Jon Bower, a government scientist, had problems believing his monitoring equipment - the air was so bad that the the readings had jumped off the scale. The Department of Health said it did not expect the 'very poor air quality' to affect many people because 'the effects are most unlikely to mean that health has been permanently damaged'. By Monday, 16 December, however, up to 160 Londoners had died in the smog and thousands of others had experienced serious breathing difficulties.
Last week, a committee of government health advisers met in secret to hear the results of an investigation into the great smog of December 1991 - the most deadly air-pollution incident in Britain for 30 years. They were told that Jon Bower and his colleagues had detected dramatic increases in toxic compounds from traffic exhausts between 12 and 15 December. One lung irritant - nitrogen dioxide - reached the highest levels since records of this pollutant began in 1976.
'The first thing you say to yourself is 'is this real?',' said Mr Bower, who works at the Government's new National Environmental Technology Centre at AEA Technology in Culham, Oxfordshire. 'The reading went off the top of our computer-graph. We had to make sure it was right before telling the Department of Environment.'
On receiving the reading that Friday, the Department of Environment briefed journalists on the likelihood of very poor air quality over some British cities, saying it was advisable for motorists not to use their cars in London unless it was absolutely necessary. Ministers, however, made no pronouncements about motorists staying at home or using public transport, despite nitrogen dioxide levels from cars reaching a record 423 parts per billion - more than twice the limit thought to be dangerous by the World Health Organisation.
The Department of Health's advice, meanwhile, was reassuring: 'Most people will not be affected,' it said, 'but asthma sufferers and those exercising outdoors may experience coughing, wheezing and shortness of breath. These effects can be reduced by limiting exercise out of doors and avoiding busy streets. It is not necessary to wear a smog mask, and nobody should worry about unavoidable exposure, for example, when going to work . . . It is stressed that 'very poor' air quality is experienced only rarely in the UK, and even at the high levels currently being measured it is not expected that many people will be affected significantly.'
An unpublished scientific study of the four-day pollution incident suggests this was wishful thinking. Professor Ross Anderson of St George's Hospital, south-west London, the main author of the study, says the death rate in London during that week was 10 per cent higher than expected - equivalent to about 160 extra deaths. He worked out the figure by comparing deaths in the same week the year before, and in the week before the appearance of the smog.
He also said that the death rate from asthma and lung disease was more than 20 per cent higher than expected and there was a 14 per cent increase in cardiovascular deaths compared with typical periods in previous years. The implication - although not proven - was that the dramatic increase in air pollution caused or contributed to the deaths of 160 people whose existing respiratory problems had been severely exacerbated. Smog has been part of our culture for generations. The 17th-century diarist John Evelyn campaigned against a 'hellish and dismall cloud of sea-coal', deploring 'with just indignation that this glorious and ancient city should wrap her stately head in clouds of smoke and sulphur, so full of stink and darkness'. Charles Dickens, Claude Monet and Arthur Conan Doyle have all made ample use of a London smog as a dramatic backdrop to mystery and suspense.
But the smog of December 1991 was very different to the peasoupers of the past. The smogs that made London infamous were largely due to the burning of coal, which produced sooty clouds of sulphurous gases. Those of modern times are primarily the product of traffic exhausts, which emit mainly nitrogen oxides and far smaller particles that cannot be seen with the naked eye but which can penetrate deep into the lungs.
There is nevertheless a common factor behind the old and new smogs - the weather. Just as the weather in December 1952, when an estimated 4,000 Londoners died in a sooty smog, was cold and still, so it was in December 1991. On both occasions an anticylone had settled over the country, creating a 'temperature inversion' above the capital. This is when a layer of relatively warm air sits like a lid over bitterly cold air trapped below. Smoke from coal fires or traffic fumes build up because they cannot rise above the warmer air draped like a duvet over the city.
What was exceptional about the incident in December 1991 was the length of time the temperature inversion lasted without any significant winds to blow it and the pollution away. In fact, as Jon Bower and his colleauges found, London was lucky not to have suffered an even worse incident. If it was not for a very brief wind that rose up late on Friday, 13 December, the nitrogen dioxide levels could have risen to an astonishing 700 parts per billion. 'There did appear a little blip in the pollution as a slight wind picked up,' Mr Bower said.
Nitrogen dioxide quickly dissolves in droplets of water in the air to form an acidic mist. When inhaled, the acid irritates the moist lining of the respiratory tissues, causing inflamation and the production of sticky mucus. Breathing becomes difficult and people with asthma, lung disease or heart problems suffer most, sometimes with fatal consequences.
'A lot of people with impaired breathing are normally all right, provided they are not tipped over the edge by pollution,' said Simon Wolff, a toxicologist at University College Hospital in London who gave evidence last week to a House of Commons select committee on the medical effects of traffic fumes.
Dr Wolff, like an increasing number of scientists, is concerned about tiny particles - called PM10 - emitted by car and lorries. PM10 are tiny bits of soot emitted from engines. They are coated in toxic chemicals that are highly reactive, Dr Wolff said. They penetrate deep into the lungs because they are too small for the respiratory system's natural filters to eject them. 'They sit there in the fluid of the lungs causing huge amounts of dangerous oxidation,' Dr Wolff said.
However it is only recently that scientists have begun monitoring levels of the particulates over British cities. But there were no monitors in place to record PM10 levels over London during the December 1991 incident.
Other British conurbations experience smog, particularly in the Midlands and North-west, but on a much smaller scale than London. Fiona Weir, the air pollution campaigner at Friends of the Earth, is critical of the overall monitoring of air pollution in Britain, arguing that concentrations of pollutants in heavy traffic are not being recorded as required by an EC environmental directive. She also criticised the 'complacency' of government departments in giving half-hearted warnings of impending 'poor air' quality. When she rang the Department of Environment pollution helpline recently she was told to turn down her central heating thermostat and press down the lids on any paint tins in the house. 'It's all part of the Government's reluctance to admit that cars are at fault,' she said.
But government officials cannot blame the December 1991 incident on anything other than London's traffic - Mr Bower's monitors have seen to that.