Smogs usually developed during periods of calm weather. In Britain, November was often the worst month for fogs, especially long lasting thick fogs. During the winter months emissions of smoke and sulphur dioxide pollution were much greater in urban areas than during the summer months due to the burning of mainly coal for heat. Smoke particles trapped in the fog gave it a yellow / black colour and these smogs often settled over cities for many days. Wind speeds would be low at these times causing the smoke and fog to stagnate; hence pollution levels would increase near ground level.

During smog periods the effects on human health were very evident particularly when smogs persisted for several days. Many people suffered respiratory problems and increased deaths were recorded, notably those relating to bronchial causes. One of the major London smogs occurred in December 1892. It lasted for 3 days and resulted in approximately 1000 excess deaths. London became quite famous for its smogs. By the 19th century, many visitors to London came to see the capital in the fog.

Despite gradual improvements in air quality during the 20th century, one of the worst smog episodes in Britain occurred in London on December 4th 1952. The Great London Smog lasted for five days and results in approximately four thousand more deaths than usual. In response to the Great London Smog, the Government passed its first Clean Air Act in 1956, which aimed to control domestic sources of smoke pollution by introducing smokeless zones. In addition, the introduction of cleaner coals led to a reduction in sulphur dioxide pollution.