Evaluation of the effect of heat stress on cognitive performance and physiological parameters of the students

heat stress is the most common occupational risk factors and can adversely affect the health and efficiency of the individuals. Exposure to extreme heat in the workplace can cause heat-related illnesses as well as increased risk of accidents and related injuries. The previous Studies have shown that heat stress can lead to some physiological responses and affect cognitive function. But these studies have not reached a clear and definitive conclusion. There have been several studies on the effect of heat stress on heart rate and blood pressure. According to the results of the most studies, heat exposure can increases heart rate, but the results of the researches on the effect of heat exposure on blood pressure are not the same. According to the results of the most studies, heat stress can leads to decrease in cognitive performance; however, some studies have shown that cognitive performance does not change or in some cases increase under heat stress conditions. Due to the differences in the results of the previous studies in this area, further studies are needed to clarify these results. The purpose of this study was to investigate the effect of heat stress on cognitive performance, heart rate and blood pressure of the students. 

In this experimental study, 18 volunteer students (9 girls and 9 boys) from the school of health and safety at the Shahid Beheshti University of Medical Sciences with the mean age of 21.11 year and standard deviation of 1.71 year and body mass index of 23.23 kilogram per square meter and standard deviation of 1.44 kilogram per square meter participated. This study was conducted in the laboratory of heat and humidity control. Each participant was randomly exposed to one of the test conditions with 3 different Wet Bulb Globe Temperature. According to the factorial method, there were six different Wet Bulb Globe Temperature conditions in which the subjects were tested in one of these six states. A total of 54 experiments were performed on the subjects. Before the test, all the participants received written consent and they all became familiar with the stages of the experiment. Prior to the test, the participants completed a questionnaire of the inclusion. The criteria for entering the study included age 20-24, undergraduate education, body mass index (BMI) of 23±2 kg/m2, no cardiovascular and mental illnesses, normal heart rate and blood pressure and no drug consumption. In order to examine the mental health of the individuals, they were asked to complete the General Health Questionnaire, developed by Goldberg (1972), before the test. Those who obtained the total score of 0 to 22 were classified in the group without any symptoms or least limit and included in the study. The participants were asked to have sufficient sleep and rest and a normal diet, without coffee or caffeine, before the test. The subjects were also asked to wear simple linen or cotton clothes with light colors to take into account the effect of Clo on heat stress for all the participants. Unwillingness to continue to cooperate during the test was also considered as an exclusion criterion of the study. After selecting the samples based on inclusion criteria, other interfering factors, including noise and lighting were evaluated and controlled. The lighting equal to 450 lux and noise pressure level equal to 35dB were recorded and controlled.Participants were exposed to three different thermal conditions (Wet Bulb Globe Temperature of 29°C as lower than the exposure limit, Wet Bulb Globe Temperature of 32°C as the exposure limit for the non-acclimatized people to warm environments, and Wet Bulb Globe Temperature of 35°C as more than the exposure limit for the non-acclimatized people) for 30 minutes, and their cognitive performance was evaluated in these three thermal conditions. The relative humidity in all stages of the study was 50%. To provide different temperature conditions, two EFHA2200 model electrical fan heaters, manufactured by ARASTEH, as well as two Beurer humidifiers were used. To measure the relative humidity, the GM8910 humidifier (BENETECH Company, China) was used with a precision of ± %5, and to measure the WBGT index, the digital QUESTEMP10 WBGT meter was used with a sensitivity of about one tenth degree Celsius. All the devices were calibrated before the test. To measure the WBGT index, the device was placed at a distance of about 10 cm from the body of the participants in the waist area (0.7 m), and after about a minute, the numbers shown by the device as a WBGT indicator were read.To assess the effect of heat stress on cognitive performance of the participants, a 2018 Integrated Visual and Auditory Continuous Performance test was used and the scores of attention scales including selective attention, focused attention, divided attention, alternating attention and sustained attention were measured in both visual and auditory domains. The IVA test is a computerized test for sustained attention and response control. The participants were exposed to pseudo-randomly ordered series visual and auditory stimuli of numbers 1 and 2, and they were asked to click the mouse when they saw or heard the number 1. With an initial explanation of how to do the test, the entire test lasted 11 minutes. The IVA results were presented in six primary scales (three response control scales and three scales of attention) in the visual and auditory areas. The response control scales are as follows: (Prudence, Consistency and Stamina). The attention scales are as follows: (Vigilance, Focus and Speed). In order to investigate the effect of heat stress on physiological parameters, blood pressure and heart rate of the individuals was also measured before and after exposure to heat. To Measure the blood pressure and heart rate of the subjects, digital Blood Pressure Monitor was used. Its accuracy for measuring blood pressure was ±3 mmHg and for measuring heart rate was ±%5. SPSS software version 22 was used to analyze the results. To analyze the relationship between heat stress and heart rate, blood pressure and attention scales resulting from the Integrated Visual and Auditory Continuous Performance test, the Generalized Estimating Equations test (GEE) was used.

According to the results of this study, with increase in Wet Bulb Globe Temperature, the heart rate of the participants significantly increased in relative to pre-exposure conditions (79.72 beat per minutes with SD of 6.83 in Wet Bulb Globe Temperature of 22°C and 90.05 beat per minutes with SD of 8.71 in Wet Bulb Globe Temperature of 35°C, p<0.05). Also, with increase in Wet Bulb Globe Temperature, the systolic blood pressure of the participants significantly increased in relative to pre-exposure conditions (103.77 mmHg with SD of 11.62 in Wet Bulb Globe Temperature of 22°C and 109.00 mmHg with SD of 11.05 in Wet Bulb Globe Temperature of 35°C, p<0.05). With increase in Wet Bulb Globe Temperature, their diastolic blood pressure significantly decreased in relative to basic conditions (72.16 mmHg with SD of 4.84 in Wet Bulb Globe Temperature of 22°C and  64.94 mmHg with SD of 5.94in Wet Bulb Globe Temperature of 35°C, p<0.05). With increasing heat stress, the mean of six attention scales in visual domain including visual attention, visual selective attention, visual focused attention, visual divided attention, visual alternating attention, and visual sustained attention steadily decreased. However, this decrease was significant only for two scales including visual attention scale and visual alternating attention at 35°C (p<0.05). The relationship between heat stress and other attention scales in the visual domain was not significant (p>0.05). With increasing heat stress, the mean of two scales of in auditory domain including auditory selective attention and auditory alternating attention steadily decreased, and this decrease was significant at the Wet Bulb Globe Temperature of 35°C (p<0.05). The auditory attention scale was also significantly decreased at the Wet Bulb Globe Temperature of 35°C (p<0.05). No significant relationship was found between heat stress and other auditory attention scales (p> 0.05).

The results of this study showed that heat stress has a strong and significant relationship with physiological parameters such as heart rate and systolic and diastolic blood pressure. According to the results of this study, heat stress can increase the heart rate and systolic blood pressure and decrease diastolic blood pressure; so Changes in heart rate and blood pressure can act as a good indicator for the heat strain. Exposure to excessive high temperatures can reduce attention and consequently decrease cognitive performance. One of the limitations of this study was the difficulty of doing it in real work environment. Considering that in industrial and real working environment, many interfering factors such as risk factors of the work environment, family and psychological conditions of the subjects, nutrition, different work shifts and time of sleep and rest can affect the results of the study, it will be difficult to select similar samples that meet all of the requirements of this study. However, studies similar to the present study need to be carried out in industrial environments and real work conditions in order to increase the applicability of the results of study in work environments.