Carbon dioxide is not the enemy

A gas your body makes and carries

Your cells produce carbon dioxide, or CO2, all the time, and your blood carries it to the lungs so it can leave with the air. A college physiology textbook, the one from OpenStax, describes how it travels: about 70 percent as bicarbonate, around 20 percent bound to hemoglobin, and 7 to 10 percent dissolved in plasma.

Inside red blood cells, an enzyme joins CO2 with water to form carbonic acid, which splits into bicarbonate and hydrogen ions. That's why CO2 and the acidity of your blood move together: more CO2, slightly more acidic blood; less CO2, slightly more alkaline.

CO2, not oxygen, is what makes you breathe

It sounds backward, but the same textbook is plain about it: the major stimulus for breathing isn't the concentration of oxygen but the concentration of carbon dioxide in the blood. When CO2 rises, hydrogen ions rise, and sensors in the brain called central chemoreceptors pick that up and increase your breathing.

There are other sensors in the carotid arteries and the aortic arch. Those do respond to oxygen, but only when it drops a lot: the textbook puts it at about 60 mm Hg or less.

When you hold your breath, the growing urge to breathe is mostly CO2 building up.

The Bohr effect, stated carefully

In 1904, Christian Bohr, Karl Hasselbalch and August Krogh published a finding they titled as a biologically important relationship. They saw that CO2 makes hemoglobin hold less oxygen, especially when oxygen pressure is low. The effect weakens when oxygen pressure is high, as it is in the lungs.

The result is elegant. A working muscle has little oxygen and plenty of CO2: right there, hemoglobin lets go of oxygen more easily. In the lungs, where CO2 leaves, it picks oxygen back up almost unhindered. Current references define it the same way: hemoglobin's affinity for oxygen drops when CO2 rises or pH falls.

The Bohr effect describes a fine adjustment that already happens by itself in a body breathing normally. It doesn't follow that building up CO2 on purpose improves your oxygenation: none of the sources cited here says so.

What happens when you overbreathe

Hyperventilating means breathing faster or deeper than your body needs. MedlinePlus, from the US National Library of Medicine, explains it in a line: breathing out too much CO2 lowers its level in your blood, and that causes many of the symptoms. They include dizziness, feeling unable to catch your breath, a pounding heartbeat, numbness or tingling in the arms or around the mouth, and muscle spasms in the hands and feet.

With less CO2, your blood turns more alkaline. Blood vessels in the brain respond by narrowing. A 2019 review reports that in healthy volunteers, blood flow to the brain drops by about 30 percent when arterial CO2 goes from about 41 to about 25 mm Hg. On top of that, hemoglobin grips oxygen more tightly: the Bohr effect in reverse.

There's the paradox. In a healthy person, hemoglobin already leaves the lungs nearly full: saturation generally runs from 95 to 99 percent. Overbreathing adds almost no oxygen to your blood, and it does take blood flow away from your brain.

Never in the water

This is the case where low CO2 is truly dangerous. An expert report published in Experimental Physiology, led by Mike Tipton of the University of Portsmouth, explains that hyperventilating before you go under removes CO2 from your body and delays the urge to surface and breathe. Oxygen keeps getting used up while the warning never comes. The result can be blacking out underwater and drowning. Their advice is blunt: never hyperventilate before submerging.

For the same reason, the fast breathings on this site, such as bellows breath (bhastrika), breath of fire or Deep breathing rounds, show a notice before they start. They're done sitting or lying down, never in water or while driving.

What this means for your practice

• Slow doesn't mean big. If you take fewer breaths, each one grows a little to make up for it. If you take in far more air than you need, you're hyperventilating even at a slow pace.
• Dizziness and tingling are the signal. Stop, breathe at your own pace and, if you want to go on, use smaller breaths.
• On training CO2 tolerance. A 2017 review cites one study in which, at six breaths a minute, the body's response to high CO2 was smaller than at a spontaneous pace. That was measured during practice. We found no solid sources for the promises about training that tolerance with holds or endurance tests, so we don't make them here.
• When to get help. MedlinePlus says to contact a provider if it's your first episode of rapid breathing, if it comes with pain, fever or bleeding, or if it doesn't ease. Rapid breathing that's new, severe or unexplained can be an emergency.