Maintaining Positive Pressure in Civil Defence Shelters During Isolation (Lock-Up) Mode

The Challenge of Maintaining Overpressure in Lock-up Mode Without External Source of Air

Positive pressure, or overpressure, is one of the fundamental principles of CBRN protection in civil defence shelters and other protected facilities.

During normal filtered ventilation, the principle is relatively simple: outside air is drawn through the CBRN filtration system and supplied into the protected space. The supplied airflow creates a controlled pressure differential between the shelter and the external environment.

As long as the pressure inside the shelter remains higher than outside, unavoidable leakage through small imperfections in doors, seals, cable and pipe penetrations, ducts and construction joints tends to flow outward rather than inward. This reduces the possibility of contaminated external air entering the protected area.

The situation becomes significantly more complicated when the shelter enters complete isolation, also known as lock-up or shut-off mode.

In this condition, the connection with the outside atmosphere is closed. No conventional ventilation air is introduced into the protected area.

The engineering question therefore becomes:

How can positive pressure be maintained when there is no normal incoming airflow?

Several different approaches are used internationally. They should be understood separately because maintaining pressure and maintaining a breathable atmosphere are related, but fundamentally different engineering tasks.

Why Isolation Mode May Be Required

A CBRN filtration system is designed to remove specific contaminants from incoming air. However, there may be situations in which the external atmosphere is unsuitable for introduction into the shelter even through the installed filtration system.

Examples may include:

• exceptionally high concentrations of toxic industrial chemicals;

• major fires producing carbon monoxide and complex combustion products;

• chemical accidents;

• uncertainty regarding the contaminant present outside;

• flooding or temporary obstruction of the external air intake;

• situations exceeding the designed filtration capability;

• specific CBRN scenarios requiring temporary complete isolation.

During such periods, external air intakes are closed and the shelter operates as a sealed environment.

Finnish civil defence guidance recognizes a closed or shut-off operating condition (sulkutila) as one of the shelter's operating modes.

Once the shelter is isolated, two separate problems immediately need to be considered:

1. Maintaining positive pressure - Air lost through unavoidable leakage must somehow be compensated.

2. Maintaining a breathable internal atmosphere - Occupants consume oxygen and continuously produce carbon dioxide, heat and moisture.

Solving only one of these problems does not necessarily solve the other.

Approach 1 – Compressed Air for Overpressure Maintenance

One of the most direct engineering solutions is to store compressed breathable air in high-pressure cylinders or tanks.

During isolation, compressed air is gradually released into the protected space through pressure-reduction and control equipment.

The system can be manually controlled or automatically regulated according to the differential pressure between the shelter and the external environment.

When pressure falls because of leakage, additional compressed air is introduced.

This approach has also historically been specified in Eastern European shelter engineering. For example, Russian technical rules for complete shelter isolation describe maintaining positive pressure using compressed air from cylinders, with the required airflow determined according to the specified overpressure and characteristics of the shelter envelope.

Advantages

The major advantage is complete independence from the external atmosphere.

Even if outside air contains extremely high concentrations of hazardous substances, the shelter does not have to introduce that atmosphere in order to compensate for leakage.

The system is also conceptually straightforward:

Compressed-air storage → pressure regulator → control valve → shelter

Automatic versions can use differential-pressure sensors to maintain the required pressure range.

Limitations

The major limitation is storage capacity.

Every cubic metre of air leaking from the shelter eventually has to be replaced. Consequently:

The tighter the shelter, the longer the compressed-air reserve can maintain pressure.

A poorly sealed shelter may consume a large compressed-air reserve surprisingly quickly.

This makes airtightness testing extremely important.

Compressed-air storage also requires space, cylinders or pressure vessels, regulators, manifolds, inspection and periodic maintenance.

Approach 2 – Controlled Introduction of Specially Treated Make-Up Air

A second approach is to avoid absolute isolation while operating with a very small controlled quantity of outside air.

This concept has been used in different forms in Eastern European and other protected-facility designs.

Instead of supplying the normal ventilation airflow required for occupants, the system introduces only the quantity necessary to compensate for leakage, maintain the required overpressure and, depending on the design, assist with atmospheric management.

However, the incoming air must be treated according to the actual external hazard.

Treatment may therefore involve several independent stages.

Carbon monoxide treatment

Where fire or combustion products are a credible threat, CO requires dedicated treatment.

Conventional CBRN filtration should not automatically be assumed to provide adequate protection against carbon monoxide.

CO treatment may require dedicated catalytic media and carefully controlled operating conditions.

Find out about Atmas carbon monoxide filters for shelters

Carbon dioxide management

CO₂ is a completely different problem.

A conventional CBRN filter does not solve the accumulation of carbon dioxide generated by occupants inside a sealed shelter.

