NBC air filtration systems

Your shelter is a life support system

When you hold your breath, oxygen decreases and carbon dioxide builds up in your body. The same thing happens in shelters without proper ventilation.

NBC Air Filtration Systems – published on July 24, 2020

The key factors for selecting an NBC air filtration system

  • The best NBC air filtration systems are made in Europe and Israel. They are the only ones available to civilians that have third party testing and certification for their hundreds of thousands of installations worldwide.
  • There are American and British made filtration systems that were designed for the civilian market. They are not independently tested and have a small fraction of the activated carbon that is required for days or weeks of constant use. These systems cost almost as much as the Israeli and European systems and their limited carbon capacity is a significant inherent design weakness. Their small blowers just cannot move air through any more carbon.


What is an NBC air filtration system?

An NBC air filtration system is a purpose-built wide spectrum air filtration system that is designed to draw air from the outside, filter it, and expel it into an enclosed space with enough force to create positive relative pressure. Let’s break this definition down:

  • NBC: Nuclear, Biological, and Chemical. This is a common acronym for weapons of mass destruction, but the military uses CBRN: Chemical, Biological, Radiological, and Nuclear.
  • Purpose-built: it was designed from the start to function as a bomb shelter air filtration system. There have been kits sold to add an intake hose to room air filters. These should be avoided. There is no way to upgrade a room air filter into an NBC air filtration system.
  • Wide spectrum: they filter out both particulates (radioactive fallout and biological aerosols) and gases (chemical weapons and industrial accidents). The activated carbon is usually impregnated with certain chemicals to increase its efficiency against the higher volatility warfare gases.
  • Draw air from the outside: they have an intake hose that draws outside air into the filtration system.
  • Positive relative pressure: when the filter is in operation, it pressurizes the air inside the shelter so it has a higher pressure than the outside air pressure. This will ensure that if there are any small leaks, the filtered air inside the shelter will flow outward and prevent unfiltered outside air from leaking inside. Without “overpressure,” the seals on the doors and hatches in your shelter must not leak. With overpressure, they can leak a little without exposing the shelter to unfiltered (toxic) outside air.


NBC air filtration system for a bomb shelter

Matching your shelter’s air to Earth’s atmosphere

Human respiration converts oxygen into carbon dioxide. The atmosphere has about 20% oxygen and 0.04% carbon dioxide. In the shelter, we need the oxygen to stay above 19.5% (195,000 parts per million) and the carbon dioxide to stay below 0.2% (2,000 parts per million). You should have alarms for both low oxygen and high carbon dioxide in your shelter. Be sure to understand that there are many low cost carbon monoxide (with an “m”) available, but we are concerned with carbon dioxide (with a “d”) when discussing ventilation. The complete list of recommended life support indicators in shelters is: thermometer, humidity meter, differential pressure gauge, smoke alarm, low oxygen detector, carbon monoxide alarm, carbon dioxide alarm, and a radon meter. Two or three of these instruments are sometimes combined into one unit. There are Amazon links below that will allow you to read or receive a warning for all of these variables in your shelter air quality.

Full disclosure: as an Amazon Associate I earn from qualifying purchases. The convenient links below will take you directly to the items on Amazon and help support this website. Thank you very much!

The cycle of life

Shelters require that oxygen rich air is introduced and then exhausted back outside so the carbon dioxide and moisture that the occupants exhale is removed. Without proper ventilation, humans will die of carbon dioxide poisoning from their exhaled breaths before oxygen deprivation. The oxygen in your shelter will be continuously replenished when the filtration system is in operation. As long as the air is introduced at one end of your shelter and exhausts out the other end, the carbon dioxide and moisture that the occupants exhale will be removed with the air that goes out. We want the filtered air to transit the entire shelter before it is exhausted. We do not want to short circuit the ventilation in your shelter by locating the filtration system and the exhaust blast valve too close together. Please see the article on bomb shelter plans for the recommended shelter layouts that incorporate these principals.

