The word 'pollutant' originally meant something different, but its continual use has polluted the literal meaning.
I've been a student of the pollutive effects of motor vehicles in particular, ever since the oil crisis of the seventies and the sudden imposition of add-on pollution controls. Superceded later by inbuilt engineering advances.
Some years ago, it was the (motor vehicle) nitrous oxide emissions that were the true pollutants, because those gases combined with atmospheric chemicals to cause toxic smog. Nitrous oxides were the truly bad guys. And sulphur dioxide which ends up as acid rain.
In this context, carbon MONoxide, while a deadly poison, was not a high contributor to pollution in that it was not a major part of the smog problem. The carbon DIoxide emitted is effectivley inert. It does not combine with anything to make it a pollutant in true meaning of the word.
From Wikepedia; The largest part of most combustion gas is nitrogen, water vapor, and carbon dioxide. These are not toxic or noxious. A relatively small part of combustion gas is undesirable noxious or toxic substances, such as carbon monoxide from incomplete combustion, hydrocarbons from unburnt fuel, nitrogen oxides from excessive combustion temperatures, Ozone (O3), and particulate matter (mostly soot). Black soot is nearly pure carbon.
When you see a short burst of black smoke from a diesel train or truck as it moves off, that is almost all pure carbon soot, which quickly falls to earth and is effectively harmless.
Motor engineers have made incredible strides in controlling the output of the smog-causing nasties, although unfortunately such advances are not newsworthy and few of the general public would have any idea.
Another fact never mentioned is that the purest form of carbon itself is....the diamond. A diamond is almost pure carbon and nothing else.
