The science

What happens when you hold your breath?

By GyroscopeUpdated November 15, 202616 sources

When you hold your breath, carbon dioxide builds and the urge to breathe grows. Keep going and your diaphragm starts to twitch on its own, and then a reflex forces the next breath, usually while your oxygen is still close to normal. Breathe fast first and that order flips, which is how people black out without warning.

The gasp at the end of a hold feels like your body running out of oxygen. It feels natural to think a long hold means big lungs, and that pushing past the urge is a kind of training. Breath-holding games in pools rest on the same idea, that your body will warn you in time. In an ordinary hold the warning comes from carbon dioxide, and your oxygen has barely moved when it arrives. The carbon dioxide alarm keeps an ordinary hold safe, and it’s also the part you can switch off by breathing hard first.

What happens in the first minute?

Your chest stops moving, but your brainstem keeps going. The physiologist Parkes, at the University of Birmingham in England, found in a 2006 review that the brain’s breathing rhythm keeps running through a hold. You keep your chest still on top of it. Meanwhile your cells keep making carbon dioxide and using oxygen, and the urge to breathe grows as carbon dioxide builds.

In 2016, Parkes’s team at Queen Elizabeth Hospital in Birmingham taught 15 women with breast cancer, aged 37 to 74, to hold their breath for radiotherapy. None had any breath-holding experience. Their very first hold, after a big breath of air, lasted 42 seconds on average, and nobody lasted under 29. Blood pressure rose steadily through each hold, by about 7 points (mmHg) a minute on air, and came back to normal within about 20 seconds of breathing again.

Why does your diaphragm start twitching?

If you keep holding past the first urge, your diaphragm starts to contract on its own, and the contractions grow stronger the longer you hold. A 2021 review of breath-hold training by Elia and colleagues lists tolerance of those growing contractions, and of the urge to breathe, as one of the things that separates long holders from everyone else. After his 29-minute world record in 2025, the Croatian freediver Vitomir Maričić said his body got worse and worse, “especially for my diaphragm, because of the contractions.”

Parkes thinks the diaphragm may help end the hold too. When the phrenic or vagus nerves on both sides were paralyzed, holds lasted longer, which points to signals from the diaphragm muscle as part of the trigger.

What ends a hold?

The breath that ends a hold is called the breakpoint, and Parkes found no single cause for it. Big lungs, extra oxygen and low carbon dioxide all stretch a hold. The opposite shortens it, and so does working harder. His team’s 2016 study shows what extra oxygen, low carbon dioxide and practice did for the same 15 women.

At the end of those 5-minute holds, the women’s oxygen and carbon dioxide were both still normal. The woman with the longest hold, 6.6 minutes, stopped just as her blood pressure reached the study’s safety limit, with her oxygen still at 97 percent and her carbon dioxide at 30 mmHg, below normal. Her oxygen and carbon dioxide can’t explain why she stopped, which fits Parkes’s view that the breakpoint has more than one cause. None of this is for copying at home, since the low carbon dioxide came from a ventilator in a hospital.

A simple way to picture it

Think of a long drive in a car with two gauges, temperature and fuel, and a warning light on the temperature gauge only. Carbon dioxide is the temperature and oxygen is the fuel. On a normal drive the engine heats up long before the tank runs dry, so the warning light keeps you safe. Breathing hard before a hold is like starting with an ice-cold engine. The light comes on much later, and the tank can hit empty first.

What is the diving response?

Hold your breath with your face in cold water and your body does something extra. Your heart slows, the blood vessels in your arms and legs tighten, and blood is steered toward the heart and brain, which makes your oxygen last longer. Elia’s review says every human has this diving response, but trained divers get more of it. Their heart rate starts to fall within 10 to 20 seconds and can drop below about 35 beats a minute. In people who don’t dive, it starts later, at 50 to 70 seconds, and gets down to about 50.

Holding your breath brings it on by itself, and cold water on the face makes it stronger through the cold sensors of the face. In Parkes’s hospital study, with the women lying dry, their heart rates didn’t slow at all. Panneton and Gan, two researchers who study the reflex in animals, have suggested it may be the most powerful automatic reflex known.

Your spleen joins in too. It stores about 10 percent of your red blood cells, and after three to five maximal holds it squeezes them out into the blood. Each hold after that starts with a little more oxygen on board, so the holds that follow can last longer.

Why does breathing fast first make it dangerous?

Fast breathing drains carbon dioxide, so the alarm starts from far below its usual point while oxygen starts about where it always does. In a 2021 Swiss study, 15 amateur runners did three rounds of 30 deep breaths, each followed by a hold on empty lungs. Their holds lasted 74, 101 and 119 seconds on average, round by round, and finger oxygen fell to 60 percent by the end of the holds. In a 2014 Dutch study of people trained in the same kind of breathing, oxygen dropped to around 50 percent for about 10 seconds at a time.

In water, a blackout like that can be fatal. A CDC report found 16 people in New York State who blacked out underwater from dangerous breath holding between 1988 and 2011, and four of them died. All four were advanced or expert swimmers aged 17 to 22, and all four had hyperventilated on purpose. In 2022 the American Red Cross, the YMCA of the USA and USA Swimming said in a joint statement that water depth isn’t a factor in what hyperventilation and long holds do to the body. They call it hypoxic blackout, and their swim instructors limit swimmers to a single breath before going under.

What does a finger oximeter show during a long hold?

On an ordinary hold the number barely moves. After air holds of about a minute, the women in Parkes’s study bottomed out at 98 percent, and the low point came about 15 seconds after they’d started breathing again. The number trails behind the hold. In a 2026 Swedish case report, a trained freediver lost consciousness after a 100-meter swim underwater in a pool. A forehead sensor caught oxygen at 51 percent in the seconds after the diver surfaced.

The lows after fast breathing would send you to the emergency room if you saw them at rest. oxygen.guide explains what the number means and what counts as low.

How long can a person hold their breath?

Trained freedivers go far past Parkes’s beginners. AIDA, the international freediving federation, lists the men’s static apnea record as 11 minutes 35 seconds, set by Stéphane Mifsud of France on June 8, 2009. Guinness World Records lists Maričić at 29 minutes 3 seconds underwater, set in Opatija, Croatia, on June 14, 2025. He took one last breath from an oxygen tank before going under, with AIDA judges and a support team watching him the whole time.

What about the BOLT test?

The BOLT test is a much gentler hold. You breathe out normally and stop at the first clear urge to breathe, so you’re timing when the carbon dioxide alarm first speaks up. It never goes near the contractions or the breakpoint. What’s a good BOLT score? has the coaching bands, and the breath hold test times it for you. In 49 elite speedskaters, the score had no link to their fitness tests, as a long breath hold means you’re fit explains.

Where the explanation stops

Parkes wrote in 2006 that no simple explanation fit everything known about the breakpoint, and the women with normal blood gases at the end of 5-minute holds show why. Nobody knows exactly why the diaphragm starts contracting when it does. And the point of blackout is often assumed to be near 50 percent oxygen, an estimate drawn from freedivers.