## 2 Materials and Methods
### Case Studies  
In order to compare the effects of different natural ventilation strategies applied in educational centres for reducing the risk of contagion, four schools were selected for measurement campaigns: two in southern Spain (Sevilla and Cordoba) and two in northern Italy (Milan). The locations of the four case studies are presented in Figure 1. The case studies were selected by taking into account the most common building (the class-corridor-class structure) and constructive typology in Mediterranean schools [27]. Significant variations in the climate can be found between northern and southern locations in the Mediterranean, leading to substantial differences in general habits in the use of heating and ventilation systems in buildings. Therefore, the case studies selected were in locations with distinct natural ventilation strategies and heating usage. To focus the research on these differences, specific measurement periods were defined for the four case studies, aiming to ensure similar outdoor conditions. Table 1 summarises the main data for the four schools. In each school, measurements were taken in two classrooms with different orientations over a one-week period with occupancy. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**School** & **1** & \multicolumn{2}{c}{**2**} & \multicolumn{2}{c}{**3**} & \multicolumn{2}{c}{**4**} \\
**Classroom** & **1.1** & **1.2** & **2.1** & **2.2** & **3.1** & **3.2** & **4.1** & **4.2** \\ \hline
**Location** & Sevilla (Spain) & \multicolumn{2}{c}{Cordoba (Spain)} & \multicolumn{2}{c}{Milano (Italy)} & \multicolumn{2}{c}{Milano (Italy)} \\
**Altitude (m)** & 42 & 217 & 120 & 120 & 120 \\
**Year of** & 1973 & 1962 & 1973 & 1957 \\
**construction** & & & & & & \\
**Facade** & Brick cavity wall & Brick cavity wall & Prefabricated panel & Brick cavity wall \\  & (unsimulated) & (unsimulated) & (3-5 cm insulation) & (unsimulated) & (unsimulated) \\
**Windows** & Aluminium frame, & Aluminium frame, & Aluminium frame, & Aluminium frame, & Aluminium frame, \\  & single glazing & single glazing & double glazing & double glazing & double glazing \\
**Orientation** & Northeast Southwest & East West & Southwest & Northeast & East East \\
**Floor area (m2)** & 47.2 & 47.2 & 52.2 & 52.2 & 50.3 & 47.6 & 48.8 & 48.8 \\
**Volume (m3)** & 141.6 & 141.6 & 156.6 & 156.6 & 150.9 & 142.8 & 156.2 & 156.2 \\
**Student age** & 15-16 & 14-15 & 16–17 & 16–17 & 10–11 & 9–10 & 13–14 & 13–14 \\
**Occupation** & 28 & 28 & 30 & 30 & 24 & 20 & 22 \\
**N. of people/m3** & 0.198 & 0.198 & 0.192 & 0.192 & 0.159 & 0.168 & 0.128 & 0.141 \\
**Ventilation system** & Natural & Natural & Natural & Natural & Natural & Natural \\
**Natural ventilation** & & & & & & & \\
**strategy** & All open & All open & All open & open & open & open \\
**Heating system** & Radiators & Radiators & Radiators & Radiators & Radiators & Radiators & Radiators \\
**Heating pattern** & & & & & & & \\
**(\% occupied hours)** & 30 & 30 & 30 & 100 & 100 & 100 & 100 \\
**Monitoring period** & & & & & & & \\
**(dd/mm/yy)** & & & & & & & \\
**Sensor ID** & & SENSONET & SENSONET & WSD00 TH2 CO & WSD00 TH2 CO \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the case studies in Spain (cases 1 and 2) and Italy (cases 3 and 4). Figure 1: Case studies location: 1—Sevilla (Spain), 2—Cordoba (Spain), 3 and 4—Milano (Italy). All four schools are located in compact urban areas, and the morphological conditions of the eight classrooms are similar, with a floor area of between 47 and 52 m\({}^{2}\) and a ceiling height of between 3 and 3.28 m. However, the classrooms in the case studies in southern Spain have higher occupancy and older students, resulting in a slightly higher thermal load and contaminant levels. Figure 2 shows the typology of the classrooms through plans and images. To establish behavioural patterns in the different classrooms, teachers were asked to complete a survey detailing the times and durations of periods with open windows, as well as the use of heating.