The average level of co2 in the world is about 410ppm.
One of the biggest emitters of co2 in the world is 7.5 billion humans just breathing.
Every time we exhale, that exhaled area co2 is at an incredibly high co2 40,000 ppm.
https://en.wikipedia.org/wiki/Carbon_dioxide#Human_physiology
The combined effect of co2 from everyone breathing out does the coal/gas electricity emissions of most small-sized countries.
That's why in your classroom/bedroom with no windows or doors/vents open gets very high and can make you sleepy.
In a concealed area like a car its 4,000ppm (ten times outside).
In submarines etc the max level is set at 8000ppm co2.
https://cdn-images-1.medium.com/max/2000/1*GvZptuCpo6ul_sng7CbPQg.png
All mammals are well adapted to co2 levels (and plants love it for photosynthesis https://youtu.be/0C9yLPbqTVw ), what would be great to see in all vehicles are cars that emit none of the other nasty gases, if they could just emit pure co2 then you could consider them just heavy breathing humans in the world.
A car that emits just pure co2 and nothing else would be a great achievement in its self, believe it or not. We have been over conditioned with co2 when there are plenty of bad gases out there from car emissions that should go away first.
https://www.youtube.com/watch?v=KZN6QuAdxLI
[youtube]KZN6QuAdxLI[/youtube]
https://youtu.be/XVUAgcSCP_U
[youtube]XVUAgcSCP_U[/youtube]