Cortisol and interleukin-6 responses during intermittent exercise in two different hot environments with equivalent WBGT

Blood marker concentrations such as cortisol (COR) and interleukin (IL)-6 are commonly used to evaluate the physiological strain associated with work in the heat. It is unclear, however, if hot environments of an equivalent thermal stress, as defined by a similar wet bulb globe temperature (WBGT), result in similar response patterns. This study examined markers of neuroendocrine (COR) and immune (IL-6) responses, as well as the cardiovascular and thermal responses, relative to changes in body heat content measured by whole-body direct calorimetry during work in two different hot environments with equivalent WBGT. Eight males performed a 2-hr heavy intermittent exercise protocol (six 15-min bouts of cycling at a constant rate of metabolic heat production (360W) interspersed by 5-min rest periods) in Hot/Dry (46°C, 10% relative humidity [RH]) and Warm/Humid (33°C, 60% RH) conditions (WBGT ～ 29°C). Whole-body evaporative and dry heat exchange, change in body heat content (ΔH(b)), rectal temperature (T(re)), and heart rate were measured continuously. Venous blood was obtained at rest (PRE) and the end of each exercise bout for the measurement of changes in plasma volume (PV), plasma protein (an estimate of plasma water changes), COR, and IL-6. Ratings of perceived exertion and thermal sensation were measured during the last minute of each exercise bout. No differences existed for ΔH(b), heart rate, T(re),%ΔPV, plasma protein concentration, perceptual strain (thermal sensation, perceived exertion), and COR between the Hot/Dry and Warm/Humid conditions. IL-6 exhibited an interaction effect (p = 0.041), such that greater increases were observed in the Hot/Dry (Δ = 1.61 pg·mL(-1)) compared with the Warm/Humid (Δ = 0.64 pg·mL(-1)) environment. These findings indicate that work performed in two different hot environments with equivalent WBGT resulted in similar levels of thermal, cardiovascular, and perceptual strain, which support the use of the WBGT stress index. However, the greater IL-6 response in the Hot/Dry requires further research to elucidate the effects of different hot environments and work intensities.     

Heat exposure study in the workplace in a glass manufacturing unit in India

The heat exposure for working conditions in coastal areas of tropical and subtropical countries like India is a crucial factor in improved qualitative and quantitative production. The hot climate augments the heat exposure close to sources like furnaces. In the present work heat exposure to workers in glass manufacturing units in a coastal area of India has been assessed. The Wet Bulb Globe Temperature (WBGT), the Corrected Effective Temperature (CET) and Mean Radiant Temperature (MRT) were measured. The WBGT values much exceeded ACGIH TLVs. A revision of these standards to suit tropical and subtropical conditions is required. The recommended durations of work and rest have been estimated.     

Criteria for assessing severely hot environments: from the WBGT index to the PHS (predicted heat strain) model

BACKGROUND: The present study deals with the main methods for assessment of hot environments: i.e., WBGT, SWreq and PHS. It is stressed how the WBGT index, which is strictly empirical, although a very practical tool for the assessment of the hot environments, can only be used for a rough evaluation of heat stress, and especially for a not very high metabolic rate (M<175 W/m2). On the contrary, the SWreq method, which is based on both subject-environment heat exchange and the effect of clothing, allows a better assessment of the work situation with a general reduction of the exposure limits with respect to WBGT, especially in non-uniform environments (ta not equal to tr). However, it should be noted that application of SWreq is required by the ISO standard 7243 when the WBGT limit values are exceeded. METHODS: In this study interest was extensively focused on the "Predicted Heat Strain" method, highlighting via a special software the differences in heat stress assessment related to this new approach, which will be adopted by the ISO in the next revision of standard 7933. RESULTS: The PHS method, unlike SWreq, allows the prediction of the time-response of the main physiological variables of interest (i.e., skin temperature, core temperature and sweat rate). Moreover thanks to better modelling of heat exchanges, the PHS method allows account to be taken of both movement and clothing effects, resulting in even more reduced exposure.     

