The greatest risk lies in the massive emissions of smoke and heat in enclosed spaces. Since the personnel present are often only passing by, they might be unaware of the particularities of the design of the tunnel. Put simply, when they come across a fire, they are already inside. Visibility is key to avoiding disorientation at critical moments, i.e. where speed is the prevailing factor to avoid asphyxia.
This is related to very high fire loads (several MW), extreme temperatures, and an abundance of toxic smoke. Protection with a water-mist system refrigerates the area while also facilitating evacuation.
Protection must also take into consideration the cooling of the tube structure, that is, to avoid structural damages which compromise its integrity.
The identification of risks must include a detailed study of the infrastructure and its design characteristics, as well as those of the environment (location, accessibility, response time, etc.), and of traffic conditions in the tunnel.
Thus, their characteristics vary based on the longitude and number of tubes, the sense of traffic, the natural/emergency ventilation system, the structure, the lining used, uneven levels, different pressure levels, etc.