Magnetic-field exposures of cable splicers in electrical network distribution vaults.

The purposes of the research reported here were to quantify the power-frequency magnetic-field exposures of cable splicers while they were performing tasks in energized network distribution vaults and to compare these exposures with occupational exposure guideline levels. Network vaults supply electricity to commercial and residential urban areas as well as to large buildings. Participating workers wore a personal exposure monitor at the waist, kept a simple diary to record their work location, and recorded information about the vaults and tasks performed. To capture temporal variability, a stationary meter was deployed in the vault during a task. Load current in the vault was measured. To examine temporal variability over long time periods, stationary meters were deployed in selected vaults for one month. Data were collected during 77 tasks in 69 vaults for 191 person-tasks, representing approximately 400 hours of in-vault personal exposure data. Highest exposures were observed in tasks performed near secondary conductors. Personal exposure variability arises principally from worker movement and activities in the vaults, not from load variability during a task. Maximum field during a person-task exceeded the International Committee on Non-Ionizing Radiation Protection (ICNIRP) (0.42 millitesla) and the American Conference of Governmental Industrial Hygienists (ACGIH) (1.0 millitesla) guideline levels during 14 percent and 8 percent of the person-tasks, respectively. The mean of measurements during a person-task exceeded those guideline levels during 4 percent and 2 percent of the person-tasks, respectively. A large number of person-tasks (40%) had measured fields above the ACGIH recommended limit of 0.1 millitesla for workers with pacemakers or other implanted devices. Based on the frequency and duration of their high exposures, cable splicers working in network distribution vaults are one of the most highly exposed groups in the electric utility industry. Selective assignment of work location and task could minimize the likelihood of exposures for vault workers exceeding guideline limits for wearers of pacemakers or other implanted devices. Scheduling vault tasks during off-peak hours (nights and weekends) may reduce exposures. However, even during these periods exposures in certain vaults can still exceed guideline levels.

Long-term effects of 60-Hz electric vs. magnetic fields on IL-1 and IL-2 activity in sheep.

This study was designed to assess the effect of exposure to long-term extremely low-frequency electric and magnetic fields (ELF-EMF) from a 500 kV transmission line on IL-1 and IL-2 activity in sheep. The primary hypothesis was that the reduction in IL-1 activity observed in our two previous short-term studies (10 months) was due to EMF exposure from this transmission line. To repeat and expand these studies and to characterize the components of EMF responsible for the previously observed reduction in IL-1 activity, the current experiment examined not only the effect of exposure to electric and magnetic fields, but also the magnetic field component alone. In the current study, IL-2 was examined to characterize the effects of EMF exposure on an indicator of T cell responses. 45 Suffolk ewe lambs were randomized into three groups of 15 animals each. One group of animals was placed in the EMF pen, located directly beneath the transmission line. A second group was placed in the shielded MF (magnetic field only) pen, also directly beneath the transmission line. The third group of animals was placed in the control pen located several hundred meters away from the transmission line. During the 27 month exposure period, blood samples were taken from all animals monthly. When the data were analyzed collectively over time, no significant differences between the groups were found for IL-1 or IL-2 activity. In previous studies ewe lambs of 8-10 weeks of age were used as the study animals and significant differences in IL-1 activity were observed after exposure of these animals to EMF at mean magnetic fields of 3.5-3.8 microT (35-38 mG) and mean electric fields of 5.2-5.8 kV/m. At the start of the current study EMF levels were reduced as compared to previous studies. One interpretation of the current data is that magnetic field strength and age of the animals may be important variables in determining whether EMF exposure will affect IL-1 activity.

Evaluation of biological effects, dosimetric models, and exposure assessment related to ELF electric- and magnetic-field guidelines.

