The greatest danger in free diving, shallow-water blackout (SWB) is a result of a variety of factors. The predominant factors are the body's adaptations to apnea and pressure changes during moderately deep dives. When a freediver ascends from a dive, the volume of the lungs expands while the amount of oxygen does not, resulting in a rapid net decrease in the concentration of oxygen available. At this point, oxygen will actually run down the concentration gradient from the blood into the lungs.
The extreme oxygen deprivation at this point causes much of the brain to essentially shut-off, resulting in the blackout. There is no warning at the time.
Individuals who have some free diving experience are most at risk, as they have been training themselves towards increased periods without breath; however, all free divers should be alert to this danger. Never dive without a partner.