Thomas E. Bernard

Rationale for a Personal Monitor for Heat Strain

Worker heat-stress exposures can be controlled for short periods above the threshold limit value (TLV) by self-assessment, if the worker can avoid overexposure based on excessive heart rate and/or excessive core temperature. A socially acceptable surrogate for core temperature and a measure of heart rate are objective measures that can increase the reliability of the self-assessment decision. This article describes a surface-mounted temperature sensor developed to indicate when rectal temperature reaches a safe limit. Protective criteria were established for temperature sensor alert limits. A fixed threshold for heart rate may cause premature alerts during bursts of activity and miss lower, but sustained, heart rates that represent significant physiological strain. For these reasons, heart rate criteria based on seven moving-time averages also were developed. The criteria are based on a relationship between heart rate and endurance time. The temperature sensor and heart rate criteria form the basis of a real-time personal monitor for heat strain.

WBGT Clothing Adjustments for Four Clothing Ensembles Under Three Relative Humidity Levels

Threshold limit values for heat stress and strain are based on an upper limit wet bulb globe temperature (WBGT) for ordinary work clothes, with clothing adjustment factors (CAF) for other clothing ensembles. The purpose of this study was to determine the CAF for four clothing ensembles (Cotton Coveralls, Tyvek® 1424 Coveralls, NexGen® Coveralls, and Tychem QC® Coveralls) against a baseline of cotton work clothes and to determine what effect relative humidity may have. A climatic chamber was used to slowly increase the level of heat stress by increasing air temperature at three levels of relative humidity (20%, 50%, and 70%). Study participants wore one of the five ensembles while walking on a treadmill at a moderate metabolic rate of 155 W m-2 (about 300 W). Physiological data and environmental data were collected. When the participants core temperature reached a steady state, the dry bulb temperature was increased at constant relative humidity. The point at which the core temperature began to increase was defined as the inflection point. The environmental temperature recorded 5 min before the inflection point was used to calculate the critical WBGT for each ensemble. A three−way analysis of variance with ensemble by humidity protocol interactions and a multiple comparison test were used to make comparisons among the mean values. Only the vapor−barrier ensemble (Tychem QC) demonstrated an interaction with humidity level. The following CAFs are proposed: Cotton Coveralls (0°C–WBGT), Tyvek 1424 Coveralls (+1), NexGen Coveralls (+2), and Tychem QC Coveralls (+10).

Prediction of Workplace Wet Bulb Global Temperature

The wet bulb globe temperature (WBGT) is the de facto standard to assess environmental contributions to heat stress. A practical problem emerges when the heat stress conditions vary over many locations or during the day. To address this problem, investigators have suggested empirical relationships and thermodynamic models. The purpose of this effort was to examine a thermodynamic model in the laboratory and to predict WBGTs in an aluminum smelter by both the empirical and thermodynamic models. In the laboratory, there was no real difference between the experimental data and the thermodynamic model. In the application to an aluminum smelter, there was a small overall tendency for the predicted values to be greater than the actual values, but there was no practical difference between the models. The empirical model provided a good match with a slight over-prediction by 0.5 degree C with a standard deviation of 3.0 degrees C. For the same data, the thermodynamic model had an average over-prediction of 0.7 degree C with a standard deviation of 2.8 degrees C. Either method of predicting WBGT was effective. The empirical method required less computation and was conceptually simpler.

WBGT clothing adjustment factors for four clothing ensembles and the effects of metabolic demands.

This study measured the clothing adjustment factors (CAFs) for four clothing ensembles (Cotton Coveralls, Tyvek 1427 Coveralls, NexGen Coveralls, and Tychem QC Coveralls; all coveralls were worn without hoods) against a baseline of cotton work clothes to determine whether the CAFs would be affected by the metabolic rate. Fifteen participants wore one of the five ensembles while walking on a treadmill at low, moderate, and high rates of work in an environment maintained at 50% relative humidity. A climatic chamber was used to slowly increase the level of heat stress by increasing air temperature. When the participants core temperature reached a steady-state, the dry bulb temperature was increased. The point at which the core temperature began to increase was defined as the inflection point, and the WBGT recorded 5 min before the inflection point was the critical WBGT for each ensemble. A three-way mixed effects linear model with ensemble by metabolic rate category interactions demonstrated that the CAF did not change with metabolic rate, so CAFs can be used over a wide range of metabolic rates. The data at the moderate metabolic rate were combined with data on 14 participants from a previous study under the same conditions. The CAFs in °C WBGT were 0 for cotton coveralls, 1.0 for Tyvek 1422A, and 2.5 for NexGen. Although the value of 7.5 for Tychem QC was found, the recommendation remained at 10 to account for the effects of humidity. The standard error for the determination of WBGT crit at 50% relative humidity was 1.60°C WBGT.

