Asphyxia is caused by the lack of oxygen
The body requires air containing about 20% oxygen by volume for normal breathing. Oxygen concentrations below 16% are generally considered unsafe for humans and relative asphyxia will result.
Oxygen deficiency starts to exist when the concentration is less than 19.5% by volume. The symptoms of oxygen deficiency are mental confusion, loss of muscle coordination, unconsciousness and eventual cessation of breathing. Danger areas include confined spaces such as cargo tanks, void spaces and cargo pump-rooms.
The onset of mental confusion takes only 12 to 40 seconds, depending on the degree of deficiency. Judgement is then seriously impaired and escape from an unsafe atmosphere is unlikely without assistance.
Potential causes for lack of oxygen are:
- Displacement of oxygen by another gas (nitrogen!)
- Decomposition of organic cargo leading to displacement or consumption of oxygen
- Consumption of oxygen by corrosion or rusting of steel
- Inerted tanks are lacking oxygen by definition!
Precautions are best described in the enclosed space entry permit procedure, which is on board of every tanker. The aforementioned spaces may only be entered after verification that sufficient oxygen is present and confirmation that no oxygen is displaced or consumed during the entry. As an additional precaution the oxygen content should be monitored during the entry.