A second man has died following an underwater training accident at a public pool on Staten Island. The New York Daily News reports that 21-year-old off-duty lifeguard Jonathan Proce died Sunday at New York Presbyterian Hospital following an exercise at Lyons Pool last Wednesday in which he and his friend, Bohdan Vitenko, also 21, were practicing underwater breath-holding.

The two were found unconscious and in cardiac arrest in three feet of water at the bottom of the pool; Vitenko died later that day. Two lifeguards have been pulled from their duties after failing to notice Proce and Vitenko, according to The New York Post. Approximately 20 other swimmers were in the pool at the at the time, the paper reports.

Proce was bound for the U.S. Air Force, while Vitenko had dreams of becoming a Navy SEAL. They were regulars at the pool, reportedly participating in a grueling workout routine that included swimming and underwater sit-ups. It is not clear if the men were following an official training program or had developed their own workouts. Either way, the military advises against certain breath-holding exercises or swimming underwater at length to avoid "shallow water blackout," which can lead to drowning.

According to ShallowWaterBlackoutPrevention.org - an awareness and education site - the condition occurs because of low carbon dioxide and low oxygen (which triggers unconsciousness). Hyperventilation done before breath-holding lowers carbon dioxide abnormally, allowing individuals to hold their breath longer. But the lower carbon dioxide levels rob the body of its built-in mechanism to tell the breath-holder to breathe before going unconscious and taking water into the lungs.

Additionally, "because of the hypoxia, one feels euphoric and empowered to continue breath-holding," the site states. Unlike regular drowning, where six to eight minutes can elapse before brain damage and death, brain damage and death caused by shallow water blackout can occur within two and a half minutes. More information about shallow water blackout can be found on the Aquatic Safety Research Group's website.

In 2008, the National Swimming Pool Foundation warned that "anyone who practices competitive, repetitive underwater breath-holding is at risk for shallow water blackout. Once submerged underwater, the swimmer may be hidden from the view of lifeguards by surface glare and ripples/waves on the surface. A series of events is then triggered, including the inhalation of water, possible convulsions and ultimately cardiac arrest and death."