FEMA 453 Design Guidance for Shelters and Safe Rooms

m Class 2. This class also includes air filtration, but with little
or no internal pressure. Without positive pressure, the safe
room does not prevent the infiltration of contaminated air.
A Class 2 Safe Room may be ventilated or unventilated. In an
unventilated Class 2 Safe Room, air is drawn from inside the
safe room, filtered, and discharged inside it. In a ventilated
Class 2 Safe Room, air is drawn from outside but at a flow rate
too small to create a measurable differential pressure.
m Class 3. This class has no air-filtering capability and is
unventilated. It is a basic safe room that derives protection only
by retained clean air within its tight enclosure. Use of the Class
3 Safe Room is commonly referred to as sheltering-in-place.
Table 3-1: Comparison of the Three General Classes of Toxic-agent Safe Rooms
Class Protection Cost Advantages and Limitations
1. Ventilated and
pressurized with
filtered air
2. Filtration with
little or no
pressurization
The Class 1 Safe Room provides the highest level of protection for
most chemicals, but the lowest level of protection for those chemicals
that are not filterable. It is also the most expensive option. Its
disadvantage is that it does not protect against a limited number
of toxic gases that cannot be filtered by conventional gas filters/
adsorbers.
Although the Class 2 Safe Room employs air filters, it does not
prevent the infiltration of outdoor air driven by natural forces
CBR ThReaT pRoTeCTion
high high
medium medium
3. Unventilated, no
filtration low low
protection has no time limits, but it provides no
protection against some toxic chemicals of high
vapor pressure.
Unventilated Class is protective against all gases,
but protection diminishes with duration of exposure
(and against non-filterable gases).
protective against all agents, but protection
diminishes with time of exposure. Carbon dioxide
buildup may limit time in the shelter.
3-3