Blacking Out While Freediving | Freedive Earth

Blacking Out While Freediving

Losing consciousness underwater is one of the greatest risks of freediving. When freediving is performed incorrectly, divers risk blacking out due to hypoxia, or low levels of oxygen. Losing consciousness out of the water is not inherently dangerous, but an underwater black out can be life-threatening. A freediver who loses consciousness underwater will reflexively inhale, and may drown without prompt assistance.

To avoid the risk of a freediving black out, divers must understand their physiological limitations, as well as understand what causes a black out in the first place. In freediving physiology basics, I explained that most black outs occur when divers hyperventilate or push their limits too quickly.

Gas Laws That Affect a Freediver:

To understand shallow water black out, a freediver must first understand how the gases in his lungs become more concentrated as he descends in the water. There are two laws of physics which apply to the oxygen, nitrogen, and other gases in a freediver's lungs.

  • • Boyle's law: The volume of a gas decreases as the surrounding pressure is increased. The the concentration of the gas increases with surrounding pressure.

As a freediver descends, the increased water pressure causes the gas in his lungs to compress and become more concentrated. This means that the concentration, and therefore the partial pressure, of oxygen in the diver's lungs increases as he descends.

  • Dalton's law: The total pressure of a mixture of gasses is equal to the sum of the partial pressure of each of the different gases in the mixture. Each gas acts alone, and is not influenced by the presence of other gases.

Dalton's law explains that the concentration of oxygen and nitrogen in a diver's lungs increase independently of each other. The presence of nitrogen and carbon dioxide in the diver's lungs in no way limits the concentration of oxygen.

The Concentration of Gases in a Freediver's Lungs Increases With Depth:

Air is made up of approximately 21% oxygen and 79% nitrogen. On the surface, the total air pressure is 1 atmosphere (ata). The partial pressure of oxygen about is 0.21 ata and the partial pressure of nitrogen is approximately 0.79 ata. As a freediver descends, the pressure surrounding his body increases, as do the partial pressures of the gases in his lungs.

Partial Pressures of Oxygen and Nitrogen at Common Depths:



* Learn how pressure increases with depth

Shallow Water Black Out in Freediving

Shallow water black out occurs when a diver ascends to a shallow depth and falls unconscious from a lack of oxygen. How can a diver lack oxygen during ascent when he had sufficient oxygen at depth? The concentration of oxygen in his lungs decreases as he ascends.

When a freediver is deep underwater, the oxygen in his lungs is more concentrated. His body can easily metabolize this highly concentrated oxygen to fuel his brain and muscles. He will feel that he has sufficient oxygen.

As he ascends, the pressure surrounding the freediver decreases, as does the concentration of oxygen in his lungs. If the diver has used too much oxygen during his freedive, the concentration of oxygen in his lungs may fall so low that his body can no longer metabolize the oxygen, causing him to lose consciousness. Most freedivers will lose consciousness if the partial pressure of oxygen in their lungs falls below 0.1 ata.

Example: A diver starts a freedive with 0.21 ata of oxygen in his lungs. He descends to 40 meters and the partial pressure of oxygen increases to 1.05 ata. While he spends time at 40 meters, his body metabolizes some oxygen and the partial pressure of oxygen falls to 0.40 ata. This partial pressure of oxygen is still high enough for him to function properly at 40 meters, in fact, it is higher than the partial pressure of oxygen on the surface. As the diver ascends, the partial pressure of oxygen in his lungs falls. At slightly less than 10 meters of depth, the partial pressure of oxygen in his lungs is less that 0.1 ata and he will fall unconscious.

The greatest change in water pressure occurs near the surface. Black outs due to low partial pressures of oxygen are most common at the 10 meters closest to the surface, as the partial pressure of oxygen in the freediver's lungs drops most rapidly during the final ascent.

Avoiding a Shallow Water Black Out While Freediving:

Some behaviors increase the risk of shallow water black out in freediving, while others help to prevent a crisis.

1. Avoid hyperventilation: it lowers a diver's carbon dioxide levels to the point that he may not feel the urge to ascend in time to avoid plummeting oxygen levels.

2. Avoid hanging on the descent line at a shallow depth: this causes the oxygen in a diver's body to exit his system very quickly. Without getting into technical details, do not stop and rest underwater at a shallow depth at the end of a freedive.

3. Avoid stopping and hanging at the deepest part of the dive: doing so will cause the oxygen in the freedivers system to move quickly to the diver's tissues, where it is metabolized. A diver who stays too long at the deepest point of his freedive may not have enough oxygen to safely surface. 

4. Always weight yourself properly: a diver who experiences a shallow water black out will sink if he is negatively buoyant. Be sure to weight yourself so that you are positively buoyant during the last 10 meters of a freedive. This will facilitate buddy rescue and emergency management. 

5. Always freedive with a buddy: your freediving buddy should dive down to meet you at the for the last 10 meters of you ascent in order to be on hand in case of a black out.

The Take-Home Message About Shallow Water Black Outs and Freediving

With training and proper freediving procedures, divers should be able to avoid a shallow water black out. A freediver should train his body to tolerate low oxygen levels using static breath holds before attempting to increase his depth and dynamic freediving times. Pushing his limits gradually allows a diver to learn his personal tolerance for low oxygen levels in a safe manner. Understanding the risks of freediving and learning to avoid them is one of the reasons that it is always important that a novice freediver undergoes professional freediving instruction.


Guide category: 
Guide tags: