Today is the day sub's air runs out

Eight o'clock this morning is the crucial moment for the 118 sailors trapped at the bottom of the Barents Sea in the Russian submarine Kursk since Monday.

That is when the clock strikes midnight in Russia, heralding August 18 - the day the air runs out, according to the best calculations of the Russian Navy.

While Navy top brass were meeting last night to discuss yet another rescue plan, and the British and Norwegians were rushing to their aid, many experts were predicting that the most likely end to this saga would be tragedy on a grand scale.

Four rescue attempts have failed and time, at best, is running out fast - the British will not get to the Barents Sea until tomorrow night at the earliest and it will take a week for the Russians to attempt their ambitious and dangerous plan to raise the submarine using giant pontoons.

At worst, time has already run out. There has been no sign of life on the stricken submarine since a weak SOS message was rapped on the hull two days ago.

One thing is certain: the men trapped in the submarine will have battled against rising carbon dioxide levels and the cold over the past four days.

Normal air contains 21 per cent oxygen but people can operate and breathe quite normally on levels as low as 8 per cent, says Carol House, of the environmental medicine unit at the Institute of Naval Medicine in Portsmouth, England.

The real danger comes from a buildup of suffocating levels of carbon dioxide.

"Normal levels of carbon dioxide in air are 4 per cent but once they reach 5 per cent, people experience headaches and nausea, and will pass out when they reach 10 per cent," says Ms House.

That process will have been slowed because the Kursk is almost certainly fitted with carbon dioxide "scrubbers," which use soda lime to absorb the gas.

Another great threat to the crew is the cold, with temperatures of between 2 and 4 deg. Hypothermia will set in as body temperatures slip below 37 deg.

At 35 deg, metabolism will slow and crewmen will become lethargic and confused. At 32 deg, they will become unconscious and their hearts will eventually stop.

Despite the overwhelming cause for alarm, many in the Russian Navy have not given up.

"The rescue operation is continuing, but strong underwater currents and almost nil visibility are making it tough," said a Navy spokesman.

"There still is hope. We're sending down all three of the rescue capsules."

Herald Online stories: Russian sub in distress

Russian Centre for Arms Control: OSKAR subs

World Navies Today: Russian subs

Russian Navy official website