This is good news but it still leaves 3.4m people in the UK suffering from the condition

Dr Martyn Partridge, National Asthma Campaign

Doctors found that the incidence of asthma, bronchitis and respiratory illnesses in general increased between 1989 and 1993.

However, their figures showed that cases actually fell after 1993 and have fallen steadily since then.

They said the reversal could not be associated with changes to the management of asthma by GPs or to environmental changes.

Writing in the latest issue of Thorax, they state: "The occurrence of downward trends in asthma and bronchitis cannot be linked to changes in factors which have previously been considered responsible for the increase in the community burden of asthma.

'Less passive smoking'

"Increased exposure to allergens, petrochemical polution, and dietary factors did not suddenly change for the better in 1993."

Dr Douglas Fleming, from the RCGP research unit in Birmingham, said the fall could not be attributed to just one factor.

"We think the reduction may be related to a reduced increased in the number of respiratory infections while clement weather, and less passive smoking, for instance, may have contributed."

Dr Martyn Partridge, chief medical adviser to the National Asthma Campaign, said the fall could be a result of asthma reaching its natural peak.

"Most studies have shown a steady increase in the number of people with asthma over the period of the mid 1980s to early 1990s.

"Clearly that was unlikely to go on increasing, for it is unlikely that the whole population would be genetically susceptible to the disease."

He added: "If true, this is good news but it still leaves 3.4m people in the UK suffering from the condition."