Asthma: environmental and occupational factors.

Asthma is in several ways a difficult disease to study. Generally arising in childhood, its pattern is often one of remission and relapse; at any point there are difficulties in translating its characteristic, clinical features into an operational definition. Geographical and temporal patterns in its distribution - whereby the disease appears to have increased in frequency in more 'westernised' countries -suggest strong environmental determinants in its causation although there are, too, undoubted and important genetic influences on both its incidence and presentation. Recent aetiological research has concentrated on the function of allergen exposure or on the role of early-life microbial contact that may regulate the development of a range of childhood allergies, including asthma. To date the 'hygiene hypothesis' offers the most efficient explanation for the distribution of the disease in time and place although convincing evidence for it remains elusive.