Oxygen reduction is a fire prevention technology used with increasing frequency in a variety of sectors, particularly in information technology (IT and server rooms), warehouses (e.g. small-load carrier, hazardous material and deep freeze stores) and archives. These systems reduce the oxygen content of the air to levels that will prevent the outbreak of fire, based on the materials being stored or the equipment being protected.
Such technology is commonly known as “fire prevention systems” or “oxygen reduction systems”.
Fires break out in Germany roughly every two minutes, and many of them cause serious damage. Whether warehouses, data centres or archives, every business needs absolutely reliable fire protection. Oxygen reduction systems were developed to protect sensitive areas not only from fire, but also from toxic smoke gases and damage from extinguishing media.
Conventional fire protection systems are passive, meaning that they only react once a fire has broken out. Preventative oxygen reduction systems, on the other hand, begin working before fires start: They introduce nitrogen into the air in closed rooms in order to reduce oxygen concentration levels continuously, thereby creating an atmosphere in which it is practically impossible for fires to develop or spread.
Three components are necessary for fires to develop: oxygen, heat energy and fuel. If one of these three components is taken away, there is no way a fire can break out. Fire prevention technology is based on this principle. By reducing the oxygen content of the air, it literally “takes the fire’s breath away”.
The flammability of nearly all combustible materials is directly correlated to the oxygen concentration of the surrounding air, so reducing oxygen concentration levels reduces their flammability accordingly. If the oxygen content in the room air is lower, igniting a combustible material will require a great deal more energy. The energy required for ignition to occur is higher than the amount needed to maintain a fire. Lowering oxygen concentration levels significantly slows down the chemical and physical processes involved in ignition. As a result, in oxygen-reduced environments, there is far less chance of fire outbreak than under normal conditions, and fires will have far less opportunity to spread. If oxygen concentrations in the ambient air are kept below a certain limit value, it will no longer be possible for a material to catch fire spontaneously. This limit value is material-specific, and is known as the ignition threshold.
With solid materials, even lowering oxygen concentration levels below 20.9 vol-% 02 is enough to provide increased fire protection, as it reduces the intensity of combustion reaction. Once the design concentration level has been reached, fire protection is fully established.
Oxygen sensors continually monitor concentration levels within the protected area, ensuring that they remain at the predefined value. At least two oxygen sensors are used to do this monitoring; they are installed at different locations within the protected area, to provide redundant measurement results.
Nitrogen is fed into the protected area in order to reduce the oxygen concentration levels below the specific ignition level of the material under protection. Adding nitrogen changes the composition of the air, pushing its oxygen content down until the remaining oxygen levels are no longer sufficient to maintain a fire or allow it to spread.
Using nitrogen as an inert gas has significant benefits: nitrogen is not toxic, and at 78.09 vol.-%, it is the primary component of normal air. As such, oxygen reduction systems using nitrogen can achieve maximum and/or total fire protection while ensuring that it is still safe for people to enter the protected areas. Nitrogen’s natural properties ensure that it will distribute homogeneously with either constant or varying oxygen concentration levels, so that these levels will remain uniform throughout the entire protected area.
Because nitrogen makes up such a high proportion of ambient air, the nitrogen needed to operate the oxygen reduction system can be generated directly on site. The WAGNER Group GmbH’s OxyReduct® systems physically separate room air into nitrogen and oxygen molecules using one of two methods: membrane technology or activated carbon.
The membrane method is based on the physical property that different gases diffuse through materials at different rates. Here, the system uses the different diffusion speeds of the main components of air— nitrogen (N2), oxygen (O2) and water vapour (H2O)—to produce a nitrogen stream. In this method, pressurised air is pushed through a bundle of polymer fibres, also known as hollow-fibre membranes, in an aluminium tube. A separation material is applied to the outer surfaces of these hollow-fibre membranes. Water vapour and oxygen can diffuse through the material easily, but nitrogen passes through it much more slowly. When air passes through the interiors of these hollow fibres, water vapour and oxygen diffuse rapidly out through the walls, whereas most of the nitrogen is retained inside the fibres. As such, the nitrogen concentration of the air increases dramatically as it passes through the hollow fibres.
