Asphyxiation

The body requires air containing about 20% oxygen by volume for the purpose of 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. The timescale for the onset of mental confusion varies between 12 and 40 seconds depending on the degree of deficiency. This is most critical because judgment is seriously impaired and escape from an unsafe atmosphere is unlikely without assistance.
Danger areas include confined spaces such as cargo tanks, void spaces and cargo pump-rooms.

A potential cause for lack of oxygen is: 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 described best 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.