How long people can live on a limited oxygen supply and what happens when it starts to run out, according to an expert

• A submersible that takes tourists to the Titanic wreckage went missing on Sunday.
• The oxygen supply is likely to run out on Thursday afternoon, the US Coast Guard said.

A submersible that takes tourists to the Titanic wreckage had not been found on Tuesday after it went missing in the North Atlantic on Sunday.

The US Coast Guard said that the crew onboard the Titan submersible lost contact with its mothership an hour and 45 minutes after submerging on Sunday morning. A submersible is unable to get to the bottom of the ocean and back without its mothership — unlike a submarine.

On Monday afternoon, the US Coast Guard estimated the submersible had between 70 to 96 hours of oxygen — the passengers' lifeline — left, and would likely run out on Thursday afternoon, Eastern Time, at the latest.

Cold people use more oxygen

The range of hours is due to a couple of factors, Mike Tipton, head of the extreme environments laboratory at Portsmouth University, UK, told Insider.

It might not be clear how much oxygen was on the vessel to begin with, but assuming that is known, the variation is likely due to the rate at which oxygen is consumed. How quickly we use oxygen depends on what we're doing, he said.

Exercising on a bike, for instance, uses about two liters of oxygen a minute, according to Tipton. However, sitting at rest doing as little as possible brings you down to about a third of a liter, and the passengers aboard the Titan submersible have enough experience onboard to know how to conserve oxygen.

But if the submersible has lost power it could be sitting on the seabed at 40°F, causing people onboard to get cold and start to shiver. This could increase oxygen consumption by around three times, according to Tipton.

"Shivering is a form of exercise that consumes up to about a liter of oxygen per minute," he said.

A human can survive for three minutes without oxygen

If the submersible starts to run out of oxygen, those onboard could experience symptoms of restlessness, headaches, confusion, increased heart rate, shortness of breath, blue fingertips, and eventually loss of consciousness, Tipton said.

Insider previously reported that the body can only last for around three minutes without any oxygen — any longer than that and you could get brain damage.

If the submersible loses power, the passengers onboard could face further issues, such as carbon dioxide poisoning.

Tipton explained that the system for producing oxygen onboard the submersible, known as a scrubber, will use chemicals that absorb CO2 from the atmosphere. This can require electricity to make the air pass over it.

"If that pump stopped, it becomes much less efficient and the CO2 level starts to build up in the atmosphere."

Symptoms of CO2 poisoning include shortness of breath, headaches, disorientation, confusion, and seizures. Eventually, the level of CO2 can become life-threatening, he said.

Tipton said there are a lot of "ifs and buts" as to how long the passengers could survive onboard the submersible, considering it isn't clear what has happened.