I’ll take a wild stab at a 777-200 cabin being ~1000m3. 0.5% of that is 5m3 which is equivalent to ~10kg of CO2 at STP, should you want to get to that concentration. This assumes it all stays in cabin while the concentration is building up, which with the packs and/or fans running is highly unlikely. To get to a point where you’d physically notice it, you’d need 5-10x as much, which is a s@%# load of dry ice!
In other words, yes, we are aware of CO2 and it appears in Dangerous Goods legislation and is subject to the constraints thereof, but no, it’s not a major source of death in the sky. Carbon monoxide, OTOH...