Photochemical oxidants are formed through the concentration of a variety of highly reactive gases in the atmosphere and are often implicated in problems of smog, crop damage and the degradation of works of art. Smog is air pollution consisting of smoke and fog. Photochemical 'smog' is composed of a number of toxic compounds, including ozone, nitrogen dioxide, and small particles, which are often referred to as oxidants. They are called secondary pollutants because they are formed in the atmosphere as a result of reactions between certain organic compounds such as hydrocarbons and nitrogen oxides. These two types of substance are the primary pollutants or 'precursors' to the components of photochemical smog. The main types are London smog and Los Angeles smog. The London type consists mainly of sulphur compounds, aerosols and carbon monoxide, leading to bronchial irritation, coughing and a marked rise in mortality. Following radical "smoke-free zone" measures, London now experiences incidences of photochemical pollution-haze, rather than heavy smog. The Los Angeles type is induced by photochemical action of sunlight, consisting mainly of organic compounds, nitrogen oxide and carbon monoxide; it causes temporary eye irritation, but the other effects on humans are not known. Under normal environmental conditions, vegetation can be injured.
As the result of volcanic action resulting in the discharge of gases (especially sulphur dioxide) into the atmosphere, an effect similar to urban smog is created. It can affect people with asthma, allergies and sinus, and can contributes to acid precipitation. Such volcanic smog is also known as vog.
Photochemical smog is now a rather common, regional-scale phenomenon in many parts of the world. The principal man-made sources of photochemical smog are emissions from motor vehicles and photochemical reactions of oxides of nitrogen and reactive hydrocarbons. The severity of smog is most often judged by the ground level concentration of ozone. The results can include possible asthmatic attacks or impaired pulmonary function in diseased people and lachrymation. Damage can occur to materials; visibility is reduced and amenity is affected. Photochemical pollution has been associated with impaired performance of athletes, increased likelihood of car accidents, and absenteeism in work and school. In 1984, the widespread damage to German forests is suspected has having been due to the impact of photochemical oxidants in combination with a wide range of physiological factors.
Urban air pollution is increasingly due to road traffic and is a growing environmental problem in many countries. Epidemiological studies have found statistically significant links between air pollution and indicators of both acute and chronic ill health. The resulting annual health costs and loss of wellbeing have surprised many observers: figures of FF 50 000 million (EUR 7600 million) have been put forward for France, while the figure for Switzerland is Sw.fr. 1600 million (EUR 1000 million).
While urban air pollution is coming under control in some countries, the situation is deteriorating rapidly in many heavily industrialized cities in developing countries. In China, for example, smoke and small particles from burning coal cause more than 50 000 premature deaths and 400 000 new cases of chronic bronchitis a year in 11 of its largest cities (World Bank 1997), and private car circulation has been restricted in some cities in South America and Europe in attempts to reduce harmful levels of air pollution. Worldwide, more than 1 000 million urban residents are exposed to health-threatening levels of air pollution (Schwele 1995).
Urban air pollution problems are reaching crisis dimensions in many of the megacities of the developing world, and the health of many urban dwellers has been impaired.