What was the London smog?

The Great London Smog (SMoke + fOG) was the result of coal smoke from fired heating during an unusual cold period combining with the heavy fogs of London. Although smog was not uncommon in London this was an unprecedented event, it lasted from Friday 5 December to Tuesday 9 December 1952. The coal smoke contained sulfur dioxide ans a well as organic (coal tar) compounds which made the mixture acidic and resulted in breathing problems for many. About 4,000 people were known to have died as a result of the fog, this estimate could be considerably loweer than the actual number.