Posted on March 31, 2010 10:02 am

Air Filters


Outside air
ventilation is a very critical requirement for all shelters. Any extended use of a sealed shelter will
require constant air changes. There have been numerous medical problems related to fallout
shelters that were traced to stale air, not radiation poisoning. Older Department of Defense
shelter design guidelines recommended 4 cubic feet per minute (CFM) of outside air ventilation per
shelter occupant, but most shelter designers today feel this is the threshold for carbon dioxide
sickness and much higher ventilation rates are needed. Since outside ventilation air is also
necessary to reduce high humidity levels and overheating in any confined area, a more realistic
value of 10 CFM per person in colder climates and 30 to 40 CFM per person in hotter climates is more

Several manufacturers offer a rotary hand operated blower designed to ventilate underground shelters continuously, but
these can be ineffective in larger shelters due to their low flow rates and the need for manual
cranking 24 hours per day. There are several homemade manual ventilator designs similar to the
old blacksmith forge bellows available on the Internet.

All shelter air intakes, exhaust vents, and piping should include a manual internal shut-off valve or gate. You should
also understand that a typical tornado or hurricane can tear off or bend over all pipes and vents
sticking up out of the ground or through a concrete floor slab. Locate all vents to minimize the
potential for wind damage and provide good structural support. Wind speeds and air pressures
related to a nuclear explosion can be multiple times higher than the most severe hurricane. A turned-down
steel pipe intake vent will minimize the risk of vandalism or rainwater flooding a below grade
shelter. In general, a properly designed air intake will keep air velocities low to avoid pulling in any
falling radioactive dust particles. Air intakes should be turned-down steel pipes located high
enough above any radioactive dust that has settled out and covered the ground.

A combination HEPA filter and an activated charcoal filter are needed to make ventilation air totally safe from biological
and radioactive contaminates. All outside air intake and exhaust vents should be manually closed off during the initial threat of any attack, then
reopened when the threat is reduced. Many of the automatic spring loaded blast valves manufactured to protect the external openings of military shelters
will no longer operate when required after providing a home for birds, rodents, and insects for years.

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