[Vitani]

Even though they are gasses, Carbon Monoxide is "heavier" than Oxygen, so that's one reason why. Plus Carbon Monoxide(high concentrations of it), can knock you out, which leads can lead to death.

It will lead to death, unless someone saves you!

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http://en.wikipedia.org/wiki/Dihydrogen_monoxide_hoax

The Dihydrogen monoxide is a lie!