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Planning and PreparednessAll buildings are designed and built to protect the occupants from the effects of the natural environment. This "barrier" effect is incomplete, however, in that no building is air tight. Every building contains openings through which winds, fans, and natural convection allow the exchange of indoor air with outdoor air. These openings are both intentional -- windows, doors, vents and fresh air intakes -- and unintentional -- cracks, joints, seams and pores. In general, these openings, their locations, the forces that drive the exchange of air through them, and the presence of any air filters determine the protection a building provides against an external release of hazardous airborne materials.Under normal circumstances and operation, a building provides occupants with little protection against the external release of an airborne hazard. This is because outdoor air is continuously introduced to the building through its openings to provide a comfortable, healthy indoor environment. A building can provide substantial protection against agents released externally (i.e. toxic clouds) only if the flow of outdoor air into the building is temporarily suspended, filtered or minimized. |
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