Evaluation of nonuniform 60-hertz magnetic-field exposures for compliance with guidelines

Magnetic-field exposures are considered in compliance with guidelines if they do not cause the induced electric field or current density to exceed basic restrictions that are based on possible adverse biological responses. Magnetic-field guidelines provide induction models for extrapolating from external field exposures to basic restrictions and vice versa. However, the uniform-field exposures used in these models do not reflect the nonuniform fields often encountered in actual high-field exposures. The purposes of this study were to investigate the relationships between external magnetic-field exposures and induced electric fields in nonuniform 60-hertz fields and to present a method for evaluating the compliance of such exposures with guidelines. Induction factors provide the induced electric field per unit of incident magnetic field. They represent a means of extrapolating from external field exposure to a peak induced electric field. Uniform and nonuniform field induction factors were computed for homogeneous ellipses and ellipsoids, and for an anatomically correct heterogeneous human model. Computations were carried out for three orthogonal uniform fields and for related nonuniform fields at varying distances from three line sources. Analytic expressions were used to compute induced peak electric fields for homogeneous models in uniform fields. A scalar-potential finite-difference method computed induced quantities for all models at 3.6-mm resolution in uniform and nonuniform fields. Equivalent uniform magnetic fields that produce the same peak electric field as a nonuniform field with a known maximum field were derived from the induction factors. To evaluate a nonuniform field exposure for compliance, the equivalent uniform field for the exposure is estimated based on the magnitude of the maximum surface field and the distance from the line source. Compliance is achieved if the equivalent uniform magnetic field is below the magnetic-field limit. Equivalent uniform magnetic-field exposures are computed for two actual electric utility tasks, as examples.     

Assessing compliance with power-frequency magnetic-field guidelines

Several organizations have established guidelines for occupational exposure to power-frequency magnetic fields. At 60 hertz, exposure limits are 1.0 millitesla (mT) for the American Conference of Governmental Industrial Hygienists, 0.42 mT for the International Committee on Non-ionizing Radiation Protection, and 1.6 mT for the National Radiation Protection Board guidelines. Adoption of the current guidelines as mandatory standards could dramatically affect electric-utility work practices. Two large personal-exposure monitoring projects have characterized exposures of overhead lineworkers and cable splicers while they perform various tasks near energized conductors. Personal exposure measurements indicate that these two groups are very likely the most highly exposed among utility workers and often experience fields above guideline levels. In addition, survey measurements in utility environments have identified locations where other workers may experience exposures at guideline levels. The nature of high-field exposure scenarios in the utility industry suggests a simple practical method for determining compliance with basic restrictions on internal induced current density [< 10 milliamperes per square meter (mA m(-2))] in the presence of fields that exceed limits to magnetic fields external to the body.     

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.     

Evaluating occupational 60-hertz electric-field exposures for guideline compliance

This article examines determination of compliance of 60-Hz electric-field exposures with occupational guideline limits. The guidelines are expressed as a limit on the unperturbed electric field without allowance for the severity of potential spark discharges. A line worker on a 500-kV transmission-line tower provided a practical example of an occupational exposure. In this realistic case, the worker's posture, the uniformity of the field, and the field orientation differed from the guideline exposure scenario of standing erect in a vertical uniform field. An accurate estimate of the unperturbed nonuniform fields in the climbing space of a lattice steel structure was computed using Monte Carlo methods that modeled surface and spatial electric fields on and near standard geometrical elements. Fields were computed at 20 points in a three-dimensional array, simulating the location of the human body on the tower. We estimated the average unperturbed electric field, space potential, induced short-circuit current, induced open-circuit voltage, and the stored charge and energy available for a discharge over a range of capacitances to ground. The on-tower exposure parameters were compared with those from the idealistic guideline exposure scenario. The average electric field of 24.4 kV/m for the on-tower exposure exceeded the limit of 20 kV/m stated in the recently adopted IEEE Standard C95.6 2002. However, the charge available for a spark discharge during the on-tower exposure was less than that for the guideline exposure scenario. Thus, for an exposure limit based on a constant-charge criterion for adverse reaction to spark discharges, guideline on-tower exposure would be below the limit established for the guideline exposure scenario. Evaluation of electric-field exposures in terms of the charge associated with spark discharges provides a means of comparing any electric-field exposure scenario with the ideal guideline scenario in terms of an effects-related physical quantity. This approach is consistent with the exposure limit/basic restriction methodology that employs a basic restriction on a physical quantity as the ultimate determinant of compliance.     

