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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Two previous epidemiologic studies reported an association between the maximum magnetic field exposure logged during a 24-h measurement period and risk of miscarriage. A hypothesis was put forth which argued that the observed association may be the result of behavioral differences between women with healthy pregnancies (less physically active) and women with miscarriage. We analyzed four existing data sets with power-frequency magnetic-field personal exposure (PE) measurements to investigate the characteristics of peak-exposure measures. We found that the value of the measured maximum magnetic-field exposure varied inversely with the sampling interval between magnetic-field measurements and that maximum values demonstrated less stability over time in repeated measurements, compared to time-weighted average and 95th and 99th -percentile values. We also found that the number of activity categories entered by study subjects could be used to estimate the proportion of subjects with exposure above various threshold values. Exposure metrics based on maximum values exceeding thresholds tend to classify active people into higher exposure categories. These findings are consistent with the hypothesis suggesting that the association between maximum magnetic fields and miscarriage are possibly the result of behavioral differences between women with healthy pregnancies and women who experience miscarriages. Thus, generalization from a given study to more global exposure characterization should be made with particular caution and with due consideration to sampling interval and other characteristics of the measurement protocol potentially influencing the measured maximum. Future epidemiologic studies of peak magnetic field exposure and spontaneous abortion should carefully evaluate the potential confounding effect of the women's activity level during pregnancy.

Magnetic-field exposures are considered in compliance with guidelines if they do not cause the induced electric field or current density to exceed basic restrictions that are based on possible adverse biological responses. Magnetic-field guidelines provide induction models for extrapolating from external field exposures to basic restrictions and vice versa. However, the uniform-field exposures used in these models do not reflect the nonuniform fields often encountered in actual high-field exposures. The purposes of this study were to investigate the relationships between external magnetic-field exposures and induced electric fields in nonuniform 60-hertz fields and to present a method for evaluating the compliance of such exposures with guidelines. Induction factors provide the induced electric field per unit of incident magnetic field. They represent a means of extrapolating from external field exposure to a peak induced electric field. Uniform and nonuniform field induction factors were computed for homogeneous ellipses and ellipsoids, and for an anatomically correct heterogeneous human model. Computations were carried out for three orthogonal uniform fields and for related nonuniform fields at varying distances from three line sources. Analytic expressions were used to compute induced peak electric fields for homogeneous models in uniform fields. A scalar-potential finite-difference method computed induced quantities for all models at 3.6-mm resolution in uniform and nonuniform fields. Equivalent uniform magnetic fields that produce the same peak electric field as a nonuniform field with a known maximum field were derived from the induction factors. To evaluate a nonuniform field exposure for compliance, the equivalent uniform field for the exposure is estimated based on the magnitude of the maximum surface field and the distance from the line source. Compliance is achieved if the equivalent uniform magnetic field is below the magnetic-field limit. Equivalent uniform magnetic-field exposures are computed for two actual electric utility tasks, as examples.

Power-frequency electric and magnetic fields are known to exhibit marked temporal variation, yet in the absence of clear biological indications, the most appropriate summary indices for use in epidemiologic studies are unknown. In order to assess the statistical patterns among candidate indices, data on 4383 worker-days for magnetic fields and 2082 worker-days for electric fields collected for the Electric and Magnetic Field Project for Electric Utilities using the EMDEX meter [Bracken (1990): Palo Alto, CA: Electric Power Research Institute] were analyzed. We examined correlations at the individual and job title group levels among indices of exposure to both electric and magnetic fields, including the arithmetic mean, geometric mean, median, 20th and 90th percentiles, time above lower cutoffs of 20 V/m and 0.2 microT, and time above higher cutoffs of 100 V/m and 2.0 microT. For both electric and magnetic fields, the arithmetic mean was highly correlated with the 90th percentile; moderately correlated with the geometric mean, median, and lower and higher cutoff scores; and weakly correlated with the 20th percentile. Electric and magnetic field indices were generally weakly correlated with one another. Rank-order correlation coefficients were consistently greater than product-moment correlation coefficients. Job title group summary scores showed higher correlations among electric field indices and magnetic field indices and between electric and magnetic field indices than was found for individual worker-days, with only the 20th percentile clearly independent of the others. These results suggest that individuals' exposures are adequately characterized by a measure of central tendency for electric and magnetic fields, such as the arithmetic or geometric mean, and an indicator of a lower threshold or cutoff for each field type, such as the 20th percentile or proportion of time above 20 V/m or 0.2 microT. A single measure of central tendency for each type of field appears to be adequate when exposures are assessed at the job title level.

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.

Several organizations worldwide have issued guidelines to limit occupational and public exposure to electric and magnetic fields and contact currents in the extremely low frequency range (<3 kilohertz). In this paper, we evaluate relevant developments in biological and health research, computational methods for estimating dosimetric quantities, and exposure assessment, all with an emphasis on the power frequency (60 hertz in North America, 50 hertz in Europe). The aim of each guideline is to prevent acute neural effects of induced electric fields. An evaluation of epidemiological and laboratory studies of neurobiological effects identified peripheral nerve stimulation as the response most suitable for establishing a magnetic-field guideline. Key endpoints that merit further study include reversal of evoked potentials; cardiovascular function, as measured by heart rate and heart rate variability; and sleep patterns. High-resolution computations of induced electric fields and current densities in anatomically correct human models are now achieved with finite-difference methods. The validity and limitations of these models have been demonstrated by computations in regular geometric shapes, using both analytic and numeric computations. Calculated values for average dosimetric quantities are typically within a few percent for the two approaches. However, maximum induced quantities are considerably overestimated by numerical methods, particularly at air interfaces. Overestimates are less pronounced for the upper 99th percentile level of a dosimetric quantity, making this measure a more useful indicator of maximum dose. Neural stimulation thresholds are dependent on the electric field around the excitable cell rather than on the current density, making the former preferable for expression of basic restrictions based on nervous system function. Furthermore, modeling data indicate that the induced electric field is much less strongly influenced by tissue conductivity than is the induced current density. In the electric utility industry, most magnetic-field exposures at or near guideline levels occur in highly nonuniform fields. Two methods are described for simplified estimation of induced quantities in such fields, with each method using as input modeling results for uniform field exposure. These methods have practical value for assessing occupational exposures relative to guideline levels.

Combining publicly available data with an expert panel is a viable method to construct an electric-shock MF JEM. This JEM will allow an evaluation of association between electric shocks and neurodegenerative diseases.