## 1 Introduction
Although the energy consumption issue and other economic issues are out of the scope of the present study, it is worth mentioning a recent work that provides an objective evaluation of different air purifiers based on their overall cost. In this work, among others, it is reported that \"_replacement filters and electricity costs generate most of the cost of running air purifiers_\" [20]. TVOC is defined as the sum of all the individual volatile organic compounds (VOC) and can be considerate as an indicator of IAQ. Indoor sources of TVOC include emissions from building material, personal care products and indoor chemical reactions. Proposed air quality guideline (AQG) values lie between 300 and 600 \(\mu\)g m\({}^{-3}\) (73.35-146.7 ppb, assuming 25 \({}^{\circ}\)C, 1 atm, and 100 g mol\({}^{-1}\)), averaged over 8 h [21; 22]. However, TVOC as a metric reveals little regarding the nature and the possible health effects of the individual organic compounds. Indoor CO\({}_{2}\) concentrations are a simple indicator of human presence and the emission of bio-effluents in classrooms. There is a direct correlation between indoor CO\({}_{2}\) levels and the number of occupants, their age, and their metabolic rate [11]. In a review article, the authors conclude that \"_Although it is not possible to say with confidence whether CO\({}_{2}\) alone is responsible for health effects at low exposures (<5000 ppm) and whether it is itself a pollutant, the existing guideline CO\({}_{2}\) concentrations can be indicative of ventilation, human bio-effluent and indoor air pollution concentrations, and therefore, the current consensus that <1000 ppm, 1000-1500 ppm and >1500 ppm represent good, moderate and poor indoor air quality, respectively, seems appropriate_\" [23; 24]. Another issue that affects academic performance is the thermal comfort in teaching rooms, apart from the concentrations of air pollutants, but this perspective is out of the scope of the present study [25; 26]. The World Health Organization (WHO) has published in 2010 guidelines for indoor air quality for some selected pollutants with indoor sources [27]. The updated WHO Global Air Quality Guidelines (AQGs) [14] are based on the latest scientific findings on the short- or long-term adverse health effects of air pollution. Some new guideline concentration values are introduced for controlling atmospheric pollutants (for example, for O\({}_{3}\)). For others, the guideline values are updated to lower values than those set previously, e.g., for CO, NO\({}_{2}\), and PM\({}_{2.5}\) and PM\({}_{10}\). The new guidelines are applicable to both outdoor and indoor environments. However they are not yet adopted as standards by the regulatory authorities in most countries, as well as in Europe. The proposed long-term AQG level for O\({}_{3}\) is 60 \(\upmu\)gm\({}^{-3}\) (30.6 ppb, STP), as a warm season average of daily maximum 8 h concentrations, and an 8 h average of 100 \(\upmu\)gm\({}^{-3}\) (50.9 ppb, STP). For NO\({}_{2}\), an AQG value of 25 \(\upmu\)gm\({}^{-3}\) (13.30 ppb), as a 24 h average concentration, was newly introduced. The proposed AQG level for PM\({}_{2.5}\) is a 24 h average concentration of 15 \(\upmu\)gm\({}^{-3}\) and annual average of 5 \(\upmu\)gm\({}^{-3}\). The corresponding levels for PM\({}_{10}\) are a 24 h average concentration of 45 \(\upmu\)gm\({}^{-3}\) and annual average of 15 \(\upmu\)gm\({}^{-3}\). For CO, the 24 h average concentration is 4 mgm\({}^{-3}\) (3.48 ppm), the 8 h average is 10 mgm\({}^{-3}\) (8.70 ppm), and the 1 h average is 35 mgm\({}^{-3}\) (30.43 ppm). The objective of the present study was to evaluate basic parameters that contribute to the IAQ in a naturally ventilated lecture classroom (recently refurbished to be airtight), a training laboratory with computers, and an office of a professor (not refurbished). These parameters included the following: PM mass concentrations in five sizes range; CO, CO\({}_{2}\), O\({}_{3}\), NO, NO\({}_{2}\), and TVOC concentrations; along with temperature and relative humidity.