CO₂ Tolerance: The Performance Variable Nobody Is Training

Most athletes tracking their performance are looking at the wrong gas.

Oxygen gets all the attention. VO2 max, oxygen saturation, breathing protocols designed to flood the system with O2 — the entire performance conversation is built around getting more oxygen in.

But here's what that misses: your body's ability to actually use oxygen isn't limited by how much you inhale. It's limited by how much CO₂ you can tolerate.

This is called the Bohr Effect, and it's been established physiology since 1904. Oxygen is released from haemoglobin into working muscles not in response to oxygen levels, but in response to CO₂. The higher your CO₂ tolerance, the more efficiently oxygen is offloaded where it's actually needed — in the muscles, during the moments that matter most.

Which means two athletes with identical aerobic capacity can perform very differently, depending on how well their system handles CO₂ under load.

Low CO₂ tolerance shows up in predictable ways. The urge to breathe hard before the actual oxygen demand justifies it. Mouth breathing under moderate intensity. A feeling of air hunger that doesn't match the effort. A system that signals distress before the body is actually in distress.

This creates a performance ceiling that has nothing to do with fitness.

The good news: CO₂ tolerance is trainable. Unlike many physiological variables that require months of adaptation, breath training targeting CO₂ response can produce measurable shifts in relatively short time frames — in the pace at which air hunger kicks in, in breathing economy during sustained effort, and in the ability to stay composed when the system is under load.

The BOLT score — a simple breath hold test developed by Patrick McKeown — gives a rough but useful baseline. Most recreational athletes score between 20 and 30 seconds. Elite endurance athletes often sit above 40. The score correlates closely with breathing efficiency during exercise, and it tends to move meaningfully with consistent training.

This is not supplementary work. It's not a recovery tool or a mindfulness practice.

It's a performance variable — one that most coaches never assess and most athletes never train.

Which means for anyone serious about finding the edge, it's usually one of the most available ones left.