Occupational heat stress under high-heat furnace work environments - a comprehensive review on devel

Abstract

Keywords: Furnace Work; High-heat Environment; Workers; Heat Strain; Assessment; Harmful Impacts

Introduction

In developing countries, heat stress is often an unacknowledged occupational health hazard [1, 2]. Excessive hot environments are usually wide-spread in foundries, iron and steel industries, glass manufacturing units, rubber processing, coke ovens, mining sites, and several other industrial sectors [2, 3]. Heat stress impacts the human physiological mechanism in terms of change in heart rate, skin temperature, core body temperature, and body mass loss due to sweating and further impose suppressions on users’ health and work productivity [4]. As compared to developed countries, workers employed in the micro small and medium enterprises (MSME) sector are mostly uneducated and unaware of this occupational health hazard where poverty also plays a vital role. Most industries falling under the MSME sector are highly labour intensive due to lack of modernistic equipment’s and not much concerned about labour’s health and safety. In developing countries, less information is available on the combined effect of workplace heat exposure and climatic conditions [1-3, 5]. Climatic zones like tropical and subtropical regions having higher ambient temperature and humidity values may impose greater risk of heat-related illness and safety threats among workers’ employed in developing countries, having low and medium source incomes [5-8].

A prolonged period of heat exposure could results in consequent performance loss among workers; particularly functioning in an indoor high heat work-environment and negatively impacts their productive capacity [9-11]. The aim of the present work is to gain insights on the factors affecting the heat stress exposure among workers employed in furnace work-sectors, available ISO standards, and suitable heat stress assessment strategies being widely used for such hot work environments. Moreover, various physiological parameters as an effective indicators of heat strain have also been addressed. Present work also summarizes several heat stress assessment studies (conducted by researchers around the globe; particularly in developing countries) related to high-heat work environments in a rigorous manner during the past two decades. The influence of occupational heat stress on workers’ productivity, negative health impacts, and role of design interventions in ameliorating hot stressful conditions have also been taken into consideration in a novel systematic approach. In present approach, relevant articles and publications have been searched from leading literature databases i.e. Scopus, PubMed, Science direct, and Google scholar. For searching strategy, several keywords like heat stress, heat strain, hot work environments, heat exposure, furnace, foundry, worker health and safety have been used. From identified literature studies, relevant articles

Figure 2. Summary of the thermal stress assessment in the individual studies.

associated with the concerned field were sorted out. The effective literature roadmap for the present work have been described briefly in Figure 1. Also, Figure 2 summarizes the considered thermal assessment studies related to different high heat work sectors like steel plants, foundries, rolling mills, manufacturing sector, glass manufacturing units, bakeries, brick kilns, mining sites, and also laboratory based environment studies. Present review study would be beneficial for occupational health practitioners, industrialists, and policy makers (particularly in developing countries) in analysing the prevalent hot stressful conditions under high heat work sectors and providing pathway to reduce occupational heat stress exposures upto the desired permissible limits.

Factors Affecting The HEAT Stress Exposure

There are six main agents that accord to the heat stress exposure experienced by the user under hot stressful work conditions like furnace environment. These six factors includes four environmental (air temperature, radiant temperature, air speed and relative humidity) and two individual factors (physical work activity and clothing worn) [12]. There are other personal factors like gender, age, psychosocial factors, health issues and poor habits (consumption of alcohol, tobacco) which also contributes to this occupational health hazard [2, 6]. Furnace is considered as a dominant source of radiant heat. The radiant and convective heat gains by the workers close to the furnace combined with climatic conditions leads to several heat related disorders like heat rashes, cramps, dehydration, exhaustion, and even heat stroke [13]. In furnace work industries, the temperature range widely depends on the type of work operation. Table 1 describes the furnace temperature range (in degree Celsius) for such different work-sectors.

Table 1. Furnace temperature range for different worksectors Work-Sector

Glass Industry [14, 15] 1400oC to 1600oC Rolling Mill Furnace [16, 17] Upto 1200oC Blast Furnace [18, 19]

Environmental and Personal Factors Air temperature is one of the main factor of importance (Figure 3) while considering heat stress, which represents the workers’ surrounding air temperature. The other important environmental factor of concern is radiant temperature as furnace is a dominant source of radiant heat. Radiant and convective heat gain by the workers close to the furnace leads to heat disorders. Radiant temperature is much more influential than air temperature for a furnace work environment, as it greatly affects the heat exchange mechanism of worker with its surroundings. Another factor, which influence the heat exchange mechanism is humidity; we generally consider relative humidity (RH) which is the ratio of actual amount of water vapor present to the maximum quantity in the air at that temperature. RH varies considerably for an indoor work environment and is of great importance as a higher humid work environment would prevent the sweat evaporation from the skin surface which would results in increase of worker’s core body temperature (CBT). Also, air velocity corresponds to the air flow surrounding the worker

