The impact of indoor air temperature on the executive functions of human brain and the physiological responses of body

Introduction

The office indoor environment quality plays an important role in the health, comfort, welfare and performance of office workers.¹ This dynamic interaction between workers and the environment, as a key issue in almost all work settings, causes physiological and psychological responses among workers, hence influencing their comfort, efficiency, productivity, safety and health.²

The impact of environmental factors on human performance has been studied over the past decades. Researchers first focused more on manual work, while recent studies have mainly focused on the office work which puts a lot of burden on workers’ mental capacity. Various researches have shown that the indoor environment significantly affects human performance. Therefore, modifying the indoor environments is a very important factor in studying the human performance. The indoor environment includes several factors such as temperature, lighting, noise, etc.³

Exposure to different air temperatures (temperature variation), the exposure time and differences in participants’ compliance with heat and nutrition are among the factors that may affect human performance.⁴ Therefore, it is highly difficult to assess the relationship between exposure to heat and cognitive function.⁴ Researchers generally agree that very high and very low air temperatures can affect the human performance.^2,5,6 Other studies have confirmed the impact of air temperature on human performance.^3,6-13 Obviously, human body warns that there is no heat balance between the body and the environment when it feels hot or cold. Such imbalance can in turn effect human’s health, performance and comfort.¹⁴

Studies have also shown that, among environmental factors, air temperature and relative humidity have a greater impact on humans because high environmental air temperature results in a great deal of strain on the cardiovascular system to maintain body temperature.¹⁵

Recent studies have also reported that the increase in air temperature (from 22.8°C to 30.6°C) reduces the performance by 5% due to thermal stress.¹⁶ Some other studies have shown that performance decreases by 2% due to a 1ºC increase in the air temperature.^17,18

Some studies have shown that air temperature exercises various effects (e.g. physiological and cognitive effects) in different work environments.¹⁹ Some studies have demonstrated that air temperature can affect the heart rate variability (HRV).^7,20,21

The executive functions of the brain can be considered as the cognitive flexibility and the management of intervening elements in goal-oriented behaviors and prediction of the consequences of a function,²² which may be affected by exposure to air temperature and negatively affect the cognitive function of the individual.

Valuable information has been published regarding exposure to heat and cognitive function over the past two decades, but there is little empirical evidence for further understanding the issue.⁴ Considering the importance of air temperature as a key environmental factor affecting the indoor environments of offices and industrial control rooms and given that office workers spend 90% of their work time in indoor environments,¹ the imbalance between temperature level and human task can have the dire consequences, in terms of thermal comfort and performance at work. The difference between this study and other studies is that carrying out the task of working memory and physiological indices are measured at the same time during exposure to different temperatures, which can be reflected in the actual changes in these indices. Therefore, the present study aimed to investigate the effect of indoor air temperature on the executive functions of the brain via the working memory test (at three cognitive levels) and on the physiological responses in the office work station.

Materials and Methods

Participants

In this study, sample size was computed by formula down:

Using similar studies and for α = 5% and β = 95% sample size by rate of abscission 20% was estimated 35 participant male students of Hamadan University of Medical Sciences with age range between 20 to 30 years (mean age 22.96 [SD: 2.96] years) were selected (pre-exposure and post-exposure) as the sample for the study under laboratory conditions. First of all, the subjects were screened for visual and hearing health. The visual test was carried out by E chart.²³ On the other hand, the hearing health, was conducted by an audiometer (MEVOX ASB15). Upon interviewing the participants, those with a history of neuropsychiatric diseases and the use of drugs that might affect the nervous system were excluded from the study (through interviews with participants). They were also asked to have enough sleep on the night before the test, not to use caffeine and any other stimulant for 12 hours before the study,²⁴ and not to smoke. Only male volunteers were selected in this study to control the gender effects. All of the participants were paid for the time they allocated to the study so that they could have further motivation. After the sample members were selected, informed consent for participation in the study was obtained from them and they were assured they could withdraw from the study at any stage without consequences.

Experimental Procedure

This study was carried out in an air-conditioning chamber with the dimensions of (L × W × H = 3.70 × 2.40 × 2.70 m). The chamber’s walls were covered with brightly colored polyurethane foam (it is an excellent sound absorber), so that it could be psychologically suitable.²⁵ The chamber’s floor was covered with cream-colored carpet. There were in this chamber a desk, a chair and a computer which had been ergonomically designed²⁶ like the workroom of an office operator (Figure 1), so that the distance from the monitor to the operator was one meter. Moreover, the minimum lighting in the chamber was at least 300 lux,²⁷ supplied by two LEDs.

Figure 1. Experimental setup in Air conditioning chamber.

The subjects were asked to specify the exact date and time they were ready to conduct the tests, which were performed in four separate sessions. Each person completed all tests in 50 minutes (each day with a constant temperature) and at air temperatures of 18°C, 22°C, 26°C and 30°C. The air temperature of 18 °C is cool and the temperature of 22°C is natural, while 26°C is a little higher than natural. The temperature of 30°C is not unrealistically high for naturally ventilated office environments under summer conditions. The ambient air temperature was checked every ten minutes around the body to resolve any possible difference. The background noise of the chamber was 55 dB (A) and had a low frequency associated with the air-conditioning fans in the chamber (similar to the natural noise in the offices). Before the tests, the participants were trained in two sessions on the test procedure and how to work with the N-back software.²⁸ All of the people were working while sitting and the order of doing the job was from low to high workload. The participants in this study were required to wear ordinary clothes such as shirts, pants, underwear, socks and shoes. The air temperature was also under controlled conditions (in different stages of the experiment) with the constant relative humidity of 50% in all conditions and the air flow rate of 0.15 m/s. Each task lasted for 5 minutes and each stimulus was displayed for 2500 ms. There were 120 stimuli at all levels of task executed during exposure to different air temperatures (Figure 2).

