Abstract
Introduction
Hot environment is common in several workplaces. Workers exposed to hot environments are at high risk of various heat-related disorders and safety problems [1–5]. In general, humans adapt to repeated exposure to heat stress to maintain body temperature, reduce physiological strain, and improve comfort and/or work capabilities, as well as reduce susceptibility to thermal injury [6]. Humans have noticeable capability to adapt to heat stress, and given adequate water and protection from the direct sunrays, healthy acclimatized persons can tolerate extended exposure to virtually any natural weather-related heat stress [7]. Heat stress results from the interaction of environmental conditions (temperature, humidity, and direct sunrays), physical workload (body heat production), and worn gear (clothing, equipment, and shoes) that impedes heat loss [8]. Environmental heat stress and exercise interact synergistically to increase strain on physiological systems [9]. This strain manifests itself in the form of high core and skin temperatures, excessive cardiovascular stress, and reduced performance. Heat acclimatization generates biological adaptations that reduce these adverse effects of heat stress. Individuals become acclimatized to heat through repeated exposures that are sufficiently stressful to elevate both core and skin temperatures, and result in significant perspiration. These biological adaptations are a result of integrated changes in thermoregulatory control, fluid balance, and cardiovascular responses [10].
Heat acclimatization is defined as repeated heat exposure of the body for few days. According to previous studies, about 1 to 2 weeks of daily heat exposure is needed to gain adaptation that reduces physiological strain to heart rate, body temperature, and oxygen consumption, and helps to improve physical work capabilities under a hot environment [11, 12]. The objective of this study was to estimate the effects of heat stress and the amount of acclimatization reported in hot climate countries. This study was approved by the Human Participants Review Sub-committee of the Institutional Review Board of King Saud University.
Methodology
Participants
Eighteen male workers from an industrial population participated in this experiment. Their mean age was 26.23 ± 4.1 years, mean weight was 72.56 ± 5.76 kg, and mean stature was 171.12 ± 4.74 cm. None of the participants had previously experienced any musculoskeletal disorders or any cardiac symptoms, or been receiving prescriptions of any medication that might have altered their heat intolerance. All the participants were not trained in hot environments prior to the tests; therefore, they were considered to be heat-non-acclimatized participants [13, 14]. All the participants were compensated for their time.
Experimental design
A one-way repeated-measures analysis of variance (ANOVA) design, with one independent variable and two dependent variables, was utilized in this study. Nine days of exposure to a hot environment (wet-bulb globe temperature [WBGT], 30°C) was the independent variable. The participants’ cardiac costs (CC) and increment aural-canal temperatures were the dependent variables.
Experimental procedures
After medical screening, each participant was informed about the purpose and procedures of the experiment, and then signed a consent form prior to the experiment. Afterward, the participant’s anthropometric dimensions (stature, acromion height, standing iliac crest height, knuckle height, knee height, forearm grip distance, chest depth, chest width, and abdominal depth) and weight were measured. Finally, a participation schedule was established. All the participants were requested not to participate in any other physical activities that might cause tiredness before execution of the experiment. All the experimental sessions were conducted inside a heat chamber installed in the Ergonomics Laboratory of the Department of Industrial Engineering, King Saud University. The environmental condition inside the laboratory was a room temperature of 22–25°C and relative humidity of 50–55%. Sessions for all the participants were conducted at the same hour of the day between 8 : 00 a.m. and 17 : 00 p.m.
Each participant performed a 100-minute-long training and acclimatization session daily for 10 consecutive days [12, 13]. On the first day, each participant was trained in a moderate environment with a WBGT of 20°C in order to familiarize with the experimental protocol before being exposed to the 30°C WBGT. For the next nine sessions, each participant was exposed to 30°C WBGT. Heat acclimatization sessions were started from the second to the tenth day, and data recorded for these 10 days were used for the analysis.
At the beginning of each session, the participant was asked to rest on a chair outside the heat chamber for 3 minutes to remove any physical fatigue. Resting heart rate and aural canal temperature data were collected prior to entering the heat chamber. Stability in resting heart rate and resting aural-canal temperature was the only condition required before going inside the heat chamber.
After 60 minutes of each session, each participant was asked to perform stretching exercises for about 40 minutes. The stretching exercises included standing floor touch, sitting toe touch, alternating toe touch, spinal stretch, and back rotation. In standing floor touch, the participant tried to touch the floor by bending at the waist while keeping the knees straight. In sitting toe touch, the participant sat on the floor with feet spread and touched one foot and then the other. In alternating toe touch, the participant stood with the feet apart, bending at the waist and touching the right toe with the left hand, and then stood erect before bending again and touching the left toe with the right hand. In the spinal stretch, the participant placed both hands and knees on the floor, raised the back, and then bent the elbows and lowered the chest toward the floor. The average of the last 5 minutes of heart rates and aural-canal temperatures were considered as working heart rates and aural-canal temperatures.
Measured responses and used equipment
Heat chamber
All the acclimatization and experimental sessions were performed inside a self-controlled heat chamber (3.2 × 2.2 × 2.2 m; Cliphyco Company, China). In addition, a heat stress WBGT meter (HT30, Extech Instruments, USA) was used to ensure WBGT consistency at the corners of the heat chamber. Two room conditions were considered in this study (20°C and 30°C WBGT). These two temperatures were used in previous studies and served as guidelines to represent a comfortable hot environment [14–16].
