The Open Public Health Journal




ISSN: 1874-9445 ― Volume 12, 2019
RESEARCH ARTICLE

Evaluation of Occupational Exposure to Heat Stress and Physiological Responses of Workers in the Rolling Industry



Farough Mohammadian1, Ali Salehi Sahl Abadi1, Omid Giahi2, 3, Jamshid Khoubi2, 3, Amin Allah Zarei4, Gholamheidar Teimori Boghsani4, *, Sedigheh Abbaspour5
1 Student Research Committee, Department of Occupational Health Engineering, School of Public Health and Safety, Shahid Beheshti University of Medical Sciences, Tehran, Iran
2 Environmental Health Research Center, Kurdistan University of Medical Sciences, Sanandaj, Iran
3 Department of Occupational Health Engineering, School of Health, Kurdistan University of Medical Sciences, Sanandaj, Iran
4 Department of Environmental Health, School of Health, Torbat Heydariyeh University of Medical Sciences, Torbat Heydariyeh, Iran
5 Department of Nursing, School of Nursing and Midwifery, Torbat Heydariyeh University of Medical Sciences, Torbat Heydariyeh, Iran

Abstract

Background and Objective:

Many indices are used to assess occupational heat stress at the work environments. The aim of this study was to evaluate heat stress using Wet Bulb Globe Temperature (WBGT) index and Discomfort Index (DI), and by measuring physiological responses in the workers of the rolling industry and comparing the appropriateness of these indices for measuring heat stress.

Methods:

This cross-sectional study was carried out on 90 healthy workers of a rolling industry in eastern Tehran in 2017. Physiological parameters of core body temperature (Tcr) and Heart Rate (HR) were measured during the working shift according to ISO9886 standard. At the same time, environmental variables such as the natural wet temperature (Tnw), dry temperature (Td) and globe temperature (Tg) were measured and recorded at the workstations. Then, the DI and the WBGT indices were calculated using the related formulae. Data were analyzed using SPSS v. 21, t-test and Pearson correlation coefficient.

Results:

The mean heat stress indices were significantly higher in working conditions than resting conditions, and there was a significant difference between the physiological parameters of Tcr and HR in resting and working conditions (P<0.001). According to the screening criteria of DI, 43.3% of the workers were exposed to the moderate level and 56.7% to the severe level of heat stress. There was a significant difference between the mean WBGT and the Threshold limit values (t= 4.903, P<0.001). Pearson correlation test showed that there was a significant and direct linear relationship between the WBGT and the physiological parameters of Tcr and HR (r=0.317, P=0.002; r=0.434, P<0.001, respectively). The DI index had a significant and direct linear relationship with HR; (r=0.229, P=0.03).

Conclusion:

The results showed that WBGT is a more appropriate index for evaluating the heat stress of workers in the rolling industry. High heat stress levels at the workstations along with heavy physical activity are health risks for workers in this industry; therefore, interventions must be undertaken to reduce exposure.

Keywords: Heat stress, Wet bulb globe temperature, Discomfort index, Physiological parameters, Rolling industry.


Article Information


Identifiers and Pagination:

Year: 2019
Volume: 12
First Page: 114
Last Page: 120
Publisher Id: TOPHJ-12-114
DOI: 10.2174/1874944501912010114

Article History:

Received Date: 28/12/2018
Revision Received Date: 16/02/2019
Acceptance Date: 06/03/2019
Electronic publication date: 28/03/2019
Collection year: 2019

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© 2019 Mohammadian et al.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: https://creativecommons.org/licenses/by/4.0/legalcode. This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


* Address correspondence to this author at the Department of Environmental Health, School of Health, Torbat Heydariyeh University of Medical Sciences, Torbat Heydariyeh, Iran; Tel: +9851-5222611; E-mail: teimorigh1@gmail.com