Airflow, pressure, and air exchanges

Let’s discuss the properties of the filtered air that your system puts out. A stream of air has both pressure and volume that can be measured and/or calculated. Air behaves like a fluid and exerts pressure in all directions. You will need to know the filtered output of your filtration system in cubic feet per minute (CFM) or cubic meters per hour (CMH) and the volume of your shelter in the same unit of measure. Companies who manufacture NBC air filtration systems advertise the volume of air their systems put out. Be sure and use their “filtered” air rating and not their “fresh” air rating when calculating your airflow requirements. Fresh = unfiltered.

How much air does your shelter require?

This comes down to two specifications: air supply per person and air changes per hour. The air supply per person is based on how much oxygen each person will use. The air changes per hour is based on how much carbon dioxide and moisture that the shelter occupants exhale. It needs to be flushed from the shelter.

Air supply per person

This is how to calculate the filtered air requirements for your shelter. The US military minimum specification for collective protection is 5 cubic feet per minute (CFM) per person. This converts to 8.5 cubic meters per hour (CMH) or 2.3 liters per second (L/S). The Israeli minimum specification is 3.5 CFM per person (5.9 CMH or 0.24 L/S). The Israelis have extensive experience with manufacturing and using NBC air filtration systems, but they tend to only use them for a few hours at a time. Most people building or installing underground shelters plan on sheltering in place for days or weeks.

Because of this, 5 CFM per person should be the minimum. The minimum guideline from American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) for motel rooms, offices, and bedrooms is also 5 CFM. The advertised filtered air rating for NBC filtration systems is usually the “system” output without the resistance of the ventilation pipes or the exhaust overpressure blast valve. Anything that resists the airflow will reduce it. The airflow is in a circuit. If you choke it down anywhere, you choke it down everywhere.

There needs to be a margin of safety built into whatever size filtration system you install. It’s easiest to stay with the 5 CFM number and factor in extra occupants. If you have four people in your family and a filtration system that puts out 88 CFM, you have over a 100% safety margin (5 CFM x 4 = 20 CFM minimum). If your filter becomes clogged with particulates and the airflow goes down or your neighbor and his family show up and your occupancy increases, you still have enough air to remain in your shelter. Minimum air supply per person = 5 CFM (8.5 CMH, 2.3 L/S).


The word “overpressure” is just an expression for positive relative pressure inside a protected space. Air pressure inside a pressure vessel like your vehicle’s tire is simply stored energy. When you puncture that tire, this energy is released outward because there is more pressure inside the tire (~30 PSI) than outside (~14.5 PSI). Modern battlefield vehicles like tanks and armored troop carriers maintain positive pressure in their crew compartments to resist outside pressure from wind gusts to push toxic air through the door seals. Shelters also need to have overpressure and there are specifications for how much pressure is recommended:

The US Military specifies a minimum positive relative pressure for collective protection areas of 0.3 inches of water gauge (7.62 mm of water gauge or about 75 Pascals). Water gauge is a small unit of measure for pressure. There are 27.7 inches (703.5 mm) of water gauge in one pound per square inch (PSI). This 0.3 inch specification corresponds to a 25 mile per hour (40 km) wind gust. Air behaves like a fluid and exerts pressure in all directions. Wind gusts will act directly on the seals in the doors and hatches. As with the airflow and air changes, we want a margin of safety with the overpressure. A differential pressure gauge will show the difference in pressure between your shelter and the outside. Minimum sustained pressure differential between your shelter and the outside: 0.3 (7.62 mm) inches of water gauge.

At Amazon:

A differential pressure gauge and the installation kit.

