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By Maggie Fox

Everyone’s probably seen or done it at least once at the pool: the “hey watch this!” trick, such as when two New York City men first took a series of fast, deep breaths, then held their breath underwater in 2011. Both drowned.

It’s not unusual, the New York City health officials report.

They call it “dangerous underwater breath-holding behavior” or DUBB for short. Other people may call it just plain dumb, but in fact most of the cases involved serious training. The victims may not have realized just how much danger they were putting themselves in.

“In all four fatalities, the decedents were aged 17–22 years, known to be advanced to expert swimmers."

New York’s Amanda Levy and colleagues report on 16 such cases in the Centers for Disease Control and Prevention’s weekly report on death, disease and accidents. Four people drowned.

The worst cases didn’t involve kids just horsing around. “In all four fatalities, the decedents were aged 17–22 years, known to be advanced to expert swimmers, and engaged in intentional hyperventilation,” Levy’s team writes.

“Drownings associated with DUBBs can occur at any water depth and be caused by many disparate factors,” they wrote.

It’s well known what happens. Hyperventilation or holding the breath before going under pushes carbon dioxide out of the body. That in turn slows the signal to the brain that makes the urge to breathe irresistible.

“The ‘blackout’ is caused by the drop in partial pressure of oxygen in arterial blood gas,” they wrote.

In the 2011 case, the young men were training for an advanced military fitness test. “After alter­nating between push-ups and swimming laps, the swimmers began intentional hyperventilation and submersion breath-control exercises. Minutes later, both swimmers were found submerged underwater and not moving,” Levy’s team writes.

Military training also killed a teenager. “An advanced-level, teenage, male swim­mer with no preexisting health conditions and experience working as a lifeguard was training for his goal to join the U.S. Navy Seals,” Levy’s team wrote.

“He was observed by pool staff performing breath-holding exercises and underwater lap swimming."

“He was observed by pool staff performing breath-holding exercises and underwater lap swimming. He repeatedly submerged himself for extended periods of time, until it was noticed that he was unconscious.” The lifeguards were unable to resuscitate him.

In another case, a teenaged boy was playing a breath-holding game with friends. “The swimmer fell unconscious underwater and his friends alerted lifeguards,” Levy’s team wrote. This time, the swimmer lived.

It’s probably more common than people think, Levy’s team writes. Research suggests there is no witness to half of all drownings, and people who practice breath-holding underwater may do it regularly.

Drowning is a common cause of death, CDC says. About 10 people die every day from drowning in the U.S., 80 percent of them males.