The evaluation of workplaces subjected to heat stress: can ISO 7933 (1989) adequately describe heat strain in industrial workplaces?

The International Standard ISO 7933 (1989) Hot environments--Analytical determination and interpretation of thermal stress using calculation of required sweat rate has been proposed for the evaluation of climatic stress within the European system of CEN standards. Comparison of results of studies performed in climatic chambers and those in the field with the predictions of ISO 7933 show that there are considerable problems in using this index in practice. In its present state of development, ISO 7933 seems to be rather a step towards a useable index for evaluating climatic conditions rather than an established climatic index which is applicable in practice. Within the CEN standards, the deficiencies of ISO 7933 are reflected mainly by a restriction of the limits of application within EN 12 515 (1997) which is based on ISO 7933.     

Appropriateness of international heat stress standards for use in tropical agricultural environments.

Where a danger to health from heat stress is identified, standards allow decisions for implementing measures to reduce the heat stress to be made. These standards, specifically ISO 7243 (Wet Bulb Globe Temperature Index, WBGT) and ISO 7933 (Sweat Required, SWreq) were designed with European and American subjects, primarily for use in those countries. While the scope of the standards is international, little consideration has been made about how valid and usable they are in industrially developing countries. This investigation evaluated ISO 7933 and ISO 7243 in terms of validity and usability. A tropical agricultural task was simulated; 16 subjects plucking tea leaves for 2 h, (ta = tr = 37.18 degrees C; va = 0.16 m/s; rh = 70.17%). While ISO 7243 was valid (if slightly over protective) and usable, ISO 7933 was over protective and underestimated sweat and evaporation rates in its predictions. The discrepancies between predicted and observed results were attributed primarily to the calculations related to clothing in the standard. Furthermore, ISO 7933 was unusable without a computer; in regions where access to such technology may be limited, a simpler method of presentation is required.     

PREDICTOL: a computer program to determine the thermophysiological duration limited exposures in various climatic conditions

PREDICTOL is a PC program used to determine the thermophysiological duration limited exposures (DLE) in humans, nude or clothed, submitted to various climatic conditions (hot and cold climates) at rest or during a physical exercise. DLE are determined following different standards of the International Standardization Organization (ISO), especially ISO 7933 for hot environment and ISO-TR 11079 for cold environment. The original aspect of this program is that it can be used whatever the climatic conditions. The program presents two modes: an educational interactive mode and a scenario mode. The educational interactive mode demonstrates the thermophysiological effects, expressed as DLE, of different parameter changes (temperature, humidity, wind speed, metabolic heat production by physical exercise, clothing insulation and water vapor permeability). The scenario mode determines DLE for given various linked sequences as encountered in occupational, military or even recreational activities, each sequence being characterized by its climatic conditions, physical activities performed and by physical clothing properties. DLE given by PREDICTOL are correlated to those obtained in various controlled climatic laboratory conditions (r = 0.86; P < 0.001). PREDICTOL is written in Visual Basic 6.0. A "help menu" is provided to explain the use of the program and give information concerning the equations used to calculate both the thermal balance and DLE.     

Estimation of heat stress in Tanzania by using ISO heat stress indices

The aim of this study was to evaluate the usefulness of the ISO heat stress standards in estimating the heat stress and strain in workplaces in Tanzania. Another aim was to select and to develop simplified methods for measuring physiological parameters in developing countries. The methods were tested in four hot factories and at a construction site. It seems that in tropical working environments the climatic conditions for which the ISO 7933 standard is applicable are too narrow. For instance, the mean skin temperature was incorrectly estimated by ISO 7933. An approximate analysis of the working situation can nevertheless be carried out by assuming the mean skin temperature to be 34.5 degrees C. During the study, heat stress and strain were not as high as expected; deep body temperatures were usually lower than 38 degrees C, sweat rates lower than 400 g/h and heart rates below 100 beats/min for about 72% of the measuring time. This is due to the job rotation of the workers and the long rest periods, because the number of workers is large in the factories, and the weather was not at its hottest during the survey.     

