We just discussed this and the new mask does not allow rebreathing of exhaled air because no air is exhaled back into the bag, in fact, it's designed to specifically prevent this. It's a pretty simple concept: the pure oxygen is pumped only into the bag, the valve at the top of the bag is anti-return, i.e. you can only pull the oxygen out, not breathe your breath air in.
In addition, the two valves on the mask itself are also anti-return, but in the opposite direction, i.e. you can only breathe your inhaled air out through the vents, NOT pull ambient air in to the mask. Basically, these masks, if worn properly and secured around the face, mean you will only breathe the oxygen from the concentrator.
Quite a while ago when in the HBOT, I tested to see if the air coming from the oxygen concentrator was sufficient for breathing by putting the end of the tube (no mask) into my mouth and only breathing what the concentrator provided, after a few breaths it wasn't really enough, for me at least. I was also aware that, while using the mask, I was drawing ambient air around the sides to supplement that from the concentrator.
So basically, it seems that the flow from the concentrator provided with the HBOT by the Macey Pan company, which while under pressure is about 6 liters per minute, isn't really sufficient alone.
The question then is whether or not breathing pure oxygen is necessary for optimal effects from the HBOT, or if a mix, of maybe 70-80% oxygen (concentrator plus the oxygen in the ambient air combined) is sufficient.