The effectiveness of this separation method is mainly dependent on:
Lower flow speeds and/or higher pressure differences between the inside and outside of the hollow-fibre membranes increase the purity of the resulting nitrogen stream. The target oxygen concentration level in the nitrogen stream within the protected areas is 5 vol-%. The membrane method of separating nitrogen and oxygen molecules provides a continuous flow rate.
Its technical implementation involves combining bundles of these hollow membranes into membrane modules, each with a defined nitrogen capacity at a specified purity level. Several such modules can be installed into a nitrogen generator, so that different capacities of nitrogen (m3/h) can be achieved. The generator contains an oxygen analyser that checks the purity of the nitrogen stream within the protected area by continuously measuring its residual oxygen content and transmitting this measurement to a central control unit.
The process of generating nitrogen using activated carbon takes advantage of the different speeds at which atmospheric oxygen and nitrogen bind to specially treated activated carbon.
In this procedure, the activated carbon’s structure is altered to give it an extremely large surface area with a large number of micro- and submicropores (diameter < 1 nm). The resulting pellets are referred to as a Carbon Molecular Sieve (CMS). Oxygen molecules in the air diffuse significantly faster into pores of this size than do nitrogen molecules. This increases the nitrogen content of the air around the pellets.
The speed at which the molecules bind depends on pressure. At maximum pressure, the pellets reach their saturation point after around 60 seconds, at which time desorption (oxygen extraction) must be initiated. This is achieved by lowering the pressure to match the pressure in the surrounding air (PSA technique) / to about 100mbar (abs. - VPSA technique) using a vacuum pump. This releases the bound oxygen from the pores once more.
The technical implementation of this procedure involves filling the activated carbon into pressure tanks, and then initiating the absorption (oxygen binding) process by streaming air through the tanks at 7 bar (PSA) / 1.2 bar (VPSA) overpressure. Doing this draws the oxygen out of the air stream; the resulting nitrogen stream is then fed into the protected area. The desorption process then follows. In order to maintain a steady stream of nitrogen, two containers are connected to a pressurised air source in alternating sequence, such that one container is always generating nitrogen while the other is undergoing materials cleaning. The system switches back and forth between the two tanks once every 60 seconds or so.
VPSA and PSA systems are used to protect very large-volume areas, such as automated high-bay warehouses. They produce large volumes of nitrogen and stand for exceptional performance in continuous operations; the VPSA system is the newer and more energy-optimised of the two methods.
Oxygen reduction systems’ areas of application range from IT centres, safes and archives to large storage facilities such as automated high-bay warehouses, hazardous materials storage, and deep-freeze storage areas. There are no restrictions on oxygen reduction system use as regards the volume of the space being protected. Oxygen reduction systems are particularly suitable for areas in which solutions using conventional extinguishing technology would be problematic or infeasible, for example shielded or low-temperature areas. They are also effective in protecting inaccessible areas,
and completely eliminate the risk of damage of the type that can result with conventional extinguishing agents. As such, oxygen reduction systems are a particularly suitable fire protection method for technical rooms and equipment that are essential to the company’s continued existence, as damage-related downtime is minimal in comparison to conventional extinguishing technology. Fire prevention systems are also an effective method of protecting areas containing high-value items that could be destroyed by water, extinguishing foam or extinguishing powder, such as museums, archives or libraries.
The only substances they cannot protect effectively are those that can burn even in non-oxygenated environments, or that do not need oxygen to produce exothermic reactions.
Protection objectives with oxygen reduction systems include:
Regardless of the size of the area they are protecting, oxygen reduction systems work by preventing materials from igniting and fires from spreading. Their fire-prevention mechanism is based on technology that measures oxygen levels within the entire protected area and lowers them through controlled release of nitrogen. An integrated early smoke detection system helps detect and report incipient fire outbreaks quickly. The system then cuts off energy supply to the fire, taking away the energy required to support the ignition process so that the fire cannot develop further.
