FEMA 453
Design Guidance for
Shelters and Safe Rooms

FEMA document number 453

Design Guidance for Shelters and Safe Rooms, selected excerpts from chapter three. Full document here.


This chapter describes how to add CBR protection capability to a shelter or safe room.

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).

Class 1. In a Class 1 Safe Room, air is drawn from outside the room, filtered, and discharged inside the room at a rate sufficient to produce an internal pressure. The safe room is thus ventilated with filtered air, eliminating the constraints related to carbon dioxide accumulation. The internal pressure produced with filtered air prevents infiltration of outside air through leakage paths.

There are also ventilated Class 2 Safe Rooms and essentially these are ones for which the filter unit has inadequate capacity to produce a measurable overpressure with the size of the selected safe room. In essence, the filter units are over-rated by the filter unit manufacturer. Generally, if a filter unit capacity in cfm is less than one-fourth the area (in square feet) of the selected safe room, depending on the type of construction, it will not produce a measurable overpressure. Matching the filter unit capacity to safe room size for Class 1 (pressurized) Safe Rooms is addressed in Section

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. Selecting a Filter Unit for a Class 1 Safe Room. Generally, filter units available commercially are not designed to standards that ensure protection against highly toxic chemical, biological, and  adiological materials. Some may provide very little protection, particularly if the manufacturer is not experienced in designing and building ultra-high efficiency filter units. Minimum requirements for the Class 1 applications are listed below. In purchasing a filter unit, certifications relative to the following requirements should be provided by the vendor:

  • The filter unit must have both a HEPA filter and an ultra-highefficiency gas adsorber in series.
  • The adsorber must contain carbon impregnated ASZM-TEDA or the equivalent. Carbon mesh size should be 12×30 or 8×16.
  • The adsorber must have efficiency of at least 99.999 percent for physically adsorbed chemical agents and 99.9 percent for chemisorbed agents.
  • The adsorber must have a total capacity of 300,000 milligram (mg)-minutes per cubic meter for physically adsorbed chemical agents. Sizing the Filter Unit for Pressurization. If a filter unit is undersized (i.e., it provides inadequate flow for pressurization), the result is substantially lower protection factors and the system becomes a ventilated Class 2 Safe Room. Filter unit(s) must be sized to provide makeup air at a flow rate sufficient to produce a pressure of at least 0.1 inch water gauge (iwg) in the shelter for protected zones of one or two stories. Taller buildings require an internal pressure higher than 0.1 iwg to overcome the buoyancy pressures that result in extreme weather conditions (i.e., large temperature differences between the  inside and outside of the safe room).

Pressure gauge. For Class 1 Safe Rooms, the pressure gauge is the indicator that the system is operating properly. This gauge displays the pressure in the safe room relative to outdoors or outside the safe room indoors. If the reference pressure is measured indoors, the readings can be subject to variations caused by fan pressures unless other building heating, ventilation, and air conditioning (HVAC) fans are turned off when the safe room is in use. Reading the reference pressure outdoors can be subject to positive and negative variations caused by air flows over and around the building. If the pressure sensor is outdoors, it should be shielded from the wind. Indoors is the best location if the building HVAC fans are turned off when the safe room is in use.

Auxiliary or Battery Power. Class 3 Safe Rooms do not require electrical power to protect their occupants. Class 1 and Class 2 Safe Rooms require power for the air-filtration units to protect at a higher level than Class 3. If power is lost in a Class 1 or Class 2 Safe Room, it will continue to protect at the level of a Class 3 Safe Room as long as the room remains sealed.