Exposure variability in the workplace: its implications for the assessment of compliance

Day-to-day variations of occupational exposures have important implications for the industrial hygienist trying to assess compliance with an occupational exposure limit. As only a limited number of samples are taken during an observation period, extrapolations are required to estimate exposures over the unsampled period. Compliance may be evaluated using estimates of the geometric mean (GM) and the geometric standard deviation (GSD) to calculate a confidence interval around the mean exposure and compare this interval to a limit value, assuming a lognormal distribution of exposures over time. These confidence intervals are very sensitive to the estimate of GSD. Hence, the questions of when to sample and how many samples to take for a reliable assessment of exposure variability (GSD) are the focus of this paper. Analyses of simulated exposure-time series and 420 data sets of personal exposures with three or more measurements obtained from actual workplaces demonstrate that the small number of samples usually collected during surveys leads to biased estimates of the variance of the exposure distribution. There is a high likelihood of an underestimate of variance, which rapidly increases if 8-hr time-weighted average samples are collected on consecutive days or within a week. The results indicate that in 80% of the within-week exposure-time series, the estimated GSD may be too low, even up to a factor of 2. Evidence is presented that autocorrelation is a likely explanation for the bias observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Equipment grounding affects contact current exposure: a case study of sewing machines

Exposure to contact current may occur when the body is in contact with two conductive surfaces with different electrical potentials. To date, no published data that describe such exposures or electrical conditions that may predispose to such exposures exist. Our investigation into contact current exposure included (i) a small sample of workers in a garment production facility with modern well-grounded equipment performing normal work tasks and (ii) a single individual simulating garment production tasks in a sewing machine repair facility with substandard equipment grounding. In both cases, we deployed a newly developed personal monitor that records contact current events at the power frequency of 60 Hz. The personal monitoring data suggested that more frequent exposure occurs in association with, and probably because of, poorer grounding practices. This preliminary conclusion was validated with controlled laboratory measurements of potentials to reference ground on specific locations of four sewing machines with different grounding characteristics. Propensity to exposure was greater in the two machines with inferior grounding characteristics, and increased in the other two when deprived of their grounding connections. Contact currents at or below threshold-of-perception levels can produce electric fields within tissues that may plausibly produce biological effects. On this basis, such exposures have been under-investigated relative to the far greater attention accorded to occupational electric and magnetic fields.     

Video exposure assessments demonstrate excessive laboratory formaldehyde exposures

Video exposure assessments were conducted in a comparative anatomy laboratory using formaldehyde-preserved sharks and cats. Work in the facility using time-integrated samplers indicated personal and area concentrations generally below the current OSHA permissible exposure limit. However, complaints about room air quality were frequent and routine. Using a photoionization detector with an integral data logger, total ionizables present were sampled as a surrogate for formaldehyde. After synchronizing time tracks from the datalogger concentrations with simultaneously created videotapes of laboratory tasks, composite video exposure overlays were generated. Use of this video exposure method revealed very short-lived, excessively high peak exposure events, whereas conventional time-weighted averages indicated the majority (30/32) of personal exposures were below the OSHA limit of 0.75 ppm. These legally acceptable exposure levels were associated with self-reported symptoms of burning nose and eyes and eye irritation. Thus, transient peak formaldehyde concentrations not detected by longer term averaging studies could be responsible for the health effects reported. The video exposure monitoring method demonstrated that close dissection work, opening peritoneal cavities, and specimen selection activities were most likely the causes of elevated student exposures. Teaching assistants' exposures were the highest, exceeding OSHA limits on several occasions. The utility of the video monitoring method for conducting enhanced, critical task exposure assessments is discussed.     