1. INTRODUCTION

Heat stress is one of the most harmful physical pollutants in the work environment, which is less studied among various harmful agents [1Nassiri P, Monazzam MR, Golbabaei F, Shamsipour A, Arabalibeik H, Abbasinia M, et al. Personal risk factors during heat stress exposure in workplace. Health and Safety at Work 2017; 7(2): 163-80. URL: http://jhsw.tums.ac.ir/article-1-5658-en.html]. It is a profound damaging agent at hot workplaces if a worker is exposed to severe heat and humidity, radiations, physical contact with hot objects and heavy physical activity for a long time [1Nassiri P, Monazzam MR, Golbabaei F, Shamsipour A, Arabalibeik H, Abbasinia M, et al. Personal risk factors during heat stress exposure in workplace. Health and Safety at Work 2017; 7(2): 163-80. URL: http://jhsw.tums.ac.ir/article-1-5658-en.html, 2Ansari M, Mazloumi A, Abbassinia M, Farhang Dehghan S, Golbabaei F. Heat stress and its impact on the workers’ cortisol concentration: A case study in a metal melding industry. Health and Safety at Work 2014; 4(2): 59-68. URL: http://jhsw.tums.ac.ir/ article-1-5147-en.html]. Working in the rolling industry is a dangerous activity in terms of heat stress [3Azari G, Mofidi AA, Meshkani M, Tabatabaei Ghamshe F, Daviantalab AH. Evaluation of heat stress in the working position 60 of furnace workers in one of the forging industries in the summer. Journal of health and safety at work 2012; 1(2): 51-8.http://jhsw.tums.ac.ir/article-1-24-en.html].

Most jobs need to be carried out in hot environments, which can affect the cooling system of the body when doing heavy physical work. Once the body is not able to maintain homeostasis between the captured and lost heat, heat damage can occur, which may get acute and even fatal in some cases [4Andersen AB. Validation of the USF Safe Exposure Time Equation for Heat Stress: University of South Florida 2011. https://scholarcommons.usf.edu/etd/2985]. Types of mild to severe heat disorders include: rash, heat cramp, syncope, heat exhaustion, and hyperthermia [5Tian Z, Zhu N, Zheng G, Wei H. Experimental study on physiological and psychological effects of heat acclimatization in extreme hot environments. Build Environ 2011; 46(10): 2033-41.[http://dx.doi.org/10.1016/j.buildenv.2011.04.027] ]. In addition, exposure to heat can cause disorders in mental health, increased medical expenditures and work accidents, reduced productivity and reduced physical capacity [5Tian Z, Zhu N, Zheng G, Wei H. Experimental study on physiological and psychological effects of heat acclimatization in extreme hot environments. Build Environ 2011; 46(10): 2033-41.[http://dx.doi.org/10.1016/j.buildenv.2011.04.027] -7Golbabaei F, Zakerian SA, Fouladi Dehaghi B, et al. Heat stress and physical capacity: A case study of semi-professional footballers. Iran J Public Health 2014; 43(3): 355-61. https://europepmc.org/abstract /pmc/pmc4419174[PMID: 25988096] ].

In order to ensure the health of workers, the heat stress and changes in physiological parameters under these conditions should be studied [8Lu S, Zhu N. Experimental research on physiological index at the heat tolerance limits in China. Build Environ 2007; 42(12): 4016-21.[http://dx.doi.org/10.1016/j.buildenv.2006.06.029] ]. Evaluation of the heat stress in work environments can be performed by measuring environmental parameters and then their impact on the body by using a heat stress index or other indices [9Wang F, Gao C, Kuklane K, Holmér I. Effects of various protective clothing and thermal environments on heat strain of unacclimated men: The PHS (predicted heat strain) model revisited. Ind Health 2013; 51(3): 266-74.[http://dx.doi.org/10.2486/indhealth.2012-0073] [PMID: 23385435] ]. Many attempts were made in the past century to develop indices that can adequately describe the amount of heat stress. Accordingly, a heat index has been developed to accurately predict the physiological strain of individuals in all thermal environments [10Criteria for a recommended standard: Occupational exposure to hot environments 1986.]. While a lot of research has been done to determine a final index, there is still much debate as to which index is more efficient and more correlated with physiological strains [11Brake R, Bates G. A valid method for comparing rational and empirical heat stress indices. Ann Occup Hyg 2002; 46(2): 165-74.[http://dx.doi.org/10.1093/annhyg/mef030] [PMID: 12074026] ].

There is a dire to define a reliable and comfortable tool for monitoring of stressful conditions and prevent health damage to enhance the efficiency of workers in hot working environments. Many thermal indices have been developed for the evaluation of heat stress, and some of them have been specifically designed for industrial workplaces [12Farshad A, Montazer S, Monazzam MR, Eyvazlou M, Mirkazemi R. Heat stress level among construction workers. Iran J Public Health 2014; 43(4): 492-8.https://europepmc.org/abstract/MED/26005660[PMID: 26005660] ] and may not be suitable for small workshops such as the rolling ones.

