How freediving research is pushing blackout prevention forward

A 19-year-old freediver’s death from a shallow water blackout has driven a major push to understand — and eventually warn against — the sudden loss of oxygen that can strike even experienced swimmers. Scientists are now combining portable monitoring, lab simul

At 19, Tucker Francis was living his dream.

He’d spent years chasing the ocean after a childhood adventure: about a decade earlier. he sailed around the northwest Atlantic with his family. and his passion for the water only grew. He loved snorkeling and recreational freediving — a loose term for diving deep underwater with no breathing gear. In practice, that meant heading down after colorful creatures he spotted and staying underwater as long as possible.

Then, during a 2017 snorkel trip in the U.S. Virgin Islands, Francis did one last freedive. He didn’t resurface. The boat’s captain found his body 10 meters down an hour later.

Investigators later determined Francis had suffered a hypoxic blackout, also often called a shallow water blackout. The mechanism was stark: he passed out when his brain couldn’t get enough oxygen. a problem that can come on without warning even among experienced swimmers. Once unconscious, the body sinks and the lungs can fill with water.

The Francis family didn’t want that story to end the same way for others. They decided to seek research that could reduce the risk of shallow water blackouts among freedivers — and turned to integrative physiologist Erika Schagatay of Mid Sweden University in Östersund.

Schagatay studies people who risk blacking out every day as they dive deeper than the Statue of Liberty is tall: competitive freedivers. also known as breath-hold divers or apneists. With help. the best of these divers can hold their breath for nearly 30 minutes — about as long as it takes to sing “The 12 Days of Christmas” 10 times.

Across cultures, breath-holding is nothing new. Indigenous peoples in Japan. Korea. the Philippines. Indonesia and elsewhere have been freediving to forage underwater for sea urchins. fish. seaweed and shellfish for thousands of years. Competitive freediving, though, didn’t get off the ground until the 1970s. Today, an estimated 4 million people around the world like to test how far they can go underwater in one breath.

What Schagatay and others are trying to do now is translate that fascination into something safer — before oxygen becomes the trap.

Schagatay’s work leans on a simple reality: in a deep breath-hold dive. the body’s chemistry changes fast. and the warning signs can be hard to feel. At a depth of 70 meters, water pressure shrinks the lungs to about the size of a soda bottle. Blood oozes into the thorax, which caves in, leaving skin flapping around the rib cage. Go deeper, and blood leaving the brain resembles black sludge because it contains so little oxygen.

Freedivers usually return to the surface. no harm done — which is why Bailey calls the experience “rewriting the medical textbooks.” It’s also why the stakes are so brutal when it doesn’t work. Shallow water blackout is insidious even for swimmers who have done everything right: an experienced swimmer can still pass out. and once unconscious. the body can sink and the lungs can fill.

One of the families’ hardest questions is whether science can predict when that moment is approaching — not just describe it after the fact.

Competitive records can look like proof that the body can be pushed indefinitely. In 1976, French diver Jacques Mayol set the competitive freediving record with a 101-meter descent. In 2023, the deepest descent with fins reached 136 meters. In 2025, a swimmer using a fin reached 326.5 meters. Another diver surpassed a total time of 11 minutes underwater while not moving in 2009. And by breathing pure oxygen in advance of his attempt. a different stationary apneist reached 29 minutes in 2025. setting a controversial world record.

But the research community isn’t building safety on triumph alone. After reaching 72 meters in 2013. American freediver Nicholas Mevoli surfaced and soon began bleeding from the mouth; he died later that day. And Russian freediver Andrey Matveenko died last November of complications after blacking out during a training dive at a world championship competition in Greece in September.

Shallow blackout is also not confined to deep-water specialists. Britt Jackson. executive director of Underwater Hypoxic Blackout Prevention. a nonprofit based in Norcross. Ga. says the risk extends to many other experienced swimmers — including synchronized swimmers. as well as underwater rugby and hockey players.

Pinning down exact numbers remains difficult. Jackson notes that there’s usually no distinction made between blackouts and other forms of drowning. Still. by combing the internet and investigating drowning death reports. her group has documented 110 blackout deaths over the last two decades. She calls it “just a drop in the bucket. ” and says more than 50 such deaths supposedly occurred in Hawaii alone in the last decade.

