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Safety on the job

Walking in space is one of the most exciting jobs in the world—and one of the riskiest.

Two of the most serious potential dangers facing astronauts during spacewalks are decompression sickness and the loss of pressurization in their suits.

Decompression sickness— or "the bends"— can happen when someone moves too rapidly from a high pressure to a low pressure. It occurs when nitrogen dissolved in blood under high pressure comes bubbling out under reduced pressure. (The nitrogen comes from breathing air, which is 80% nitrogen and 20% oxygen.) The bubbles can collect in joints, causing extreme pain and even death if not treated promptly.

This can happen to divers who surface too quickly from a deep dive and in astronauts as a result of going from a higher pressure in the Space Shuttle to a lower pressure in the space suit.

The Shuttle is pressurized at 14.7 pounds per square inch (psi)—equivalent to ground level pressure on Earth. The suit is at 4.3 psi, which allows astronauts greater dexterity in doing their tasks, especially with their hands, and reduces the fatigue caused by working in an inflated suit. If the suit pressure were higher, it would make their jobs that much harder.

To reduce the risk of decompression sickness, the Shuttle pressure is dropped to 10.2 psi the day before a spacewalk is planned. This causes nitrogen to slowly flush out of the astronauts’ blood. Then, after they get into their suits, they sit in the airlock breathing pure oxygen to eliminate any remaining nitrogen.

This process can last from 40 to 75 minutes, depending on how long the Shuttle was at 10.2 psi. Without this initial reduction in the Shuttle cabin pressure, pre-breathing could take as long as four hours—a wait that would be quite uncomfortable in the bulky spacesuits.

A similar procedure will be followed on the Space Station, except that only the airlock will be reduced to 10.2 psi, leaving the rest of the Station at 14.7 psi. The astronauts who are doing the spacewalk will "camp out" in the equipment storage area of the airlock overnight at the lowered pressure, which will start purging nitrogen from their blood and reduce the time needed for pre-breathing pure oxygen.

Because of the large number of spacewalks needed to build the Station, scientists are investigating other methods, such as exercise, that might further reduce pre-breathing times.