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Robert Kavet

Publications and source records attributed to Robert Kavet.

10 recordsLinked to original sources
PubMed2003

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.

Abdomen↗
PubMed2003

Childhood leukemia: electric and magnetic fields as possible risk factors.

Numerous epidemiologic studies have reported associations between measures of power-line electric or magnetic fields (EMFs) and childhood leukemia. The basis for such associations remains unexplained. In children, acute lymphoblastic leukemia represents approximately three-quarters of all U.S. leukemia types. Some risk factors for childhood leukemia have been established, and others are suspected. Pathogenesis, as investigated in animal models, is consistent with the multistep model of acute leukemia development. Studies of carcinogenicity in animals, however, are overwhelmingly negative and do not support the hypothesis that EMF exposure is a significant risk factor for hematopoietic neoplasia. We may fail to observe effects from EMFs because, from a mechanistic perspective, the effects of EMFs on biology are very weak. Cells and organs function despite many sources of chemical "noise" (e.g., stochastic, temperature, concentration, mechanical, and electrical noise), which exceed the induced EMF "signal" by a large factor. However, the inability to detect EMF effects in bioassay systems may be caused by the choice made for "EMF exposure." "Contact currents" or "contact voltages" have been proposed as a novel exposure metric, because their magnitude is related to measured power-line magnetic fields. A contact current occurs when a person touches two conductive surfaces at different voltages. Modeled analyses support contact currents as a plausible metric because of correlations with residential magnetic fields and opportunity for exposure. The possible role of contact currents as an explanatory variable in the reported associations between EMFs and childhood leukemia will need to be clarified by further measurements, biophysical analyses, bioassay studies, and epidemiology.

Animals↗
PubMed2002

Occupational magnetic field exposure and cardiovascular mortality in a cohort of electric utility workers.

In electric utility workers, occupational exposure to magnetic fields has previously been associated with mortality from acute myocardial infarction (AMI) and arrhythmia but not from chronic coronary heart disease (CCHD) or atherosclerosis. To investigate these health endpoints further, the authors examined mortality from AMI (n = 407) and CCHD (n = 369) in a cohort of 35,391 male workers at the Southern California Edison Company between 1960 and 1992. Exposure was estimated according to duration of employment in occupations associated with high levels of magnetic field exposure and was calculated as cumulative exposure to magnetic fields expressed in micro-Tesla-years. Adjustment was made for age, calendar time, socioeconomic status, race, and worker status (active or inactive). The authors found that men working longer in high-exposure occupations or working as electricians, linemen, or power plant operators had no increased risk of dying from either AMI or CCHD compared with men who never worked in high-exposure occupations. For cumulative exposure, no association was observed with mortality from AMI (rate ratio per 1 micro T-year = 1.01, 95% confidence interval: 0.99, 1.02) or CCHD (rate ratio per 1 micro T-year = 1.00, 95% confidence interval: 0.99, 1.02). These results, indicating no exposure-related risk increase for AMI mortality, do not confirm previous results.

Adult↗
PubMed2002

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

We measured magnetic fields and two sources of contact current in 36 homes in Pittsfield, MA. The first source, V(P-W), is the voltage due to current in the grounding wire, which extends from the service panel neutral to the water service line. This voltage can cause contact current to flow upon simultaneous contact with a metallic part of the water system, such as the faucet, and the frame of an appliance, which is connected to the panel neutral through the equipment-grounding conductor. The second is V(W-E), the voltage between the water pipe and earth, attributable to ground currents in the water system and magnetic induction from nearby power lines. In homes with conductive water systems and drains, V(W-E) can produce a voltage between the faucet and drain, which may produce contact current into an individual contacting the faucet while immersed in a bathtub. V(P-W) was not strongly correlated to the magnetic field (both log transformed) (r = 0.28; P < 0.1). On the other hand, V(W-E) was correlated to the residential magnetic field (both log transformed) (r = 0.54; P < 0.001), with the highest voltages occurring in homes near high voltage transmission lines, most likely due to magnetic induction on the grounding system. This correlation, combined with both frequent exposure opportunity for bathing children and substantial dose to bone marrow resulting from contact, lead us to suggest that contact current due to V(W-E) could explain the association between high residential magnetic fields and childhood leukemia.

Child↗
PubMed2002

Exposure guidelines for low-frequency electric and magnetic fields: report from the Brussels workshop.