Air exchanges

The US Environmental Protection Agency recommends a minimum of 0.35 air changes per hour in homes. This is one air change every three hours. It is a very low specification based on minimum indoor air quality. Most homes have at least 3 air changes every hour. Air quality is a good measure for establishing how many air changes your shelter should have every hour. Since the airflow removes carbon dioxide, moisture, and odors, the air change rate is directly connected to air quality. ASHRAE recommends four changes an hour for all enclosed space and it goes up from there for certain places like kitchens, laundry rooms, and hospitals. Cooking food, washing clothes, and caring for sick people are all things that need to happen in shelters as well. But we can’t just turn up the filter when cooking food. Most NBC air filtration systems do not allow for adjustable airflow as a safety precaution. They are either on or off. This prevents a situation where not enough air is being supplied for the occupants, yet the filter is on and making noise. If it’s moving air, it’s moving the maximum amount. Air that has been properly filtered for nuclear, biological, and chemical agents is expensive so we are going to give two recommendations. Just like with the air supply per person above, we like to see a safety margin added to these numbers: a desired minimum of 4 air changes per hour and an absolute minimum of 2 air changes per hour.

An option to avoid

There are some expensive options that allow an enclosed space to not require true ventilation. This equipment was developed for the mining industry but has recently been marketed for bomb shelters. As mines got deeper, refuge chambers were required to keep the miners alive while waiting for rescue that might take days to reach them. These refuge chambers did not have access to outside air and the air in the mine could be filled with toxic gases or very fine mineral dust that will quickly overwhelm any filtration system. Equipment was developed that facilitates the absorption of carbon dioxide by chemical consumables and bottled oxygen is used. But these systems do not produce the overpressure that helps to seal toxins out of your shelter and the cost of their consumables is significant. These systems should only be used as they were designed: a last ditch option. They should only be used as a backup to an NBC air filtration system.

How NBC air filtration protects you

The filtration system converts electrical energy to air pressure. This pressure and the airflow that it provides is a valuable commodity that protects you in several ways:

  1. Preventing toxic leaks in the air filtration system. The blower (fan) is the last component in the filter housing. This puts everything before the blower in negative pressure relative to the air pressure in the shelter. The intake blast valve, the intake hose, and the filter cartridge are all before the blower. If there are any leaks in these components, filtered air from the shelter will be drawn into the filtration system instead of unfiltered air being expelled outward into your shelter. The filtration system contains unfiltered (toxic) air, but it protects itself (and you) by creating a pressure differential between the inside and outside of the filtration system housing and components.
  2. Preventing toxic leaks into the shelter. Everything after the blower is in positive pressure. This includes all of your shelter up to the exhaust overpressure blast valve. This overpressure will ensure that if there are any small leaks in the shelter, the filtered air will flow outward instead of unfiltered air flowing inward. The filtration system protects the entire shelter (and you) by creating a pressure differential between the inside of the shelter and the outside world.
  3. You can (and should) use the airflow from the NBC air filtration system to flush a decontamination airlock. This airlock needs to be the last space that the airflow transits. Please see the articles on shelter layouts and airlocks for more information. The filtration system protects you by flushing the airlock to remove unfiltered (toxic) air before you open the inner door.


Air filtration – particulates and gases

Solids and gases are two types of matter. Small solids that are in gases (like dust in the air) are called particulates. Particulates are sieved out in progressively finer filter media and then the air is drawn through a bed of tightly packed activated carbon to remove harmful gases. Some events like forest fires and nuclear explosions release both particulates and gases that must be removed from the air you breathe.

Particulate air filtration

The particulates must be removed from the air before it is drawn through the carbon. There is usually a pre-filter integrated into the intake blast valve to filter the large particles and extend the life of the HEPA filter. The minimum requirement by United States standards is to remove at least 99.97% of airborne particles 0.3 micrometers (μm) in diameter. The Israeli and European filtration systems have similar standards.

The particulates that most concerning are radioactive fallout and biological aerosols. These are half of the “N” and all of the “B” in “NBC air filtration system” (Nuclear and Biological). The other half of “N” is a gas called radioactive iodine that is released during nuclear events that the carbon adsorbs.

Radioactive fallout consists of small particles of dust and ash that are created during a nuclear detonation. These particulates are thrown up into the atmosphere by the blast, and then fall back to earth. Fallout has different sizes and timelines. Local fallout is an acute danger that starts shortly after the detonation and consists mostly of the larger particles. It can extend far beyond the other effects of a nuclear detonation (direct radiation, blast, and heat waves). What makes it an acute danger is the amount of radiation. It has more mass than the finer particles and can accumulate rapidly on flat surfaces. Global fallout is a chronic danger. It comes from the finer particles are thrown up into the upper atmosphere that can take months or years to fall out.

