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Freediving Blackout Explained – Why It Happens and How To Prevent It

If you have ever seen a freediving blackout you might think it looks quite scary. It can be dangerous if water enters the airways and should be avoided where possible. However, with the right safety protocols in place, a freediver will recover from a blackout quite quickly, and have little to no memory of it. Let’s take a look at what exactly a freediving blackout is, what causes it, and how it can be avoided.

What is freediving blackout?

A freediving blackout is when a freediver becomes too hypoxic (blood oxygen level becomes too low) that she/he experiences unconsciousness. This can happen in deep water, in shallow water, or on the surface. Various factors can increase the likelihood of a hypoxic blackout including diet, fatigue, hyperventilation, and pushing your limits too far.

Types of freediving blackouts

Deep water blackout

Deep water blackouts are quite rare and might occur if freedivers performs a dive that is well out of their capabilities, if they hyperventilate before diving, or if they are extremely tired or sick.

Shallow water blackout

Experiencing unconsciousness in freediving is much more common in shallow water than in deep water, this is because the last part of the dive is the most challenging for the freediver.
Another reason for this is that the absolute pressure of the water surrounding the diver drops as they get shallower and achieve positive buoyancy, and the partial pressure of oxygen in the lungs also drops; The lungs expand and pull oxygen from the blood, making it less able to reach the brain.

Surface blackout

Many blackouts occur on the surface, this is due to the same reasons as a shallow water blackout; A freediver pushes their hypoxic limits just a little too much along with the change in pressures as they ascend causes a blackout.

Dynamic/static apnea blackout

Many freedivers will perform static or dynamic dives in shallow water like a swimming pool where pressure is not a factor. However, if the diver hyperventilates, or pushes their limits too far, they can become very hypoxic and a blackout can occur.

Shallow water blackout in pool
A girl lost consciousness in the pool – Image source:

What causes a blackout?

A freediving blackout is caused by becoming too hypoxic (low levels of oxygen in the blood). Not enough oxygen reaches the brain in the human body which causes a hypoxic blackout. Factors that make a blackout more likely include:


Hyperventilation is a manipulation of the breath (breathing heavy and fast), and it reduces the amount of carbon dioxide in the blood. This can make a breath hold feel nicer as it postpones the urge to breathe.

However, hyperventilating does not extend the time it takes to become hypoxic, so if a freediver hyperventilates they might overestimate the amount of time they have left before a blackout occurs. This is why hyperventilating is not recommended for freediving.

Stress and Tiredness

Being tired or nervous can increase your likelihood of experiencing blackout. This is because such conditions make your heart to beat faster, which burns oxygen faster too.
It’s important to become as relaxed as possible before freediving underwater and one should not go freediving if feeling unwell, exhausted or experiencing a high level of stress as it increases the risks associated with freediving.

Pushing past your limits

Some freedivers might choose to add big jumps of depth to their personal bests because of an exciting dive site they want to explore, showing off to others, or because they are going to a national record in a competition perhaps.

If you are going on a dive but are unsure whether you are capable to reach the new depth you set, it might make you nervous which can add to your risk of passing out. This of course does not mean that you should not set new goals to reach greater depths and times but be aware of and respect your limits.

Improper recovery breaths

Recovery breaths should be performed after a breath hold. You learn how to perform proper recovery breaths during a freediving course, but it essentially involves a big inhale, a hold of around one second, and then a passive exhale, repeated until you feel completely normal with no chance of blacking out. This type of breathing replenishes oxygen levels quickly and can fend off a blackout as long as they are performed properly and for a long enough time.

What does a blackout feel like?

Many freedivers who have experienced a blackout have said that it feels quite euphoric. Many experience symptoms including colorful visuals, a feeling of tingling or of relaxation.
Surprisingly, for most people a blackout is not a negative feeling, and usually the freediver will wake up remembering very little about what happened. They will seem confused about how they got to the surface and why everyone looks worried about them.

Blackout rescue

Receiving training from an experienced instructor is essential to learn proper freediving safety protocols around how to rescue someone from passing out while freediving. However this is not a professional guidance, here is a quick overview of how to perform blackout rescues:

Deep water/shallow water blackouts

If a freediver has a blackout underwater, as a safety diver you should hold the diver’s mouth shut and keep their chin down to ensure their airway is closed, use your other hand on the back of their head to pull the diver to the surface.

Once on the surface, remove their mask/nose clip, lanyard, and weight belt, tap them on the collar bone, and talk to them until they come around. If the diver does not regain consciousness within a few seconds, check for breathing, keep their airway out of the water, and perform rescue breaths while towing them to the shore.

