Oxygen Deficiency

The potential for oxygen deficiencies must be considered as entry into or work in oxygen-deficient atmospheres (ODA) is dangerous. An oxygen deficient environment is one where the atmosphere is below 19.5% oxygen level. A safe oxygen level means an oxygen content in air of between 19.5% -23.5% (Safe Work Australia).

Air purifying/filtering respirators such as filtering facepiece/disposable, reusable half and full face, Powered Air Purifying Respirators (PAPR) i.e. any respirator that uses a filter/cartridge to capture an airborne contaminant, must NOT be used in an oxygen deficient environment. A suitable supplied air respirator e.g. Self Contained Breathing Apparatus (SCBA) or airline with emergency breathing device function must be used.

Such atmospheres may exist in confined spaces, e.g. as a result of a chemical reaction or displacement. These conditions generally exist in areas with limited ventilation, e.g. tanks, vats, reaction vessels, wells, tunnels, pipes, conduits, access holes, silage pits and deep earthen trenches. Safe procedures for working in confined spaces are not covered in this tool but refer to your respective country/state/territory codes of practice as linked further below.

There are four basic mechanisms by which oxygen-deficient atmospheres, or their equivalent, may occur. These are: 

  1. Ascending to altitude.
  2. Chemical reaction with solid products, e.g. the formation of rust or other corrosion products.
  3. Chemical reaction gaseous, e.g. respiration, fire.
  4. Displacement (dilution) by some other gas or vapour.

Of these, only the first two may result in an oxygen deficiency without the introduction of, or an increase in concentration of, some other gas or vapour. In the latter two cases, a significant oxygen deficiency will usually be caused with the simultaneous production of a toxic concentration of such a gas or vapour.

At high altitudes, although oxygen percent by volume remains constant, the amount of oxygen available for breathing is reduced owing to the lower pressure. This may produce effects similar to those of lack of oxygen by action of a simple asphyxiant. 

Earth’s atmosphere has an essentially fixed composition of the gases (% Volume)

  • Nitrogen (78.09%)
  • Oxygen (20.95%)
  • Carbon Dioxide (0.04%)
  • Other gases (0.92%)

Other gases present in small amounts include neon, helium, and krypton. Water vapour, an important constituent of the normal atmosphere, may be present up to 5% of the total volume. No single definition (value) of an oxygen-deficient atmosphere has been universally accepted. Where a definition is required however, local legislation should be followed. A safe oxygen range is defined in AS/NZS 2865

The symptoms of oxygen deficiency depend on the oxygen concentration present. It is difficult for the exposed person to realize the effect of oxygen deficiency. Lack of oxygen can cause gradual depression of the central nervous system. This affects powers of discrimination, logic and hearing, with attendant muscular weaknesses and lack of coordination. Extremely low levels of oxygen will lead to death in minutes without intervention.

In AS/NZS 1715:2009 Table 4.6 as below provides Selection consideration of use in oxygen deficiency or IDLH atmosphere or unknown concentrations of contaminants.

AS/NZS 1715:2009 Table 4.6

Selection Considerations of use in oxygen deficiency or IDLH atmosphere or unknown concentrations of contaminants
Expected Use Equipment Remarks
Escape SCBA, quick fill type Mines-continuous supply/rechargeable for

long escape route
Work, routine Full facepiece airline with EBD Limited mobility and distance to source of

respirable air
Work, including rescue use

 

 ·         Compressed air SCBA duration: > 15 minutes

·         Compressed oxygen SCBA

·         Air-line with SCBA

 ·         Limited Duration

·         Specialist training essential

·         Limited mobility and distance to source of respirable air
Special response HAZMAT incident. No O₂ deficiency Combined gas/vapour and particulate P3 full facepiece, PAPR or air-line with filter Assessed as suitable by emergency service personnel
Work, including rescue use

Special response HAZMAT incident. O₂ deficiency

 Compressed air SCBA duration: > 15 minutes

 
Work in Confined Space Refer to relevant Regulator Confined Space code of practice ·         If oxygen deficiency is suspected then supplied air equipment is suitable

·         Limited duration only

·         Specialist training essential

 

Reference/s for further information and detail:

  • AS/NZS 1715:2009 – Selection, use and maintenance – Section 3.7
  • SA/SNZ TS ISO 16975.1:2023 – Selection, use and maintenance – Section 7.3.2.4

Further Australia & New Zealand Confined Space Resources