Several organizations worldwide have issued guidelines to limit occupational and public exposure to electric and magnetic fields and contact currents in the extremely low frequency range (<3 kilohertz). In this paper, we evaluate relevant developments in biological and health research, computational methods for estimating dosimetric quantities, and exposure assessment, all with an emphasis on the power frequency (60 hertz in North America, 50 hertz in Europe). The aim of each guideline is to prevent acute neural effects of induced electric fields. An evaluation of epidemiological and laboratory studies of neurobiological effects identified peripheral nerve stimulation as the response most suitable for establishing a magnetic-field guideline. Key endpoints that merit further study include reversal of evoked potentials; cardiovascular function, as measured by heart rate and heart rate variability; and sleep patterns. High-resolution computations of induced electric fields and current densities in anatomically correct human models are now achieved with finite-difference methods. The validity and limitations of these models have been demonstrated by computations in regular geometric shapes, using both analytic and numeric computations. Calculated values for average dosimetric quantities are typically within a few percent for the two approaches. However, maximum induced quantities are considerably overestimated by numerical methods, particularly at air interfaces. Overestimates are less pronounced for the upper 99th percentile level of a dosimetric quantity, making this measure a more useful indicator of maximum dose. Neural stimulation thresholds are dependent on the electric field around the excitable cell rather than on the current density, making the former preferable for expression of basic restrictions based on nervous system function. Furthermore, modeling data indicate that the induced electric field is much less strongly influenced by tissue conductivity than is the induced current density. In the electric utility industry, most magnetic-field exposures at or near guideline levels occur in highly nonuniform fields. Two methods are described for simplified estimation of induced quantities in such fields, with each method using as input modeling results for uniform field exposure. These methods have practical value for assessing occupational exposures relative to guideline levels.

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.

Summary and evaluation of guidelines for occupational exposure to power frequency electric and magnetic fields.

Major U.S. and international guidelines for limiting occupational exposures to EMF are evaluated. These safety guidelines are designed to prevent short-term adverse effects by maintaining bulk-tissue current densities below 10 mA m(-2) (rms). Above this level, effects of induced currents and electric fields can include stimulation of neural and cardiac tissues. The models and input data used by guideline-setting organizations to relate 50/60-Hz magnetic-field exposures to induced current densities differ significantly. In order to develop a better understanding of such differences, the current densities derived from exposure guideline models are compared to minimum thresholds for cardiac stimulation and fibrillation. The nominal minimum thresholds for cardiac stimulation and ventricular fibrillation are 100 times and 200 times greater, respectively, than the current density of 10 mA m(-2) used as a dosimetric limit. However, the assumed relationship between the 10 mA m(-2) dose limit and magnetic field exposure limits introduces additional uncertainty. The ratios between the threshold for cardiac stimulation and the calculated induced current density at the exposure limit vary between a low of 50 and a high of 526, depending upon the guideline. These ratios, as indicators of implicit safety factors, are larger than those recommended to protect against adverse effects of induced current density, including cardiac stimulation, in magnetic resonance imaging or against adverse effects of toxic chemical exposures. This review and assessment of EMF occupational exposure guidelines suggests that several scientific and compliance issues remain ambiguous or unresolved. Recommendations are made for guideline organizations to strengthen and clarify the scientific basis for the guideline process. These recommendations include the documentation of supporting data, development of operational definitions for guidelines, examination of dosimetric models, clarification of safety factors, and identification of high priority topics for future research.

Variability and consistency of electric and magnetic field occupational exposure measurements.

There is widespread scientific and public interest in possible health effects from exposure to electric and magnetic fields at frequencies associated with electricity use. Electric and magnetic field exposure assessment presents specific problems, among which are the inherent variability in exposure, the lack of robust statistical summary measures, and the lack of an accepted metric based on biological response. These pose challenges in defining distinct exposure groups, a basic goal for exposure assessments used in epidemiological studies. This paper explores the extent to which distinct electric and magnetic field exposure groups can be defined, by examining the variability and consistency of occupational electric and magnetic field exposure measurements among studies and within individual studies. Principal analyses are made by job titles because they are the most frequently used descriptors for stratifying occupational exposures to electric and magnetic fields. Methodological issues affecting the degree of consistency in measured electric and magnetic field exposures among occupational environments are also examined. Exposures by job title reported from electric and magnetic field measurement studies are summarized by general job category and industry. Analyses are performed both within and between job categories. Distributions of daily measured exposures for job categories taken from three large studies in the U.S. electric utility industry are compared to investigate consistency of exposures at a more detailed level. Analyses of reported personal exposure measurements from many studies and countries are consistent with less rigorous observations made heretofore on the basis of individual studies. In these studies, significantly elevated electric and magnetic field exposures are found in the electrician, lineworker, and substation worker categories; significantly elevated magnetic field exposures are also noted in the generation worker category; and magnetic field exposures in these groups are consistent across countries. Analyses within and among the elevated exposure job categories indicate that there are no significant differences between them. Among the studies, it is not possible to distinguish between exposures for well-defined groups within the categories, such as between transmission lineworkers and distribution lineworkers in the lineworker category; between generation operators and generation mechanics; or between substation operators and substation maintenance workers. This information provides a context for past studies and will help future efforts to define distinct occupational exposure groups exposed to electric and magnetic fields. Compilations of measured personal exposure data by industry and job title have been prepared as appendices (available from the author upon request).

