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Inert Gas Risks - Oxygen Deficiency
Oxygen deficiency can arise when using gases such as nitrogen, carbon dioxide and argon, unless good practice is observed.
This page specifies the dangers and outlines simple precautions to be taken so that every user can employ these gases with confidence and without danger.
Composition of air
The approximate volumetric composition of air is:
| Gas | Abbreviation | % |
|---|---|---|
| Oxygen | 02 | 21% |
| Nitrogen | N2 | 78% |
| Argon | Ar | 1% |
- atmospheric gases are non-toxic but alterations in their concentrations - especially that of oxygen - have an effect upon life and combustion processes
- if good practice is not observed accidents may happen as changes in concentration cannot be easily detected by the human senses
- oxygen is not flammable but it does support combustion
- nitrogen and argon inhibit combustion
- when these gases are in the liquid state it is necessary to bear in mind the very low temperatures involved (below -180°C). They can rapidly cause cold burns and make certain materials sufficiently brittle to lead to structural failure
Hazards from oxygen deficiency
Oxygen is essential to life and it is therefore vital that adequate oxygen is present in any atmosphere being breathed.
While a healthy person may survive a short exposure to air with an oxygen content as low as about 16%, no one should ever be asked to endanger his or her life by breathing such an atmosphere.
Oxygen deficiency cannot readily be detected by human senses. Victims are usually unaware of the danger they are in and may even have a feeling of wellbeing.
Symptoms
Human beings vary considerably in their reactions to oxygen deficiency and it is therefore not possible to lay down hard and fast rules.
A general indication of what is likely to happen is given in the table below. It should be appreciated that some individuals may react very differently and the reaction may vary due to the presence of other gases, especially carbon dioxide.
| Oxygen content (vol %) | Effects and symptoms (at atmospheric pressure) |
|---|---|
| 11-16 | Diminution of physical and intellectual performance without the person being aware of this |
| 8-11 | Possibility of fainting without prior warning |
| 6-8 | Fainting within a few minutes, resuscitation possible if carried out immediately |
| 0-6 | Fainting almost immediate |
Below 11% oxygen there is a risk of death due to asphyxia unless the person is resuscitated immediately. In general, oxygen deficiency can lead to:
- loss of mental alertness
- distortion of judgement
- brain damage (after a relatively short time)
Causes and avoidance of oxygen deficiency
Oxygen deficiency is best guarded against by careful attention to the following points:
Leakage of gases other than oxygen
- this leads automatically to oxygen deficiency. Newly assembled equipment which uses inert or any other gas should be thoroughly leak-checked by a timed gas pressure drop test. This must be supplemented by testing with an approved leak-test fluid which is compatible with the equipment for which it is being used. Alternatively a solution of 1% Teepol in demineralised water may be used
- all equipment, for instance piping and hose connections, should be properly fitted. Hoses and other equipment should be kept leak-tight and be protected from damage
- all maintenance and repair work should be carried out by experienced and fully skilled personnel
- when the work period is over, the cylinder valve or piped supply stop valve must be turned off. This is in order to avoid possible leakage in the time between the end of one working period and the beginning of the next
- the valves on welding equipment should not be relied upon for turning off the gas supply
- gas cylinders in use should be protected against being knocked or dropped
Spillage of liquid gases
A small amount of liquid can lead to the formation of a large amount of gas.
Consequently, liquid spillage can rapidly cause oxygen deficiency in confined spaces, pits, etc.
Tanks and equipment for the storage and handling of liquid gases should be inspected carefully and maintained in accordance with the relevant regulations or recommendations.
Vent outlets
Vented gases are often deficient in oxygen and work should not be carried out in such atmospheres.
Purging and cryogenic processes
Oxygen deficiency will arise when preparing plant items (such as vessels) for repair by purging with nitrogen or other inert gases.
Processes such as food cooling, ground freezing, cryogenic surgery and blood plasma preservation lead automatically to oxygen-deficient atmospheres.
People should not enter such areas, even if the atmosphere is only slightly deficient in oxygen, unless adequate breathing equipment is used.
Welding and heating processes
All gas welding and heating processes involve taking oxygen from the air and can lead to a deficiency unless the workspace is sufficiently ventilated.