and is one another influential factor desirable for relieving heat stress in a furnace work environment. Still air results in heating up of the work environment, whereas moving air supports heat loss by convection and similarly physical movement of the workers results in increase of air flow, but depends on the activity level [12, 28]. Being warm blooded creatures, our body generates heat continuously and the rate at which this heat is generated is given in terms of “metabolic rate”. Usually it depends on the activity of the worker and given in terms of “met”, (1 met = 58.2 W/m2) [28]. Metabolic rate depends on different physical characteristics of a person and the level of muscular work intensity level (ISO 9886, 2004). Most of the previous research studies have classified the metabolic rate for highheat furnace related work operations under moderate and heavy workload categories [6, 14-16, 27, 29, 30]. ISO 8996 standard provides a detailed estimation of the same based on different parameters like heart rate, weight, body mass index (BMI) and classifies work category (low, moderate, high) based on the metabolic rate [31]. Metabolic rate varies significantly for different type of activities (like resting, sedentary, standing, domestic work, industrial work) and is different from person to person. Metabolic heat (in W) is evaluated from the metabolic rate by multiplying it with the body surface area (in m2). Du Bois provides one such estimation to calculate the body surface area and suggests it varies from 1.3 m2 to 2.2 m2; whereas for an average adult it is considered as 1.8 m2 [32]. Suppose a furnace worker with average surface area equal to 1.8 m2, performing a moderate level work activity (2.0 met); will be producing metabolic heat equal to (58.2 x 2.0 x 1.8) 209.52 W. Metabolic rate has a critical impact on the workers thermal comfort; as

furnace work has been classified as moderate to high level work. As more heat is produced and it needs to be dissipated, so that the body’s inner temperature doesn’t exceeds the permissible limit values. The other important personal factor is the clothing worn; providing insulation effect which gives resistance to heat transfer. If the insulation level of clothing is too high, then it will generate immense heat stress even if the work conditions are not very hot. It is represented in terms of “clo”, where 1 clo corresponds to 0.155 m2 oC/W [33]. Clothing factor is of great concern particularly in work conditions similar to furnace industries, as it affects the sweating rate which in turns imbalances the body heat exchange mechanism, especially when personal protective equipment (PPE) is worn by the worker [34]. But on the other side, it also protects the worker in cold environments and mainly depends on the actual work conditions. ISO 9920 standard provides an estimation of thermal insulation values for various clothing ensembles (in “clo”) and vapor permeability resistance; which in turns impacts the thermal strain experienced by the worker under respective work-conditions [33]. However, few other personal factors which are not directly related, but research studies revealed that factors like psychological ability, age, gender, health conditions, and other poor work practices also contributes to the thermal discomfort [2-7]. Table 2 describes the considered metabolic rate and clothing insulation values in related thermal assessment studies under different work environments. The heat exchange between the human and its surroundings has been represented by heat balance equation which relates the heat production rate to rate of heat lost. The combined heat loss by radiation, convection and conductions

Figure 3. Dominant factors for Heat Stress under high-heat work environment.

Table 2. Metabolic rate and clothing insulation values considered in related work-studies First Author

HEAT Stress Assessment

For heat stress assessment, several indices have been developed over the past century which includes the environmental factors and personal factors or combination of both [41]. There are several indices which include the physiological parameters (heart rate (HR), skin temperature (Tsk), body mass loss due to sweating, core body temperature) into consideration. Parsons [34] suggested a three step approach which involves ISO 7243 standard for direct assessment based on monitoring and control of hot environments; ISO 7933 (rational indices approach) for analysing the heat exchange between the worker and its surroundings followed by ISO9886 standard that describes the principles of physiological measurement which may be used in the establishment of personal monitoring systems of workers exposed to hot environments. This three step approach could be beneficial in conducting more in-depth analysis of the hot work conditions to which the workers are exposed; by considering the effect of environmental

factors, metabolic rate, clothing insulation, sweat evaporation and physiological variables. Suitable ISO Standards There are several ISO standards (with recent modifications) available which are suitable for evaluating heat stress parameters in hot and humid work environments; as depicted in Table 3. ISO 7243 provides an initial screening or heat stress exposure assessment for hot work environment. To validate the screening results, physiological parameters can be evaluated based on the ISO 9886 standard. For more in-depth analysis ISO 7933 must be used, which provides better estimation of the thermal strain experienced by the worker [34]. There are also two supporting standards i.e. ISO 9920 and ISO 8996, which provides thermal insulation values for different types of clothing worn and metabolic rate based on the physical work activity performed. Heat Stress Indices for Hot and Humid Work Environment Heat stress indices can be divided into three main categories i.e. rational indices (based on the heat exchange

equation), empirical indices (relating to objective and subjective strain) and direct indices (involving direct measurements) [46]. Based on research studies, several heat stress indices that have been found suitable for monitoring and evaluating the thermal stress conditions under high-heat work environments, which are described as: Wet Bulb Globe Temperature (WBGT): WBGT is an empirical index; widely used and validated index for the assessment of heat stress in hot work environments considering the combined effects of air temperature, humidity, air velocity and radiation by measuring natural wet-bulb temperature (Tnw), dry bulb temperature (Ta) and radiant effects using globe temperature (Tg); in degree Celsius for both indoor and outdoor work conditions. Equation (4) and (5) shows the respective expressions without and with solar load effect [46]. For Indoor work environment; WBGT = 0.7Tnw + 0.3Tg

Table 3. ISO Standards suitable for evaluating heat stress parameters in hot and humid work environments ISO Standard

It provides a method for the assessment of heat stress in hot For Indoor environment; work environments considering the effect of radiant temperature WBGT = 0.7T + 0.3T nw g using Globe temperature (Tg), natural wet bulb temperature (Tnw) For outdoor environment; and ambient air temperature (Ta). The modified standard also WBGT = 0.7Tnw + 0.3Tg + 0.1Ta includes the combined effects of clothing-worn and metabolic rate classifications based on the work-activity performed; and also provides the work rest cycle for a task based on the ACGIH guidelines. This standard explains different measurement techniques for Heart Rate, Skin Temperature, Core Body indicators of several physical parameters like heart rate, skin Temperature, Body mass loss (sweating) temperature, Body mass loss due to sweating and core body temperature characterizing the respective thermal environments.

