Atmospheric Hazards: Oxygen, Flammable and Toxic Gases
This domain tests the atmospheres that kill in confined spaces: oxygen deficiency and enrichment, flammable gases and vapours, and toxic gases such as hydrogen sulphide and carbon monoxide.
Oxygen
- Normal air is 20.9% oxygen; the usual entry range is 19.5% to 23.5%.
- Deficiency is caused by inert gas purging, oxidation such as rusting, combustion, bacterial decay, displacement by heavier gases in pits, and poor ventilation.
- Effects: 15–19% reduced coordination; 12–14% headache, dizziness and poor judgement; 10–12% rapid loss of consciousness; 6–8% death within minutes.
- Enrichment makes materials burn violently; clothing and hair can become saturated, and oil or grease on equipment is dangerous.
Flammable gases and vapours
Characterised by flashpoint, lower and upper explosive limits, and vapour density. Methane is lighter than air and collects at the top; LPG and solvent vapours are heavier and collect at the bottom. A common entry action level is 10% LEL, with evacuation at 20% LEL or as specified in the code of practice. Never purge with a flammable gas. Sources include sludge, residues, solvents, hot work and battery charging.
Toxic gases
- Hydrogen sulphide - colourless, heavier than air, rotten-egg smell at low levels; olfactory fatigue above about 100 ppm; immediate collapse at high concentrations; accumulates in sewers and pits.
- Carbon monoxide - colourless, odourless chemical asphyxiant; binds haemoglobin roughly 200–250 times more strongly than oxygen; produced by combustion, engines and welding.
- Carbon dioxide - heavier than air, asphyxiant and toxic at high concentration; found in sewers, fermentation and dry ice.
- Others: ammonia, chlorine, sulphur dioxide, solvent vapours, welding fume, diesel exhaust and dust.
Worked example
A worker descends a 4 m pit and collapses without warning; a colleague climbs down and collapses too. The likely cause is hydrogen sulphide above 100 ppm, which causes immediate loss of consciousness. The correct response is not to enter: raise the alarm, rig the tripod, ventilate, and wait for BA-equipped rescuers. Two deaths from one hazard is the classic pattern of untrained rescue.
Traps that cost marks
- Relying on smell to judge hydrogen sulphide.
- Believing oxygen enrichment is harmless.
- Testing only for flammables and omitting H2S and CO sensors.
- Forgetting carbon monoxide from engines and welding.