The level of protection that an oxygen reduction system needs to establish and maintain in order to ensure fire protection depends on the ignition thresholds of the materials within the protected area.
The substances in the protected area are documented and identified so that their ignition thresholds can be determined. If these values are unknown for any materials or objects, they must be determined using established testing procedures. The substance with the lowest ignition threshold determines the system’s design concentration. For group-related risks, the fire scenario is delineated in terms of known material configurations. This makes it possible to summarise such objects as a group and assign them a fixed design concentration value.
The WAGNER Group GmbH specialises in systems engineering, and has developed and realized special protection concepts for a variety of applications. After conducting a risk analysis and identifying protection goals, WAGNER uses these as the basis for creating an OxyReduct® fire prevention solution tailored to the customer's specific needs. WAGNER has developed and defined the following basic protection concepts:
Regulation Concept I - permanent reduction of oxygen concentration levels
By continuously reducing oxygen concentration levels, the OxyReduct® fire prevention system prevents fires from developing or spreading. Oxygen concentration levels are defined based on the ignition thresholds of the materials present, then lowered to the target concentration in a controlled manner and held there, creating a highly fire-retardant atmosphere.
Regulation Concept II – Oxygen reduction with two adjustable levels
OxyReduct® can also adjust oxygen levels completely automatically at specific times. For example, O2 concentration levels within the protected area might be reduced only slightly during the day, to 17% vol, ensuring that the area remains freely accessible. During night and weekend hours, the oxygen concentration is then reduced to the second level (14.6% vol), ensuring maximum fire protection during periods without supervision.
Regulation Concept III - Quick reduction
The OxyReduct® fire prevention system lowers atmospheric oxygen concentration to 17% volume in a controlled manner, thus significantly restricting fire behaviour. It then works in tandem with an early fire detection system, and rapidly lowers oxygen levels if alarms are triggered. It does this by using nitrogen tanks (nitrogen reservoirs) to lower oxygen concentration to a level that can extinguish the fire. The system can maintain this level almost indefinitely in order to prevent re-ignition. This protection concept is perfect for applications involving areas that are difficult or impossible for fire brigades to access.
Regulation Concept IV - two-stage quick reduction
When the fire detection system triggers a pre-alarm, the system rapidly accesses its nitrogen reservoir to reduce room oxygen concentration to a specified level (e.g., 15.8% vol); the OxyReduct® fire prevention system then maintains this concentration level continuously. Detection of additional smoke triggers the second stage, wherein oxygen concentration is reduced even further, below the ignition threshold (e.g., 13.8% vol), and then kept at that level.
Advantages of rapid reduction:
Oxygen reduction systems lower oxygen levels within the protected area by feeding in nitrogen generated on site. Generally, the entire system includes the following modules:
• Compressed air generation,
• Nitrogen generation,
• Central control unit,
• Oxygen measuring sensors,
• Notification and signalling devices,
• Supply network.
All system components are dimensioned large enough to provide sufficient nitrogen capacity to supply the entire protected area (or all protected areas, with a multi-area system) at the same time.
The compressed air generation and filtering components help condition the air for optimal processing in nitrogen generation.
Compressed air is generated with (for example) a screw-type compressor. This step involves pressurising intake air to the necessary level of overpressure. The compressed air is also cooled and the pressure dew point lowered.
The filtration step cleans residual oil and other particles from the compressed air using particle filters and activated carbon adsorbate. This maximises the quality of the compressed air, thereby ensuring the nitrogen generators’ long-term durability.
Regulating the pressure of the processed air to a procedure-specific level ensures optimum effectiveness of the nitrogen generation process. At this operating point, the system generates the maximum quantity of nitrogen per kilowatt hour of electrical energy used.
Nitrogen can be generated using any of several technical processes, depending on the quantity required. What all of these procedures have in common is that they involve separating pressurised intake air into its individual components. The resulting nitrogen-enriched gas stream is then conveyed to the protected area through a pipeline, and then fed in through outlet openings.
