The aim of this project is to increase awareness of the hazard of oxygen depletion, and to increase the level of control measures used.

Oxygen deficiencies (hypoxia) result in a reduction in the oxygen saturation of the blood (anoxia), leading to a retardation of the oxidising processes in the brain, and consequently to disturbances of the central nervous system. Early signs that a person is working in an oxygen-poor atmosphere, and is beginning to develop anoxia, are that the pulse and respiration rate increase as the body attempts to compensate for the reduced oxygen levels. These signs are often accompanied by a lack of muscle coordination, insensitivity to pain, emotional changes and fatigue. If any of these symptoms appear in situations where asphyxia is possible, immediately remove the affected person to the open air, and follow up with resuscitation if necessary. It is important to realise that the victim may not be aware that he or she is being asphyxiated. More severe depletion can lead to nausea, vomiting, loss of consciousness, convulsions, respiratory collapse, and death within just a few minutes.

Oxygen deficiency is most commonly associated with working in mines, sewers, deep excavations, wells, silos, poorly ventilated confined spaces and at high altitudes.

Oxygen depletion is often associated with the presence of significant levels of atmospheric contaminants that may or may not be toxic, e.g. hydrogen sulphide, carbon monoxide, nitrogen and carbon dioxide.

What is oxygen depletion?

By volume, fresh air is composed of 78.1% nitrogen, 20.9% oxygen, 0.033% carbon dioxide and about 0.9% inert gases, the most abundant of which is argon.

Technically the oxygen content can be considered to be depleted if it falls measurably below 20.9%, and people should not work in conditions where the level drops below 19.5%.

How do oxygen levels become depleted?

Oxygen levels can diminish due to physical displacement, biological activity, chemical activity or naturally as a result of altitude. Although the composition of air remains constant with altitude, the amount of air available to breathe reduces with altitude such that at 2000m there is only approximately 80% of the air available as at sea level.

Physical displacement - refers to displacement of air, and consequently oxygen, by some other gas, vapour, fume, or mist, thus lowering the concentration or proportion of oxygen present. Liquid nitrogen, and other cryogenic^[1] fluids, will displace air as they evaporate. In the case of liquid nitrogen, upon evaporation it will occupy a volume of approximately 700 times the volume of the liquid from which it has evaporated. Physical displacement can also result from the use of compressed gases, internal combustion engines and burning of heating fuels (the latter two both produce carbon dioxide, carbon monoxide and water vapour), and the release of steam from an industrial or commercial process.

Biological activity - in the context of oxygen depletion, refers to microbial activity (yeasts, moulds, bacteria, algae) that utilises oxygen, and generally releases carbon dioxide and/or other metabolites. Microbial activity can take place anywhere and is normally not a problem from a clean air point of view, unless it takes place in a confined space with inadequate ventilation, such as fermentation vessels, effluent tanks, sewers, silos and pits.

Chemical activity - in the context of oxygen depletion, refers to any chemical process that uses oxygen. One such example is oxidation of steel or iron, causing rust to occur. Where this occurs in a confined space, the oxygen content of the atmosphere will be reduced.

Is oxygen depletion a problem in New Zealand workplaces?

Oxygen depletion, confined spaces and toxic atmospheres are very much interrelated subjects.

While possibly not a major problem in New Zealand, oxygen depletion must always be considered a possibility in confined spaces. Confined spaces will also often harbour significant quantities of 'confined space contaminants'. Confined space issues continue to be a significant contributor to the serious harm figures arising in New Zealand workplaces.

