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Freediving and Hypoxia: A Deadly Combination

Freediving without a dive partner could wind up being the last dive trip you ever take – just take Valentine Thomas’s word for it.


The globetrotting freediver and spearfishing fanatic recently had a run-in with cerebral hypoxia during a deep dive in the Caribbean. Cerebral hypoxia happens when the brain is completely deprived of oxygen, and leads to something freedivers call a shallow-water blackout.


Thomas’s harrowing experience was a textbook case of the dangers that occur when freedivers run out of air. According to, a shallow-water blackout can happen to anyone participating in the sport, and without the right support group and fellow divers watching your back, it doesn’t take long for things to turn deadly.


So what causes freedivers to lose oxygen levels in the brain so quickly they run the risk of passing out underwater?


The risk for hypoxia while freediving is real


Losing consciousness underwater is an inherent risk for every freediver competing today. It doesn’t matter if you’re new to the sport or surpassing the thousand-dive mark: no one is impervious to hypoxia.


Just take the case of Natalia Molchanova, a freediving veteran who was regarded by many as the world’s greatest freediver. Her mysterious disappearance followed by her presumed death while on a solo dive off the coast of Spain last year shocked the freediving world. It also served as a somber reminder of just how dangerous underwater blackouts can be.


So what causes a freediver to blackout in the first place? More often than not, three factors contribute to shallow-water blackouts: a lack of oxygen in the blood, rapid depressurization on ascent, and hyperventilation (also known as self-induced hypocapnia).


We asked Thomas to describe how all this plays out during a freedive. “When [you’re ascending] and you’re out of air, the pressure gets lower and lower the closer you get to the surface,” Thomas told us. “Your lungs are going to expand back to their normal size, so when you’re between zero and 10 meters your lungs are back to normal . . . but you still don’t have air.”


The space between 10 meters and the surface is when Thomas says freedivers are most at risk for blacking out. The sudden drop in pressure on the lungs paired with oxygen depletion is the perfect combination for losing consciousness.


Were Thomas diving alone, she may not be alive today.


“As soon as I reached the surface I blacked out,” she said. “My buddy grabbed me and took my mask off and woke me up. I was out for probably two, three seconds tops.”


She also went on to describe the lack of motion control that frequently accompanies a shallow-water blackout. This symptom is known in the freediving community as the samba for its resemblance to a loose-limbed form of dancing.


“People say it looks like you’re dancing, kind of losing control of your limbs and things like that. You start kicking around [and acting] funny. In my case, my right arm started twitching and making weird movements. That’s when I realized I’m really out of air right now.”


How can freedivers prevent cerebral hypoxia?


For Valentine Thomas, diving safely is all about learning and adhering to freediving best practices.


“Always dive with buddies,” she said, “because no matter what happens, if he’s there to watch your back then you should be all right.”


Thomas said training with a professional freediver and attending a freediving class covers the basics while teaching freediving enthusiasts the best methods for avoiding disaster below the surface.


“When you do that type of training they cover how to act [while] putting you in uneasy situations so you don’t panic. It’s all about safety. Of course, you learn to go a little deeper because you’re [calm] in the water after learning the techniques, but most importantly you learn about safety. Safety boosts your confidence and confidence helps you go a little deeper.”


What’s Valentine Thomas’s freediving advice for newbies?


While freediving is one of the riskiest aquatic activities, it’s no reason to give up on the sport altogether. Freediving can be incredibly rewarding – as long as newcomers understand the risks involved.


“If you dive within your limits then you should be fine,” she said. Still, even after extensive training and countless dives, Thomas suffered from the inevitable shallow-water blackout during a recent spearfishing expedition.


“I did a dive to about 85 feet – instead of breathing up,” she said. “I came back up from a dive and I was breathing up when I saw a fish down. Instead of taking an extra two minutes to breath up I decided to take a dive straight away.”


The lesson? Don’t let the promise of a big fish lure you into deeper waters without taking the necessary precautions and breathing up first.


After all, there are plenty of other fish in the sea.


Catch Valentine’s firsthand account of her blackout here.


To follow Valentine Thomas on her spearfishing adventures, be sure to check her out on Instagram.

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