I got injured often when I was little.
Sometimes I had to.
Too much oxygen too fast can make you faint, or go into shock.
Only a few gasps was enough to make me calm down enough to cry.
It really does work, and it is absolutely safe.
A couple of times I tried to not use it, to see what would happen.
The grown ups were more persistent than me.

The bag retains your carbon dioxide & you breath it back in, so it helps to regulate your oxygen/carbon dioxide levels so you get back to breathing more normal

@LadyMarissa has it correct about the Oxygen – CO2 issue.

Does it work? Yes. It worked for me. It’s really not a psychological thing at all. it’s a chemical thing.

Hyperventilation does not cause your oxygen levels to go to dangerous levels.
Hyperventilation will drop your blood levels of CO2 and give you a mild respiratory alkolosis. It won’t make you go into shock. You can get lightheaded. The bag thing really doesn’t do much physiologically although you will rebreath a little CO2. It does give the sufferer something to do focus on so they can calm down.

Hello.
Hyperventilation is normal and can be worrying at times,There are many reasons that can cause hyperventilation from,A cold/Flu/Stress/Anxiety and constant worrying,Overexertion,Working your body too intense,too fast,without giving your body time to warm up/prepare.Muscular systerm,Central nervous system fatigue could cause this.A overexerted chest,or inflamed muscles in and around the chest and ribs can cause a tight feel causing anxiety,heart attack symptoms,then shortness of the breath,to hyperventilation.

If someone is hyperventilating it can be a scary experience with many symptoms.The breathing in the bag trick does help psychologically as in the back of our mind we know that doctors,Paramedics and other health professionals recommend this which in return makes us feel safe/Safer.

The bag also helps one when breathing in as when we feel we are gasping,Choking,our brains tend to focus in the moment and on the symptoms and the feeling,When one uses a bag to breathe, you hear the bag and the air moving in and out and the sound of the bag inflating and deflating.this itself can take our mind off focus.