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Consistency Assessment of Heat Stress Monitoring of Outdoor Workers Using Sweat Rate Measurement and Wet Bulb Globe Temperature WBGT in Arid and Semi-Arid Climates
A, Rahimi Forushani
A. et al. Consistency Assessment of Heat Stress Monitoring of Outdoor Workers Using Sweat Rate Measurement and Wet Bulb Globe Temperature WBGT in Arid and Semi-Arid Climates,
Online ahead of Print
;In Press(In Press):e12910.
Heat stress as a major problem that strikes a lot of people, especially outdoor workers, throughout the world. Many parts of Iran have arid and semi- arid climates, in which high air temperature and radiant temperature, along with usually low humidity, cause hazardous conditions for outdoor workers in hot months. Therefore, in order to take every preventive measure to protect exposed workers against heat related disorders, it is indispensable to select an appropriate index that accurately relates environmental parameters to physiological responses.
This investigation aimed at investigating two heat stress and strain indices of wet bulb globe temperature-WBGT- and sweat rate, respectively, in order to examine their consistencies in arid and semi- arid climates of Iran.
A total of 272 outdoor workers from different jobs were randomly selected and their sweat rate was calculated via a defined protocol three times a day. Simultaneously, environmental parameters, as well as WBGT index, were recorded for each working station. This study was performed during spring and summer.
The obtained results based on Kappa value showed a weak agreement between two standard indices. In addition, evaluation of outdoor workers using WBGT compared to sweat rate was accompanied by an overestimation. Moreover, based on mean and standard amounts of sweat rate, there were no cases exceeding the presented reference value regarding sweat rate at any time of the study.
It seems that sweat rate standard level at a shift work may need some modifications related to the real condition of work in arid and semi-arid climates of Iran. In addition, judgment based on monitoring sweat rate alone, as a physiological response to heat in these climates, can be probably accompanied by underestimation
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Heat stress is a major problem for many types of people including the elderly, newborns as well as children, pregnant women, and also outdoor workers. It can cause a lot of disorders and illnesses ranging from simple fatigue, lethargy, and weakness to some hazardous illnesses such as heat stroke or heat syncope (
1). There is a lot of work that should be performed in outdoor environments. Therefore, many workers must inevitably work outside ( 1). Outdoor workers spend more than one third of every day in outdoor settings and can be exposed to heat stress with deferent intensity ( 2- 4). According to the main mission of occupational hygienist, monitoring occupational health hazards and protection of workers from them is necessary. Hence, the hygienist needs reliable and applicable tools, methods, and equipment.
There are several heat stress and strain indices for monitoring and evaluation of thermal condition in different climates (
5- 7). Some of them such as wet bulb globe temperature (WBGT) have been validated and used as a popular index worldwide. However, in spite of wide usability and applicability of WBGT, it is also associated with some limitations, which make it only a screening indicator for thermal conditions. For example, some deficacy of WBGT index efficiency in very hot and humid condition can be observed ( 8). Therefore, detailed analysis through accurate rational indices such as predicted heat strain (PHS) or heat related physiological responses are strongly recommended whenever primary heat stress condition is observed ( 9). Core temperature, especially rectal temperature as well as sweat rate, are two parameters that create the best correlation in accordance with changes in environmental thermal condition ( 9, 10). However, applicability and adoption of both were restricted in real field studies due to the very sophisticated and difficult measurement methods (in the case of sweat rate) and also ethical and cultural issues (in the case of rectal temperature). Due to the priority of heat related physiological responses and reliance of indices on thermal equilibrium equation of the human body on one hand, and limitations associated with empirical indices such as dependency on climate, community and condition, which are developed on the other hand. The present study aims at answering these two questions: 1) considering some difficulties in measuring sweat rate, can it be an accurate judgment if evaluation of thermal condition in different climates is performed using only sweat rate monitoring? 2) how is the consistency of WBGT as a valid and popular heat stress index and sweat rate as one of the most important heat related physiological responses in environmental conditions with different humidity and air temperature? In fact, answering these questions can help in accurate evaluation and judgment of different climates, as well as taking preventive and protective measures against heat related disorders and illnesses.
