THE CLAIM

If you’re hyperventilating, breathe into a paper bag.

THE FACTS

Grabbing a bag and breathing into it repeatedly, known in medical literature as “rebreathing,” has long been recommended to ease rapid, uncontrolled breathing.

But most medical studies and experts suggest that the method, though accepted, is dangerous and should be retired.

The idea behind it is to increase carbon dioxide levels. Hyperventilation causes the body to expel too much carbon dioxide, and “rebreathing” exhaled air helps restore that lost gas.

The problem is that several medical conditions, like asthma and heart attacks, can be confused with hyperventilation. In such cases, reducing oxygen and increasing carbon dioxide can be deadly. One study in The Annals of Emergency Medicine described three cases in which people having heart attacks thought, wrongly, that they were hyperventilating and died after losing oxygen while breathing into bags.

Another study said using a bag was no better at easing hyperventilation than using an open tube. For better results, experts say, stay calm and practice breathing slowly and deliberately.