These indices can be categorized in the three groups of rational, empirical, and direct indices [2Ansari M, Mazloumi A, Abbassinia M, Farhang Dehghan S, Golbabaei F. Heat stress and its impact on the workers’ cortisol concentration: A case study in a metal melding industry. Health and Safety at Work 2014; 4(2): 59-68. URL: http://jhsw.tums.ac.ir/ article-1-5147-en.html]. The use of many of these indices necessitates the measurement of environmental variables, occupational conditions and various physiological parameters. Moreover, in order to measure some of these indices, there is a need for some special tools; the calculation process is also complex and requires the use of the computer. Among available thermal indices, the use of direct one was simple and easy with calculations based on environmental variables [13Liang C, Zheng G, Zhu N, Tian Z, Lu S, Chen Y. A new environmental heat stress index for indoor hot and humid environments based on Cox regression. Build Environ 2011; 46(12): 2472-9.[http://dx.doi.org/10.1016/j.buildenv.2011.06.013] ].

The Wet Bulb Globe Temperature (WBGT) is a simple heat stress index, which requires the natural wet temperature (Tnw), dry temperature (Td) and globe temperature (Tg) measurements. This index has been introduced as a global standard and is applied to control the heat stress in many working environments [14Parsons K. Heat stress standard ISO 7243 and its global application. Ind Health 2006; 44(3): 368-79.[http://dx.doi.org/10.2486/indhealth.44.368] [PMID: 16922180] ]; being recommended by the International Standard Organization (ISO 7243) for the rapid assessment of hot environments [15 ISO7243. Hot environments-estimation of the heat stress on working man, based on the WBGT-index (wet bulb globe temperature). Geneva: ISO. 1989[http://dx.doi.org/10.2486/indhealth.44.368] [PMID: 16922180] ]. It is the only index that measures Tg. On the other hand, the American Conference of Governmental Industrial Hygienists (ACGIH) has determined the limits of heat exposure, (Table 1) [16TLVs and BEls: Threshold limit values for chemical substances and physical agents and biological exposure indices USA 2004.], approved by the Occupational Health and Safety Organization (OSHA) and the American Industrial Hygiene Association (AIHA) [17Sawka M, Wenger C, Montain S, et al. Heat stress control and heat casualty management 2003.https://armypubs.army.mil/epubs/DR_pubs/DR_a/pdf/web/tbmed507.pdf[http://dx.doi.org/10.1037/e458312006-001] ]. The WBGT index, despite its extensive use, has a number of limitations [18Budd GM. Wet-bulb globe temperature (WBGT)-its history and its limitations. J Sci Med Sport 2008; 11(1): 20-32.[http://dx.doi.org/10.1016/j.jsams.2007.07.003] [PMID: 17765661] ].

Discomfort Index (DI) is a direct one that has been recommended for more than four decades along with WBGT for rapid assessment of hot environments. It is simple and does not require complicated tools, with easy results’ interpretation [19Epstein Y, Moran DS. Thermal comfort and the heat stress indices. Ind Health 2006; 44(3): 388-98.[http://dx.doi.org/10.2486/indhealth.44.388] [PMID: 16922182] ]. It was introduced by Thom and modified by Sahar et al. (1962). The parameters of Td and Tnw are needed to measure DI index. It has a high correlation with the WBGT index (R2 = 0.947) [19Epstein Y, Moran DS. Thermal comfort and the heat stress indices. Ind Health 2006; 44(3): 388-98.[http://dx.doi.org/10.2486/indhealth.44.388] [PMID: 16922182] ]. DI values ​​are quite similar to WBGT index. Specified criteria have been estimated for the heat stress and its relation with the heat sensation based on a large number of studies in a wide range of community groups and in different climatic conditions [19Epstein Y, Moran DS. Thermal comfort and the heat stress indices. Ind Health 2006; 44(3): 388-98.[http://dx.doi.org/10.2486/indhealth.44.388] [PMID: 16922182] ]:

  • For the DI values ​​under 22, there is no heat stress.
  • For DI values between 22 and 24, most people have a mild heat stress sensation.
  • For DI values between 24 and 28, the heat stress is relatively heavy, and people feel very hot and physical work may be interrupted.
  • For DI values above 28, the heat load is extreme, body temperature cannot be maintained during physical work resulting in high risk of heat-related disorders.

