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Hyperventilation can cause loss of CO2 from the blood leading to faint-headedness and panic. https://medlineplus.gov/ency/article/003071.htm#:~:text=Excessive%20breathing%20creates%20a%20low,such%20as%20bleeding%20or%20infection. Breathing into a bag increases the CO2 into the blood (as you are rebreathing old breath) and restoring the balance.

When people hyperventilate breathing into a bag works by putting some of the lost carbon dioxide back into your lungs and body. This helps to balance oxygen flow in your body

This is technique to control hyperventilation, such as in a panic attack. When you hyperventilate, your oxygen (O2) levels go up and your cardon dioxides (CO2) levels drop. In a healthy state, you need a certain O2 and CO2 balance, but hyperventilating tips the scale to having more O2. If you have too much O2, your blood PH changes and becomes too alkaline. If you remember the book or movie The Andromeda Strain, the researches discovered that the killer virus does not survive in alkaline environment, so an infected person had to hyperventilate in order to not drop dead.

The idea is that by breathing in and out of a paper bag, you would inhale more CO2. The extra CO2 balances with the extra O2 you get from the hyperventilating. I don't think the breathing in and out of the bag necessarily helps calm the panic attack, but it does help prevent the panic attack from creating further medical complications.

The current medical opinion is a bit divided. The science behind it still makes sense, but some doctors might argue that this technique might cause other types of harm. An incorrect use of the paper bag might lead to too much CO2 and not enough O2, which causes another danger. This technique can also be unsafe for those with certain medical conditions.

Perhaps not the main intent, but filling up the bag on the exhale and draining it all the way on the inhale helped people to focus on deep breaths instead of shallow, quick breaths.

Why does the blood alkalinity matter? Isn’t it just the over saturation of O2 that causes issues?

Your body only knows to breathe because of increasing CO2 concentration, so unless you tape the bag to your face, you are likely to know when the air in the bag is too stale. Too much or too little O2 isn't something you can detect.

That is also why hyperventilating and holding your breath can cause you to fall unconscious, you run out of oxygen without hitting CO2 levels which tell your body to breathe. Never try to hyperventilate in order to hold your breath longer, doubly so if it's to dive underwater.

IT is NOT the over saturation of O2. Theres a relative plateau you reach as you hyperventilate in terms of how much O2 you can accumulate (and its pretty close to what you live at normally) but your body is able to blow off a tremendous amount of CO2, which is an acidic compound. As you blow it off your blood becomes more alkaline, which causes some temporary problems-- dizziness, weakness, more anxiety, and carpopedal spasms. If you ever see someone in a BAD panic attack they will sometimes feel cramping in the feet and hands, or even contract the muscles in their feet and hands. It tends to make them panic more and slowing down their breathing is pretty much the only way to turn it around.

Divers actually take advantage of this sometimes, they hyperventilate before a dive to suppress their respiratory drive by blowing off CO2, but it can be kind of a risky technique because your oxygen level doesnt really rise that much.

O2 toxicity tends to only happen with overdelivery of oxygen, on room air its pretty difficult to accomplish.

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People sometimes breathe much faster than is healthy. This is called hyperventilation and can be caused e.g. by anxiety or stress. When you hyperventilate, you're saturating your blood with oxygen, and depleting it of carbon dioxide (CO2). CO2 dissolved in liquid (such as blood) makes an acid called carbonic acid. Without CO2 in your blood, you're missing this acid and your blood becomes more alkaline, which causes a syndrome called hypocapnia, which is associated with symptoms such as dizziness, fainting and (more) anxiety.

So, to sum up: people who are hyperventilating run a risk of having too little CO2 in their blood. The idea behind breathing into a paper bag is that you would breathe out air with more CO2 in it (compared to regular air), and then by breathing this back in you would raise your CO2 levels.

There are two reasons why this method is no longer recommended: (1) it's rather ineffective and (2) it can be very dangerous:

1. Experiments have shown that CO2-levels barely increase faster by rebreathing exhaled air, and no more than a placebo condition where people believed they were rebreathing, but were in fact breathing normal air. Why does a placebo method affect CO2 blood levels? Well, because simply relaxing and breathing more slowly is an effective method of raising CO2, and a placebo treatment can help reassure people that they will be okay. They believe that it will help, and that makes them relax.
2. Symptoms resembling those of hyperventilation can be caused by conditions such as asthma attacks or heart attacks. And sometimes people who hyperventilate do so because they are actually low on oxygen (due to some other medical reason). In all these cases, making people breathe into a paper bag can be very dangerous, and even lethal, as you are depriving them of much-needed oxygen.

So the takeaway is that at a minimum, you should exercise an abundance of caution when attempting the paper bag method. Never have someone else hold the bag for you (let alone hold a bag over another person's mouth if you think they are hyperventilating). Never use this method if you suffer from any kind of heart or lung problems, or if you've ever head a stroke, embolism, blood clots, etc. Only breathe into the bag for a few breaths (most sources recommend no more than 6-12 breaths). And avoid using this method if you're not sure that you're hyperventilating.

People panicking breathe too fast, and breathe out more co2 than your body is used to. This makes you feel weird (light-headedness, fingers tingling, that sort of thing).

The idea is that re breathing the air you just breathed out should slow the co2 loss and make you feel better. My understanding of why we stopped doing it is its just a lot more effective to get the person to slow their breathing by talking them through it, and the paper bag thing never practically worked that well (while the theory is sound, in practice you have to strike a reasonably fine balance in having a rebreathing circuit open enough to allow plenty of oxygen in but still retain co2, and that's just really tricky).

No. Over-saturation of O2, i.e. hyperoxia, certainly isn't healthy but the symptoms aren't as acute, and the partial pressure of oxygen in normal air simply isn't high enough to cause oxygen toxicity (at sea level pO2 is about 21 kPa and toxicity only occurs above 30 kPa). That is, no matter how fast you breathe, in normal air you can never raise your blood oxygen to toxic levels.

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