Why do astronauts need a special suit to breathe?

Human breathing isn't just about having oxygen nearby. Lungs work through pressure differences: gas moves into the blood because the partial pressure of oxygen in the alveolus is higher than in the capillary. In a vacuum that pressure is zero, and the exchange reverses: oxygen escapes from the blood into the lungs and from there to the outside. That's why a mask, like on an airplane, wouldn't be enough; the entire body needs to be enclosed in a pressurized volume.

The suit used for spacewalks from the International Space Station solves the problem with pure oxygen at a pressure of about 29.6 kPa (4.3 psi), roughly a third of sea-level atmospheric pressure. Using pure oxygen allows that total pressure to be lowered without oxygen running short, and a low pressure is key to keeping the suit from stiffening up so that joints and gloves remain workable.

In a sealed enclosure, the first thing to become dangerous isn't a lack of oxygen but the buildup of the carbon dioxide that's exhaled. Concentrations of just a few percentage points already cause headaches, disorientation and impaired judgment. The life-support backpack continuously circulates the gas, passing it through a chemical absorbent that captures CO2 and also controls humidity, which would otherwise fog up the visor.

Lowering the pressure has a physiological cost. Suddenly switching from the station's air to the suit's 29.6 kPa would release nitrogen bubbles into the blood, the same decompression sickness that affects divers. To avoid this, the crew breathes pure oxygen for hours before going out and does light exercise, a pre-breathing protocol that purges the nitrogen dissolved in tissue. Only once that process is complete can the hatch be opened.