Is no longer increasing in some countries, but the reasons for this are unclear

A broad consensus exists that in most Western countries the prevalence of asthma increased over the last four decades of the 20th century. This is based largely on repeat studies of school age children. Evidence is emerging that in recent years this trend has flattened or fallen in some countries. For example, as part of the UK arm of the international study of asthma and allergies in childhood (ISAAC), repeat studies found that self reported symptoms of asthma in 13-14 year old children had fallen by about 20% in the United Kingdom between 1995 and 2002.1 This trend was also observed in the health survey for England between 1996 and 2001.2 Over the same period a similar fall in symptoms of asthma in 6-7 year old children reported by parents was seen in Melbourne.3 On the other hand, the only available repeat survey of preschool children noted …