[rescue] Replacement for canned air?

[Phil Stracchino]

On Thu, Apr 08, 2004 at 02:17:14PM -0600, Geoff Koehler wrote:
> Bottled N2 is not that bad, because it only can dilute the oxygen in
> the air.  Nitrogen is 70% of air, so if you increase that
> concentration to 75% or so (decreasing the relative amount of O2), you
> can handle it for a while before things go black.   Acually, it sort
> of fades to grey, then black.  Its just O2 starvation, but nitrogen is
> not poisonous in itself.  CO2, however, is only 0.35% of air, and is a
> poisonous gas.  Its not as bad as CO, but is poisonous none the less.
> One breath of pure CO2 will knock you out instantly.  Several more
> will kill you. 


This is a common misconception.

CO2 is not poisonous (unlike CO).  The ordinary air you breathe
typically contains, if memory serves, about 0.4% CO2 when you inhale it,
and around 2% by the time you exhale it.  It will, however, asphyxiate
you at comparatively low concentrations because of the way that the
human (well, not just human, any hemoglobin-based) respiratory system
works. The problem is that if the partial pressure of CO2 is more than
about half that of oxygen, CO2 stops diffusing *out* of your bloodstream
in your lungs in favor of O2, and starts diffusing *into* the
bloodstream instead.  You don't die from being poisoned as such; you die
of anoxia.

Nevertheless, the end result is much the same; you end up dead either
way.  The difference is that if you pass out because of a high CO2
concentration, all that's necessary to revive you (before any physical
brain damage occurs, at any rate) is to get you out of the high-CO2
environment and let you breathe clean air again.  The CO2 will diffuse
back out of your bloodstream in a hurry and oxygen will start
circulating again.

With CO, on the other hand, even at extremely low concentrations it
bonds chemically to the hemoglobin in your blood and actively displaces
oxygen from it.  Instead of merely becoming saturated with CO2 instead
of O2, your blood becomes unable to carry oxygen.  The best remedy for
serious CO exposure, iirc, is a hyperbaric chamber with a pure-oxygen
atmosphere.  About 2 bars of oxygen will displace the CO back out of the
hemoglobin and oxidise it to CO2.

Notice what we're saying here; breathing pure oxygen is bloody dangerous
in itself.  It's sorta like those bug sprays that say "Atropine is
antidotal" ... this is don't-scare-the-customer marketingspeak for
"DANGER: CONTAINS NERVE GAS."  Under normal circumstances, you'd never
want atropine in your system because it's such nasty shit, but where
nerve gases are concerned, it's the lesser of two evils.


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