Normal air consist of oxygen, CO₂ and nitrogen, with traces of water etc.
Now imagine displacing all gasses except oxygen with water vapour. By water vapour I don't mean hot steam or fog. Just gaseous water.
The question is would it be beneficial or harmful to human, and how much so? And assuming it's not deadly, in which circumstances could it be useful?
One thing I am wondering in particular is deep dive and high altitude applications. Having no/low content of nitrogen in such an artificial air mixture would help to avoid decompression sickness. And one would think water vapour is at least cheaper than pure oxygen (also might be safer and have less side effects).