A 60-year-old man fell into New York's Long Island Sound, pulled himself out - and then died several hours later, apparently of drowning.  Emergency doctors today called it a case of secondary drowning, something very unusual.

The man, Tommy Mollo of Yonkers, N.Y., fell off the back of a friend's boat Saturday morning while helping move it between slips at a marina in nearby New Rochelle, WABC-TV reported. Mollo returned to his apartment and told his wife he felt ill. She called 911 and emergency workers took Mollo to a hospital, where he was pronounced dead at 12:05 p.m., the station reported.

An ER doctor told the station that water got into Mollo's lungs when he fell overboard, which led to subsequent breathing difficulties that could have been exacerbated by medical issues he already had.

Mollo's case represents a rare occurrence of a relatively rare phenomenon, beginning with his self-rescue, emergency room doctors said.

Secondary drowning typically occurs "when one is immersed in water, they almost drown, water successfully enters the lungs, and then they are rescued," said Dr. Gabe Wilson, associate director of emergency medicine at St. Luke's-Roosevelt Hospital in New York City. "Conceivably water could be inhaled while one still had the means to pull themselves out, but it would certainly be a rare occurrence as usually panic sets in by then."

Wilson cited one study that showed secondary drownings make up 5 percent of overall drownings in children and teens. "There is no great data for adults," he told ABCNews.com.

The lag between the time water enters  the lungs and begins to cause problems can range from one to 48 hours, he said. "Because onset can be rapid, it is not known whether there are predictable warning signs." As a result, anyone who experiences an episode of near-drowning should be evaluated in an emergency department and "possibly observed for 24 hours," Wilson said.

Lung damage from secondary drowning occurs when water comes into direct contact with the cells lining the lungs, interfering with their ability to supply needed oxygen to the body and to take away carbon dioxide, a gaseous waste product.

This damage can be particularly severe when delicate lung tissues are flooded with salty ocean water, like that of Long Island Sound. The water "tends to pull fluid from the body into the lungs," said Dr. Larry Baraff, associate director of the emergency department at the Ronald Reagan UCLA Medical Center. When fluid moves into the lungs, it "takes up space where the air would be."

In Mollo's case, he said, "it's conceivable that the drowning episode and lack of oxygen led to a heart problem, like a cardiac arrhythmia or a myocardial infarct (heart attack)."

However, he said secondary drownings are survivable with fast-enough medical attention.

"If you make it to the hospital alive, it's very unusual to die from drowning," Baraff said. Survivors of near-drownings who arrive at the ER in what seems to be good shape will undergo monitoring "just to make sure they don't get worse."

Those who are in distress can be put on a ventilator. Doctors then use pressure to "force fluid out of the lungs so oxygen can get back in."