Kottke has a post with an aerial photograph of Edinburgh in 1920. We don't know the time of year or day; we don't know whether the conditions were normal or abnormal. But what it suggests to me is a memory, a memory of the great London smog of 1952 (most recently dramatized in BBC's The Crown and of reading about the PA smog of 1948.
Using coal to heat houses, as we did our house when I was growing up, produced smoke which killed, most dramatically in the right (wrong) geographic and climactic conditions. That problem has been solved, at least for home heating.