I see bomber pilots in WWII movies wearing oxygen masks with "balloons" dangling from them.?

Question

What is the function of these balloons?

Are they supposed to be inflated?

In the movies, they never are.

Answer 1

Absolutely nothing to do with hyperventilating.

Its called a BLB Mask

Since exhaled human breath contains about 16% oxygen (as opposed to about 21% in ambient air), the exhaled breath could be breathed back into the ‘rebreathing bag’ where it could effectively be recycled instead of being breathed out into ambient air (and thereby wasted). This permitted lower flow rates of inlet oxygen, among other things—a decided benefit that allowed available oxygen supplies to be extended by a significant margin. Otherwise, the BLB Mask allowed the wearer to talk, drink, and even eat without interrupting the supply of oxygen.

Answer 2

I believe they inflated and deflated if they were in use.I guess it would show if the mask was working and/or you or your buddy was breathing.

Answer 3

Instead of having those bags now, military aircraft have a switch that selects Normal or 100% oxygen.

Answer 4

Cherokee is right it is a re-breather bag just like the bags on airline oxygen masks. Maybe you have noticed that during the preflight announcement on airline flights the attendants say not to worry if your bag does not inflate because that is normal.

Answer 5

The idea was that they would hold some of the CO2 when a crewmember exhales.  Since one was breathing 100% O2, this would help prevent hyperventilating.  It was found not to work really well and was another place of potential mask leaks-and was abandoned.