## 5 Study Cases
### Results and Discussion  
Figure 6 shows the computed CIAQ Risk Indexes for 15 studied classrooms. A horizontal yellow line has been added to indicate the reference value of CIAQ = 36.65, obtained for the CL-X1 case. This serves as a benchmark for comparing the results of the other cases. The results indicate that the potential occupancy, occupants' activities and the operation ventilation rate capacity of the classroom are the key factors influencing the developed Risk Index. These findings align with the importance weights of input features affecting IAQ as presented by [PERSON] et al. (2023) [45].  
Implementing the developed approach, educators and facility managers can ensure that the design, architecture, and installation systems of the classroom are optimized to maintain healthy air quality levels and/or prevent CO\({}_{2}\) concentrations from exceeding permissible limits, thereby creating a safer and more conducive learning environment for students and staff. For the latter purpose, the following measures can be implemented: (1) limitation of space occupancy, occupants' age, or activity, (2) increase in ventilation rates (VR) or (3) a combination of these options.  
The first option involves limiting the number of occupants in the room to ensure that CO\({}_{2}\) concentration remains below the limit. Proper allocation of students, taking into account the specific characteristics of each classroom and the occupancy-related factors, can significantly enhance safety and efficiency. By strategically assigning students to classrooms that are best suited for their age group and activities, and by managing the time spent in each space, schools can minimize potential risks. This measure is necessary in spaces where the maximum achievable VR is limited, such as classrooms with mechanical ventilation systems that are undersized relative to the required or recommended air changes per hour (ACH) values, or where ventilation limitations arise from design characteristics.  
Figure 7 demonstrates the positive effect on the exposure index obtained by reducing a number of students in the classroom (CL-D2) or time spent there (CL-A2). Nevertheless, many teaching spaces are designed for high occupancy with low \(V\!R\)s, so severely limiting  
Figure 6: Computed CIAQ Risk Indexes for the studied cases.  
the number of occupants can lead to underutilization, potentially resulting in the inability to accommodate all enrolled students. Therefore, the alternative option entails increasing the VR in spaces where the CO2 concentration limit is exceeded while maintaining 100% occupancy. This measure is feasible in mechanically ventilated spaces where the size of the ventilation system can be adjusted. Similarly, to Figure 7, the influence of the ventilation capacity on the vulnerability index (CL-G2/G3) can be observed in Figure 8. However, achieving ACH values above 10 through natural ventilation is challenging, which is the predominant system used in the analyzed classrooms. This situation is similar in other European countries, where most schools (86%) use natural ventilation; 7% use assisted ventilation, and 7% use mechanical ventilation [11; 46]. Thus, a combination of measurements might lead to a more suitable solution, i.e., adjustment of the ventilation strategy coupled with the selection of the group of a certain age or selection of the suitable group (in terms of the size) of students for the given classroom along with the control of activity level [47]. In fact, the magnitude of the CIAQ Risk Index could be significantly affected by establishing the required activity level for a particular classroom; this phenomenon is considered by the hazard index, as it is possible to note in Figure 9 (CL-E2/E3).  
Figure 8: Computed CIAQ Risk Indexes along with the vulnerability indexes for the studied cases.  
Figure 7: Computed CIAQ Risk Indexes along with the exposure indexes for the studied cases.