Controlling CO₂ Levels in Shelters and Bunkers
A shelter must provide both physical protection and a breathable internal atmosphere. Strong walls, gastight doors and protective filtration are essential, but the air inside must also remain suitable throughout the period of occupancy.
Every person consumes oxygen and produces carbon dioxide through breathing. Adequate outside-air replacement removes this carbon dioxide and replenishes the oxygen consumed. When the supply stops or becomes insufficient, CO₂ can accumulate while oxygen levels decline.
Excessive CO₂ can affect occupants’ wellbeing and ability to function. At high concentrations, it can become life-threatening. Because occupants cannot reliably detect rising CO₂ through their senses, atmospheric monitoring is an important part of shelter operation.
Two conditions require particular attention: operation without external air and operation with insufficient air replacement.
Lock-up (isolation) Mode - When No External Air Is Supplied
A shelter may need to be temporarily isolated from the outside atmosphere. This condition is commonly called isolation, lock-up or close-up mode.
Isolation may be necessary when external contamination exceeds the capabilities of the installed filtration system, or when fire, flooding or damage prevents safe use of the air intake. During this period, the shelter must maintain suitable internal conditions without its normal outside-air supply.
Occupants continue producing carbon dioxide after the intake is closed. Without ventilation or dedicated removal, CO₂ remains inside and its concentration rises. The rate of accumulation depends on the number of occupants, their physical activity and the available internal air volume.
A larger air volume can slow this increase, but it does not prevent it. The permitted isolation period must therefore be established during design, taking into account the shelter’s occupancy and available life-support equipment.
When the CBRN Filtration System Provides Insufficient Airflow for Adequate Air Exchange
CO₂ can also accumulate while a CBRN filtration system is operating. A running unit does not necessarily mean that enough outside air is reaching the occupied space.
An undersized system may filter incoming air against the contaminants it is designed for, yet deliver too little replacement air for the shelter’s volume and occupancy. Filtration capability and airflow capacity must therefore be assessed together.
The available airflow depends on the complete installation. Protective filters, ducts, valves and other components create resistance, which affects how much air reaches the shelter. Restricted intakes, filter loading and equipment faults can further reduce performance.
Selecting equipment only by its maximum fan capacity can lead to insufficient air replacement. The relevant figure is the airflow delivered through the complete system in the required operating mode, particularly during CBRN filtration.
Air Replacement and Air Changes per Hour
Air replacement is commonly expressed as air changes per hour, or ACH. This describes the volume of outside air supplied each hour in relation to the shelter’s internal air volume.
Adequate outside-air replacement removes the CO₂ produced by occupants and replenishes the oxygen they consume. The required airflow must therefore be matched to both the shelter’s volume and its planned occupancy.
Where the applicable shelter standard requires at least two air changes per hour, the system must deliver this rate in the specified operating mode. A lower rate of one air change per hour may be used where explicitly permitted for the relevant shelter category and operating conditions.
These requirements refer to suitable outside air supplied through the appropriate ventilation or filtration system. The design must also consider airflow distribution so that replacement air reaches the occupied areas effectively.
Requirements differ between countries and shelter classifications. This is discussed further in Civil Defence Shelter Standards: Why Requirements Differ Between Countries.
Air Circulation Is Not Air Replacement
A fan moving air around the shelter can improve distribution, but it does not replenish oxygen or remove the carbon dioxide generated by occupants.
Recirculated air should not be counted as outside-air replacement when assessing ACH. A high internal circulation rate can otherwise give a misleading impression of ventilation performance.
Recirculation through a dedicated CO₂ scrubber serves a different purpose. The scrubber removes carbon dioxide from the air passing through it, allowing treated air to return to the shelter. Oxygen replenishment must still be addressed separately.
The Role of CO₂ Scrubbers
CO₂ scrubbers are used where ventilation alone cannot adequately control carbon dioxide, particularly during extended isolation or restricted outside-air supply.
Selecting a scrubber requires consideration of both its removal rate and its total absorption capacity. The equipment must remove CO₂ quickly enough to manage the occupants’ output, while the available absorbent must last for the planned operating period.
These are separate requirements. A large quantity of absorbent does not compensate for insufficient removal performance, and a high removal rate does not guarantee a long operating time without sufficient absorbent capacity.