The EMF Exposure Guidelines Science Workshop was held to evaluate the information base for guidelines for electric and magnetic field exposures at extremely low frequencies, to identify research needs, and to discuss how best to apply scientific knowledge in developing exposure recommendations. Although the existing guideline values are based on electrostimulation of nerve and muscle tissues, guidelines must also consider controversial studies of potential health effects from chronic exposures at environmental levels that are far too weak to cause electrostimulation. The size of the safety factor applied in specifying exposure limits reflects a variety of factors that introduce uncertainties. These include confidence in existing dose-effect relationships, population diversity, the reliability and precision of techniques to control over-exposure, and completeness of the information base, particularly regarding long-term effects. Specific research questions that can change the level of uncertainty meaningfully were identified in these areas: biophysical mechanisms of electrostimulation; the range of thresholds throughout exposed populations; cancer causation in children and adults, including a possible role for contact currents; cognitive, behavioral, and physiological effects on the central nervous system; improved dosimetry; and better understanding of electromagnetic interference with implanted biomedical devices such as pacemakers, defibrillators, and physiological monitors. This report introduces ten papers from the workshop that address these and related topics in detail.

Biophysical Phenomena↗
PubMed2002

Candidate sites of action for microdosimetry associated with exposure to extremely-low-frequency magnetic fields, electric fields and contact currents.

Recent advances enable one to apply numerical techniques to anatomically-correct human models to compute current densities and electric fields in tissue due to exposure to electric fields, magnetic fields, or contact currents. These methods have proved to be informative in estimating exceedance of basic restrictions prescribed by exposure guideline organizations. To date, the analyses have been conducted with a resolution on the order of millimeters. However, these techniques have future roles to play at higher levels of resolution at those sites in target tissues suspected of transducing local electric fields into biological responses. Two specific cases in which high resolution "microdosimetry" would yield value involve (a) residential settings and childhood leukemia and (b) worker exposure and cardiovascular disease. Recent research suggests that residential contact currents on the order of microamperes can produce biologically significant dose (expressed as the local electric field) to the bone marrow of a child. Microdosimetry would focus on pluripotent progenitor cells resident in the marrow compartment, as well as anatomic features that distinguish a child's from an adult's marrow. Laboratory and epidemiologic research has suggested that magnetic field exposure may affect heart rate variability, a measure reflective of autonomic nervous system control of cardiac activity. Given the physical attributes of the central nervous system and the sites that could serve as substrates for field interactions, future microdosimetry addressing heart rate variability effects may be well-advised to focus on the electrically excitable dendritic arborizations of neurons. In both cases, microdosimetry will help shed light on primary interactions in tissue.

Adult↗
PubMed2002

Pacemaker interference by magnetic fields at power line frequencies.

Human exposure to external 50/60-Hz electric and magnetic fields induces electric fields within the body. These induced fields can cause interference with implanted pacemakers. In the case of exposure to magnetic fields, the pacemaker leads are subject to induced electromotive forces, with current return paths being provided by the conducting body tissues. Modern computing resources used in conjunction with millimeter-scale human body conductivity models make numerical modeling a viable technique for examining any such interference. In this paper, an existing well-verified scalar-potential finite-difference frequency-domain code is modified to handle thin conducting wires embedded in the body. The effects of each wire can be included numerically by a simple modification to the existing code. Results are computed for two pacemaker lead insertion paths, terminating at either atrial or ventricular electrodes in the heart. Computations are performed for three orthogonal 60-Hz magnetic field orientations. Comparison with simplified estimates from Faraday's law applied directly to extracorporeal loops representing unipolar leads underscores problems associated with this simplified approach. Numerically estimated electromagnetic interference (EMI) levels under the worst case scenarios are about 40 microT for atrial electrodes, and 140 microT for ventricular electrodes. These methods could also be applied to studying EMI with other implanted devices such as cardiac defibrillators.

Electromagnetic Fields↗
PubMed2002

Pacemaker interference by 60-Hz contact currents.

Contact currents occur when a person touches conductive surfaces at different potentials, thereby completing a path for current flow through the body. Such currents provide an additional coupling mechanism between the human body and external low-frequency fields. The resulting fields induced in the body can cause interference with implanted cardiac pacemakers. Modern computing resources used in conjunction with millimeter-scale human body conductivity models make numerical modeling a viable technique for examining any such interference. An existing well-verified scalar-potential finite-difference frequency-domain code has recently been modified to allow for combined current and voltage electrode sources, as well as to allow for implanted wires. Here, this code is used to evaluate the potential for cardiac pacemaker interference by contact currents in a variety of configurations. These include current injection into either hand, and extraction via: 1) the opposite hand; 2) the soles of both feet; or 3) the opposite hand and both feet. Pacemaker generator placement in both the left and right pectoral areas is considered in conjunction with atrial and ventricular electrodes. In addition, the effects of realistically implanted unipolar pacemaker leads with typical lumped resistance values of either 20 kohms and 100 kohms are investigated. It is found that the 60-Hz contact current interference thresholds for typical sensitivity settings of unipolar cardiac pacemaker range from 24 to 45 microA. Voltage and electric field dosimetry are also used to provide crude threshold estimates for bipolar pacemaker interference. The estimated contact current thresholds range from 63 to 340 microA for bipolar pacemakers.

Adult↗
PubMedDate not supplied

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.

Electric Conductivity↗