ASTHMA IS A worldwide problem. The prevalence of asthma in children varies from 0 to 30 per cent in different countries. There is strong evidence which suggests that the prevalence is increasing, but the available data is insufficient to determine the likely causes for this increase, or for the variations among countries.
The increase in the prevalence of asthma may be due to changes in the indoor or outdoor environment, especially allergens in the air, particularly domestic mites and occupational allergens. The presence of mites in the indoor environment and the insulation of houses are other important causes. The presence of a large number of insects like moths or cockroaches can also trigger asthma symptoms. Climate is also an important factor because it directly affects the amount of allergen present in the environment. For example, a damp and warm climate favours the growth of mites and moulds.
The increase in the prevalence of allergy and asthma may also be due to air pollution or tobacco smoking. Passive smoking in infancy can also cause allergic sensitization to common allergens.
Urbanization appears to be correlated with an increase in the prevalence of asthma in some populations. For example, in the more polluted cities of Sweden, the prevalence of allergy has increased. Similar observations have been made in Chile and in South America where school children living in heavily polluted areas are more susceptible to asthma than those living in less polluted areas. The role of air pollution in causing asthma and allergy was studied by comparing the prevalence of asthma and other allergic disorders in school children living in two German cities: Leipzig with its heavy industrial pollution, and Munich, with its heavy automobile traffic. Asthma and allergy were more prevalent in Munich, while bronchitis was more prevalent in Leipzig.