Contact voltage measured in residences: implications to the association between magnetic fields and childhood leukemia

The relationship between residential magnetic fields and contact voltage: a pooled analysis

It has been suggested that residential exposure to contact currents may be more directly associated with the potential for an increased risk of leukemia in childhood than magnetic fields. Contact current exposure occurs when a child contacts a bathtub's water fixtures, which are usually contiguous with a residence's electrical ground, and when the drainpipe is conductive. The Northern California Childhood Leukemia Study (NCCLS) is the only epidemiological study known to address whether contact current may confound the reported association between residential magnetic fields and childhood leukemia. The study contributed contact voltage and magnetic-field data for over 500 residences of leukemia cases and control children. We combined these data with the results of previous measurement studies of contact voltage in other communities to conduct an analysis of the relationship of magnetic fields with contact voltage for a total sample of 702 residences. The Spearman correlation of magnetic field with contact voltage was 0.29 (Spearman, P < 0.0001). Magnetic-field and contact voltage data were both divided into tertiles, with an upper magnetic-field cutpoint of 0.3 μT suggested by values used in epidemiological results and an upper contact voltage cutpoint of 60 mV based on dosimetric considerations. Expressed as an exposure odds ratios (EOR), we report an association of contact voltage with magnetic fields of 15.1 (95% CI 3.6-61) as well as a statistically significant positive trend across magnetic-field strata (EOR of 4.2 per stratum with 95% CI 2.4-7.4). The associations appear to be large enough to support the possibility that contact current could be responsible for the association of childhood leukemia with magnetic fields.

Exposure to electrical contact currents and the risk of childhood leukemia

The objectives of this study were to examine the association between contact current exposure and the risk of childhood leukemia and to investigate the relationship between residential contact currents and magnetic fields. Indoor and outdoor contact voltage and magnetic-field measurements were collected for the diagnosis residence of 245 cases and 269 controls recruited in the Northern California Childhood Leukemia Study (2000-2007). Logistic regression techniques produced odds ratios (OR) adjusted for age, sex, Hispanic ethnicity, mother's race and household income. No statistically significant associations were seen between childhood leukemia and indoor contact voltage level [exposure ≥90th percentile (10.5 mV): OR  =  0.83, 95% confidence interval (CI): 0.45, 1.54], outdoor contact voltage level [exposure ≥90th percentile (291.2 mV): OR  =  0.89, 95% CI: 0.48, 1.63], or indoor magnetic-field levels (>0.20 µT: OR  =  0.76, 95% CI: 0.30, 1.93). Contact voltage was weakly correlated with magnetic field; correlation coefficients were r  =  0.10 (P  =  0.02) for indoor contact voltage and r  =  0.15 (P  =  0.001) for outdoor contact voltage. In conclusion, in this California population, there was no evidence of an association between childhood leukemia and exposure to contact currents or magnetic fields and a weak correlation between measures of contact current and magnetic fields.

Residential magnetic fields and measures of neutral-to-earth voltage: variability within and between residences

The objectives of this study were to characterize temporal patterns of magnetic fields (Bavg) and two measures of neutral-to-earth voltage: the voltage between the water line and earth (VW-E), and the voltage between bathtub plumbing fixtures and the drain (Vbath). The latter is a source of exposure to contact current in bathing children that has been proposed to explain the reported association between power-frequency magnetic fields and childhood leukemia. These quantities were measured each minute in a sample of 15 single-detached residences in San Jose, CA. Generally, Bavg, VW-E, and Vbath were positively correlated with each other within residences, and displayed similar diurnal patterns. Weekday and weekend patterns displayed qualitative differences that reflect the more scheduled workday for weekdays, and a less structured pattern for weekends. When pooled with two prior measurement studies, positive associations across residences between Bavg and both VW-E and Vbath were observed. Home designs over the past 30-40 years have lead to a decreasing prevalence of Vbath as conductive drains have been swapped out for non-conductive materials. Nonetheless, the observed relationships within and across residences indicate that contact current has the characteristics of a factor that could explain the association of magnetic fields with childhood leukemia.

