11 JANUARY 1963, Page 14


Si,---The figures Mr. Jackson gives (Spectator, December 28) for pollutants produced by motor vehicles are misleading. The major constituent of the exhaust gas is the nitrogen remaining when the oxy- Len in the air is exhausted by combustion of the fuel. Apart from nitrogen the major products of combus- tion are carbon dioxide and water vapour. Because combustion is incomplete, carbon monoxide and hydrocarbons are also present. The concentrations of the respective pollutants vary considerably with the. engine operation, i.e. according to whether it is accelerating, cruising, etc., but normally the propor- tion of carbon monoxide is 1 to 3 per cent. (not 85 per cent.) and of hydrocarbons less than 1 per cent: (not 11 per cent.). Carbon monoxide may increase to over 10 per cent with the engine idling and when the engine needs adjustment, but even so the total volimie of the gas is small. • Oxides of nitrogen are also produced in the com- bustion process but their concentration is nowhere near 3 per cent.; it does not normally exceed 0.1 per cent, and is dsually lower than this.

Although niotor vehicles represent a source of pollution which is on the increase in this country and should not he ignored, it must be borne in mind that the Los Angeles type of smog is Produced by the action of prolonged sunlight on some of the gaseous pollutants. Without a major climatic change, there- fore, such a smog is unlikely to occur in the fore- seeable future in this country.