Why are we not harmed by the 78% of nitrogen in our atmosphere as it is poisonous?

Question: Why are we not harmed by the 78% of nitrogen in our atmosphere as it is poisonous?
- Keywords:
  chemistry,
  earth,
  gas,
  harmful
Asked by higgsboson to Zoe on 11 Nov 2014.
- Zoe Roberts answered on 11 Nov 2014:
  
  Good question. You are correct in thinking that pure nitrogen is poisonous. However the mixture of nitrogen, oxygen, carbon dioxide, argon and other gasses is just right to sustain human life.

Question: Why are we not harmed by the 78% of nitrogen in our atmosphere as it is poisonous?