Poor conditions and other obstacles mean rescue divers are facing the "daunting task" of rescuing passengers who may have found air pockets inside a ferry that sank off South Korea's southern coast.
But even if they have air to breathe, the threat of hypothermia is reducing their chances of survival.
"It’s a very difficult operation and while there may be survivors, even if they're found, it's going to be hazardous to bring them to the surface," said Kim Petersen, governor emeritus of the Maritime Security Council and president and chairman of Security Dynamics.
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Twenty-five people, including a female crew member, at least five students and two teachers, were confirmed dead by South Korean coast guard officials Friday. But more than 270 passengers — many of them high school students — were missing and many are presumed dead. Officials put the number of survivors at 179.
The 146-metre Sewol now sits — with just part of its keel visible — in waters off Mokpo, about 470 kilometres from Seoul. The interior of that ship is the focus of the desperate search for survivors.
Currently, there are 178 divers feeling their way around inside the vessel, diving in teams, running multiple safety lines, searching in a grid fashion as they look for openings that could lead to voids where pockets of air could sustain life, Petersen said.
The working assumption, Petersen said, is that some passengers were able to find air pockets in the vessel. But with divers planning to pump oxygen into the ship to help any survivors, the question is whether or not they're still alive.
Divers started pumping air into the submerged vessel early on Friday. It is unclear whether the air is for a rescue or salvage operation.
Prof. Gordon Giesbrecht, associate dean of kinesiology and recreations management at the University of Manitoba, said food shouldn't be an issue, as someone could live for weeks without it. Although the general rule is that people can only last three days without water, some survivors of disasters have shown they can live longer,
This means passengers on the vessel would die either by asphyxiation or hypothermia, he said. Surviving hypothermia could depend on how extensively the passengers have been immersed in water.
With water temperatures ranging from 10 C to 12 C, Giesbrecht said it's unlikely that someone almost completely immersed in water could survive past 24 hours.
"People could be in enclosed spaces with trapped air anywhere from full immersion up to the neck, to very little immersion. And in that case, people could survive for days."
However, with poor visibility and a strong current, divers are facing huge challenges finding surviving passengers.
"It’s just a daunting task," Petersen said.
Any survivor who is found may be injured, suffering from hypothermia, and now have to be moved from a void deep inside the ship to the surface using scuba equipment they will most likely be unfamiliar with.
"The water is 10 C. It’s murky, it’s going to be pitch dark, it has to be the most frightening environment these desperate students could find themselves in, if there are any that’s still alive," Petersen said.
"And then having to move through an environment that’s going to have numerous obstacles, for example metal edges, broken glass and then coupled with the current — all of which makes for a daunting rescue scenario."
Petersen said it may take 15 to 20 minutes to follow the safety lines back through the maze of hallways and debris that would ultimately lead to the surface.
Rescue crew may also try to raise the vessel using a crane to try to extract any survivors, but that might require more time than would be available, Petersen said.
Once a survivor has been found, Giesbrecht said there’s no rush to get them out, even if they’re suffering from hypothermia.
"When you're going to rescue somebody, speed isn’t the issue. If they’re still alive, they’re in a very stable situation so you can take your time, knowing that in an hour their temperature won't drop."
The rescuers should take the time to calm the passenger down, he said.
"You’ve got the person, they’re alive. If it took you an hour to get the person out of there, who cares."
"Basically you’re going to teach somebody how to scuba dive in the worst possible conditions. Cold water, dark, survival situation. So you really have to take your time," Giesbrecht said.