On Earth as in space, people have oxygen in their blood. People also have another element in their blood called nitrogen. On Earth, nitrogen takes up 78 percent of the air we breathe. This gas is harmless when we breathe it on Earth.

In space, however, breathing nitrogen would cause air bubbles in the blood and be very painful. The pure oxygen that astronauts take in during prebreathing flushes all of the nitrogen out. Sunita Williams explains this process in her journal.

From the Journal of Sunita Williams

“In order to go out the door of the ship safely, we go through a pretty thorough process called pre-breathe. In space, if you have nitrogen in your blood, it creates air bubbles. This can cause serious pain. To breathe safely in a spacesuit, there can only be oxygen in the blood. To do this, we spend time breathing in only pure oxygen. We can’t get medical care to help us out. So, in this line of work, we need to try to eliminate this kind of pain caused by air bubbles in the blood.”