The combined effect of exposure to ambient heat and body heat production due to metabolism can cause heat strain in the body. Direct methods for assessing heat exposure are based on the measurement of the body's heat strain; a body's response to heat stress measurable by physiological parameters [20Varley F. A study of heat stress exposures and interventions for mine rescue workers. Transactions 2004; 316: 133-42. https://www.cdc.gov/niosh/mining/UserFiles/works/pdfs/asohs.pdf]. Various physiological parameters for assessing the heat strain include core body temperature (Tcr), Heart Rate (HR), dehydration and Urine Specific Gravity (USG) [21Joubert D, Bates G. Occupational heat exposure Part 2: The measurement of heat exposure (stress and strain) in the occupational environment. Occupational Health Southern Africa 2008; 2-6. http://www.occhealth.co.za/?/viewArticle/993]. HR is a primary indicator for a physiological strain especially during work and rest [22Lee J-Y, Nakao K, Takahashi N, Son S-Y, Bakri I, Tochihara Y. Validity of infrared tympanic temperature for the evaluation of heat strain while wearing impermeable protective clothing in hot environments. Ind Health 2011; 49(6): 714-25.[http://dx.doi.org/10.2486/indhealth.MS1291] [PMID: 22020015] ]. The oral, eardrum, rectal and the urine temperatures can be recorded as Tcr [23ISO9886. Evaluation of thermal strain by physiological measurements. Geneva: International Organization for Standardization, 2004.]; an important criterion for the adverse health effects. The eardrum temperature is a good alternative to internal body temperature at the workplace, being non-invasive and promptly responsive to alterations in body temperature [22Lee J-Y, Nakao K, Takahashi N, Son S-Y, Bakri I, Tochihara Y. Validity of infrared tympanic temperature for the evaluation of heat strain while wearing impermeable protective clothing in hot environments. Ind Health 2011; 49(6): 714-25.[http://dx.doi.org/10.2486/indhealth.MS1291] [PMID: 22020015] ].

Skimpy research is available on evaluation of thermal indices as well as heat stress in Iran; thus requires further studies. On the other hand, considering the health of workers, evaluation of heat stress in different work environments, especially small and indoor workshops, which can be of great importance from in terms of occupational health as it plays an important role in the better analysis of working conditions and occupational health standards. The aim of this study was to evaluate the heat stress of workers in the rolling industry using DI and WBGT and comparing their relations with the physiological responses of Tcr and HR.

2. MATERIALS AND METHODS

2.1. Research Design

This descriptive-analytical cross-sectional study was carried out on workers of a rolling industry located in eastern Tehran in summers of 2017. This industry consists of various cutting equipment (n=17), melting furnaces (n=24), lathing machine (n=12), power posts (n=7), and scales (n=3), and production line (n=27) included; hydraulic pitch roller, scissors, stands, vertical loops, cooling, and knot. In this study, all workers were included in the study in a census. The inclusion criteria were; heat adaptation (more than one year of work experience), no cardiovascular disease, thyroid disease, hypertension, diabetes, febrile illnesses, ear infections, no antidiuretic drug consumption, and not taking any drugs that may affect HR.

On the day before making measurements, the objectives of the study and need for observation of instructions like resting at night was described to all subjects. According to the workshop conditions, and after coordination with a number of subjects, each working unit was monitored. During measurements, it was tried to measure the parameters on subjects under the same conditions and, considering the working duration of personnel, total measurement time was considered to be 2 hours for each person. The demographic variables were collected by a questionnaire containing information like age, height, weight, disease history, clothing type and color, health conditions, type of activity, use of gloves and socks, heat status, smoking, type of medications and the number of off days.

2.2. Measuring Environmental Variables

The environmental variables of Tnw, Td and Tg were measured and recorded by the WBGT digital device model BK 874 made in the UK and at the workstations according to ISO 7243 standards [14Parsons K. Heat stress standard ISO 7243 and its global application. Ind Health 2006; 44(3): 368-79.[http://dx.doi.org/10.2486/indhealth.44.368] [PMID: 16922180] ]. To determine air velocity (Va) the VT50 digital heat anemometer made in France was used. The calibration of the devices was approved prior to measurement and by competent authorities. Measurements were carried out at the nearest point to the workplace of individuals; and given the uniformity of conditions, each measurement was generalized for all employees.

