## 3 Numerical Results
### Dynamics of Combustion Gas Accumulation (CO and CO\({}_{2}\)) in the Built Environment  
In addition to analyzing thermal and visibility parameters, evaluating occupant risk during a fire requires the strict monitoring of toxic gas concentrations produced by combustion--especially carbon monoxide (CO) and carbon dioxide (CO\({}_{2}\)). According to the specialized literature [10], exposure for 15 min to CO concentrations in the range of 1000-1200 ppm (0.1-0.12%) is associated with a high risk of lethal intoxication. For CO\({}_{2}\), the safety threshold is exceeded at concentrations above 4% (\(\approx\)40,000 ppm), which impairs individuals' physiological ability to evacuate.  
The CFD simulation data presented in Figures 18 and 19 reveal a clear differentiation between the scenarios in terms of toxicological risk. In scenarios S0 and S2, the values recorded at monitoring point P2 show a rapid exceedance of lethal thresholds for both CO and CO\({}_{2}\). In scenario S0, the CO concentrations exceed 0.12% after only 116 s, while in S2 this occurs at 155 s (Figure 18). The critical CO\({}_{2}\) level of 4% is reached at 130 s in S0 and 170 s in S2 (Figure 19). This trend indicates an accelerated accumulation of toxic gases due to the lack of efficient ventilation and the inability to evacuate safely. Conversely, scenario S1 maintains CO and CO\({}_{2}\) concentrations at significantly lower levels. Although slight increases in CO above 0.12% are observed at isolated moments, the levels do not reach dangerous thresholds and do not persist for critical durations. This confirms the effectiveness of unidirectional pressurization in limiting the spread of combustion gases. Notably, CO becomes toxic before CO\({}_{2}\), as evidenced by the chronology of threshold exceedance in all scenarios. This behavior highlights the importance of differential monitoring and rapid response, as CO--being odorless and neurotoxic even at low concentrations--poses the primary risk factor during the initial minutes of a fire.  
Figure 18: Variation in CO concentrations at point P2 in the stairwell for scenarios S0, S1, and S2.  
CO and soot emissions in a mechanically ventilated fire are influenced by the ventilation rate and the size of the fuel container, with underventilation favoring CO production, while effective ventilation leads to increased soot generation [52]. In this context, at points P3 and P4, mechanical positive pressure ventilation provides complete protection against the accumulation of toxic gases, maintaining the CO and CO\({}_{2}\) concentrations at zero throughout the simulation. Among the three simulated scenarios, only S1 ensures that the CO and CO\({}_{2}\) concentrations remain within acceptable limits for safe evacuation. In scenarios S0 and S2, the toxic gases rapidly exceed critical thresholds, confirming the inadequacy of natural ventilation (S0) and the instability of airflow in the top-down fan configuration (S2). These findings support the conclusion that monitoring toxic gases--especially CO--and ensuring a controlled, unidirectional airflow are essential requirements for the effective protection of occupants during fire events.