HOUSTON (AP) – International space station astronauts pulled an alarm and donned protective gear Monday after smelling a foul odor that turned out to be a harmful chemical leaking from an oxygen vent, NASA said.

“We don’t exactly know the nature of the spill … but the crew is doing well,” said Mike Suffredini, NASA’s space station program manager. “It’s not a life-threatening material.”

The crew first reported smoke but it turned out to be an irritant, potassium hydroxide, leaking from an oxygen vent, Mr. Suffredini said.

The crew donned surgical gloves and masks but did not have to put on gas or oxygen masks, Mr. Suffredini said.

NASA declared a spacecraft emergency for only the second time in the eight-year history of the station. The first time was for a false alarm of an ammonia spill.

NASA initially said that the crew in the orbiting lab 220 miles above Earth had been working on a Russian oxygen-generating system known as the Elektron. But Mr. Suffredini said no work on the system had been scheduled at that time.

The Elektron was activated at 6:30 a.m. EDT and shut down about a half hour later. Russian cosmonaut Pavel Vinogradov reported the leak to Mission Control in Russia at 7:23 a.m. EDT.

Mr. Vinogradov described the liquid as transparent, “like distilled water.”

“At first, small-sized bubbles escaped, drops, four or five,” Mr. Vinogradov said.

American astronaut Jeff Williams described the smell of burning rubber, but Mission Control in Houston said that odor likely came from the overheating of a rubber gasket.

“That also jibes with the visible smoke coming from the rubber gasket,” Mr. Williams said.

The station’s third crew member is Thomas Reiter of the European Space Agency, who arrived for his six-month stay in July aboard space shuttle Discovery. Mssrs. Williams and Vinogradov are slated to return to Earth at the end of the month.

Because the station’s emergency system was activated, the ventilation system was shut down, but ground operations reactivated it a short time later. Astronauts used a charcoal air-scrubbing device to remove the offensive smell and Mr. Williams said the odor “decreased significantly.”

The potassium hydroxide, a corrosive that can cause serious burns and can be harmful if inhaled, was cleaned up with towels and wrapped up in two rubber bags, Mr. Suffredini said.

Potassium hydroxide can be used to power batteries and is also known as potash lye.

The Elektron system has given the space station headaches before. It had operated on-and-off for months before breaking down last spring. In June, the crew tried to reactivate it, with mixed results, after replacing a hydrogen vent valve outside during a spacewalk.

The failure of the Elektron, which looks like a water heater, had no impact on operations at the space station.

The international space station was in the middle of a revolving door of visitors. Space shuttle Atlantis’ six astronauts departed on Sunday and a Russian Soyuz vehicle carrying two new station crew members and space tourist Anousheh Ansari were expected to arrive on Wednesday.

Early Monday, Atlantis astronauts attached a boom to the shuttle’s robotic arm and inspected for damage to the shuttle’s wings and nose. This is part of the post-Columbia accident routine for shuttles, in which astronauts look for the type of heat shield cuts and tears that caused the fatal shuttle accident in 2003.

The inspection was conducted by pilot Chris Ferguson and astronauts Dan Burbank, a Connecticut native, and Steve MacLean while the shuttle stayed dozens of miles away from the station in the same relative orbit. If the astronauts find the type of damage that could cause a deadly accident, the shuttle can return to the station. Earlier inspections showed the heat shield was in good condition.

At the same time, astronauts examined and tried to fix what may be a minor leaky valve used for dumping water overboard.

Mission Control praised Atlantis for completing its main mission of adding a 17 1/2-ton addition, including a pair of 115-foot-long solar wings, to the space station.