A total of 272 healthy outdoor male workers were randomly selected from 5 climates with different environmental characteristics. For sampling, a two-stage protocol was used. First, based on environmental characteristics of each region, two categories were defined (arid and semi- arid). Then, based on the number of clusters (provinces of the country in each) category, 158 subjects were randomly selected from arid and 114 subjects from semi- arid regions. They were between 18 and 62 years old and all of them had at least a one year job experience and so they were assumed to have been acclimatized. Workers having disorders and history of illnesses such as hypertension, renal, cardiovascular, skin disorders, and any disorders causing excess sweat rate were excluded from the study. Demographic characteristics of subjects were gathered via a tailor-made questionnaire and the role of each of them was examined corresponding to sweat rate in the present study. Subjects participated voluntarily in the study and were informed of the main objectives of the investigation. Moreover, they were allowed to withdraw from the tests at any time of the study.
2.2. Personal and Environmental Parameters
Thermal insulation of workers’ clothes and the level of work load were estimated according to ISO 9920 (2004) and ISO 8996 (1989), respectively. An advanced calibrated heat stress monitor (Casella Microtherm WBGT, UK) was applied for recording dry air temperature (ta), natural wet temperature (tnw), globe temperature (tg) and WBGT index, and a digital manometer (Lotron PHB 318, Taiwan) for relative humidity. Air velocity was also measured using a thermal anemometer (Kimo, Franch). All parameters were recorded 3 times in a 4-hour interval from 8:00 a.m. to 12:00 a.m. and mean and standard deviation of each of them between 8:00 a.m. and 10:00 a.m. and 10:00 a.m. to 12.00 a.m. were considered in later analysis.
2.3. Sweat Rate Measurement
Sweat rate was calculated from weight differences before and after heat exposure was adjusted for water intake and urine output. A scale (Weigh-Sanova, Iran) with an accuracy of ± 10 grams at the range of 20 to 130 kg on site was used to weigh each subject at the beginning and end of each time interval. The measurements were done 3 times a day at about 8:00 a.m., 10: 00 a.m., and 12:00 a.m. The total sweat rate was determined using the following formula:
Total sweat rate = primarily weight without clothes - (secondary weight without clothes - drink - urine).
In addition to total sweat rate, total dehydration was computed in terms of grams, for doing comparisons between obtained results and the standard values. For this purpose, the amount of sweat rate in terms of g.h
-1 was multiplied to duration of exposure (two hour intervals in this study). Thus, the total dehydration in terms of grams was achieved. 2.4. Statistical Tests and Analysis
Statistical tests were conducted using SPSS software (version 20). The data were statistically analyzed by independent samples t-test, Spearman’s correlation coefficient test, Kappa agreement coefficient, and linear and quadratic regression for fitting line in scatters were used to examine consistency between the indices. Differences were considered significant at P < 0.05.
Tables 1 and 2 show demographic characteristics of subjects and range of environmental parameters in different climates.
Table 1. Demographic Characteristics of Subjects in Two Climates (n = 272)
Variable Arid Climate (n = 158) Semi-Arid Climate (n = 114) 95% CI P Value b Lower Upper Age, y 35.56 ± 9.31 35.73 ± 9.41 -1.77 1.42 0.83 Job experience, y 10.61 ± 7.26 12.61 ± 7.95 -3.29 -0.71 0.002 Metabolic rate, w 331.98 ± 50.90 318.67 ± 40.93 5.56 21.06 0.001 thermal insulation of clothes, clo 0.81 ± 0.19 0.84 ± 0.17 -0.05 0.01 0.18 Body are, m -2 1.87 ± 0.17 1.88 ± 0.19 -0.04 0.01 0.36 BMI, (cm. kg -2 24.73 ± 4.03 24.99 ± 4.45 -0.97 0.45 0.48
aValues are expressed as mean ± SD.
bIndependent samples T-test.
Table 2. Environmental Parameters in Arid and Semi-Arid Climates (n = 544)
Variable Arid Climate (n = 316) Semi-Arid Climate (n = 228) 95% CI P Value b Lower Upper Air temperature, °C 30.62 ± 6.18 32.08 ± 4.58 -2.36 -0.55 0.002 Natural wet temperature, °C 19.65 ± 2.94 22.56 ± 2.58 -3.37 -2.43 < 0.001 Globe bulb temperature, °C 34.78 ± 6.31 35.43 ± 5.15 -1.61 0.32 0.19 Relative humidity, % 40.46 ± 12.02 46.23 ± 9.21 -7.56 -3.98 < 0.001 Partial vapor pressure, KPa 1.71 ± 0.28 2.18 ± 0.36 -0.52 -0.41 < 0.001 Air velocity, m.s -1 1.36 ± 1.01 1.13 ± 1.81 -0.03 0.48 0.08
aValues are expressed as mean ± SD.
bIndependent samples T-test.
