Jose wrote:

There was an earlier message concerning somebody breathing nitrogen by
accident (I presume at sea level) and going into immediate convulsions.

Nobody said immediate.

The point is you can't entirely empty you lungs by just exhaling, the
human body just isn't built for this. So there's still oxygen in the
lungs even when you take one or two breathes of pure helium (or
nitrogen, for that matter). If however you continue to breathe pure
helium, you will loose consisiousness pretty quickly.

If the environmental pressure falls, this is an entirely different
situation.

Stefan