FEMA 453 Design Guidance for Shelters and Safe Rooms

of wind and buoyancy. It therefore provides a lower level of
protection than a Class 1. If exposed to an unfilterable gas, the
unventilated Class 2 Safe Room retains a level of protection
provided by the sealed enclosure. The unventilated Class 2 Safe
Room would thus not have a complete loss of protection as could
occur with the gas penetrating the filter of a Class 1 or Class 2
ventilated Safe Room.
The Class 3 Safe Room, with no air filtration, is the simplest
and lowest in cost. It can be prepared with permanent sealing
measures or with the quick application of expedient sealing techniques
such as applying duct tape over the gap at the bottom of
the door or over the bathroom exhaust fan grille. The disadvantage
is that there is no intentional ventilation; therefore, this class
of safe room cannot conform to ventilation requirements of other
types of emergency shelters.
Most safe rooms are designed as standby systems; that is, certain
actions must be taken to make them protective when a hazardous
condition occurs or is expected. They do not provide protection
on a continuous basis. Merely tightening a room or weatherizing a
building does not increase the protection to the occupants. Making
the safe room protective requires turning off fans, air conditioners,
and combustion heaters as well as closing doors and windows. It
may also involve closing off supply, return, or exhaust ducts or
temporarily sealing them with duct tape. In a residence, taking
these actions is relatively simple and can be done quickly. In an office
building, doing so usually requires more time and planning,
as there may be several switches for air-handling units and exhaust
fans, which may be at diverse locations around the building.
Unventilated safe rooms have been widely used in shelteringin-place
to protect against accidental releases of industrial
chemicals. Local authorities make the decision on whether to
shelter-in-place or evacuate based on conditions, the likely duration
of the hazard, and the time needed to evacuate. Although
sheltering-in-place (i.e., use of an unventilated safe room) is applicable
for relatively short durations, experience shows that it
3- CBR ThReaT pRoTeCTion