Introduction

1. Prior to the late 1960s, domestic burning of coal in the United Kingdom's towns and cities was an important contributor to the dense winter fogs, known as smoke fogs or, colloquially, as smogs. A particularly severe four day episode in London in 1952 drew attention to the effects of such smogs on human health, it being estimated that, during and shortly after the episode, some 4000 excess deaths occurred in the Greater London area as a result of the pollution and associated weather conditions. The severity of this effect was such that legislation, the Clean Air Act 1956, was enacted to control the burning of smoky fuels in towns and cities. Thereafter the air in the cities of the United Kingdom improved and smogs became a thing of the past. For a long time it was believed that the problem of urban air pollution had been solved. However, two factors have combined to make it necessary to review this opinion. First, the increase in volume of traffic in our cities has drawn attention to a different and increasing source of pollution and, second, recent studies in other countries, particularly the United States, have shown that modern urban air pollution may still be causing effects on health, even at concentrations far lower than those recorded during the 1950s and 1960s.
2. The original studies of the effects of the London smogs identified two pollutants, sulphur dioxide and smoke particles, as the ones most likely to have been responsible for the excess number of deaths. It was not possible to separate their effects and British and European legislation subsequently has required both to be measured and their concentrations in the air controlled simultaneously. The two pollutants usually occurred at the same time, since both are produced by coal burning. However, more recently, as motor vehicles have become the major source of urban particulate pollution, oxides of nitrogen have become the main associated urban pollutants while sulphur dioxide has become less important. The effects of sulphur dioxide have been considered in a previous report and in this report we consider the effects of particles.
3. In each of its previous reports the Panel has considered the effects of a single, well-defined chemical substance. This report differs in that it considers a pollutant, characterised by its physical properties, which may be of different chemical constitution depending on its source, and which is always a complex mixture of chemicals. In fact, particles as measured in the air are defined by the method of measurement. Originally, they were measured in the United Kingdom by the "Black Smoke" method, whereby air was drawn through a filter paper and the blackness of the stain measured. This method is still widely used and gives a good indication of the concentration of particles produced by coal burning. It is, however, somewhat less useful for quantifying the particles produced by motor traffic, or those produced by reactions between gases in the air. Increasingly particles are being measured by a method that determines the mass of that fraction which is considered most likely to be deposited in the lung. These particles are called PM₁₀ (Particulate Matter less than 10 µm in diameter¹).
4. In this report the Panel discuss the sources of PM₁₀ and their chemical and physical properties, the method by which they are measured and monitored, and the concentrations currently found in the United Kingdom. We discuss the evidence associating increases in their concentration in the air with adverse effects on health, and conclude by recommending an Air Quality Standard for the United Kingdom intended to reduce the magnitude of such effects.