EPIDEMIOLOGY

Ambiguities over the definition of asthma make both estimates of the prevalence of asthma and comparisons between studies difficult to interpret. Comparisons made within a study are, for this reason more informative. In children both asthma and wheeze appear to be commoner in boys with convergence of the rates during adolescence.1,2 There is regional variation in rates within countries and this is particularly marked in developing countries with low prevalence of disease in poor rural communities.3-6 Urban areas have a higher prevalence2 and racial differences have been noted in some studies. In the United States blacks have higher rates of both wheeze and asthma,2,7 but the differences are less clear cut in New Zealand8 and the United Kingdom.9 A lack of any association with social class has been reported from Australia, the United States, New Zealand, England and Wales, though some earlier studies have suggested an association with higher social class and Dawson et al. suggested that more severe disease was more common in poorer children.10