Heat exposure in the Canadian workplace

Exposure to excessive heat is a physical hazard that threatens Canadian workers. As patterns of global climate change suggest an increased frequency of heat waves, the potential impact of these extreme climate events on the health and well‐being of the Canadian workforce is a new and growing challenge. Increasingly, industries rely on available technology and information to ensure the safety of their workers. Current Canadian labor codes in all provinces employ the guidelines recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) that are Threshold Limit Values (TLVs) based upon Wet Bulb Globe Temperature (WBGT). The TLVs are set so that core body temperature of the workers supposedly does not exceed 38.0°C. Legislation in most Canadian provinces also requires employers to install engineering and administrative controls to reduce the heat stress risk of their working environment should it exceed the levels permissible under the WBGT system. There are however severe limitations using the WGBT system because it only directly evaluates the environmental parameters and merely incorporates personal factors such as clothing insulation and metabolic heat production through simple correction factors for broadly generalized groups. An improved awareness of the strengths and limitations of TLVs and the WGBT index can minimize preventable measurement errors and improve their utilization in workplaces. Work is on‐going, particularly in the European Union to develop an improved individualized heat stress risk assessment tool. More work is required to improve the predictive capacity of these indices. Am. J. Ind. Med. 53:842–853, 2010. © 2010 Wiley‐Liss, Inc.     

Challenges to temperature regulation when working in hot environments

The focus of this review is upon acute exposure to hot environments and the accompanying physiological changes. The target audience includes physiologists, physicians and occupational health and safety practitioners. Using the principles of thermodynamics, the avenues for human heat exchange are explored, leading to an evaluation of some methods used to assess thermally-stressful environments. In particular, there is a critique of the wet-bulb globe temperature (WBGT) index, and an overview of an alternative means by which such assessments may be undertaken (the heat stress index). These principles and methods are combined to illustrate how one may evaluate the risk of heat illness. Three general areas of research are briefly reviewed: the physiological impact of wearing thermal protective clothing, heat adaptation (acclimation) and whole-body pre-cooling. These topics are considered as potential pre-exposure techniques that may be used to reduce the threat of hyperthermia, or to enhance work performance in the heat.     

An evaluation of heat stress indices (ISO 7243, ISO/DIS 7933) in the prediction of heat strain in unacclimated men

Summary The ISO 7243 heat stress standard based on the wet bulb globe temperature (WBGT) heat stress index and the analytical standard proposal ISO/DIS 7933 were evaluated in eight physically trained and eight untrained, unacclimated men during prolonged light exercise carried out while wearing industrial work clothing. The exercise tests were done in a thermoneutral (20°C/40%), a warm humid (30°C/80% humidity), and a hot dry (40°C/20% humidity) environment. Both of the standards were effective in predicting the excessive thermal strain observed in the dry and humid heat (WBGT, 28°C). In dry heat, neither of the standards took into account the higher sweating capacities of the physically trained men. The large inter-individual variability in the physiological responses to heat stress questioned the calculations of definite allowable exposure times (ISO/DIS 7933).     

热环境允许暴露时间的工效学研究

采用热交换方法分析了18个高温作业环境工人的热紧张。根据由作业条件所决定的必需皮肤湿度、必需的出汗率与生理可能达到的皮肤湿度、出汗率计算工人在热环境的允许暴露时间。评价方法适用于对劳动条件做精确分析和制定劳动作息制度参考。     

Comparison of heat stress measuring techniques in a steel mill

Three heat stress measurement devices, currently used to assess heat exposure in the workplace, were compared in indoor and outdoor environments at a steel mill in Orem, Utah, during the month of July, 1982. Sixty sets of environmental data from a total of fourteen different test locations were collected and analyzed. Significance tests, linear regression equations and correlation coefficients were calculated to determine comparability and relationships between standard WBGT Index and Botsball (WGT) and also between standard WBGT Index and an electronic WBGT Index Meter. The results of the significance testing between the standard WBGT, Botsball, and electronically generated WBGT showed that the electronically generated WBGT means were statistically much closer to the standard WBGT values than were the Botsball (WGT) results. The statistical analysis performed shows that a high correlation of variation exists between the standard WBGT and both the Botsball and the electronically generated WBGT.     

The thermal work limit is a simple reliable heat index for the protection of workers in thermally stressful environments

BACKGROUND: Workers in many industries are exposed to thermally stressful work environments. Protection of the health of workers without unnecessarily compromising productivity requires the adoption of a heat index that is both reliable and easy to use. OBJECTIVES: To evaluate the Thermal Work Limit (TWL), in a controlled environment and under field conditions, against these criteria. METHODS: Volunteers performed graded work in a controlled thermal environment to determine the limiting workload for the conditions. Core temperature and heart rate were monitored as indicators of thermoregulation. In the field study, outdoor workers were monitored for signs of physiological strain in thermal environments which were characterized using both the traditional Wet Bulb Globe Temperature (WBGT) and the TWL. Abilities of each of these indices to accurately reflect the thermal stress on workers were evaluated. RESULTS: In the controlled environment, the TWL was found to reliably predict the limiting workload. In the field study, TWL was a more appropriate and realistic index than WBGT, which was found to be excessively conservative. CONCLUSIONS: The results confirm previously published studies evaluating TWL in underground environments, which have led to its widespread adoption in the Australian mining industry. The study extends the applicability of TWL to outdoor environments and generates management guidelines for its implementation.     

