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. Also we need to know the relationship between numbers of molecules, vol, pressure and temp. Teachers make many gas laws only to confuse you. But if there is just the tinyest amount of sulfur dioxide, for example, then there may not be much of a health effect. It is not a small animal.