Case studies - Oxygen depletion

1. A farmer used a ladder to descend into a relatively new offal pit to recover an implement. After recovering the tool and near the top he fell backwards into the pit. He had died by the time rescue services arrived. Gas measurements taken some days later, considered likely to approximate those at the time of the accident, showed an oxygen level of 3.2%. There were also significant amounts of methane (a non-toxic asphyxiant) and very low levels of hydrogen sulphide present. Significant carbon monoxide (CO) readings found were thought to be due to cross-reactivity of the CO sensor with carbon dioxide, which, although not measured at the time, would have very likely been present at thousands of parts per million. The National Institute for Occupational Safety and Health (NIOSH) in the U.S. states that at 6%, breathing is difficult and death occurs in minutes.
2. In Victoria, a hotel worker was overcome and died as a result of exposure to an oxygen-deficient atmosphere in a hotel cellar. Carbon dioxide and sometimes nitrogen are used to provide a pressure head for tapping off beverages such as beer and soft drinks. In a poorly ventilated area, like a cellar, the oxygen in the air can be diluted by gases from a leaking system, and a person entering the area can be overcome without warning. Death will occur within 3 minutes. Anyone who spontaneously attempts to rescue a victim in these circumstances is also likely to become a victim themselves. In cellar situations such as this, a maintenance programme that includes regular inspection and testing of the gas system, by competent service contractors, is essential. Consideration should also be given to the installation of natural or mechanical ventilation as a practicable step to reduce the accumulation of gases in the event of a leak. Where there is no functional ventilation in place, the cellar should be fitted with an oxygen alarm to warn if breathing zone levels of oxygen drop below 19.5%, in which case no person should enter the area. In the absence of adequate ventilation or an alarm, the cellar must be regarded as a confined space with insufficient oxygen, and entry should involve a full confined space entry procedure.
3. A welder in Western Australia was overcome when he entered a 750mm pipe fabrication where argon had been used to create a gas shield behind an external weld zone. A second worker also encountered difficulties when he went to assist after the alarm was raised by an observer. The pipe fabrication in question was several metres long and one section had been dammed and flooded with argon gas. Upon completion of the welding, the dams were vented and the argon expelled from the weld area. The welder then entered the pipe to remove the dams, unaware that the expelled argon had accumulated in another section of the fabrication. There was a failure to implement established confined space procedures, despite the need for an observer being recognised. Had the system been implemented, a risk assessment would have identified potential hazards and control measures would have been completed, test equipment would have been provided to test and monitor the atmosphere, and employees would have been aware of the actions to be taken in the event of an emergency. (Source: Government of Western Australia, Department Consumer and Employment Protection)
4. A cellar-hand was overcome by carbon dioxide after entering a 4,500 litre wine vat through a 380 mm opening at the top of the wine vat containing crushed grape skins and seeds. The juice of the crushed grapes had been drained off through the drainer at the bottom of the tank. The atmosphere was inert due to the presence of large amounts of carbon dioxide. The contributing factors included a lack of scientific equipment to test the wine vat's internal atmosphere and the employee appeared to lack an knowledge of the risks associated with carbon dioxide, including the rapidity of symptoms, onset of euphoria, loss of muscle control and death within minutes. (Source: Government of Western Australia, Department Consumer and Employment Protection)
5. An employee of a fruit packer collapsed when he was working in a controlled atmosphere room. It is believed that the atmosphere contained no more than 2% oxygen.
6. While assessing maintenance work required in a large vessel known as a pregasser, a milk powder shift fitter removed a manhole cover, and lay on his back with his head and shoulders in the vessel, to inspect the proposed job. The vessel, while in use, had been filled with nitrogen to prevent spoilage of the product. Although the gas valve was "off" at the time of the accident, there was still residual nitrogen in the vessel. The worker became unconscious a short time later, but was revived and made a full recovery following evacuation to hospital. The employer had developed a draft confined space entry procedure, but this had not been implemented as an interim measure. Had it been implemented, it is likely that the accident would have been avoided. The use of gas detection equipment was a practicable step that should have been taken.
7. Three men died in February 1999 in an oxygen-deficient tidal manhole in a 300 mm sewer pipeline in Auckland. It is believed that one of the men entered the manhole, to remedy an apparent blockage, and collapsed shortly afterwards. The second man entered to effect a rescue, but it appears that he got into difficulty, the third man entered to assist him. The job of removing such blockages was normally done from above ground using a water blasting technique. It was speculated, based on the evidence available, that they experienced some difficulty in manipulating the hose into the blocked pipeline from above. There were three separate companies involved in the operation - all three had written policies in place at the time of the accident, consistent with the standard for confined space work, but safety equipment had been left at one of the business's depots. Because there was insufficient gear to clear the blockage, it was agreed that the work would not be done that day. However, one of the three men remained on site and decided to descend into the manhole. Testing of the atmosphere was undertaken some five days after the event. This revealed an atmosphere severely deficient in oxygen and containing other gases including hydrogen sulphide, carbon dioxide, carbon monoxide and methane. The pattern of readings suggested that less oxygen was present at high tide. The pathology report concluded that death resulted from hydrogen sulphide intoxication whilst working in a sewer.