2.3. Measuring Physiological Parameters

With environmental variables, physiological parameters of tympanic temperature, as the Tcr, and HR were measured simultaneously in two phases according to ISO 9886 standards [24ISO9886. Ergonomics-Evaluation of Thermal Strain by Physiological Measurements Second Edition. Geneva: International Standard Organization. 2004:1-21.]. In the first phase, after 30 minutes in the break room, at 20, 25 and 30 minutes, the physiological parameters including the Tcr, and HR were measured and their mean values ​​were recorded as basic information. In the second phase, after measurements were done at the resting state and the worker got back to the workplace, the physiological parameters of the Tcr, and the HR were measured at 20, 40 and 60 minutes, and their means were taken. The eardrum temperature was measured by a digital thermometer FT70 (beurer) made in Germany. The accuracy of the device is ±0.2°C and its range is 34 to 43°C. The measurements of the tympanic temperature were made in the right ear. HR was measured by using LAICA MD 6132 made in Italy. This device is fastened to the arm of the subject by the cuff pressure and is able to measure the HR of people along with blood pressure. Workers' activities were carefully monitored and recorded to estimate the metabolic rate of individuals. The metabolism rate of individuals during the work shift was determined by the classification tables of the ISO8996 standard. Then, using Eq. 1, the mean of metabolism rate was calculated for the subjects.

(1)

Mi: Metabolism rate of each activity, Ti: Time of each activity, T: the duration of work in a working shift, : Mean metabolism rate in each work shift in W/m2.

In order to estimate the thermal insulation of clothing (Clo), workers' clothes were carefully investigated at work. Clothing type (long or short sleeves) and its thermal insulation were determined based on the ISO 9920 standard [25ISO9920. Ergonomics of the thermal environment–Estimation of thermal insulation and water vapour resistance of a clothing ensemble. International Organization for Standardization Geneva, Switzerland. 2007.].

2.4. Measuring Heat Stress Indices

In order to calculate the WBGT index at indoor and outdoor environments, Eqs.2 and 3 were used, respectively.

(2)
(3)

Considering that the work environment was somewhat inconsistent in some of the workstations, the measurement of the WBGT index was performed at three heights of ankle, waist and head, and the WBGT value was calculated using Eq.4 [15 ISO7243. Hot environments-estimation of the heat stress on working man, based on the WBGT-index (wet bulb globe temperature). Geneva: ISO. 1989[http://dx.doi.org/10.2486/indhealth.44.368] [PMID: 16922180] ].

(4)

Eq. 5 was used to calculate DI index [19Epstein Y, Moran DS. Thermal comfort and the heat stress indices. Ind Health 2006; 44(3): 388-98.[http://dx.doi.org/10.2486/indhealth.44.388] [PMID: 16922182] ].

(5)

2.5. Statistical Analysis

Statistical analysis was performed using SPSS software (version 21, SPSS Inc., Chicago, IL, USA). Mean and standard deviation/SD (Minimum/Maximum) were reported for quantitative variables and frequency (percentage) for qualitative ones. Data were analyzed using T-test and Pearson correlation coefficient. The level of statistical significance was considered at P<0.05.

Table 1
Screening criteria for heat stress exposure based on ACGIH standard (WBGT values in °C).


Table 2
Mean demographic variables of respondents (n=90).


Table 3
Environmental variables measured at workplace of participants.


Table 4
Physiological parameters of subjects.


Table 5
Heat stress indices at workstations.


3. RESULTS

The demographic variables of the subjects studied are shown in Table 2. The mean age of subjects was 35.2 ± 8.44 years, weight 75.71 ± 12.06 kg and height 173.51 ± 7.33 cm. The mean metabolism rate of workers during the work was moderate and 182.94 ± 34.42 watts per square meter (w/m2). About 41 (45.6%) of workers were wearing the overalls. The mean environmental variables were higher in working conditions than that of resting conditions. The mean of measured environmental variables by relaxation and working state of the subjects are presented in Table 3.

