[Air pollution and asthma in children]

The prevalence of asthma and allergic diseases has increased world-wide during the last quarter of the 20th century, particularly among children and adolescents. No change common to all sites where asthma has increased throughout the world has been identified, suggesting that this 'epidemic' phenomenon is likely due to multiple factors. The following have been most discussed: exposure to indoor and outdoor allergens, modification of the patterns of respiratory infections, decreasing trends of physical activity, evolution in the make-up of environmental irritants, including tobacco smoke and urban air toxicants. In this review, we point out the role of exposure to air pollutants, in addition to and in combination with other asthma enhancers or precipitators. Whereas concentrations of the 'classical' air quality indicators (SO2, CO) have more or less steadily decreased, asthma prevalence augmented in developed countries during the same period. However, the nature of the air pollution mix has deeply evolved, and should also be considered. Ambient air concentrations of industrial and house heating combustion sources of pollutants in the city have substantially decreased, but by contrast the concentrations of various ultrafine particles have increased. Now, there is in vitro and in vivo evidence that exposure to urban air particles, and particularly to diesel exhausts, elicits chronic oxidative stress and repeated inflammatory responses, so that they may enhance allergic inflammation and airway hyper-responsiveness. Several epidemiological studies suggested an association between traffic density close to places of children's residence and prevalence of respiratory symptoms, and more specifically of asthma or allergic rhinitis symptoms in them. Chronic exposure during infancy to traffic-related pollutants may accelerate or even provoke, among genetically sensitive subjects, disruption of the normal regulatory and repair processes eventually contributing to the increase of asthma incidence.