Shelter sellers talk about filters. The number that decides whether a family can stay below grade is airflow per person, and it changes with the season and the number of people inside.
Start with the people
A shelter's air load comes from the bodies in it, not the size of the room. Each person adds carbon dioxide, heat and water vapor every minute. A large shelter with two people is easy to ventilate; a small one with eight people in July is not. That is why every sizing question starts with who will be inside, for how long, and in which season.
The four numbers that matter
Oak Ridge National Laboratory's shelter ventilation research, published in Nuclear War Survival Skills, gives the range:
- About 1 cfm per person is the floor, even in cold weather, to keep exhaled carbon dioxide from building to dangerous levels.
- 3 cfm per person was FEMA's historic minimum.
- 12 cfm per person was not enough in a Navy summer test: conditions became difficult and a third of the men developed heat rash.
- About 40 cfm per person is the Oak Ridge guidance for hot weather, to hold the temperature rise to a couple of degrees.
What that means for your household
| Household | Cold-weather floor (1 cfm each) | FEMA historic minimum (3 cfm) | Navy test level (12 cfm) | Hot-weather guidance (40 cfm) |
|---|---|---|---|---|
| 4 people | 4 cfm | 12 cfm | 48 cfm | 160 cfm |
| 6 people | 6 cfm | 18 cfm | 72 cfm | 240 cfm |
| 12 people | 12 cfm | 36 cfm | 144 cfm | 480 cfm |
The gap between the first and last columns is the whole story. A family of six that is comfortable on 18 cfm in January can need 240 cfm in July. A system sized for the brochure, not the season, fails in summer.
Filter mode is a different number
When the air outside is contaminated, every cubic foot must pass through the NBC filter chain, and that chain has a rated flow. Filtered airflow is usually far lower than open ventilation. Our Premium unit, a Swiss VA-150 class system, moves about 176 cfm of fresh air in ventilation mode but about 88 cfm through the filters. In filter mode, the job of removing heat and humidity moves to a separate cooling and dehumidification system sized for full occupancy. See heat and humidity.
Pressure and power
Filtered air also has to keep the shelter at a slight positive pressure, so leaks blow out rather than in. Switzerland, which builds shelters for essentially its whole population, specifies the chain: a protected intake, blast valve and pre-filter, the ventilation unit, the gas filter, and an overpressure valve at the entrance. All of it needs power. Every Styrkholm air system has battery backup and a hand crank, and we size the battery to run the blower through an outage rather than relying on cranking alone.
What to ask
Ask any builder three things: the fresh-air rate per person at full occupancy in summer, the filtered flow rate in filter mode, and how heat is removed when the filters are running. Compare our three tiers on the air systems page, or size a shelter with the Shelter Configurator.
Key takeaways
- Size air by people, not by square feet.
- About 1 cfm per person keeps carbon dioxide in check in cold weather; summer heat needs up to 40.
- NBC filtration runs at a lower rate than ventilation, so heat and humidity need their own system.
- Ask any builder for the cfm per person at full occupancy in July.
- Battery backup and a hand crank keep air moving when the power fails.
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.