The body notices &quot;suffocation&quot; through detecting carbon dioxide levels.  Simply replacing oxygen with another inert gas (like nitrogen) does not trigger the suffocation response in the body and so you essentially just exhale all of your oxygen and go hypoxic.  People react differently to hypoxia, some people just pass out while others get the giggles before they pass out.  Then you die.<p>You would not even know it is happening most likely.<p>This is why confined spaces are so dangerous and governed so strictly by OSHA.  A lot of times you will pass out within one or two breaths if you inhale something with very low oxygen levels (&lt;15%) and then you die and anyone who follows you in to rescue you dies.