In the middle of those uncertainties, Schagatay’s team is trying to create something more concrete: measurements that can be turned into a warning.

Schagatay has also learned that what divers do right before submerging can quietly shift the odds. One problem she noticed — especially among beginners — is that they involuntarily hyperventilate right before they submerge. That increases the risk of blacking out because it expels CO2 that would otherwise trigger the body’s urgent “BREATHE NOW” command.

“Carbon dioxide is our best friend,” says Juan Valdivia-Valdivia, a neurosurgeon at BayCare Medical Group in Tampa, Fla., who practices freediving.

When CO2 rises, the diaphragm begins to spasm to force inhalation. Freedivers can learn to control the impulse — but if someone hyperventilates, Schagatay says, the body may not start that spasm response. Divers can “extend the dive too long,” and they’re more likely to black out.

That’s where the lab work becomes intensely practical.

In Östersund, Schagatay is setting up experiments near the Swedish national biathlon center. In the same building as her lab. Sweden’s top Nordic skiers are studied on a custom treadmill. and there’s a chamber where oxygen content and temperature can be controlled to create near Everest-like conditions. A room one floor above holds a stationary bike with a basin of water positioned above the handlebars.

The setup is designed to test a common safety practice among swimmers or spearfishers who repeatedly submerge to explore coral reefs or attempt to catch fish. After surfacing to take a breath, they tend to rest on the surface twice as long as they spent underwater, before diving again.

The question was whether that rest length is enough to replenish oxygen — or whether it leaves divers vulnerable over time.

In the lab, volunteers pedal slowly as a substitute for swimming motion. They place their face in the water and hold their breath for a fixed duration. then pull out and breathe normally for twice as long as they were underwater. They do this 10 times. After each submersion, they exhale into a tube attached to a machine that measures oxygen and CO2. They also wear an oxygen meter on a finger and an electrocardiogram monitor around the chest. After a few submersions, the researchers collect a finger prick blood sample to measure lactic acid accumulation.

Schagatay and Pernett reported in Helsinki last year at the annual meeting of the European Underwater and Baromedical Society that the double rest time technique is sufficient in only about 70 percent of the study participants. That means almost a third do not replenish their oxygen adequately and could be at greater risk of blacking out. The team plans to publish those results later this year.

The findings help explain why oxygen alone isn’t the whole story. Good freedivers tend to have large lungs. and how long oxygen stores last varies depending on metabolic rate and — to some extent — heart rate. Experienced divers also tend to have low heart rates. But once the body runs low on oxygen, lactic acid begins to accumulate, and recovery needs can stretch longer.

Pernett says beginner divers or those facing rough conditions need to pace out their dives even more, breathing on the surface three times as long as the time submerged.

There’s also a biological “superpower” involved: the spleen. Studies show that under stress, this organ contracts and floods the blood with red blood cells that carry oxygen. This contraction is part of what’s known as the human dive reflex.

When the mammalian body submerges in water, the reflex aims to maintain oxygen for the brain. The heart rate slows. Blood pressure in the arteries increases. Capillaries in the limbs constrict, shifting blood flow to the body’s core.

Ilardo says the spleen boost during the dive reflex is so critical that the Bajau — an Indigenous freediving group in Indonesia. Malaysia. and the Philippines — have evolved bigger spleens to help forage. They carry a genetic mutation that affects spleen growth. Ilardo and colleagues reported this in 2018 in Cell.

Training appears to matter even beyond that evolutionary advantage. Schagatay’s team previously showed that practice leads to a stronger dive response in the Bajau.

Ilardo’s team also reported last year in Cell Reports that the Haenyeo. a group of Korean women who freedive to depths of about 10 meters to harvest seafood. have bigger spleens than their land-based peers. Ilardo says this seems tied to lifestyle: they start as youngsters and continue through pregnancies and well into their ninth decade. So far, scientists have found no genetic component to their spleen size difference.

But the Francis family’s aim isn’t about chasing evolution or maximizing performance for its own sake. It’s about warnings.

With new funding from the Francis family, Schagatay built a portable lab she sets up in places such as Dahab, Egypt. There, along the Red Sea, a 100-meter-deep sinkhole called the Blue Hole attracts freedivers of all skill levels.

