Researchers at the Air Monitoring Group, part of the National Centre for Environmental Technology, forecast that air quality in Britain should improve over the next couple of days as the wind shifts to bring cleaner air in from the Atlantic.
John Bower, the head of the group, said: 'We do not expect a recurrence of the high levels of ozone that we had over the weekend. We are forecasting better air quality because the meteorology is changing.'
In London, ozone is the main summer pollutant and levels tend to peak in the afternoon - between about 2pm and 5pm. Concentrations of nitrogen dioxide, the other significant contributor to poor air quality, reach their maximum levels in the early evening, after the rush hour.
But the air quality is better this year than in the hot dry summers of the late 1980s. 'In 1989-90, pollution levels were substantially higher,' Mr Bower said, 'and in December 1991, nitrogen dioxide levels were four times higher than we have seen at peak periods in London this summer.'
Ozone levels much higher than London's are being recorded in some parts of the Austrian and Swiss Alps.
Although the weather over the past few weeks has been exceptional for Britain, it would be accepted as the norm in most continental European countries every summer. Yet while Britain worries that the weather and its associated pollution have triggered asthma attacks, few European cities have recorded seasonal summer outbreaks of asthma in previous years.
What has changed is scientists' ability to measure pollutants in the atmosphere and the publication of air quality data. Measures of air quality, updated every hour, are available on Teletext and Ceefax or on a freephone hotline established by the Department of the Environment. A DoE spokesman said: 'It is the most comprehensive system in Europe - the only one that gives hourly bulletins.' But he conceded that, 'because of that, by publishing more information, we have been able to allow people to be more alert and have heightened interest.'