Esshup's mention of the fireball inside a 40-foot tunnel still has me spooked.
Ehh..Explosions are no biggie
Seriously though, any solvent or sealant is probably going to require a fresh air breathing system to be applied in a 48"x40' foot coffin...
All you really need to do is change air per concentration of vapor based on deposit/evaporation/curing rates and keep it at 25% of the LEL (lower explosive limits) Need to know what actually makes up the stuff you are applying to calculate this, then you can get the required amount of air flow to break it down so it is not an issue.
It's all about eliminating high concentrations and dispersing them in a somewhat safe manner.
Fresh Air rig also required.
