A lack of oxygen
The air that we breathe contains around 21% oxygen and, at that level, people can work without difficulty. If the oxygen level falls below 10% it will cause breathing difficulties, unconsciousness and possibly death.
A reduction in oxygen can be caused by the following.
- Hot works or machinery that burns up the oxygen.
- People breathing.
- Rust (oxidisation) inside enclosed tanks.
Too much oxygen
If the air contains too much oxygen this can be a major hazard. Organic materials (such as oil and wood) will be able to catch fire and burn more easily, and ordinary materials (such as paper and clothing) will burn more fiercely.
An increase of only 4% oxygen is enough to create a hazard. This may happen by accident.
- In oxyacetylene and oxypropane processes, sometimes not all of the oxygen supplied to a cutting torch is used. Some may be released, increasing the atmospheric oxygen above the normal 21%.
- Leakage from torches or hoses may go unnoticed (such as during meal breaks or overnight). For this reason, they should be removed and properly isolated at every break time.
Depending upon the work activity, any space can become a confined space
Toxic and flammable atmospheres
Oxygen can be replaced by asphyxiating, toxic or flammable gases in the following ways.
- By stirring up sludge or slurry in excavations, which may release methane or hydrogen sulphide.
- Natural methane venting from the ground or rotting vegetation.
- Using substances that give off fumes or vapours (such as solvents, paints and resins).
- Sewage, giving off hydrogen sulphide (which smells like rotten eggs and will drive out or dilute the oxygen in the air).
- Chalky ground, which gives off carbon dioxide, especially when wetted by acidic rain.
- People breathing out carbon dioxide.
- Gases (such as LPG, methane or oxygen enrichment) which build up to form a highly flammable atmosphere.
An overturned tanker or a large spill may release petrol or dangerous chemicals into the drainage system. The vapours can travel hundreds of metres.
Apart from the hazards mentioned, other dangers may arise within a confined space. Some examples are listed below.
- Using electrical and mechanical equipment.
- Extremes of heat, which can have harmful effects and may be intensified in a confined space.
- High humidity levels, which can interfere with the body’s natural cooling mechanism, preventing sweat from evaporating.
- Excessive sweating, which will cause the body to lose vital salts.
- Difficulty getting into, or out of, and working in a confined space, which may involve working at height.
The potential hazard of an inrush of water, gas, sludge and so on, due to a failure of walls or barriers, or leakage from valves, flanges or blanks, must all be considered at the risk assessment stage.