Thermal characteristics of clothing ensembles for use in Heat Stress analysis

The Heat Stress Index was an early model for the assessment of heat stress. The International Organization for Standardization (ISO) standard for required sweat rate is the current generation of heat balance methods for occupational heat stress. The method assumes cotton clothing and works adequately for cotton/polyester blends. To extend the usefulness of the model, the thermal characteristics of a variety of commercially available and prototype protective clothing ensembles have been determined for application in the ISO method. The fundamental principle for assessing thermal characteristics of work clothing is establishing the critical environmental conditions in which test subjects were just able to maintain thermal equilibrium. Critical conditions were found for warm, humid conditions; hot, dry conditions; intermediate conditions of temperature and humidity; and/or moderate conditions in which metabolic rate was increased to a limiting thermal load. Typically, five subjects at each condition for each ensemble were used. Metabolic rate, average skin temperature, and the environmental conditions (air temperature and vapor pressure) were noted at the critical conditions, and the total insulation was estimated for each ensemble. From these values, the total evaporative resistance, the clothing factor for dry heat exchange (CFcl), and the clothing factor for evaporative cooling (CFpcl) were determined. When compared with reports of others on thermal characteristics the results agreed when pumping factors and clothing wetness were considered. The result was higher than expected values for CFcl and lower values for CFpcl.

PHYSIOLOGICAL EVALUATION OF LIQUID-BARRIER, VAPOR-PERMEABLE PROTECTIVE CLOTHING ENSEMBLES FOR WORK IN HOT ENVIRONMENTS

Work clothes using fabrics with vapor-transmitting characteristics are in limited use in various industrial applications, and there is a growing interest in their purported ability to help reduce heat stress. This study was performed to compare two vapor-transmitting ensembles with other clothing ensembles previously tested. The evaluation was based on an established experimental protocol that determines the critical values of air temperature and water-vapor pressure so that an individual maintains thermal balance, while controlling other factors that contribute to worker heat stress (e.g., air motion and metabolism). There were no differences between the two vapor-transmitting garments in their effects on worker heat stress. When compared to the results of other studies, the two vapor-transmitting garments had critical environmental characteristics similar to two layers of cotton coveralls and they performed better from a heat stress standpoint than a disposable vapor-barrier suit worn over cloth coveralls.

Maximum Sustainable Work Rate for Five Protective Clothing Ensembles with Respect to Moisture Vapor Transmission Rate and Air Permeability

Abstract The fabrics associated with protective clothing affect heat stress, which influences productivity and risks of heat-related disorders. This study compared the work limiting effects of five protective coveralls and a semiclothed condition (t-shirt and shorts). Two fabric characteristics determined from bench tests, moisture vapor transmission rate (MVTR), and air permeability were also examined as possible predictors of ensemble performance. A progressive metabolic rate protocol was used where environmentalconditions (Tdb = 32°C; Tpwb = 26°C) were held constant while treadmill speed was slowly increased. The limiting metabolic rate to just maintain thermal equilibrium was the critical point. At this point, critical speed and critical metabolic rate were noted and total evaporative resistance was calculated for each ensemble. Five acclimatized subjects wore each of the six clothing conditions in a random order. Statistically significant differences were found among the five protective garments and a semiclothed ensemble for critical treadmill speed (Scrit ), critical metabolic rate (M crit ), and total evaporative resistance (R e − t ). The semiclothed condition (S crit = 1.77 m/sec; M crit = 580 W; R e−t = 0.0099 kPa m 2 /W) and ensembles made from spunbonded, melt blown, spunbonded polypropylene (SMS) (1.72 m/sec; 560 W; 0.0135 kPa m2/W) and spunbonded polypropylene (1.67 m/sec; 550 W; 0.0126 kPa m2/W) were able to support higher work rates than fabrics made from Tyvek 1422-A (a nonwoven spunbonded olefin) (1.48 m/sec; 470 W; 0.0183 kPa m 2 /W) and a microporous film supported by spunbonded polypropylene (1.34 m/sec; 420 W; 0.0231 kPa m2/W). A tightly woven polyester ensemble (1.59 m/sec; 510 W; 0.0130 kPa m2/W) had intermediate values and was not significantly different from either group. Air permeability was a better predictor of fabric work limiting performance than MVTR. An air permeability on the order of 10,000 L/min cm 2 bar would have little effect on maximum sustainable work.

