Experts believe that Shallow Water Blackout is the #1 cause of swimming-related deaths in physically fit swimmers. Shallow water blackout is due to an insufficient amount of carbon dioxide to activate the body's natural impulse to breathe. Swimmers who practice prolonged underwater breath-holding are at risk. When the oxygen level in the blood runs low before the carbon dioxide level rises to the point that triggers the breathing reflex, the swimmer loses consciousness. The swimmer never actually feels as though a breath is needed. Once submerged underwater, the swimmer is hidden from the view of lifeguards by surface glare and ripples/waves on the surface. There is no surface or underwater struggle and therefore, very difficult for a lifeguard to notice. Lifeguards and Aquatic Facilities should never allow long breath holding at their pools.  It sets up the lifeguard to think the person is "just playing and holding their breath" because they see it as normal behavior when actually the person is unconscious.