So, this is a Prometheus-motivated question for context.

Now, carbon dioxide is a natural biproduct of us, and most things I've heard have said that carbon dioxide as a gas isn't dangerous, but since it isn't oxygen, you can asphyxiate when exposed to, say, all carbon dioxide.

However, I can also find some things that talk about Hypercapnia, which is too much oxygen in the blood, and symptoms that correspond to the amount of oxygen in the air. This seems to run counter to carbon dioxide just causing asphyxiation.

Where is the health risk from carbon dioxide? Is there an actual amount of carbon dioxide that is dangerous, or is this health risk simply from presuming that if most of the atmosphere is nitrogen, there's only so much of the rest of the atmosphere that could be oxygen? If we had enough oxygen in the atmosphere, could we conceivably have much more carbon dioxide? For example, if we had 21% oxygen, but only 60% nitrogen and 19% carbon dioxide, say (to go with dramatically higher carbon dioxide than the few percent I see as health risk) would that be something someone could breathe because it has plenty of oxygen still?