Anywhere, when a person has to enter a confined space like the tunnel described above, oxygen and other gas levels have to be checked first and take measures. As the issue in the story is of national importance, personnel enter inside, get exposed to the gas. Still, they go ahead and one of the person in the team, an expert in bomb disposal safely makes it inactive. Though the person gets unconscious, brought out by team members, finally survives and receives a medal.
The lesson for the readers is that, persons in tunnels can risk their lives due to gas or lack of oxygen. Though, initially I thought that the author should have considered safety aspects while continuing with the story, on brighter side, those who understand the dangers faced by the characters will atleast remember the same.
