Yes, Polycarp, “by the time it’s poisonous you’re already dead” is a very useful definition. 
There are still grey areas re:cause and effect of COPD narcosis. Though KarlGauss stated things differently than I did, there are still some gaps in knowledge. My teaching in med school was COPD patients with high levels of pCO2 relied on high CO2 to the baroreceptors and medulla oblangata to keep breathing. This turns out to be oversimplistic, and KarlGauss is more correct when he reverses my statement.
More than you want to know…
http://emj.bmjjournals.com/cgi/content/full/18/5/333
There was a horrifying incident with CO[sub]2[/sub] in the 1980s in Africa. A lake had a huge amount of CO[sub]2[/sub] dissolved in the water, a test tube of it was filled with bubbles, it was natural seltzer. I think because of decomposition of carbonate rocks in some stratum of the lakebed. In the middle of the night, an enormous bubble of CO[sub]2[/sub] belched out of the lake and because it was heavier than air it moved over the ground and killed everyone in a nearby village in their sleep. A whole village silently wiped out all at once.
Psychonaut.
I’ve been exposed to a .3% concentration for extended periods while on an FBM submarine. This is not enough to be immediately bad for you but the people on the boat would notice when it got this high.
A lot of people would get headaches and some others, including myself, would be unable to sleep deeply. You just tend to doze. Concentration also suffers.
These are all very subjective but they were widely reported.
There was also an underwater urban legend that the excess CO2 made your blood acidic and it would start dissolving calcium out of your bones. As I say, that was probably nothing more than an urban legend for people underwater. 
I hope that was what you were looking for.
Regards
Testy