The installation must also allow air from the occupied areas to reach the scrubber effectively. Internal partitions, equipment arrangements and poorly circulated areas should be considered when planning the system.
Atmas offers CO₂ removal systems and scrubbers in different capacities and configurations for shelters, bunkers and other enclosed spaces.
Different Types of CO₂ Removal Systems
CO₂ removal systems can be divided into two main groups: systems with replaceable absorbents and systems that regenerate their capture material for repeated use.
The choice depends on occupancy, isolation time, frequency of use, available power, maintenance requirements and storage space.
Replaceable Absorbent Filters
These systems pass shelter air through filters containing soda lime or other purpose-designed chemical absorbents that capture carbon dioxide. Once the material reaches its usable capacity, the filter or absorbent is replaced.
Soda lime is an established and widely used solution. For many shelter applications, replaceable filters provide a cost-effective approach with relatively straightforward equipment and maintenance. They do not require an energy-intensive regeneration process during operation, although power is normally needed to move air through the filters.
Other chemical formulations may be used where particular performance, weight or storage requirements apply. The material must be specifically designed for CO₂ removal. Ordinary activated-carbon filters should not be assumed to perform this function.
Absorbent capacity and stored reserves must match the expected occupancy and isolation period. The design must also account for airflow resistance, storage conditions and the heat and moisture associated with the absorption process.
Regenerative Systems
Regenerative systems use capture materials that can be restored and reused. Examples include amine-based systems and molecular sieves used in specialized life-support applications, including submarines and spacecraft.
During regeneration, the captured CO₂ is released using heat, vacuum or changes in pressure, depending on the technology. It must then be discharged outside the protected atmosphere or collected for further handling. Regeneration does not destroy the carbon dioxide, so its disposal must be included in the design.
These systems generally involve higher initial costs and more complex controls and maintenance than replaceable-filter systems. Regeneration can also create significant power and cooling demands. For shelters used occasionally, these requirements may outweigh the benefit of reducing replacement absorbents.
Regenerative systems are not inherently limited to small air volumes. Their suitability depends on the required removal capacity and operating conditions. They may be appropriate for prolonged or frequent operation, where storing and replacing large quantities of absorbent becomes less practical.
Monitoring and Maintenance
The installed airflow should be measured during commissioning in the relevant operating modes. This confirms whether the complete system delivers the performance assumed during design.
During occupancy, CO₂ monitoring shows whether carbon dioxide is accumulating under actual conditions. A running fan or a positive pressure reading cannot establish this on its own.
Sensors should be positioned appropriately and maintained according to the manufacturer’s instructions. Alarm settings and the actions required when an alarm occurs should form part of the shelter’s operating procedures.
For shelters equipped with scrubbers, preparation also includes checking equipment condition, absorbent availability and the power or manual operating arrangements provided for the intended period of use.
Oxygen Maintenance
When sufficient suitable outside air is supplied, ventilation replenishes the oxygen consumed by occupants. During isolation or inadequate air supply, an independent oxygen or breathable-air supply may be required.
A CO₂ scrubber does not replenish oxygen, and adding oxygen does not remove accumulated carbon dioxide. Both functions must be considered together, with appropriate monitoring and controls.
These principles are explained in Closeup Mode in Shelters: Air Regeneration and Supply Principles. Related equipment is presented on the Oxygen and Air Supply Systems page.
Overpressure Maintenance
Overpressure helps reduce the entry of contaminated external air by directing leakage outward. However, maintaining positive pressure does not confirm that CO₂ and oxygen concentrations are acceptable.
When outside-air supply stops, pressure maintenance requires separate consideration. This is covered in Maintaining Positive Pressure in Civil Defence Shelters During Isolation.
Selecting the Right System for the Shelter
Shelter air management should begin with the expected occupancy, required operating time and emergency conditions the shelter is intended to withstand. These determine the necessary outside-air supply, CO₂ removal capacity and supporting equipment.
The design should account for normal ventilation, CBRN filtration and isolation wherever these modes are required. Each mode places different demands on the equipment and must have clear operating limits.
Atmas Group provides shelter filtration and air management solutions, including CBRN filtration, CO₂ removal, oxygen supply, atmospheric monitoring and overpressure control for new shelters and modernization projects.
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