"CO is certainly combustible"?? - really???
CO (carbon monoxide) is the result of incomplete combustion of methane CH4 which was one of the constituents of coal gas (as detailed by Noreen in 2008). Complete combustion in a proper oxgen supply generates CO2 - carbon DIoxide: less poisonous, but you can still get CO2 narcosis. Yes, CO does bind to haemoglobin very tightly, preventing oxygen transfer and binding, whereas carbon dioxide is much more easily displaced by oxygen to form oxyhaemaoglobin.
The "Dirty Old" bit apllies to the blackening by soot (carbon particles) of everything in the area, as well as to any atmospheric pollution. It wasn't the only Dirty old Town - my home town was called "Auld Reekie" after all!