Oxygen and CO₂: Understanding the Gases That Control Your Breath-Hold
Every freediver eventually faces the same burning question—literally. That intense urge to breathe, the diaphragm contractions, the growing discomfort as a breath-hold continues. What's actually happening inside your body? And more importantly, what does it mean for your safety?
Understanding oxygen and carbon dioxide dynamics isn't just interesting physiology—it's essential knowledge that separates safe freedivers from those at risk of blackout. The science explains why hyperventilation kills, why the urge to breathe can deceive you, and how to train your tolerance without crossing dangerous lines.
The Two-Gas System
Your body runs on a delicate balance between two gases:
Oxygen (O₂): The fuel your cells need to produce energy. You start a breath-hold with lungs full of it, and your body steadily consumes it throughout the dive.
Carbon Dioxide (CO₂): The waste product of cellular metabolism. As your body uses oxygen, it produces carbon dioxide, which accumulates in your blood and tissues.
These two gases are inversely related during a breath-hold: oxygen falls while carbon dioxide rises. But here's the critical insight that every freediver must understand—your body monitors these two gases very differently.
Why CO₂ Drives the Urge to Breathe
The urge to breathe is primarily triggered by rising carbon dioxide levels, not falling oxygen.
Your body has highly sensitive chemoreceptors—specialised cells in your brainstem and carotid arteries—that detect even small changes in blood CO₂. When carbon dioxide rises, these receptors trigger increasingly urgent signals to breathe. This is the discomfort you feel during a breath-hold: the tightness, the contractions, the overwhelming desire to inhale.
Oxygen, by contrast, doesn't create strong warning sensations until levels drop dangerously low. For most healthy people, the first sign of critically low oxygen is visual disturbance (greyout) or unconsciousness—there's no gradual discomfort warning you that blackout is imminent.
This asymmetry is the foundation of freediving safety—and the mechanism behind most freediving fatalities. Understand the fatal consequences in our shallow water blackout guide.
The Breath-Hold Timeline
Here's what happens during a typical breath-hold:
Phase 1: Easy Phase
In the first 30-60 seconds for beginners, oxygen levels remain high, CO₂ is building but hasn't triggered strong urges, and you feel relatively comfortable.
Phase 2: Struggle Phase
As discomfort begins, CO₂ has accumulated enough to trigger chemoreceptor response. You experience the "urge to breathe" and diaphragm contractions begin (involuntary breathing movements). Oxygen is still adequate for consciousness.
Phase 3: Critical Phase
As you approach your limits, CO₂ continues rising, intensifying discomfort. Oxygen may be dropping toward critical levels. Contractions become stronger and more frequent. This is where most people wisely breathe.
Phase 4: Danger Zone
If breath-hold continues despite intense urge, oxygen can drop to levels incompatible with consciousness. Hypoxic blackout becomes possible. The gap between "uncomfortable" and "unconscious" can be seconds.
The Hyperventilation Trap
Hyperventilation—taking rapid, deep breaths before a dive—is one of the most dangerous practices in breath-hold diving. Understanding why requires grasping the CO₂/O₂ relationship.
What Hyperventilation Does
Hyperventilation rapidly blows off carbon dioxide from your blood, creating a condition called hypocapnia (low CO₂). Here's the critical problem:
It delays the urge to breathe because CO₂ starts abnormally low
It barely increases oxygen stores because haemoglobin is already 97-99% saturated
It removes your warning system without meaningfully extending safe breath-hold time
The Deadly Consequence
With the urge to breathe delayed, you can swim until oxygen drops to critical levels without any warning. The reduced CO₂ at the start means it takes longer to accumulate enough CO₂ to trigger breathing urges. Meanwhile, oxygen consumption continues at normal rates.
The result: a diver can lose consciousness suddenly, with no gradual warning, because their CO₂-based warning system was artificially suppressed while their oxygen supply ran out.
Learn proper breathing techniques that don't involve hyperventilation.
Ascent Blackout: The Pressure Factor
Depth adds another dangerous variable to the oxygen equation.
Partial Pressure and Depth
At depth, increased ambient pressure raises the partial pressure of oxygen in your lungs. This keeps more oxygen dissolved in your blood, potentially masking how depleted your oxygen stores actually are. You might feel relatively comfortable at 20 metres with oxygen levels that would cause problems at the surface.
The Dangerous Ascent
As you ascend, pressure drops rapidly. The oxygen partial pressure in your lungs falls, and oxygen can actually diffuse from your blood back into your lungs—the reverse of normal respiration. This is called "reverse diffusion."
The last 10 metres of ascent represent the greatest proportional pressure change (from 2 ATA to 1 ATA—a 50% reduction). This is why the vast majority of freediving blackouts occur in the final metres before the surface.
