Over the years, indoor environmental quality (IEQ) has been widely studied from a variety of perspectives: from the first experiments to assess the physiological response of individuals, it has become a human-centered concept [
1]. IEQ is a broader concept that includes many factors, such as thermal conditions, indoor air quality (IAQ), lighting, and acoustics inside a building, which affect the health and well-being and productivity of occupants. IAQ quantifies the minimum acceptable air quality that promotes human health, comfort, and productivity. This includes the removal of harmful pollutants entering the living space from the outside or those that form inside the space. The level of IEQ depends on many complex interconnected parameters [
2,
3] and reflects the performance of the building in relation to the health and well-being of its occupants [
4]. It is well-known that IEQ can be influenced by residents and their activities in the indoor environment [
5]. It is very important to develop better control techniques for HVAC systems to ensure occupant-controlled energy and comfort management, given the importance of energy savings in buildings [
6]. The relationship between the level of satisfaction of the population with the conditions of the indoor environment, and their productivity in the workplace is well established. However, the relationship between building renovation decisions and the level of satisfaction of the population with the IEQ is insufficiently explored [
7]. User comfort and power consumption become two opposing entities, leading to pareto-optimal control lines when designing HVAC systems [
8]. The difference in the energy performance of buildings is a well-known phenomenon [
9]. Sustainable building design should be in line with even more urgent energy-saving requirements—as in nearly zero-energy buildings (NZEB)—and a high level of IEQ [
10]. Addressing the two common challenges to building performance—reducing the carbon footprint associated with providing a comfortable indoor environment and improving the health and well-being of residents—requires a more comprehensive understanding of how the indoor environment of buildings works [
11]. To understand the future of sustainable buildings, it is important to recognize that services such as HVAC and lighting are provided to create suitably comfortable conditions for staff to be productive [
12]. The COVID-19 (SARS-CoV-2) pandemic has significantly affected our daily lives [
13]. Its rapid spread and subsequent deaths worldwide led to the declaration of a pandemic situation in the world at the end of December 2019 [
14]. According to a study [
15], people generally spend more than 60% of their time at home [
16] and the rest of their time at work, school, and/or commuting, leading to about 90% or more of their time indoors. Because of the occurrence of the COVID-19 pandemic, it can be stated that almost 100% of the time people spend indoors [
17]. Concerns about the spread of the virus in confined spaces due to insufficient ventilation have gradually raised the need to improve IEQ which depends significantly on the performance of the building’s outlets, lighting, and HVAC systems [
18]. As people spend more time indoors, it is important to identify the relationship between IEQ and health of the building’s occupants [
19]. Public health strategies to reduce indoor transmission, such as ventilation and centralized insulation, will be beneficial for the prevention and control of COVID-19 [
20]. The design of a modern working environment must take into account a high level of spatial and technological change by providing responsive heating and air quality systems. Residents of the building will require internal conditions to support computer-intensive activities as well as paperwork [
21]. Occupancy is a key input variable for HVAC sizing in buildings. However, HVAC designers usually estimate occupancy data based on assumptions that rarely reflect actual situations. As a result, these assumptions can lead to undersized or oversized HVAC systems that either provide too low or too high peak loads or ventilation airflows than is actually necessary to meet IEQ requirements during building operation [
22]. According to a study by Borgstein et al. [
23], buildings do not regularly operate at an optimal level and often do not meet project forecasts. These failures affect energy efficiency and ensure adequate IEQ and user satisfaction. This study points out that several of the buildings failed on a basic level to provide basic conditions of IEQ, as demonstrated by the 13 different failure modes such as thermal comfort not met by undersized HVAC systems; no external air supply (or insufficient external air); unbalanced air distribution for cooling and so on. Only five of the 33 buildings studied did not report a single issue related to environmental quality. Most issues were related to the HVAC systems, which either do not provide thermal comfort or are not properly controlled. A study also emphasizes that the most common problem of unbalanced air distribution for cooling was found almost exclusively in large buildings. In terms of IEQ and building service performance, user satisfaction with greenery is significantly higher than in terms of thermal comfort, IAQ, equipment, operation, and maintenance. In addition, factors influencing the energy consumption of buildings are analyzed in order to provide guidance on further improving the performance of green buildings during the design and operation phases [
24]. The results of another study [
25] suggest that building managers could pay less attention to people living in energy saving measures, while paying more attention to communicating with ordinary people in order to raise their awareness of energy savings. The results of this study also revealed that wasting residents have the greatest potential to narrow gaps, which can be realized by combining communications and HVAC system equipment with zone control. According to a study by Leyten and Kurvers [
26], the robustness of an office building or HVAC system can be defined as the degree to which a building or system fulfills its design purpose in a real situation. Insufficient robustness can be caused by hypersensitivity to deviations from design assumptions, unrealistic maintenance requirements, integration of heating and ventilation, regulation of supply air volumes, and lack of transparency for residents and building management. As a study states [
27], assessment tools for office buildings are very important. The work done in the study [
27] could lead to the development of an IEQ model that reflects the user’s opinion. The use of variables used in the calculation of a building’s energy performance to calculate IEQ is an important step in the development of IEQ methodologies, if they are to be compared between energy consumption and the comfort of the occupants. The importance of the office environment to the comfort, productivity, and health of workers cannot be overstated [
28]. The results of a Polish study [
29] show that the importance of indoor air quality should not be forgotten when considering energy-saving strategies. Their simulations prove that for instance a garage attached to a house with more air tightness can be harmful to human health. The results of this study encourage research into different impacts strategies to increase energy efficiency and maintain appropriate indoor air quality inside residential buildings. The results of another study [
30] prove that the location of the furnace influences contaminant accumulation and migration. Such simulations can be an important tool when designing a ventilation system with respect to the furnace to improve the removal of dangerous substances.
The main goal of this study is an investigation of IEQ under different heating conditions in the meeting room of the ABC KLIMA office building in Košice in Eastern Slovakia. In Slovakia, there is not enough measurement of IEQ to assess the extent to which building users are exposed to high concentrations. The second aim of the study is to show that the central air handling unit by using the filter technology with high-efficiency (more than 99%) has low indoor/outdoor concentration ratios (I/O ratios). Therefore, outdoor PM concentrations do not affect the level of PM occurrence in the indoor environment.