The results of the physiological parameters of subjects are illustrated in Table 4. The mean Tcr and HR were higher in working state than the resting state (P < 0.001).

Table 5 shows the mean of the calculated heat stress indices at rest and during work states. The mean DI and WBGT were significantly higher at work (P < 0.001).

Table 6 shows the exposure of respondents to heat stress based on DI criteria. About 39(43.3%) subjects experienced moderate and 51 (56.7%) severe levels of exposure to heat, and there was no mild level of exposure to heat stress. The workers of melting and then production line were more than exposed to heat stress. The average allowed limits of heat exposure was 28.32±0.38°C, most subjects were exposed to heat stress and there was a significant difference between the average WBGT and the threshold limit values (t=4.903, P<0.001).

Pearson correlation test showed that there was no significant correlation between metabolism with age and weight of the subjects (r = 0.023; r = 0.203, P <0.05, respectively). The results of the correlation coefficient related to heat stress indices and physiological parameters are presented in Table 7. Pearson correlation test revealed a direct linear and significant relationship of WBGT with Tcr and HR. Pearson correlation test showed a significant direct linear relationship between the DI index and HR with r value of 0.229. The highest Pearson correlation coefficient was found between WBGT and HR (r = 0.434).

4. DISCUSSION

Heat stress at indoor work environments may occur by local exposure to heat sources, and may increase due to physical activity and clothing. Subjects who have been working for a long time in a hot environment with same conditions usually acclimatize to heat after a while due to physiological changes, which depends upon the level of activity as well as the duration of heat exposure. It is estimated that most people acclimatize gradually to the heat within 4 and 14 days [26Joubert D, Bates G. Occupational heat exposure Part 1: The physiological consequences of heat exposure in the occupational environment. Occupational Health Southern Africa 2008; 2.5. http://www.occhealth.co.za/?/viewArticle/992]. Improved heat tolerance is due to the ability to increase sweat and reduce HR. Sweating occurs in the lower core, with a reduction in skin temperatures and the amount of salt in sweat [27Sawka MN, Castellani JW, Pandolf KB, Young AJ. Human adaptations to heat and cold stress. DTIC Document 2002. http://www.dtic.mil/dtic/tr/fulltext/u2/p012427.pdf, 28Lugo-Amador NM, Rothenhaus T, Moyer P. Heat-related illness. Emerg Med Clin North Am 2004; 22(2): 315-327, viii.[http://dx.doi.org/10.1016/j.emc.2004.01.004] [PMID: 15163570] ].

Table 6
Exposure to heat stress based on DI index criteria among study groups.


Table 7
Pearson's correlation coefficient between heat stress and physiological indices.


This study was carried out on heat-acclimatized workers in the rolling industry with an aim of evaluation of occupational exposure to heat stress using the DI and the WBGT indices. The working conditions of most workers were in indoor environments, although the doors of the eastern and western sides were always open and the working environment has a public ventilation system. Moreover, the workers of cutting, power and scales units were also working in the outdoor environments. The results based on DI index criteria and WBGT index showed that most of the workers were exposed to heat stress. The values of WBGT were significantly different from the threshold limit values. In the study by Hamerezaee et al., workers in the steel industry with WBGTTWA 35.76 0C were exposed to higher values than the allowed limits. They also experienced more heat stresses than the present study [29Hamerezaee M, Dehghan SF, Golbabaei F, Fathi A, Barzegar L, Heidarnejad N. Assessment of semen quality among workers exposed to heat stress: A cross-sectional study in a steel industry. Saf Health Work 2018; 9(2): 232-5.[http://dx.doi.org/10.1016/j.shaw.2017.07.003] [PMID: 29928539] ]. According to Krishnamurthy et al. about 90% of the measured WBGT in steel workers exceeded the threshold limit values, which was related to the work units with radiation heat and moderate workload [30Krishnamurthy M, Ramalingam P, Perumal K, et al. Occupational heat stress impacts on health and productivity in a steel industry in southern India. Saf Health Work 2017; 8(1): 99-104.[http://dx.doi.org/10.1016/j.shaw.2016.08.005] [PMID: 28344848] ].

