alt Hacker News1% (10,000 ppm) is enough for the person to become aware something is odd through drowsiness or an elevated heart rate.
I don't think it's too far-fetched for a quarter of that to cause subconscious cognitive effects, that could be measured in tests.
The air in your lungs sits around 40,000ppm or 4% carbon dioxide.
In every breath you remove about 25% of the oxygen from the air in your lungs, which is why mouth-to-mouth resuscitation works, at all. Most of the oxygen is still in there.
To be clear, that 25% represents a change in oxygen level from around 21% to around 16%, so the few tenths of a percent change in carbon dioxide just isn't a huge amount.