The presence of large quantities of nuclei leads to the formation of fog as temperatures fall and relative humidity increases. During the worst conditions in cities such as Lon- don, the droplets of dilute sulphuric acid were covered film of oily impurities that gave the by a urban fog its particular taste and colour (the pea- souper). Although pollution concentrations within the smog largely emanate from local p sources, the wind circulation can spread the pollutants urban throughout the city. This because of the occurs urban heat island over the city centre which pro- duces rising air (a low-pressure area) into which cooler air from the outskirts is drawn. The smog wil remain for as long as the anticyclone persists. Stationary sources of pollution are the main causes of smoky sulphurous smogs, but the auto-mobile is chiefly responsible for photochemical smogs, which are a product of the action of sunlight on hydrocarbons and oxides of nitrogen present in vehicle exhausts. The photochemical smog is com- posed of ozone, aldehydes and peroxyacetyl nitrate (PAN). Collectively referred to as oxidants. these cause eye irritation and coughing. visibil- reduced ity and damage to vegetation. Athens experiences some of the worst trafic pollution in Europe with the n an ochre-coloured cloud containing dan- gerous levels of nitrogen oxide and ozone, while in California 97 per cent of populated areas have experienced reduced visibility and 70 per cent eye 49 irritation The motor vehicle is also the chief cause of lead pollution in the urban atmosphere. Lead i extremely toxic and accumulates in the body more rapidly than it is excreted, leading to damage to kidneys. It has also been implicated in slow-learning problems in children. Between 20 per cent and 50 per cent of lead entering the body comes from the the air we breathe, and people living near roads typically exhibit higher levels of blood lead than elsewhere in the city. Many governments have now introduced legislation to reduce the lead content in petrol. There however, a range of other metals are, eg. mercury, zinc, chromium and cadmium) whose potentially harmful effect on human beings has yet to be examined adequately