Cardiovascular and thermal consequences of protective clothing: a comparison of clothed and unclothed states

We have undertaken a laboratory-based examination of the cardiovascular and thermal impact of wearing thermal (heat) protective clothing during fatiguing exercise in the heat. Seven males completed semi-recumbent, intermittent cycling (39.6 degrees C, 45% relative humidity) wearing either protective clothing or shorts (control). Mean core and skin temperatures, cardiac frequency (f(c)), stroke volume (Q), cardiac output (Q), arterial pressure, forearm blood flow (Q(f)), plasma volume change, and sweat rates were measured. In the clothed trials, subjects experienced significantly shorter times to fatigue (52.5 vs. 58.9 min), at lower peak work rates (204.3 vs. 277.4 W), and with higher core (37.9 degrees vs. 37.5 degrees C) and mean skin temperatures (37.3 degrees vs. 36.9 degrees C). There was a significant interaction between time and clothing on f(c), such that, over time, the clothing effect became more powerful. Clothing had a significant main affect on Q, but not Q, indicating the higher Q was chronotropically driven. Despite a greater sweat loss when clothed (923.0 vs. 547.1 g.m(-2) x h(-1); P<0.05), Q(f) and plasma volume change remained equivalent. Protective clothing reduced exercise tolerance, but did not affect overall cardiovascular function, at the point of volitional fatigue. It was concluded that, during moderately heavy, semi-recumbent exercise under hot, dry conditions, the strain on the unclothed body was already high, such that the additional stress imparted by the clothing ensemble represented a negligible, further impact upon cardiovascular stability.     

The assessment of heat radiation

Approximately 900 climatic chamber experiments were performed with 16 male subjects to study the thermal strain at climates including increased heat radiation. Based on the reactions of heart rate, rectal temperature and sweat rate, a heat stress index was developed for the assessment of climates with effective heat radiation intensities up to 1400 W m⁻². The index considers different combinations of dry air temperature (5–55°C), globe temperature (25–76°C), mean radiant temperature (25–160°C), air velocity (0.5–2.0 m s⁻¹), clothing, physical work load and directions of radiation and air flow.

The index integrates combinations of the variables producing the same degree of thermal strain into a single value. This value indicates the temperature of the physiologically equivalent climate in which air and radiant temperature are equal. It can be determined from a simple formula or from correspondent graphs.

In comparison, the international recommended heat stress indices are less capable to evaluate heat radiation correctly. The incorporation of the new partial index into the used indices may improve substantially their physiological validity in the assessment of climates with radiant heat stress.

Relevance to industry

The goal of this paper is to provide an improved assessment of thermal stress in working environments in which heat radiation is an important heat stress factor.     

Gender-related difference in sweat loss and its impact on exposure limits to heat stress

The aim of this study was to evaluate the validity of the recently proposed predicted heat strain (PHS) model with respect to gender, particularly in relation to the estimation of admissible exposure durations for work in hot environments. Two experiments addressing the effects of the climatic conditions inside vehicles during summer with altogether 96 females and 114 males were analysed. Each subject was exposed for 2 h to a typical summer climate. Highly significant gender-related differences were found for the observed sweat rates. When adjusting the sweat rate to anthropometric measures the differences remained statistically significant, even if females and males were matched for body surface area or surface-to-mass ratio. There is much evidence in the literature that gender-related differences in sweat loss diminish once they are adjusted to anthropometric variables or to physical fitness. However, thermal indices like the PHS model rely on absolute values of the physiological response to heat stress. Therefore, a systematic overestimation of the sweat loss for women could imply an underestimation of body heat storage and core temperature accordingly. A possible approach to revised limit values in the PHS model could be to reduce the values of the maximum sweat rate for women.

