Short answer. A shelter needs far more air to stay cool than to stay breathable. Swiss rules set 3 m³ per hour per person for hand-cranked, filtered emergency airflow in their largest small unit 1. The U.S. FEMA minimum reported by Cresson Kearny is 3 cfm (about 5 m³/h) per person, but a crowded shelter in hot, humid weather needs about 40 cfm (about 68 m³/h) 4.
Why does a closed shelter need ventilation at all?
Three things build up when people stay in a sealed room for hours or days: exhaled carbon dioxide, body heat, and water vapour from breath and sweat.
Carbon dioxide is the most familiar. Outdoor air now holds about 426 ppm of CO2 (global monthly mean, July 2026) 6. U.S. occupational limits allow 5,000 ppm averaged over a work shift, 30,000 ppm as a short-term limit, and treat 40,000 ppm as immediately dangerous to life or health 5. Listed symptoms of overexposure include headache, dizziness, restlessness and breathing difficulty 5. Kearny, who joined the Oak Ridge civil defense project in 1964, gives about 1 cfm (about 1.7 m³/h) of outdoor air per person as the amount needed just to keep exhaled CO2 from becoming dangerously concentrated, and names headaches and deeper breathing as the first signs 4.
Heat and humidity are usually the harder problem. Kearny describes how floors, walls and ceilings absorb body heat for a while, after which a fully occupied shelter without adequate forced ventilation can become so hot and humid in warm weather that occupants collapse, and some shelters can become dangerously hot within a few hours 4. CO2 sets the floor; heat sets the real requirement.
How much air per person do official sources give?
Figures differ because they answer different questions: the bare minimum to survive, the volume a filter is rated for, or the airflow needed to keep heat bearable. Metric conversions in brackets are arithmetic (1 cfm ≈ 1.7 m³/h).
| Source | Situation | Airflow per person |
|---|---|---|
| Kearny | Prevent dangerous CO2 build-up, cold weather | 1 cfm (≈ 1.7 m³/h) 4 |
| Swiss FOCP/BABS, TPH-11 | Emergency hand-cranked mode through the gas filter, VA 300 unit | 3 m³/h 1 |
| FEMA minimum, as reported by Kearny (1987) | Minimum ventilation rate per occupant for U.S. shelters | 3 cfm (≈ 5.1 m³/h) 4 |
| Kearny (Navy test, August 1962) | Summer air at this rate still produced difficult conditions | 12 cfm (≈ 20 m³/h) 4 |
| Kearny | Small permanent shelter, cooler parts of the U.S. | at least 15 cfm (≈ 25 m³/h) 4 |
| Kearny | Small permanent shelter, most of the U.S. | at least 30 cfm (≈ 51 m³/h) 4 |
| Kearny | Hot, humid heat wave; keep effective temperature rise to 2 °F (more if the sun heats walls or ceiling) | 40 cfm (≈ 68 m³/h) 4 |
The spread is large: from about 1.7 to about 68 m³/h per person, a factor of forty. The lowest value only keeps exhaled CO2 in check; the highest is meant to carry away body heat and water vapour in a heat wave 4.
How does the Swiss system handle fresh air versus filtered air?
Switzerland's standardised shelters define the airflow modes precisely. The model number of each small ventilation unit (VA 40, 75, 150, 300) equals its nominal airflow in m³/h, the volume its gas filter is designed for 1. In filter mode, used when outside air is contaminated, the airflow equals that nominal value 1. In fresh-air mode, used when outside air is clean, the unit must deliver at least double the nominal airflow without the gas filter 1. Emergency mode is hand-cranked through the gas filter and delivers the nominal airflow, except on the VA 300, where it is 200 m³/h, specified as 3 m³/h per person 1.
The brochure for residents calls the ventilation system "the lungs of your shelter" and lists its parts: an air intake, a blast valve and pre-filter, a blower, an NBC (gas) filter and an overpressure blast valve at the entrance 2. The overpressure keeps unfiltered air from leaking in 2. Each shelter provides at least 1 m² of floor area and 2.5 m³ of volume per occupant 2. FOCP states that ventilation and filter systems older than 40 years must be replaced because their efficiency declines 3.
Manual or powered ventilation: which is better?
Official Swiss practice is both. The ventilation unit can be operated both electrically and manually 2, and the specification defines a hand-cranked emergency mode 1. It caps the power needed at the crank, including emergency lighting, at 65 W for the VA 40, 85 W for the VA 75 and 135 W for the VA 150 and VA 300 in emergency mode 1. Residents preparing a shelter are told to check that the blue fresh-air mark on the air volume indicator can be reached by hand "without much effort" 2.
Kearny adds two cautions about powered systems. Most permanent shelters have centrifugal blowers that could not deliver enough air in hot weather for every planned occupant 4. In a Navy test, a high-speed electric ventilating pump plus friction in pipes and filters warmed the incoming air by 3 °F, which he warned could be disastrous in a heat wave 4. Natural "chimney" ventilation can be enough in cold weather but fails in hot weather 4.
Any engine-driven generator brings its own danger. CDC says never to use a generator or any gasoline-powered engine inside a home, basement or garage, or less than 20 feet from a window, door or vent, and to use a battery-powered or battery-backup carbon monoxide detector 7. Smoking also produces carbon monoxide and should not be allowed when ventilation is low 4.
Sizing a fan, an intake or a filter for a specific shelter is engineering work. A qualified engineer should do it, using the local climate, occupancy and threat.
What official agencies recommend
- Switzerland (FOCP/BABS). Standardised ventilation units with filter, fresh-air and hand-cranked emergency modes; fresh-air mode at least double the filter airflow; emergency airflow of the VA 300 set at 3 m³/h per person 1. Residents must check the blower, pre-filter, hand crank and overpressure valves before moving in 2, and must not alter the ventilation system 3.
