Fog, or mist, can be defined as water droplets suspended in the air just over the surface of the ground, in other words a very low level cloud. The water droplets form around minute airborne dust and smoke particles. Smog is where the level of soot and other airborne pollutants is much higher - so blocking out much more incoming sunlight, reducing visibility much more that ordinary fog, and can cause serious respiratory problems.

In medieval London, pollution from coal burning was seen as such a serious matter that a commission was established in 1285 to investigate the problem. In 1307 legislation was introduced to prevent the use of sea coal in kilns and by blacksmiths, but as wood became scarce and expensive London relied more and more on imported coal. By the end of Elizabeth I's reign in 1603, coal consumption in the city had risen to more than 50,000 tons a year. In 1661, the diarist and proto-environmentalist John Evelyn published a diatribe against air pollution in London: Fumifugium, or The Inconvenience of the Aer and the Smoak of London Dissipated. In another book, A Character of England, he wrote that London was

cloaked in such a cloud of sea-coal, as if there be a resemblance of hell upon earth, it is in this volcano in a foggy day: this pestilential smoke which corrodes the very iron, and spoils all the moveables, leaving a soot on all things that it lights; and so fatally seizing on the lungs of the inhabitants, that cough and consumption spare no man.

Before the railways coal was invariably transported to the capital by boat hence the term sea coal in the above quote. The 1746 map by John Rocque shows that the Vauxhall area could boast coal wharves as well as timer yards on the Surrey bank of the Thames.

The travellers who came to London in the early 19th century were shocked by its soot-caked buildings and oppressively fuggy air. Dickens in Bleak House (1852-3) wrote

[There was] fog everywhere, fog up the river where it flows among green aits and meadows - fog down the river, where it rolls defiled among the tiers of shipping and the waterside pollutions of a great (and dirty) city. Fog on the Essex marshes, fog on the Kentish heights. Fog creeping into the cabooses of collier-brigs; fog lying out in the yards, and hovering in the rigging of great ships ... Fog in the eyes and throats of ancient Greenwich pensioners, wheezing by the firesides of their wards; fog in the stem and bowl of the afternoon pipe of the wrathful skipper, down in the close cabin; fog cruelly pinching the toes and fingers of his shivering little 'prentice boy on the deck.

Police stopping traffic for a Fire Engine during a London Fog

In the 1890s, smoke control legislation was embodied in the Public Health (London) Act of 1891, but the Act did not apply to domestic chimneys. In 1905, Dr H A Des Voeux of the Coal Smoke Abatement Society coined the term 'smog' - a mixture of smoke and fog. In 1912, the Lancet estimated that 76,000 tons of soot fell on London every year. In 1926, the Public Health (Smoke Abatement) Act was passed, but once again ignored the problem of domestic fuel

During December 1952 unusual weather conditions created the ideal situations for the formation of a major fog. Things became so bad that a performance of La Traviata at Sadler's Wells had to be abandoned when the audience could no longer see the stage! Pedestrians found their skin and clothing caked with filthy particles, vehicle windscreens were obscured by a film of black slime. Cattle at the Smithfield Show at Earl's Court were taken sick and 13 had to be destroyed. Some 12,000 people died as a result of the fog. The subsequent outcry produced the Clean Air Act of 1956, which, by controlling domestic smoke output for the first time, banished the 'pea-souper' fogs that had become synonymous with London.