The survival rates for disaster victims buried under the debris of broken concrete and twisted metal vary widely, depending on their circumstances, age, pre-existing health conditions and availability of drinkable water, experts say.

"It all depends on their overall health to start with, the injuries they have, and their access to water," said Dr. Eric Weinstein, a disaster medicine specialist in South Carolina and part of a urban search and rescue team there.

As rescue efforts continue in Elliot Lake, Ont., officials have said there appears to be little hope of finding more survivors from the wreckage of the mall.  At least one person was killed after part of the mall's roof caved in on Saturday afternoon and 12 people are still unaccounted for.

Ontario Provincial Police Staff Sgt. Jim Bock, head of the Urban Search and Rescue Unit, said that survival rates "depend on the injuries … and on the structure of the environment.

"There are time frames, but every situation is different. So if I was to quote 24 hours, 72 hours — I haven't been there, haven't seen the layout of the land. So it's very hard to say."

The City of Toronto's Heavy Urban Search and Rescue website reports that the rate of survival for people trapped in a collapsed building "drops day by day over the first four to five days, after which the prospects of survival are extremely unlikely."

"According to one source, 81 per cent of those rescued on the first day are likely to survive. This rate drops to 34 per cent on the second day and falls to only seven per cent by the fifth day," the website says.

Survivors who defy the odds

But natural disasters of the past are replete with stories of people being pulled from the rubble days after a catastrophe.

"We've had anecdotal cases coming out of Haiti, out of Mexico City, out of some of the other major earthquake situations where you find people coming out from the debris after several days," said Sean Tracey, of the Canadian Centre for Emergency Preparedness. "A lot of times they have access to water in there or other means to help sustain them."

Darlene Etienne, 16, was pulled out of the rubble by French rescue workers 15 days after the Haiti earthquake struck, having survived by drinking some water remaining from a bathroom.

But others have been able to last days without water. A 69-year-old woman survived a week, trapped in the rubble and apparently without access to any fluids.

Obviously food and water play an important role in survival rates. People can go without food for weeks, possibly  30 to 40 days according to some survival guides. But water is more essential. Experts seem to vary on the number of days one can last without water. Some have suggested two to three days maximum, but others say it can be a little longer.

"Five days [without water]. Seven days, then you start getting into some kidney issues," Weinstein said.

Lack of water leads to severe dehydration, kidney failure and death.

The very young and very old are more vulnerable, Weinstein said, as are those with underlying medical problems, such as asthma, diabetes, HIV and cancer, he said.

Survivors who have been hit by debris often suffer from a crush injury or crush syndrome, also known as rhabdomyolysis. This occurs when a severely injured muscle releases the protein myoglobin into the bloodstream, which can lead to kidney damage or kidney failure.

"The crush injury is a major problem in the field," Weinstein said, adding that continued pressure on a limb from a piece of debris also increases the risk of tissue death, or gangrene.

With files from Zach Dubinsky