Anthropometric measures
A SiberHegner GPM Anthropological Instrument (DKSH Switzerland Ltd., Zurich, Switzerland) with an adjustable stool was used to measure the participants’ anthropometric dimensions. This instrument consists of the following: fixed anthropometer (0–2100 mm with straight probes and curved measuring branches), sliding caliper (length of 0–200 mm with a depth of 0–50 mm), spreading caliper with rounded ends (0–600 mm), fiberglass tape (Dean, 0–1500 mm), and balance scale (Seca 708, 0.1–200 ± 0.1 kg).
Heart rate
Electrocardiography (ECG) signals were recorded by using an ECG monitor (CASSY Laboratory, EKG box No. 524049 and Sensor-CASSY No. 524010, Leybold Didactic Gmbh, Germany). The instrument was calibrated according to the manufacturer’s instructions. Cardiac cost (CC), which is the difference between heart rate (beats/min) at work and heart rate (beats/min) at rest, was calculated and considered for statistical analysis in this study [17].
Aural canal temperature
Aural canal temperature was recorded by using a Braun ear thermometer (ITR 4520, Braun Company, Germany). Increment in working body temperature (the difference between the working and resting aural-canal temperatures) was calculated and considered for statistical analysis in this study [18].
Blood pressure
Blood pressure was measured by using an Omron MX2 Digital Automatic upper arm blood pressure monitor, Japan. This measure was not used in the statistical analysis; it was used only to monitor for any adverse conditions in the participants’ health that might have arisen during experiment execution.
Experimental variables
The independent variable in this study was time (number of days for acclimatization). The dependent variables were CC and incremental aural-canal temperature. Cardiac cost (CC) was calculated as the difference between working and resting heart rates (beats/min) measured during days of progress in the hot environment (WBGT, 30°C). Incremental aural-canal temperature was calculated as the difference between the working and resting aural-canal temperatures measured during days of progress in the hot environment (WBGT, 30°C). Changes in CC and incremental aural-canal temperature were considered indicators of the heat acclimatization gained [18]. Heat acclimatization gained when the physiological measures during days of progress in the hot environment (WBGT, 30°C) was significantly stabilized. The physiological responses in this study were heart rate and aural-canal temperature expressed in CC and incremental aural-canal temperatures.
Data analyses
The Statistical Package for the Social Sciences software (SPSS) version 22 (www.spss.com) was used for the statistical analysis (one-way repeated-measures ANOVA test). The Tukey test was used to identify which levels of the independent variable have significant effect on the dependent variables. The Shapiro-Wilk test was implemented to test data normality [19]. The statistical significance was set at a confidence level of 95%.
Results
Incremental aural-canal temperatures
The results showed that time (days) has a significant effect on the participants’ incremental aural-canal temperature (°C) as presented in Table 1. Increasing the room temperature had a significant effect on the participants’ incremental aural-canal temperatures, as the participants’ aural-canal temperatures increased significantly starting on the first day of exposure to heat when compared with the participants’ aural-canal temperatures at room temperature (WBGT, 20°C), as shown in Table 1 and Fig. 1. Then, a significant trend of decreasing incremental aural-canal temperatures until the sixth day of heat exposure was observed. The participants’ incremental aural-canal temperatures of the last 3 days were considered stable.
Cardiac costs
Results showed that time (days) has a significant effect on the participants’ cardiac cost (beats/min), as presented in Table 1. Increasing the room temperature had a significant effect on the participants’ CC, as the participants’ CC increased significantly starting on the first day of exposure to the heat when compared with the CC computed at room temperature (WBGT, 20°C), as shown in Table 1 and Fig. 2. Then, a significant trend of decrease in CC until the fourth day of exposure to the heat was observed. The participants’ CC in the last 5 days was considered stable.
Discussion and conclusions
The participants improved their acclimatization significantly after the fourth and fifth day of exposure to the heat sessions, based on their physiological responses (i.e., CC and incremental aural-canal temperatures). We suggest that faster acclimatization occurred in this study because of the hot climate of the country where the study participants lived and worked. Evidence of faster acclimatization of working populations similar to the present study cohort was found.
Over the course of the research investigation, some limitations were identified. Therefore, the results of this research are limited to young healthy male adult workers who lived and worked in countries with the same weather conditions as Saudi Arabia. The limitations of this study emphasized the significance of future research that would investigate the reliability of extending the results of this study to different worker populations who live and work in countries with the same weather conditions as Saudi Arabia. Examples of these worker populations include different sex or age groups, and workers who are not from relatively hot countries but moved to work in a hot country.
The results of this study were difficult to compare to the results of the studies of Ramadan [14], and Hafez and Ayoub [15] because of the differences in study protocols and participant population. Finally, this research presented useful information regarding heat acclimatization characteristics of a specific worker group in hot climate countries. This information was not available in the literature.
Conflict of interest
The authors have no conflict of interest to report.
Footnotes
Acknowledgments
The authors acknowledge the research center of the College of Engineering at King Saud University for its support. In addition, the authors acknowledge the reviewers for their valuable comments and recommendations that contributed significantly to the quality of this article.