The biggest threat to submarine passengers now may be declining oxygen levels

As search and rescue teams search the North Atlantic for the missing submarine Titan, one overriding question could determine the fate of the five passengers: how much oxygen is left?

The submarine contains a finite amount of oxygen and cannot generate more in any way. Once consumed, passengers are left without air to breathe. The craft is estimated to have started on Sunday with about 96 hours of breathing air supply; Wednesday morning, the admiral of the US Coast Guard is in charge of the search said in a broadcast interview that the remaining amount had probably shrunk to about 20 hours.

There’s no way to say more precisely how much is left.

Assuming the ship is still intact underwater, according to Dr. David Cornfield, a pulmonologist at Stanford University, several variables could extend the survival time of the five people on board.

If they can stay calm and take less deep and frequent breaths, they can take several hours. “They can change the curve very modestly,” said Dr. Cornfield. For example, if they could slow their breathing enough to gain 10 percent more time, that would add nine hours of survival time to the possible window for rescue.

The level of carbon dioxide, an invisible gas exhaled during breathing, also affects survival time. If the carbon dioxide builds up too high, people on board can become drowsy, become unconscious and eventually die. The Titan is said to be equipped with a scrubber, or filter, intended to remove excess carbon dioxide from the air inside the enclosed craft.

The submarine is a tight fit for a pilot and four crew members: 22 feet long, 9.2 feet wide, and 8.3 feet high. Its small size is intended to enable submarine expeditions at a relatively low cost, but experts have warned of structural risks and other concerns about the craft’s reliability. David Pogue, a CBS reporter and former New York Times technology columnist who has been aboard the Titan, described the interior as “about the size of a minivan.”

Images of OceanGate Expeditions, the company that operates Titan, shows a interior resembling a metal tube. Passengers sit against the curved walls; there are no chairs for them and little room to stand or move.