Air sampling of nickel in a refinery

Air monitoring was conducted in a nickel base metal refinery to determine compliance with occupational exposure limits. The hypothesis stated that levels of airborne dust may pose a risk to worker health if compared to the relevant exposure limits. Exposure limits for nickel species are set for the inhalable nickel dust fraction. Personal air samples, representative of three selected areas were collected in the workers' breathing zones, using the Institute of Occupational Medicine (IOM) samplers. Real-time personal samples were collected randomly over a two-month period in three nickel production areas. Filter papers were treated gravimetrically and were analysed for soluble and insoluble nickel through inductive coupled plasma-mass spectrometry (ICP-MS). Measured concentrations were expressed as time weighted average exposure concentrations. Results were compared to South African occupational exposure limits (OELs) and to the threshold limit values (TLVs) set by the American Conference of Governmental Industrial Hygienists (ACGIH) to determine compliance. Statistical compliance was also determined using the National Institute for Occupational Safety and Health procedure as prescribed by South Africa's Hazardous Chemical Substances Regulations in 1995. In two of the areas it was found that exposure concentrations complied with the OELs. Some exposures exceeded the OEL values and most exposures exceeded the TLV values in the other area concerned. A comprehensive health risk assessment needs to be conducted to determine the cause of non-compliance.     

A methodological approach for measuring personal exposure to noise]

A criterion is proposed for planning personal noise exposure surveys. For measurements taken at random over entire work shifts, the aim is to achieve preset confidence limits of the arithmetic mean of daily personal exposure (i.e., +/- 2 dBA or +/- 1 dBA). Five or six measurements are sufficient to estimate the standard deviation. By following simple rules to calculate the confidence limits for a population with unknown variance, it is possible to fix the number of measurements N with sufficient accuracy to achieve this goal. This number is a function of daily personal exposure variability. The choice of the confidence limits determines the accuracy of the personal noise exposure assessment, provided that it is the exponential mean of N measurements. This method allows the survey to be planned according to the desired accuracy of the final result. An example is provided of a survey carried out in a refractory brick factory, which shows that if the daily personal exposure range is 4-6 dBA, a 5-measurement survey is sufficient to achieve a +/- 2 dBA confidence interval, while 10-12 measurements are necessary to achieve a +/- 1 dBA confidence interval. If the range is 14-15 dBA the same results are achieved with 12-14 measurement in the first case and with about 50 in the second. The latter results shows that if the survey is stopped after 5 measurements, the probability of accepting a value of integrated personal noise exposure outside the N-integrated +/- 1 dBA range may reach 50%, mainly among groups of workers with greater variability in daily exposure.     

The rules of the game: An analysis of Osha's enforcement strategy

Although OSHA promulgates standards for chemical exposures on the basis of workers' relative risk of acquiring chronic disease, it interprets the limits as absolute levels never to be exceeded. This poses a dilemma to the gathering of useful information which can be used to assess and reduce exposures, because employers can maximize compliance outcomes by minimizing exposure monitoring. The result is that, in the absence of adequate information, the working population's true risk of acquiring disease is essentially unknown and average exposures of several times the permissible exposure limit (PEL) can be declared in compliance. The dilemma could be resolved if OSHA would define compliance in terms of one or more parameters of the cumulative exposure distribution. Since OSHA's current method of assessing risks is based upon average exposures, enforcement of the PEL as the limit of each worker's true mean exposure would be internally consistent. This change could have the effect of encouraging employers to monitor exposures since the confidence interval surrounding the mean can be narrowed with increased sample size.     

