PURPOSE OF THE STUDY.
To summarize the literature on the effect of childhood exposure to traffic-related air pollution (TRAP) on the development of asthma and allergies.
Birth cohort studies that examined the association between TRAP exposure in early childhood and development of respiratory or allergic outcomes.
Medline, Embase, and ISI Web of Science databases were searched using key words related to TRAP and respiratory or allergic outcomes and data were extracted using a standardized form. Only English-language studies were considered. Meta-analysis was done to analyze the association between exposures to various components of traffic, including PM2.5, NO2, and “black carbon” and the outcomes of asthma incidence and allergic sensitization separately. Analyses were further stratified by the age at which asthma incidence was measured.
Nineteen articles describing 11 birth cohorts fit the inclusion criteria. Increased exposure to NO2, PM2.5, and black carbon showed significant associations with increased asthma incidence, especially at younger ages, but there was substantial variability between studies, particularly for the studies of NO2 and PM2.5. Increased exposure to NO2 and PM2.5 was also associated with increased risk of sensitization to outdoor, but not indoor, aeroallergens, again with substantial variability between studies. Meta-analysis of the association between TRAP and hay fever, eczema, or wheeze could not be performed.
Increased exposure to TRAP was found to be associated with increased risk of incident asthma and allergic disease, but substantial heterogeneity was found between studies. The authors call for more large, long-term studies investigating the effects of age of exposure on these outcomes and looking at long-term outcomes.
Traffic-related air pollution is a known risk factor for asthma and allergic diseases, and this study confirms that the weight of the literature suggests that specific components of TRAP are related to incident asthma and sensitization to outdoor aeroallergens. Some of the studies showed that respiratory effects can occur at levels of these pollutants below guidelines recommended by the World Health Organization, suggesting that regulation may need to be focused on further lowering TRAP below current thresholds. Future research is needed to identify thresholds for these and other pollutants where health effects are expected.
- Copyright © 2015 by the American Academy of Pediatrics