simply selected those with more intense and temporally stable exposures. However, if intensity or temporal stability was the critical parameter, then one might also expect to observe a trend of decreasing mean 6-OHMS excretion among those with 2 hours or less of substation/3-phase work or among those with 1-phase exposures. A trend of decreasing mean 6-OHMS excretion was observed only among those with more than 2 hours of substation/3-phase work, even though a gradient of exposure across tertiles and similar magnitudes of magnetic field intensity or temporal stability were observed among subjects in each group of substation/3-phase and 1-phase activity. Clearly, further investigation of magnetic field exposures in substations and in the vicinity of 3-phase and 1-phase conductors is needed. The intensity, temporal stability, and degree of magnetic field polarization in each environment should be quantitatively assessed along with other potentially relevant magnetic field parameters, such as high frequency transients and harmonic content.
Temporally stable magnetic field exposures that occurred in substation/3-phase environments were more strongly associated with decreased mean 6-OHMS excretion than magnetic field intensity, as measured by the geometric mean. These findings are consistent with previous studies in electric utility workers that indicated decreased 6-OHMS excretion in response to temporally stable magnetic field exposures. 17–18 The importance of temporally stable magnetic field exposures in eliciting biological effects was originally described by Litovitz and coworkers. 33 The basis for the biological activity of temporally stable exposures remains unexplained but may provide a clue as to the fundamental mechanism of interaction between 60-Hz magnetic fields and melatonin production. Kruglikov and Dertinger 34 indicate that a highly correlated exposure is required for stochastic resonance at a cellular level. However, further work is required to determine whether such a mechanism might mediate the effects of temporally stable magnetic field exposures on 6-OHMS excretion in humans.
Studies performed in rats by Kato and coworkers indicated that circularly polarized magnetic fields were more effective at inducing melatonin suppression than linearly polarized fields. 24–27 They observed decreased circulating melatonin concentrations in rats when using 1.4 μT circularly polarized magnetic fields. 24,25,27 The same group reported that chronic exposure to a horizontally polarized magnetic field was effective at a higher intensity of 5 μT but not at 1 μT. 26–27 Linearly polarized 50/60-Hz magnetic fields have been effective at reducing circulating melatonin levels in other rodent studies, 35–38 although results have been inconsistent. 39–42 Sheep penned under a 3-phase transmission line had no noticeable changes in circulating melatonin levels after 6 to 10 months of exposure.42a Field polarization at ground level under the power lines was not reported, although a large axial ratio (ie, close to linear polarization) would have been expected.27–28 Inasmuch as no other laboratory has attempted to evaluate the effects of field polarization on magnetic field induced melatonin suppression in experimental animals, the role of this parameter remains undefined.
The characterization of human biological responses to 60-Hz magnetic fields is critical for determining whether concern over potential health effects is warranted. Melatonin suppression is a plausible link to increased cancer risks that have been associated with such exposures. Results from the present analysis suggest that magnetic field induced melatonin suppression seems to be enhanced by work in substations and with energized 3-phase conductors. Failure to characterize magnetic field polarization or other potentially important modifying factors 18,49 may partially explain the inconsistent findings reported to date. Recently developed personal exposure devices are now available to evaluate the role of field polarization and other biologically based exposure parameters on human 6-OHMS excretion. 50 Reduced melatonin secretion may serve as an important model for understanding human biological responses to magnetic field exposures.
The authors gratefully acknowledge the cooperation of the participating utilities, their employees who participated in this study, and their representatives. Urinary 6-OHMS assays were performed under the direction of Dr Terry Nett, Director of the Radioimmunoassay Laboratory for the Colorado State University Animal Research and Biotechnology Laboratories.
In particular, the authors thank Ms Jeanette Haddock for assistance with data collection, Ms Xiao Ming Sha for assistance with the 6-OHMS assay, Drs Lee Wilke and Martin Fettman for assistance with the creatinine assays, and Mr Travers Ichinose and Dr Annette Bachand for assistance with data processing. Dr Scott Davis of the Fred Hutchinson Cancer Research Center provided the design for adaptation of the light meters to the EMDEX monitors. Battelle Pacific Northwest Laboratories and Platte River Power Authority provided light meters. Mr Ken Webster provided computer programming assistance.
This work was supported by research grant no. 1 R01ES08117 from the National Institute of Environmental Health Sciences, National Institutes of Health, Bethesda, Maryland.
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