Magnetic field exposure among utility workers.

The Electric and Magnetic Field Measurement Project for Utilities--the Electric Power Research Institute (EPRI) Electric and Magnetic Field Digital Exposure (EMDEX) Project (the EPRI EMDEX Project)--was a multifaceted project that entailed technology transfer, measurement protocol design, data management, and exposure assessment analyses. This paper addresses one specific objective of the project: the collection, analysis, and documentation of power-frequency magnetic field exposures for a diverse population of utility workers. Field exposure data measured by an EMDEX system were collected by volunteer utility employees at 59 sites in four countries between September, 1988, and September, 1989. Specially designed sampling procedures and data collection protocols were used to ensure uniform implementation across sites. Volunteers within 13 job classifications recorded which of eight work or three nonwork environments they occupied while wearing an EMDEX meter. Approximately 50,000 hours of magnetic field exposure records taken at 10 s intervals were obtained, about 70% of which were from work environments. Exposures and time spent in environments were analyzed by primary work environment, by occupied environment, and by job classification. Generally, for utility-specific job classifications related to the generation, transmission, and distribution of electricity, the field and exposure measurements in terms of workday mean field were higher than in more general occupations. The job classifications with the highest (median workday mean) exposure were substation operators (0.7 microT) and electricians (0.5 microT). Total variance also tended to be largest for utility-specific job classifications. For these workers, the contributions of between-worker and within-worker variances to total variance were about the same. Measurements in utility-specific environments were higher than in more general environments. Estimates of time-integrated exposure indicated that utility-specific job classifications received about one-half or more of their total exposure on the job. The nonwork field and exposure distributions for workers in all job categories were comparable with median nonworkday means of about 0.09 microT.

Correlations among indices of electric and magnetic field exposure in electric utility workers.

Power-frequency electric and magnetic fields are known to exhibit marked temporal variation, yet in the absence of clear biological indications, the most appropriate summary indices for use in epidemiologic studies are unknown. In order to assess the statistical patterns among candidate indices, data on 4383 worker-days for magnetic fields and 2082 worker-days for electric fields collected for the Electric and Magnetic Field Project for Electric Utilities using the EMDEX meter [Bracken (1990): Palo Alto, CA: Electric Power Research Institute] were analyzed. We examined correlations at the individual and job title group levels among indices of exposure to both electric and magnetic fields, including the arithmetic mean, geometric mean, median, 20th and 90th percentiles, time above lower cutoffs of 20 V/m and 0.2 microT, and time above higher cutoffs of 100 V/m and 2.0 microT. For both electric and magnetic fields, the arithmetic mean was highly correlated with the 90th percentile; moderately correlated with the geometric mean, median, and lower and higher cutoff scores; and weakly correlated with the 20th percentile. Electric and magnetic field indices were generally weakly correlated with one another. Rank-order correlation coefficients were consistently greater than product-moment correlation coefficients. Job title group summary scores showed higher correlations among electric field indices and magnetic field indices and between electric and magnetic field indices than was found for individual worker-days, with only the 20th percentile clearly independent of the others. These results suggest that individuals' exposures are adequately characterized by a measure of central tendency for electric and magnetic fields, such as the arithmetic or geometric mean, and an indicator of a lower threshold or cutoff for each field type, such as the 20th percentile or proportion of time above 20 V/m or 0.2 microT. A single measure of central tendency for each type of field appears to be adequate when exposures are assessed at the job title level.

Exposure assessment for power frequency electric and magnetic fields.