In systems with several sub-sections, selector valves feed the nitrogen stream into only the area or areas in which the oxygen concentration activation threshold has been exceeded. The system detects deviations from this measured value and reports them to the central control unit as faults. The other gases, as oxygen-enriched air, are conveyed outside through a second pipeline.
Oxygen concentration levels are continuously measured within the protected area. In multi-area systems, these measurements are performed separately for each sub-area. The control system reads these measurements and regulates the system in such a way that the preset target levels are established and maintained. If desired, the system can also record oxygen levels in the operating room. System status and error messages are displayed to personnel, and can be forwarded as needed.
The air needed to generate the nitrogen stream is compressed to the optimum level of overpressure for that specific procedure. This is usually achieved using electrically motorised, oil-lubricated rotary screw compressors, which are capable of continuous operation and can produce a constant, pulse-free stream of compressed air.
The air used in pressurisation is either drawn straight from the operating room or fed in from outside via ducts. Under favourable ambient conditions, filtration components in front of the compressor can maximise the service life of the compressor stage. Safety-related measurement values, such as
Compressor end temperature, dewpoint and filter loading are monitored by the pressure generation unit. If limit values are exceeded, it generates an error message, and puts the engine in safety mode if needed.
After leaving the compressor stage, the pressurised air is cleaned of residual oil (oil separator). The oil is cooled, filtered and returned to the compressor circuit.
The now nearly oil-free compressed air is cooled to a pressure dew point of +3 C using a cold-compressed air dryer. This causes most of the humidity contained in the previously warm compressed air to condense out. This condensate contains additional traces of oil contamination. An oil-water separator cleans this condensate to the point that the water phase can be initiated in a drainage system. Before leaving the compressed air generator, the pressurised air is passed through a microfilter to remove any remaining impurities.
If a suitable pressurised air or nitrogen supply is already available on the premises, it can be used to supply the oxygen reduction system. Predefined quality and quantity requirements apply to these sources.
In order to realise the applications and protection goals described, oxygen reduction systems contain components to generate the necessary quantities of nitrogen, to regulate the system, and to detect pyrolysis.
The system can also be fitted with a nitrogen reservoir tank for purposes of lowering oxygen levels rapidly.
A single, central oxygen reduction system can be used to protect either one area or several, i.e., structurally separate rooms and facilities. The multi-area system principle thus makes it possible to set different oxygen concentration levels in each of the sub-areas.
In essence, oxygen reduction systems consist of the following components:
Pressurised air supply
A continuous stream of pressurised air is generated on site using normal ambient air. This independent supply of pressurised air provides the necessary quantities of pressurised air without restrictions, ensuring that the entire system is ready for operation at any time.
The compressed air must meet specified quality criteria. Filtration elements remove particles and oil vapour from the pressurised air stream. They also detect any pollutants penetrating the compressor and stop the system before they can damage the nitrogen generator.
The pressurised air is divided into its individual components, which are drawn off separately in order to create a stream of nitrogen. This nitrogen stream has a very low pressure dewpoint and a fixed concentration of residual oxygen, which is monitored continuously. If this measured value exceeds the set limit threshold, a notification message is sent to the central control unit.
Central control unit
The control unit is equipped with analogue and digital inputs and outputs in accordance with the size of the system. It records values measured within the protected area and the operating room, along with status information on the individual system parts.
It activates components in accordance with its programming. It displays system status information, and triggers alarms or warning messages whenever impermissible conditions are detected. The control unit can be used to change settings and read out additional information. By default, the battery-powered reserve energy supply is designed to supply four hours of emergency power, but can also be set up to provide 30 or 72 hours of power (in accordance with DIN VDE 0833-2).
Oxygen measurement system (O2 sensors)
Oxygen sensors within the protected area and the operating room continuously measure the oxygen content of the room air. Using at least two sensors in each protected area allows the sensor system to self-monitor. These measurements are forwarded to the central control unit for processing. If oxygen sensors are used to regulate hold times in connection with quick-release mechanisms, the sensors must be demonstrably resistant against fire aerosols. This ensures that the hold time can be maintained until intervention forces can begin combating the problem, but for no less than one hour. The control lines for operating the oxygen sensors must be designed with an emphasis on preserving function.
