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).

For what it's worth...

----------
> From: Morgan Olsson <mrt@INAME.COM>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: ph and CO2 measurers?
> Date: Wednesday, March 25, 1998 8:35 AM
>
> >>concentration of just 10% will cause unconsciousness - remember this
when
> >
> >   Are you confusing this with carbon MONoxide?  Your expired breath
> >   typically contains 5% CO2.
>
> In school I learned 7% CO2 is dangerous.
> CO is *much* more dangerous, rather poison.
>
> /Morgan
> /  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \
> \  mrt@iname.com, ph: +46 (0)414 70741; fax +46 (0)414 70331    /