Time trends in exposure measurements from OSHA compliance inspections of the pulp and paper industry

Time trends in employee exposures to the air contaminants measured by the Occupational Safety and Health Administration (OSHA) during compliance inspections of pulp and paper manufacturing facilities conducted between 1979 and 1997 were evaluated based on the measurement results stored in the OSHA Integrated Management Information System (IMIS) database. The IMIS database is among the largest sources of occupational exposure measurements available for occupational health research in the United States. The IMIS database contains the results of 3,568 personal time-weighted average (TWA) measurements for 171 air contaminants made at 524 establishments in Standard Industrial Classification (SIC) 26. An analysis of these measurements revealed an overall decrease in the total number of measurements made per year since 1991, and a decrease in the percentage of measurements by year that exceeded the OSHA permissible exposure limits (PELs). Linear regression analyses detected decreasing trends in the geometric mean concentrations by year for 33 of the 36 agents analyzed.     

Occupational and residential 60-Hz electromagnetic fields and high-frequency electric transients: exposure assessment using a new dosimeter

One problem that has limited past epidemiologic studies of cancer and exposure to extremely low-frequency (0-100 Hz) electric and magnetic fields has been the lack of adequate methods for assessing personal exposure to these fields. A new 60-Hz electromagnetic field dosimeter was tested to assess occupational and residential exposures of a group of electrical utility workers and a comparison background group over a 7-day period. Comparing work periods only, utility workers' exposures were significantly higher than background levels by a factor of about 10 for electric (E) and magnetic (B) fields and by a factor of 171 for high-frequency transient electric (HFTE) fields. When overall weekly time-weighted averages combining work and nonwork exposures were compared, ratios of the exposed to background groups were lower. B and HFTE exposure ratios remained statistically significant, with values of 3.5 and 58, respectively, whereas the electric field exposure ratio was no longer significant, with a value of 1.7. E-field exposures of the background group were the highest during the nonwork period, probably reflecting the use of electrical appliances at home. Residential E- and B-field exposures were in the same range as published results from other surveys, whereas occupational E-field exposures tended to be lower than exposures reported in other studies. The high variability associated with occupational exposures probably accounts for the latter discrepancy. Worker acceptance of wearing the dosimeter was good: 95% of participants found it to be of little or no inconvenience while at work. At home, 37% found the device to be inconvenient in its present form but would not object to wearing a slightly smaller and lighter dosimeter.     

Personal exposure monitoring wearing protocol compliance: an initial assessment of quantitative measurement

Personal exposure sampling provides the most accurate and representative assessment of exposure to a pollutant, but only if measures are implemented to minimize exposure misclassification and reduce confounders that may cause misinterpretation of the collected data. Poor compliance with personal sampler wearing protocols can create positive or negative biases in the reported exposure concentrations, depending on proximity of the participant or the personal sampler to the pollutant source when the monitor was not worn as instructed. This paper presents an initial quantitative examination of personal exposure monitor wearing protocol compliance during a longitudinal particulate matter personal exposure monitoring study of senior citizens of compromise health in North Carolina. Wearing compliance varied between participants because of gender or employment status, but not longitudinally or between cohorts. A minimum waking wearing compliance threshold, 0.4 for this study of senior citizens, is suggested to define when personal exposure measurements are representative of a participant's exposure. The ability to define a minimum threshold indicates data weighting techniques may be used to estimate a participant's exposure assuming perfect protocol compliance.     

Exposure to extremely low frequency magnetic fields among working women and homemakers.

Given concerns with potential health effects of exposure to magnetic fields, the goal of this study was to examine the magnitude and sources of occupational and residential exposure to extremely low frequency (primarily 60 Hz) magnetic fields among women. Exposure to 60 Hz magnetic fields was surveyed among cases and controls recruited for a study of breast cancer in 25 counties in North Carolina. The 273 women who participated wore an integrating personal magnetic-field exposure meter (AMEX 3-D) that measured their time-weighted average (TWA) exposure. A questionnaire was administered to determine the duration and frequency of electric appliance and machinery use. The geometric mean (GM) of the TWA exposure for employed women was 0.138 microT (range 0.022-3.636 microT) and for homemakers 0.113 microT (range 0.022-0.403 microT). Women working in manufacturing and industrial facilities had the highest exposure (GM 0.265 microT, range 0.054-3.436 microT), while nurses and health technicians (GM 0.134 microT, range 0.032-0.285 microT) and teachers and school administrators (GM 0.099 microT, range 0.035-0.673 microT) had the lowest exposures. Job titles, unless very limited in scope and/or environment, self-reported information about equipment use, potential exposure sources, time, and distance were not good predictors of magnetic-field exposure. Furthermore, the results show that occupations previously observed to have increased risk of breast cancer, such as teachers, nurses, administrative support, and housewives, did not have elevated average magnetic field exposures. Therefore, it is questionable whether exposure to power frequency magnetic fields is the cause of the increased breast cancer risk seen in these occupations.     

