Asthma.

From an epidemiological point of view four issues are briefly emphasized here: the definition of asthma, time trends and regional differences, and risk factors for asthma. Furthermore, I will focus upon a few aspects regarding the relation between exercise and asthma. The definition of asthma has presented problems for those involved in studying the disease in general populations, where the estimates rely solely on reported asthma and respiratory symptoms. A standardized questionnaire does not fully overcome differences in languages and interpretations of the concepts of asthma and wheeze over time and in different communities. Although the belief of an upward trend of childhood asthma seems to be widely accepted, studies with the proper methodology to investigate this issue are scarce. The prevalence of asthma varies within and between countries. There is no clear urban rural gradient, and the prevalence of atopy appears to be higher in western than in eastern Europe. Amazingly little is still known about genetic and environmental factors that are causally related to the manifestation of allergic disorders in previously asymptomatic individuals. The effect of exercise on asthma may be many-sided. Children with asthma can actively take part in sport. However, to what extent extensive exercise may affect the occurrence of bronchial responsiveness is unclear, and among athletes exercise may be a risk factor for the development of "athlete's asthma". In conclusion, epidemiological studies have to date not reached their potential due to the lack of longitudinal studies including objective measures of exposures and health outcomes. Causal inference is limited, and further studies designed to answer specific research questions are needed.