Heat stress standard ISO 7243 and its global application

This paper presents heat stress Standard ISO 7243, which is based upon the wet bulb globe temperature index (WBGT), and considers its suitability for use worldwide. The origins of the WBGT index are considered and how it is used in ISO 7243 and across the world as a simple index for monitoring and assessing hot environments. The standard (and index) has validity, reliability and usability. It is limited in application by consideration of estimating metabolic heat and the effects of clothing. Use of the standard also requires interpretation in terms of how it is used. Management systems, involving risk assessments, that take account of context and culture, are required to ensure successful use of the standard and global applicability. For use outdoors, a WBGT equation that includes solar absorptivity is recommended. A 'clothed WBGT' is proposed to account for the effects of clothing. It is concluded that as a simple assessment method, ISO 7243 has face validity and within limits is applicable worldwide.     

Laboratory evaluation of permissible exposure limits for men in hot environments

Forty-six industrial workers completed a total of 653 one-hour work bouts requiring an average of 122-235 kcal/M2/hr in an environmental chamber maintained at heat stress levels ranging from 8-37 degrees C wet bulb globe temperature (WBGT). Heart rates (HR) and rectal temperatures (Tre) were measured at the end of each work bout. Environmental heat stress levels were divided into two groups - those above and those below the permissible exposure limits (PEL). The PEL is the proposed maximum environmental thermal stress to which industrial workers can be exposed without endangering their health. The number of observations in each of these regions was further divided into those which were above the recommended limits of a World Health Organization study group (HR less than or equal to 110 bpm, Tre less than or equal to 38.0 degrees C) and those which were not. The number of "safe" (HR less than or equal to 110 bpm, Tre less than or equal to 38.0 degrees C) observations in environments with heat stress less than or equal to the PEL ranged from 100% to 2.4% depending on subject acclimatization and work rate. The degree of protection was always less in the winter than in the summer and was less for higher work rates. Men who normally worked in hot environments had fewer "excessive" HR's and Tre's than those who did not.     

Thermal responses to intermittent work in selected enironments.

Acclaimed subjects exercised intermittently for 8 hrs in 25 to 50 degrees environments. Thermal and metalobic determinations were made. Results showed the time-weighted WBGT heat strain index for intermittent work needs re-evaluation. For acclimated subjects working steadily at 350 kcal/hr, 30% max, the ULPZ is more than 33% WBGT.     

Body heat storage during intermittent work in hot-dry and warm-wet environments

We examined heat balance using an American Conference of Governmental Industrial Hygienists threshold limit value allocated exercise protocol in hot-dry (HD; 46 °C, 10% relative humidity (RH)) and warm-wet (WW; 33 °C, 60% RH) environments of equivalent WBGT (29 °C) for different clothing ensembles. Whole-body heat exchange and changes in body heat content (ΔH(b)) were measured using simultaneous direct whole-body and indirect calorimetry. Eight males performed six 15-min cycling periods at a constant rate of metabolic heat production (360 W) interspersed by 5-min rest periods for six experimental trials: HD and WW environments for a seminude control (CON), modified work uniform (MWU, moisture permeable top and work pants), and standard work uniform (SWU, work coveralls and cotton undergarments). Whole-body evaporative and dry heat exchange, rectal temperature (T(re)), and heart rate were measured continuously. The cumulative ΔH(b) during the 2 h intermittent exercise protocol was similar between HD and WW environments for each of the clothing ensembles (CON, 387 ± 55 vs. 435 ± 49 kJ; MWU, 485 ± 58 vs. 531 ± 61 kJ; SWU, 585 ± 74 vs. 660 ± 54 kJ, respectively). Similarly, no differences in T(re) (CON, 37.67 ± 0.07 vs. 37.48 ± 0.08 °C; MWU, 37.73 ± 0.08 vs. 37.53 ± 0.09 °C; SWU, 38.01 ± 0.09 vs. 37.94 ± 0.05 °C) or heat rate (CON, 93 ± 3 vs. 84 ± 3 beats·min?1; MWU, 102 ± 5 vs. 95 ± 9 beats·min?1; SWU, 119 ± 8 vs. 110 ± 9 beats·min?1) were observed at the end of the 2 h intermittent exercise protocol in HD vs. WW environments, respectively. We showed similar levels of thermal and cardiovascular strain for intermittent work performed in high heat stress conditions of varying environmental conditions but similar WBGT.     

