Deadliest gas on the world?

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Hospitals often make patients breathe pure oxygen as a medical treatment if there's an emergency that requires it. It doesn't kill them. (High concentrations of it are toxic when breathed under pressure though, which is why deep sea diving requires the use of special gas mixtures)

Also oxygen itself is not flammable, no matter what the media say. "Burning" is the reaction of something with oxygen. (For example carbon reacts with oxygen and creates carbondioxide in the process) Since oxygen can't react with itself, it is not flammable. Of course a high concentration of oxygen makes starting a fire more easy.
(source: my chemistry teacher)

In case of the smoker who burned to death, I'd say that the high concentration of oxygen he was breathing created a sudden darting flame from his cigarette which he then inhaled. But what was actually burning wasn't the oxygen itself but the tobacco and paper from the cigarette.