It evaluates the thermal stress experienced by a subject in a hot Required Sweat Rate, Maximum Sweat and humid work conditions. It gives a prediction of the sweat Rate, Predicted core body temperature, rate and the internal core temperature (as indicators of thermal Thermal strain strain), it provides a computer based program for the evaluation of predicted heat strain (PHS) model.

This standard provides different methods for the determination Metabolic Rate (W/m2) of metabolic rate for different work tasks/activities, work cycles in the context of various climatic working environment; categorizing them under “Low, Moderate, High and Very-High workloads”.

This standard provides an estimation of thermal insulation values Clothing factors: clothing thermal of various clothing ensembles (in terms of “clo”) like thermal insulation values (clo), clothing vapor insulation and water vapor resistance; which in turns effect the resistance (evaporative resistance) thermal strain experienced by the subject with respect to the working conditions.

ISO 7243 standard also considers the spatial and temporal variations as given in equations (6) and (7) by evaluating the WBGT w.r.t three different heights levels like ankle height (0.1m), waist height (1.1m) and head height (1.7m) and also based on the different time intervals using time weighted average formula. It also takes into account the effect of clothing ensembles worn by the workers in terms of clothing adjustment values (CAV) and classifies the physical work activity of the workers based on the metabolic rate (resting, low metabolic rate, moderate, high, or very high metabolic rate). These adjustments provide the effective WBGT index reference values (threshold limit values (TLVs)) for acclimatized and non-acclimatized workers based on the physical activity [42]. WBGTSpatial = [WBGTHead +

Physiological Strain Index (PSI): PSI is an empirical heat stress index that indicates the physiological strain experienced by a person in a hot environment by evaluating two physiological parameters [47] i.e. rectal temperature (Tre) and heart rate (HR). It indicates the physiological strain based on a 10 point universal scale (with PSI values varying from 0 to 10) and respective thermal strain level as follows; 0 – 2 (No/little), 3– 4 (low), 5 – 6 (Moderate), 7 – 8 (High), and 9 – 10 (Very High). It considers the combined effect of both thermoregulatory and cardiovascular system, while evaluating the thermal strain. The physiological strain index is expressed by equation (8):

Discomfort Index (DI): The development of a direct indices tool called the “discomfort index” based on dry bulb temperature (Td) and wet bulb temperature (Tw) in oF; with some correction factor relates the thermal degree of discomfort perceived by the user in a work environment [49]. Higher the value of DI results in higher degree of discomfort. The different levels of discomfort are given as: DI > 70 (people feel discomfort), DI > 75 (over half of the population will feel uncomfortable), when DI = 79 (everyone will be uncomfortable), and DI ≥ 80 (discomfort becomes more serious). The Discomfort Index is given in equation (9) ­followed by the cumulative discomfort index [50] in equation (10).

Where; Tret is simultaneous rectal temperature measurement taken at any time (in oC) Tre0 is the initial rectal temperature measurement (in oC) HRt is the simultaneous heart rate measurement taken at any time (in bpm) HR0 is the initial heart rate measurement (in bpm) Based on the PSI index, the maximum allowable rise in rectal temperature during heat exposure is 3oC (36.5oC to 39.5oC) and maximum allowable elevation for heart rate (HR) is 120 bpm (60 to 180 bpm). PSI can be evaluated for any time of exposure, while a user is performing any work task or at rest/recovery period, as it involves the measurement of only two physiological parameters, which reduces the scope of error [48].

Heat stress index (HSI): HSI (proposed by Belding and Hatch, 1955) evaluates the heat stress in a hot and humid work environment based on the heat exchange equation between the human and its surroundings [51]. HSI evaluates the heat strain experienced by the user in a thermal work environment based on the evaluation of metabolic rate (M), radiant load (R), convective load (C), and maximum evaporative cooling (E). HSI provides a single value which indicates the heat strain level experienced by the user in a thermal environment [52]. The Heat Stress Index (HSI) equation is given by equation (11):  Ereq  HSI =  ×100  Emax 

Convective heat exchange (C) = 7.0 × V 0.6 × (Tair − Tskin )

Where:Tskin = 35°C, Vapor pressure of skin (Pskin) = 42 mm of Hg One another expression for HSI relating the metabolic rate (M) (in Btu/h), dry bulb (Tdb) and wall surface (Tw) temperatures (in °F), air velocity (Var) (in ft/min) and vapor pressure (pa) (in mm of Hg, where 42 mm of Hg represents the skin vapor pressure at 95°F based on relationship with the experimental studies is given as: HSI =

