A period of unusually cold weather, combined with an anticyclone and windless conditions, collected airborne pollutants—mostly arising from the use of coal—to form a thick layer of smog over the city. It lasted from Friday 5 December to Tuesday 9 December , then dispersed quickly when the weather changed. It caused major disruption by reducing visibility and even penetrating indoor areas, far more severely than previous smog events, called " pea-soupers ". Government medical reports in the weeks following the event estimated that up to 4, people had died as a direct result of the smog  and , more were made ill by the smog's effects on the human respiratory tract. More recent research suggests that the total number of fatalities may have been considerably greater, with estimates of between 10, and 12, deaths. London has suffered since the 13th century from poor air quality  and this worsened in the s.
Urban air pollutants arise from a wide variety of sources although they are mainly a result of combustion processes. The largest source in most urban areas is motor vehicles and industry. The impact of transport on urban air quality will be looked at in more detail in Section 4, whilst the impact of industry will be studied in Section 5. Fact File 3 identifies some of the urban air quality issues today.
Imagine smog so thick that you can't see your feet as you walk through it; so impenetrable that it blots out the sun; so toxic that it stings your eyes and leaves you gasping for breath. It may sound like the backdrop to some post-apocalyptic nightmare, but on 5 December , this terrifying scenario became the reality for the people of London. On that cold, clear day in , Londoners huddled around their coal fires for warmth. But while the smoke would normally disperse into the atmosphere, an anticyclone hanging over the region created an inversion — trapping the pollution close to the ground and leading to the formation of a sulphurous, toxic shroud that would blanket the capital for the next five days.
Air quality: particulate pollution, photochemical smog and pollution reduction policies. The quality of air within urban areas is often of a much poorer quality than that of surrounding rural areas. This is particularly pronounced in industrial cities and in cities within LICs and LDCs where environmental standards applied to heavy industries and vehicles are much lower than in richer nations.