Out of the Fifties smog and into the Nineties exhaust fumes

It was probably the worst air pollution incident in British history. For five days in December 1952 a thick, yellowish ''pea-souper'' hung over London. By the time the pall rose, an estimated 4,000 people had died from the smog.

The first sign that something serious was happening came when casualty wards began filling up with people complaining of breathing difficulties and heart problems. The second was when undertakers began to run out of coffins and florists had trouble meeting the demand for wreaths.

The worst comparable air pollution incident in recent times occurred in December 1991, again in London and again during the same weather conditions - still, cold air trapped close to the ground - that helped to cause the 1952 disaster. A similar statistical analysis concluded that about 160 extra people had died of respiratory or cardiovascular problems, probably but not necessarily as a result of poor air quality.

Even during this exceptionally dirty episode, when levels of nitrogen dioxide from car exhausts reached twice the limits recommended by the World Health Organisation, the number of casualties was far smaller than in 1952. Some scientists point out that the figure of 160 extra deaths is just on the limits of what would be statistically significant for a city with a population of millions.

Air pollution in the 1990s is very different from the choking, sulphurous smogs of 40 years ago. Now the principal problems stem from the nitrogen dioxide and extremely fine particles emitted principally from car exhausts. An additional problem in sunny weather is the production of ground-level ozone, an extremely irritant gas for the lungs, as a result of a complex cocktail of chemical reactions produced by the action of sunlight on car fumes.

Jon Ayres, consultant in respiratory medicine at the Heartlands Hospital, Birmingham, and an adviser to the National Asthma Campaign, says there is a lot of ''guesswork'' about what effect this form of pollution has on health. There is no doubt that ground-level ozone can trigger asthma attacks but usually only in sufferers who are already more severely prone to attacks anyway.

Even at high levels of ozone, Dr Ayres says, people do not flood into casualty wards in the way they did in the 1950s. The problem is ''very often overstated''.

The extremely fine particles of today's airborne pollution - 10 millionths of a metre in diameter - are causing an additional health concern. These particles, mostly emitted in vehicle exhausts, are made of a wide variety of materials and can find their way deep into the lungs, where they may cause intense irritation resulting in inflammation and breathing difficulties.

David Coggon, a respiratory scientist at the Medical Research Council's Environmental Epidemiology Unit in Southampton, says that although there is an association between fine particles and respiratory or cardiovascular disease, ''it is not always consistent.

''We're just not in a position where we can say unequivocally that fine particles are the cause of a specific disease. But the evidence indicates that there seems to be a problem.''

The real difficulty with present-day air quality is finding out whether there are serious long-term effects on health. The extra 160 deaths in 1991 could have happened anyway a few weeks or months later because these people may already have been seriously ill.

Air pollutants are not the health risk that some pressure groups make them out to be, Dr Coggon says. ''Some individuals do experience symptoms but the effects are very small compared to, say, the health effects of smoking. The general public has a rather distorted view of the health risk of air pollution.''

The public is genuinely worried about the rise in asthma among children and the corresponding increase in the level of traffic pollution. But the science of cause and effect is not simple and more work needs to be done to resolve the issue. However, whatever the health risks from today's air pollution they are not as great as they were 40 years ago, before clean-air legislation.