Very early fire detection
Early fire detection systems are used to detect and report incipient fires quickly. Because ignition sources are less effective in oxygen-reduced environments, only very small quantities of fire energy and pyrolysis products are produced, so conventional fire alarm systems are unsuitable for use in detecting active ignition sources within these protected areas. The sensors used must have Class A response sensitivity in accordance with EN 54, Part 20. Air sampling smoke detectors are particularly suitable for this application. Preferably, these should be connected to a continuously manned post, from which action can be taken immediately. Otherwise, a central fire alarm system control unit can be used. The operator of the oxygen reduction system must instruct external or internal firefighting forces that, if the air sampling smoke detector is triggered, they should avoid ventilating the protected area as long as the fire brigade has sufficient visibility. The protective atmosphere will prevent flames from forming and keep an incipient fire at the smouldering stage. Ventilating the protected area would supply it with oxygen, which would unnecessarily facilitate fire development.
The nitrogen stream created using the nitrogen generator is piped into the protected area, while the oxygen-enriched air is directed safely outside. This nitrogen is then fed into the protected area through a distributor pipe network.
Electrical cable network
The electrical cable network is planned and installed in accordance with all applicable technical rules and regulations.
Oxygen reduction systems with “quick-release” mechanisms are equipped with an additional nitrogen tank. If a fire breaks out, this tank serves to lower oxygen levels within the protection area within seconds in order to extinguish the fire. As such, there are two parts to the system: Part 1 represents the VdS (Organisation of Property Insurers)-approved oxygen reduction system, and should be configured as per VdS 3527; Part 2 consists of an extinguishing system as per VdS 2380. System Part 2 may deviate in terms of flooding time and target concentration; these deviations are to be verified in experiments with VdS.
System Part 1 should be set with an upper limit of no higher than 17.2 Vol.-% O2. If the system is to be configured using higher upper limits, Part 2 must be designed as a conventional extinguishing system in accordance with VdS 2380, without the deviations described above. If the upper limit is set <17.2 Vol.-%, the initial concentration can also be reduced accordingly. System Part 1 must use appropriate measures as per VdS 3527 (alarm values, safety concept, etc.) to ensure that this O2 level is maintained. If this initial concentration is set lower, the reserve quantity of extinguishing gas in System Part 2 can be lowered accordingly as well.
System structure and components
200 bar/300 bar nitrogen extinguishing system components are used for storing nitrogen, distributing the extinguishing agent, and feeding the extinguishing agent into the protected areas. Only those system versions without pneumatic/mechanical delay equipment are used.
With VdS-compliant systems, System Part 2 is always fitted with an evacuation alarm in accordance with VdS 2380, with electrically time-delayed triggering of the extinguishing process. This is necessary for purposes of property protection. To ensure redundant alarms in the event of fires, when the message to switch the operating concentration is sent to the central controller, it triggers the evacuation alarm in the affected area.
Oxygen reduction systems with quick release can be designed as single or multi-zone installations. In multi-zone installations, selector valves are used to direct the quick-release nitrogen into the protected area where fire has been detected. If the oxygen reduction system is protecting areas of different sizes, the reserve nitrogen will be group-controlled using VdS-approved components, so that only the necessary quantity of nitrogen is supplied to the protected area.
Supply pipe network
The pipe network used to supply the quick-release extinguishing gas is to be planned and constructed in accordance with VdS 2380.
Fire alarm technology for activating quick release
In the event of fire, System Part 2 must be triggered by a fire detection system as defined in VdS 2095. The quick-release mechanism is triggered via a VdS-approved fire detection control panel for regulating extinguishing systems, or with an electrical control device.
