Residential Magnetic Fields, Light-at-Night, and Nocturnal Urinary 6-Sulfatoxymelatonin Concentration in Women
Scott Davis1,2,
William T. Kaune3,
Dana K. Mirick1,
Chu Chen1 and
Richard G. Stevens4
1 Program in Epidemiology, Division of Public Health Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA.
2 Department of Epidemiology, School of Public Health and Community Medicine, University of Washington, Seattle, WA.
3 EM Factors, Richland, WA.
4 Department of Community Medicine, University of Connecticut Health Center, Farmington, CT.
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ABSTRACT
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Exposure to 60-Hz magnetic fields may increase breast cancer risk by suppressing the normal nocturnal rise in melatonin. This 19941996 Washington State study investigated whether such exposure was associated with lower nocturnal urinary concentration of 6-sulfatoxymelatonin in 203 women aged 2074 years with no history of breast cancer. Each woman was interviewed and provided data on the following for a 72-hour period at two different seasons of the year: 1) magnetic field and ambient light measured every 30 seconds in her bedroom, 2) personal magnetic field measured at 30-second intervals, and 3) complete nighttime urine samples on three consecutive nights. Lower nocturnal urinary 6-sulfatoxymelatonin level was associated with more hours of daylight, older age, higher body mass index, current alcohol consumption, and current use of medications classified as beta blockers, calcium channel blockers, or psychotropics. After adjustment for these factors, higher bedroom magnetic field level was associated with significantly lower urinary concentration of 6-sulfatoxymelatonin during the same night, primarily in women who used these medications and during times of the year with the fewest hours of darkness. These results suggest that exposure to nighttime residential 60-Hz magnetic fields can depress the normal nocturnal rise in melatonin.
breast neoplasms; carcinogens, environmental; circadian rhythm; electricity; electromagnetic fields; melatonin
Abbreviations:
BMI, body mass index
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INTRODUCTION
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It has been suggested that exposure to 60-Hz magnetic fields may increase the risk of breast cancer by suppressing the normal nocturnal rise in melatonin production and release (1
), thereby resulting in increased levels of circulating estrogen. Several lines of inquiry have been pursued to investigate a possible link between pineal function, circulating estrogen level, and breast cancer risk (summarized by Stevens and Davis and by Brainard et al. (2
, 3
)). Other than limited evidence that blood melatonin levels are reduced in human volunteers exposed to magnetic fields (4
), there have been few studies of the effect of magnetic field exposure on pineal function in humans. Although some results from experimental and occupational studies suggest that nocturnal melatonin levels can be reduced by exposure to magnetic fields, the evidence thus far is inconsistent and incomplete (5



10
). More importantly, it remains unknown whether such exposures can alter the endogenous hormonal environment in women in a manner that might be important in the etiology of breast cancer. Therefore, the present study was undertaken in 19941996 to investigate, for the first known time in women, whether exposure to magnetic fields and/or light-at-night is associated with lower nocturnal concentration of the primary metabolite of melatonin found in the urine (6-sulfatoxymelatonin).
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MATERIALS AND METHODS
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Study participants
Participants were women aged 2074 years selected from a group of 591 women in King and Snohomish counties in Washington State who participated as controls in a case-control study of breast cancer and exposure to electromagnetic fields (11
). The women initially were identified by random digit dialing (12
). A sample was selected to provide approximately equal representation of the highest and lowest bedroom magnetic field exposures. Of the 31 women still living in homes classified as Very High Current Configuration according to the scheme developed by Wertheimer and Leeper (13
) to approximate exposures inside a residence on the basis of external wiring configurations, 26 (84 percent) agreed to participate. Each of the remaining 556 potentially eligible women was ordered by the mean magnetic field level measured in her bedroom over a 48-hour continuous period during her participation in the case-control study. Eighty-three (82 percent) of the 101 eligible women who had the highest measured exposures and 94 (85 percent) of the 110 eligible women who had the lowest measured exposures agreed to participate. The institutional review board approved the protocol for contacting potential participants and the manner in which informed consent was obtained.
Data collection and laboratory methods
Data collection consisted of the elements described in table 1 for a 72-hour measurement period. The entire protocol was repeated approximately 3 or 6 months later, based on random assignment, to provide measurements in different seasons of the year. This study design made it possible to investigate the effects of different lengths of daily darkness on any potential association between magnetic field exposure and urinary 6-sulfatoxymelatonin level. The study took place over approximately 14 months.
