## 4 Discussion
### People's Presence  
In Campaign 1, the maximum people in the Lab was 12 students and that in the office was 2-3 persons. The effect of human debris and particle resuspension is depicted in Figure 2a,b. Walking-induced particle resuspension depends not only on the size of the  
Figure 5: Campaign 3, Class: The effect on PM mass concentrations (**a**) and on gaseous air pollutants (**b**) of air purifier, people’s presence, and windows or doors being opened during lectures (S = students present, W cl or W op = windows closed or open, DC or D op = door closed or open, O = empty room, and Pur C or Pur op = air purifier closed or open). The dotted rectangle depicts the operation of the air purifier. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multicolumn{3}{c}{**Campaign 2**} & \multicolumn{8}{c}{**Campaign 3 (by Event *)**} \\ \hline
**Variable** & & & _S, DC, W cl, Pur C_ & _S, DC, W cl, Pur op_ & _S, DC, Wop, Pur C_ & _O, DC, Wop, Pur C_ & _O, DC, Wop, Pur C_ \\ \hline \multicolumn{3}{c}{**PM (\(\mu\)g m\({}^{-3}\))**} \\ \hline  & Mean & SD & Mean & SD & Mean & SD & Mean & SD & Mean & SD \\ PM\({}_{1}\) & 11.31 & 5.81 & 7.35 & 0.98 & 10.62 & 4.01 & 8.11 & 3.08 & 5.40 & 1.27 \\ PM\({}_{2.5}\) & 14.32 & 7.52 & 14.26 & 3.18 & 16.50 & 4.65 & 12.57 & 3.52 & 7.69 & 1.81 \\ PM\({}_{4}\) & 20.35 & 13.31 & 45.31 & 14.66 & 42.41 & 11.67 & 32.41 & 7.54 & 16.19 & 4.61 \\ PM\({}_{7}\) & 25.18 & 19.83 & 95.67 & 33.25 & 85.38 & 26.58 & 68.51 & 18.95 & 29.44 & 10.24 \\ PM\({}_{10}\) & 27.03 & 23.05 & 119.32 & 44.00 & 108.94 & 36.80 & 88.26 & 28.33 & 35.66 & 13.88 \\ TSP & 31.20 & 32.07 & 176.33 & 68.97 & 160.70 & 60.43 & 145.61 & 58.47 & 45.51 & 19.42 \\ \multicolumn{3}{c}{**Gaseous pollutants**} \\ TVOC (ppb) & 128.77 & 30.54 & 194.58 & 38.79 & 134.41 & 32.10 & 197.86 & 17.72 & 73.64 & 11.92 \\ CO (ppb) & 495.62 & 192.88 & 672.92 & 487.49 & 705.33 & 351.93 & 360.57 & 36.99 & 119.79 & 32.60 \\ CO\({}_{2}\) (ppm) & 641.97 & 421.53 & 1273.92 & 554.04 & 2166.37 & 534.78 & 1559.86 & 178.67 & 684.64 & 190.85 \\ O\({}_{3}\) (ppb) & 8.23 & 16.66 & 28.33 & 22.50 & 18.15 & 13.31 & 25.67 & 12.70 & 27.30 & 10.67 \\ \multicolumn{3}{c}{**Microclimatic conditions**} \\ AT (\({}^{\circ}\)C) & 18.66 & 2.12 & 24.87 & 1.25 & 26.05 & 0.93 & 24.40 & 0.21 & 23.65 & 0.28 \\ RH (\%) & 41.13 & 9.32 & 46.00 & 5.45 & 49.67 & 5.11 & 43.44 & 0.66 & 32.86 & 2.13 \\ \hline \hline \end{tabular}  
* S = students present, W cl or Wop = windows closed or open, DC or D op = door closed or open, O = empty room, Pur C or Pur op = air purifier closed or open.  
\end{table} 
Table 3: Descriptive statistics of indoor and outdoor atmospheric pollutant concentrations along with air temperature and RH (Campaign 2 and 3). particles, but also on the flooring material, the walking rate, and the RH [33]. In these rooms (no carpeting), re-suspension was significant in all size ranges [9]. The effect of the human activity on PM concentrations was more pronounced in the case of the 12 students that entered the Lab to use the computers: PM mass concentration increased during the class from 2.2 times (PM\({}_{1}\)) to 5.1 times (PM\({}_{10}\)) on average from their background levels. Air temperature was increased by 25% and RH decreased by 24% for the same period of time. Peak PM concentrations were recorded mainly in the Lab, when people entered the room. This can be attributed not only to the movement of people, but also to the increased atmospheric turbulence emerged from the opening of the door [34]. As can be seen in Figure 2, the PM concentrations had their lowest levels inside the Lab, when it was empty and there were no PCs in operation. The air exchange rate during these times was low because doors and the window were kept closed. In Campaign 2, the number of people in the Class was usually 10 to 12 and, once, 30 students were present. Although human presence was minimized during this period, owing to COVID-19 restrictions, the PM concentrations remained higher than in the Office or in the Lab, even after the Class was empty and closed.