Cold storage warehouses present significant challenges for conventional water-based fire protection systems due to large quantities of combustible commodities, high-bay rack storage and complex storage arrangements. When automated storage and retrieval systems are introduced, the challenge may become greater due to denser storage configurations, narrower flue spaces and limited manual access to the seat of a fire.

Low temperatures create additional complications for water-based systems. Because wet-pipe sprinkler systems cannot generally be used in areas subject to freezing, cold storage warehouses commonly rely on dry-pipe or double-interlock pre-action systems. These systems introduce additional equipment and maintenance requirements, as well as concerns involving corrosion, condensation, ice plugs and delayed water delivery. 

In a double-interlock pre-action system, fire detection and sprinkler operation must occur before the valve opens and water enters the piping. The fire may continue to develop while the system completes this sequence and water travels through the piping to the operating sprinklers.

Oxygen reduction represents a fundamentally different fire protection strategy. 

Rather than waiting for a developing fire to activate a suppression system, an oxygen-reduction system continuously maintains the enclosure at a concentration below a predetermined level, where the anticipated commodities are not expected to support a spreading fire. 

Unlike water, foam, clean agent or other suppression systems that respond after a fire develops, oxygen reduction is a fire prevention strategy. Ignition and localized combustion may still occur, but the reduced-oxygen atmosphere is intended to prevent sustained fire growth and propagation through the stored commodities. 

How oxygen reduction works

The fire tetrahedron identifies four elements necessary to sustain combustion: fuel, heat, oxygen and a chemical chain reaction. Oxygen reduction systems address one of these elements by maintaining the protected enclosure below the limiting oxygen concentration for fire propagation. This is the oxygen concentration below which the anticipated commodity and storage arrangement are not expected to support sustained fire spread.

Normal atmospheric air contains approximately 78% nitrogen, 20.9% oxygen, 0.9% argon and small quantities of carbon dioxide and other gases. An oxygen reduction system uses nitrogen generation equipment to separate nitrogen from ambient air. The nitrogen-enriched air is then introduced into the protected enclosure, reducing the oxygen concentration within the space.

 Oxygen sensors are installed throughout the protected area to continuously monitor atmospheric conditions. The system controls the nitrogen generation equipment to maintain oxygen within the required operating range. Alarms notify facility personnel if the oxygen concentration rises above the fire-prevention design threshold, falls below established personnel safety limits or otherwise moves outside the allowable range.

 Oxygen reduction should not be confused with a total-flooding, inert-gas, clean-agent system designed in accordance with NFPA 2001, Standard on Clean Agent Fire Extinguishing Systems. A clean agent system discharges after fire detection to achieve an extinguishing concentration. An oxygen reduction system continuously maintains a reduced-oxygen atmosphere before an ignition occurs. 

The protected space may still contain enough oxygen to support localized combustion, but the atmosphere is intended to prevent a sustained fire from propagating through the stored commodities.

 The required oxygen concentration is not the same for every warehouse. It depends on the commodity composition, corrugated packaging, plastic content, storage configuration, flue spaces, potential ignition sources and the fire-test methodology used to establish the limiting oxygen concentration.

Why freezers may be better candidates than coolers

Coolers and freezers do not present identical applications for oxygen-reduction fire prevention because their operating conditions, enclosure integrity and occupancy patterns differ. 

The more important distinction, however, may be how the space is operated. A highly automated freezer is often tightly enclosed to reduce refrigeration losses, uses controlled product-transfer openings and has limited routine personnel occupancy. These characteristics also support an oxygen reduction system’s ability to maintain a uniform design concentration. 

A conventional cooler may involve frequent door cycling, forklift traffic, manual order picking and employees working within the space for extended periods. Air infiltration increases nitrogen demand and makes it more difficult to maintain the required oxygen concentration, while routine occupancy creates substantial worker safety concerns.

 For these reasons, oxygen reduction appears more suited to a tightly enclosed, highly automated and normally unoccupied freezer than to a routinely occupied cooler. The terms “cooler” and “freezer” alone, however, are not sufficient to determine suitability. Enclosure integrity, access frequency, storage arrangement and expected personnel occupancy must be evaluated for each facility.

 Reliability, maintenance and emergency planning

The reliability and capacity of an oxygen reduction system depend heavily on enclosure integrity and normal facility operations. Air leakage, frequent door cycling and doors remaining open for extended periods allow nitrogen-enriched air to escape and ambient air to enter the protected enclosure. These conditions increase the system’s nitrogen demand and must be considered when sizing the nitrogen generation equipment. Poor enclosure integrity may also make it difficult for the system to consistently maintain the required oxygen concentration.

 An oxygen reduction system is a continuously operating mechanical and control system. Its effectiveness, therefore, depends on reliable electrical service, properly sized equipment and appropriate redundancy. The design should consider backup electrical power, redundant nitrogen generation capacity and the duration for which the protected enclosure can remain below the required oxygen concentration following a power or equipment failure. 

Where electrical reliability is a concern, the system may need to be connected to an emergency or legally required standby power source, as determined by the system design and applicable approval criteria.

 The facility must also understand the importance of inspection, testing and maintenance. Oxygen sensors require periodic calibration and replacement in accordance with the manufacturer’s instructions. Nitrogen generators, compressors, controls, alarms and associated mechanical equipment must be maintained to ensure continued operation. Very early warning fire detection, such as an air-aspirating smoke detection system, must also be monitored, tested and maintained so incipient combustion or a developing fire is identified promptly.