Pilot measurements of ELF contact currents in some electric utility occupations

Contact currents from touching objects with different voltages can produce electric fields within the body that produce neurological and other biological effects. To begin measuring these exposures among electric utility workers, a new contact current meter (CCM) was tested in a pilot study at Southern California Edison. The CCM was worn for 82 full-shift measurements by 76 volunteers from eight occupations who did not work directly with energized electrical equipment. The volunteers were exposed to an average of 285.8 contact current events above the meter's 1-microA threshold, but most of these were electrostatic spark discharges. Fourteen employees experienced an average of 135.1 contact currents events whose primary frequency was 60 Hz. Using a circuit model of the human body, the average contact currents going from arm to arm was 9.8 microA (maximum = 178.0 microA), and the average going down the torso was 25.5 microA (maximum = 662.0). The maximum exposures were experienced by a technical support employee working in a substation. All measurements in this pilot study were below the 3000 microA maximum permissible exposure for contact currents set by the Institute of Electrical and Electronic Engineers (IEEE). Combining these current measurements with the results of high-resolution dosimetry, the internal electric fields averaged an estimated 1.7 mV/m in the heart (maximum = 21.0 mV/m), and 1.9 mV/m in the hematopoietic bone marrow in the torso (maximum = 56.5 mV/m). These internal electric fields from contact currents are below the basic restriction of 943 mV/m in the IEEE exposure standards but are above 1 mV/m, a level where biological effects have been often reported in laboratory studies. Safety concerns limited the measurements to de-energized equipment, so we did not obtain data on work in energized high-voltage environments, the most likely sources of high contact currents. This pilot study identified other improvements to the contact current meter that would make it better able to measure exposures in future health studies.

Leukemia attributable to residential magnetic fields: results from analyses allowing for study biases

Nearly every epidemiologic study of residential magnetic fields and childhood leukemia has exhibited a positive association. Nonetheless, because these studies suffer from various methodologic limitations and there is no known plausible mechanism of action, it remains uncertain as to how much, if any, of these associations are causal. Furthermore, because the observed associations are small and involve only the highest and most infrequent levels of exposure, it is believed that the public health impact of an effect would be small. We present some formal analyses of the impact of power-frequency residential magnetic-field exposure (as measured by attributable fractions), accounting for our uncertainties about study biases as well as uncertainties about exposure distribution. These analyses support the idea that the public health impact of residential fields is likely to be limited, but both no impact and a substantial impact remain possibilities in light of the available data.

Biophysical Mechanisms: A Component in the Weight of Evidence for Health Effects of Power-Frequency Electric and Magnetic Fields

Comparatively high exposures to power-frequency electric and magnetic fields produce established biological effects that are explained by accepted mechanisms and that form the basis of exposure guidelines. Lower exposures to magnetic fields (< 1 microT average in the home) are classified as "possibly carcinogenic" on the basis of epidemiological studies of childhood leukemia. This classification takes into consideration largely negative laboratory data. Lack of biophysical mechanisms operating at such low levels also argues against causality. We survey around 20 biophysical mechanisms that have been proposed to explain effects at such low levels, with particular emphasis on plausibility: the principle that to produce biological effects, a mechanism must produce a "signal" larger than the "noise" that exists naturally. Some of the mechanisms are impossible, and some require specific conditions for which there is limited or no evidence as to their existence in a way that would make them relevant to human exposure. Others are predicted to become plausible above some level of field. We conclude that effects below 5 microT are implausible. At about 50 microT, no specific mechanism has been identified, but the basic problem of implausibility is removed. Above about 500 microT, there are established or likely effects from accepted mechanisms. The absence of a plausible biophysical mechanism at lower fields cannot be taken as proof that health effects of environmental electric and magnetic fields are impossible. Nevertheless, it is a relevant consideration in assessing the overall evidence on these fields.

Selection bias and its implications for case-control studies: a case study of magnetic field exposure and childhood leukaemia

Based on the epidemiological association between residential exposure to extremely low frequency-magnetic fields (ELF-MF) and childhood leukaemia, the International Agency for Research on Cancer classified ELF-MF as a possible human carcinogen. Since clear supportive laboratory evidence is lacking and biophysical plausibility of carcinogenicity of MFs is questioned, a causal relationship between childhood leukaemia and magnetic field exposure is not established. Among the alternative explanations, selection bias in epidemiological studies of MFs seems to be the most plausible hypothesis. In reviewing the epidemiological literature on ELF-MF exposure and childhood leukaemia, we found evidence both for and against the existence of selection bias. To evaluate the potential for selection bias, we examined the relationship of socioeconomic status to subject participation and exposure to MFs. We find that, often, reporting of selection processes in itself is biased and incomplete, making the interpretation and evaluation of a potential for bias difficult. However, if present, such a bias would have wide implications for case-control studies in general. We call for better reporting and for evaluation of the potential for selection bias in all case-control studies, as well as, for the development of novel methods in control selection and recruitment.

