Analyses of magnetic-field peak-exposure summary measures.

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.

Wire coding in the EMF and Breast Cancer on Long Island Study: relationship to magnetic fields.

The Electromagnetic Fields and Breast Cancer on Long Island Study (EBCLIS) is a large population-based case-control study investigating possible associations between magnetic fields and breast cancer, and includes a comprehensive set of in-home measurements. We investigated the reproducibility of wire codes, their relation to 24-h measurements of residential magnetic fields, and potential influences, such as housing characteristics, in homes of the 1161 EBCLIS participants. Replicate wire coding was performed in homes originally categorized as having very high current configurations (VHCC) in the Wertheimer-Leeper (W-L) wire coding scheme, and a random sample of other homes (235 residences). Reproducibility was very high, with a kappa statistic of 0.83 (95% confidence interval (CI)=0.77-0.89) for the five-category W-L wire codes and 0.91 (95% CI=0.86-0.95) for the three-category Kaune-Savitz (K-S) codes. As levels of W-L and K-S wire codes increased, the mean and median 24-h levels of broadband and harmonic fields in the residences also increased, indicating an association between wire codes and magnetic fields measurements. Regions of Long Island with the highest percentage of homes built before 1950 had the highest percentage of higher current configuration homes, as well as the highest average 24-h broadband and harmonic measurements. Adjustment for age of the home and region did not affect the relation between wire codes and measured magnetic fields. Our results indicate that: (a). a high reproducibility in wire coding was achieved, (b). wire codes were correlated with magnetic fields, and (c). wire code levels were related to the age of the home. The high level of reproducibility suggests that, in our case-control analyses, there will be minimal bias due to misclassification of wire code categories. Results also suggest that wire codes are a proxy measure, to some degree, for current in-home magnetic field measurements in this study.

Results of a multisite study of U.S. residential magnetic fields.

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 rho>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, rho>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; between-visit 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.

Rate of occurrence of transient magnetic field events in U.S. residences.

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.

Residential wire codes: reproducibility and relation with measured magnetic fields.

OBJECTIVES: To investigate the reproducibility of wire codes to characterise residential power line configurations and to determine the extent to which wire codes provide a proxy measure of residential magnetic field strength in a case-control study of childhood leukaemia conducted in nine states within the United States. METHODS: Misclassification of wire codes was assessed with independent measurements by two technicians for 187 residences. The association between categories of wire code and measured level of magnetic field was evaluated in 858 residences with both a wire code measurement and a 24 hour measurement of the magnetic field in the bedroom. The strength of the association between category of wire code and risk of leukaemia was examined in two regions with different average levels of magnetic field in homes with high categories of wire code. RESULTS: The reproducibility of any of three different classifications of wire codes was excellent (kappa > or = 0.89). Mean and median magnetic fields, and the percentage of homes with high magnetic fields increased with increasing category for each of the wire code classification schemes. The size of the odds ratios for risk of leukaemia and high categories of wire code did not reflect the mean levels of the magnetic field in those categories in two study regions. CONCLUSION: Misclassification of categories of wire code is not a major source of bias in the study. Wire codes provide a proxy measure of exposure to residential magnetic fields. If magnetic fields were a risk factor for leukaemia, however, there would be some attenuation of risk estimates based on wire codes because of misclassification of exposure to magnetic fields at both extremes of the wire code range. The lack of an association between high categories of wire code and risk of leukaemia cannot be explained by a failure of the wire code classification schemes to estimate exposure to magnetic fields in the study area.

Problems and priorities in epidemiologic research on human health effects related to wiring code and electric and magnetic fields.

Because of a reported excess of cancers among children living near power lines, there is some concern that electric and magnetic fields (EMFs) induced by electric power sources may affect human health, and this possibility has provoked considerable controversy. The scientific question of whether there are such health effects is far from resolved. Building upon a set of detailed reviews of the available evidence, this paper proposes research priorities and places particular emphasis on epidemiologic research. The most pressing need is to verify the validity of the claim that childhood cancer risk is affected by the type of wiring code in the vicinity of the household. More useful work can be done to verify this in the areas in which such studies have already been carried out, and additional studies should be done elsewhere. Methodological investigation of the interrelationships among different measures and proxies for EMF is needed, and this could feed back to influence the type of EMF measures used in epidemiologic studies. Studies of cancer among adults in relation to EMFs in the work place are needed. Of lower priority are studies of adverse reproductive outcomes in relation to parental EMF exposure and studies of the neurobehavioral impact of chronic EMF exposure. This article also discusses the structural impediments of conducting environmental epidemiology research and argues that bold, large-scale epidemiologic monitoring systems are needed. There is a discussion of the interface between epidemiology and public policy in a topic area as controversial as EMFs.

Childhood cancer in relation to a modified residential wire code.

Several studies have found associations between wire configuration codes, a proxy for historical residential magnetic field exposure, and childhood cancer. The Wertheimer-Leeper coding method was modified by eliminating the distinction between thick and thin primaries, distinguishing only between open and spun secondaries, and reducing the number of categories from five to three. The association between the modified code and measured magnetic fields was similar to the association with the original wire code. The modified code was used to reanalyze data from a case-control study of childhood cancer in the Denver metropolitan area. In the original study, cases were diagnosed from 1976 to 1983 among children under age 15 and compared to controls selected through random digit dialing. Wire codes for the residence at diagnosis yielded imprecise elevations of two and above for very high current configuration homes or modest 1.5-fold elevations for a dichotomous wire code. In contrast, the modified Wertheimer-Leeper code generated risk estimates that were both precise and markedly elevated for the high wire code (HWC) compared to low wire code (LWC) classifications, with medium wire code (MWC) showing little or no increase in risk. High wire code yielded odds ratios of 1.9 for total cancers (95% CI: 1.1-3.2), 2.9 for leukemias (95% CI: 1.5-5.5), and 2.5 for brain cancer (95% CI: 1.1-5.5) that were not confounded by measured potential risk factors for childhood cancer. These risk estimates are larger than the dichotomized results and more precise than those from the original five-level wire code, though limitations in the original study remain, particularly potential control selection bias.(ABSTRACT TRUNCATED AT 250 WORDS)