On Wed, 25 Mar 1998 10:10:56 -0700 Justin Crooks <jcrooks@WYOMING.COM>
writes:
>I have seen studies where up to 35% CO2 was mixed with sufficient
>oxygen,
>and breathed comfortably for quite some time.  I think the only danger
>of
>CO2 is that it typically is the byproduct of an oxygen-consuming
>reaction,
>and therefore there is insufficient oxygen to sustain consciousness
>(and/or
>life).

If you could quote those studies, it would be more believable.  I did a
little research, and found that information on C02 is rather scarce in
the industrial hygiene literature, probably because C02 poisoning is not
a major problem.  According to the Conference of Governmental Industrial
Hygienists, Inc's 1980 book "Documentation of TLV's" (Threshold Limit
Values ?), a 7-10% concentration of CO2 will cause unconsciousness within
a few minutes, and presumably subsequent death.  This is based on a 1931
German study.  The recommended limit is 0.5% averaged over 8 hours, and
1.5% for short intervals (15 minutes).  Higher limits (up to 5%) are
considered acceptable for people meeting certain medical qualifications.
Perhaps people who are constantly exposed to high concentrations (e.g.
Biosphere 2 residents) would be able to adapt to a 10% concentration.
Spacecraft are designed for about 0.6% or less.

I found no mention of higher than normal O2 concentration providing an
increase in CO2 tolerance.  From the limited physiology that I know, I
would think that the amount of oxygen (as replacement for nitrogen)
combined with the C02 would have little effect.  Two independent types of
perfusion are at work in the lungs.  One perfusion gets oxygen in, and
depends on the oxygen level in the breathed air being sufficiently high.
The other gets C02 out, and depends on the C02 level in the breathed air
being sufficiently low.  Above a certain level, the body is unable to
compensate for the acidity of the blood caused by dissolved C02.

However, perfusion is driven by partial pressures, so an atmosphere of
35% CO2 and 65% O2 may be tolerable if the total pressure is about 1/4 of
normal atmospheric (not likely to happen in the typical basement "farm"
except maybe in case of a tornado).  This would be roughly the same C02
and O2 partial pressures found in normal-pressure air with 8% C02.

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