Discussion

The thermal ambience of furnace workforce is influenced by the combination of several factors (six dominant factors that affects the human thermal response i.e. four environmental and two personal factors) like clothing, climatic conditions, and physical activity performed [63, 64]. As compared to developed countries, several research studies have been conducted under various industrial sectors, but there is still a lot of research work required to be executed in developing countries; as the climatic/geographical conditions and target population differs from one country to another. So, there is need and scope to explore various environmental factors affecting thermal ambience in a rigorous manner. A research conducted in South-east Asia revealed that workers employed under several work sectors were exposed to extreme level of thermal stress, resulting in production loss and serious health issues among users [7]. In India, during hot summer season, millions of underprivileged workers are strained by harsh thermal workconditions with subsequent health challenges, reduced productivity and daily incomes [6]. One such research study related to manufacturing sector in India, found that there was significant reduction in workers’ productivity due to rise in ambient temperatures at the workplace [8]. The hot and humid conditions leads to excessive perspiration and makes the working conditions tougher and unfavourable for human working [65]. Table 5 summarizes the research work of several authors related to thermal stress evaluation for workers employed under various high-heat work-sectors. From assessment studies (depicted in Table 5), it was observed that occupational heat impacts have received limited attention in developing countries despite workers being a vulnerable sub-population to climate change and very few appropriate studies have been reported on heat stress under high heat furnace work environments. In developing countries, there’s lack of research considering the combined effect of climatic conditions and occupational heat

exposures on worker’s health and productivity. Mostly, previous studies are based on the assessment-orientedapproach rather than action-oriented-approach. There is need for adaptation research with purposive action focused on improving the workplace thermal conditions rather than just providing assessment and suggestions. Decline in Work-Productivity and Financial burden Workers employed under industrial work-sectors are often exposed to excess levels of heat stress with resultant reduction in work-capacity and serious occupational health consequences due to climatic change and several other parameters under high-heat work conditions. In developing countries, less information is available on the combined effect of climatic and industrial heat exposure. This occupational heat exposure has a negative impact on the workers well-being; which declines their work efficiency and in turns affect the production of the plant. Kjellstrom et al. [1] stated that climate change results in reduced work capacity in heat-exposed work environment and found this as a major issue for developing countries in achieving economic and social development. It was analysed that work capacity rapidly reduces, as WBGT exceeds from 26oC to 30oC. However, developed countries having less economic constraints could implement specific adaptation measures in terms of occupational heat exposures using expensive methods/techniques as compared to low-income (developing) countries. Mohamed and Srinavin [70] proposed a productivity model with respect to dominant thermal environment parameters; relating work-productivity with PMV thermal comfort index for three different work categories (i.e. Light, moderate, and heavy construction work tasks). Authors presented three robust regression models (as described in equation) covering major variability of the productivity with respect to thermal work-conditions. PL = 102 − 0.80 * PMV − 1.84 * PMV 2

PH = 83 + 21.64 * PMV − 9.53 * PMV 2 + 0.91 * PMV 3 (35) Kjellstrom et al. [71] reported that rise in occupational heat exposure due to climate change may lead to labor productivity losses with subsequent economic declines. Authors proposed a productivity versus heat stress based graph model for acclimatized workers; relating work capacity (in %; maximum hour duration that user should be engaged working) as function of WBGT value for different work-intensity levels (200 W, 300 W, 400 W, 500 W; ranging from light to very-high workloads). Ismail et al. [72] evaluated the combined effect of temperature and relative humidity on worker productivity in an automotive

Table 5. Related assessment studies for high-heat work environments in developing countries Author/ Year

WBGT, CET, MRT

The heat stress indices values exceeded much more than the permissible limit values (average WBGT = 40 oC); author suggested revising the existing standards based on the local climatic conditions.

Evaluating heat stress exposure levels using selected indices to assess the thermal health hazards present at a glass manufacturing plant

WBGT, CET, HSI

Assessing occupational heat exposure level under two different sections in a glass industry using environmental variables and different heat stress indices (i.e. WBGT, CET, and HSI)

The relationship between all the evaluated environmental parameters (expect air velocity) with respect to different body heights (head, abdomen and ankle) was found to be insignificant. However, positive correlation was observed between the WBGT, HSI, and CET.

PMV, PPD

(PMV: Predicted Mean Vote; PPD: Predicted percentage of dissatisfied)

To identify the association between the PMV and thermal sensation (S) among workers employed in metal industry and developing a linear multiple regression equation.

It was observed that the mean radiant temperature, air-temperature, and metabolic rate could positively influence the employees thermal comfort sensations.

The study aims to evaluate occupational thermal stress and subsequent impacts on workers’ health and productiveness; by collecting qualitative and quantitative data from 18 different workplaces, under both organized and unorganized work-sectors.

No significant difference was observed among the respective organized and unorganized work-sectors. During summer and winter time-periods, around 82% and 42% workers were affected by higher WBGT values than the recommended limits.

Proposing design interventions based on decreasing the radiant heat exposure generating from a blast furnace followed by analysing the effectiveness of proposed design controls using WBGT, MRT indices and CBT.