Perry et al. stated that to provide an optimal index based on physiological parameters, HR and Tcr were best parameters for estimating WBGT, DI and Required Sweat Rate index (SWreq) indices [31Logan PW, Bernard TE. Heat stress and strain in an aluminum smelter. Am Ind Hyg Assoc J 1999; 60(5): 659-65.[http://dx.doi.org/10.1080/00028899908984488] [PMID: 10529998] ]. In this study, the physiological parameters of Tcr and HR were used to study the physiological responses of rolling industry workers. The results showed that the mean heat stress indices of DI and WBGT were significantly higher in working than at rest. Also, there was a significant difference between the mean Tcr and HR in both the working and the rest states. Heat stress is the net heat load that enters into the body of a person due to exposure to environmental variables, metabolic processes and clothing requirements, and the heat strain is the individual's physiological responses to environmental conditions; this response augments by increasing the amount of exposure to heat stress [32 ACGIH. Heat Stress and Strain. American Conference of Governmental Industrial Hygienists, 2001]. The physiological strain caused by heat stress is affected by four environmental variables: air temperature, humidity, air flow rate and thermal radiation, as well as energy consumption due to individual physical activity and thermal properties of the cloth (Clo). Besides, heat adaptation, hydration and body posture at work also affect the physiological strain caused by heat stress [33Kampmann B, Bröde P. Physiological Responses to Temperature and Humidity compared with Predictions of PHS and WBGT Environmental Ergonomics XIII 2009; 54-8.].

Ahasan et al. aimed at monitoring the heat stress of workers in the rolling steel industry, the Tcr and HR of workers were increased who worked in high-temperature units and radiation workshops [34Ahasan R, Basahel A, Haque W, Islam E. Monitoring thermal stress of steel Rre-rolling mill workers. Ergon Int J 2018; 2(2): 000132. https://medwinpublishers.com/EOIJ/EOIJ16000132.pdf[http://dx.doi.org/10.23880/EOIJ-16000132] ]. Latzka et al. investigated the influence of physical activity on HR increase during exercise in hot laboratory conditions, the data showed that HR increased with the intensity of physical activity [35Latzka WA, Sawka MN, Montain SJ, et al. Hyperhydration: tolerance and cardiovascular effects during uncompensable exercise-heat stress. J Appl Physiol 1998; 84(6): 1858-64.[http://dx.doi.org/10.1152/jappl.1998.84.6.1858] [PMID: 9609777] ]. In the study of Maw et al. regarding effects of hot environment on Tcr, HR, and physiological strain, it was concluded that with increasing WBGT, HR, oral temperature, and physiological strain increased [36Maw GJ, Boutcher SH, Taylor NA. Ratings of perceived exertion and affect in hot and cool environments. Eur J Appl Physiol Occup Physiol 1993; 67(2): 174-9.[http://dx.doi.org/10.1007/BF00376663] [PMID: 8223525] ]. Dehghan et al. examined the correlation between the WBGT and Physiological Strain Index (PSI), their results showed that by increasing the WBGT index, the PSI also increased, an inference consistent with the findings of the present study [37Habibi P, Dehghan H, Rezaei S, Maghsoudi K. Relationship between physiological strain index and wet bulb globe temperature index in women in the climate chamber. J Health Sys Res 2014; 10(3): 442-9. http://hsr.mui.ac.ir/index.php/jhsr/article/view/1618].

