What Is Freediver Blackout?

Freedivers and spearfishers can blackout at different points in the water column and all of these are currently collectively labeled with the awkward and confusing term ‘shallow water blackout.’ While not a perfect solution, DiveWise prefers to use the phrase Freediver Blackout to provide a little more clarity. Below are the types of blackouts freedivers can experience.

  •  Static apnea blackout occurs at the surface when a motionless diver holds her breath long enough for the circulating oxygen to fall below that required for the brain to maintain consciousness.  It involves no pressure changes in the body and is usually performed to enhance breath-hold time.  It should never be practiced alone, but under strict safety protocols with a safety beside the diver.
  • Shallow water blackout occurs on ascent from depth when circulating oxygen stores fall below that required to maintain consciousness.  The diver is at the greatest risk of blacking out 30 ft below the surface and shallower as he ascends.
  • Surface blackout occurs when a diver with low levels of circulating oxygen has surfaced after ascending from depth and has begun breathing, yet blacks out before the inhaled oxygen has time to reach the brain.
  • Deep water blackout is uncommon but it does happen, and at depths greater than 30 ft, likely triggered by high CO2 levels and low O2.

This animated diagram helps to explain the term shallow water blackout as it applies generally to the sport of freediving and spearfishing.


As a diver descends to depth the increasing water pressure causes an increase in overall oxygen pressure, even as oxygen is being consumed. However, during ascent with water pressure and oxygen pressure decreasing, and oxygen supplies running low at the end of the dive, freediver blackout can come on quickly with little or no warning. The diver will experience the greatest decrease in oxygen pressure at about 30 ft below the surface and shallower. After the diver inhales air at the surface, it takes about 20 heartbeats (or 30 seconds) for the fresh oxygen to travel to the brain. Until this circuit is complete, the diver continues to be in danger of blackout even though he is breathing and may have even signaled to his buddy that he is “okay.”

Freediver blackout is insidious. Some survivors have described the experience as a beautiful way to die. No suffering, no warning, no clue death was near. It is this feature of freediver blackout that makes it so deadly.