kHz Magnetic Fields Research Articles

Wearable wireless power systems for ‘ME-BIT’ magnetoelectric-powered bio implants

Objective. Compared to biomedical devices with implanted batteries, wirelessly powered technologies can be longer-lasting, less invasive, safer, and can be miniaturized to access difficult-to-reach areas of the body. Magnetic fields are an attractive wireless power transfer modality for such bioelectronic applications because they suffer negligible absorption and reflection in biological tissues. However, current solutions using magnetic fields for mm sized implants either operate at high frequencies (500 kHz) or require high magnetic field strengths (10 mT), which restricts the amount of power that can be transferred safely through tissue and limits the development of wearable power transmitter systems. Magnetoelectric (ME) materials have recently been shown to provide a wireless power solution for mm-sized neural stimulators. These ME transducers convert low magnitude (1 mT) and low-frequency (∼300 kHz) magnetic fields into electric fields that can power custom integrated circuits or stimulate nearby tissue. Approach. Here we demonstrate a battery-powered wearable magnetic field generator that can power a miniaturized MagnetoElectric-powered Bio ImplanT ‘ME-BIT’ that functions as a neural stimulator. The wearable transmitter weighs less than 0.5 lbs and has an approximate battery life of 37 h. Main results. We demonstrate the ability to power a millimeter-sized prototype ‘ME-BIT’ at a distance of 4 cm with enough energy to electrically stimulate a rat sciatic nerve. We also find that the system performs well under translational misalignment and identify safe operating ranges according to the specific absorption rate limits set by the IEEE Std 95.1-2019. Significance. These results validate the feasibility of a wearable system that can power miniaturized ME implants that can be used for different neuromodulation applications.

Flexible screen-printed coils for wireless power transfer using low-frequency magnetic fields

We report the fabrication and testing of a multilayer flexible screen-printed coil that can be used to generate low-frequency (<1 kHz) magnetic fields for applications such as electrodynamic wireless power transmission (EWPT). Compared to traditional coil-forming methods, screen printing enables rapid fabrication of thin, light weight, conformable, and flexible structures. The flexibility is demonstrated by repeated bending of a stack of up to three layers for a curvature (inverse bending radius) of 53.2 . We also demonstrate wireless power transmission of 1 mW of power at a distance of 4 cm between the flexible receiver and transmitter. The effect of the flexibility is observed through a 32% power increase when the receiver is bent concavely and a 20% output power decrease when the stack is bent convexly. It is also seen that the insertion of a 0.2 mm-thick soft-magnetic material sheet increased the magnetic field by 41% from its original value. Mechanical cycling was also performed for the stack. After 10 000 bending cycles, a 16.4% increase in the coil resistance was measured for a 0.33 Hz frequency and 78% increase for a 1 Hz cycling frequency, respectively. Screen printed 3D flexible transmitter coils can enable the incorporation of EWPT for wireless charging in applications such as healthcare and consumer-based electronics.

Carcinogenicity of intermediate frequency magnetic field in Tg.rasH2 mice.

Although the likelihood of exposure to leaking intermediate frequency magnetic fields (MFs) from electronic devices, such as induction‐heating and wireless power transfer systems, has increased, biological data assessing the health risks associated with human exposure remain insufficient. We examined the carcinogenicity of a 20 kHz MF, a typical frequency produced by induction‐heating cookers, using a transgenic rasH2 mouse model. Twenty‐five male and female CByB6F1‐Tg(HRAS)2Jic mice were exposed to a 0.20 mT, 20 kHz MF (22 h/day) or sham‐exposed for 26 weeks. As a positive control, 10 male and female rasH2 mice from the same batch were administered a single intraperitoneal injection of 75 mg/kg N‐methyl‐N‐nitrosourea. A blinded histopathological evaluation was performed, and the same experiments were conducted twice, independently, to confirm the reproducibility of the results. Histopathological examination revealed that spontaneous neoplastic lesions, such as splenic hemangiosarcomas and gastric squamous cell papillomas, were less (1–3 per group) in the MF‐ and sham‐exposed groups. The frequency of the neoplastic lesions was not significantly different between the groups. Eight to ten mice in each positive‐control group exhibited malignant lymphoma. The outcomes were consistent between duplicated experiments, which indicates lack of carcinogenicity of 20 kHz MF in the rasH2 mouse model. Bioelectromagnetics. © 2019 The Authors. Bioelectromagnetics Published by Wiley Periodicals, Inc.

