Not a crisis, but action is needed

The haze of photochemical smog that collects over cities has been seen by most people during hot summer spells. Media reports on adverse health effects abound, and in Britain last week the environment minister asked motorists to leave their cars at home. Is this wise advice or overreaction?

The fact that air pollution can be harmful has been accepted in Britain since 4000 excess deaths were recorded during the smog in December 1952.1 The public outcry that followed led to the Clean Air Act of 1957. Air quality improved to the extent that the Clean Air Council was abolished in 1979, the Medical Research Council's Air Pollution Unit was closed a year later, and the monitoring network was largely disbanded.

Classic industrial air pollution was made up largely of smoke and sulphur dioxide from the burning of coal and generally occurred in winter. In Britain this has mainly been replaced by pollution from the combustion of petrol and diesel, which predominantly takes place in vehicles. Vehicle exhausts emit oxides of nitrogen (particularly nitrogen dioxide), carbon monoxide, hydrocarbons, particulates, and lead. On hot, still days these accumulate over cities, together with low concentrations of industrial pollutants, and undergo further reactions catalysed …