Earlier quoted context omitted.
This page suggests that Inergen (unless there is a thing called Intergen that I'm confusing it with?) mixes with air to lower the oxygen content, but still remain breathable: http://www.tyco.no/products/Gaseous-Fire-Suppression/inergen...
The resultant 12% oxygen is 60% of normal atmospheric air. It's still breathable, if you're an average healthy person with healthy cardiac and pulmonary function, sitting at rest - but it's not healthy, it's enough that you're going to start seeing systemic responses. The reduction in partial pressure of oxygen being sucked into the lungs will also cause an increase in the partial pressure of carbon dioxide and inert…
To put that in perspective, that's like being sent to the top of Pikes Peak (4.3km / 14,000') in seconds. Pilots flying that high in unpressurized aircraft are required to have oxygen masks. Most people will develop altitude sickness when rapidly subjected to that.
When you consider the potential for stress or panic in this kind of scenario, hypoxia emerges as a very real threat even for the young and healthy.