Using design control interventions, the MRT and WBGT values decreased by 26.5°C and 5.2°C respectively; While the workers’ CBT reduced by 2.6°C.

(CET: Corrected Effective Temperature; MRT: Mean Radiant Temperature)

20. Male

Workers involved in intensive physical work activities reported more heatrelated health morbidities and reduced productivity. Significant association was observed among the workers subjective thermal perception on health and productivity impacts under the existing thermal work-conditions.

Methodology

The objective is to find an optimal heat stress index for brick kilns workers engaged in different work activities, by comparing the results of different thermal indices

Workers engaged in indoor kiln were exposed to higher environmental parameters values as compared to outdoor; except for air-velocity.

Skin temperature, blood pressure, heart rate, ear-carotid artery temperature, oral temperature. (ET: Effective

WBGT and DI were found to be highly correlated with each other. WBGT was suggested as the optimal index for Brick kilns work environment.

Analysing health and productivity loss among workers employed in high heat work conditions, due to various thermal stress factors.

90% of WBGT values exceed the TLV limits. Maximum value of WBGT (41.7oC) was found in the coke oven area and minimum (27.2oC) in the control room zone. Around 10.6% workers reported productivity loss (in terms of not achieving specified targets) due to high thermal stress.

PMV, PPD

Statistically significant associations were observed between WBGT and carotid artery temperature, skin temperature, and oral temperature.

WBGT, PSI, HSI; Physiological parameters (Heart Rate, CBT: Ear canal temperature)

Computational fluid dynamics (CFD) simulation has been used to evaluate the existing human thermal comfort level among workers employed in plastics manufacturing company based on PMV-PPD calculations; followed by suggesting CAD simulation based design modifications focussed on increasing the thermal comfort level among workers.

Replacing existing ceiling glass material; from clear glass to reflective clear glass (6 mm thickness; with 23% transmittance).

Examined the impacts of occupational heat stress on the health and productivity of steel plant workers during the summer season.

Average WBGT, PSI and HSI values were found to be 30.89 ± 1.1 oC, 3.15 ± 0.64 and 118.5 ± 18.61%.

The design modifications resulted in decrease of PMV by 0.64 point (existing PMV: 1.83 to 2.82; modified PMV: 1.63 to 2.18). With PPD resulted 13.8% increase in thermal comfort (existing PPD: 68.9% TO 98%, modified PPD: 58.2% TO 84.2%) and ambient temperature decrease by 4oC.

It was observed that work activities close to furnaces and hot rolling area were exposed to high levels of heat stress. For heavy workload, PMV-productivity model indicated a productivity loss ranging from 20 to 30 % respectively. Higher correlation was observed for HSI with the heart rate. However, a significant relationship was found for WBGT and PSI indices with the core body temperature.

Methodology

Heat stress evaluations among bakery workers using WBGT and PSI indices; comparison with national and international limits followed by suggesting control measures.

WBGT index values were found to be higher than the TLVs specified by American Conference of Government Industrial Hygienists (ACGIH) and Egyptian Environmental Law (EEL); for acclimated workers performing moderate work activities.

Physiological parameters (heart Hot dessert rate, CBT: oral temperature) climate Egypt

Around 42% workers were exposed to high heat stress values; with PSI ranging from low to moderate level. Bolghanabadia et al. 2019 [61]

103 Bakery workers Iran, Tropical and Subtropical Steppe Climate

WBGT, Heat strain score index (HSSI); oral and drumhead temperature, heart rate, BMI

50. Mine

Investigating the heat strain rate among three different group of bakery workers (i.e. bakers, bread grabber, and pastry makers) under existing physical conditions followed by analysing relationship between physiological variables

WBGT exceeded the TLVs for 80% workers with WBGT Mean (SD) value of 28.69 (1.41). Oral and drum-head temperatures were highest among workers engaged in baking activity, as compared to other two groups. Significant relationship was observed between WBGT and HSSI (p-value < 0.001, r = 0.61); and with other considered physiological variables, except BMI.

Evaluating the thermal stress exposure among workers employed in a rolling mill; using WBGT and DI followed by analysing their relationship with the physiological parameters i.e. CBT and HR.

Significant differences were observed for physiological parameters (i.e. CBT, HR) in between resting and working conditions. The mean scores for DI and WBGT were reported as: DI Mean (SD): 28.28oC (1.11); WBGT Mean (SD): 29.05oC (1.27). Highest correlation was observed between WBGT and heart rate followed by CBT.

UTCI, TSI, WBGT, WBDT (Wet bulb dry temperature); Physiological parameters (blood pressure, HR, CBT: tympanic temperature, and skin temperature) (UTCI: Universal Thermal Climate Index)

Determining environmental and workers’ physiological responses followed by evaluating the respective indices (i.e. UTCI, WBGT, WBDT, and TSI) and examining the correlation among the physiological responses and respective thermal stress indices.

The average values (with SD) for UTCI, WBGT, WBDT, and TSI indices were 34.59°C (1.51), 24.59°C (0.62), 28.37°C (0.82), and 31.51°C (0.7). Strongest association was observed between UTCI and WBDT indices followed by WBGT and WBDT. No significant correlation was observed among the evaluated indices and workers physiological responses (p-value > 0.05), except only significant association observed for WBGT and TSI indices with the skin temperature.

