Shallow Water Blackout: The Risks of Freediving

Freediving is an amazing activity, but it also has a lot of risks involved with it. By far the greatest of those risks is freediving blackout. This is also commonly referred to as “shallow water blackout.” Divers have become increasingly concerned with preventing shallow water blackout, so much so that swimmers and doctors have formed organizations to increase awareness about shallow water blackouts. Shallow water blackouts are a danger, but they are also an avoidable danger. The best way to avoid a shallow water blackout is to understand why they occur in the first place.  

What is shallow water blackout and how does it happen?

Shallow water blackout occurs when a person loses consciousness under water due to a lack of oxygen. To help you understand how shallow water blackout occurs, it is best to break it down into three stages.

Stage 1: The carbon dioxide levels in the diver’s blood decrease


  • People engaging in freediving tend to hyperventilate, either on purpose (because it helps them stay underwater longer) or because they are exerting themselves trying to dive deeper.
  • Hyperventilation lowers the levels of carbon dioxide levels in the blood. This is known as Hypocapnia.

Stage 2: The diver begins to use oxygen


  • The way the body works is that as the body uses oxygen, it becomes carbon dioxide. The body detects carbon dioxide and uses that to determine when it needs more oxygen.
  • As the diver swims, their body uses oxygen and turns it into carbon dioxide.
  • But, because the diver had lower levels of carbon dioxide in their blood, the body is effectively unable to detect that it needs oxygen.

Stage 3: The diver loses consciousness

  • Since the body is unable to determine that it needs oxygen, the brain will slowly become oxygen deprived.
  • Most people will just slip into unconsciousness and will not get a warning that they need oxygen.

What makes shallow water blackout so dangerous is that in most cases there is no way to alert someone else for help. With regular drowning, the victim at least has a chance to thrash around in the water. But, in a shallow water blackout, the victim simply sinks to the bottom and drowns silently, meaning that the only chance for help is if someone happens to see the person sinking. There are two good methods for avoiding a shallow water blackout. First off, don’t hyperventilate. Secondly, don’t go freediving on your own.