Relevance to industry

Work in hot environments is still present at many industrial workplaces, particulary in underdeveloped countries. International standards provide exposure limits for the prevention of heat disorders. The results of this study indicate that these limits should be revised with respect to gender-related differences in sweat loss.     

Comparison and validation of heat stress indices in experimental studies

The present study aimed at (1) deriving the best methodology for using heat stress indices in fluctuating ambient conditions, (2) assessing various ways for improving the prediction ability of the Required Sweat Rate index (SW_req). The data base included the results from five experimental series involving 32 volunteers; these series involved environmental variations at constant metabolic rate, work-rest cycles in constant ambient conditions and alternate periods of work in heat and rest in neutral conditions. Observed and predicted variations in sweat rate were compared. During exposures to rapid changes in climatic conditions, the best prediction of the body sweat loss was provided by a model involving the simulation of the response of skin temperature and sweat rate to a step change in ambient conditions. The model used exponential weighting algorithms with time constants of 3 and 10 minutes respectively for these two variables. The study showed also that the prediction accuracy of SW_req index can be significantly improved by adopting. a new expression for the calculation of the evaporative efficiency of sweating, by predicting the mean T_sk value as a function of the parameters of the work situation and by applying the exponential weighting to the predicted values of sweat rate. Among the various heat stress indices tested in the study, the Required Sweat Rate Index gave the best approximation of the body sweat loss.     

Prediction of strain for intermittent heat exposures

Under intermittent heat exposures, predictions of body sweat loss ()Δm) could be derived either from the ambient conditions or from their time-weighted average. To compare the accuracy of these two procedures, five subjects exercised, semi-nude, for 120min at constant work load (50 W) under six environmental conditions: four conditions with 20 min duration ‘square pulse(rsquo; variations in either air temperature (T _a or ambient vapour pressure (P _a, and two conditions with constant T _a and P _a, levels. Selected ambient conditions involved skin wettedness (w) levels ranging from 0·35 up to 1. The recorded Δm were compared to the values predicted (PΔm) using the ‘Analytical determination of thermal stress’ model (ISO-DP 7933). Results showed that PΔm were lower than observed Δm by 2 to 10% depending on the condition. This trend could mainly be ascribed to the proposed relation between the evaporative efficiency of sweating and wettedness levels. For exposures consisting of T _a variations, the prediction error was not markedly different whether a time-weighted average of the ambient variables was used or not For exposures consisting of P _a, variations, the expected dripping of sweat during the humid exposures was limited by the short duration of these periods and the averaging procedure gave the best prediction of the actual sweat loss. It is concluded that under intermittent heat stress conditions, a time-weighted average of the ambient conditions may be used for thermal strain predictions, provided that the heat exposure periods are of short duration.     

Statistical methods for testing the conforms nee of products to user performance standards

User performance testing is an essential part of the development of information technology products. Usability engineering techniques and also methods included in the new International Organization for Standardization (ISO) draft standard for VDTs involve testing products against a standard or benchmark. This paper highlights some of the statistical and methodological problems involved in conformance testing and outlines appropriate procedures. These include the use of sequential tests which, in comparison with conventional tests, can achieve a radical reduction in the number of subjects required.     

Heat stress indices: A review paper

This paper represents a review of the thermal indices commonly used for assessing heat stress conditions in an environment, e.g., Corrected Effective Temperature, New Effective Temperature, Heat Stress Index, Wet Bulb Globe Temperature and Wet Globe Temperature. The advantages and disadvantages of the indices are included, as well as an example of calculations and procedures required to determine the resulting values of each heat stress index. Separate sections for Programmed Hand Held Calculators, the Mean Equivalence Lines and the International Organization for Standardization (ISO) standards are also provided in the paper.     

Statistical methods for testing the conforms nee of products to user performance standards

Abstract

User performance testing is an essential part of the development of information technology products. Usability engineering techniques and also methods included in the new International Organization for Standardization (ISO) draft standard for VDTs involve testing products against a standard or benchmark. This paper highlights some of the statistical and methodological problems involved in conformance testing and outlines appropriate procedures. These include the use of sequential tests which, in comparison with conventional tests, can achieve a radical reduction in the number of subjects required.     

