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SESK: Stuff Everyone Should Know · sesk.info/swim-no-hyperventilating-breath-hold

Why shouldn't swimmers breathe fast and deep before underwater distance contests?

It can cause a blackout underwater, without warning.

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  1. It makes your goggles fog up
  2. It can cause you to black out underwater ✔
  3. It makes you float too much to go under

Why: Hyperventilating lowers the carbon dioxide that tells your body to breathe. You lose the urge to come up, and you can pass out underwater. The Red Cross warns about this hypoxic blackout.

Huh, didn't know that: The Red Cross notes that the people holding underwater distance contests are often very good swimmers.

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References

Answer: It can cause you to black out underwater. Hyperventilating lowers the carbon dioxide that tells your body to breathe. You lose the urge to come up, and you can pass out underwater. The Red Cross warns about this hypoxic blackout.

  1. American Red Cross: Shallow Water and Hypoxic Blackout www.redcross.org/take-a-class/resources/articles/shallow-water-hypoxic-blackout
    Hypoxic Blackout: Hyperventilation occurs and you blackout.
    Checked 2026-10-12.
  2. American Red Cross: Water Safety www.redcross.org/get-help/how-to-prepare-for-emergencies/types-of-emergencies/water-safety.html
    Understand the dangers of hyperventilation and hypoxic blackout.
    Checked 2026-10-12.

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