# A Systematic Literature Review of Non-Invasive Indoor Thermal Discomfort Detection
## 1 Introduction  
Over the last 50 years, people have become increasingly bound to the indoor workplace. The regular worker spends around 35 hours per week in front of the computer, at brainstorming sessions and meetings. Due to this fact, there is a need to provide good indoor environment quality, not only because it will result in fewer sick leave periods but also because it will result in improved productivity. A number of studies have been conducted to determine ways in which indoor comfort can bring an increase in quality and productivity among the employees [1; 2; 3; 4; 5; 6; 7], and even more studies have been conducted to find a way to reduce energy consumption while still providing comfortable indoor conditions [8; 9; 10; 11; 12; 13]. In general, comfort can be divided into three groups: physical, functionaland psychological. The given review is focused only on indoor thermal comfort which is a part of physical comfort within indoor environment quality (IEQ) [14]. It is an important topic due to current climate change conditions, abnormality in temperature peaks concerning different seasons and general overheating and over-cooling challenges. Indoor thermal comfort is assured by the combination of different aspects such as clothing insulation, levels of activity, radiation exchange, air temperature, air movement and humidity [15]. A variety of these factors can be predefined by age, sex, diet or even clothing requirements at the workplace [16; 17]. Therefore, a number of the studies have been performed in order to find a correlation between measurable parameters and actual comfort. Thermal comfort is traditionally evaluated by using predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) models (indexes). The PMV model developed by [PERSON] [18] represents the mean thermal sensation vote on a standard 7-point scale:  
* 3 Hot
* 2 Warm
* 1 Slightly warm
* 0 Neutral
* 1 Slightly cold
* 2 Cool
* 3 Cold  
It is also referred to as the seven-point ASHRAE thermal sensation scale. The PPD is a quantitative measure of the thermal comfort for a group of people within a given thermal environment [9]. Both PPD and PMV were adopted by several international standards and guidelines, such as ASHRAE Standard 55 [19] and ISO 7730 [20]. The PMV model was widely used for different building types and within different climate zones around the world, which resulted in a deviation between predicted and actual thermal sensations. Such a deviation may be explained by the fact that the given model was developed in a laboratory setting, which means it contains constraints that are not typical for buildings in real life [21]. Due to given circumstances, the adaptive thermal comfort model was proposed in 1970 [22]. [PERSON] et al. [23] suggested a feedback approach for thermal comfort field surveys' interpretation, which was based on the fact that unpleasant sensations would lead to physical reactions and cause a change in the comfort control system itself. [PERSON] and [PERSON] [24] suggested a list of physiological, psychological, social and behavioral actions which can restore comfort conditions in response to cold or heat. Since people live in different climate zones, they can experience different perceptions of the indoor thermal environment, and as a result, have their levels of \"usual\" indoor temperature altered. The same can be applied for seasonal outdoor temperature change. If a person enters a room with indoor temperature 21 \({}^{\circ}\)C, he/she would feel warmer if the outside temperature is \(-1\)\({}^{\circ}\)C but a totally different response will accrue if it is summer and the outside temperature is 30 \({}^{\circ}\)C.  
That is why many studies have been conducted for thermal comfort evaluation on a personal level. Such studies are focused more on creating numerical thermal comfort models that take into account cultural identities, human body heat exchange, metabolic activities and other parameters which define each person individually [25].