But with enough supplies for the two-person Expedition Eight crew of Mike Foale and Alexander Kaleri to last at least another six months and no sign yet the leak is growing, station managers say they are not alarmed by the situation.

"We're going to take a very measured and methodical approach to sorting through this problem. There is no risk to the vehicle or crew," Mike Suffredini, NASA's space station operations and integration manager, told reporters on Tuesday. "We're in no particular hurry to overreact."

The leak was first detected by flight controllers on Jan. 2, but a second look at the recorded data prompted by the discovery showed that the leak may have begun as early as Dec. 22.

Officials say they still aren't completely sure they have a leak because the station's air pressure is not constant, either from day to day or from place to place within the station, but it does seem likely something is happening.

The average daily leak rate is about five times greater than what the station is expected to naturally leak.

"It's a pretty subtle change. Engineers are taking it very seriously," Suffredini said.

Applying pressure

Normally the air pressure is kept at a sea level standard of 14.7 psi. The average pressure now has dropped to about 14.2 psi and officials are close to deciding whether or not they will use a supply of nitrogen to repressurize the cabin.

Suffredini said the station could continue to depressurize down to about 13.9 psi before there would be some concerns with how well the station hardware would work. A drop to 13.5 psi might be tolerated under certain situations, but even that number is not a concern for the health of the crew.

The air pressure would probably have to drop below 10.2 psi before the crew's health would be a concern and they would be ordered home in their Soyuz spacecraft. But the station's systems likely would have failed long before that and prompted a bailout, Suffredini said.

An initial hunt for the source of a leak hasn't turned up anything yet. Foale and Kaleri used an ultrasound measuring tool to "listen" for signs of an air leak around obviously suspect places as hatches and windows.

Some of the next search steps would involve closing hatches between the U.S. and Russian segments, or even individual modules, in an attempt to rule out various locations.

Suffredini said the Russians have formed a commission to look into the air leak. Their top suspect is the Vozduk carbon dioxide removal system which had a history of causing such leaks on space station Mir.

The crew will spend time during the next few days inspecting the Vozduk's many valves, seals and pipes -- some of which lead to the vacuum of space.

Clearing the air

Unrelated to the air pressure concerns, but definitely a variable in the situation, is the availability of oxygen at the station.

The primary system for generating oxygen at the outpost is a Russian device called the Elektron, which takes water and separates it into hydrogen and oxygen. The hydrogen is dumped overboard and the oxygen fed into the crew cabin.

The system has been troublesome and during the past few days completely failed, Suffredini said, prompting the crew to rely on igniting a candle-like combustion device that gives off oxygen as it burns.

There are enough of the candles to last about two months, and there are additional supplies of oxygen in the Quest airlock and docked Russian spacecraft if needed.

Fortunately there also is spare Elektron hardware onboard which the Expedition Eight crew can use to replace the faulty system -- procedures for which Mission Control is working on in preparation for sending to the crew.

Meanwhile, a Progress freighter is being prepared for launch from the Baikonur Cosmodrome at 6:58 a.m. EST (1158 GMT) Jan. 29. It is to carry a fresh load of fuel, food and supplies to the station, docking their two days later at 8:19 a.m. EST (1319 GMT) on Jan. 31.

Should the cargo need to be altered to help solve any technical issues at the station there still is time to work that, Suffredini said.