Sweat rate and dehydration have been shown in
Table 3. Reference values of each parameter are also presented. These values correspond to 8 hours of exposure. According to the two-hour duration of exposure in this study, the reference values were equal to 1,300 g for dehydration and 250 g.h -1 for sweat rate.
Table 3. Total Dehydration and Sweat Rate of Outdoor Workers
Parameter During 8:00 to 10:00 During 10:00 to 12:00 Reference Value b Dehydration, g 410.84 ± 282.72 711.72 ± 457.65 3250 (5200) c Sweat rate, g.h -1 205.42 ± 141.36 355.86 ± 228.82 650 (1000)
aValues are expressed as mean ± SD.
bThe values in the bracket are the reference value for acclimatized persons. For acclimatized subjects, the maximum sweat rate is, on average, is 25% greater than for un-acclimatized subjects ( 6).
cThe presented values are for 8 hours exposure, so, exposure time must be considered.
The results of heat stress monitoring using environmental WBGT and then effective WBGT, Environmental WBGT plus cloth adjustment factor (CAF), as well as consistency assessment of WBGT and sweat rate have been shown in
Tables 4 - 6 and Figure 1.
Table 4. Heat Stress Monitoring Results Based on WBGT
Parameter During 8:00 to 10:00 During 10:00 to 12:00 Environmental WBGT, °C 23.46 ± 3.23 26.15 ± 3.60 Effective WBGT a , °C 24.68 ± 3.47 28.13 ± 4.35
aEnvironmental WBGT plus clothes adjustment factor (CAF).
Table 5. Number of Agreement and Disagreement Cases Observed Between Evaluation Based on WBGT and Sweat Rate
WBGT, °C a Sweat Rate, g.h -1 b Kappa Value c Permissible Impermissible Between 8 to 10 (n = 272) NC d Acceptable 269 3 Unacceptable 0 0 Between 10 to 12 (n = 272) 0.13 Acceptable 170 10 Unacceptable 77 15 Between 8 to 12 (n = 544) 0.18 Acceptable 439 13 Unacceptable 77 15
aReference value of WBGT according to ACGIH was selected equal to 28 (°C) assuming moderate work, normal clothes insulation, continuous work and acclimatized workers.
bReference value of sweat rate was selected equal to 1,000 g.h -1 assuming all subjects were acclimatized.
cKappa agreement coefficient.
dNo statistics are computed because categorized WBGT is a constant.
Table 6. Correlation of WBGT and Sweat Rate for Evaluation of Thermal Environments in Different Climates
Index N Nominal Category a Site Sweat Rate g.h -1 Mean ± SD r P Value b WBGT, °C 100 Arid, cool and warm to very warm region 1 296.55 ± 247.60 0.284 < 0.001 108 Semi-arid, moderate and very warm region 2 252.35 ± 190.50 0.364 < 0.001 120 Semi-arid, cool and warm region 3 251.07 ± 160.37 0.244 < 0.001 120 Arid, cool and warm region 4 227.50 ± 157.00 0.355 < 0.001 96 Arid, cool and very warm region 5 399.26 ± 225.47 0.478 < 0.001
aCool in the nominal category is one of the properties of winter weather conditions, not necessarily spring and summer weather conditions which is emphasized in this study ( 11).
bCorrelation is significant at the 0.01 level (2-tailed), Spearman’s test.
Figure 1. Linear and polynomial line for sweat rate in different air temperature and relative humidity
The main object of this study was to emphasize the degree of agreement between heat stress evaluation by using WBGT and the sweat rate, two valid and adopted thermal indices, for heat exposure monitoring in arid and semi- arid regions.
