“carbon monoxide”
You just gave an excellent explanation for why homo sapiens should not live in large cities.
Notice that the “health experts” never talk about that!
Still the levels of carbon monoxide in polluted urban atmospheres is not really that high and the levels of carboxyhemoglobin in urban non smokers is not statistically significantly different than rural non smokers. Yet if you smoke or use wood to heat your home, the levels of carbon monoxide rise quickly.