Stephen King writes in Danse Macabre that any good horror story must touch upon a source of realistic horror for the reader by probing his or her own fears. With that in mind, let's take a look at the Killer Fog of London. Most of that smoke was pollution, caused by local factories and open coal fires used to heat houses. Add to that the fact that London had just scrapped all of its electric tramcars and replaced them with diesel buses, and you had a massive amount of pollution that, attached to the fog, settled down on London. Trapped by the surrounding hills and a stagnant mass of warm air above it, it would remain in London for over a week.
Pollution can bring about sickness and death in a myriad of ways.
For example, high ozone levels can cause cardiac arrhythmia. But the main problem with the fog was a more basic one. By adding particles to the air and reducing oxygen, the fog made it harder for people just to breathe. Doctors in London reported people coming into the hospitals gasping for breath. But we do know this.