FEMA 453 Design Guidance for Shelters and Safe Rooms

model building codes all have provisions that allow occupancies
as concentrated as 5 square feet per person. The American Society
of Heating, Refrigeration, and Air-Conditioning Engineers
(ASHRAE) recommends that a minimum head room of 6.5 feet
and a minimum of 65 cubic feet of net volume be provided per
shelter occupant. Net volume shall be determined using the net
area calculated for the space.
ASHRAE Ventilation Standard 62-1981, Ventilation for Smoking-Permitted
Areas defines minimum outdoor air supply rates for various
types of occupancy. These rates have been arrived at through a
consensus of experts working in the field. A minimum rate of 5
cfm per person for sedentary activity and normal diet holds the
carbon dioxide (CO 2 ) level in a space at 0.25 percent under steady
state conditions. Although normal healthy people tolerate 0.5
percent CO 2 without undesirable symptoms and nuclear submarines
sometimes operate with 1 percent CO 2 in the atmosphere,
a level of 0.25 percent provides a safety factor for increased
activity, unusual occupancy load, or reduced ventilation. The
ASHRAE Handbook 1982 Applications Environmental Control for
Survival states that carbon dioxide concentration should not exceed
3 percent by volume and preferably should be maintained
below 0.5 percent. For a sedentary man, 3 cfm per person of
fresh air would maintain a CO 2 concentration of 0.5 percent.
1.7.1.1 Tornado or Short-term Shelter Square Footage Recommendations.
Historical data indicate that tornado shelters will
typically have a maximum occupancy time of 2 hours. Because the
occupancy time is so short, many items that are needed for the
comfort of occupants for longer durations (in hurricane shelters)
are not recommended for a tornado shelter. FEMA 361, Section
8.2 recommends a minimum of 5 square feet per person for tornado
shelters. However, other circumstances and human factors
may require the shelter to accommodate persons who require
more than 5 square feet. Square footage recommendations for
persons with special needs are presented below; these recommendations
are the same as those provided in the FEMA 1999 National
Performance Criteria for Tornado Shelters:
1-34 design considerations