Effect of pre-and post-exam stress levels on thermal sensation of students

Abstract

The Predicted Mean Vote and Predicted Percentage of Dissatisfied (PMV/PPD) method is used worldwide to assess thermal comfort. The PMV/PPD method traditionally depends on four environmental parameters; air temperature, relative humidity, mean radiant temperature and air velocity, and two personal parameters; metabolic rate and clothing insulation. However, accurate modelling of thermal comfort requires consideration of psychological impacts, as well as associated physical responses to the environment. This paper investigates the effect of one of the psychological parameters; stress level on the thermal sensation of students for male and female which can be a sufficient limitation of the accuracy of thermal comfort/sensation models. Actual Thermal Sensation (ATS) and Profile of Mood States (POMS) are used to examine the effect of stress level on the thermal sensation. Pre-test-Post-test Control (PPC) experimental design is conducted on the students in a university, Ankara, Turkey, which has a Csb type climate zone according to Köppen-Geiger climate classification. First, students are split into two random groups; control and experimental groups. The students in experimental group are requested to attend exam while the students in control group read their favourite books. Then, students are subjected to pre-and post-exam surveys in order to understand the relationship between stress level and ATS. As a supportive analysis, Heart Rate (HR) and Skin Temperature (ST) are also included in the study as sympathetic responses of occupants to the thermal discomfort due to stress. Smart wristbands and infrared thermometers are used to measure Heart Rate and Skin Temperature of the students. Results showed that there is a difference between control group and experimental group before the exam (pre-test) except the ST of females. After the exam (post-test), there are no significant differences between two groups.

Introduction

Thermal comfort analysis is mainly based on thermal balance of the human body with the environment as indicated in methods, such as Fanger’s Predicted Mean Vote (PMV) -Percentage of Predicted Dissatisfied (PPD) model [1], and adaptive thermal comfort approach [2]. In conditioned buildings, Fanger’s method includes four environmental variables such as air temperature, relative humidity, mean radiant temperature and air velocity, and personal parameters including clothing value and activity level as shown in Fig. 1 [1]. In naturally ventilated buildings, psychological adaptation, which implies changes in the physiological responses resulting from repeated exposure, is taken into account in the adaptive thermal comfort model [3], [4]. However, there are still discrepancies on both thermal comfort methods for obtaining thermal sensation of occupants in conditioned and unconditioned buildings [5]. Although thermal comfort/sensation is generally considered to be a physiological phenomenon, psychological processes such as stress level, feelings and expectations modify the actual thermal sensation of occupants [1], [2], [6], [7] (Fig. 1). Psychological variables can be counted as tension, anger, fatigue, depression, esteem-related affect, vigour, and confusion (Fig. 1) [6]. Stress level is a key psychological factor that causes changes in mental and physical conditions [8]. Nowadays, people face with excessively high level of stress which damages their health [9]. Considering almost 74% of people is overwhelmed in stress, understanding the complex relationship between stress level and thermal sensation is essential since it enhances the thermal acceptability and satisfaction of the occupants [9], [10].

Fanger’s PMV/PPD model describes heat balance equations between environment and human body as indicated in [1], [11]. The model refers a thermal scale which runs from cold (-3) to hot (+3) as given in Table 1. ASHRAE 55 [12] accepts 0 value of PMV as neutral with a tolerance of ± 0.5 while 90% of occupants feel thermally satisfied with their environment. On the other hand, Predicted Percentage of Dissatisfied (PPD) shows the percentage of occupants which will be dissatisfied with the environment [1], [12].

Even though ASHRAE 55 [12] defines thermal comfort as “the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation”, Fanger’s model does not take into account “state of the mind”. Considering thermal comfort is not only determined by the physical state but also by the state of mind, it is worth to remind that psychological parameters of thermal comfort are prominent omissions in the Fanger’s model.On the other hand, adaptive thermal comfort approach gives occupants opportunity to adapt their environments according to their own thermal preferences [3], [4]. Thus, occupants have a wider tolerance of variations in indoor thermal conditions [4]. These psychological adaptive processes can be presumed reasons of limitations on thermal comfort models. For instance, Jones [13] stated that absence of psychological variables in thermal comfort models is the biggest limitation on accuracy since Actual Thermal Sensation (ATS) can be highly related to the psychological variables as well as physical parameters. Singh et al. [14] reported that the reason of discrepancies between thermal comfort models was psychological factors in unconditioned buildings. In another study by Zabetian and Kheyroddin [15], psychological factors on thermal comfort were investigated for the occupants in urban spaces. The authors concluded that improving the privacy, the level of site familiarity, the amount of love and affection to an urban place improved thermal comfort of the occupants. Similarly, Park et al. [16] studied the effects of psychological responses of subjects to thermal comfort in urban and forest zones. The authors examined 168 subjects in 14 different zones by using environmental parameters in Fanger’s PMV/PPD model while psychological parameters such as fatigue, tension and anxiety were obtained by the Profile of Mood States (POMS) questionnaire. The authors concluded that positive psychology of subjects in forest areas improved their thermal comfort. Rohles [17] examined thermal comfort of two groups in different chambers at the same temperature of 18 °C with and without heater. However, the heater was never operated during the experiments. Both groups felt warmer since they were informed that there was a heater in the room before the experiments. In other words, occupants felt warmer without any changes of thermal parameters in the chamber which indicated that psychology highly affects thermal comfort.

Measuring and estimating psychological parameters are quite difficult. Many authors used POMS questionnaires to obtain psychology of occupants [16], [18], [19], [20]. POMS is a standard validated psychological test formulated by McNair et al. [6] which contains 65 words/statements that describe the feelings of occupants. However, the accuracy of questionnaires is always a question mark. The questions should be easy to understand by occupants since they can get easily bored and give incorrect answers. Considering a questionnaire with 65 statements, it is logical to say that the implementation of POMS method is time-consuming and inconvenient especially in a numerical thermal comfort model. For this reason, some authors suggested that Heart Rate (HR) and Skin Temperature (ST) could be used as indicators of psychology besides POMS questionnaire [21]. Different hormones are produced from by hypothalamus under stress and blood flow is increased to carry extra heat produced to the surface to increase convective heat loss [22], [23], [24]. Huizenga et al. [25] used ST as a thermal sensation parameter and found that the ST fluctuated significantly when the occupants were subjected to thermal asymmetries in an environment. Similarly, Yao et al. [26] proved that there is a relation between the ST and thermal comfort and they suggested to consider the ST as a psychological parameter in thermal comfort models. Sympathetic activity of occupants plays an important role in thermal sensation of occupants. Liu et al. [27] found that the HR of occupants affects their thermal comfort. The authors investigated 33 subjects (22 male and 12 female college students) and concluded that students felt uncomfortable when their sympathetic nerve activities were dominant. In another study by Islam [28], gender differences on the HR were investigated in detail. Fifteen subjects (11 males and 4 females) wore five different clothing ensembles at different metabolic rates. The authors revealed that the HR of females was 12 bpm higher than males. Although there are numerous studies that deal with the impact of physical parameters on thermal comfort/sensation in the literature, a few studies exist focusing on how psychology affects thermal sensation including gender differences.

This study differs from past studies by hypothesizing that the effect of one of the psychological parameter; stress level on thermal sensation could vary with gender difference since the responses of females and males to changes of environmental and psychological parameters are different. Ersan et al. [29] indicated that exams are the most stress provoking factor for the students. To this aim, field experiments were conducted to define pre- and post-exam SL and its effect to the thermal sensation by measuring the HR and ST.