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5.2 SHELTERING IN PLACE

In normal operations, a building does little to protectoccupants from airborne hazards outside the building becauseoutdoor
air must be continuously introduced to provide a comfortable,healthy indoor environment. However, a building can providesubstantial protection against agents released outdoors if the flowof fresh air is filtered/cleaned, or temporarily interrupted or re-duced. Interrupting the flow of fresh air is the principle appliedin the protective action known as sheltering in place.The advantageof sheltering in place is that it can be implemented rapidly. Thedisadvantage is that its protection is variable and di- minisheswith the duration of the hazard. Sheltering requires that twodistinct actions be taken without delay to maximize the passiveprotection a building provides:

❍ First, reduce the indoor-outdoor air exchange ratebefore
the hazardous plume arrives. This is achieved by closing allwindows and doors, and turning off all fans, air conditioners, andcombustion heaters.

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

The level of protection that can be attained by sheltering inplace is substantial, but it is less than can be provided by higheffi- ciency filtration of the fresh air introduced into thebuilding. 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 protectionit provides. In most cases, air conditioners and combustion heaterscannot be operated while sheltering in place because operating themincreases the indoor-outdoor exchange of air.

❍ The duration of exposure. Protection varies with time,diminishing as the time of exposure increases. Sheltering in placeis, 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 inthe building after the hazardous plume has passed also affects thelevel of protection. Because the building slowly purgescontaminants that have entered it, at some point during plumepassage, the concentration inside exceeds the concentrationoutside. Maximum protection is attained by

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

❍ Natural filtering. Some filtering occurs when the agent isdeposited in the building shell or upon interior surfaces as airpasses into and out of the building. The tighter the building, thegreater the effect of this natural filtering.