Examples of such spaces are
Oxygen shortage in these spaces affects the brain faster than any other part of the body and since oxygen content of the atmosphere falls below 21% in such enclosed spaces, breathing become faster and with more effort. When oxygen supply is below 16% the brain is quickly affected and at 10% unconsciousness is unavoidable and the individual will die if not revived and removed to fresh atmosphere. At oxygen levels below 5% unconsciousness is immediate with irreversible brain damage.
Ventilation has to be carried out before entry is permitted into an enclosed space. This may be either Forced ventilation, where at least two air changes are done before entry is made or Natural ventilation where the space is allowed to breath for at least 24 hours.
In double bottom tanks, ventilation is ensured by filling the compartment with clean seawater and then pumping it out. Regardless of the ventilation method employed, entry must be made only after the tests have shown that the atmosphere inside is breathable - contains 21% oxygen and no noxious hydrocarbon or other toxic gases
Gas tests are done to ensure safety. Oxygen is depleted by oxidization and tanks containing amounts of rusty colored water are highly dangerous. ...