WBGT had a direct and significant relationship with Tcr and HR, with it a better correlation with HR than the Tcr. DI index had a direct correlation with the Tcr and HR as well, association with the latter being significant. Fahad et al. conducted a study on steel industry workers in Turkey, in which although WBGT index had a high correlation with physiological parameters, it had an even higher correlation with the Tcr which was consistent with our study results [38karim Fahed A, Ozkaymak M, Ahmed S. Impacts of heat exposure on workers’ health and performance at steel plant in Turkey. Eng Sci Tech, Int J 2018; 21(4): 745-52.https://doi.org/10.1016/j.jestch.2018.05.005]. A higher correlation with HR in such work environments can be related to the working conditions of workers who are under a higher physical burden. WBGT had a higher correlation with the physiological parameters than the DI, similar results were depicted by Fahad et al. [38karim Fahed A, Ozkaymak M, Ahmed S. Impacts of heat exposure on workers’ health and performance at steel plant in Turkey. Eng Sci Tech, Int J 2018; 21(4): 745-52.https://doi.org/10.1016/j.jestch.2018.05.005]. Hamerezaee et al. showed that the WBGT is a more appropriate index than Predicted Heat Strain (PHS) for the evaluation of heat stress among steel industry workers [29Hamerezaee M, Dehghan SF, Golbabaei F, Fathi A, Barzegar L, Heidarnejad N. Assessment of semen quality among workers exposed to heat stress: A cross-sectional study in a steel industry. Saf Health Work 2018; 9(2): 232-5.[http://dx.doi.org/10.1016/j.shaw.2017.07.003] [PMID: 29928539] ]. In the study of Golbabaei et al., the WBGT index showed a better correlation with physiological parameters of HR, blood pressure and the skin temperature (Tsk) compared to the DI index; results being consistent with this study [39Golbabaei F, Monazam Esmaieli MR, Hemmatjou R, Yaaghoub P, Reza G, Hosseini M. Comparing the Heat Stress (DI, WBGT, SW) Indices and the Men Physiological Parameters in Hot and Humid Environment. Salamat va Muhit 2012; 5(2): 245-52. http://ijhe.tums.ac.ir/article-1-31-en.html]. According to Chen et al. physiological parameters of HR, systolic and diastolic blood pressure and Tsk were most correlated with the WBGT index and the physiological parameter of Tcr had the highest correlation with the SWreq among steel workers [40Chen ML, Chen CJ, Yeh WY, Huang JW, Mao IF. Heat stress evaluation and worker fatigue in a steel plant. AIHA J (Fairfax, Va) 2003; 64(3): 352-9.[http://dx.doi.org/10.1080/15428110308984827] [PMID: 12809541] ]. In a study by Brahmapurkar et al., most workers in the glass factory were exposed to heat stress, and Tg was higher than Td at all work units [41Brahmapurkar K, Lanjewaar A, Zodpey S, Brahmapurkar V, Khakse G, Thakre S, et al. Heat stress and its effect in glass factory workers of central India. Int J Eng Res Technol (Ahmedabad) 2012; 1(8): 2278-0181. https://www.ijert.org/phocadownload/V1I8/IJERTV1IS8577.pdf]. Also, in the present study, the average Tg was higher than the Td, so the radiation heat could be one of the main causes of heat stress.

It should be noted that each of the thermal stress indices has its own advantages and disadvantages, and WBGT index is no exception to this rule, despite its high acceptability and widespread use [ 18Budd GM. Wet-bulb globe temperature (WBGT)-its history and its limitations. J Sci Med Sport 2008; 11(1): 20-32.[http://dx.doi.org/10.1016/j.jsams.2007.07.003] [PMID: 17765661] ]. Industries that are exposed to heat, they are required to provide the device, and by using it correctly, effective measures can be taken to reduce heat stress of work environments.

This study was conducted at a short interval, which could be among the limitations of the present study. However, in industries like rolling with regard to the working conditions, heat stress can exist in all seasons of the year, and it is a permanent problem. Due to the fact that heat-related complications can be prevented, due consideration should be made to promote the culture of occupational health among workers. This strategy will have a good impact on the health of industrial workers. Health measures such as the supply of cool drinks, schedules for rest periods and training of workers need to be considered.

CONCLUSION

In the rolling industry, heat stress exceeds the recommended exposure limits in many workstations. Due to the heavy physical activity of workers, interventional measures should be taken to reduce exposure. Also, WBGT index showed a better correlation with the physiological parameters of Tcr and HR, which is easy to use and can be used to assess the risk of heat stress and to determine the health hazards of workers exposed to heat.

ETHICAL APPROVAL AND CONSENT TO PARTICIPATE

The study was approved by the student research committee of the Shahid Beheshti University of Medical Sciences, Tehran, Iran. No. 1396/65558.

HUMAN AND ANIMAL RIGHTS

No animals were used in this research. All human research procedures followed were in accordance with the ethical standards of the committee responsible for human experimentation (institutional and national), and with the Helsinki Declaration of 1975, as revised in 2013.

CONSENT FOR PUBLICATION

Informed consent was obtained from all subjects prior to data collection.

CONFLICT OF INTEREST

The authors declare no conflict of interest, financial or otherwise.

ACKNOWLEDGEMENTS

The authors would like to thank the student research committee of Shahid Beheshti University of Medical Sciences and all the subjects who cooperated in the implementation of the research.

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