common terms and definitions

Explanations of the places and equipment in a shelter

Understand what everything is called when designing and building your shelter!

Common Bomb Shelter Terms – published on August 8, 2020

Protected spaces

  1. Safe room: a room that shelters the occupants from the effects of a severe weather event, a weapon of mass destruction (WMD) event, or an attack by a malicious person.
  2. Collective protection: an envelope (room) that is isolated from airborne toxins by a nuclear, biological, and chemical (NBC) air filtration system. Collective protection spaces are rated by airflow and air changes; which determines the occupancy limit of the space.
  3. Panic room: a fortified room that is hardened against breaching by malicious people. Steel and/or concrete are the usual building materials for panic rooms, but ballistic panels made from fiberglass, Kevlar, or Spectra may be used. Panic rooms are rated by their ballistic resistance.
  4. Bomb shelter: a space that is hardened against the effects of explosions. Bomb shelters are rated by how much pressure they can resist. This is usually given in bars (atmospheres) or pounds per square inch (PSI).
  5. Fallout shelter: a space that is resistant to the effects of radioactive fallout. Fallout shelters are rated by their resistance to radiation which is determined primarily by the amount of mass (weight) between the occupants and the source of the radiation (fallout).
  6. Storm shelter: a space that offers protection from severe weather. Areas that are prone to tornadoes should have storm shelters underground and areas that are prone to hurricane water surges or floods should have storm shelters above ground.

There is some overlap on these definitions. The most effective shelters are ones that incorporates all of these features.

Shelter terminology

Places in a shelter
  1. Airlock: a small room with two doors that prevents unfiltered air from entering your shelter. See the article on airlocks to understand everything you need to know about them.
  2. Disinfecting station: a place with a sink, shower, and/or other means to remove toxins from your clothes and body before entering your shelter.
  3. Mechanical room: a room that contains equipment like an NBC air filtration system, a heat pump, a carbon dioxide scrubber, etc.
  4. Approach: the path, stairs, landing, and/or tunnel that leads to the entrance of a shelter.
  5. Landing: a flat area at the bottom of stairs that allows a blast door swing outward.
  6. Hatch: a blast or ballistic resistant “door” that is horizontal when closed and vertical when open. You enter a submarine through a hatch.
  7. Pass-through: a small door that allows items like phones, keys, ammo, or money to be passed through a wall. Also known as a “firing port.”
  8. Anteroom: a separate room in a shelter that combines the airflow from multiple sources like the intake blast valves shown below. The anteroom is sealed up and several NBC air filtration systems draw air from the room.

These blast valves are mounted in an external wall of a anteroom:

Blast valves in an anteroom

Please see the article on bomb shelter plans for examples of some of these terms.

Equipment in a shelter

  1. HVAC: Heating, Ventilation, and Air Conditioning. This is an umbrella acronym that encompasses all of the components used in commercial and residential applications – ducts, heat pumps, air conditioners, range hoods, bathroom fans, etc. NBC air filtration systems are not normally considered to be general HVAC equipment. They are specialized filtration systems.
  2. NBC filtration systems: Nuclear, Biological, and Chemical air filters.
  3. CBRN filtration systems: a military acronym for Chemical, Biological, Radiological, and Nuclear. It is used interchangeably with NBC, but it breaks out radiation sources like dirty bombs from nuclear detonations.

Sealed spaces like bomb shelters require mechanical (powered) air ventilation.

NBC air filtration systems

These are the components in an NBC air filtration system:

  1. Ventilation pipes: the pipes that run to the surface to bring air into, and out of, an underground shelter. These are usually not included with the NBC air filtration systems and must be purchased or fabricated separately.
  2. Blower: The air mover in an NBC air filtration system. Like a fan, but with a shroud that surrounds the impellor and makes it more efficient by keeping the air in front of the blades instead of having it be thrown outward by centrifugal force.
  3. Filter canister: A round steel vessel that contains the HEPA filter and the bed of activated carbon. It is a separate component that is designed to be easily replaceable when the HEPA is fully loaded with particulates and/or the carbon has reached the breakthrough point and is no longer adsorbing gases.
  4. Intake blast valve: an automatic valve that shuts when presented with a high pressure event. Intake blast valves usually mount on the wall above the filtration system and have an integrated pre-filter inside their steel housing.
  5. Overpressure blast valve: the exhaust valve that is mounted away from the filtration system. The air flows from the filtration system to the overpressure blast valve and then out of the shelter through the exhaust ventilation pipe. They valves also have a one way check valve that only lets air go out so that if there is a wind gust outside, unfiltered air will not back up into the shelter through the exhaust ventilation pipe.
  6. Backup hand crank: a handle that fits onto the blower motor shaft and allows the occupants of a shelter to manually move air through the NBC air filtration system. This handle is only used when there is no power for the filtration system. Usually after the grid goes down and backup generators and/or batteries are exhausted.