CO₂ management normally requires a dedicated CO₂ removal system, or scrubber.

This distinction is important:

CBRN filtration ≠ CO removal ≠ CO₂ removal.

They address different contaminants and should be treated as separate engineering functions.

Find out about Atmas carbon dioxide (CO2) systems for shelters

CBRN filtration

Particle and gas filtration can protect against radioactive particles, biological aerosols and specific chemical agents for which the filtration system has been designed.

Find out about Atmas CBRN filtration systems for shelters

Advantages of controlled make-up air

Compared with relying entirely on stored compressed air, controlled make-up air can potentially allow significantly longer operation because the available air volume is not restricted by the number of cylinders installed inside the facility.

Limitation

The concept depends entirely on the ability of the treatment system to safely process the outside atmosphere.

If the contaminant concentration or type exceeds the capability of the installed treatment system, complete isolation may still be necessary.

For this reason, a well-designed protected facility may incorporate both complete isolation and treated-air operating modes.

Approach 3 – Passive Thermal Pressurization in Highly Airtight Shelters

A third and particularly interesting phenomenon is relevant to highly airtight shelters, including the Finnish civil defence shelter concept.

When people occupy a completely sealed shelter, they continuously release heat.

Electrical equipment, lighting and other internal sources can produce additional heat.

As the internal air temperature increases, the enclosed air expands. In an approximately fixed-volume and highly airtight enclosure, this thermal expansion can cause the internal pressure to increase relative to the outside atmosphere.

In simplified terms:

Occupants → heat generation → increasing internal air temperature → thermal expansion → positive pressure

This can contribute to maintaining positive pressure without introducing outside air.

However, this phenomenon must be understood correctly.

It is a passive effect, not an active overpressure system

Occupant heat should not be presented as equivalent to a compressed-air pressure-maintenance system.

The pressure generated depends on several variables, including:

• shelter airtightness;

• internal volume;

• number of occupants;

• initial internal and external temperatures;

• heat transfer through the shelter structure;

• leakage rate;

• duration of isolation;

• thermal mass of the structure.

Once thermal equilibrium begins to develop, additional pressure generation becomes increasingly limited.

Pressure may also decrease if the shelter cools or if leakage exceeds the pressure effect generated by thermal expansion.

For these reasons, thermal pressurization is better understood as a passive characteristic of a very airtight shelter during isolation, rather than as a continuously controllable source of overpressure.

The Critical Limitation of Complete Isolation

There is an even more important issue.

Heating the existing air does not regenerate it.

People inside the shelter continuously:

consume O₂ → produce CO₂ → produce heat → produce moisture.

Finnish technical literature specifically identifies occupant CO₂ generation, O₂ consumption and humidity as factors limiting the duration of the shelter shut-off phase.

Therefore, even if a sealed shelter successfully maintains positive pressure, this does not mean that occupants can remain safely inside indefinitely.

This leads to one of the most important principles in shelter air-management design:

Overpressure and Air Regeneration Are Not the Same Thing

A shelter can have adequate positive pressure while having an unacceptable internal CO₂ concentration.

Likewise, oxygen can be introduced into a shelter without necessarily providing enough total gas volume to compensate for leakage and maintain the required pressure differential.

For prolonged isolation, several parameters must therefore be managed simultaneously.

O₂ – Oxygen

Occupants continuously consume oxygen. For extended isolation, oxygen may need to be supplied from an independent source.

Possible solutions include compressed breathable air, controlled oxygen supply or other engineered oxygen-generation solutions.

CO₂ – Carbon Dioxide

Occupants continuously produce carbon dioxide.

For prolonged lock-up operation, CO₂ generally has to be removed using a dedicated CO₂ scrubber or equivalent air-regeneration technology.

CO – Carbon Monoxide

CO is not generated by normal human respiration but may become relevant because of fires, combustion processes, engines or contamination entering from outside.

Where CO is part of the design threat, dedicated CO-removal technology may be required.

Temperature

People and equipment continuously release heat.

Without external ventilation, the internal temperature can progressively increase and may eventually become a limiting factor for occupancy.

Humidity

Human respiration and perspiration continuously introduce moisture into the shelter.

Humidity therefore increases during prolonged isolation and may require condensation control, cooling or dehumidification.

Overpressure

Finally, air lost through leakage must be replaced or otherwise compensated for if controlled positive pressure is required throughout the isolation period.

A More Complete Engineering Solution for Long-Term Isolation

For shelters required to remain completely isolated for extended periods, a more comprehensive air-regeneration concept may therefore include:

CO₂ removal + controlled O₂ supply + overpressure maintenance + air circulation + cooling/dehumidification + continuous atmospheric monitoring.

