Injury or death due to oxygen deficiency is a common hazard in the maritime industry. An oxygen-deficient atmosphere has less than 19.5% available oxygen (O2) and any atmosphere with less than 20.8 % oxygen should not be entered.
Confined spaces, if not properly monitored, can create hazards for workers and rescuers. The oxygen level in a confined space can decrease because of work being done, such as welding, cutting, or brazing; or, it can be decreased by certain chemical reactions (rusting,...
https://safety4sea.com/cm-oxygen-deficiency-the-silent-killer/