The operator must have his or her oxygen-reduction system(s) checked by the manufacturer, or by persons trained in accordance with manufacturer instructions. This must be done immediately following any unusual incidents that may have negatively impacted safety. If defects are discovered that constitute a potential danger to persons, the oxygen-reduction system must be taken out of operation.
The operator of the oxygen-reduction system must ensure that any identified defects are repaired.
The operator must have the manufacturing company or an authorised person conduct an acceptance test on the oxygen reduction system immediately following system installation or significant changes to the system. This test must take place prior to commissioning.
The operator must have proper system functionality tested by the manufacturer or a qualified person at least once per year. Special operational circumstances may make it necessary to carry out more frequent inspections.
Record of inspections
The results of the inspections must be recorded in an inspection report. Records of the acceptance tests must be kept throughout the entire service life of the fire prevention system. Regular inspection records must be kept for at least four years. These may be stored on computer data carriers. The documents must be presented to the competent supervisory authorities upon request.
Being in an oxygen-reduced atmosphere is comparable to being at a high altitude. The significant physiological factor is the oxygen partial pressure (p02). From an occupational-health perspective, actual altitude ( = hypobaric hypoxia) and oxygen reduction ( = isobaric hypoxia) can be considered identical (relevant differences only occur above 6,300 ̴m / below ̴9% O2 as a result of differences in breathing mechanics due to lower air viscosity).
Depending on the oxygen concentration selected and the duration of stay, oxygen-reduced air can also cause symptoms of acute altitude sickness (headaches, tiredness, nausea, loss of appetite, dizziness), but only after at least five to six hours of uninterrupted time in a room with oxygen concentrations under 14% (at sea level).
Longer exposure to significantly reduced oxygen levels (below 11% vol) can result in higher rates of error in visual tasks and logical thinking, along with slower reaction times and limited coordination. For physically demanding work, a loss of performance of -10 % per 2 % of O2 reduction, starting from 17.4 vol %, must be taken into account in work scheduling.
Among other things, the lowered oxygen content of the air, and the resulting lower oxygen partial pressure, can be a hazard to employees with advanced heart and circulatory disease, respiratory and lung disorders, or blood disorders. The degree of risk depends on the severity of the disorder and the oxygen concentration. Generally speaking, people who do not experience respiratory distress following moderate exertion (e.g., walking up stairs) will not be at an increased risk in environments with oxygen concentrations as low as 14.8% vol.
Individual workplace analyses must be conducted to determine all measures necessary for extreme hypoxia (< 13% vol.). For control reasons, the oxygen concentration can be stabilised at ± 0.2 vol %. This fluctuation range is physiologically irrelevant and can therefore be accepted from a personal safety perspective.
As fire prevention systems based on oxygen reduction by means of nitrogen infeed appeared on the market and began increasing in popularity, the Deutsche Gesetzliche Unfallversicherung (German Statutory Accident Insurance) issued Directive BGI/GUV-I 5162, Working in Oxygen-Reduced Environments, based on a study it commissioned, experiences reported in the field, and other international research efforts. The University of Munich’s study shows that it is possible to remain in a reduced-oxygen environment without health risks, but such environments are to be classified based on the degree of oxygen reduction.
Oxygen-reduced areas can be divided into four risk classes, depending on the potential hazard involved:
Certain framework conditions must be in place for work performed in reduced-oxygen spaces.
The following measures are recommended for persons entering oxygen-reduced environments:
Risk class 1 (O2 concentration ≥ 17.0% vol.)
Risk class 2 (O2 concentration < 17 ~ 14.8% vol.)
Employees who experience discomfort when working in an oxygen-reduced environment should leave the oxygen-reduced area immediately. If they feel completely recovered within 30 minutes, they may re-enter the area. If not, or if their symptoms return upon re-entering the oxygen-reduced environment, they should undergo physical examination before entering the area again.
Risk class 3 (O2 concentration < 14.8 ~ 13.0% vol.)