Characterizing historical industrial hygiene data: a case study involving benzene exposures at a chemical manufacturing facility (1976-1987)

This article describes how nearly 3700 air samples of benzene collected in a typical chemical manufacturing (acetic acid) facility in the United States from 1976 to 1987 were used to characterize daily time-weighted average (TWA) exposure levels. We found that those workers directly involved in manufacturing operations had likely TWA exposures to benzene of about 2.0 ppm from 1976-1981 and about 1.0 ppm from 1982-1987. These results are consistent with the improved industrial hygiene programs at chemical facilities, which often occurred following the adoption of stricter occupational exposure limits. Additionally, about 97% of all personal TWA samples had reported benzene concentrations less than 10 ppm, which was the permissible exposure limit in place prior to 1987. Because one of the primary objectives of historical workplace air sampling efforts was to understand the source of release of contaminants, a large number of short-term (typically about 1 min) area samples were also collected. Although these types of samples are often not useful for predicting human exposure without time-motion information, airborne benzene concentrations were about five- to tenfold higher for many of the short-term area samples than for the personal TWA measurements. The methodology presented here should be useful for evaluating industrial hygiene data collected after the early 1970s (after the promulgation of OSHA), and our findings support prior reports that large corporations in the United States have tended to reduce workplace exposures to airborne contaminants consistent with historical changes in occupational exposure limits.     

Interpersonal and daily variability of personal exposures to nitrogen dioxide and sulfur dioxide

Chronic exposure is often assessed using a single measurement per individual or group. However, daily levels of personal exposure can vary greatly. Chronic exposure classification by a single measurement could be significantly affected by the interpersonal and daily variations of exposures. The purpose of this study is to determine the effect of using a single personal exposure measurement on estimating long-term exposure. This study used measurements of consecutive 14 daily personal exposures to nitrogen dioxide (NO(2)) and sulfur dioxide (SO(2)) of 50 individuals in Yeochun, Korea. The daily personal exposures were measured by passive samplers. Personal exposure to NO(2) was associated with gas cooking, and personal exposure to SO(2) was associated with ambient air pollution. Mixed effects models indicated that daily variability was greater than interpersonal variability for both the pollutants. Effectiveness of using single-day personal measurements for long-term population mean exposure was supported by relatively consistent daily population averages, but multiple-day measurements might be warranted for characterizing individual exposures or high-end population exposures such as the 95th percentile. Although classification of high and low exposure groups by 1-day exposure and by 14-day exposure produced similar group totals, 20% of individual NO(2) exposures and 31% of individual SO(2) exposures were misclassified using 1-day exposures. Average values of 1-day exposure and 14-day exposure were significantly different, but the difference decreased by an increase in the number of measurements for the short-term exposure. The findings were similar for both NO(2) and SO(2), although the two air pollutants have different sources and behaviors.     

A tale of two limits

As they do with other potentially hazardous agents, occupational health and safety professionals in the United States take a two-pronged approach to controlling noise exposure. We assure compliance with OSHA's regulatory limits to satisfy lawyers and compliance officers while using TLVs to protect worker health. Unfortunately, using both the TLV and OSHA limits for noise exposure may involve considerably more work than for most chemical exposures. Using both limits for noise may require different measuring equipment or multiple measurements of exposure. Using the TLV also may require noise measurement at the abdomen, as well as the ear and consideration of concomitant chemical exposures. Finally, using the TLV requires the OHS professional to confront the fact that no exposure value can claim to protect all workers and that employee rotation may result in more hearing loss than would exposing a smaller group of workers to more noise. These facts make it clear why OHS professionals should never use exposure limits as just numbers. Rather, we strive to understand the basis for the limits we are using, to be sure we understand the limitations of those limits and how they might uniquely affect our workers in their work environments.     

