Tunnels

Tunnel systems are enclosed areas that become packed with people at predictable times during the day, like during rush hour. Air currents above ground, as well as those generated by the movement of cars or trains through the structure, accelerate the spread of smoke or gases through the tunnel and up to street level through the ventilation system, exposing many people to toxic fumes. This presents significant challenges in a fire emergency, but in any kind of incident in which air quality is compromised.