- at least once every three years for employees under 40
- once annually for employees 40 and older
In view of the ongoing debate within several countries (Austria, Great Britain, France, Germany), it is important to stress that (mild) hypoxia generally poses no danger. Five important factors must be taken into account when determining a risk profile for hypoxia exposure:
Extremely short exposure
Extremely short exposure generally occurs at altitudes between 1,800m and 2,500m, for a period of anywhere between a few minutes and a few hours.
In fire protection rooms equipped with hypoxia systems that create isobaric hypoxia of 14.8-17% oxygen concentration (± 0.2), employees are exposed to an equivalent altitude of 1,700-2,600m (based on ICAO standard atmosphere, see Fig. 2). This altitude is within the range of so-called “threshold altitudes”, meaning the altitudes at which the body shows initial reactions to hypoxia. Depending on the system, this threshold altitude varies between 1,500m (slightly increased resting pulse rate) and 2,400 m (increased concentrations of erythropoietin serum).
As such, altitudes around this threshold do not represent a hypoxic danger to healthy people. Persons with moderate chronic illnesses are at no danger, either. Possible risks for seriously ill persons will be covered later.
Under certain special conditions, employees may be exposed to altitudes between 2,700m and 3,800m in fire protection rooms. Such exposure is limited to no more than a few hours, and frequently lasts less than 60 minutes. It is typical of such exposure situations that employees are free to leave the hypoxia area at any time if they feel unwell.
Several popular activities expose people to altitudes that are even higher, e.g., skiing at 3,800m (Europe) or over 4,000m (USA), or road traffic at nearly 3,000m (Europe), over 4,000m (USA and Tibet), or over 5,000m (South America). In such situations, the main problem is often the change in pressure, especially for children or people with upper respiratory infections.
The longest-duration exposures of this type (“extremely short exposure”) are long-distance flights, though in some cases these can also be called “limited exposure” (see below). Some data suggests that several airlines pressurise their cabins to altitudes higher than the ICAO’s specified limit of 2,400m, particularly in modern aircraft. Generally, such exposure only lasts a few hours. At altitudes of up to 3,000m (or even higher), there is no danger of developing altitude sickness within this period of time. Acute pressure changes can be the main problem for this group, particularly for people with colds. Generally speaking, everyone feels well at this altitude, even pregnant women and children. The only exceptions are persons with severe pre-existing conditions (see below).
Within this “extremely short exposure” group, there is one small subgroup for which there are special conditions: People who train others to acclimatise for extremely high-altitude expeditions, particularly mountaineers, or employees undergoing such pre-acclimatisation for work at very high altitudes. This type of pre-acclimatisation is being performed more and more frequently in isobaric hypoxia facilities. Participants are exposed to altitudes of 5,300m or greater. In most cases, such exposure is limited to anywhere between a few minutes and half an hour. The particular advantage of isobaric hypoxia lies in the fact that participants can easily return to a normal atmosphere at any time if they feel unwell.
Persons with certain pre-existing conditions could potentially develop serious health problems at such altitudes, whereas healthy people generally tolerate exposure of this duration well: Exposure time is too short for participants to develop acute altitude sickness, and also too short to cause related neurological problems. In aviation medicine, this period of time is referred to as the “time of useful consciousness.”
Beginning at 1,500m above sea level, maximum working capacity decreases 10-15% for each 1,000m in altitude, with physically fit people experiencing the greatest proportional performance losses.
Since work done at high altitudes is usually relatively low-exertion (estimated at 0.5-1.0 W/kg body weight), this effect normally poses no limitations. With very strenuous work performed above 3,000m, O2 becomes a progressively more limiting factor, and people performing such intense work are unable to stabilise their SaO2 at a level that would be expected for the corresponding height at rest. As a result, their SaO2 levels drop.
This type of work should only be performed by healthy persons, and even they must be presumed to have a limited (lower) working capacity when planning the necessary activities and resources.
Reduction in maximum working capacity at increasing altitudes: -7.0% per 7,000 m for people with low fitness levels; up to 7.5% per 7,000 m for people with high fitness levels, starting at 7,500m altitude.
7,500 m: “Threshold altitude”
5,300 m: Threshold of full acclimatisation / long-term stay.