Thermal limits of men in moderate to heavy work in tropical farming

The farmers in tropical climate are exposed to high heat during the summer months. The study examined the physiological strains of farmers (N=26) to six combined exposures of work and high heat, with moderate and heavy work (26 to 50%, and 51 to 75% VO(2max)) and three ambient conditions, i.e., 34.4 to 42.2 degrees C WBGT (inside) in an environmental chamber. While the cardio-respiratory responses and Tcr were predominantly influenced by the work severity (p<0.001), the environmental warmth greatly influenced the sweating response (p<0.001). The importance was placed on the segmental Tsk as the first rank indicator of the bodily heat strain. Both the environmental warmth and work severity had independent discernable effects on the dynamic equilibrium of the central and peripheral mechanism to regulate the body temperature. The segmental and compartmental (core, muscle, fat and skin) heat balance analysis indicated the span of convergence of the segmental core and muscle temperatures to the divergence of skin and fat temperatures (CORE-SHELL) as a quantitative estimate of the segmental gradient for heat transfer. The summation of heat exchange across the compartments and segments yielded the transient change in Tcr (0.06 to 0.12 degrees C/min), with significant difference between the moderate and heavy work. The Tcr of 39 degrees C was taken as the limit of tolerance for the farmers, and by defining the criteria limit of Tcr of approximately 2.5 degrees C gradient from the basal Tcr and the rate of change in Tcr, the tolerance times were estimated. Corollary to the development of ISO 7933 standard (PHS index), the predictions of tolerance times from the transient change in Tcr or the exponential relationship with the WBGT (tolerance time, min = 1,841 e (-0.103 WBGT)) were useful to suggest the protective limit for men at work in extremely hot environment. The simplicity of prediction lies in using WBGT as a criterion. The exponential equation estimated the tolerance time of 55 min at 34 degrees C WBGT, and up to 38 degrees C WBGT, the decrease in tolerance time was 4 to 5 min per degree increase in environmental warmth. Beyond 38 degrees C WBGT, the estimated tolerance time decreased by 2 to 3 min per degree increase in WBGT. Further optimization and validation of the knowledge for men and women farmers in different age groups will have application in managing heat illnesses and disorders in tropical farming.     

Assessment of the risk of heat disorders encountered during work in hot conditions

OBJECTIVE: To co-ordinate the work of the main European research teams in the field of thermal factors in order to develop and improve significantly the methods presently available for assessing the risks of heat disorders encountered during work in hot conditions. METHOD: Each item from the required sweat rate model was reviewed on the basis of the most recent literature. A database with 1,113 laboratory and field experiments, covering the whole range of hot working conditions, was assembled and used for the validation. RESULTS: Influence of clothing ensemble on heat exchange: methods and formulas were developed that take into account the dynamic effects associated with forced convection and the pumping effect associated with body movements and exercise. Prediction of the average skin temperature: the model used in the required sweat rate standard ISO 7933 was extended to cover more severe conditions with high radiation and high humidity and different clothing and take into account the rectal temperature for the prediction of the skin temperature. Criteria for estimating acceptable exposure times in hot work environments: criteria were reviewed and updated concerning the maximum increase in core temperature and the acceptable water loss, for acclimatised and nonacclimatised subjects. These limits are intended to protect 95% of the population. Measuring strategy: a strategy was developed to assess the risks in any working situation with varying conditions of climate, metabolic rate or clothing. A detailed methodology was developed in three stages: an "observation" method for the recognition of the conditions that might lead to thermal stress; an "analysis" method for evaluating the problem and optimising the solutions; and an "expert" method for in-depth analysis of the working situation when needed. Validation: the different results were used to prepare a revision of the interpretation procedure proposed in the ISO standard 7933. We validated the modified approaches using the database. This involved the whole range of conditions for which the model was extended, namely conditions with high and low radiation, humidity and air velocity as well as fluctuating conditions. Based on these results, the predicted heat strain model was developed: it is presently proposed as an ISO and CEN standard.     

