It so happens that I am working on a water-treatment document for a "discerning" facility and I have been intrigued by a similar term, "espace confiné" which might be taken to mean "vessel" or "enclosed space" but which, for me, in context, means "confined atmosphere" (particularly relating to hot-work permits). Of course the two concepts are intimately linked, and given the "quality" of the document I am working from, it is hardly surprising to consider that the writers might easily have got their ideas mixed up!

The term most appropriate will depend on what they are actually talking about. For instance, a person working in an empty oil tank is in a "confined atmosphere" if the danger is that he might suffocate (a small pool of oil on the floor of the tank is a greater danger than a much larger amount of oil in the open air). If the danger is that, in the event of an emergency, he might have difficulty getting out of the tank, then it is a "confined space". As I say, the two are closely related, but "confined atmosphere" might be less applicable if the danger is of a "confined space" - even an open-topped space - filling with water more or less quickly, for instance.