At the height of the epidemic, some London hospitals ran out of medication for asthmatics. Startled doctors at one hospital at first thought there had been an escape of poison gas, as at Bhopal in India 10 years ago.
'We suspect that this is the biggest documented outbreak anywhere in the world,' said Dr Virginia Murray, of the National Poisons Unit at Guy's and St Thomas's Hospitals. Asthma experts believe that air pollution may have been partly responsible, though unusually high pollen counts must have played a role.
Doctors and public health officials have joined forces to investigate how many people experienced acute breathing difficulties - and how many died - in the outbreak, which they say stretched 'from Bristol to Cambridge and Barnsley to Canterbury'.
One of the group, Dr Martyn Partridge, said that 'hundreds upon hundreds' of people had to go to hospital, but that this would have represented 'only the tip of the iceberg' of those affected.
One in seven children in Britain now has asthma - the only treatable, chronic disease to be advancing in Western countries - and 2,000 Britons die of it each year. Scientists are convinced that air pollution by ozone and nitrogen dioxide, mainly from car exhausts, is exacerbating the epidemic.
This summer's unprecedented outbreak began after violent thunderstorms on 24 June. Dr Murray said that the National Poisons Unit was called in after 55 people attended the accident and emergency department at Whipps Cross Hospital, east London, during that evening and the next morning. The hospital thought there might have been a leak of poison gas.
The unit discovered that there had been a similar rush of cases - 10 times the average - across London during the same period. There were 96 emergency cases at Newham Hospital, 91 at King George's Hopital, Ilford, and 62 at the Royal London, Whitechapel.
Further investigation revealed there had been similar outbreaks over much of the country. Several hospitals ran out of nebulisers and medicines for treating asthma. 'Accident and emergency units had to cope with the equivalent of a plane crash near every hospital over a wide area,' said Dr Partridge. Similar but smaller outbreaks had followed thunderstorms in the past. There was one in Birmingham and two in Melbourne, Australia, during the 1980s.
Dr John Ayres, consultant chest physician at Birmingham's Heartlands Hospital, who investigated the city's outbreak, says that certain thunderstorms after periods of dry weather seem to release highly allergenic spores and pollen. He believes high pollen levels and air pollution before the storms may make people more susceptible to bad attacks of the disease.
Dr Malcolm Green, chairman of the British Lung Foundation and a chest consultant at the Royal Brompton Hospital, said: 'An astonishing outbreak like this means that something serious is going on. A likely scenario is that high pollution levels in the preceding weeks sensitised people's lungs.
'We have had plenty of thunderstorms in the past which have not led to such episodes. There must be other factors. Air pollution has been increasing, and this is a warning that we must do something about it.'
Concern is growing about air quality in Britain. It is only a month since it was revealed that on one day in December 1991, freezing smog killed 160 people in the London area. On that occasion, too, hundreds more experienced breathing difficulties.
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