# Effects of Night Ventilation on Indoor Air Quality in Educational Buildings--A Field Study
## 4 Discussion  
The measurements were carried out in different seasons during the time period of 2019-2020. The buildings were selected from the different property owners to measure buildings that do not have the previously reported IAQ problems. In the measurement campaign, \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**TVOC** & \multicolumn{2}{c}{**1 Week**} & \multicolumn{2}{c}{**Weekday 8–16**} & \multicolumn{2}{c}{**Weekday 6–8**} \\ \hline
**[ppb]** & **avg** & **sd** & **avg** & **sd** & **avg** & **sd** \\ \hline \hline \multicolumn{7}{c}{classroom in primary school 1} \\ \hline pre-started & 238 & 85 & 330 & 96 & 167 & 31 \\ continuous & 210 & 87 & 325 & 23 & 165 & 26 \\ intermittent & 287 & 129 & 398 & 154 & 195 & 79 \\ \hline \hline \multicolumn{7}{c}{playroom in kindergarten 1} \\ \hline pre-started & 223 & 75 & 222 & 14 & 177 & 12 \\ continuous & 191 & 95 & 341 & 40 & 226 & 42 \\ intermittent & 276 & 112 & 278 & 94 & 205 & 39 \\ \multicolumn{7}{c}{playroom in kindergarten 2} \\ \hline pre-started & 186 & 62 & 263 & 15 & 130 & 2.1 \\ continuous & 180 & 87 & 317 & 70 & 154 & 23 \\ intermittent & 212 & 92 & 311 & 53 & 153 & 27 \\ \hline \hline \multicolumn{7}{c}{classroom in secondary school} \\ \hline pre-started & 195 & 46 & 202 & 23 & 155 & 19 \\ continuous & 194 & 79 & 272 & 88 & 199 & 61 \\ max & & & & & \\ continuous & 187 & 91 & 285 & 89 & 150 & 38 \\ min & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Averaged TVOC concentration in different time periods. Figure 9: Measured particulate matter in first-floor group workspace when the ventilation started in the morning. The ventilation airflow rate was 2 L/s-m\({}^{2}\). The vertical line shows the start of mechanical ventilation. The black arrow shows a trend of decrease. 5 buildings in autumn, 2 buildings in winter, 2 buildings in spring, and 2 buildings in summer season were measured. The field measurements lasted altogether 4-6 weeks in each building, and the choice of buildings had to be compromised in terms of cost and total time frame by measuring one or two buildings simultaneously. Consequently, the number of measured buildings was only 11, and therefore, the statistical representativeness was poor. The reliability of statistical evaluation can usually require 20-30 buildings in field measurements. Besides, the field measurements were made at different times of the year, which means that the buildings of the same type could not be directly compared with each other. In practice, the real challenge was to adjust exactly similar comparison cases, which is a rather well-known limitation in field experiments in non-controlled real indoor environments. However, the measuring instruments and analysis methods were of high quality, leading to reasonable observations in each building. As a result, new knowledge has been processed on indoor air quality and microbial concentrations in educational buildings. The results indicate that a selection of the night-time ventilation method may have a smaller effect on the indoor air quality than that of a normal variation of those measured quantities. This was believed to have an association with indoor pollutant generation, interior usage, and season conditions. Consequently, the systematic similarities were not recognized with those night ventilation methods. The hypotheses were proposed on how indoor environments should be ventilated during unoccupied hours. The first hypothesis was related to what is the effect of night ventilation on morning indoor air quality. The results show that night ventilation is not necessary if the occupied ventilation is started 1-2 h before occupied periods. A 1-2-h flushing period should be also required after the space usage in the evenings. The common TVOC sources are personal, teaching, or cleaning products as well as interior building materials [39]. Furthermore, the cosmetics used by students may significantly increase TVOC levels [40]. Consequently, the daily space activities affect greatly the TVOC levels.