Sweat distribution and perceived wetness across the human foot: the effect of shoes and exercise intensity

Abstract

This study investigates foot sweat distribution with and without shoes and the relationship between foot sweat distribution and perceived wetness to enhance guidance for footwear design. Fourteen females performed low-intensity running with nude feet and low- and high-intensity running with shoes (55%VO_2max and 75%VO_2max, respectively) on separate occasions. Right foot sweat rates were measured at 14 regions using absorbent material applied during the last 5?min of each work intensity. Perceptual responses were recorded for the body, foot and four foot regions. Foot sweat production was 22% greater nude (p?<?.001) and with shoes did not increase with exercise intensity (p?=?.14). Highest sweat rates were observed at the medial ankle and dorsal regions; lowest sweat rates at the toes. Perceptions of wetness and foot discomfort did not correspond with regions of high sweat production or low skin temperature but rather seemed dominated by tactile interactions caused by foot movement within the shoe.

Practitioner summary: This study provides a detailed view of foot sweat distribution for female runners with and without shoes, providing important guidance for sock and footwear design. Importantly, perceptions of wetness and foot discomfort did not correspond with areas of high sweat production. Instead tactile interactions between the foot, sock/shoe play an important role.

Abbreviations: VO_2max: maximal oxygen consumption; HR: heart rate; RH: relative humidity; GSL: gross sweat loss; Nude-I1: without socks and shoes, low intensity running; Shod-I1: with socks and shoes, low intensity running; Shod-I2: with socks and shoes, high intensity running     

An investigation of heat stress effects on time-sharing performance

A study was conducted to investigate the effects of heat stress on time-sharing performance. Twelve participants performed three dual-task scenarios and a multiple-task scenario for 2 h in each of six climates. The climates were obtained by generating each of three wet bulb globe temperatures (WBGT; 22, 28 and 34 degrees C) with two relative humidity levels (30 and 70%). The dual tasks selected from the Criterion Task Set (CTS) were: (1) display monitoring with mathematical processing; (2) memory search with mathematical processing; and (3) unstable tracking with memory search. The multiple task scenario was generated using the SYNTASK software. The results indicated a significant heat stress effect on CTS display monitoring and unstable tracking performance and on the SYNTASK visual monitoring and auditory discrimination tasks. Additionally, at 34 degrees C WBGT, 70% relative humidity was more detrimental to performance than 30% relative humidity. Results were interpreted using the Maximal Adaptability Model and Shingledecker's information processing stage/resource framework. To describe the results in an orderly manner, the authors propose the concept of heat stress selectivity effects. In addition, the results were used to evaluate whether the most recent NIOSH recommended heat stress standard, which is based solely on physiological and medical criteria, protects time-sharing performance. It was concluded that the NIOSH criterion does offer protection up to 28 degrees C WBGT.     

An investigation of heat stress effects on time-sharing performance

A study was conducted to investigate the effects of heat stress on time-sharing performance. Twelve participants performed three dual-task scenarios and a multiple-task scenario for 2 h in each of six climates. The climates were obtained by generating each of three wet bulb globe temperatures (WBGT; 22, 28 and 34°C) with two relative humidity levels (30 and 70%). The dual tasks selected from the Criterion Task Set (CTS) were: (1) display monitoring with mathematical processing; (2) memory search with mathematical processing; and (3) unstable tracking with memory search. The multiple task scenario was generated using the SYNTASK software. The results indicated a significant heat stress effect on CTS display monitoring and unstable tracking performance and on the SYNTASK visual monitoring and auditory discrimination tasks. Additionally, at 34°C WBGT, 70% relative humidity was more detrimental to performance than 30% relative humidity. Results were interpreted using the Maximal Adaptability Model and Shingledecker's information processing stage/resource framework. To describe the results in an orderly manner, the authors propose the concept of heat stress selectivity effects. In addition, the results were used to evaluate whether the most recent NIOSH recommended heat stress standard, which is based solely on physiological and medical criteria, protects time-sharing performance. It was concluded that the NIOSH criterion does offer protection up to 28°C WBGT.     

