Shallow Water Blackout: Warning Issued About Breath-Holding Pool Danger

— -- Whitner Milner was a sport spear fisherman who was training in his family's backyard pool to hold his breath for longer periods when something went wrong. That's when Rhonda Milner found her son at the bottom of the Atlanta pool on April 17, 2011.

“And then I, you know, tried to stir him and didn't realize I couldn't,” she said.

Whitner Milner, 25, had passed out and drowned. An avid spear fisherman and accomplished free diver, he’d fallen victim to shallow water blackout, in which people who are trying to hold their breaths for an extended period essentially pass out underwater because of oxygen deprivation to the brain.

Warning signs are going up at pools across the country, and New York City and Santa Barbara are among the first cities in the country to ban extended breath-holding in public pools, Reuters has reported.

“Good Morning America” talked with Tom Griffiths, president and founder of Aquatic Safety Research, about the little-known hazard, and we asked how long was too long to hold your breath underwater.

“We usually say 30 seconds is about the limit. Certainly we don’t want people holding their breath for more than a minute because a child can drown within 90 seconds," Griffiths said.

Several shallow water blackouts have been caught on underwater surveillance video. In one video, a 13-year-old boy struggles to stay conscious, then slowly sinks to the bottom of the pool. Helps comes just in time. In another video, an experienced 18-year-old swimmer can be seen sinking seconds before being rescued.

Shallow water blackout-related deaths can be hard to track, but at least seven cases have been documented across the country, according to figures from the New York City Department of Health. Griffiths told “GMA” that most shallow water blackouts happen in water that’s between three and four feet deep, not at the deep end of the pool.

Milner founded Shallow Water Blackout Prevention to raise awareness about the hazard that claimed the life of her son.

“I had never heard of shallow water blackout before our son died,” she said a YouTube video in which she tells her son’s story.

Michael Phelps, the decorated Olympic gold medal-winning swimmer, is also lending his voice to raise awareness of the issue.

"If shallow water blackout occurs, it’s often fatal. But through education and understanding, it can be 100 percent preventable,” he said in a 2014 public service announcement by USA Swimming, the national governing body for competitive swimming in the United States.