To study the effect of magnetic fields on the risk of miscarriage, we conducted a population-based prospective cohort study among pregnant women within a large health maintenance organization. All women with a positive pregnancy test at less than 10 weeks of gestation and residing in the San Francisco area were contacted for participation in the study. We conducted in-person interviews to obtain information on risk factors for miscarriage and other potential confounders. All participants were also asked to wear a magnetic field-measuring meter for 24 hours and to keep a diary of their activities. Pregnancy outcomes were obtained for all participants by searching the health maintenance organization's databases, reviewing medical charts, and telephone follow-up. We used the Cox proportional hazard model for examining the magnetic field-miscarriage association. A total of 969 subjects were included in the final analyses. Although we did not observe an association between miscarriage risk and the average magnetic field level, miscarriage risk increased with an increasing level of maximum magnetic field exposure with a threshold around 16 milligauss (mG). The rate ratio (RR) associated with magnetic field exposure > or = 16 mG (vs <16 mG) was 1.8 [95% confidence interval (CI) = 1.2-2.7]. The risk remained elevated for levels (in tertiles) of maximum magnetic field exposure > or = 16 mG. The association was stronger for early miscarriages (<10 weeks of gestation) (RR = 2.2, 95% CI = 1.2-4.0) and among "susceptible" women with multiple prior fetal losses or subfertility (RR = 3.1, 95% CI = 1.3-7.7). After excluding women who indicated that their daily activity pattern during the measurements did not represent their typical daily activity during pregnancy, the association was strengthened; RR = 2.9 (95% CI = 1.6-5.3) for maximum magnetic field exposure > or = 16 mG, RR = 5.7 (95% CI = 2.1-15.7) for early miscarriage, and RR = 4.0 (95% CI = 1.4-11.5) among the susceptible women. Our findings provide strong prospective evidence that prenatal maximum magnetic field exposure above a certain level (possibly around 16 mG) may be associated with miscarriage risk. This observed association is unlikely to be due to uncontrolled biases or unmeasured confounders.

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This paper describes the study design, measurement protocols, and results of a project examining residential magnetic -field exposures at eight sites across the contiguous United States. The goal of the project was to investigate surrogates that have been used in epidemiologic studies to characterize residential magnetic -field exposure. These surrogates include: personal -exposure ( PE ), fixed -location long -term ( LT ), and outside and inside point -in -time ( PIT ) magnetic -field measurements; net -service ( or ground current ) measurements; and the ''wire -code category'' of the residence. ( The latter is a surrogate for magnetic -field exposure based on the nature and proximity of electric power lines outside the house. ) Measurements were conducted on four visits to each of eight sites between January 1994 and June 1997 for a study population of 218 single -unit detached dwellings. Information on the residence, residents, and neighborhood was collected. A simple random sample of 392 single -unit detached dwellings at the sites was used to create a weighted sample of houses representative of the population of single -unit residences. The correlations among the various types of 60 -Hz magnetic -field measures were relatively strong ( Pearson r > 0.74, Spearman > 0.78 ). Variability of PE and LT measurements, as measured by the standard deviations during a visit, was independent of wire -code category. Visit means for PE, LT, and outside and inside PIT were well correlated over periods between visits of from 1.5 to 20 months ( r > 0.62, > 0.76 ). These results support the use of survey measurements ( less demanding than personal monitoring ) to represent exposure that occurred up to 20 months in the past. The principal component of the total variance in PE measurements was the between -house variance; betweenvisit and between -site variances were generally less important. This supports the sampling of many houses with relatively few visits in residential exposure characterization studies. There was a trend for presumably higher wire -code categories to be associated with higher field summary measures for all summary measures related to magnetic -field magnitude, including PE and LT resultant, and inside and outside resultant ( 60 Hz ) and harmonics. However, because of the overlap in field levels between categories, wire code was not a good predictor of magnetic -field levels, accounting for less than 21% of the variance in magnetic -field measurements.