Figure 2. Different stages of measuring physiological indices and mental performance.

When performing each task, the subjects were given a break while sitting on the chair so that they could prepare themselves for the next stage (in order to reduce the effect of fatigue). In order to prevent defects in measurements, the subjects were asked to have a minimum movement and focus on the task. After installation of electrocardiogram (ECG) electrodes and tying the respiration belt around the subjects’ waists, the signals were measured before, during and after the execution of the N-back task for 5 minutes with open eyes.

Results

Discussion

The results of this study showed that air temperature affects the executive functions of the brain and the physiological indices. The study also suggests that participants had better performance (accuracy) in 22°C compared to 30°C and 18°C. In other words, the air temperature was more pleasant in the neutral air temperature range (22°C) and the participants had a good thermal comfort at these air temperatures, so their performance was less affected by the air temperature.

The results of previous studies have shown that performance is less affected at neutral to slightly warm temperatures,^35‚36 which is consistent with the results of this study. Studies have also revealed that hot environments are more risky than cold environments; therefore, it is recommended that the ambient air temperature conditions range should before slightly cold to neutral air temperatures,³ which is consistent with the results of the study. Seppänen and colleagues’⁶ studies on the work in various air temperature showed that the highest efficiency was associated with neutral air temperature, which is consistent with the results of this study. Many studies have shown that an increase of 1°C leads to a decrease in performance by 2%, and no change in performance has been observed in the moderate air temperature range. This has also been corroborated by Tanabe et al.¹⁸

The results of this study showed that with an increase of 1°C ambient temperature compared to comfort temperature (22°C), the accuracy of response performance in terms of low workload, medium workload and high workload declined by 1.39%, 1.45% and 1.31% respectively. On the other hand, the results of this study showed that with a reduction of 1°C in ambient temperature compared to comfort temperature (22°C), the accuracy of responses performance in terms of low workload, medium workload and high workload declined by 0.54%, 1.97% and 2.57% respectively. These results indicate that workplace authorities should concentrate on employee safety, as well as the efficiency and quality of their work, especially in control rooms, offices and other similar environments.

The results of this study were not consistent with the findings of Fang et al³⁶ and Kahl.³⁷ Perhaps this difference is caused by laboratory conditions, gender or even the kind of task that participants performed while being exposed to various temperature. Another main reason for this contradiction may be the participants’ gender. In our study, all participants were male, while in Fang et al study, all of them were female, and in Kahl’s research, 140 of the participants were female and only 36 were male. The results showed that, on average, subjects decreased their response times in the low and high temperature (18°C and 30°C) compared the other temperature conditions. A shorter response time corresponds to a greater stress from the subject, according to the arousal theory.³⁷

On the other hand, the human body reacts to environmental stimuli and there is a continuous and dynamic interaction between the human body and the environment, which creates a psychological and physiological strain.² A study conducted by Yao et al²⁰ showed that HRV has a close relationship with thermal comfort. Our study also displayed that the air temperature affects the mean of HR indicators, LF/HF, SDNN, RMMSD and respiration rates in performance conditions at different levels. As a result, there was a significant difference between the mean HR changes, with the highest level of the obtained significance being observed in air temperatures of 30°C and 22°C at all levels of task execution. Liu et al²¹ concluded that the HR changes at high air temperatures was higher than that in moderate air temperatures, which is consistent with the findings of this study.

This study also showed that there were greater changes in LF/HR ratio of HRs when participants were exposed to undesirable air temperatures (high and low air temperatures) than when they were exposed to moderate air temperatures. This may be due to heat feeling discomfort at these air temperatures. This finding is consistent with the results of previous studies.^7,20,38 The present study also showed that the changes in the mean SDNN and RMSSD in exposure to unsuitable conditions (30°C and18°C) were greater than those in the exposure to 22°C. In this study, the changes in the mean RSP were higher in 30°C than in 22°C, so that there was a significant difference between the changes in the RSP in 30°C as compared to the changes in 22°C, which is consistent with the results of the study conducted by Liu et al.²¹ In general, changes observed in physiological parameters with increasing and decreasing air temperature relative to the thermal comfort temperature (22°C) indicate that warm temperatures and cold temperatures have imposed stress on the participants.

Conclusion

It can be concluded from the laboratory assessment of the effect of air temperature on the executive functions of the brain and the physiological parameters that:

Brain executive functions in high and low air temperatures are more influenced by the air temperature than the moderate air temperature, which may reduce accuracy and increase the error in sensitive work environments that required more attention.

High and low air temperatures significantly increased the participants’ HR, LF/HF and respiratory rates, which can have a represents stress and high mental fatigue and negative long-term impact on their health.

Undesirable high and low air temperatures caused significant changes in the SDNN and RMSSD indices, which have not been uniformly changed at different air temperatures.

In moderate air temperatures (22°C), the LF/HF ratio, which represents the sympathetic and parasympathetic equilibrium to the vagal one, is approximately close to one, indicating that, in this temperature, the participants had a better thermal comfort, so that they had a good performance (accuracy).

Ethical approval

This study was approved by the Research Ethics Committee of Hamadan University of Medical Sciences (grant No. 9510146069). All participants signed the consent form and they were ensured of the confidentiality of data collection prior to the study.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

MM and MA conceptualized the study, designed data collection methodology, and led manuscript development. AMA collected all the data and wrote the first draft of the manuscript. RG contributed to the writing of the manuscript. LT conducted data analyses.

Acknowledgments

We greatly appreciate the students of Hamadan University of Medical Sciences for participation in this project. This paper has been extracted from a Ph.D. dissertation at Hamadan University of Medical Sciences.

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