Heat generation from magnetic fluids under alternating current magnetic field or induction coil for hyperthermia-based cancer therapy: Basic principle

Superparamagnetic particles (SUPs) have been used in many applications in the area of hyperthermia-based cancer treatment, as a magnetic resonance imaging contrast agent, as a carrier for drug, in the removal of toxic ions, etc. When SUPs are dispersed in liquid, they can experience Brownian motion and Neel's spin relaxations. In the presence of direct current magnetic field, SUPs do not show a hysteresis loop. Because of this, they are unable to produce heat. However, in alternating current magnetic field (AMF) of a few kHz and small magnetic fields, they can generate heat. For the treatment of cancer, hyperthermia temperature of 43°C is required, at which temperature cancer cells can be killed selectively, but normal cells can survive. The theory behind heat generation from SUPs in the presence of AMF will be discussed in this work.

Testing and application of an integrated fluxgate sensor DRV425

Abstract The article presents testing of a miniature fluxgate sensor developed and manufactured by Texas Instruments as well as an application of the sensor in a compact magnetic field probe with USB interface and LabView based software. Several basic properties of the sensor were evaluated and compared with the datasheet specifications. Offset stability measurements have indicated some possible problems for applications that require very high DC and low-frequency precision. Jumps in offset were present for some of the devices, frequency and duty of the jumps varying between individual pieces and with respect to the supply voltage. Magnetic probe for fast and handy measurements of DC and AC (up to 30 kHz) magnetic fields (up to ±1 mT) is also presented. The probe contains 24-bit analog to digital converter, microcontroller and high-speed USB interface which allows a live view of the signal in time and spectral charts. The probe proved to be very useful for the DRV425 debugging, application development, EMC measurements and magnetic field demonstrations activities.

Genotoxicity of intermediate frequency magnetic fields in vitro and in vivo

We assessed genotoxic effects of intermediate frequency magnetic fields (MF) in vitro and in vivo. Rat primary astrocytes were exposed for 24 h to a 7.5 kHz MF at a magnetic flux density of 30 or 300 µT. Male C57BL/6 J mice were exposed continuously for 5 weeks to a 7.5 kHz MF at 12 or 120 μT, and blood samples were collected for the genotoxicity assays. To evaluate possible co-genotoxicity, the in vitro experiments included combined exposure with menadione (an agent that induces mitochondrial superoxide production and DNA damage) and methyl methanesulfonate (an alkylating agent). DNA damage and DNA repair (in vitro) were measured using the alkaline Comet assay and formation of micronuclei was assessed microscopically (in vivo) or using flow cytometry (in vitro). The results did not support genotoxicity or co-genotoxicity of 7.5 kHz MFs at magnetic flux densities up to 300 µT in vitro or in vivo. On the contrary, there was some evidence that exposure to 7.5 kHz MFs might reduce the level of genetic damage. Strongest indication of any biological effects was obtained from measurements of relative cell number, which was significantly and consistently increased after MF exposure in all in vitro experiments. Health implications of this finding are unclear, but it suggests that 7.5 kHz MFs may stimulate cell proliferation or suppress cell death.

Behavioural phenotypes in mice after prenatal and early postnatal exposure to intermediate frequency magnetic fields