Evaluation of the environmental stress index (ESI) for hot/dry and hot/wet climates

Recently, a novel environmental stress index (ESI) which is composed from commonly used meteorological variables: ambient temperature (T(a)), relative humidity (RH), and solar radiation (SR) was suggested as follows: ESI = 0.63T(a)-0.03RH+0.002SR+0.0054(T(a) x RH)-0.073(0.1+SR)(-1); (degrees C) The purpose of the present study was to evaluate and validate the ESI for hot dry and hot wet climatic conditions. The ESI was applied to large meteorological databases from 2 different locations resembling hot/wet and hot/dry climates. Data analysis revealed high correlation between ESI and the wet bulb globe temperature (WBGT) index for each of the two databases: P < 0.05, R2 = 0.985 and 0.982, for the hot/dry and hot/wet conditions, respectively. Therefore, it is concluded that ESI, which is constructed from fast response and commonly used weather variables (T(a), RH, SR), and also found in a microsensor format is validated for hot/dry and hot/wet zones and as a potential index to serve as an alternative to the WBGT for heat category assessment.     

Calculating Workplace WBGT from Meteorological Data: A Tool for Climate Change Assessment

The WBGT heat stress index has been well tested under a variety of climatic conditions and quantitative links have been established between WBGT and the work-rest cycles needed to prevent heat stress effects at the workplace. While there are more specific methods based on individual physiological measurements to determine heat strain in an individual worker, the WBGT index is used in international and national standards to specify workplace heat stress risks. In order to assess time trends of occupational heat exposure at population level, weather station records or climate modelling are the most widely available data sources. The prescribed method to measure WBGT requires special equipment which is not used at weather stations. We compared published methods to calculate outdoor and indoor WBGT from standard climate data, such as air temperature, dew point temperature, wind speed and solar radiation. Specific criteria for recommending a method were developed and original measurements were used to evaluate the different methods. We recommend the method of Liljegren et al. (2008) for calculating outdoor WBGT and the method by Bernard et al. (1999) for indoor WBGT when estimating climate change impacts on occupational heat stress at a population level.     

Heat strain and heat stress for workers wearing protective suits at a hazardous waste site

In order to evaluate the effects of heat stress when full body protective suits are worn, heart rates, oral temperatures and environmental parameters were measured for five unacclimatized male workers (25-33 years of age) who performed sampling activities during hazardous waste clean-up operations. The protective ensembles included laminated PVC-Tyvec chemical resistant hood suits with rubber boots, gloves full facepiece dual cartridge respirators and hard hats. For comparison, measurements also were performed when the men worked at a similar level of activity while they wore ordinary work clothes. A comparison of the heart rates for the men working with and without suits indicated that wearing the suits imposed a heat stress equivalent to adding 6 degrees to 11 degrees C (11 degrees to 20 degrees F) to the ambient WBGT index. A similar result was obtained by calculating the WBGT in the microclimate inside the suits and comparing it to the ambient WBGT. These results indicate the following: 1) there exists a significant risk of heat injury during hazardous waste work when full body protective clothing is worn, and 2) threshold limit values for heat stress established by the ACGIH must be lowered substantially before extending them to cover workers under these conditions.     

Prediction of rectal temperature by the Questemp II personal heat strain monitor under low and moderate heat stress

This study assessed the use of aural canal temperature measured with the Questemp II personal heat strain monitor (Tq) relative to rectal temperature (Tre) during simulated industrial work in three different wet bulb globe temperatures (WBGT). Sixteen subjects performed walking and arm curl exercise at a rate of 300 kcal/hour for 4 hours while wearing Saranex protective coveralls in 18, 23, and 27 degrees C WBGT environments and wearing the Questemp II. Correlations were determined between Tre and Tq for the three conditions and for all conditions combined. Pearson r values were 0.48 (18 degrees C WBGT), 0.42 (23 degrees C WBGT), 0.38 (27 degrees WBGT), and 0.50 (all trials). Because a major concern is safe maximum core body temperature, means and standard deviations for differences between Tre and Tq were assessed at peak temperatures to determine the predictability of Tre from Tq solely at these points. Large standard deviations in delta values relative to a small overall tolerable temperature range ruled out the use of Tq in this manner. Based on the current data, aural canal temperature as measured with the Questemp II did not provide an accurate reflection of Tre across time nor at peak core temperatures during low to moderate heat strain.