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TABLE 1. Data elements studied to determine a possible association between exposure to magnetic fields and/or light-at-night and urinary 6-sulfatoxymelatonin concentration, Washington State, 19941996
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The volume of urine was determined, and each sample was assayed for creatinine concentration based on a kinetic modification of the Jaffe reaction using the Roche Reagent for Creatinine (Roche Diagnostic Systems, Nutley, New Jersey). Urinary concentrations of the primary metabolite of melatonin, 6-sulfatoxymelatonin, were determined by using commercially available radioimmunoassay kits (CIDtech Research Inc., Mississauga, Ontario, Canada). The assay was run in duplicate with 500 µl of diluted sample. Each run included the kit control provided by the manufacturer and an in-house control using a urine sample provided by a volunteer at the beginning of the study was used. Assay sensitivity was 0.5 ng/ml urine, and intra- and interassay percent coefficients of variation were approximately 9 and 13 percent, respectively.
Statistical methods
Nine exposure variables were defined prior to analysis to characterize a participant's exposure to magnetic fields. The following three reflected exposure to magnetic fields in the bedroom at night: 1) mean nighttime bedroom magnetic field exposure, 2) proportion of nighttime bedroom magnetic field measurements
0.2 µT, and 3) short-term variability in the bedroom magnetic field. Nighttime was defined for each subject and each night as the time period between the last void before going to bed and the first void the next morning. Mutually exclusive 10-minute time blocks were used to group exposure measurements. A statistic (Y) was computed to characterize short-term variability in the bedroom magnetic field, as follows:
where Bk, k = 1, 2, ... , 20 are the 20 values of the measured magnetic field recorded at 30-second intervals during a 10-minute time period. This statistic, when divided by
(i.e., the number of measurement intervals 1), is equal to the "rate-of-change metric" introduced by Yost (14
). The bedroom variability statistic was defined as the average of all 10-minute Y statistics during the nighttime period.
Three variables reflected personal exposure to magnetic fields on a continual (24-hour) basis: 1) mean 24-hour personal magnetic field exposure, 2) proportion of 24-hour personal magnetic field measurements
0.2 µT, and 3) short-term variability in the personal magnetic field (defined as above). In addition, one variable was constructed to reflect the wire code configuration assigned to each participant's residence according to the scheme developed by Wertheimer and Leeper (13
). Two variables were defined to characterize exposure to light-at-night: 1) proportion of nighttime bedroom light measurements
10 lux and 2) number of times the subject reported getting up and turning on a light.
Additional factors known or suspected to affect melatonin levels were specified a priori for use in covariate adjustment. Primary covariates were defined as those well established from previous research to affect melatonin levels and included participant age, menopausal status, and duration of darkness. Participants were classified as premenopausal if they reported regular or irregular periods, did not use hormone replacement therapy, and had not undergone a hysterectomy with oophorectomy; otherwise, they were classified as postmenopausal. Fifteen participants were considered "indeterminate" according to this definition and were classified as premenopausal if aged <50 years and as postmenopausal otherwise. Numbers of hours of darkness each night that urine was collected were obtained from the US Naval Observatory for the Seattle, Washington, metropolitan area. Secondary covariates were defined as factors suspected to affect melatonin levels and included the following: 1) alcohol consumption within the previous 24 hours; 2) current or recent (within the last 30 days) use of an electric blanket; 3) smoking within the last 7 days; 4) body mass index (BMI; weight (kg)/height (m)2), categorized according to the Surgeon General's Report on Nutrition and Health (15
); 5) height; and 6) nightly use of any medications categorized as a beta blocker, calcium channel blocker, or psychotropic. To ascertain medication use, participants listed any medications used during each night of urine collection. A pharmacologist subsequently grouped the reported medications into these three categories, which were selected on the basis of evidence that drugs of these types can affect melatonin levels (16
18
).
