Many shelters are sold with a vent pipe or two and a small fan. With a family inside, heat and humidity, not oxygen, become the danger within hours.
People are heaters
Every person at rest gives off heat and water vapor. In a closed room with a family inside, temperature and humidity rise steadily. The ventilation research done at Oak Ridge National Laboratory warned that fully occupied shelters without enough forced ventilation could become so hot and humid in warm weather that occupants would collapse, sometimes within a few hours.
What the tests found
FEMA's historic minimum was 3 cubic feet of outside air per minute per person. In a Navy shelter test at 12 cfm per person in hot weather, conditions still became difficult, and 34 of 99 men developed heat rash. The Oak Ridge guidance is about 40 cfm per person in hot weather to keep the temperature rise to about 2°F. In cold weather, about 1 cfm per person is still needed just to keep exhaled carbon dioxide from reaching dangerous levels.
Clean air, not just more air
Moving air is only half the job. Air brought into a shelter during a fallout, chemical or biological event must be filtered. Switzerland specifies the chain for its shelters: a protected intake, a blast valve and pre-filter, a ventilation unit, an NBC gas filter, and an overpressure valve at the entrance, with airlocks on larger shelters. Positive pressure keeps unfiltered air from leaking in.
What to ask
Ask any builder for the fresh-air rate per person at full occupancy in summer, the filter classes, and what happens when the power is out. Read NBC air filtration explained and 12 Questions to Ask Any Bunker Builder.
Key takeaways
- People in a closed room produce heat and moisture continuously.
- In Navy tests at 12 cfm per person, conditions became difficult and a third of the men developed heat rash.
- Oak Ridge guidance is about 40 cfm per person in hot weather.
- Even in cold weather, about 1 cfm per person is needed to keep carbon dioxide from building up.
- Protection requires the full filtered chain: blast valve, pre-filter, blower, NBC filter, overpressure.
This guide is general information to help you plan. It is not engineering, legal or water-rights advice, and requirements vary by state and county. Every Styrkholm project is designed by licensed professionals for the specific site.