Be sure and see the article on NBC air filtration systems to understand everything about these systems.

Detectors, gauges, and alarms
  1. Thermometer
  2. Humidity meter
  3. Differential pressure gauge
  4. Smoke alarm
  5. Low oxygen detector
  6. Carbon monoxide alarm (with an “M” – produced from incomplete combustion)
  7. Carbon dioxide alarm (with a “D” – one of the elements humans exhale)
  8. Radon detector

These are the recommended life support indicators for shelters. Be aware of the difference between carbon monoxide and carbon dioxide. They are not interchangeable. Carbon monoxide detectors are inexpensive and usually combined with smoke detectors. Carbon dioxide detectors cost several  times more. The instruments with digital or analog gauges need to be mounted where they can be easily read. The ones with alarms need to be heard.

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These links are to a complete set of the recommended life support indicators listed above.

Shelter construction methods

Effective underground shelters can be built from reinforced concrete or fabricated steel. Here are the usual shelter construction methods:

  1. Prefabricated shelter: a shelter that is built in a factory and delivered to the installation site. Usually fabricated from steel, fiberglass is also used. This is a steel shelter fabricated from a large culvert:
  2. Poured concrete shelter: a shelter that has been made from concrete poured into forms. The usual method is to pour the floor slab first, build the wall forms on top of it, pour the walls, build the ceiling forms on the walls, and pour the ceiling. These three pours may be combined for some installations.
  3. Block wall concrete shelter: similar to the poured concrete shelter, this construction method uses masonry blocks instead of the poured walls. Rebar and concrete must go into the voids of the blocks.
  4. Composite panel shelter: usually constructed above ground as a panic room, ballistic panels made from fiberglass, Kevlar, or Spectra are used to fortify a room against an attack from a malicious person.

The first three are shelters that are installed separate from a residence. A basement shelter has many advantages, but suffers from a lack of radiation shielding in most installations. Be sure and see the article on safe room location when deciding where to place your shelter on your property.

Installation of a shelter

  1. Excavation: removing the soil in preparation for installing an underground shelter (digging a hole).
  2. Backfilling: replacing the soil on top of the shelter in the hole.
  3. Overburden: the mass (weight) of the soil, buildings, and/or vehicles on top of an underground shelter.
  4. Packed earth: commonly used as shielding to protect a shelter from radiation. It must be mechanically compacted to increase the density.
  5. Soil compaction: the natural density of soil. The deeper the soil, the denser it is. The natural soil compaction at the bottom of a hole should not be disturbed prior to an underground shelter being installed or constructed.
  6. Soil permeability: the property of soil to transit air and water through it.
  7. Water table: the depth of the water underground. Large flat valleys have water tables, many other places have dynamic flows of water underground that vary in depth with the seasons.
  8. Bedrock: Solid rock that is under the layer of soil. It can be removed with machinery and explosives to increase the depth of a hole.

The water table and the bedrock level are two issues that can have a big impact on where you can place your underground shelter.