Since urine samples and exposure data were collected for each participant on 3 consecutive days during two different sessions, each subject contributed up to 6 days of exposure and melatonin data. Individual observations were highly correlated; thus, a repeated-measures analysis was required. The presence of exposure variables and covariates that changed with each measurement day and/or session required that these variables be considered time dependent in the repeated-measures setting. All analyses used the SAS MIXED procedure to fit linear regression models with correlated error structure to account for the correlation of the repeated measurements on each subject (19


23
). The error structure had restricted maximum likelihood estimation of a 6 x 6 covariance matrix with three unknown parameters: day-to-day variation, session-to-session variation, and residual error. For all regression analyses, the response variable was the natural log transformation of 6-sulfatoxymelatonin concentration normalized to creatinine (nanograms of melatonin divided by milligrams of creatinine). Mean nighttime bedroom magnetic field, short-term variability in the bedroom magnetic field, mean 24-hour magnetic field, and short-term variability in the 24-hour magnetic field were transformed by using the natural log transformation. The proportions of nighttime bedroom and 24-hour personal magnetic field measurements
0.2 µT, and the proportion of nighttime bedroom light measurements
10 lux, were transformed by using a modified definition of the logit transformation (24
). If there was an indication of an exposure effect on 6-sulfatoxymelatonin concentration, an interaction term between the exposure measure of interest and the number of hours of darkness was added to the model to investigate exposure effect modification by different lengths of daily darkness.
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RESULTS
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Descriptive characteristics
Of the 203 participants enrolled in the first measurement session, 200 provided complete (72-hour) bedroom meter measurements, 201 provided complete personal meter measurements, 168 provided three complete urine samples, and 200 provided at least one urine sample. Three subjects became ineligible for the second measurement session because they changed residences between sessions. Of the remaining 200 women, 198 provided complete bedroom measurements, 200 provided complete personal measurements, 171 provided three complete urine samples, and 195 provided at least one urine sample. Most data losses were accounted for by partial urine collection. Samples from participants who reported spillage of urine that the laboratory technician estimated to represent >2 percent of the total volume were deemed unusable (n = 37 samples, 21 in session 1 and 16 in session 2). There were eight samples from five subjects whose concentrations of 6-sulfatoxymelatonin were below the detectable limits of the assay (0.5 ng melatonin/ml urine). These samples were assigned one half the value of the detectable limit of the assay before being normalized to creatinine level. Samples from both measurement sessions for one participant and one measurement session for two participants were excluded from the analysis because they reported using melatonin supplements at least once during the measurement period, which resulted in 6-sulfatoxymelatonin levels that were elevated more than 100 times over the unsupplemented values.
Table 2 shows the distributions of two of the three primary covariates and the secondary covariates. The distribution of number of hours of daily darkness was slightly U-shaped (data not shown), reflecting a slightly greater number of total sample days occurring at the extremes of the range of daily darkness (8.0 hours at the summer solstice and 15.6 hours at the winter solstice).
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TABLE 2. Distribution of participants in a study of exposure to magnetic fields and light-at-night, according to primary and secondary covariates, Washington State, 19941996
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Urinary 6-sulfatoxymelatonin concentration
A total of 1,106 nocturnal urine samples were available for analysis. The distribution of urinary 6-sulfatoxymelatonin concentration was right skewed, meaning that the majority of the measurements were at the low end of the scale (mean, 19.3 ng/mg creatinine; standard deviation, 12.8). The concentration varied somewhat according to the season of the year; each season was defined as the 3 months centered on the respective equinox or solstice. Concentrations were highest in winter (mean, 22.5 ng/mg creatinine) and lowest in summer (mean,16.8 ng/mg creatinine). Spring and fall values were intermediate (means, 20.1 and 18.1, respectively). Urinary 6-sulfatoxymelatonin concentrations were highly and significantly correlated from day to day within each measurement session (Spearman's rank correlation coefficients: 0.90 and 0.85, respectively for the two sessions; p < 0.0001 for both) as well as between measurement sessions (Spearman's rank correlation coefficient: 0.75; p < 0.0001).
Measures of exposure to magnetic fields and light-at-night
Table 3 summarizes the measures of exposure to magnetic fields and light-at-night. Mean nighttime bedroom magnetic field levels were low; half of the subjects had mean levels of <0.04 µT. Mean 24-hour personal magnetic field levels were higher. The distributions for both measures were highly right skewed. The distribution of the proportion of nighttime bedroom measurements
0.2 µT was also highly right skewed, with more than 70 percent of the nights having no time
0.2 µT. However, only two measurement days had 24-hour personal magnetic field levels with no time
0.2 µT. For both nighttime bedroom and 24-hour personal measurements, mean magnetic field levels and proportions of levels
0.2 µT were highly and significantly correlated from day to day within each measurement session.
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TABLE 3. Descriptive summary of measures of exposure to magnetic fields and light-at-night, Washington State, 19941996
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The distributions of the variables reflecting short-term variability in bedroom and 24-hour personal magnetic field levels were similar for all four seasons. Values were significantly correlated day to day within each measurement session; bedroom measures were more highly correlated than 24-hour measures. Wire code classification of the homes of the 203 subjects was distributed as follows (data not shown): 57 lived in homes classified as Very Low Current Configuration, 72 in UnderGround, 20 in Ordinary Low Current Configuration, 28 in Ordinary High Current Configuration, and 26 in Very High Current Configuration. For each of the four seasons, the lowest two categories accounted for the greatest proportion of subjects.
