A69 A systematic review and meta-analysis exploring the associations between exposure to traffic-related air pollution and the development of childhood asthma

Abstract

Abstract Background Asthma is a common chronic condition affecting some 300 million people worldwide, and is the most common chronic illness of childhood. In recent decades, asthma prevalence has increased at a rate which outstrips the pace of genomic variation and changes in environmental exposures are thought to be partially responsible. The association between children’s exposure to traffic-related pollution and the development of asthma remains unclear. Methods This systematic review attempts to clarify this association. EMBASE, MEDLINE and Transport Database were searched for relevant studies published since 1996, using keywords. 2,619 unique references were identified and screened by title/abstract against pre-specified eligibility criteria. Observational studies, investigating the risks of developing asthma associated with exposure to traffic-related pollution, in children under the age of nineteen were included. 61 references were screened using the full paper and 21 met all inclusion criteria. 30 data-fields relating to the populations, exposures and outcomes were extracted from each study. The quality of eligible studies is being assessed against established standards and some outcome measures will be combined in meta-analyses. Results The research area is young, with a substantial part of its evidence-base coming from 3 cohort studies. Overall, the included studies reported conflicting outcomes. Those supporting the notion of a causal link between traffic-related pollutants and asthma found small effect estimates. Asthma definitions varied, yet 11 studies reported physician-diagnosis. Exposure was estimated using various metrics ranging from the crudest road-proximity, employed in 8 studies, to the most sophisticated dispersion-modelling, employed in 4. The majority of the exposure metrics encountered had limited capacity in capturing the heterogeneous spatio-temporal nature of traffic-related pollution, which may have caused an underestimation of the exposure effect. Effect estimates between NO2, the most studied pollutant, and the development of asthma were mostly positive, yet inconsistent and largely insignificant. There was evidence of gender differences. Other susceptibility factors included ethnicity and exposure during infancy. Studies suggested that exposure was associated with asthma even in areas that met the air quality standards. Conclusions A limited number of studies investigated the contribution of traffic-related pollution to the onset of childhood asthma and what is now known about these associations may only be part of the picture. Although effect estimates for traffic-pollutants tended to be generally low, the risk posed on populations is likely to be significant considering the large proportion of people exposed to traffic. One explanation of the pattern of NO2 effects is that it could act as a marker of a mixture or of more toxic traffic-pollutants, for example ultra-fine particles. Multidisciplinary collaboration bringing together more refined exposure metrics and health data should identify more robust associations.