Smog

During the 19^th and early 20^th centuries the Industrial Revolution in Britain witnessed a dramatic increase in air pollutant emissions associated with the burning of coal. Such pollution gave rise to the dramatic smog episodes known as "pea soupers", which became commonplace in many of Britains inner cities. In 1905 Dr. H.A. Des Voeux used the term smog to describe the conditions of sooty or smoky fogs. These smogs were also known as "great stinking fogs".

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 19^th century, many visitors to London came to see the capital in the fog.

Despite gradual improvements in air quality during the 20^th century, one of the worst smog episodes in Britain occurred in London on December 4^th 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.

Today, smoke and sulphur dioxide pollution in cities is much lower than in the past, as a result of legislation to control pollution emissions, and cleaner emission technology. However, the rapid growth of tranpsort in the last 20 years has resulted in a different kind of pollution in urban areas. Photochemical smogs build up during warm sunny days in summer, when air pollutants are trapped at ground level. High levels of ozone, a secondary pollutant, may be produced as nitrogen oxides and hydrocarbons react in the presence of sunlight. Pollution concentrations may become very high, leading to poor air quality.