Electromagnetic fields are ubiquitous in the environment. Human exposure to intermediate frequency (IF) fields is increasing due to applications like electronic article surveillance systems, wireless power transfer, and induction heating cooking hobs. However, there are limited data on possible health effects of exposure to IF magnetic fields (MF). In the present study, we set out to assess cognitive and behavioural effects of IF MF in mice exposed during prenatal and early postnatal periods. Pregnant female mice were exposed continuously to 7.5kHz MFs at 12 and 120μT, from mating until weaning of pups. Sham exposed pregnant mice were used as a control group. A behavioural teratology study was conducted on the male offspring at two months of age to detect possible effects on the developing nervous system. Body weight development did not differ between the exposure groups. The exposure did not alter spontaneous motor activity when exploring a novel cage or anxiety in novelty-suppressed feeding or marble burying tests. Improved performance in the Rotarod task was observed in the 12µT group, while the 120μT exposure group swam more slowly than the sham exposed group in the Morris swim navigation task. However, indices of learning and memory (path length and escape latency during task acquisition and search bias during the probe test) did not differ between the exposure groups. Furthermore, the passive avoidance task did not indicate any impairment of long-term memory over a 48h interval in the exposed groups. In a post-mortem histopathological analysis, there was no evidence for an effect of IF MF exposure on astroglial reactivity or hippocampal neurogenesis. The results suggest that the IF MF used did not have detrimental effects on spatial learning and memory or histological markers of tissue reaction. The two statistically significant findings that were observed (improved performance in the Rotarod task in the 12µT group and decreased swimming speed in the 120µT group) are likely to be chance findings, as they do not form an internally consistent, dose-dependent pattern indicative of specific developmental effects.

Behavioral testing of mice exposed to intermediate frequency magnetic fields indicates mild memory impairment.

Human exposure to intermediate frequency magnetic fields (MF) is increasing due to applications like electronic article surveillance systems and induction heating cooking hobs. However, limited data is available on their possible health effects. The present study assessed behavioral and histopathological consequences of exposing mice to 7.5 kHz MF at 12 or 120 μT for 5 weeks. No effects were observed on body weight, spontaneous activity, motor coordination, level of anxiety or aggression. In the Morris swim task, mice in the 120 μT group showed less steep learning curve than the other groups, but did not differ from controls in their search bias in the probe test. The passive avoidance task indicated a clear impairment of memory over 48 h in the 120 μT group. No effects on astroglial activation or neurogenesis were observed in the hippocampus. The mRNA expression of brain-derived neurotrophic factor did not change but expression of the proinflammatory cytokine tumor necrosis factor alpha mRNA was significantly increased in the 120 μT group. These findings suggest that 7.5 kHz MF exposure may lead to mild learning and memory impairment, possibly through an inflammatory reaction in the hippocampus.

Effects of intermediate frequency magnetic fields on male fertility indicators in mice

Human exposure to intermediate frequency (IF) fields is increasing due to new applications such as electronic article surveillance systems, wireless power transfer and induction heating cookers. However, limited data is available on effects of IF magnetic fields (MF) on male fertility function. This study was conducted to assess possible effects on fertility indicators from exposure to IF MF. Male C57BL/6J mice were exposed continuously for 5 weeks to 7.5kHz MF at 12 and 120μT. Sperm cells from cauda epididymis were analysed for motility, total sperm counts, and head abnormalities. Motile sperm cells were classified as progressive or non-progressive. Testicular spermatid heads were counted as well. The body weight development and reproductive tissue weights were not affected. No exposure-related differences were observed in sperm counts or sperm head abnormalities. Proportion of non-motile cells was significantly decreased in the 120µT group, and a corresponding increase was seen in the percentage of motile cells (significant in non-progressive motile cells). In conclusion, no adverse effects on fertility indicators were observed. Increased sperm motility is an interesting finding that needs to be confirmed in further studies.