The systems must operate together rather than independently.

For example, adding oxygen alone does not remove CO₂.

Removing CO₂ alone does not replace oxygen.

Neither process necessarily compensates for the total volume of air escaping through leakage.

Compressed breathable air can simultaneously contribute oxygen and replace leakage volume, but its operating duration is limited by storage capacity.

This is why long-duration isolation systems should be designed as an integrated life-support and pressure-management system rather than as individual pieces of equipment.

Continuous Monitoring Is Essential

A protected facility intended for isolation operation should not rely solely on theoretical calculations.

The actual internal atmosphere and pressure conditions should be monitored.

Depending on the shelter type and applicable requirements, monitoring may include:

Differential pressure – verifies that the required pressure relationship between the protected space and outside is maintained.

O₂ concentration – confirms adequate oxygen levels.

CO₂ concentration – indicates accumulation caused by occupants.

CO concentration – particularly relevant where fire or combustion contamination is part of the threat assessment.

Temperature and relative humidity – determine whether environmental conditions remain acceptable.

More advanced installations may integrate these sensors into a central air-management control system.

Airtightness Is the Foundation

All three approaches ultimately depend on one fundamental characteristic:

Shelter airtightness.

A shelter does not need to be theoretically perfect, but its leakage must be sufficiently low and predictable for the selected pressure-maintenance system.

Leakage can occur through:

• door and hatch seals;

• ventilation penetrations;

• electrical and communication cable penetrations;

• pipe penetrations;

• construction joints;

• incorrectly closed valves;

• damaged gastight components.

The greater the leakage, the greater the quantity of make-up air required.

For compressed-air systems, excessive leakage directly reduces available isolation time.

For treated make-up-air systems, it increases the required treatment capacity.

For passive thermal pressurization, excessive leakage may prevent useful positive pressure from developing at all.

This is why proper construction, gastight penetrations, maintained seals and periodic airtightness testing are fundamental to shelter performance. Finland's 2026 maintenance guidance specifically includes verification of shelter airtightness as part of functional-condition inspections.

Comparing the Three Approaches

Method	External Air Required	Pressure Method	Main Advantage	Main Limitation
Compressed breathable air	No	Active	Independent from contaminated outside atmosphere	Limited storage capacity
Treated make-up air	Yes, small controlled quantity	Active	Potentially long-duration operation	Depends on treatment capability
Thermal pressurization in highly airtight shelter	No	Passive	No dedicated pressure source required for the thermal effect	Limited, variable and not independently controllable

The methods do not necessarily have to be mutually exclusive.

A sophisticated shelter can incorporate several operating modes and change between them according to the external threat.

Conclusion

Maintaining overpressure in a civil defence shelter during complete isolation is fundamentally different from maintaining overpressure during normal CBRN filtration.

Three principal approaches can be identified.

Compressed-air systems actively replace leakage losses using stored breathable air and can operate completely independently from the external atmosphere.

Controlled treated make-up-air systems introduce a small quantity of appropriately treated external air, allowing pressure to be maintained without consuming large quantities of stored air.

Highly airtight shelters may also experience passive thermal pressurization, where heat generated by occupants and equipment warms the enclosed air and contributes to positive pressure during the initial isolation period.

Each approach has advantages and limitations.

Most importantly, maintaining overpressure does not by itself make a sealed shelter habitable.

Long-term isolation requires simultaneous management of pressure, oxygen, carbon dioxide, temperature, humidity and potentially carbon monoxide or other contaminants.

For shelters designed for extended lock-up operation, the most robust approach is therefore an integrated solution combining:

high shelter airtightness, atmospheric monitoring, CO₂ removal, controlled oxygen or breathable-air supply, thermal and humidity management, and engineered overpressure control.

The appropriate configuration should always be determined according to the shelter's volume, occupancy, leakage rate, required isolation duration, external threat assessment and applicable national or project-specific requirements.

Atmas Group Expertise in Shelter Air Management

Atmas Group provides specialized expertise in the design, modernization and integration of civil defence shelters and protected facilities, including CBRN filtration, ventilation, overpressure control and isolation-mode air management. Our capabilities cover complete solutions for maintaining safe shelter conditions, including gastight ventilation systems, blast valves, CBRN filtration, CO and CO₂ removal, compressed-air and oxygen supply, atmospheric monitoring, and pressure-control solutions. By combining engineering knowledge with tested protective technologies, Atmas supports both new-build and retrofit projects in selecting and integrating solutions according to shelter type, occupancy, required isolation time, threat assessment and applicable national requirements. Our approach considers the shelter as an integrated protective environment—combining structural protection, airtightness, ventilation, filtration, air regeneration and life-support functions to provide reliable protection under demanding emergency conditions.