Few disasters can have led to such great good as the four-day London "smog" of December 1952. Though its extreme concentrations of smoke and sulphur dioxide appear to have caused up to 4000 extra deaths,1 it provided the impetus for the Clean Air Act 1956. Many lines of evidence suggest that the pollution so strikingly reduced by this Act was associated with both acute and chronic illness.2 Air pollution has many components (see box) and may be of different types in different places.3 In this article I deal with smoke and sulphur dioxide pollution,4 mainly in Britain, and in the next I will touch on other pollutants. Severe fogs had occurred before in Britain and elsewhere; and smoke had polluted cities for centuries?in fact, the problem was past its worst well before 1952.5 In the eighteenth century Benjamin Taylor lectured about "the clouds of Smoke, Vapour, and corrupted Effluvia," correctly pointing to the importance of temperature inversions, the incomplete com? bustion of coal, and "all the Sooty particles . . . and cloud of Smoke, which if continued, ... its caustic quality would seize upon our lungs, restrain their vigour, and at last, render them totally incapable of their office."6 A century earlier the more famous John Evelyn ascribed the ill health of Londoners to the "fuliginous and filthy vapour"7; while a commission was set up to punish those who burnt coal in the City of London as long ago as 1307. Only in the present century, however, could air pollution become an important cause of illness and death: earlier its effects, though suspected, were overwhelmed by those of infectious diseases.5 Moreover, potentially lethal effects of intense fog would not be so apparent until there was a large population of susceptible people: by 1952 antibiotics enabled many people to survive with precarious respiratory and cardio? vascular systems; and it was these apparently, not the fit, who were killed by the fog.1