Concrete shelter construction terms
  1. Concrete: a mixture of water, sand, gravel, and cement. Suitable grades for building a shelter start at a compressive strength of 4,000 pounds per square inch (27.5 mPa).
  2. Concrete form: usually made from boards and wood panels, concrete forms constrain and shape the concrete after it is poured, but before it cures and becomes a solid.
  3. Slump test: a test performed on wet concrete to determine its viscosity.
  4. Screed: leveling and smoothing concrete as it cures.
  5. Rebar tie wire: pieces of wire or assemblies that are made just for tying rebar together. All of the rebar in a structure needs to be connected. When there are separate pours for the floor slab and walls, rebar is left sticking up vertically around the edges of the floor slab while it cures. Then, it’s tied to the rebar in the wall before the wall is poured. This ties in the floor rebar to the wall rebar. The wall rebar also needs to be tied into the ceiling rebar.
  6. Rebar (reinforcement bar): steel that has been formed to have ridges and embedded in concrete. The ridges provide a mechanical lock between the rebar and the concrete. Steel has a much higher tensile strength (ability to resist being stretched) than concrete. Rebar is the perfect complement to concrete which has a very low tensile strength, but has a high compressive strength. Rebar is sized by one-eighth of an inch increments and referred to as “number X” rebar. Number 4 is 1/2 inch (12.7 mm) in diameter: four eighths = 0.5 inches. Number 9 rebar is one and one eighth inch (28.575 mm) in diameter. This is rebar:

Rebar, steel reinforcement bar

Even if you are purchasing a pre-fabricated steel shelter, you may get into pouring a foundation for it.


Forces are energy that travels through objects. Our bodies, our homes, and the earth all have forces that are constantly transmitted through them. Structures need to be engineered to resist the forces that are anticipated to affect it. All of these forces are measured or calculated for engineered materials and components:

  1. Compression: pushing inward on an object from opposite directions. An example is to stack one brick on top of another. The bottom brick is under a compressive load from the force of gravity between the top brick and the earth.
  2. Tension: pulling outward on an object from opposite directions. An example is a tug-of-war contest. Two teams each take one end of a rope and try and pull the other team across a line. The rope is under tension. The term used in units of measure is “tensile strength.”
  3. Shear: a force that will cut (shear) an object. An example is a nail holding a board to a wall. The board is affected by gravity and pushes downward on the nail. But the nail is embedded in the wall and is fixed in position. There is a shear point in the nail between the wall and the board. With enough force, the board will shear off the nail right where it emerges from the wall.
  4. Torsion: the force that is exerted in a helical pattern through an object. An example is a driveshaft of a car. When the engine rotates the driveshaft, there is a line of force that is spirals around the shaft in the form of a helix. Torsion is the load going through the shaft, the force that it produces is called torque.

All of these forces can be present in one object. An example is a steel “I” beam that has both ends supported and a load placed on top of it in the center. The top of the beam is under compression, the center of the beam is under shear, and the bottom of the beam is under tension. If the load in the center of the beam is not concentric, but is place on one edge, it will impart a torsion load on the beam.

Forces in a beam

Shelter life terminology

  1. Protocol: the steps taken every time in a certain situation. Protocols should become habits so your subconscious will fall back on them when under extreme stress.
  2. Ingress: entering your shelter.
  3. Egress: exiting your shelter.
  4. Human contact categories: you, your tribe, your neighbors, unknown people, and malicious people. You should have concentric rings of loyalty for these categories from highest to lowest, respectively. Other terms for the last three categories are friendlies, bogies, and bandits, or neighbors, trespassers, and predators.
  5. Threat escalation: surveillance, detection, observation, and interaction.

Every critical procedure needs to have a written protocol that everyone understands.

Air filtration

  1. HEPA – high efficiency particulate air filter. Filters meeting the HEPA standard must meet certain levels of efficiency. The US Department of Energy mandates that 99.97% of particles whose diameter is greater than or equal to 0.3 µm be filtered out of the airflow.
  2. Activated carbon – a form of carbon that has an enormous surface area compared to its mass due to a high level of porosity. Most NBC air filtration systems use granular carbon that has been treated with certain chemicals that enhance its ability to adsorb certain gases.
  3. Adsorption – (with a “D”) this is the adhesion of molecules from a gas or a liquid to a surface. This is not absorption (with a “B”), in which a fluid is dissolved by or permeates a liquid or solid.
  4. Airflow – the amount of air supplied by an air handler (HVAC, NBC filter, etc.). Usually measured cubic feet per minute (CFM) or cubic meters per hour (CMH). These acronyms are similar and are easy to confuse when reviewing specifications of filtration systems from countries that use the metric system.
  5. Fresh air vs. filtered air – many NBC air filtration systems have a way of bypassing the filter cartridge and just pumping unfiltered air. They have this capability to provide an air supply to a protected space when there is no airborne threats – such as a severe weather event or if you are doing maintenance inside your shelter. The system airflow ratings are much lower for filtered air because the filter cartridge provides significant resistance to the airflow. When sizing your filtration system for your shelter and number of occupants, always use the filtered air rating.
  6. Pressure: energy (force) in a gas or liquid that is exerted equally in all directions. Pressure is typically measured in units of force per unit of the surface that the force acts upon. Typically measured in these units:
      • Inches of water (27.67 inches of water = 1 PSI)
      • Pounds per square inch (PSI)
      • Pounds per square foot (PSF)
      • Kilopascals (kPa)
      • Bars (1 bar = 0.987 atmospheres = 14.695 PSI)

