Well, the scientists could get together and study and propose. Our task is to understand, scientifically, what is going on. So, since the text starts with air and since we spoke of the atmosphere let us start with that. Yes, I couldn't believe how many particals someone would breathe in every breath of carbon monoxide I am not sure why carbon monoxide is used here. Should be every breath of air .
Carbon monoxide is, of course, an air pollutant.
It is also a necessary product of human biology as well. And it cannot be too hot or too cold either. It is also true that human life requires a small amout of CO2 to stimulate breathing. Molecules are very small. Evenly mixed is necessary.