Acute and subchronic toxicity of 20 kHz and 60 kHz magnetic fields in rats

Despite increasing use of intermediate frequency (IF) magnetic fields (MFs) in occupational and domestic settings, scientific evidence necessary for health risk assessments of IF MF is insufficient. Male and female Crl:CD(SD) rats (12 per sex per group) were exposed to 20 kHz, 0.20 mT(root mean square, rms) or 60 kHz, 0.10 mT(rms) sinusoidal MFs for 22 h day−1 for 14 days (acute) or 13 weeks (subchronic). Experiments were duplicated for each frequency to ensure outcome reproducibility, and examinations were blinded for quality assurance. All rats survived without significant clinical signs until the end of experiments. Some changes in body weight between the MF‐exposed and control groups were observed over the course of exposure, although the directions of the changes were inconsistent and not statistically significant after subchronic exposure. There were significant differences between MF‐exposed and control groups in some organ weights and parameters in hematology and clinical chemistry, but these were minor in magnitude and not repeated in duplicate experiments. Histopathological findings reflecting toxicity were sporadic. Frequencies of other findings were similar to historic data in this rat strain, and findings had no specific relationship to changes in organ weight or parameters of hematology and clinical chemistry in each animal. The changes observed throughout this study were considered biologically isolated and were attributable to chance associations rather than to MF exposure. The results, in particular the histopathological evidence, indicate an absence of toxicity in IF MF‐exposed rats and do not support the hypothesis that IF MF exposure produces significant toxicity. Copyright © 2015. The Authors. Journal of Applied Toxicology Published by John Wiley & Sons Ltd.

Intermediate frequency magnetic field generated by a wireless power transmission device does not cause genotoxicity in vitro.

The aim of this study was to evaluate effects of intermediate frequency magnetic fields (IFMF) generated by a wireless power transmission (WPT) based on magnetic resonance from the perspective of cellular genotoxicity on cultured human lens epithelial cells (HLECs). We evaluated the effects of exposure to 90 kHz magnetic fields at 93.36 µT on cellular genotoxicity in vitro for 2 and 4 h. The magnetic flux density is approximately 3.5 times higher than the reference level recommended by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines. For assessment of genotoxicity, we studied cellular proliferation, apoptosis and DNA damage by Cell Counting Kit-8 (CCK-8) assay, flow cytometry analysis, alkaline comet assay and phosphorylated histone H2AX (γH2AX) foci formation test. We did not detect any effect of a 90 kHz IFMF generated by WPT based on magnetic resonance on cell proliferation, apoptosis, comet assay, and γH2AX foci formation test. Our results indicated that exposure to 90 kHz IFMF generated by WPT based on magnetic resonance at 93.36 µT for 2 and 4 h does not cause detectable cellular genotoxicity.

Intermediate frequency magnetic field and chick embryotoxicity

Intermediate frequency magnetic fields (MFs) have widely been used in industrial machines and home appliances, such as induction heating cookers, although toxicity studies to evaluate the potential health risks of such fields are insufficient. In induction heating cookers, the MF source (i.e. hobs), is located near the abdominal position of a person cooking. Hence, developmental effects on the fetus may be a concern in case the person is a pregnant woman. Fertile White Leghorn eggs (60/group) were either exposed to 20 kHz, 1.1 mT(rms) or 60 kHz, 0.11 mT(rms) sinusoidal MFs for 19 days during embryogenesis. The same number of eggs served as a control group. In addition, a sham-sham experiment was conducted to validate the equality between exposure and control facilities. After exposure, embryos were examined for mortality rate and stage. Live embryos were evaluated for developmental stage and gross and skeletal anomalies. Length of upper beak and leg digits was also measured. Examinations were conducted in a blinded fashion to ensure quality assurance; experiments were triplicated for each frequency to confirm the outcome reproducibility. Mortality rate and stage, incidence of malformed embryos, and developmental variables in live embryos were found to be similar between the MF-exposed and corresponding control group. Incidence of gross anomalies such as mandibular edema and skeletal anomalies such as coccyx defects were low across the experiments, and no significant group differences were noted. In conclusion, exposure to 20 kHz or 60 kHz MF did not produce any significant teratogenic developmental effects in chick embryos.

Effect of magnetic field on Fe – Si – Al alloy properties with zero magnetostriction

A method and results for a study of the effect of permanent and variable (up to about 1.5 kHz) magnetic fields with intensity H = 0–1050 A/m on internal friction Q –1 and Young’s modulus defect ∆E/E in magnetically-soft alloy of the system Fe – Si – Al (sendast type) with mechanical vibration frequency about 105 Hz are provided. The range of strain rate amplitudes e0 within which Q –1 and ∆E/E are independent of H, and the range where they increase linearly with an increase in S0 over the whole range of change in H are determined. Results obtained are analyzed within the framework of existing relaxation theory and hysteresis mechanism of elastic vibration damping within magnetically soft materials.

