In 1945 the Director of Fuel Research in the Department of Scientific and Industrial Research surveyed the problem of atmospheric pollution. He noted ‘a gradual, but very definite improvement in many areas. There is much truth in the statement that the old type of London fog is almost unknown’. His optimism was misplaced. As peace returned the home consumption of coal rose and thin fog still hung over London for between 40 and 60 days a year. This fog was defined as giving visibility of less than a kilometre, and was not what the Director had in mind. He was thinking of the Dickensian ‘London particular’, the pea-souper or smog. * Records of this severe fog — acrid, sometimes evil-smelling, and always dense — go back in London to the seventeenth century. John Evelyn recorded one in his diary for December 1671. Twenty-five such fogs occurred in the eighteenth century, and there were no fewer than 14 in the first 40 years of the nineteenth century. There were some notable fogs, usually in association with exceptionally cold weather, between 1873 and 1892. The worst, measured by the extra mortality from bronchitis in the week after each fog, occurred in 1880, 1891 and 1892. In 1880, deaths from bronchitis were 130 per cent above normal, in 1891, 160 per cent and in 1892, 90 per cent. In the worst year, deaths from all causes were 90 per cent above average. In 1918 there was again fog, but the high mortality then was mostly attributable to the influenza epidemic. In November 1921 there were two days of fog and cold but no obvious extra deaths. In 1924, fog without cold again added litde to average deaths. The cold and fog of 1935 caused a 40 per cent rise in deaths from bronchitis. On this evidence it would seem reasonable to hope that London fog was largely a thing of the .past.