Job Level Risk Assessment Using Task Level Strain Index Scores: A Pilot Study

This paper explores 2 methods of modifying the Strain Index (SI) to assess the ergonomic risk of multi-task jobs. Twenty-eight automotive jobs (15 cases and 13 controls) were studied. The first method is based on the maximum task SI score, and the second method is modeled on the NIOSH Composite Lifting Index (CLI) algorithm, named cumulative assessment of risk to the distal upper extremity (CARD). Significant odds ratios of 11 (CI 1.7–69) and 24 (CI 2.4–240) were obtained using the modified maximum task and CARD, respectively. This indicates that modification of the SI may be useful in determining the risk of distal upper extremity injury associated with a multi-task job.

Estimation of Metabolic Rate Using Qualitative Job Descriptors

This project developed two methods to estimate metabolic rate that can be used easily and provide acceptable precision. The methods were developed from the measurement of oxygen consumption on 80 typical jobs in automotive manufacturing. The Ready-Reference Method uses three easily identified, dichotomous characteristics to classify the job into one of four levels of metabolic rate. The characteristics are based on motion of the hands, weights or forces, and walking; standard error of estimate was 68 kcal/hour. The Component Method has four terms that account for hand motion, walking/carrying, lifting, and pushing/pulling. Hand motions and lifting factors are characterized by three or four categories with a reference value to make the determination easier by using more qualitative data. Walking/carrying and pushing/pulling data are noted as typical distances traveled in one minute. The standard error of estimate for this method was 62 kcal/hour. In addition to a smaller standard error of estimate, the further advantage of the Component Method over the Ready-Reference Method is an ability to point toward the largest contributors to metabolic rate.

Short-Term Heat Stress Exposure Limits Based on Wet Bulb Globe Temperature Adjusted for Clothing and Metabolic Rate

Most heat stress exposure assessments based on wet bulb globe temperature (WBGT) consider the environmental conditions, metabolic demands, and clothing requirements, and the exposure limit is for extended work periods (e.g., a typical workday). The U.S. Navy physiological heat exposure limit (PHEL) curves and rational models of heat stress also consider time as a job risk factor so that there is a limiting time for exposures above a conventional WBGT exposure limit. The PHEL charts have not been examined for different clothing and the rational models require personal computers. The current study examined the role of clothing in short-term (time limited) exposures and proposed a relationship between a Safe Exposure Time and WBGT adjusted for clothing and metabolic rate. Twelve participants worked at a metabolic rate of 380 W in three clothing ensembles [clothing adjustment factors]: (1) work clothes (0°C-WBGT), (2) NexGen microporous coveralls (2.5°C-WBGT), and (2) vapor-barrier coveralls (6.5°C-WBGT) at five levels of heat stress (approximately at the clothing adjusted TLV plus 7.0, 8.0, 9.5, 11.5 and 15.0°C-WBGT). The combinations of metabolic rate, clothing, and environment were selected in anticipation that the participants would reach a physiological limit in less than 120 min. WBGT-based clothing adjustment factors were used to account for different clothing ensembles, and no differences were found for ensemble, which meant that the clothing adjustment factor can be used in WBGT-based time limited exposures. An equation was proposed to recommend a Safe Exposure Time for exposures under 120 min. The recommended times were longer than the PHEL times or times from a rational model of heat stress.