Wim Hof Method
Shallow Water Blackout | Wim Hof Method

What is Shallow Water Blackout?

Shallow Water Blackout refers to loss of consciousness in or under water due to a lack of oxygen, as a result of modified breathing. It’s a dangerous phenomenon that claims thousands of lives every year. How does this happen exactly, and what can you do to prevent it?

You have no doubt at one point or another tried to see how long you can hold your breath. At first you don’t feel much of anything. But as the seconds pass by, a weird and increasingly unpleasant sensation grows in your chest, and ultimately forces you to gasp for air. 

This overwhelming urge to draw breath is called the respiratory drive or breathing reflex. It’s a wonderful piece of physiological ingenuity— a built-in warning system that makes sure that we don’t deprive ourselves of that all important gas called oxygen, for longer than is good for us. And it works so well that it is in fact virtually impossible to make yourself pass out by holding your breath, under normal conditions.

And yet that is exactly what happens with shallow water blackout. So how does the breathing reflex work, and how can it be overridden?

How does the breathing reflex work?

It sounds counterintuitive, but the breathing reflex actually has nothing to do with how much oxygen you have left in your lungs. Instead your body gauges the level of its end product: carbon dioxide, or CO2. You breathe in oxygen, and you exhale CO2. When you hold your breath, the carbon has nowhere to go, and slowly builds up in your blood. When this reaches a certain threshold (which varies from person to person), socalled chemoreceptors send signals to the respiratory muscles, compelling them to contract and forcing you to breathe. The breathing reflex is therefore also called the hypercapnic drive or CO2 drive.

CO2 level is at baseline when holding your breath after regular breathing. The breath reflex comes on relatively quickly— well before oxygen levels dip dangerously low.

Hijacking the breathing reflex

When you practice WHM breathing, you slightly raise oxygen (your blood is normally already at around 95-100% oxygen saturation) and significantly lower carbon dioxide.

The breath reflex relies on detection of CO2 in the bloodstream, so if you bring CO2 down to below baseline, it will take longer for the reflex to kick in. This is why you get such long retention times on WHM breathing— you’re effectively giving yourself a head start.

CO2 level is lower than baseline when starting the breath hold. Oxygen can drop to very low levels before CO2 has built back up far enough to induce the breathing reflex.

But this also means that you’ve gotten rid of the safety mechanism that ensures that you continue breathing in a timely manner. With the breathing reflex delayed, there is nothing to warn you about low oxygen reserves, and if you hold your breath long enough you can eventually pass out. If you’re just chilling on your couch this is not too big of a deal. Your arms flop to your sides, you drool all over your pillow, and your tea gets cold. But in water — even the shallowest puddle — it suddenly becomes a major issue.

Shallow Water vs. Hypoxic Blackout

Shallow water blackout is called such because it most commonly occurs in swimming pools. But it’s a bit of a misnomer because this phenomenon strikes at greater depths all the same— notably catching free divers off guard. Most official bodies now use the term “hypoxic blackout” (hypoxia meaning low oxygen), although shallow water blackout remains the more commonly known term.

How to prevent Shallow Water Blackout

What makes shallow water blackout so dangerous is that you don’t realize it’s happening until it’s too late. You don’t see it coming unless you know about it, and unfortunately many people remain unaware about shallow water blackout, despite it making up 20% of all drownings.

We do our best to spread knowledge about this, but not everyone is affiliated with the Wim Hof Method. If you know people who practice the Wim Hof Method, please remind them of the following:

  • Deep breathing can affect motor control and, in extreme cases, lead to loss of consciousness
  • Always sit or lie down in a safe, comfortable space before practicing the techniques
  • Never practice the Wim Hof Method (or any other forms of deep) breathing ahead of or during immersion in or over water, when piloting a vehicle, or in any other situation where loss of consciousness could cause harm to yourself or others.


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