A diver can feel fine at depth, begin ascending, and lose consciousness within metres of safety. The oxygen that seemed adequate at depth becomes critically insufficient as ambient pressure drops.
Training CO₂ Tolerance Safely
Experienced freedivers develop increased tolerance to elevated carbon dioxide. This allows them to remain calm and comfortable longer into a breath-hold, delaying the intense urge to breathe.
But CO₂ tolerance training must be approached carefully.
What CO₂ Training Does
Through repeated exposure to elevated CO₂ levels, your chemoreceptors become less reactive. The same CO₂ concentration that once triggered urgent breathing signals becomes more tolerable. This is a genuine physiological adaptation.
The Critical Safety Principle
CO₂ tolerance training extends your comfort zone—it does not extend your oxygen supply. You can train yourself to ignore increasingly urgent breathing signals, but your body's oxygen consumption remains unchanged.
This is why CO₂ training is considered the safest form of breath-hold training when done correctly: you're working with your warning system, not against it. You're learning to stay calm during discomfort, not learning to push past the point where oxygen runs out.
Safe CO₂ Training Protocols
CO₂ Tables: A series of breath-holds with progressively shorter rest intervals. Breath-hold duration stays constant (typically 50-60% of maximum), but recovery time decreases, allowing CO₂ to accumulate gradually.
Key Safety Guidelines:
Never exceed 50-60% of your maximum breath-hold
Always train with a buddy who knows what you're doing
Never combine CO₂ and O₂ training in the same session
Stop immediately if you experience any unusual symptoms
One to two sessions per week maximum
Never train in water alone
Always dive with proper safety protocols—see our buddy system guide.
Recognising Your Limits
Your body provides signals throughout a breath-hold. Learning to interpret them is essential for safe freediving.
Diaphragm Contractions
These involuntary "hiccups" or breathing movements are triggered by CO₂ accumulation. They're uncomfortable but not dangerous in themselves—they're simply your body's attempt to initiate breathing.
Early contractions indicate you're entering the struggle phase. Strong, frequent contractions mean CO₂ is high, but they don't directly indicate oxygen status.
The Danger of Ignoring Signals
Trained freedivers can suppress the urge to breathe through mental control. This is both a skill and a risk. The ability to continue past strong contractions means the ability to push into oxygen-critical territory.
Elite freedivers operate in a zone where the determining factor is "critical level of hypoxemia before loss of consciousness"—meaning they've learned to push past CO₂ warnings and rely on training and experience to surface before oxygen runs out. This is not a margin recreational divers should approach.
Adaptations with Training
Regular breath-hold training produces measurable physiological changes:
Increased CO₂ Tolerance: Chemoreceptors become less reactive, allowing longer breath-holds before discomfort begins.
Enhanced Hypoxia Tolerance: Trained freedivers can maintain consciousness at oxygen saturations that would render untrained individuals unconscious. However, this margin is still narrow.
Improved Efficiency: Trained divers use oxygen more efficiently through relaxation, reduced unnecessary movement, and optimised diving technique.
Cerebrovascular Adaptations: Increased blood flow to the brain during hypoxia helps maintain consciousness during challenging breath-holds.
Learn about all the physiological adaptations in our comprehensive physiology guide.
Practical Applications
Before Every Dive
Breathe normally: Calm, relaxed breathing—never hyperventilation
Final breath: A comfortable, full breath—not maximum gasping inhalation
Mental state: Relaxed and focused, not anxious or rushed
During the Dive
Monitor sensations: Note when contractions begin, their intensity
Stay relaxed: Tension increases oxygen consumption
Plan conservatively: Always surface with margin to spare
Recovery
Breathe immediately upon surfacing: Don't delay
Hook breathing: Short, sharp inhalations with held exhalation if needed
Recovery breathing: Return to normal breathing pattern before talking or swimming
Surface interval: At minimum, 2-3 times your dive duration before the next dive
The Bottom Line
Understanding O₂ and CO₂ dynamics reveals a fundamental truth about freediving safety: your warning system (CO₂) and your life support (O₂) are two separate systems that can become dangerously desynchronised.
Respect what your body tells you. The urge to breathe exists to protect you. Training can make this urge more manageable, but it cannot make oxygen deprivation safe.
The freedivers with the longest careers aren't those who ignored their bodies' signals—they're those who learned to work with them, pushing boundaries gradually while always maintaining margins that keep them conscious and alive.
The interplay between oxygen and carbon dioxide during breath-holds is both elegant and unforgiving. Master this science, respect these gases, and you'll freedive safely for a lifetime. Ignore it, and a single mistake can be fatal—often in water shallow enough to stand in.
This article is for educational purposes and does not constitute medical advice. Always train with certified instructors and never freedive alone. Start your training journey with what to expect from your first freediving course.