The results of the study show that Iran’s climate experiences many diverse weather types in arid and semi- arid regions (
Table 2). The results of Table 3 show that according to two-hour exposure’s threshold limit values of acclimated persons for dehydration and sweat rate, only sweat rate exceeds the reference value during 10:00 a.m. to 12:00 a.m. and dehydration was under reference value during the study. The mean and standard deviation of effective WBGT exceeded reference value ( 7) at the same time ( Table 4). It is necessary to keep in mind that these values may be enhanced and the condition exacerbated if the work lasts more than 4 hours. It means that most likely the comparison of sweat rate and evaluation of heat stress using WBGT are accompanied by an overestimation in such regions. This result has been clearly revealed in Table 5, so that while sweat rate values are permissible during 8:00 a.m. to 12:00 a.m., there are a lot of cases that exceed WBGT values. In addition, obtained Kappa values were inappropriate and a weak consistency was achieved between these two indices. This finding has been achieved in several studies ( 11- 13). In a similar study, Bate et al. examined the physiological responses of construction workers in the United Arab Emirates and showed that in case of the supply of body fluids and self pacing, workers can work in the summer without serious physiological consequences ( 14). Wasterlund (1996) proposed that WBGT index and other indices related to it should not be used for estimation of physiological strain in agriculture. Continuous working without scheduled work rest regimen, high radiance, and lack of enough or available healthy and cool water for necessary rehydration can explain why the WBGT index is not applicable in such conditions ( 15). In another study done on farmers, it was revealed that from three physiological responses related to heat stress, including heart rate, raised body temperature, and sweating, only the amounts pertaining to heart rate are nearer threshold limit values and sweat rate rarely reached or exceeded threshold limit value ( 16). On the other hand, replacing the lost water is important and was strongly advised during the study ( 17). ISO 7933 (ISO, 2004) recommended a maximum water loss by sweating 5% of the body mass in order to protect 95% of the population. This threshold came from the hypothesis that the rehydration rate is 40% of the water lost in 95% of the cases ( 7). As can be seen in Figure 1, the sweat rate changes related to air temperature are not significant up to 30°C; much increase in air temperature takes place along with noticeable increase observed in sweat rate. This condition is reverse in the case of sweat rate related to relative humidity, so that a decreasing trend can be seen for sweat rate against relative humidity in the range of 20% to 50%, a nearly constant amount up to 60% and finally a mild slope increase in sweat rate can be seen by increasing the air moisture. According to these findings, it seems as expected that there are higher sweat rates in high temperature and low humidity regions such as sites 1 and 5 in this investigation ( Table 6). The main reason for this finding is due to the higher dry and radiant temperature than the skin temperature (35°C). The arid and semi- arid regions in Iran are usually desert and very hot areas with long hours of sun radiation in the center of Iran. Moisture is low in these areas and in some parts it is so low that moisture level is as low as 20% - 30% and the radiant temperatures of about 50 degrees Celsius are the characteristics of these areas. The combined effect of these two environmental factors plays a vital role in heat stress incidence in these areas. In this situation, conventional and radiation heat can be absorbed by the human body; thus, these routes are not adequate ones for heat loss from the body and the only way to release body heat is evaporation of sweat from the skin surface. Moreover, low humidity in such areas can help to improve evaporation efficiency of sweat and heat loss from the body. McArdle et al. developed the predicted 4-hour sweat rate index (P4SR), which uses sweat rate as an indicator of heat strain and predicts sweat rate for 4 hours for different combinations of metabolic rates and climatic conditions ( 18). However, it was shown that sweat production by itself does not comprehensively represent heat strain ( 19).
In another study, WBGT and predicted heat strain (PHS), were performed with the results showing that the predicted sweat rate were not different in the warm humid and hot dry environments, however, the evaporation rate was significantly lower in the warm humid environment. In the dry heat environment, neither of the indices considered the higher sweating capacities of the physically- trained men (
20). Previously, the WBGT index failed to differentiate between strenuous warm humid conditions and hot dry environments that did not result in excessive physiological strain ( 14, 21).
On the other hand, the amounts of sweat rate vary for different populations. For example Stofan et al. studied 10 members of a Florida team of football in a 2.5-h practice. Average sweat loss exceeded 4 L for the players with the history of the heat cramps (
22). Moreover, self pacing in conditions in which lack of work–rest cycles to decrease heat stress are observed can cause physiological responses to heat such as sweat rate to be in permissible or under permissible limit, despite the existence of stressful environmental conditions based on WBGT index ( 23).
It seems that the standard sweat rate level at a work shift may need some modifications related to real conditions of work in arid and semi-arid climates of Iran and judgment based on monitoring sweat rate alone, as a physiological response to heat in these climates, can be probably accompanied by underestimation.