Removal of argon, carbon dioxide and cold gas
Removal of argon, carbon dioxide and cold gas from large vessels and deep pits can be difficult due to the relatively high density of the gas compared with air.
Air introduced into the bottom of such spaces often floats up through the dense gas without displacing it. This presents a special problem in that purging is liable to take much longer than expected.
Detection of oxygen enrichment or deficiency
Measuring instruments
These indicate increases and decreases in the oxygen concentration of the ambient atmosphere and have a measuring range from 0 to 40% by volume of oxygen.
Various measuring techniques are available, giving visible and/or audible warnings and can provide continuous or discontinuous measurement. BOC can advise on the suitability of instrument to be used.
Accuracy
The accuracy of the measuring method should be such that the real oxygen value is between 19.5% and 22.5%.
Using measuring instruments
The manufacturer's operating and maintenance instructions must always be carefully followed.
In confined work environments the measuring instrument should be located as near as possible to the worker.
It is recommended that the worker has a portable measuring instrument attached to his working clothes which gives an audible and visual alarm if the oxygen content of the atmosphere is likely to deviate more than 2% from that of normal air.
Other gases
The safety of a space does not depend on oxygen content alone but can be affected by other gases such as fuel gases. These should be analysed as necessary.
Preventative measures
General considerations
Apparatus used for the manufacture, distribution and utilisation of inert gases must be installed and identified in accordance with the recommendations of the industrial gas industry, and must comply with the applicable regulations.
Any leak must be dealt with by adequately trained staff using specific equipment.
Information should be available to all personnel on the actions to be taken by staff and first aiders in the event of an incident.
Operating personnel must at all times obey works rules and regulations and, where called for, protective equipment must be worn.
Breathing equipment
Application
Appropriate breathing equipment is essential in situations where oxygen deficiency can arise and on no account should rescue be attempted without proper equipment and adequate training in its use.
Breathing equipment is not required for oxygen-rich situations.
Types of equipment
Absorbent types of respirator give no assistance whatsoever in an oxygen-deficient atmosphere.
Recommended types of breathing equipment are:
- self-contained breathing apparatus using air cylinders. When wearing this apparatus, it may be difficult to enter manholes
- fresh air masks where the respirator is connected via a tube of adequate length and diameter to a clean compressed air supply or to a region where the atmosphere is of satisfactory composition to support life
Double manning
Where personnel have to work in confined spaces which may become subject to atmospheric oxygen enrichment or deficiency, a watcher must be stationed immediately outside the confined space entrance.
The watcher should hold the rope of a rescue harness attached to the person working in the confined space and should, if necessary, have a winch available.
Analysis
Before people enter a space which may be subject to oxygen enrichment or deficiency the atmosphere should be analysed for oxygen. Free entry is permissible only if the oxygen concentration is between 20 and 22%.
If there is any possibility of a change in concentration, anyone entering such a space must be issued with a personal continuous oxygen-measuring device giving an audible alarm when the oxygen concentration in the atmosphere varies outside the safe limits.
Information and training
All people who work in spaces where oxygen deficiency or enrichment can occur should be given adequate instructions as to the risks involved, special attention being drawn to the nature of the risks, the rapidity of their effects and that the operator may be unaware of the potential danger he is exposed to.
Practical risk-reduction training should be given.
Blanking and ventilation
- any vessel which is connected to a gas source other than air containing 21% oxygen must be disconnected from such a source by the removal of a section of pipe, by the use of a spectacle plate or by inserting blanking spades
- the space should be thoroughly ventilated so as to maintain a normal atmosphere before and during entry
- reliance on the closure of valves to prevent oxygen enrichment or deficiency is not sufficient
- permission to enter such a space may be given only after the issue of a permit certificate signed by the responsible person
Oxygen deficiency – first aid
- remove the patient to the open air without delay and keep him warm
- administer oxygen from an automatic resuscitator or supply artificial respiration by an approved method
- summon medical assistance and continue treatment until the patient revives or professional medical assistance is available
- ensure that all rescue personnel have adequate supplies of oxygen or air from self-contained breathing apparatus or a fresh air line
Entry into confined spaces
Guidance is given in the HSE Approved Code of Practice called 'Safe work in confined spaces' available from HSE Books. It also contains the Confined Space Regulations 1997.
