Introduction

1. Sulphur dioxide is formed by the combination of one atom of sulphur and two atoms of oxygen. At normal temperature and pressure it is a gas. It dissolves in water to give an acidic solution which is readily oxidised to sulphuric acid. It is one of the gases released into the atmosphere by volcanic activity which, together with the oxidation of dimethyl sulphide released from marine organisms, constitutes the major natural source of sulphur dioxide. However, in the United Kingdom, the predominant source of sulphur dioxide is from the combustion of sulphur-containing fossil fuels, principally coal and heavy oils.
2. Sulphur dioxide was one of the components of the dense fogs that occurred in industrial cities in the nineteenth century and earlier decades of the present century. The combination of this gas and sooty particles was derived from the widespread burning of coal in homes and factories, and it gave rise to the term smoke fog or 'smog' coined by Des Voeux in 1905. Public awareness of the effects on health was increased in the 1950s following the major smog episode in London in 1952. Such episodes were shown to be associated with increased numbers of episodes of chest illness and large numbers of excess deaths, mainly among elderly people with heart and lung disease.
3. The occurrence of these episodes led to the passing, in 1956, of the United Kingdom's first Clean Air Act, with a subsequent reduction in the use of coal as a domestic and industrial fuel in cities and consequent substantial reductions in the concentrations of smoke and sulphur dioxide in the urban atmosphere. In parallel, there was a shift towards the siting of large coal- and oil-burning power stations in rural areas, and this has meant that sulphur dioxide may now be as much a rural as an urban pollutant.
4. Sulphur dioxide is an irritant when it is inhaled, because of its acidic nature, and high concentrations may cause breathing difficulties in people exposed to it. Recent studies have shown that people suffering from asthma may be especially susceptible to the adverse effects of sulphur dioxide and that, within the range of concentrations that occur in pollution episodes, it may provoke attacks of asthma.
5. In this document, the Panel discuss briefly the sources of sulphur dioxide, methods by which it is measured in the atmosphere, the results of monitoring its concentrations in the United Kingdom, and the effects it may have on health. We consider current European limits and World Health Organization guidelines, and make recommendations for a new Air Quality Standard for the United Kingdom. In doing so, we have taken account only of the direct effects of sulphur dioxide on health and have not considered its well-known effects on vegetation, freshwaters and buildings. We believe, however, that acceptance of and compliance with our recommendations will also make a contribution towards ameliorating these other undesirable effects.