NEW YORK (AP) — For those stranded by a flood, survival depends on many factors including access to oxygen, food and water.

Catastrophic flooding along the Nepal-China border has killed hundreds and left thousands more missing. On Monday, rescuers in Nepal raced to free hundreds of workers thought to be trapped in hydropower site tunnels that were buried in mud.

Experts say the first 72 hours are critical as the chances of survival drop without access to clean water.

One important aspect is whether the tunnels had emergency protocols in case of an outage or natural disaster, said disaster epidemiologist Jennifer Horney with the University of Delaware.

If so, workers could have been trained on the location of exits. But the force of the flooding could mean mud may be caked over such exits, rendering rescue plans difficult, Horney said.

For the hundreds trapped, access to clean drinking water is critical and it's unclear whether any emergency supplies were already stocked in the tunnels.

“There just really wouldn’t be a way to to go much further than the three days without safe, drinkable water,” Horney said.

Access to breathable air is another key to survival. At least one expert said that some workers could have access to oxygen — but six days after the floods, suffocation worries grow.

“These tunnels are big enough to drive trucks through,” said Jakob Steiner, a geoscientist at the University of Graz in Austria based in Dhaka, Bangladesh. “And from what we understand, they have managed to get oxygen to some of those stuck inside and are trying to rescue them now. It’s a race for time, though, as people need food and water.”

Beyond that, the mental effects of such isolation can also take a toll.

Other important considerations include “whether there are indications of serious injuries" and whether there's a safe way to deliver water and essential supplies while rescue operations proceed, according to Thomas Chandler, director of Columbia University's National Center for Disaster Preparedness.

He cited a Himalayan tunnel collapse from 2023 caused by a landslide, where 41 Indian workers were rescued after being trapped for 17 days. They were provided food, water and oxygen through pipes.

The climate inside the tunnels may also play a role. If it's hot and humid, people could get dehydrated faster. Those who are rescued may be very weak, so supportive care is important for both physical and mental impacts, according to disaster experts.

Associated Press journalists Shristi Kafle in Kathmandu and Huizhong Wu in Bangkok contributed to this report.

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