Traffic-related environmental factors and childhood obesity: A systematic review and meta-analysis.

A growing body of research links traffic-related environmental factors to childhood obesity; however, the evidence is still inconclusive. This review aims to fill this important research gap by systematically reviewing existing research on the relationship between traffic-related environmental factors and childhood obesity. Based on the inclusion criteria, 39 studies are selected with environmental factors of interest, including traffic flow, traffic pollution, traffic noise, and traffic safety. Weight-related behaviours include active travel/transport, physical activity (PA), and intake of a high trans-fat diet or stress symptoms; weight-related outcomes are mainly body mass index (BMI) or BMI z-scores and overweight/obesity. Of 16 studies of weight-related behaviours, significant associations are reported in 11 out of 12 studies on traffic flow (two positively and nine negatively associated with PA), five out of six studies on traffic safety (four positively and one negatively associated with PA), one study on traffic pollution (positively with unhealthy food consumption), and one study on traffic noise (negatively associated with PA). Among 23 studies of weight-related outcomes, significant associations are reported in six out of 14 studies on traffic flow (five positively and one negatively associated with obesity outcome), seven out of 10 studies on traffic pollution (all positively associated with obesity outcome), and two out of five on traffic noise (all positively associated with obesity outcome). Our findings show that long-term traffic pollution is weakly positively associated with children's BMI growth, and traffic flow, pollution, and noise could affect weight-related behaviours. Associations between traffic density and noise and weight status are rather inconclusive.