Methodology

Examining the relationship among occupational thermal stress and DNA damage among steel plant workers prone to high-heat work environment.

Higher risk of DNA damage was observed among the exposed workers as compared to the control group (X2 = 47.1; p < 0.0001). Significant increase was observed in the micro nuclei (MN) frequency for the affected group as compared to control group (Adj. OR = 23.3, 95% CI 8.0–70.8);

CBT: tympanic temperature, Sweat rate, Urine specific gravity (USG), Micronuclei (MN) frequency in lymphocytes

WBGT; Blood cell count, Enzyme-linked immunosorbent assay (ELISA) WBGT; CBT: tympanic temperature, Urine Specific Gravity (USG), Renal/urologic anomalies

Among exposed group higher risk of DNA damage was associated with workers engaged in high-heat work activities (Adj. OR = 81.4; 95% CI 21.3–310.1); also having significant relationship with years of exposure (Adj. OR = 29.7; 95% CI 2.8–315.5), leading to higher induction of MN-frequency. To analyse the effect of occupational heat stress on the immunological parameters of foundry workers in Iran with reference to increase in WBGT values.

Under high heat conditions, there was decrease in the leukocytes levels and white blood cell counts among foundry workers which indicates possible decline in the workers’ immune system.

Analysing the risk of renal/ urologic anomalies (i.e. kidney stones and other structural renal anomalies) among steel workers exposed to high-heat work conditions.

For 220 workers, heat exposure values exceeded the TLVs; with WBGTMean (SD) = 33.2°C (± 3.8°C). Significant relationship was observed between the workers’ heat exposure level, rise in CBT and urine specific gravity (relating to dehydration level). 91 workers were tested for renal anomalies using portable renal ultrasound scanner. Around 33% were tested positive for having renal anomalies (i.e. kidney/ureteral stones and urethral calculi). These anomalies were found higher among heat-exposed workers (WBGT > 30oC) as compared to unexposed group and was also affected by the year of exposure (≥5 years).

industrial workstation and proposed a mathematical equation, which could predict the workers’ production rate based on the RH and WBGT temperature. Stated thermalproductivity models could be implemented in hot furnace work environments; to analyse the occupational heat exposure impacts on user’s work-capabilities. In China, 9550 work-related injury claims and insurance compensation pay-outs (for the period of January,

2011 to December, 2012) were analysed from the work related injury insurance system, indicating an insurance pay-out of 282.3 million Chinese Yuan [73]. Further, the relation between WBGT index and work-related injury claims & insurance compensation pay-outs were calculated. It was found that injury claims increased with rising WBGT values. Around, 4.8% of work-related injuries and 4.1% insurance pay-outs were attributed to heat exposure

for WBGT above threshold limit values (TLVs). Another research study examined the relationship among climatic conditions, worker’s productivity and health status under two different work conditions [74] i.e. indoor industrial work-environment (pottery industry, power plant, knife industry) and outdoor work-conditions (agricultural and construction). Qualitative analysis revealed worker’s perceived productivity loss ranging from 10% to 60% among construction and industrial workers. Beheshti et al. [75] evaluated exposure of thermal stress and its subsequent performance-loss among workers; functioning under highheat indoor work conditions. It was concluded that extreme workplace heat could significantly decrease the labour work performance and consequently, their production capacity. The performance loss attributable to occupational heat stress was determined by using performance loss versus thermal stress graph proposed by Kjellstrom et al. [71]. Venugopal et al. [7] studied occupational heat stress and its impact on the health and productivity of workers by collecting qualitative and quantitative data from 18 different workplaces in both organized and unorganized work sectors. Workers with heavy workloads reported more heatrelated health issues and reduced productivity. Another researcher analyzed the unsteady work environments in the working face of hot coal mines; with results revealing rise in skin and core temperatures with increasing levels of relative humidity. Air temperature around 22oC to 24oC was found to be a control factor to improve the thermal tolerance. The unsteady thermal condition significantly affect the mine worker’s thermal sensations and physiological responses which in turns effects the productivity [76]. Kjellstrom and Growe [77] reported impact of climate change on rising

heat stress exposure levels imposing severe health suppressions among work-force in near future and suggested utmost need for remedial solutions based on protecting workers health and productivity. Lee et al. [78] examined the negative impacts of climate change related heat stress on labor work-productivity in South Korea and concluded futuristic decline in labor productivity during middle of 21st century (2041–2070) and consequent outdoor work productivity decline of 26.1% from current scenario by the end of 21st century. Pogacar et al. [79] analysed the climate change projections based on mean air temperature, maximum air temperature (in terms of number of days) and selected thermal indices over past decades; indicating a positive trend in terms of heat stress during the summer season. Case study conducted among 400 manufacturing workers revealed stressful work conditions (with 96% workers reporting uncomfortable work environment) directly impacting their health and work-productivity. According to the International Labour Organisation (ILO) 2019 report, by 2030 heat stress will be accountable to loss of approximately 80 million Jobs around the world in which India will be worst hit expecting to loss of around 34 million full-time jobs, which will lead to large scale economic and productivity losses [80]. Negative Health Impacts on Workers In developing countries, the relationship between occupational high-heat exposure and its possible health impacts has not been clearly well established. Studies related to this occupational hazard are limited because of several challenges and constraints in developing countries. Prevalent limitations necessitates the urgency for appropriate

