Deadliest gas on the world?

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It's far more dangerous to try to ignite hydrogen than it is to ignite oxygen. You'd need an absurd amount of oxygen concentrated in your lungs to become ignited from the inside. Having a high concentration of hydrogen in your lungs would probably cause your chest cavity to burst.

All speculation, though, as I have no concrete proof that you'll explode.

Well, yes, Hydrogen and Oxygen together are a highly explosive mixture that should be kept away from sparks. (It won't explode on its own though, it still needs initial energy) Hydrogen alone without any oxygen around will not explode, not even with a spark, because there is no second "partner" for the chemical reaction.
Breathing it is not really that dangerous apart from the risk of explosion btw, I've seen my chemistry teacher do it as a trick because it makes you speak funny, just like helium ^^

Also, yes, when Hydrogen(H2) and Oxygen(O2) react (in an explosion), the result is H2O - water. I shit you not, water can be created in an explosion.