[Risk factors: environment, tobacco smoke]

Environmental factors are usually considered as risk factors for increase of asthma prevalence. They may act isolately but are frequently associated. They act either directly by inducing asthma or more likely by increasing allergenic sensitization. Geographic situation is a well known risk factor. Important differences are noted between countries. The western lifestyle is evocated, including type of alimentation, small size of siblings, increased allergen exposure in houses. Intrauterine environment may play a role, particularly tobacco smoke during pregnancy and its respiratory effects on infant. Maternal allergenic exposure during pregnancy is an important factor because of maternofetal immunologic interactions. Outdoor pollution acts by enhancing bronchial responsiveness, allergenic sensitization and worsening respiratory diseases. Its effect is probably less important in infants and small children who are living indoor most of the time. Infections seems to have a complex action. Some virus, including respiratory syncytial virus, act to induce asthma or sensitization. Other type of infections (viral ou microbial) have a protector effect. Exposure to tobacco smoke, particularly maternal smoking, is identified in all studies, as one of the most important factors to be considered in childhood asthma. Exposure to allergen increases the risk of sensitization. Its direct responsibility to induce asthma is not established. Some of recent studies are suggesting the concept of a protective effect of early exposure. As far as preventive intervention is concerned, the recognition of these factors is important to limit the prevalence of childhood asthma.