If an accident or some other incident creates contaminated air in the nearby area, everyone would be brought indoors, including those in external buildings. Building personnel would close all windows and doors and shut down the heating, ventilation, and air conditioning system (HVAC). This would create a neutral pressure in the building, meaning the contaminated air would not be drawn into the building.
Shelter-in-place is a short-term measure (measured in minutes or hours, not days) designed to use a facility and its indoor atmosphere to temporarily separate people from a hazardous outdoor environment. The alternative would be to evacuate into a hazardous situation, thereby causing harm to all involved.
The duration of a shelter-in-place will be determined by local emergency personnel.