## 3 Results and Discussion  
### Environmental Conditions  
Based on the indoor and outdoor temperature data and indoor CO\({}_{2}\) levels recorded in the different case studies, boxplot graphs were used to analyse the values (Figure 3). The central mark of each box indicates the median of the measured values, with the lower and upper ends of the box representing the 25 th and 75 th percentiles, respectively. The whiskers extend to the most extreme data points, which are not considered outliers (5 th and 95 th percentiles), and outliers are represented with the \"+\" symbol.  
Figure 3: Measured data of (**a**) air temperature and (**b**) CO\({}_{2}\) levels.  
In terms of air temperature (Figure 3a), similar ranges were observed in both southern Spain (cases 1 and 2) and northern Italy (cases 3 and 4), with median values of indoor classroom temperatures ranging between 20 and 22.5 \({}^{\circ}\)C. Outdoor air temperatures are slightly lower in northern Italy, but more intensive use of heating systems and fewer hours of window opening result in slightly higher indoor temperatures than those in southern Spain.  
However, significant differences were observed between the indoor CO\({}_{2}\) values measured in the cases in southern Spain and northern Italy (Figure 3b). While in cases 1 and 2 CO\({}_{2}\) values frequently remained below 700 ppm, in cases 3 and 4, they often reached (classroom 3.2) or approached values of around 1000 ppm. In order to better understand the ventilation strategies employed in each case, the distribution of measured CO\({}_{2}\) levels in different classrooms was analysed using histograms (Figure 4). Classrooms 1.1 and 1.2 exhibited a concentrated distribution at very low CO\({}_{2}\) levels, between 400 and 500 ppm, similar to case 2.1, but with slightly higher values (500-600 ppm). Classroom 2.2 showed a distribution of measured CO\({}_{2}\) values close to normal, with the peak frequency at 600 ppm. Although the highest frequency of values around 900 ppm was found in classrooms 3.1 and 3.2, in the former, most measurements were below this value, unlike in the latter, where most exceeded this value. For classrooms 4.1 and 4.2, the distribution was less clear, and several peaks of similar frequency, around 600 ppm and 800 ppm (case 4.1) and 1000 ppm (case 4.2), were observed.  
Comparing the variables measured in this work with other studies, [PERSON] et al. [17] detected similar average temperatures (around 22 \({}^{\circ}\)C) in mild seasons in northern Spain, although with relatively higher maximum values (approximately 29 \({}^{\circ}\)C). Something similar occurs with the average CO\({}_{2}\) concentration levels (of around 550 ppm) found in northern [17] and central Spain [39], which are close to the median values measured in the  
Figure 4: CO\({}_{2}\) level histograms.  
Spanish cases of this work (but lower than those of the Italian cases). Nevertheless, the maximum value detected by [PERSON] et al. (2446 ppm) is significantly higher than those measured in all the case studies of this work, probably due to less intensive ventilation. In central Italy, [PERSON] et al. [19] found the average levels of CO\({}_{2}\) to be around 1500 ppm, slightly higher than those obtained in the most unfavourable case of this work (case 3), but with similar average indoor temperatures (around 22 \({}^{\circ}\)C). In northern Italy during a mild period, [PERSON] et al. [26] measured CO\({}_{2}\) concentration levels generally between 500 and 1500 ppm and indoor temperatures between 20 and 24 \({}^{\circ}\)C (very similar to the Italian case studies of this work).