Sealed shut
Sheltering in-place is used if a migrating toxic gas or vapor could quickly overtake unprotected or evacuating citizens, or if evacuation would create problems that would outweigh its usefulness. The amount of protection from sheltering in-place depends mainly with the air tightness of the facility and the length of time the facility is exposed to a hazard.Even the most weather-tight structure will slowly allow contaminated air to enter, however, sealing windows, doors, and vents with plastic sheeting, duct tape and other barrier materials can further reduce infiltration of contaminated air into a facility.