EMF and health

Electric and magnetic fields are ubiquitous in the modern society, and concerns have been expressed regarding possible adverse effects of these exposures. This review covers epidemiologic research on health effects of exposures to static, extremely low-frequency (ELF), and radio frequency (RF) fields. Research on ELF fields has been performed for more than two decades, and the methodology and quality of studies have improved over time. Studies have consistently shown increased risk for childhood leukemia associated with ELF magnetic fields, whereas ELF fields most likely are not a risk factor for breast cancer and cardiovascular disease. There are still inadequate data for other outcomes. More recently, focus has shifted toward RF exposures from mobile telephony. There are no persuasive data suggesting a health risk, but this research field is still immature with regard to the quantity and quality of available data. This technology is constantly changing and there is a need for continued research on this issue. Almost no epidemiologic data are available for static fields.

Research strategies for magnetic fields and cancer

Widespread concerns about whether electric and magnetic fields (EMF) could adversely affect human health have been raised in epidemiologic studies reported since the 1980s. Possible EMF health effects have been widely publicized in the popular press since that time. We consider here three possible mechanisms of action of EMF on childhood leukemia. We identify the first as "magnetic fields": this hypothesis relates the average level of magnetic field to the incidence of childhood leukemia. We identify a second, recently proposed, mechanism as "contact current": this hypothesis relates the low voltage and consequent current that occurs on the domestic water pipe, due to U.S. grounding practices, as a source for exposure of children. The third hypothesis is that the relationship observed is spurious. Using a modified example taken from the work of Von Winterfeldt and Keeney, we use Decision Analysis to estimate the value of information for distinguishing between the three hypotheses. We believe that this improves on the usual process for deciding on research budgets. Depending on which hypothesis we favor a priori, the value of being informed ranges from US 101 dollars to US 233 dollars per "problem household." Since there could be as many as 2 million such households, the value of information for resolving this issue could approach half a billion dollars! We find that there is no value of information for finding the odds ratio given the contact current hypothesis. In writing this article, we have consciously kept the computations as simple as possible so as to engage the reader's attention and interest. In a penultimate section, we suggest numerous possible extensions for a group interested in discussing and deciding on the value of research on the relationship between magnetic fields and cancer.

Childhood leukemia and EMF: Review of the epidemiologic evidence

All populations are exposed to varying degrees of electromagnetic fields (EMF); in this study we consider only extremely low frequency (ELF) and radio frequency (RF) fields. After the first study of ELF and childhood leukemia in 1979, intensive epidemiologic investigation has sought to shed light on the potential relation between EMF and childhood leukemia. Consistent associations from epidemiologic studies and two pooled analyses have been the basis for the classification of ELF as a possible carcinogen by the International Agency for Research on Cancer (IARC). The study of RF is still in its infancy and little is known about residential RF exposure or its potential effects on childhood leukemia. The purpose of this study, presented at the WHO Workshop on Sensitivity of Children to EMF in Istanbul, Turkey in June 2004, is to review and critically assess the epidemiologic evidence on EMF and childhood leukemia.

Contact current hypothesis: Summary of results to date

Research conducted over the past 5 years has addressed the hypothesis that the reported association between residential magnetic fields and childhood leukemia may be explained by exposure to contact current. The use of multi-grounded neutrals in electrical distribution and residential electrical wiring systems in the United States results in a voltage on a residence's water line relative to earth that in turn creates a voltage between the water fixtures of a bathtub, sink, or shower and the drain, if the latter is made of conductive material. A bathing child may thus be exposed to contact current upon manual contact with the faucet, spout, or water stream. Dosimetry modeling indicates that modest and realistically anticipated currents (10s of microA) can produce electric fields in bone marrow (100s of mV/m) sufficient to overcome questions of biophysical plausibility. Both measurements in two regions of the United States and computer modeling of typical single-residence US neighborhoods indicate that residences with average magnetic fields in the high tail of the magnetic field distribution are more likely than residences with lower fields to also have higher contact voltage. The association of residential magnetic fields with contact voltage, the dosimetry results, and the indication from a behavioral survey that children tend to engage in behavior that results in exposure all support the hypothesis. Further research is needed to characterize electrical systems in other nations to determine whether contact current exposure occurs and whether it is associated with residential magnetic fields.

