In-vehicle levels of air pollution recorded inside cars commuting across the US-Mexico border at the San Diego/Tijuana border crossing

The border port of entry between San Diego and Tijuana is the world's busiest land border crossing. Many area residents commute across the border by car, and frequently encounter long delays at the border. Exposure to air pollution during commuting can represent a significant contribution to a person's daily pollutant exposure. We measured air pollution inside vehicles crossing the US/Mexico border. Subjects were recruited who regularly commuted across the border (n = 65). Air pollution monitors were placed inside vehicles prior to the commute at Universidad Autónoma de Baja California in Tijuana, and removed at the other side of the border at San Diego State University. We measured fine (PM2.5) and ultrafine (<0.1 um) particulates, black carbon, PAHs, carbon monoxide, and CO2. Vehicle location was tracked using GPS, and participants used a camera, voice recorder and a questionnaire to record conditions during the commute. While waiting at the border, carbon monoxide levels in-vehicle were always significantly elevated as compared to other parts of the commute. PM2.5 levels inside vehicles were approximately twice as high on the Mexican side and in the border wait as compared to the US side. Black carbon and particle-bound PAHs levels reached high peak levels during the border wait, though average concentrations were not significantly higher than during driving in Tijuana. Ultrafine particle concentrations were also elevated when driving. Exposure to air pollution while driving could represent a potentially important source of pollutant exposure for many commuters in the San Diego-Tijuana border region.