Sorry Tom, after haveing a look at the articles I agree with Fishy, it is toxic. In high concentrations it will suffocate you but in lower concentrations it will mess with your blood chemistry and kill you.
Extract from HSE documentation:
"CO2 has been shown to exhibit a level of
toxicity related to the concentration and time of exposure...
...CO2 is commonly thought of as posing a threat to life through asphyxiation when it displaces the oxygen in air down to dangerously low levels. For CO2 to reduce the oxygen concentration in air down to a level that is immediately dangerous to life, the CO2 concentration would need to be in the order of 50% v/v. Evidence shows, however, that CO2 does create an immediate threat to life at a concentration of only 15% in air due to the
toxicological impact it has on the body when inhaled at this concentration".
I do stand by my comments about CO2 at the concentrations used for fixed fire extinguishing systems - which are typically far in excess of those mentioned as being hazardous in the above documents (your typical portable fire extinguishers are a different matter entirely) - my take from the literature is that a few breaths at anything above 25% will be enough for quite a good proportion of us to be rendered unconscious within seconds. No asphyxiation alarm will help if it takes you a long time to get out of the location, unless you've got immediate access to emergency BA. As I said, I would never enter one of these locations unless a) it was so small that I was absolutely sure that I'd be able to get out within seconds and b) someone competent assured me that there had been no release of CO2 in that location. That's an entirely personal choice - not suggesting that others ought to have the same opinion!
