A "fresh air bell" you can breathe in still can mean many things.
To give an example just considering O2:
As a healthy person you can breathe air with oxygen contents between 14% and 100% at sea level quite normally for quite some time. Certainly considering extreme circumstances it could be considered fresh air. At 10 meters depth or at the about 2 bar air pressure in the air bell as low as 8% oxygen might, considering the circumstances, possibly still be perceived as fresh air. When freediving however, you might suddenly notice a big difference between normal air with 21% O2 and higher or lower O2. Low O2 concentrations might give you a big blackout on the way up to the surface and high O2 gives other problems.
This is just talking O2 and there is much more to it. When freediving you do extreme things with the air you breathe and carry in your lungs so you should set much higher standards then others to what you consider as "normal fresh air".
Be careful and take care with such general terms when doing things like freediving.
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