Lack of teratological effects in rats exposed to 20 or 60 kHz magnetic fields.

BACKGROUND: A risk assessment of magnetic field (MF) exposure conducted by the World Health Organization indicated the need for biological studies on primary hazard identification and quantitative risk evaluation of intermediate frequency (300 Hz–100 kHz) MFs. Because induction heating cookers generate such MFs for cooking, reproductive and developmental effects are a concern due to the close proximity of the fields' source to a cook's abdomen. METHODS: Pregnant Crl:CD(SD) rats (25/group) were exposed to a 20 kHz, 0.2 mT(rms) or 60 kHz, 0.1 mT(rms) sinusoidal MF or sham-exposed for 22 hr/day during organogenesis, and their fetuses were examined for malformations on gestation day 20. All teratological evaluations were conducted in a blind fashion, and experiments were duplicated for each frequency to confirm consistency of experimental outcomes. RESULTS: No exposure-related changes were found in clinical signs, gross pathology, or number of implantation losses. The number of live fetuses and low-body-weight fetuses as well as the incidence of external, visceral, and skeletal malformations in the fetuses did not indicate significant differences between MF-exposed and sham-exposed groups. Although some fetuses showed isolated changes in sex ratio and skeletal variation and ossification, such changes were neither reproduced in duplicate experiments nor were they common to specific field frequencies. CONCLUSIONS: Exposure of rats to MFs during organogenesis did not show significant reproducible teratogenicity under experimental conditions. Present findings do not support the hypothesis that intermediate frequency MF exposure after implantation carries a significant risk for developing mammalian fetuses. Birth Defects Res (Part B) 92:469–477, 2011. © 2011 Wiley Periodicals, Inc.

Chronic exposure of Sprague-Dawley rats to 20 kHz triangular magnetic fields

Purpose: As a continuing study of 20 kHz triangular magnetic fields (MF) [Lee et al. ], we investigated the chronic toxicity and possible health effects of exposure to 20 kHz MF at the flux density of 30 μT, which is the limit standard for the occupational population in South Korea, with the use of Sprague-Dawley rats.Materials and methods: Rats were exposed to 20 kHz triangular MF at 30 μT Root Mean Square for 8 h/day for 18 months. Body and organ weights were measured and urinalysis, hematological and blood biochemistry analyses were performed in individual animals. Histopathological evaluation was also performed for the brain, thymus, lung, heart, liver, kidney, intestine and reproductive organs, including tumour tissue.Results: The mortality patterns in male or female rats exposed to magnetic fields were compared to the mortality patterns found in sex-matched sham control animals. Significant alteration of body weight was not observed with MF exposure. No significant differences were seen in sham-exposed and MF-exposed animals based on urological factors, hematological factors and blood biochemistry. Total tumour incidence was not different between sham-exposed and MF-exposed animals.Conclusion: Our results suggest that chronic exposure to 20 kHz triangular MF with 30 μT flux density did not increase toxicity in rats.

Intermediate frequency magnetic fields generated by an induction heating (IH) cooktop do not affect genotoxicities and expression of heat shock proteins

The aim of this study is to evaluate the effects of intermediate frequency (IF) fields generated by induction heating (IH) cooktops from the perspective of cellular genotoxicity and stress responses. We evaluated the effects of exposure to 23 kHz magnetic fields at 6.05 mT(rms) for 2 h on cellular genotoxicity and stress responses in vitro. The maximum output power in most IH cooktops is at this frequency. The magnetic flux density is approximately 1000 times higher than the reference level in the International Commission on Non-ionising Radiation Protection (ICNIRP) guidelines. For assessment of genotoxicity, we studied cell growth, comet assay, micronucleus formation and hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene mutation. Heat shock protein (Hsp) 27, 70, 105 and phosphorylated Hsp27 were evaluated as indicators of the stress responses. We did not detect any effects of the IF magnetic fields on cell growth, comet assay, micronucleus formation, HPRT gene mutation, expression of phosphorylated Hsp27, or nuclear translocation of Hsp27, 70 or 105. Our results indicate that exposure to an IF magnetic field at 6.05 mT(rms) for 2 h does not cause detectable cellular genotoxicity, and does not induce detectable cellular stress.