In some experiments, amateurs and skilled freedivers repeatedly held their breath for specified time periods on land. In others, they swam successively greater lengths underwater in a pool. Schagatay compared land-based results with underwater swims to see how well dry-land tests reflect a diver’s in-water abilities.

If the timing of breath-holding on land can predict time safely spent underwater, she says, it could allow researchers to predict how long one can hold the breath safely.

The work also takes advantage of the monitoring equipment Schagatay began using even before the Francis family approached her six years ago. She had been attending freediving competitions and outfitting athletes with monitors that tracked their oxygen consumption by measuring oxygen in the blood. She also tracked heart rate. lung volume and the rise of carbon dioxide levels in the body as measured in exhaled air.

Schagatay’s goal has been to understand what the body experiences and how it copes as oxygen dwindles and CO2 rises.

Along the way. the research is also feeding into other medical areas — lung and heart function. brain aging. and conditions linked to oxygen deprivation. Pernett says freediving training models how low oxygen in the blood affects the body. a deficit that appears in lung infections. chronic obstructive pulmonary disease. lung scarring or thickening and sleep apnea. He suggests training exercises that expand the lungs and take in more air could help asthmatics. and that people with sleep apnea might benefit if they have a strong dive reflex.

The heart is another thread. The low oxygen and high CO2 and lactic acid levels associated with deep dives seem to predispose athletes to irregular heartbeats, Pernett says, and he and others think similar changes may increase risk in nondivers.

Bradycardia — a slow heart rate — is seen differently through the lens of the dive reflex. Schagatay suggests that in an ambulance, responders shouldn’t try to cure the problem. “The [low] heart rate is the body trying to cope.”

For Bailey. freedivers are also a way to study the blood-brain barrier — a “cellular and molecular ‘wall’” that keeps toxins out of the brain. His subjects tend to be very healthy, which lets his team decode what happens when the barrier is stressed. During very long breath-holds. Bailey says blood pressure and blood flow rise sharply to deliver oxygen to a brain with a “one heck of an appetite” for that gas. In his work with freedivers. he found that low oxygen stresses the brain. causing a temporary disruption of the blood-brain barrier. When it breaks down, Bailey says, harmful proteins can slip into brain tissue. Over time, repeated stress can lead to effects that resemble neurodegeneration, both Bailey’s and Schagatay’s teams have shown.

Bailey describes the whole process as “accelerated model” thinking. “Freediving may act as a kind of accelerated model of brain aging,” he says, “It’s like stress-testing a machine to see where it cracks.”

But for the family, the human need comes first, even if the science branches out.

On the way to a warning device, Tucker’s father, Peter Francis, describes the hope and the uncertainty in the same breath. Schagatay “is doing terrific work,” he says. “But whether it will lead to a device, I don’t know.”

Schagatay’s experiments have provided much of the data needed to compute when a person’s blood oxygen approaches critically low levels. Turning that knowledge into a practical device is still daunting. Schagatay says there are some 30 factors that affect these things.

Divers in whom oxygen drops early in the dive face greater risk of blacking out, even in moderately deep dives that freedivers don’t feel are risky. They don’t reach the surface in time.

Pernett has also been working on devices that detect when someone about to dive underwater is hyperventilating, though he says more testing is needed.

In the meantime, Pernett says there’s “a lot [that can be done] with education.” He adds: “I don’t think there’s a chance that a device can be the only solution.”

The Francis family agrees. “We wouldn’t suggest that people not freedive,” Peter Francis says, “but there’s certain things you have to do to do it safely, not least of which is to make darn sure that you have somebody watching you who can pull you out.”

Valdivia-Valdivia knows how quickly rescue can change everything. He has blacked out but stayed alive because safety divers nearby immediately pinched his nostrils shut. covered his mouth to close the airways. and quickly brought him to the surface. “I didn’t know it was happening until I was rescued.”.

“I wish more people would be aware [of blackouts],” he says. “Freediving can give you a very wholesome life, or it can take your life.”

For Schagatay. the research is about turning the ocean’s allure into an evidence-backed form of safety — one that doesn’t rely on luck. or on the hope that a body will always respond in time. For the Francis family. it’s also about Tucker’s life continuing in the most practical way possible: a warning device. a watch-sized safeguard. and the insistence that no one should have to disappear 10 meters down before science catches up.

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