What makes breathing in and out of a paper bag so helpful?

Question

When you are nervous, why do they tell you to breathe out of a paper bag?

Answer 1

The problem with hyperventilation is not breathing in too much oxygen, it is breating out too much carbon dioxide. The nitrogen will make you light-headed too...and both of those can lead to dizziness and passing out.

A paper bag forces you to breath back the carbon dioxide you exhale, lowering the ratio of nitrogen at the same time. It helps to prevent your body from triggering a sympathetic response, working to curb your rapid heart rate.....so that is how the old trick works.