Intermediate frequency magnetic fields generated by an induction heating (IH) cooktop do not affect genotoxicities and expression of heat shock proteins

Purpose: The aim of this study is to evaluate the effects of intermediate frequency (IF) fields generated by induction heating (IH) cooktops from the perspective of cellular genotoxicity and stress responses.Materials and methods: We evaluated the effects of exposure to 23 kHz magnetic fields at 6.05 mTrms for 2 h on cellular genotoxicity and stress responses in vitro. The maximum output power in most IH cooktops is at this frequency. The magnetic flux density is approximately 1000 times higher than the reference level in the International Commission on Non-ionising Radiation Protection (ICNIRP) guidelines. For assessment of genotoxicity, we studied cell growth, comet assay, micronucleus formation and hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene mutation. Heat shock protein (Hsp) 27, 70, 105 and phosphorylated Hsp27 were evaluated as indicators of the stress responses.Results: We did not detect any effects of the IF magnetic fields on cell growth, comet assay, micronucleus formation, HPRT gene mutation, expression of phosphorylated Hsp27, or nuclear translocation of Hsp27, 70 or 105.Conclusions: Our results indicate that exposure to an IF magnetic field at 6.05 mTrms for 2 h does not cause detectable cellular genotoxicity, and does not induce detectable cellular stress.

Effect of Human Model Height and Sex on Induced Current Dosimetry in Household Induction Heater Users

Induced currents in the high-resolution, anatomical human models are numerically calculated by the impedance method. The human models are supposed to be exposed to highly inhomogeneous 20.9 kHz magnetic fields from a household induction heater (IH). In the case of the adult models, the currents ranging from 5 to 19 mA/m2 are induced for between the shoulder and lower abdomen. Meanwhile, in the case of the child models, the currents ranging from 5 to 21 mA/m2 are induced for between the head and abdomen. In particular, the induced currents near the brain tissue are almost the same as those near the abdomen. When the induced currents in the central nervous system tissues are considered, the induced currents in the child model are 2.1 to 6.9 times as large as those in the adult model under the same B-field exposure environment. These results suggest the importance of further investigation intended for a pregnant female who uses the IH as well as for a child (or the IH users of small standing height).

Long-term exposure of Sprague Dawley rats to 20 kHz triangular magnetic fields

Purpose: There are only a few reports on harmful effects of 20 kHz sine waves; however, it is essential to comprehensively evaluate the potentially harmful effect of triangular signals at the same frequency. Therefore, in this study, effects of long-term exposure to 20 kHz magnetic fields was examined.Materials and methods: Eighty Sprague Dawley rats were divided into two groups (half male and female in each sham and exposed groups), and they were exposed to 20 kHz triangular magnetic fields at 6.25 μT rms for 8 h/day for 12 or 18 months. Urinalysis [pH, glucose, protein, ketone bodies, red blood cells (RBC), nitrogen, bilirubin, urobilinogen, and specific gravity], hematological analysis (RBC, hemoglobin, hematocrit, thrombocyte count, and leucocyte count), blood biochemistry (total protein, blood urea nitrogen, creatinine, glucose, total bilirubin, total cholesterol, aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase), and histopathological analysis of organs (thymus, stomach, intestine, liver, kidney, testis, ovary, spleen, brain, heart, and lung) were performed.Results: No significant differences were seen between 20 kHz magnetic-fields exposed rats and sham-exposed rats in body and organ weights, hematological analysis, blood biochemistry, urinalysis data, and histopathological examination, except for the numbers of neutrophiles and lymphocytes in female rats. The number of neutrophiles was significantly increased in female rats on the 12th month after exposure, and the number of lymphocytes in female rats was significantly decreased on the 18th month.Conclusion: Long-term exposure of rats to 20 kHz triangular magnetic fields did not induce any significantly harmful effects, except changes in neutrophiles at 12 months and lymphocytes at 18 months of exposure in female rats. These hematological changes need to be investigated again at a higher intensity of 20 kHz magnetic fields.

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.