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5.2 SHELTERING IN PLACE

In normal operations, a building does little to protect occupants from airborne hazards outside the building because outdoor
air must be continuously introduced to provide a comfortable, healthy indoor environment. However, a building can provide substantial protection against agents released outdoors if the flow of fresh air is filtered/cleaned, or temporarily interrupted or re- duced. Interrupting the flow of fresh air is the principle applied in the protective action known as sheltering in place.The advantage of sheltering in place is that it can be implemented rapidly. The disadvantage is that its protection is variable and di- minishes with the duration of the hazard. Sheltering requires that two distinct actions be taken without delay to maximize the passive protection a building provides:

❍ First, reduce the indoor-outdoor air exchange rate before
the hazardous plume arrives. This is achieved by closing all windows and doors, and turning off all fans, air conditioners, and combustion heaters.

❍ Second, increase the indoor-outdoor air exchange rate as soon as the hazardous plume has passed. This is achieved by opening all windows and doors, and turning on all fans to ventilate the building.

The level of protection that can be attained by sheltering in place is substantial, but it is less than can be provided by high effi- ciency filtration of the fresh air introduced into the building. The amount of protection varies with:

❍ The building’s air exchange rate. The tighter the building (i.e., the lower the air exchange rate), the greater the protection it provides. In most cases, air conditioners and combustion heaters cannot be operated while sheltering in place because operating them increases the indoor-outdoor exchange of air.

❍ The duration of exposure. Protection varies with time, diminishing as the time of exposure increases. Sheltering in place is, therefore, suitable only for exposures of short duration, roughly 2 hours or less, depending on conditions.

❍ Purging or period of occupancy. How long occupants remain in the building after the hazardous plume has passed also affects the level of protection. Because the building slowly purges contaminants that have entered it, at some point during plume passage, the concentration inside exceeds the concentration outside. Maximum protection is attained by

increasing the air exchange rate after plume passage or by exiting the building into clean air.

❍ Natural filtering. Some filtering occurs when the agent is deposited in the building shell or upon interior surfaces as air passes into and out of the building. The tighter the building, the greater the effect of this natural filtering.

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