Biological aerosols are expelled when people speak, sneeze, or exhale. These airborne body fluids can be spread through duct systems in buildings and homes as well as the intake ventilation pipe of your shelter. They are mostly liquid, but behave like solid particulates in the atmosphere. NBC air filtration systems normally have a HEPA filter that is equal to a P100 respirator (99.97% of 0.3 micrometer particulates). The threat of biological warfare agents like weaponized anthrax seems to be minimal these days, but NBC filtration systems are designed to protect the occupants of collective protection spaces from them.

Gas adsorption

This is the “C” for “chemical” in NBC air filtration. Activated carbon is responsible for “filtering” gases out of the air. Gases are not really filtered; they are adsorbed onto the surface of the carbon molecule by molecule. Note that “adsorb” with a “d” is not the same process as “absorb” with a “b.” A sponge absorbs liquid by drawing it inside. The old cathode ray tube televisions “adsorb” dust onto the surface of the glass. Because it’s not happening at the molecular level, this isn’t true adsorption, but it’s a good analogy of how the carbon is electrically charged (activated) to attract the molecules of gases that are present in the air moving through the carbon bed. The granular carbon used in NBC filtration systems is tightly packed in the filter canister, which is designed to prevent blow-by of the stream of air in case the carbon settles.

Activated carbon

The bed of carbon is always after the particulate filters in the airflow. This is to keep particulates from adhering to the surface of the carbon. The three variables with carbon that we are concerned about here are:

  1. How much carbon.
  2. The type of carbon
  3. The residency time of the air in the carbon.

The tightly packed granular carbon bed has much more resistance to the airflow than the particulate filters. When NBC air filtration systems are designed, the blower pressure and volume need to be sized to match the resistance of the filter media. This is why NBC air filtration systems must be purpose-built. The designer must balance airflow, pressure, noise, reliability, and price.

Residency time

The process of adsorption requires that every molecule of gas must find a molecule of carbon to attach to. This does not happen instantly. The US military specification for the residency time of the air in the bed of carbon is one quarter (0.25) of a second. This may not seem like very much time, but it’s something the filtration system manufacturers must allow for. Both the mesh size and the bed depth of the carbon have to be engineered so the carbon has enough time for the adsorption process. The speed of the air inside the filter canister is measured or calculated and the carbon bed depth is determined by the residency time at this speed.

Types of carbon

There are different kinds of carbon used in NBC air filtration systems. Plain carbon is effective against many gases, but most manufacturers impregnate their carbon with additional chemicals that allow for a wider spectrum of adsorption. The US military uses a carbon called ASZM-TEDA. It is only manufactured by Calgon Carbon, but the filtration systems made in Israel and Europe use similar carbon. The type of carbon can be over-hyped. Even plain activated carbon will adsorb many of the gases we are likely to encounter. One filter manufacturer spills a lot of digital ink to say that ASZM-TEDA is the only kind of carbon that is acceptable. However, their filtration system only has 5.5 pounds of activated carbon. This is not enough by about a factor of ten. Once your carbon is saturated,you have breakthrough:


Carbon has an extremely high surface to mass ratio, but when the entire surface has molecules of gases on it, it can no longer adsorb chemicals. This is called the breakthrough point because it happens rapidly. The atmosphere contains water vapor, which is a true gas that will be adsorbed by the carbon. Because of this, you should only run air through your filter canister for bi-annual testing and keep the filter canister sealed up when not in use.

How much is carbon is in your NBC air filtration system?

The more carbon in your filtration system, the longer it will adsorb gases before reaching the breakthrough point. There are two American made NBC air filtration systems. One has about 5.5 pounds and the other has about 9 pounds of carbon. Both systems are rated at a filtered airflow of 60 CFM by the manufacturer. The Andair VA150 is a popular NBC filtration system made in Switzerland. It has 60 pounds of carbon and is rated for 88 CFM of filtered air.

