FEMA 453 Design Guidance for Shelters and Safe Rooms

3.1 oVeRVieW
This chapter describes how to add CBR protection capability
to a shelter or safe room.
CBR ThReaT pRoTeCTion
CBR THReaT pRoTeCTion 3
A CBR safe room protects its occupants from contaminated
air outside it by providing clean, breathable air in two ways: (1)
by trapping air inside the room and minimizing the air exchange
(an unventilated safe room) and (2) by passing contaminated air
through a filter to purify it as it is supplied to the room (a ventilated
safe room).
Unventilated safe rooms that are tightly sealed cannot be occupied
for long periods without the risk of high carbon dioxide
levels. This constraint does not apply to ventilated safe rooms,
which can be designed to provide filtered and conditioned fresh
air at any desired rate. Ventilated safe rooms can therefore be
used on a routine basis, although most are designed as standby systems,
not for continuous, routine use.
Obtaining protection from an unventilated safe room can be as
simple as selecting a relatively tight room, entering it, and closing
the door. This procedure is commonly referred to as expedient
sheltering-in-place. In this simple form, a safe room protects its
occupants by retaining a volume of clean air and minimizing the
infiltration of contaminated outdoor air. In practice, however, a
safe room is not perfectly tight. The natural forces of wind and
buoyancy act on small, distributed leakage paths to exchange air
between the inside and outside.
As contaminated air infiltrates a safe room, the level of protection
to the occupants diminishes with time. With infiltration in a sustained
exposure, the concentration of toxic vapor, gas, or aerosol
in the safe room may actually exceed the concentration outdoors
because the sealed safe room tends to retain the airborne
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