It's ~420 PPM now. https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_at...

Do you mean to imply something in such a small concentration can’t have any adverse effect - because in that case, would you breathe the same concentration of hydrogen cyanide in air?

(Just a hint - don’t do that)

CO2 is 0.04% of the earth's atmosphere, amazing!

79% Nitrogen

21% Oxygen

0.9% Argon

0.1% Other (includes CO2)