Editing Skein of Worlds:The Material Plane (section)

==== Air ====
Air provides, well... air. Air fluctuates a good deal as the winds come and go. Areas with too little air tend to make breathing difficult. This might manifest in any number of ways, from a simple lack of air, to stagnant air, to a lot of dust or water vapor suspended in what air there is. Areas with too much air tend to be very windy. Air vortices tend to form in large, high clouds.

Air exists even underground and underwater. Areas underwater with too little air can't support sea life very well. Areas underwater with too much air tend to manifest as underwater fountains of bubbles and the like.