The distributions of both the proportion of nighttime bedroom light measurements
10 lux and the number of times the subject reported getting up and turning on a light were highly right skewed. The distributions of both measures were similar for all four seasons and were significantly correlated day to day within measurement session, although only moderately so.
Effects of exposure to magnetic fields and light-at-night
Table 4 summarizes the parameter estimates and the estimated difference in nocturnal urinary 6-sulfatoxymelatonin level per unit change in each of the primary and secondary covariates. The estimated effects of each covariate were consistent across all models, regardless of the exposure term in the model (listed in table 5). Participant age was significantly inversely associated with urinary 6-sulfatoxymelatonin concentration: each additional year in age was associated with approximately a 1 percent lower nocturnal urinary 6-sulfatoxymelatonin level. Adding a quadratic term for age did not materially alter the relation. Number of hours of darkness was directly associated with nocturnal 6-sulfatoxymelatonin level: each additional hour of darkness was associated with a 2 percent higher 6-sulfatoxymelatonin concentration. In none of the models was menopausal status associated with 6-sulfatoxymelatonin.
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TABLE 4. Parameter estimates and estimated difference in nocturnal urinary 6-sulfatoxymelatonin concentration for primary and secondary covariates, evaluated as a group, Washington State, 19941996
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TABLE 5. Parameter estimates from regression analyses between log 6-sulfatoxymelatonin and nine indicators of exposure to residential magnetic fields and light-at-night, Washington State, 19941996
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Relative to the lowest BMI category, each higher category was associated with approximately an 8 percent lower nocturnal 6-sulfatoxymelatonin concentration. Reported use of beta blockers, calcium channel blockers, or psychotropic medications was associated with approximately a 28 percent lower nocturnal urinary 6-sulfatoxymelatonin concentration. Consumption of alcohol in the 24 hours preceding morning urine collection was consistently related to a lower 6-sulfatoxymelatonin concentration, but at suggestive levels of significance (this relation is explored further in reference (25
)). The following factors were not significantly associated with 6-sulfatoxymelatonin levels in any of the analyses and thus were excluded from subsequent models: current or recent use of an electric blanket, measured magnetic field from an electric blanket if used, smoking, and height.
After adjustment for the primary covariates, higher mean nighttime bedroom magnetic field level was associated with lower 6-sulfatoxymelatonin level in the urine at night (table 5). Inclusion of an interaction term between number of hours of darkness and mean nighttime magnetic field revealed that the overall effect changed with different lengths of daily darkness (i.e., at different times of the year). Higher mean nighttime magnetic field was significantly associated with lower 6-sulfatoxymelatonin concentration until the length of darkness exceeded about 12 hours, the vernal and autumnal equinoxes (table 6).
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TABLE 6. Parameter estimates from regression analyses between log 6-sulfatoxymelatonin and nighttime bedroom magnetic field level, Washington State, 19941996
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Further adjusting this model for BMI category or alcohol consumption did not materially change the relation between mean nighttime magnetic field level and nocturnal 6-sulfatoxymelatonin concentration. However, adjustment for medication use did alter this relation. Mean nighttime magnetic field remained inversely related to 6-sulfatoxymelatonin concentration, but, overall, this effect was no longer significant. Inclusion of an interaction term between mean nighttime magnetic field and medication use showed that the effect of exposure differed between those who did and did not use medications that might affect melatonin. Among subjects who reported taking medications that can affect melatonin level, there was a significant exposure effect that changed in a continuous manner with number of hours of darkness. A statistically significant inverse association was observed between mean nighttime magnetic field and 6-sulfatoxymelatonin concentration until length of darkness exceeded about 12 hours (table 6). Among those who did not report taking such medications, there was an indication of an inverse association between nighttime magnetic field and 6-sulfatoxymelatonin concentration, but this association was nonsignificant regardless of the number of hours of darkness. This relation is displayed graphically in figure 1 for both users and nonusers of these medications.

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FIGURE 1. Estimated effect (and 95% confidence interval), using the dose-response slopes presented in table 6, of log mean nighttime bedroom magnetic field on log urinary 6-sulfatoxymelatonin concentration for female nonusers (top panel) and users (bottom panel) of medications that can affect melatonin levels, Washington State, 19941996. Test of no overall difference between the two regression lines, p = 0.1, using the chi-square reduced model likelihood ratio test.
