## 6 Analysis and Discussion of the Results
#### 6.1.7 \(\text{FED}_{\text{toxic gases}}\) Value  
The above-mentioned results showed that the BEB fire worsens--compared to the burning ICEB--the environmental conditions along the escape route used by the tunnel users who are downstream of the bus in flames, especially in terms of concentrations of toxic gases (i.e., HF, CO, and CO\({}_{2}\)) and, to a lesser degree, in terms of heat (i.e., temperatures and radiant heat fluxes) and visibility distances. Therefore, in this section, the \(\text{FED}_{\text{toxic gases}}\) parameter is also computed and used for a comparison with the tenability limit of 0.1 [42]. Figure 11: Spatial distributions downstream of the fire related to the visibility distance at the height of breathing along the escape route toward Portal \(B\) for both BEB and ICEB fires, in both cases of ventilation investigated. Figure 10: Spatial distributions downstream of the fire related to the radiant heat flux at the height of breathing along the escape route toward Portal \(B\) for both BEB and ICEB fires, in both cases of ventilation investigated. In both cases of ventilation investigated, from Figure 12, we can see how the BEB fire leads to much higher values of the \(\text{FED}_{\text{toxic gases}}\) parameter downstream of the bus in flames than the burning ICEB. From Figure 12, considering the scenario of the naturally ventilated tunnel, it can also be seen that during the burning of the BEB, the values of the \(\text{FED}_{\text{toxic gases}}\) parameter exceed 0.1 at a distance from the fire center between 140 and 235 m, reaching a peak of approximately 0.15 at about 185 m downstream of the center of the bus in flames. In the event of the ICEB fire, the values of the \(\text{FED}_{\text{toxic gases}}\) parameter are above the tenability limit at a distance from the fire center in the range of 160-215 m, with a peak of about 0.12 detected at approximately 195 m away from the center of the burning bus. Nevertheless, the \(\text{FED}_{\text{toxic gases}}\) corresponding to the last user escaping from the tunnel through Portal \(B\) is \(>\)0.1 only in the case of the BEB fire. With reference to the mechanical ventilation, Figure 12 shows that during the BEB fire, the values of the \(\text{FED}_{\text{toxic gases}}\) parameter are higher than the tenability limit both in the proximity of the bus in flames and at a distance from its center between 60 m and 145 m, with a peak of approximately 0.51 measured at about 20 m away from the fire center. In contrast, for the burning ICEB, the \(\text{FED}_{\text{toxic gases}}\) parameter exceeds 0.1 both in the vicinity of the burning bus and at a distance from its center between 75 m and 135 m, achieving a peak of about 0.37 at approximately 20 m downstream of the center of the bus in flames. Therefore, also in this case, the last person escaping from the tunnel through Portal \(B\) is subjected to \(\text{FED}_{\text{toxic gases}}\)\(>\) 0.1 only during the BEB fire. In light of this, it can be concluded that in both cases of natural and longitudinal mechanical ventilation, the combustion of a BEB in a bi-directional road tunnel might cause--compared to an ICEB fire--a greater reduction in the safety level of users. Environmental Situations in the Tunnel to Which the Firefighters are Exposed (after \(t\) = 15 min from the Fire Start)  
In this section, to assess if the firefighters are exposed to sustainable environmental conditions when they enter the tunnel to extinguish the burning bus and/or help the tunnel users, the spatial distributions related to the HF, CO, and CO\({}_{2}\) levels, temperature, radiant heat flux, visibility distance, and \(\text{FED}_{\text{toxic gases}}\) at the breathing height along the escape route both upstream and downstream of the bus in flames are reported. It is to be stressed that firefighters are expected to enter the tunnel from Portal \(A\) (i.e., in the same direction as the air flow generated by natural or mechanical ventilation).