Particles in the Air: The Enigma of Urban Air Pollution

Abstract

As British cities grew during the Industrial Revolution, so they became increasingly polluted by the smoke from factory and domestic coal combustion. During winter time, episodes of temperature inversion kept the smoke close to ground level and gave rise to the famous smogs of the first half of this century, culminating in the London smog of December 1952, which was associated with some 4000 excess deaths in 1 weekl. Knowledge of an association between cold weather and excess deaths was not new, meteorologists having studied it from the middle of the nineteenth century2, but in the early twentieth century it became apparent that cold alone did not explain the excess and that the concurrence of fogs was a likely additional factor3. The 1952 message that air pollution is bad for you, delivered so close to Parliament itself, had a dramatic effect and smoke control in towns and cities was enforced by the Clean Air Act of 1956. The pioneering Medical Research Council (MRC) air pollution unit was also set up at St Bartholomew's Hospital Medical School under Professor Pat Lawther and an important series of studies of the interactions of air pollutants and health, notably by Lawther and Waller, began4. This was the heyday of the 'English disease', chronic bronchitis, and the researchers concentrated (understandably) on the lung as a target for pollutants. They did, however, recognize that deaths during these episodes were often from heart disease. The legislation was so successful that smoke and sulphur dioxide concentrations in our cities fell progressively through the following three decades, and the MRC unit was eventually closed, its work apparently done. Why the excess deaths occurred has never convincingly been explained but it has been assumed that air pollution episodes, acting as the grim reaper, harvest those who, on account of severe heart or lung disease, are about to die. The closure of the MRC unit occurred at a time of rapid increase in the numbers of vehicles on our roads. As the smokey chimneys disappeared, so the streets filled with cars and lorries and their attendant exhaust fumes and gases. This increase in motor vehicles has accelerated, although fortunately there has not been a corresponding rise in air pollutants because engine and fuel technology has to date kept pace. However, the fall in air pollutants has been checked and the centres of some cities have again become unpleasant places to live in or to pass through during large parts of the day. In appropriate weather conditions, episodes of quite severe air pollution have started to return, although the constituents are not the same as those associated with the urban smogs of earlier years. And epidemiologists have once again shown associations between modem air pollution, disease and death5-7.