That answer may be familiar to enclosed space victims.
The air we breathe consists of several major components. Oxygen, critical for human life makes up about 20.9% of that air. That majority of air is made up of nitrogen – close to 78%! The remaining 1% largely consists of argon (~0.93%) and carbon dioxide (~0.04%). It doesn’t seem that much could go wrong with this atmosphere, but taking away critical components or adding toxic substances can kill us in an instant.