Near-road traffic pollutant levels in relation to traffic characteristics in Tijuana, Baja California

Spending a significant amount of time in proximity to vehicular traffic has adverse effects on human health, especially to vulnerable populations such as young children. Concentrations of many traffic-related pollutants decrease rapidly with increasing distance from the road, underscoring the fact that fixed-site monitors for air pollutants in communities may not capture traffic-related exposures important to human health. Recent research reporting near-road levels of air pollution has mainly been carried out in locations such as California where regulatory interventions have improved vehicle emissions; however, few roadside pollutant measurement studies have been done in developing nations. The impact of traffic-related pollution in these nations may be greater since vehicular fleets may be older and more polluting. We measured roadside pollutants at over fifty sites in the city of Tijuana, BC. In addition, to assess the potential exposure to a vulnerable sub-group, we measured roadside concentrations of these pollutants in proximity to elementary schools. The relationship between traffic and pollutants was analyzed by performing twenty-minute simultaneous roadside measurements of carbon monoxide, fine particulate matter (PM2.5), ultrafine particulate matter (UFP, PM < 0.1) and black carbon. Traffic was measured by video recording the sampling session at each site and performing traffic counts. Roadside pollutant levels were extremely high in some areas, for example ultrafine particle concentrations exceeded 100,000pt/cc at some locations. Ultrafine particle concentrations were highly correlated with traffic counts, black carbon but to a lesser extent and PM2.5 least correlated. Carbon monoxide was highly correlated with both ultrafine particles (rho = 0.734) and black carbon (rho = 0.743) making it a possible lower-cost surrogate for near-traffic exposure measurements in epidemiologic and other studies. These findings suggest that traffic related-air pollution may pose a significant risk to Tijuana residents exposed near roadways, especially children, and that public health interventions are warranted.