Figure 4. Possible negative impacts on workers’ health and safety.

assessment studies; for analysing the existing conditions more effectively and provide valid conclusions based on improving the worker’s social wellbeing. During summer season, lot of indigent labourers are influenced by extreme high-heat work ambience, which results in severe health risks followed by reduced productivity and economic burden [1, 5–8]. Krishnamurthy et al. [6] evaluated the negative impacts of heat stress on the health and productivity of workers working under high heat industries in southern India and determined the dehydration status of the worker by interpreting urine color with a urine color chart. Around 90% of WBGT index values exceeded the TLV limits and 70% of workers reported change in urine color and volume indicating dehydration/lack of periodic fluid consumption. Varghese et al. [81] suggested a positive association among the hot climatic conditions and heat related occupational injuries, with possible negative impacts involving fatigue, psychomotor performance loss, reduced concentration, and lack of alertness. Xiaodong et al. [82] identified that the period from 14:00 to 15:00 PM as the most hazardous for workers throughout the day and impose high heat stress on the human body. Also, Nunfamioi et al. [83] observed that inadequate prevention and control policies, adversely affect workers’ health and safety as well as their productive capacity and social well-being. Venugopal et al. [68] reported that apart from occupational heat-related morbidities, the issues of damages at sub-cellular level (DNA damage) is prevalent among workers exposed to high working temperatures. Higher risk of DNA damage was observed among the exposed workers as compared to the control group; with significant increase observed for the micro nuclei (MN) frequency in lymphocytes of 120 steel plant workers (exposed group) as compared to unexposed workers (control group). Among exposed group, higher risk was associated with workers engaged in high-heat work activities (also having significant relationship with years of exposure); leading to higher induction of MN-frequency. Jafari et al. [30] analysed the effect of occupational heat stress on the immunological parameters of foundry workers in Iran. For higher WBGT index values, there was prominent decrease in the white blood cell counts and lymphocyte levels among the exposed group with relative increase in neutrophils levels and the neutrophil – lymphocyte ratio, which negatively affects and weakens the workers’ immune system. Figure 4 summarizes the possible negative impacts of occupational heat stress on workers’ well-being. Role of Control Interventions in Ameliorating Heat Stress The role of design control interventions could be considered beneficial in improving the thermal ambience of a work environment, which may include engineering control interventions like improving ventilation design, installing reflective thermal barriers, sensor based intelligence and even simulation based control studies. These design control

interventions may be taken as an effective and important control measure in reducing the exposure levels upto the desired permissible limits [84-88]. One study utilized engineering control interventions (air-cooler design) to improve the workplace condition among operators working in evaporator assembly area of an electronics company. Study aimed at improving the work productivity based on reducing the percentage of dissatisfaction (PPD) among operators [9]. HSI & WBGT measurements were considered at 11 work locations, where PPD value were measured before and after to validate the design intervention using CBE thermal comfort tool. The results indicated significant decrease in the heat stress measurements. Sugiono et al. [67] utilized computational fluid dynamics (CFD) simulation (in ANSYS Fluent) to evaluate the existing thermal comfort level among workers employed in a plastics manufacturing plant and suggested CAD based design modification (by replacing existing ceiling glass material with reflective clear glass; having 6 mm thickness and 23% transmittance). Proposed modification indicated thermal comfort improvements among workers based on predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) evaluations (where ambient temperature decreased by 4oC, PMV decreased by 0.64 point and PPD showed 13.8% increase in thermal comfort). Giahia et al. [18] study aims to design and implement radiant heat controls, using a heat absorbing system (i.e. cooling tower with water circulation) in the furnace body and installing steel framed structure (with multi-layered reflective aluminum covering) in the workstation followed by analyzing the efficiency of the employed interventions using valid and standardized indices of radiant heat. Heat stress indices and core body temperature were measured before and after to validate the design interventions. Engineering control interventions were proved to be effective in reducing radiant heat by decreasing the mean radiant temperature (MRT) and WBGT values by 26.5 oC and 5.2 oC respectively, whereas workers’ core body temperature decreased by 2.6 oC. Mohammadyan et al. [89] implemented a designed cool spot with double layer insulation in order to reduce the WBGT and MRT levels among workers in a foundry industry. The implemented design intervention resulted in reducing the WBGT value (from 29.6 oC to

Methodology

“CFD Simulation Approach” Kamar et al. [90] Non-air conditioned Mosque building; Malaysia

In present study, authors used CFD based design strategy for improving the thermal comfort inside a large spaced mosque building. Field measurement followed by 3D-CAD model of Mosque building was analysed using CFD approach for different suggested design models.

Author proposed four suitable design modifications: Installing Exhaust fans (with 1-m diameter) on Mosque building walls. Four cases of suitable combination were considered: Case 1 - Roof-12 fans,

Installing ten exhaust fans (1-m diameter) at the south-side wall, at a floor height of 6 metres (m) has a potential of reducing the PMV index by 75–95% and the PPD index by 87–91%.

