The most severe air pollution ever occurred was due to sulphurous smong caused in London on December ,5, 1952 hence the name London Smog was assigned. The mixture of smoke, fog and SO2 that affected London so badly after the introduction of coal as a fuel , is chemically reducing mixture , so it is called reducing smog . Smog conditions prevailed for five days at the cost of death of 4000 people. The causes of death were Penumonia, Bronchitis and allied respiratory troubles. The peak SOX concentration was 1.3 ppm and smoke 4 mg m-3.
Components of London Smog – These are:
(i) SOX (ii) Particulates such as soot, ammonium sulphate etc. (iii) Humidity form fog and aerosols.
Fog Formation – SO2 is the major contaminant accompanied by smoke. SO2 creates a chemically reducing atmosphere because of its oxidizing nature. The reaction occurs in early hours of morning and appears to worsen shortly after sunrise (24 ̊ C) , perhaps because of photolytic and catalytic induced oxidation of SO2 into SO3 followed by combination with moisture to give acidic aerosol . Particulates of smoke from coal combustion provide the condensation nucleation sites on which fog droplets condense. Their large surface areas help in catalytic oxidation of SO2. Actually air near the ground is cooled by conduction. Higher up in the troposphere, the air is not cooled appreciably, so in the absence of winds, there is colder air underneath warmer air when temperature inversion takes place. In this belt of stable air, formation of London type smog occurs.

London smog produces much more harmful effects than the sum total of SO2 and particulates. This constitutes an example of synergistic effect. In fact, there have been several examples of exposure of industrial workers to SO2 at levels much higher than found in London smog without any ill effects. On the contrary, even a low level of SO2 with particulate can be disastrous.
The five day health disaster in Donora Pennysylvania , USA was preceeded by the formation of killer smog due to temperature inversion on Deember 26 , 1948 . This notorious smog was seen in Glass glow , Manchester , UK, Delhi , Mumbai wher sulphur rich coal was used . SO2 combines with H2S and NH3 to produce sulphites which are the main toxicants of smog. Sulphate haze, a green house gas, is known to cause climate cooling but it forms acid and depletes ozone layer . Sulphurous smong causes damage to plants, poses human health hazard and creates numerous corrosion problems.