[Myrddin]

A mask with pure nitrogen will get the job done just fine. No pain. Unconscious in 30 to 60 seconds. Seven minutes to brain death. Cheap. No physical trauma.

[mamelukesabre]

I like that.

I think you are a bit optimistic with the time though. 30 to 60 seconds? I can hold my breath for 90+ seconds ...or at least I could when I was in my 20s.

[Paleo Conservative]

A mask with pure nitrogen will get the job done just fine.

I don't think that would be legal, because the law requires an injection. Perhaps nitogen bubbles could be injected.