Specific and nonspecific obstructive lung disease in childhood: Causes of changes in the prevalence of asthma

Reversible airway obstruction in childhood includes two major groups of pat ients. those with recurrent wheezing following bronchiolitis in early child hood, and those with allergic asthma, which represents an increasingly larg e proportion of cases through the school years. Over the last 40 years of t he 20th century, allergic asthma has increased in many countries and in rel ation to several different allergens. Although this increase has differed i n magnitude in different countries and also in the social groups most affec ted, it has had several features in common. The increase generally started between 1960 and 1970, has been progressive since then, and has continued i nto the 1990s without a defined peak. Among children 5-18 years of age, the increase has predominantly been among allergic individuals. Theories about the causes of the increase in asthma have focused on two scenarios: a) tha t changes in houses combined with increased time spent indoors have increas ed exposure to relevant allergens, or b) that changes in diet, antibiotic u se, immunizations, and the pattern of infections in childhood have led to a change in immune responsiveness such that a larger section of the populati on makes T(H)2, rather than T(H)1 responses including IgE antibodies to inh alant allergens. There are, however, problems with each of these theories a nd, in particular, none of the proposed changes can explain the progressive nature of the increase over 40 years. The fact that the change in asthma h as much in common with epidemic increase in diseases such as Type II diabet es or obesity suggests that similar factors could be involved. Several line s of evidence are reviewed that suggest that the decline in physical activi ty of children, particularly those living in poverty in the United States, could have contributed to the rise in asthma. The hypothesis would be that the progressive loss of a lung-specific protective effect against wheezing has allowed allergic children to develop symptomatic asthma. What is clear is that current theories do not provide either an adequate explanation of t he increase or a practical approach to reversing the current trend.