Workplaces where oxygen depletion should be considered:

• Hospitality industry - carbon dioxide and/or nitrogen systems are used, often in poorly ventilated cellars for tapping off beverages such as beer and soft drinks. Where these systems are poorly maintained, leakages may occur leading to displacement of air.
• Brewing and winemaking - fermentation produces carbon dioxide, resulting in displacement of air in fermentation vessels. Also, vessels are charged with a carbon dioxide atmosphere to minimise spoilage due to oxidation reactions.
• Farming - offal pits and effluent tanks will often have contaminant-rich and oxygen-poor atmospheres.
• Laboratories - - In cell culture and semen storage wide use is made of cryogenic liquids, such as liquid nitrogen. With a boiling point of 196°C below zero, liquid nitrogen readily evaporates at room temperature, displacing air. Many cell culture rooms are operated at a positive pressure, by introducing filtered air - where this is the case, a build up of nitrogen is less likely.
• Waste water treatment - biological activity caused by microbes utilises oxygen and often produces toxic gases as by-products of metabolism.
• Businesses burning heating fuels - where small rooms are heated using gas or solid fuels, oxygen will be consumed. Where the burners are unflued, toxic gases, especially carbon monoxide, will accumulate.
• Engineering maintenance - commonly engineers are called upon to work inside confined spaces such as vats, vessels and tanks, where at best, atmospheres will be stale, if not dangerous due to low oxygen levels and/or high levels of toxic contaminants.
• Many types of business use compressed gases, too. These fall into 5 categories:
  1. Compressed oxygen - leaks can lead to an enriched oxygen atmosphere, increasing the risk of fire and of spontaneous combustion of oils and greases. Examples of gases containing oxygen include oxygen, Entonox (50%oxygen/50% nitrous oxide) and Heliox (mixture of helium and oxygen).
  2. Toxic gases - examples include carbon monoxide, hydrogen sulphide, ammonia (generally compressed as a liquid). Most of these toxic gases are found in research and calibration laboratory settings.
  3. Inert gases - the best examples are nitrogen, carbon dioxide and argon or helium, which are generally only hazardous because of their capacity to displace air, and thus reduce the concentration of oxygen. They are asphyxiants.
  4. Flammable asphyxiant gases - these act in the same way as the inert gases, they are asphyxiants, but they are also highly flammable. Examples include methane, acetylene and LPG.
  5. Compressed air -not a problem if compressed air leaks slowly into a workplace.
• Categories a) through d) all have the capacity to displace air.
  Category a) has the capacity to generate an enriched oxygen atmosphere.
  Categories b), c) and d) have the capacity to generate an oxygen-poor atmosphere.
  Cryogenic fluids are liquefied gases that have a normal boiling point below -238°F (-150°C)

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Footnote:

1 Cryogenic fluids are liquefied gases that have a normal boiling point below -238°F (-150°C)

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Issued by the Department of Labour, New Zealand
http://www.osh.dol.govt.nz

No. 24 - January 2007