Traffic-related air pollution in the community of San Ysidro, California, in relation to northbound delays at the US-Mexico border

The San Diego/Tijuana US-Mexico border crossing at the San Ysidro Port of Entry is the busiest in the world, with over 13 million personal vehicles crossing from Mexico in 2009. San Ysidro, California, is the US community immediately adjacent to the border crossing. More than ninety percent of San Ysidro residents are Hispanic, and the average household income is less than 60% of the San Diego regional average. With homes, schools and businesses in close proximity to the crossing and to major transportation corridors, San Ysidro is potentially highly affected by border-related traffic. This study investigated delays at the border crossing as a source of traffic-related air pollutants in San Ysidro, especially in relation to wind direction. The air pollutants particulate matter < 2.5 micrometers in diameter (PM2.5), ultrafine particulate matter, black carbon particulates and carbon monoxide were periodically sampled at rooftop locations through the course of one year. Sites include one commercial establishment and two elementary schools in San Ysidro, as well as a coastal reference site approximately 6 miles from the border crossing. Weather data and border delay times were also collected. Results indicate consistently higher daytime concentrations at the near-border measurement site for all pollutants. Pollution concentrations were higher during calm winds or when wind was blowing from the south or east. Black carbon pollution was significantly associated with the border delay times when the wind was blowing from the south or east, but not when the wind was blowing from the northwest. These results may inform planning for future public health and development activities in San Ysidro.