A portable meter for measuring low frequency currents in the human body.

A portable meter has been developed for measuring low frequency currents that flow in the human body. Although the present version of the meter was specifically designed to measure 50/60 Hz "contact currents," the principles involved can be used with other low frequency body currents. Contact currents flow when the human body provides a conductive path between objects in the environment with different electrical potentials. The range of currents the meter detects is approximately 0.4-800 microA. This provides measurements of currents from the threshold of human perception (approximately 500 microA(RMS)) down to single microampere levels. The meter has a unique design, which utilizes the human subject's body impedance as the sensing element. Some of the advantages of this approach are high sensitivity, the ability to measure current flow in the majority of the body, and relative insensitivity to the current path connection points. Current measurement accuracy varies with the accuracy of the body impedance (resistance) measurement and different techniques can be used to obtain a desired level of accuracy. Techniques are available to achieve an estimated +/-20% accuracy.

The possible role of contact current in cancer risk associated with residential magnetic fields.

Residential electrical wiring safety practices in the US result in the possibility of a small voltage (up to a few tenths of a volt) on appliance surfaces with respect to water pipes or other grounded surfaces. This "open circuit voltage" (V(OC)) will cause "contact current" to flow in a person who touches the appliance and completes an electrical circuit to ground. This paper presents data suggesting that contact current due to V(OC) is an exposure that may explain the reported associations of residential magnetic fields with childhood leukemia. Our analysis is based on a computer model of a 40 house (single-unit, detached dwelling) neighborhood with electrical service that is representative of US grounding practices. The analysis was motivated by recent research suggesting that the physical location of power lines in the backyard, in contrast to the street, may be relevant to a relationship of power lines with childhood leukemia. In the model, the highest magnetic field levels and V(OC)s were both associated with backyard lines, and the highest V(OC)s were also associated with long ground paths in the residence. Across the entire neighborhood, magnetic field exposure was highly correlated with V(OC) (r = 0.93). Dosimetric modeling indicates that, compared to a very high residential level of a uniform horizontal magnetic field (10 mu T) or a vertical electric field (100 V/m), a modest level of contact current (approximately 18 mu A) leads to considerably greater induced electric fields (> 1 mV/m) averaged across tissue, such as bone marrow and heart. The correlation of V(OC) with magnetic fields in the model, combined with the dose estimates, lead us to conclude that V(OC) is a potentially important exposure with respect to childhood leukemia risks associated with residential magnetic fields. These findings, nonetheless, may not apply to residential service used in several European countries or to the Scandinavian studies concerned with populations exposed to magnetic fields from overhead transmission lines.

Application of the case-specular method to two studies of wire codes and childhood cancers.

This paper presents the results of applying the case-specular method to two earlier studies of wire codes and childhood cancers (DA Savitz et al, Am J Epidemiol 1988;128:21-38, and SJ London et al, Am J Epidemiol 1991;9:923-937). The method compares the wire codes of case residences with the wire codes of specular residences constructed by switching the location of the case residence across the center of the street. The method was designed to discriminate between the magnetic field hypothesis, which postulates that childhood cancer is affected by magnetic fields and that wire codes are a proxy for magnetic fields, and the neighborhood hypothesis, which postulates that childhood cancer is affected by some characteristics of the neighborhood other than magnetic fields and that wire codes are a proxy for those characteristics. Although the results from the two applications of the method have limited precision, they support the results originally reported (odds ratios of around 2 for very high current configuration residences and childhood cancers) and do not support suggestions that the associations are due to confounding by socio-economic and neighborhood factors. The results leave open the question of whether or not control selection bias could have influenced the original associations, because there was no convincing evidence that the control-specular matrices were symmetric.

The residential case-specular method to study wire codes, magnetic fields, and disease.

We propose a residential case-specular method for the study of wire codes and childhood cancer. The method compares the wire codes of case residences with the wire codes of identical residences (specular residences) located in a virtual situation in which the position of the residence or the position of the power line is switched around the center of the street. It is designed to discriminate between the magnetic field hypothesis, which postulates that childhood cancer is affected by magnetic fields and that wire codes are a proxy for magnetic fields, vs the neighborhood hypothesis, which postulates that childhood cancer is affected by some characteristics of the neighborhood other than magnetic fields and wire codes are a proxy for those characteristics. The method is based on several assumptions that we tested with 400 randomly selected residences. Under certain conditions, the method also may allow effect estimation without requiring the selection of controls and the potential biases that result from control selection. The method is applicable to both past and future studies.

Modeling magnetic fields in residences: validation of the resicalc program.

RESICALC is a computer program that models magnetic fields due to currents on arbitrary arrays and configurations of electric transmission lines, primary and secondary distribution lines, and ground and neutral return pathways. The program conducts network analyses of ground/neutral currents in neighborhoods based on residential loads and impedances in the current path. Experiments were conducted at the Magnetic Field Research Facility (MFRF) in Lenox, Massachusetts to validate the program's field computation. MFRF has a simulated residential electric distribution system that permits the testing of scenarios with a broad range of electrical characteristics. The results demonstrate that the program accurately models fields from complex combinations of supply and ground/neutral currents, and shows how estimated fields may be sensitive to impedance values assigned to the ground and neutral currents. The program has usefulness as an exposure assessment tool when access to residences is not possible, and as a means to estimate fields when planning electrical facility or residential development. Careful mapping of power line and residential coordinates in the program, as well as acquiring the highest quality load and grounding data available, are critical for modeling valid exposure estimates.

Assessing historical exposures of children to power-frequency magnetic fields.

One risk factor for human cancer currently being studied is residential exposure to power-frequency magnetic fields. A key problem in such research is how best to use contemporaneous measurements to assess past magnetic-field exposures. The main goal of the research presented in this paper was to examine the effectiveness of various surrogate measures in predicting historical exposures and to determine if residential power consumptions and the loads served by neighborhood electric networks could be used to improve the accuracy of such predictions. Residential magnetic-field data were collected during 24-h periods in the spring of 1990 and, again, in the winter of 1990-1991 for 35 children living in Western Massachusetts and Northern California. Measurements included spot magnetic fields in rooms occupied by subjects for an average of one or more hours per day, 24-h recordings at locations selected to emphasize ground-current and power-line fields, personal exposures, wire codes, residential power consumptions, and loads served by neighborhood electric networks. The geometric means of time-weighted-averaged (TWA) room spot magnetic fields measured during earlier and later visits to each home were 0.052 microT and 0.060 microT, respectively. Geometric-mean personal exposures for these visits were 0.084 microT and 0.111 microT and were significantly larger. Wertheimer-Leeper wire codes were associated with exposure. These codes, TWA spot fields, and the 24-h averages of the magnetic-field recordings taken to emphasize power-line contributions were about equally effective in explaining between-home variability in personal exposures measured eight months in the past or future. In contrast, personal exposure measurements were ineffective surrogates for past or future exposure. The study yielded little evidence suggesting that residential power consumption or neighborhood electric power flow are helpful in explaining temporal changes in personal exposure.