Case 2 - west-side wall-12 fans, Case 3 - east-side wall-12 fans, Case 4 - south-side wall-10 fans

Present study aimed at enhancing the thermal work conditions inside the Railway Pantry Car Kitchen. Onsite field measurements followed by analysing proposed 3D-CAD based design modifications using CFD approach in ANSYS Fluent.

Author proposed four design modifications for pantry kitchen: Case 1–4 Exhaust fans (front wall), 2 Carriage fans (roof) Case 2–4 Exhaust fans (front wall), 2 Carriage fans (left and right side wall) Case 3–4 Exhaust fans (front wall), 4 air vents (lower front wall)

CFD results revealed that case-I design model provided a better design concept by improving air ventilation and decreasing the indoor ambient temperature during all cooking periods; as compared to the existing and other case models.

Case 4–4 Exhaust fans (front wall), 3 Carriage fans (bottom surface) “Engineering Control Interventions” Mohammadyan et al. [89]

To reduce heat stress exposure levels among workers employed in a foundry industry.

Implemented design intervention resulted in reduction of the WBGT from 29.6oC to 22.8oC, and MRT decreased from 43.8oC to 28.6oC; thereby reducing the heat stress exposure among workers.

To control heat stress exposure level among workers in foundry unit of an Iranian steel plant. WBGT measurements were performed before and after implementing control plans.

Author proposed installing double layered reflective protective shields to reduce the radiant heat exposures among workers.

Experimental results revealed reduction in radiant temperature from 44.04°C to 35.8°C and decrease in WBGT value from 28.88°C to 26.57°C

To reduce radiant heat exposure generating from a blast furnace utilizing engineering control interventions followed by analysing the efficacy of the control interventions using heat stress indices and workers’ core body temperature (CBT).

Implemented a heat absorbing system (cooling tower with water circulation) in the furnace body and installing steel framed structure (with multi-layered reflective Aluminium covering) in the workstation.

Using both interventions, the MRT and WBGT value decreased by 26.5oC and 5.2oC; while the workers CBT reduced by 2.6oC.

Methodology

Electronic manufacturing plant; Indonesia 11 industrial workers

Intervention To analyse and improve the effect of environmental conditions on workers thermal comfort by reducing the predicted percentage of dissatisfied (PPD) among employed users.

Proposed ventilation design using direct evaporative air cooler as an engineering control to improve the work place condition

After implementing the proposed engineering control intervention, PPD value predicted by CBE thermal comfort tool reduced to 34% and also HSI reduced to

Conclusion

From the present literature review, it may be concluded that heat stress is an ignored occupational health hazard significantly affecting the workers’ well-being; particularly under high-heat furnace work-sectors in developing countries. The combination of several factors (like air temperature, radiant heat, air velocity, and relative humidity) surrounding the furnace workplace (as well as the metabolic heat and worn clothing) negatively impacts the workers’ health; which declines their work efficiency and in turns affect the production capacity. ISO standards (ISO7243, ISO9886, and ISO7933) could provide more in-depth analysis of heat stress parameters under hot and humid work environments, when used in conjunction with the supporting standards (ISO9920 and ISO8996). Several indices have been developed for the assessment of heat stress exposure, but each varies depending on the considered environmental, personal, and physiological factors. Also, these indices may differ based on their suitability with respect to the particular work-conditions and different geographical locations. Previous studies reveals that WBGT index may be used as an optimal heat stress index, due to its applicability and ease of use in hot work environments. But, merely relying on a single index could generate inappropriate results. So, it must be used as an initial screening method followed by evaluating the physiological parameters (several valid indicators of thermal strain like heart rate, core body temperature, and skin temperature) based on the recommended standard guidelines and also in combination with other widely used heat stress indices like DI, PSI, HSI, TSI, TWL; which would provide better estimation of the thermal strain experienced by the worker. In developing countries, very few studies have been reported considering the combined effect of climatic conditions and industrial heat exposure on the thermal strain experienced by the worker. Several negative impacts include decreased productivity, reduced daily incomes, and consequent health risks. This chronic heat stress significantly affects the immunological parameters also; resulting in damages at the sub-cellular level (DNA damage), decrease in white blood cell counts and lymphocyte levels; which negatively impacts and weakens the workers’ immune system. Apart from this, several other health issues like dehydration, exhaustion, heat stroke, and risk of renal/ urologic anomalies (i.e. kidney stones, urethral calculi, and other structural renal anomalies) are attributable to this occupational heat stress. However, several studies reported that these heat-related morbidities could be reduced by

increased awareness for associated risks involved and the consequent financial benefits (with averting injury, poor health outcomes and lost productivity). Adequate prevention and control policies are necessary to ameliorate the productive capacity and social well-being of the exposed workers. Remedial control measures like sensor based intelligence, proper ventilation design, installing thermal reflective barriers, providing cooling spots, cooling vest-design, and other radiant control measures may prove to be effective in controlling the workplace heat stress exposure. This review study may be beneficial for industrialists, occupational health practitioners, and policy makers (particularly in developing countries) in overviewing the negative impacts of prolonged heat exposure, necessary remedial measures and suitable assessment strategies for monitoring this occupational health hazard under the prevalent high heat work-conditions.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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