A word about pressure: inflating a tire on a vehicle to 30 PSI is fast and convenient. When you take the hose off the tire, you get a little whoosh of air that doesn’t seem like much. But if your entire body were to be hit by a 30 PSI blast load, you would likely die. The equivalent wind speed to 30 pounds per square inch is over 1,200 miles per hour – far beyond the fastest wind speed ever recorded on Earth. In a 30 PSI pressure event, a standard 36 inch x 84 inch door (about 1 meter x 2 meters) will have 90,720 pounds (41,150 kg) of force acting upon it. That is 45 tons of weight hitting one door. 30 PSI is no joke – it will turn almost any above ground structure into rubble.

  1. Pressure differential: the difference in pressure between two spaces.
  2. Overpressure: a term used to describe the positive pressure inside of an enclosed space. The shelter pressure must be at least 0.3 inches of water gauge above the atmospheric pressure outside of the shelter in order to meet the requirements of a Class 1 protected space as defined by the US Army Corps of Engineers.
  3. Differential pressure gauge: a gauge that samples the air pressure inside and outside of a shelter and displays the difference. They are usually mounted inside the shelter and have a sampling tube that goes through a wall or the ceiling. The sampling tube has the same air pressure as the outside.
  4. Pressure resistance: in air ventilation and filtration it is the resistance to the airflow by the ventilation pipes, air filters, blast valves, and anything else the air has to move through or around. Pipes and ducts have known resistance based on their size, shape, and how rough their internal surfaces are. Components like blast valves and filter cartridges have a “pressure drop” rating that quantifies the resistance at certain airflows.

NBC air filtration manufacturers should provide all of these specifications upon request.

Blast protection

  1. Blast load: the amount of pressure produced by a detonation. The blast load will decrease as it is measured farther from the point of detonation. Structures (buildings, shelters, etc.) and components (blast doors, blast valves, etc.) are engineered to withstand the effects of known blast loads. A 3 bar blast rating is a typical for components like blast doors and blast valves. Blast loads are usually given in peak pressure and duration or an impulse figure that incorporates both the peak pressure and duration.
  2. Effects of a detonation: the direct effect is called the incident pressure wave. When there are surfaces (like the ground) that are not parallel or perpendicular to the incident pressure wave, they reflect the energy and are called the reflective pressure wave. When the incident and reflective pressure waves combine, it is called a mach stem. Mach stems produce very high pressures for short durations.
  3. Static load: a pressure that has an infinite duration. Usually any duration measured in seconds is considered a static load.
  4. Dynamic load: expressed as a peak pressure (in PSI or bars) and duration (in milliseconds) or an impulse. An impulse is a figure that incorporates both the peak pressure and duration.
  5. Rebound response: when a door or a hatch is presented with a positive pressure blast load, the force “seats” the door leaf against the frame. The force is transmitted from the door leaf, to the frame, to the wall. The door leaf will flex inward due to this positive pressure and store energy – like a large, flat spring. When the positive phase of the blast load is over, the energy stored in the door leaf is returned and the door leaf will spring back and “unseat” from the frame. This is called the rebound response. It is usually calculated at 50% of the seating blast load, but it can be specified at up to 100% of the blast load. This force is transmitted from the door leaf, to the hinges and latches, to the frame, to the anchorage, and then to the wall.

Blast resistant components are certified by either testing or calculations.

The take-away

You should understand these terms and definitions so you can effectively communicate (and negotiate) with your shelter supplier, builder, and/or contractor!

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Next article: Common mistakes made when designing and building shelters

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