Effects of condensation in clothing on heat transfer

A condensation theory is presented that enables the calculation of the rate of vapour transfer with its associated effects on temperature and total heat transfer inside a clothing ensemble consisting of underclothing, enclosed air, and outer garment. The model is experimentally tested by three experiments; (1) impermeable garments worn by subjects with and without plastic wrap around the skin, blocking sweat evaporation underneath the clothing; (2) comparison of heat loss in impermeable and semi-permeable garments and the associated discomfort and strain; (3) subjects working in impermeable garments in cool and warm environments at two work rates, until tolerance. The measured heat exchange and temperatures are calculated with satisfying accuracy by the model (mean error = 11, SD = 10 Wm^?2 for heat flows and 0·3 and0·9°C for temperatures, respectively). A numerical analysis shows that for total heat loss the major determinants are vapour permeability of the outer garment, skin vapour concentration and air temperature. In the cold the condensation mechanism may completely compensate for the lack of permeability of the clothing as far as heat dissipation is concerned, but in the heat impermeable clothing is more stressful.     

Effects of two kinds of clothing made from hydrophobic and hydrophilic fabrics on local sweating rates at an ambient temperature of 37°C

Abstract

The purpose of this study was to find the effects of clothing made from hydrophobic and hydrophilic fabrics on the sweating physiology in environmental conditions where only the mechanisms of wet heat loss could occur. A comparison was made of the local sweat rates from the forearm and their related physiological parameters between polyester (E) and cotton (C) clothing at an ambient temperature (T _a) of 37°C. Six female students, aged from 21 to 28 years, served as subjects. The subjects wore clothing made from either fabric E or fabric C and rested quietly for 60 min in a chair mounted on the bed scale under the influences of environmental conditions of 37°C and 60% relative humidity (rh) with an air velocity of 0 1 m-s^-l.

The major findings are summarized as follows: (1) Local sweat rates were distinctly higher in E than in C in five out of six female subjects. (2) Clothing surface temperatures at the chest level were significantly higher in C than in E. (3) The positive relationship between local sweat rates and mean skin temperature (Tsk) existed both in E and in C. However, the local sweat rates were mostly higher in E under the influences of the same T _sk. These results are discussed in terms of thermal physiology and clothing sciences. It was concluded that the different properties of moisture absorbency between E and C could play a role for sweating physiology in the environmental conditions where only the mechanism of wet heat loss could occur.     

Validation and adjustment of the mathematical prediction model for human sweat rate responses to outdoor environmental conditions

Abstract

Based on indoor laboratory studies, a mathematical model to predict sweat loss response was suggested as follows:

Under outdoor conditions this model was over estimating sweat loss response in shaded (low solar radiation) environments, and underestimating the response when solar radiation was high (open field areas). The present study was conducted in order to adjust the model to be applicable under outdoor environmental conditions. Four groups of fit acclimated subjects participated in the study. They were exposed to three climatic conditions (30°, 65% rh; 31°C, 40% rh; and 40°C, 20% rh) and three levels of metabolic rate (100, 300 and 450W) in shaded and sunny areas while wearing shorts, cotton fatigues (BDUs) or protective garments. The original predictive equation for sweat loss was adjusted for the outdoor conditions by evaluating separately the radiative heat exchange, short-wave absorption in the body and long-wave emission from the body to the atmosphere and integrating them in the required evaporation component (E_req) of the model, as follows:

where SL is solar radiation (W m ^-2), M_ethis the Stephan Boltzman constant, and /T is the effective clothing insulation coefficient. This adjustment revealed a high correlation between the measured and expected values of sweat loss { r= 099, p< 0-0001).