Most divers will not require further help but if they still do not gain consciousness, CPR and further medical help is required. Never risk your own life.

If you are unable to bring the diver to the surface without risking yourself also having a blackout, you should head to the surface to get help from other divers, or if diving on a line, you can pull the freediver up by the rope that they are attached to with their lanyard.

Dynamic/static apnea blackout/surface blackout

If a freediver experiences a blackout on the surface, or while performing a static or dynamic breath hold in a swimming pool, place the diver on their back, remove their mask/nose clip and weight belt, use your hand on the back of their head to keep their airway out of the water.

After, tap them on the collar bone while talking to them until they regain consciousness. If they do not start breathing by themselves, rescue breaths are required, and further medical help is necessary.

How to prevent/avoid blackouts when freediving?

Although a blackout is not dangerous in most cases, here are a few tips to avoid it or minimize the risk of experiencing one:

Always dive within your limits: As we already talked about, pushing your limits or diving to depths that are challenging to you will put you at a greater risk of experiencing blackout when freediving. Progress slowly and sensibly in your freediving training, add only a few feet at a time to get deeper, and only attempt dives you feel confident you can do.

Never hyperventilate: Avoid hyperventilation before a dive. Breathing slowly at a normal volume and into the belly is perfect for becoming relaxed for freediving.

Perfect your technique: Poor diving technique can make you feel tired quicker underwater and raise your heart rate which can lead to a blackout. Practice your finning/pulling/swimming technique to become as efficient as possible.

Avoid diving when stressed: As we already learnt, stress can raise the heart rate and bring on hypoxia sooner. Make sure you are properly relaxed before freediving and avoid diving if you are feeling stressed from external factors such as work or relationships.

Avoid diving in bad conditions: If you are diving in stressful conditions such as very choppy waters, strong currents, thunderstorms, or around dangerous boats/rocks you will feel more stressed and your relaxation breathing might be affected. If you don’t feel comfortable with the conditions, skip the dive session.

Practice proper recovery breaths: Proper recovery breathing after a breath hold is sometimes enough to avoid a blackout. Never skip your recovery breaths, even if you think you don’t need them.

Medical conditions that increase the risk of fainting when freediving

Some factors surrounding your health might increase your risk of a blackout. These include:

  • Dehydration
  • Low blood sugar
  • Being very overweight
  • Being very tired
  • Having cold/sickness
  • Being too cold in the water

Interesting questions about blackout in freediving

Why do freedivers pass out at the surface?

Passing out at the surface is down to two reasons: The drop in partial pressure of oxygen in the lungs as the diver gets shallower, and the fact that the final part of the dive is when the body is in need of oxygen the most.

Are there any signs of blackout before it happens?

If a freediver is about to black out underwater, they will likely start to slow down with their kicking or pulling the rope, they might look panicked, or appear fatigued. A freediver who is about to black out on the surface might experience an LMC before blacking out (loss of motor control) which includes spasms and struggling to breathe, or they might look confused before they blackout.

How long does a blackout last?

Most freedivers recover from a surface blackout in just a few seconds, more serious blackouts could last a few minutes and might require rescue breaths to get oxygen into their lungs. If a diver has a blackout underwater they will not recover until they reach the surface and are able to breathe air again.

How do you minimize the risk of shallow water blackout?

Always dive within your limits, never add huge depth to your personal best, avoid diving if you are hungover, very tired, dehydrated, or have low blood sugar.

How common is shallow water blackout?

In freediving, shallow water blackouts are fairly common amongst competitive divers and spearfishers who tend to stay for a long time underwater. Most recreational freedivers won’t experience a blackout because they tend not to push their limits quite so much, but it is important to always have a buddy close by just in case.

What should I do if I experience a blackout when freediving?

If you experience a blackout you should end your dive session and avoid diving for the rest of the day. Next time you go freediving you should reduce the depth and/or time that you were diving to when you experienced a blackout and stick within your limits. You shouldn’t experience any after-effects from a blackout except a little fatigue.

Is it safe to continue freediving after a blackout?

No, always end your dive session if you experience a blackout. Your body needs to rest and recover before trying to freedive again.

Where do most blackouts happen when freediving?

Most blackouts happen on the surface or very shallow before the diver reaches the surface. This is due to the drop in partial pressure in oxygen in the lungs and because the last part of the dive is the most challenging for the freediver.

Can you drown/die if you pass out underwater?

If you blackout underwater and no one is there to rescue you, water would enter your airways which is likely to lead to death by drowning. This is why diving with a qualified dive buddy who understands proper freediving safety is essential. You should make sure you know how to rescue another diver from a blackout, too.

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