The UK urban “smogs” in the late 19th and the early to mid 20th centuries were mainly composed of sulphur dioxide and smoke and had a marked impact on health. For example, the London smog of December 1952 was responsible for a large increase in respiratory illnesses and an estimated 4000 excess deaths, mainly among older people with heart and lung disease. However, technological, legislative and social changes (The 1956 Clean Air Act, relocation of power stations and improvements in their design, emission reduction technologies and decreased domestic use of coal) have reduced pollution considerably from industrial, power generating and domestic sources. In fact, emissions from these sources have for the most part either stopped increasing or are decreasing.
Unfortunately, the same cannot be said of emissions from petrol and diesel-powered vehicles, which emit a variety of pollutants, many of which are harmful to health. Modern catalytic converters have reduced harmful emissions from vehicles fitted with them, but increasing numbers of vehicles on our roads means that road transport has now become the most important source of air pollution in many parts of the UK and is the dominant source of pollution in cities.
Despite the improvements, days of poor air quality in cities are not quite a thing of the past. Under certain weather conditions, smog can still form. On cold, calm, winter days, for example, pollutants from industry, traffic and domestic sources can become trapped under a “lid” of cold air close to the ground and their concentrations can quickly increase, forming smog.