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To summarize these results in a more practical format, the dose-response slopes shown in table 6 were used to estimate the percentage decrease in urinary 6-sulfatoxymelatonin concentration associated with higher levels of mean nighttime bedroom magnetic field for both users and nonusers of medications that can affect melatonin levels. Table 7 displays these results for four specific days of the year corresponding to the midpoints of the four seasons. A reference level of 0.04 µT was chosen since it was the median value of magnetic field exposure for all 1,171 nights. The results showed, for example, that among women taking medications that can affect melatonin level, a twofold higher mean nighttime bedroom magnetic field level was associated with an 8 percent lower urinary 6-sulfatoxymelatonin concentration at the summer solstice.
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TABLE 7. Estimated percentage decrease in urinary 6 sulfatoxymelatonin concentration on 4 days of the year, for selected levels of mean nighttime magnetic field exposure, Washington State, 19941996
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The analysis was repeated by using both untransformed and square-root-transformed mean nighttime magnetic field in the regression model, and the results essentially were unchanged. Additionally, results were not materially different when the model was restricted to the medication users rather than all participants.
Similar results were found after adjustment for the primary covariates when exposure was characterized by the proportion of bedroom magnetic field measurements
0.2 µT (table 5). After further adjustment for the secondary covariates, for those subjects who reported taking medications that can affect melatonin level, there was a statistically significant inverse association between the proportion of nighttime magnetic field measurements
0.2 µT and 6-sulfatoxymelatonin concentration until the length of darkness exceeded about 12 hours (table 6). The association was strongest with the lowest number of hours of darkness. For subjects who did not report taking such medications, there was an indication of an inverse association, but this relation was nonsignificant regardless of the number of hours of darkness. The results were essentially unchanged when the model was restricted to medication users.
Although the logit transformation was used to help account for the fact that the proportion of nighttime magnetic field levels
0.2 µT was highly right skewed, an attempt also was made to evaluate how sensitive the results were to this aspect of the data by creating an indicator variable (yes if the measurement for any nighttime interval was
0.2 µT, no if otherwise). When this variable was used, the results were essentially unchanged.
After adjustment for the primary covariates, there was no evidence of significant differences in 6-sulfatoxymelatonin concentration associated with any of the following (table 5): 1) variability in nighttime bedroom magnetic field, 2) mean 24-hour magnetic field based on the personal measurements, 3) proportion of personal magnetic field measurements
0.2 µT, 4) variability in personal magnetic field measurements, 5) wire code configuration, 6) proportion of nighttime light levels
10 lux, and 7) reported number of times a participant got up and turned on the light. Results were unchanged when the regression models for these exposure variables were extended to adjust for the effects of the secondary covariates (results not shown).
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DISCUSSION
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This study suggests that exposure to a higher magnetic field strength, as measured in a woman's bedroom during the night, is associated with a lower concentration of 6-sulfatoxymelatonin in the woman's urine during the same night. It is noteworthy that a number of findings were consistently observed regarding factors previously reported to affect melatonin levels. A strong association was found with the number of hours of darkness. Season of the year is known to affect the nocturnal rise in melatonin (16
), due presumably to seasonal shifts in the time of the nocturnal peak in melatonin (26
). Increasing age was significantly associated with lower melatonin levels, a finding consistent with some previous studies (27
, 28
) but not all (29
). Consumption of alcohol was associated with lower melatonin levels, consistent with findings in rats (30
, 31
) and humans (32
, 33
). Finally, a substantially lower 6-sulfatoxymelatonin concentration was observed in those who reported taking classes of medications (beta blockers, calcium channel blockers, psychotropics) known to affect melatonin levels (16
18
). These results demonstrate internal consistency and suggest that the present study was capable of detecting changes in urinary melatonin levels that might be expected.
Several findings from this study suggest that there may be variability in individual susceptibility to the effects of magnetic field exposure. That is, those persons with lower baseline melatonin levels, for whatever reason, may be more susceptible to the additional effects of magnetic fields in lowering their melatonin concentration. The lower 6-sulfatoxymelatonin concentration associated with magnetic field exposure was most pronounced in those who used medications known to reduce melatonin levels. Furthermore, the strongest magnetic field effects were observed during the summer months when melatonin levels were lowest, regardless of participants' medication use. Since melatonin levels were lower at increasing ages, an exploratory analysis was conducted in which age was investigated as a potential modifier of the magnetic field effect on 6-sulfatoxymelatonin concentration. The magnitude of the magnetic field effect increased somewhat with older age, but not significantly so. Consistent with these findings are two studies that reported considerable individual variability in sensitivity to the effects of light-at-night on melatonin levels in humans (34
, 35
) and results from an experiment conducted by Graham et al. whereby reductions in melatonin associated with exposure to 20.0-µT magnetic fields were observed only in those with low baseline melatonin levels (5
). However, two subsequent experiments conducted in a similar manner failed to replicate this finding (5
, 6
).
