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5.4.2 Applying External Filtration

Applying external filtration to a building requires modifica-tions to the building’s HVAC system and electrical system, and italso usually requires minor architectural changes to reduce airleakage from the selected protective envelope. These changes arenecessary to ensure that, when the protective system is inoperation, all outside air enters the building through the filters.The air exchange that normally occurs due to wind, chimney effect,and operation of fans must be reduced to zero. This is achievedmainly by introducing filtered air at a rate sufficient to producean overpressure in the building and create an outward flow throughall cracks, pores, seams, and other openings in the buildingenvelope. For standby systems, dampers are normally required totighten the envelope in transitioning to the protec- tive mode. Thelevel of overpressure required varies with weather conditions andheight of the building.The capacity of filtration units needed forprotection is determined by the leakage characteristics and size ofthe building. The cost of installing a high efficiency filtrationsystem varies directly with the leakage rate; higher leakage rateequals higher costs, and the need for additional heating andcooling capacity for the filtered air.Filtration system capacitymust be matched to the leakage of the building to achieve maximumprotection. Fan-pressurization tests are usually performed onbuildings to determine their normalized leakage rates. Nominal dataon the leakage rates of various types of buildings are available inthe U.S. Army Corps of Engineers Engineering Technical Letter (ETL)1110-3-498, Design of Collec- tive Protection Shelters to ResistChemical, Biological, and Radiological (CBR) Agents, (February 24,1999) and can be used to estimate the leakage rate of abuilding.