Probabilistic approach to ranking sources of uncertainty in ELF magnetic field exposure limits.

There is a need to improve the biological data, dosimetry, and risk assessment methodology used for setting guidelines for occupational exposures to extreme-low-frequency magnetic fields. This paper illustrates how a probabilistic approach can be used to determine priorities for future research based upon the analysis of biological and dosimetric variables that affect stimulation of the heart by magnetically-induced currents. A model was constructed to predict a level of whole-body magnetic-field exposure below which no cardiac stimulation is expected (Bncs). For each iteration of the model, a value was selected from cardiac stimulation threshold, shape factor, and conductivity distributions by Latin Hypercube sampling, and a value for Bncs was computed. The very wide range of simulated Bncs magnetic-field values obtained indicates that there is considerable uncertainty about what constitutes a "safe" level of exposure. The results show that the major occupational-exposure guidelines are very conservative with respect to risks of cardiac stimulation. The minimum Bncs value computed (0.01 T) by either a circular-loop model or an ellipsoid dosimetric model is ten times the highest recommended workday exposure value in a guideline. The lowest 5% Bncs value calculated for a circular-loop model is about 50-times the ICNIRP occupational exposure limit; the lowest 5% Bncs value calculated for an ellipsoid model is more than 100 times the ACGIH occupational exposure limit. A new finding is that the method specified by several guidelines for determining guideline compliance when exposures occur at multiple frequencies (additive weighting of harmonics) leads to substantially lower Bncs estimates relative to a probably more valid rms-weighting method. Probabilistic sensitivity analyses indicate that lack of knowledge of the threshold for cardiac stimulation is the greatest source of uncertainty as to what is a "safe" level of exposure to extreme-low-frequency magnetic fields. Additional research to address this source of uncertainty may be expected to have the greatest potential impact to reduce overall uncertainty about safe magnetic field exposures.     

The relationship between TLV-TWA compliance and TLV-STEL compliance

The purpose of this study was to investigate the relationship between the American Conference of Governmental Industrial Hygienists' (ACGIH) time-weighted average (TLV-TWA) and short-term exposure limit (TLV-STEL) Threshold Limit Values (TLVs). It is of value to determine if one or the other of these exposure limits is inherently more stringent so that exposure monitoring strategies may be devised which efficiently use available resources and effectively control exposures to meet both exposure limits. The ACGIH short-term exposure limit (TLV-STEL) imposes three conditions on short-term (15-minute) exposures. These conditions involve exceeding the TLV-TWA and TLV-STEL levels and the time-separation of those short-term exposures that exceed the TLV-TWA level. These conditions were analyzed to produce eight unique component probabilities for TLV-STEL non-compliance. The sum of these eight components is the total probability of TLV-STEL non-compliance. Mathematical expressions for the eight probability components are derived in terms of the probability that a single 15-minute exposure exceeds the time-weighted average threshold limit value (TLV-TWA) and the geometric standard deviation of these 15-minute exposures. These expressions were applied to various hypothetical workplaces, and the relationship between TLV-TWA and TLV-STEL compliance is presented. The results showed that non-compliance of 15-minute exposures with the TLV-STEL level is only one part of overall TLV-STEL non-compliance. The additional ACGIH provisions for TLV-STEL compliance--the number of 15-minute exposures exceeding the TLV-TWA level and the number of clean periods separating such exposures--can be important factors in determining TLV-STEL compliance. It is concluded that compliance with all provisions of the ACGIH TLV-STEL adds a degree of stringency that greatly enhances the likelihood of TLV-TWA compliance for most workplace environments.