The Industrial Revolution in Great Britain can be traced back to the 1600s if not earlier. By the mid 19th century, it was well under way, and the steady growth of factories, heavy industry and steel mills in rapidly developing urban areas all had their effect on the atmosphere. In larger cities, the English fog had become the smog for which the country was once so notorious. No heavy traffic as such, but coal fired steam engines of all kinds were to be found, while gas supplies, when they began to be distributed throughout towns and cities, did not take the form of "clean" natural gas, but were made from coal, wood and other similar materials.

I don't know if that helps it "fit the theory," but to a simple chump like me, it looks like it.