What time the missing Titanic tourist submarine will run out of air

The clock is ticking down on the time still left to rescue those on board the missing Titan submarine, which vanished into the North Atlantic Ocean as it ferried five people down to the Titanic's sunken wreckage.

An international search and recuse effort is now underway as the hours tick by, ever closer to the deadline at which there will be no oxygen left to keep those on board alive.

According to the BBC, the sub's breathing air supply is expected to run out Thursday, June 22, at around 6am local time.

That means oxygen will run out for the five on board at about 11am for those watching and waiting in London on June 22.

That's about 8pm as per Australian EST on June 22, for those of you Down Under who are watching the clock tick by.

The vessel had about 96 hours of oxygen onboard when its dive began on Sunday.

The OceanGate submersible vanished one hour and 45 minutes into its dive and has not been seen since.

Hyperbaric medicine expert Dr Ken Ledez at Memorial University in St John’s, Newfoundland, explained the factors at play to the BBC.

He revealed depleting oxygen is not an exact science, and a number of factors are at play including a person's metabolism.

"It’s not like switching off a light, it’s like climbing a mountain," he said.

"They're going to do everything they can to reduce their oxygen consumption, they're going to rest, they're going to try to be as relaxed and calm as possible."

He explained that too much movement on the submarine could cause a metabolic spike and, in turn, create more unbreathable carbon dioxide.

He also added that 'hypothermia could be their friend'.

"There is a possibility if they cool down enough and lose consciousness they could live through it... The heartbeat can be really slow when cold," he told the BBC.

"[So] some may survive longer than others."

Those on board are British billionaire adventurer Hamish Harding, along with Shahzada Dawood, his son Suleman and OceanGate’s chief executive and founder Stockton Rush, reportedly together with French submersible pilot Paul-Henri Nargeolet.

The submarine was launched at 8am local time on June 18.

It was expected to resurface at 3pm, but the Titan submarine lost contact with its surface vessel one hour and 45 minutes into the dive.