In order to account for both the amount of carbon and the volume of air is moving through it; we need to make each one of these into a ratio of the quantity of carbon to volume of airflow. We will use pounds for the carbon and CFM for the airflow. We won’t convert these to metric because the end result is an index number. An index is a number derived from a formula and is used to characterize a set of data. The body mass index (BMI) is a good example. It combines height and weight to produce an index number that can be compared between different people or the same person over a period of time. Here are the three ratios of carbon to airflow:

  1. 5.5 pounds of carbon to 6o CFM
  2. 9 pounds of carbon to 60 CFM
  3. 60 pounds of carbon to 88 CFM

Now we will divide the weight of the carbon by the airflow to get an index of how long the carbon should adsorb gases:

  1. 5.5/60 = 0.09
  2. 9/60 = 0.15
  3. 60/88 = 0.68

These numbers do not represent time until breakthrough but they do allow us to compare these systems to each other. Broken out of the equations and moving the decimal two places to the right to give easier to understand whole numbers; we have an index numbers of:

  1. 9
  2. 15
  3. 68

Next we will compare these numbers by dividing the index number of the Andair system by the index number of the American made systems:

68 / 9 = 7.5
68 / 15 = 4.5

The Andair VA150 should adsorb gases 7.5 times and 4.5 times longer than the American systems, respectively. This is a significant difference that should be taken into consideration when deciding which filtration system to invest in.

What is normally included with an NBC air filtration system?

Most NBC air filtration systems include these components:
  1. An intake blast valve with an integrated pre-filter
  2. An exhaust overpressure blast valve
  3. A filter canister (be sure and get at least one extra canister)
  4. A blower (fan) assembly with a backup hand crank
  5. Connection ducts and hoses
  6. Some systems also include a lamp that is powered by the hand crank


NBC air filtration system


Overpressure blast valves

The exhaust overpressure blast valve is shown separate from the filtration system above. It needs to be mounted as far away from the filtration system as possible. There are actually three valves in this housing. The “overpressure” valve is a very light plate that is normally seated shut by gravity and/or a spring. When the filtration system is in operation, the pressure from it will automatically open this valve plate and air will be exhausted outside. The other two valves in the overpressure blast valve housing are the ones that protect you from nearby detonations. They are normally open and allow low pressure air to move in either direction through the blast valve housings. They slam shut when presented with either a high pressure blast or the following negative pressure phase. Detonations have at least two phases: a blast wave followed by a negative pressure phase. It’s not really negative pressure, but it’s less than atmospheric pressure. Explosions blow the atmosphere outward, leaving an atmospheric void. Both the intake and exhaust blast valves are steel pressure vessels that close automatically for the positive and negative phases of an explosion.

What is normally NOT included with an NBC air filtration system?

Most NBC air filtration systems also require these components for installation:
  1. Ventilation pipes. You may or may not be able to purchase these from the filtration system manufacturer. Longer pipes will have to be fabricated onsite. Be sure to use the proper diameter pipes for the filtration system you install. Steel pipes are best. Plastic pipes are vulnerable to thermal disruption. They will melt when heated and become brittle when cold. Be sure you do not exceed the manufacturer’s specifications for maximum ventilation pipe length because pipes impart resistance to the airflow.
  2. Blast valve sleeves. These are sections of steel pipe that are cast into the wall. They have flanges for bolting the blast valves on the inside and the ventilation pipes on the outside. They also feature J-hooks or steel straps on the outside of the pipe that anchor them into the concrete.
  3. If your shelter has existing walls, you will need to mount the blast valves with concrete wedge anchors. Be sure to use engineered anchors and follow the anchor manufacturer’s instructions when installing them. You will need to know the pressure rating of the concrete and the hole size in the blast valve mounting flanges to determine the correct anchors.
  4. If you have a steel shelter, you will need to weld threaded studs to the inside of the wall to attach the blast valves. Note that blast valves need to be removable for servicing. Insects might get into your ventilation pipes and build a nest up against the valves. Or, you may need to service the springs or seals in the valve. Either way, they must be able to be dismounted from the wall.

