Carbon dioxide has an underappreciated role in how breathing feels and functions. Understanding CO2 helps explain why the hold phases in box breathing are not just a pause — they are a functional part of the technique.
CO2 is the primary signal that tells the body when to breathe. When CO2 drops — as can happen with fast or anxious breathing — oxygen delivery to tissues is actually reduced (the Bohr effect: lower CO2 makes haemoglobin hold onto oxygen more tightly). This is one reason why over-breathing can paradoxically feel worse, not better.
Anxious or stressed breathing tends to be faster and shallower, which can reduce CO2 levels slightly below their optimal range. This mild CO2 reduction can itself produce sensations such as lightheadedness, tingling, or a sense of unreality — which can feed back into anxiety.
The top and bottom holds in box breathing briefly pause CO2 exchange, allowing CO2 to naturally stabilise. The slow overall pace also prevents the CO2 wash-out associated with rapid breathing. Many researchers propose this CO2 stabilisation contributes to the calming effect practitioners report.
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