- United States (FEMA minimum, via Kearny). A minimum of 3 cfm per occupant, described by Kearny as about three times the outdoor air needed to prevent CO2 headaches 4. Kearny shows that summer conditions call for far more: at least 15 to 30 cfm in small shelters, and 40 cfm in a hot, humid heat wave 4.
- United States (CDC, NIOSH). CO2 workplace limits of 5,000 ppm (time-weighted average) and an IDLH of 40,000 ppm 5; no fuel-burning engines indoors 7.
Where the Swiss and U.S. figures diverge, the gap mostly reflects purpose: the Swiss value is a filtered emergency minimum for reinforced-concrete shelters 2, while the higher Kearny values target heat removal in American summers 14.
Frequently asked questions
Can I stay in a sealed room without any ventilation?
Only briefly. Kearny notes that closing an earth-covered shelter's openings for an hour or two can keep out smoke from a nearby fire, but ventilation should not be restricted long enough for weaker occupants to overheat or develop CO2 headaches 4.
Is 3 m³/h per person enough?
It is a survival minimum. The Swiss emergency figure of 3 m³/h per person applies to filtered, hand-cranked operation 1, and the Swiss standard doubles the airflow when outside air is clean 1. In hot weather, Kearny gives far higher needs: about 40 cfm (about 68 m³/h) per person, and more where the sun heats the walls or ceiling 4.
What are the first signs that a shelter needs more air?
Headaches and deeper breathing are the first symptoms of CO2 build-up 4. NIOSH also lists dizziness, restlessness and breathing difficulty 5. Rising heat and humidity, felt most at the exhaust end, signal that heat is not being removed 4.
Can I run a generator to power the fan?
Not inside. CDC says generators and gasoline engines must never run inside a home, basement or garage, nor within 20 feet of a window, door or vent 7.
How often should a shelter ventilation unit be checked?
Swiss guidance asks residents to inspect the pre-filter, condensation trap, hand crank, airflow indicator and overpressure valves before moving in 2, and FOCP requires replacing systems older than 40 years 3.
Get a shelter assessed
Airflow, filtration and heat load depend on the room, the climate and the number of people, and their sizing belongs to a qualified engineer. For your own site, start from the risks around your home in your Phaenyx plan.
Sources
- Federal Office for Civil Protection (BABS), Switzerland — Technisches Pflichtenheft TPH-11, Kleinbelüftungsgeräte VA 40, 75, 150 und 300 (edition 1 October 2026) — https://www.babs.admin.ch/dam/de/sd-web/vmP0i9O4DOJe/tph11-de.pdf (accessed 2026-10-20)
- Federal Office for Civil Protection (FOCP), Switzerland — Shelters: their purpose, construction and utilisation (June 2023) — https://civil-protection-knowledge-network.europa.eu/system/files/2025-12/88_106_e_broschuere_vorbereitung_schutzraum.pdf (accessed 2026-10-20)
- Federal Office for Civil Protection (FOCP), Switzerland — Shelters for the population — https://www.babs.admin.ch/en/shelters-for-the-population (accessed 2026-10-20)
- Cresson H. Kearny, original edition Oak Ridge National Laboratory (1979) — Nuclear War Survival Skills, 1987 edition, Chapters 6 and 17 — https://archive.org/stream/NuclearWarSurvivalSkillsCressonKearny1987/NuclearWarSurvivalSkills_-_Cresson_Kearny_-_1987_djvu.txt (accessed 2026-10-20)
- NIOSH, CDC — Pocket Guide to Chemical Hazards: Carbon dioxide — https://www.cdc.gov/niosh/npg/npgd0103.html (accessed 2026-10-20)
- NOAA Global Monitoring Laboratory — Trends in CO2 — https://gml.noaa.gov/ccgg/trends/global.html (accessed 2026-10-20)
- CDC — What to Do to Protect Yourself During a Power Outage — https://www.cdc.gov/natural-disasters/response/what-to-do-protect-yourself-during-a-power-outage.html (accessed 2026-10-20)
Turn this into action
A plan worked out for your household.
- Water, food and equipment counted for each person and pet
- The nuclear plants and zones around your home
- Phaenyx Safe: one tap to tell your people you are safe
Sources
- Federal Office for Civil Protection (FOCP/BABS), Switzerland · Technisches Pflichtenheft TPH-11 für die Herstellung von Kleinbelüftungsgeräten der Typen VA 40, 75, 150 und 300 für Zivilschutzbauten (Ausgabe 1. Oktober 2026) checked 2026-10-20
- Federal Office for Civil Protection (FOCP), Switzerland (hosted by the EU Civil Protection Knowledge Network) · Shelters: their purpose, construction and utilisation (88.106 e, June 2023) checked 2026-10-20
- Federal Office for Civil Protection (FOCP/BABS), Switzerland · Shelters for the population checked 2026-10-20
- Cresson H. Kearny / Oregon Institute of Science and Medicine (1987 edition); Oak Ridge National Laboratory (original 1979 edition) · Nuclear War Survival Skills, 1987 edition (Chapter 6, Ventilation and Cooling of Shelters; Chapter 17, Permanent Family Fallout Shelters); original 1979 edition published by Oak Ridge National Laboratory checked 2026-10-20
- National Institute for Occupational Safety and Health (NIOSH), CDC · NIOSH Pocket Guide to Chemical Hazards: Carbon dioxide checked 2026-10-20
- NOAA Global Monitoring Laboratory · Trends in CO2 checked 2026-10-20
- Centers for Disease Control and Prevention (CDC) · What to Do to Protect Yourself During a Power Outage checked 2026-10-20