Brian Whatcott wrote:


Nitrogen displaces oxygen, so you pass out without warning.

Nitrogen does not *displace* oxygen (O2 weighs more than N2). Excess
Nitrogen will reduce the partial pressure of oxygen. Respiration is
controlled not only on CO2 concentration, but on blood pH and O2
saturation as well. N2 does not bind with hemoglobin, and does not
significantly affect blood pH. As a result, an excess of N2 will cause
further depletion of oxyhemoglobin in the red blood cells (less than 25%
is typically transferred, leaving a reserve factor which prevents your
sudden passing out scenario). This results in a higher respiratory rate
due to low O2 saturation, as well as causing a higher partial pressure
of CO2 (bicarbonate in solution), further lowering the pH of the blood,
and triggering the same reactions as does CO2. CO2 does it somewhat
more quickly is all.

It is not
otherwise toxic. So figure being underwater for a few seconds. That's
as much time as you have with N2.

I'm afraid I don't understand what this is trying to say.

With CO2 you pant.

As you will with N2. Increased CO2 will initiate higher respiratory
rates more quickly than decreased O2 saturation rates will, since the
homeostatic control sensors are more sensitive to CO2.

Both gases diffuse of course.

Which is why, with any reasonable implementation, neither N2 (although
the handling hazards of liquid N2 make it fairly useless IMO) nor CO2
are an issue.

Keith Hughes