 Written operating procedures should address equipment failures, system impairments, maintenance entry, damaged storage, dropped pallets and any other condition requiring personnel to enter or work within the protected enclosure. The procedures should specify the actions required when oxygen concentrations exceed the fire prevention design threshold or fall below established personnel safety limits.

 Emergency planning is equally important. Opening doors during firefighting operations introduces oxygen into the protected enclosure and may permit a localized fire to grow or previously limited combustion to intensify. First responders should be informed that an oxygen reduction system is installed and understand the stored-commodity hazard, atmospheric conditions and potential for renewed fire growth. 

Pre-incident planning should address fire department access, atmospheric monitoring, ventilation, electrical isolation and the conditions under which personnel may safely enter the enclosure.

 Depending on the impairment, the facility may need to suspend loading, restrict door openings or provide temporary fire protection until the system is restored.

Sustainability trade-offs and water-use considerations

Oxygen reduction systems may provide environmental and operational benefits when compared with traditional water-based fire protection systems. Where an oxygen reduction system prevents significant fire propagation, it may reduce smoke and combustion emissions, damage to stored products and potentially contaminated firefighting water runoff. 

Avoiding a major fire can also reduce the environmental impacts associated with product disposal, demolition, reconstruction and supply chain disruption.

 Where accepted as an alternative to automatic sprinkler protection, oxygen reduction may also reduce or eliminate the need for fire pumps, fire water storage tanks and other water supply infrastructure. Although automatic sprinkler systems normally consume little water unless they operate, the supporting infrastructure requires periodic inspection, testing and maintenance. 

Fire pumps require weekly or monthly no-flow testing, depending on the installation, as well as an annual full-flow performance test. The annual test requires water flow and may discharge a substantial quantity of water, depending on the test arrangement. 

 Fire water storage tanks also require periodic internal inspection. Under the 2026 edition of NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, steel tanks without corrosion protection require interior inspection every three years while other tanks require inspection every five years. 

These inspections commonly require the tank to be drained or substantially lowered, resulting in periodic water use and disposal. Tanks located in areas subject to freezing may also require electric heaters or other heating systems, creating an ongoing energy demand.

 Oxygen reduction systems impose a different environmental burden. Nitrogen generation requires continuous electrical energy and demand increases when enclosure leakage, door cycling and material-transfer openings allow nitrogen-enriched air to escape. The system also relies on compressors, controls, oxygen sensors and other equipment requiring maintenance and eventual replacement.

 For these reasons, oxygen reduction should not be characterized solely as a water conservation measure. Its sustainability and resilience benefits should be evaluated on a life cycle basis, considering continuous electrical consumption, periodic water use, tank heating, avoided infrastructure, potential fire losses and the environmental consequences of replacing damaged products and facilities.

Code acceptance and worker-safety constraints

The International Building Code and International Fire Code do not provide a prescriptive pathway for an oxygen reduction system to replace required automatic sprinkler protection. Furthermore, NFPA does not currently publish a dedicated installation standard for oxygen reduction fire prevention systems. 

A project would therefore likely require approval under the applicable code provisions for an alternative material, design, method of construction or equipment. This approval would be project-specific and require evidence demonstrating equivalent performance to the protection otherwise required by the code. 

 FM provides the most developed U.S. technical framework through Data Sheet 4-13, Oxygen Reduction Systems. The data sheet contains recommendations for FM-approved systems used as an alternative to automatic sprinkler protection, including commodity-specific limiting oxygen concentrations, continuous oxygen monitoring, nitrogen generator redundancy, enclosure integrity and inspection, and testing and maintenance requirements. FM data sheets, however, are property loss prevention guidelines rather than adopted model building or fire codes. 

 Employee exposure presents a separate regulatory concern. OSHA defines an atmosphere containing less than 19.5% oxygen as oxygen-deficient. Oxygen reduction systems may continuously maintain concentrations substantially below this threshold. 

Relevant considerations include respiratory protection, medical evaluation, employee training, atmospheric monitoring, emergency escape, rescue procedures and exposure of maintenance personnel, contractors and first responders. OSHA generally treats oxygen-deficient atmospheres as immediately dangerous to life or health unless narrowly defined exceptions are demonstrated. 

 Accordingly, FM criteria can guide system design, but model code approval and occupational safety compliance must be addressed separately.

Key factors for determining viability

Oxygen reduction fire prevention may be a credible alternative for tightly enclosed, highly automated cold storage facilities with limited routine occupancy. Its potential benefits must be weighed against continuous energy use, enclosure performance, maintenance, system reliability and emergency response requirements.

This process is not a direct substitute for automatic sprinkler protection. Its use requires commodity-specific testing, FM design criteria, project-specific code approval and careful evaluation of OSHA requirements. Facilities with frequent personnel access, significant door cycling or limited operational control may be poor candidates.

Ultimately, the viability of oxygen reduction fire prevention depends as much on how personnel enter, operate and maintain the warehouse as it does on the combustible commodities stored within it.

 Tracy Worley, PE, PMSFPE, is the fire technical director at Henderson Engineers, a national building systems design firm.