Carbon dioxide and over-breathing: why you feel dizzy

Carbon dioxide drives your breathing

Your cells make carbon dioxide all the time as they use energy, and your lungs breathe it out. Your brainstem watches the level closely. If it rises, even a little, you breathe more; if it falls, you breathe less. Oxygen sensors matter too, but in a healthy person carbon dioxide is the main control. That is why holding your breath gets uncomfortable: the urge to breathe comes from carbon dioxide building up, not from running out of oxygen.

What over-breathing does

If you breathe faster or deeper than your body needs, you blow off carbon dioxide faster than you make it. The level in your blood falls, a state called hypocapnia, and your blood becomes slightly more alkaline. That has two main effects:

• Nerves become more excitable, partly because less calcium is free in the blood. That causes tingling or pins and needles in your fingers, feet and around your mouth, and sometimes cramping of the hands.
• Blood vessels in the brain narrow, so less blood reaches it for a while. That causes dizziness, light-headedness, blurred vision and a spaced-out feeling.

Over-breathing can also cause a tight chest, a racing heart and a feeling of not getting enough air, which ironically makes people breathe even more. It happens in panic attacks, in breathing pattern disorder and in fast breathing methods such as Wim Hof breathing. See dizziness and tingling.

Why breath-holds after over-breathing are dangerous

If you over-breathe and then hold your breath, your carbon dioxide starts so low that the urge to breathe takes much longer to arrive. Meanwhile your oxygen keeps falling, and it can drop far enough for you to black out before you feel any need to breathe. On land you would usually start breathing again and come round. Under water you would drown. This is called shallow water blackout, and it kills strong, healthy swimmers. See breath-holding and water.

What about slow breathing?

Slow breathing exercises don’t normally lower your carbon dioxide, as long as the breaths stay soft rather than big. If you feel tingly or light-headed during a slow exercise, your breaths are probably too deep: make them smaller and gentler. Some methods, such as Buteyko breathing, aim to breathe a little less, though a trial found that carbon dioxide levels didn’t change even when symptoms improved.

The paper bag

Rebreathing into a paper bag raises carbon dioxide, which is why it used to be advised for hyperventilation. It is no longer recommended: it also lowers your oxygen, and an emergency doctor reported deaths in people who were given a bag when their breathlessness was actually caused by low oxygen or a heart attack. Slow, gentle breathing does the job safely.

Detox and alkalising: myths

Some breathing methods are promoted as detoxing or alkalising your body. Over-breathing does make your blood more alkaline, but only briefly, and that is what causes the unpleasant symptoms above; your body quickly corrects it. Your lungs don’t remove toxins in the way these claims suggest. Your liver and kidneys do that, and breathing harder doesn’t help them.