The Electric and Magnetic Field Measurement Project for Utilities--the Electric Power Research Institute (EPRI) Electric and Magnetic Field Digital Exposure (EMDEX) Project (the EPRI EMDEX Project)--was a multifaceted project that entailed technology transfer, measurement protocol design, data management, and exposure assessment analyses. This paper addresses one specific objective of the project: the collection, analysis, and documentation of power-frequency magnetic field exposures for a diverse population of utility workers. Field exposure data measured by an EMDEX system were collected by volunteer utility employees at 59 sites in four countries between September, 1988, and September, 1989. Specially designed sampling procedures and data collection protocols were used to ensure uniform implementation across sites. Volunteers within 13 job classifications recorded which of eight work or three nonwork environments they occupied while wearing an EMDEX meter. Approximately 50,000 hours of magnetic field exposure records taken at 10 s intervals were obtained, about 70% of which were from work environments. Exposures and time spent in environments were analyzed by primary work environment, by occupied environment, and by job classification. Generally, for utility-specific job classifications related to the generation, transmission, and distribution of electricity, the field and exposure measurements in terms of workday mean field were higher than in more general occupations. The job classifications with the highest (median workday mean) exposure were substation operators (0.7 microT) and electricians (0.5 microT). Total variance also tended to be largest for utility-specific job classifications. For these workers, the contributions of between-worker and within-worker variances to total variance were about the same. Measurements in utility-specific environments were higher than in more general environments. Estimates of time-integrated exposure indicated that utility-specific job classifications received about one-half or more of their total exposure on the job. The nonwork field and exposure distributions for workers in all job categories were comparable with median nonworkday means of about 0.09 microT.

Recent interest in the transient magnetic field events produced by electrical switching events in residential and occupational environments has been kindled by the possibility that these fields may explain observed associations between childhood cancer and wire codes. This paper reports the results of a study in which the rate of occurrence of magnetic field events with 2–200 kHz frequency content were measured over 24 h or longer periods in 156 U.S. residences. A dual‐channel meter was developed for the study that, during 20 s contiguous intervals of time, counted the number of events with peak 2–200 kHz magnetic fields exceeding thresholds of 3.3 nT and 33 nT. Transient activity exhibited a distinct diurnal rhythm similar to that followed by power frequency magnetic fields in residences. Homes that were electrically grounded to a conductive water system that extended into the street and beyond, had higher levels of 33 nT channel transient activity. Homes located in rural surroundings had less 33 nT transient activity than homes in suburban/urban areas. Finally, while transient activity was perhaps somewhat elevated in homes with OLCC, OHCC, and VHCC wire codes relative to homes with underground (UG) and VLCC codes, the elevation was the smallest in VHCC and the largest in OLCC homes. This result does not provide much support for the hypothesis that transient magnetic fields are the underlying exposure that explains the associations, observed in several epidemiologic studies, between childhood cancer and residence in homes with VHCC, but not OLCC and OHCC, wire codes. Bioelectromagnetics 21:197–213, 2000. © 2000 Wiley‐Liss, Inc.

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The purposes of the research reported here were to quantify the power-frequency magnetic-field exposures of cable splicers while they were performing tasks in energized network distribution vaults and to compare these exposures with occupational exposure guideline levels. Network vaults supply electricity to commercial and residential urban areas as well as to large buildings. Participating workers wore a personal exposure monitor at the waist, kept a simple diary to record their work location, and recorded information about the vaults and tasks performed. To capture temporal variability, a stationary meter was deployed in the vault during a task. Load current in the vault was measured. To examine temporal variability over long time periods, stationary meters were deployed in selected vaults for one month. Data were collected during 77 tasks in 69 vaults for 191 person-tasks, representing approximately 400 hours of in-vault personal exposure data. Highest exposures were observed in tasks performed near secondary conductors. Personal exposure variability arises principally from worker movement and activities in the vaults, not from load variability during a task. Maximum field during a person-task exceeded the International Committee on Non-Ionizing Radiation Protection (ICNIRP) (0.42 millitesla) and the American Conference of Governmental Industrial Hygienists (ACGIH) (1.0 millitesla) guideline levels during 14 percent and 8 percent of the person-tasks, respectively. The mean of measurements during a person-task exceeded those guideline levels during 4 percent and 2 percent of the person-tasks, respectively. A large number of person-tasks (40%) had measured fields above the ACGIH recommended limit of 0.1 millitesla for workers with pacemakers or other implanted devices. Based on the frequency and duration of their high exposures, cable splicers working in network distribution vaults are one of the most highly exposed groups in the electric utility industry. Selective assignment of work location and task could minimize the likelihood of exposures for vault workers exceeding guideline limits for wearers of pacemakers or other implanted devices. Scheduling vault tasks during off-peak hours (nights and weekends) may reduce exposures. However, even during these periods exposures in certain vaults can still exceed guideline levels.

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