6% CO2 will take out a human. So, the issue there is not just asphyxiation but toxicity. The body wants to expel CO2, but can't due to diffusion.
Halotron, per the MSDS, requires 2% to have issues with cardiac arrhythmia.
Both these gases are heavier than O2, so that can be an issue with enclosed space, as the O2 goes away.
http://amerex-fire.com/upl/downloads...i-f1d41aa3.pdf
Thus, the rule to ventilate well before entering the ER, if no breathing gear.
Of course, fire makes its own cocktail of toxic gases in a space.
Fires are just generally bad.