Electric Fields in the Human Body due to Electrostatic Discharges

Electrostatic discharges (ESDs) occur when two objects at different electric potentials come close enough to arc (spark) across the gap between them. Such discharges may be either single-event or repetitive (e.g., 60 Hz). Some studies have indicated that ESDs may be a causative factor for health effects in electric utility workers. Moreover, a hypothesis has recently been forwarded imperceptible contact currents in the human body may be responsible for health effects, most notably childhood leukemia. Numerical modeling indicates that the electric fields in human tissue resulting from typical contact currents are much greater than those induced from typical exposures to electric and magnetic fields at power line frequencies. Numerical modeling is used here to compute representative spark-discharge dosimetry in a realistic human adult model. The frequency-domain scalar potential finite difference method is applied in conjunction with the Fourier transform to assess electric fields in selected regions and tissues of interest in the body. Electric fields in such tissues as subcutaneous fat (where peripheral nerves may be excited), muscle and bone marrow are of the order of kilovolts per meter in the lower arm. The pulses, however, are of short duration (approximately 100 ns).

Experimental validation of a statistical model for evaluating the past or future magnetic field exposures of a population living near power lines

This study was designed to provide an experimental validation for a statistical model predicting past or future exposures to magnetic fields (MF) from power lines. The model estimates exposure, combining the distribution of ambient MF in the absence of power lines with the distribution of past or future MF produced by power lines. In the study, validation is carried out by comparing exposures predicted by the model with the actual measurements obtained from a large-scale epidemiological study. The comparison was made for a group of 220 women living near a 735 kV power line. Knowing that the individual arithmetic means of MF exposures follow a log-normal distribution, the Pearson correlation between the log-transformed measured means and the calculated ones was determined and found to be 0.77. Predicted values of MF exposures were slightly lower than measured values. The calculated geometric mean of the group was 0.33 microT, compared to 0.38 microT for the measured geometric mean. The present study shows good agreement between the measured MF exposure of an individual inside a house near a 735 kV line and the MF exposure calculated using a statistical model.

Association of residential magnetic fields with contact voltage

The US National Electrical Code's (NEC) requirement to ground a home's electrical service to the residential water line results in a voltage between the water line and earth, V W-E. The voltage may result from ground return current that flows into the earth via the water line or from inductive effects from other sources of magnetic fields, such as transmission lines. This voltage can, in turn, serve as a source for Vbath, the voltage between the water fixtures and conductive drain pipes sunk into the earth beneath a residence. Vbath can be a source of contact current exposure to a child touching a water fixture while bathing. Previous research has suggested that exposure to these currents could be the basis for the association between power-frequency magnetic fields and childhood leukemia. In this study, we assessed the association between measured Vbath and VW-E with the average spot-measured magnetic field, Bavg, in a sample of 191 single-family residences in the Denver metropolitan area. This area was the source of cases and controls for previous studies of electric and magnetic field (EMF) and childhood cancer. The association of both Vbath and VW-E with Bavg had upward trends across magnetic field strata (<0.1 microT (reference); 0.1-<0.3 microT; and > or = 0.3 microT). In addition, VW-E was associated with Vbath. Without further study, these results cannot be applied to multi-dwelling residences or to electrical systems prevalent in other nations. Nonetheless, when combined with the finding that contact current is a far more plausible candidate than the residential magnetic field for mediating biological effects on the basis of comparative dose to bone marrow, these associations indicate that contact current exposure deserves further study.

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.

Comparison of electric fields induced in humans and rodents by 60-Hz contact currents

Contact currents flow when a conducting object such as an animal touches conductive surfaces at different potentials. This completes a path for current flow through the body. These currents provide an additional coupling mechanism between the human body and low-frequency external fields to that due to direct induction effects. Recent research indicates that childhood exposure to residential contact currents may play a role in explaining any possible association between residential magnetic fields and childhood leukemia. To verify this hypothesis, laboratory experiments with rodents are planned. Thus, it is important to understand the relationship between fields induced in rodents and humans. Results from numerical computations are reported here. They are based on high-resolution anatomically based inhomogeneous models of adult and child male humans and male and female rats and mice, for a variety of 60-Hz contact current scenarios. It is hoped that this work will aid in the design of experiments involving rodents and in the interpretation of results as applied to humans. It is found that for geometrically similar models, the induced electric-field scales in an anticipated inverse-square manner with the geometric scaling factor. For dissimilar models, scaling can provide a crude estimate for translating induced field results between species. However, numerical modeling provides the most suitable analysis tool for more accurate estimates.