Filter manufacturers should have a recommended blast valve installation for steel shelters.


At time of order:
  1. Calculate how much airflow your shelter requires and purchase a filtration system that exceeds this airflow to give a margin of safety.
  2. Order at least one extra filter cartridge. Two extra is better. Do not break the seal on the replacement cartridges. Keep them inside the shelter with the filtration system and protect them from thermal cycles and moisture.
  3. Inquire about how the blast valves are attached to the walls. Either order wall sleeves or concrete wedge anchors.

This is also the time to address the ventilation pipes. Start with asking the manufacturer of the filtration system if they can supply them or have a recommendation where to get them.

  1. Install the filtration system as per the manufacturer’s instructions.
  2. Test the system with the filter cartridge bypassed (fresh air mode). Ensure that you have airflow throughout your shelter by watching the exhaust overpressure blast valve open when the filtration system is powered on.
  3. Attach the filter cartridge to the blower and operate the filtration system for the shortest time possible. Ensure you have the proper airflow (overpressure blast valve opening) and pressure (differential pressure gauge).
  4. Turn off the system, connect the hoses for fresh air mode, and seal up the filter canister. Water vapor in the atmosphere is the enemy of activated carbon. Run as little air as possible through one of your filter canisters and keep the other ones factory sealed.

Be sure you get the installation and operation manuals when you purchase your filtration system.

Bi-annual testing:

Pick a day that has low humidity and perform the tests in numbers two and three above. Be sure to use the same filter cartridge as the initial test and seal it up after each subsequent test. Moisture is the enemy of gas adsorption.

Turning the system on:
  1. Unseal the filter cartridge and attach the hose to it for filtered air mode.
  2. Turn the system on.
  3. Check that the exhaust overpressure blast valve opens and the shelter has at least 0.3 inches of water gauge overpressure.

The doors and hatches must be sealed shut to have overpressure.

Turning the system off:
  1. Turn the system off.
  2. Remove the hose from the filter cartridge and replace the seal on it. These protocols need to become habits.

Protect your carbon from moisture and it will protect you from toxic chemicals.


There is a big difference in NBC filtration systems that have been developed by countries that mandate shelters for their citizens and countries that do not. Israel, Switzerland, and Finland all have a rich heritage of protecting their citizens and produce effective filtration systems.

This website has no association and receives no remuneration from any of these companies:

These filtration systems are recommended:
  1. Beth El, made in Israel. They make the best NBC air filtration systems available to civilians.
  2. LUNOR or Andair (US distributor), made in Switzerland. These are an equal to Beth El in design and a close second in construction. They are easier to find and usually more affordable.
  3. Temet, (US distributor) made in Finland. These are quite good as well, but can be harder to find.

Any one of these proven systems are worth having in your shelter.

These filtration systems are not recommended:
  1. The systems made in America for the civilian market. Both have plastic housings that contain commercial HEPA filters instead of purpose designed filter canisters. For use in a static installation like bomb shelters, they need about an additional 50 pounds (22.6 kg) of carbon to handle a typical chemical threat.
  2. The system made in England for the civilian market. These are about the same as the American systems.
  3. Any homemade system. Do not stick a gas mask filter on a shop-vac and call it an NBC air filtration system. It will not provide air that is safe to breathe.

The take-away

Enclosed spaces require ventilation to sustain life. Your shelter needs an NBC air filtration system to provide it and to isolate you from airborne toxins and pathogens. The manufacturer or distributor that you purchase a filtration system from should make a recommendation for the airflow required for your shelter, but it should be double-checked using the information on this page.

The minimum specifications for the air supply in your shelter are:
  1. 5 CFM (8.5 CMH or 2.3 L/S) of filtered air per person
  2. 0.3 inches (7.62 mm) of water gauge (75 pascals) in overpressure
  3. 2 air changes per hour

All of these are minimums. More is better, up to a point. If you can double these numbers, you will probably have better indoor air quality than you have in your home.

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Stay safe!