Obviously Oxygen isn't poison per se, but too much Oxygen can cause seizures and unconsciousness. Ok, Ok... too much of everything is pretty much harmful...
Read this. Pretty interesting..
"There’s a caustic substance common to our environment whose very presence turns iron into brittle rust, dramatically increases the risk of fire and explosion, and sometimes destroys the cells of the very organisms that depend on it for survival. This substance that makes up 21% of our atmosphere is
Diatomic oxygen (O2), more widely know as just oxygen."
http://www.damninteresting.com/beware-the-dangers-of-oxygen/[/QUE]
Paula, In our atmosphere there is 78% nitrogen..21% oxygen and the remainder is made up of water vapour,carbon dioxide, neon and argon, which neutralizes the serious effects of too much oxygen :

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