In the absence of adequate ventilation the level of oxygen can be reduced surprisingly quickly by breathing and combustion processes.
Oxygen levels may also be depleted due to dilution by other gases such as carbon dioxide (also a toxic gas), nitrogen or helium, and chemical absorption by corrosion processes and similar reactions. Oxygen sensors should be used in environments where any of these potential risks exist.
When locating oxygen sensors, consideration needs to be given to the density of the diluting gas and the “breathing” zone (nose level). For example helium is lighter than air and will displace the oxygen from the ceiling downwards whereas carbon dioxide, being heavier than air, will predominately displace the oxygen below the breathing zone. Ventilation patterns must also be considered when locating sensors.
The table below shows the effect of a diluting gas on the level of oxygen
Oxygen monitors usually provide a first-level alarm when the oxygen concentration has dropped to 19% volume. Most people will begin to behave abnormally when the level reaches 17%, and hence a second alarm is usually set at this threshold. Exposure to atmospheres containing between 10% and 13% oxygen can bring about unconsciousness very rapidly; death comes very quickly if the oxygen level drops below 6% volume.
The hazard presented by oxygen deficiency is easily under-estimated; especially as risks can exist in non-industrial environments such as cellars or bars where CO2 and nitrogen are used. Oxygen depletion due to corrosion or bacterial activities presents a significant risk in confined spaces such as pipes, vessels, sewers and tunnels. Oxygen sensors are often installed in laboratories where inert gases (eg nitrogen) are stored in enclosed areas.
Increased levels of oxygen may dramatically increase the flammability of any combustible matter. If oxygen levels exceed 24% volume, even materials such as clothing which might normally just smoulder may burst into flame.
The risk from oxygen enrichment exists where pure oxygen is stored; for example in hospitals and industrial gas manufacturing and distribution plants. Oxygen sensors with rising alarms set at 23.5% volume are typically used in such environments.