A number of exposure measures were not associated with nocturnal urinary 6-sulfatoxymelatonin concentration, including 1) 24-hour personal magnetic field measurements, 2) variability in magnetic field levels, 3) two measures of light-at-night, and 4) wire code configuration. It is not clear from existing studies whether daytime exposure to magnetic fields could act on melatonin rhythms at night or whether 24-hour measurements are sensitive enough to detect such an effect if one exists. Short-term variability in magnetic field levels was investigated based on limited evidence that variation in the application of a magnetic field (e.g., intermittent vs. constant) might enhance the biologic effect of exposure (8
). Since a measure of natural light was included in the analysis, the proportion of light measurements
10 lux primarily reflected the ambient light levels in the bedroom, which were extremely low. Self-report of turning on a light during the night may be subject to a greater degree of misclassification than actual bedroom measurements of light. In this study, wiring configuration, a surrogate measure of magnetic field exposure, was poorly correlated with nighttime magnetic field measurements.
The biologic significance of the reductions in urinary 6-sulfatoxymelatonin concentration found in this study over long periods of time is unknown. Nevertheless, these results are of considerable interest in the context of mechanisms that could affect development of breast cancer. In its role as a neuroendocrine transducer, the pineal gland provides a hormonal signal that can affect release of gonadotropins (luteinizing hormone and follicle-stimulating hormone) from the pituitary (36

39
). These two hormones are critical in the biosynthesis of steroid hormones in the ovary, including estradiol (40
, 41
). Consequently, pineal function, through the reduced secretion and action of melatonin, may influence ovarian function and estrogen production and thereby result in increased levels of circulating estrogen. There is a substantial body of experimental, epidemiologic, and clinical evidence that breast cancer risk is influenced by endogenous hormones (reviewed by Bernstein and Ross and by Dao (42
, 43
)). Animal studies have repeatedly demonstrated that estrogens can induce and promote mammary tumors in rodents (43
), and numerous epidemiologic studies have found increased estrogen levels to be an important factor in determining risk of breast cancer in humans (42
).
Evidence has recently emerged that melatonin levels can be suppressed by electric and magnetic fields and by light, that manipulation of melatonin levels can affect development of mammary carcinoma in animals, and that exposure to either magnetic fields or light can enhance development of chemically induced mammary carcinoma in animals (reviewed by Stevens and Davis and by Brainard et al. (2
, 3
)). The weight of this evidence provides a framework for postulating a mechanism that might explain how exposure to power-frequency magnetic fields could influence the risk of breast cancer. The results reported here provide intriguing suggestions that exposure to magnetic fields in the home setting at night is sufficient to depress the normal nocturnal rise in circulating melatonin. In the present study, these effects were associated with relatively low levels of exposure and focus attention on the possibility that they occur primarily in persons whose melatonin levels are already low or perhaps are more susceptible to change. Additional studies designed to clarify the influence of magnetic field exposures on reproductive hormones in humans, and individual variation in baseline levels of melatonin and susceptibility to change in melatonin levels, would be most useful.
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ACKNOWLEDGMENTS
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This research was supported by the Electric Power Research Institute through contracts with EM Factors of Richland, Washington (WO 2964-25) and the Fred Hutchinson Cancer Research Center in Seattle, Washington (WO 2964-24).
The authors thank the following persons for their valuable contributions to this study: Norma Logan, project management; Laurie Ludwig, management of field operations; Elizabeth Carosso, data management; Peggy Adams Myers, contract administration; JoAnn Prunty, melatonin analysis; Betsy Gilbert and Christine Karlsen, field technicians; and Dr. Leeka Kheifets, project support at the Electric Power Research Institute.
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NOTES
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Reprint requests to Dr. Scott Davis, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North MP-474, P.O. Box 19024, Seattle, WA 98109-1024 (e-mail: sdavis{at}fhcrc.org).
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Received for publication July 6, 2000.
Accepted for publication December 20, 2000.