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, paint drying) or through bacterial action (fermentation).

Oxygen deficiency may be caused by several factors:

  • Air displaced by cargo vapors (e.g. in cargo tanks, pipelines, pump room bilges).
  • Oxygen consumed by corrosion or oxidization (e.g. in enclosed spaces, mainly ballast tanks).
  • Air displaced by fire extinguishing systems (e.g. in spaces protected, and spaces containing the fire extinguishing machinery, pump room, CO2 room).
  • Air displaced by Inert Gas (e.g. in cargo tanks, void spaces).


Symptoms and Health effects

Exposure to an oxygen deficient atmosphere can result in unconsciousness, paralysis, brain damage or even death. Initial symptoms of oxygen deficiency are: headache, drowsiness, nausea, weakness, dizziness, deep and labored breathing.


First Aid

Upon discovering a person who has suffered oxygen deficiency, it is imperative fresh air to be provided immediately, even during the rescue operation and, when necessary, start artificial respiration.



Whenever entry into a space into a possibly oxygen deficient space is required, first measure the oxygen by an oxygen meter. As an oxygen safe atmosphere is considered the one with oxygen range from 20.6% to 22% by volume.

Areas on board where oxygen deficiency may be of concern include the following locations [but ARE NOT LIΜIΤΕD to]:

  • All enclosed spaces
  • Spaces containing or connected to a fire suppression system (e.g. CO2 storage rooms)
  • Any space forming part of the Inert Gas System
  • Bilges
  • Pipes, lines etc

According to amendments to SOLAS XI-1 which entered into force in 2014, every ship is required to carry an appropriate portable atmosphere testing instrument or instruments in order to measure the level of oxygen as well as flammable gases or vapors, hydrogen sulphide and carbon monoxide prior to entry into enclosed spaces.

Additionally, suitable means shall be provided for the calibration of all such instruments. These instrument should be calibrated as required and personnel on board should be able to demonstrate competence on use and basic maintenance of them.


Key issues to check

  • Test equipment routinely in accordance with manufacturer’s instructions.
  • Check regularly for signs of damage and that calibration certificates remain valid.
  • Ensure manufacturer’s instructions are available and fully understood
  • Ensure ship’s crews are trained in the use of the particular equipment they are expected to use.