People providing information that could be taken as medical advice (even if not so intended), should have medical credentials and should state them in their post. Most legitimate experts will also understand that the whole context of a person’s clinical presentation has to be considered as the same symptom can occur for different reasons. An expert will usually not say, “this is the only correct answer.”

I cannot speak to the physiological aspects of this at all, but I can say that someone who is hyperventilating may need medical attention. Other commenters have also suggested that bag breathing can be dangerous in some circumstances.

PLEASE NOTE that if someone is having a panic attack and they are not hyperventilating, DO NOT give them a bag to breathe into as this will exacerbate their symptoms. Increased CO2 inhalation has been found to correlate with worsened symptoms in individuals experiencing acute anxiety. CO2 enriched air will actually cause anxiety in otherwise healthy people, and is sometimes used in research studies for the purpose of inducing anxiety.

To be clear: I am not saying that someone who is having a panic attack and is hyperventilating can be given a bag to breath into. I can only comment on the often mistaken assumption that bag breathing is the appropriate treatment for a panic attack. Again, please always refer to medical professionals.

Source: https://www.nature.com/articles/s41398-019-0634-z

And I am a graduate student in clinical social work.

Edited: for clarity.