"David Kazdan" wrote in message m...
Helium is almost exactly the dilutional asphyxiant that nitrogen is. It's used in deep-diving artificial atmospheres
for two reasons: Nitrogen is a weak anesthetic gas, producing intoxication at several atmospheres pressure--(nitrogen
narcosis"), and it's soluble enough in blood and other water-based body fluids to fizz out when the pressure is
released suddenly (the bends, caisson worker's disease). Helium has neither of these properties.

But breath helium in the absence of oxygen and, just like if you breath nitrogen in similar circumstances, you will
become unconscious, have an anoxic seizure, and die. It's the same mechanism as used by the suicide who surrounds
himself with non-burning natural gas in an oven. The people you've seen get away with a breath or two of it on the
residual oxygen in their lungs and blood. They can't get away with it for long, and there are a few deaths per year
in the US from people persisting in breathing toy-balloon oxygen for voice games. There are also a few deaths to
nitrous oxide breathing for intoxication, also for anoxia.

As others have noted, carbon monoxide is a metabolic poison and causes problems even in the presence of normally
adequate oxygen. Carbon dioxide in high concentrations is primarily a dilutional asphyxiant but, again as others have
noted, also has metabolic toxic effects.

David


David Kazdan, MD, PhD
Anesthesiologist
Pilot


Excellent post, David, thank you.