Over the past decade considerable data have been collected on electric and magnetic fields in occupational environments. These data have taken the form of area measurements, source characterizations, and personal exposure measurements. Occupational EMF levels are highly variable in space and time. Exposures associated with these fields exhibit similar large variations during a day, between days, and between individuals within a group. The distribution of exposure measures is skewed over several decades with only a few values occurring at the maximum field levels. The skewness of exposure measures implies that large sample sizes may be required for assessments and that multiple statistical descriptors are preferred to describe individual and group exposures. Except for the relatively few occupational settings where high voltage sources are prevalent, electric fields encountered in the workplace are probably similar to residential exposures. Consequently, high electric field exposures are essentially limited to utility environments and occupations. Within the electric utility industry, it is definitely possible to identify occupations with high electric field exposures relative to those of office workers or other groups. The highly exposed utility occupations are linemen, substation operators, and utility electricians. The distribution of electric field exposures in the utility worker population is very skewed even within a given occupation. As with electric fields, magnetic fields in the workplace appear to be comparable with residential levels, unless a clearly defined high-current source is present. Since high-current sources are more prevalent than high-voltage sources, environments with relatively high magnetic field exposures encompass a more diverse set of occupations than do those with high electric fields. Within the electric utility industry, it is possible to identify occupational environments with high magnetic field exposure relative to the office environment. Utility job categories with the highest exposures are generation facility workers, substation operators, utility linemen, and utility electricians. There are also higher exposures among traditional "electrical worker" job categories. Outside the electrical utility industry, potential sources of high occupational magnetic field exposures at ELF are induction furnaces, welding machines, electrical transportation systems, and electrical distribution vaults. However, the use of low power electrical equipment such as small motors in close proximity to workers and possibly for long periods of time could also lead to high exposure situations. Handheld survey instruments are available to perform area measurements of electric and magnetic fields at power frequencies but not aat all frequencies within the ELF range. Sophisticated personal computer-based instruments are available to characterize areas and sources across the entire frequency range.(ABSTRACT TRUNCATED AT 400 WORDS)

Exposure assessment for power frequency electric and magnetic fields (EMF) and its application to epidemiologic studies.

Epidemiologic studies suggesting possible health effects associated with exposure to electric and magnetic fields (EMF) from the transmission, distribution, and use of electricity have motivated increased interest in and attention to EMF exposure assessment. The result has been new instruments, measurement approaches, and exposure models that can improve on what has been a weakness in past epidemiologic studies, namely EMF exposure assessment. This paper presents a status report on EMF exposure assessment that emphasizes the need for incorporation of these advances in future studies. Several factors are identified that make the assignment of contemporary or retrospective EMF exposures potentially more difficult than for other environmental agents. These include: EMF is not generally detectable by humans, exposure scenarios for EMF are generally not memorable, there is no clear mechanism for EMF effects, and the pervasive nature of EMF in an industrialized society makes identification of a low-exposure group difficult. Elements of study design that are impacted by the nature of EMF exposures include: sampling and measurement strategies, summary measures of exposure, and the choice of surrogate and/or models of exposure. Consideration of these exposure assessment issues and incorporation of recent advances can improve the overall quality of epidemiologic studies with an EMF exposure component.

Experimental macroscopic dosimetry for extremely-low-frequency electric and magnetic fields.

Environmental and laboratory exposure to electric and magnetic fields (EMF) in the extremely-low-frequency range (ELF) produces electrical quantities that interact directly with the exposed biological system on a scale small compared to the size of the human body but large with respect to cellular dimensions. The purpose of this paper is to describe these macroscopic electrical quantities and their characterization through measurements on living systems and experimental models. Electric field exposure results in a total induced current, surface electric fields, internal electric fields, and internal currents. Magnetic field exposure results in internal magnetic field, internal electric fields, and internal currents. Basic properties of fields and matter determine the methods by which these quantities can be measured. Quantification or dosimetry for these parameters on a macroscopic basis can be directed to the whole body, a cross section across the body, a local surface area, or a local volume. Models of varying degrees of sophistication have been used to establish spatial distributions of external fields and internal fields and currents.

Effects of a high-voltage direct-current transmission line on beef cattle production.

Two herds of beef cattle were maintained beneath a +/- 500 kV direct-current transmission line during a 30-month period, and were compared with two similar herds maintained away from the transmission line. Exposures of animals under the line were five to 30 times greater than those of control animals, depending on the parameter of interest, with average exposure magnitudes of 5.6 kV/m, 4.1 nA/m2, and 13 k ions/cm3, respectively, for electric field, ion current, and density of ions. Productivity and health status of cows and calves were similar between lines and control treatments. Mean body mass of cows increased with maturity, from 438 kg in 1985 to 496 kg in 1987. Calf gain averaged 0.93 kg per head per day. No unusual sources of mortality were observed. Based on this confinement study, beef cattle permitted to graze in the vicinity of a high-voltage, direct-current transmission lines are not expected to experience any decrease in frequency of conception, calving, growth rate, or survival.