Project title: Evaluation of Exposures to Diesel Particulate Matter Utilizing Ambient Air Monitoring and Urinary Biomarkers Among Pedestrian Commuters who Cross the U.S.-Mexico Border at San Ysidro, CA.
Completed in: 2013 | Faculty advisor: Michael G. Yost
Background: Walk-in-line pedestrians crossing the U.S.-Mexico border northbound at the San Ysidro Port of Entry (SYPOE), referred to as “Border Commuters,” may be at an increased risk of experiencing elevated traffic-related air pollution, including diesel exhaust (DE). DE exposure has been associated with numerous adverse health effects, particularly cardiovascular and respiratory problems, including lung cancer. “Border Commuters” wait in line for extended periods and stand within 10 feet of highly concentrated traffic, particularly to diesel buses. Understanding the magnitude of traffic-related exposures is important for this vulnerable population. It was hypothesized that “Border Commuters” who reside in Tijuana, Baja California, Mexico and cross SYPOE northbound as a pedestrian will experience higher exposure to traffic-related pollutants than “Non-Border Commuters” defined as those who live and work or go to school in or near San Ysidro, California, U.S.A. and do not cross into Mexico.
Methods: Ninety-one participants were enrolled for this study; 80% were “Border Commuters” and 20% were “Non-Border Commuters.” Questionnaires, time activity diaries, and urine samples were collected from all participants. Of the “Border Commuters”, 56 personal 24-hour PM2.5, 1-nitropyrene (1-NP) - a marker for diesel exhaust – and carbon monoxide (CO) samples were collected. There were 22 at-home indoor and 14 at-home outdoor 1-NP samples collected. Additionally, area samples collected at the border included 35 days of 1-NP, black carbon (BC), CO, fine particulate matter (PM2.5) and ultrafine particulate matter (UFP). Of the “Non-Border Commuters”, 15 personal 24-hour PM2.5, 1-NP, and CO samples were collected. Additionally, 3 at-home indoor and outdoor 24-hour 1-NP samples were collected.
Results: Personal exposure to PM2.5 was nearly 2-fold higher among “Border Commuters” compared to “Non-Border Commuters” (39 ± 30 vs 21 ± 11 μg/m3, p<0.01 Mann-Whitney), while personal exposure to 1-NP was more than 8-fold higher among the “Border Commuters"(1.7 ± 2.3 vs 0.22 ± 0.21 pg/m3, p<0.01 Mann-Whitney). “Border Commuters” had a 3-fold increase exposure to CO than “Non-Border Commuters” (2.8 ± 1.8 vs 0.22 ± 0.21 ppm, p<0.01 Mann-Whitney). Two metabolites of 1-NP were readily detected in urine samples, the most abundant of which was 8-hydroxy-1-nitropyrene (8-OHNP) followed by 8-hydroxy-N-acetly-1-aminopyrene (8-OHNAAP). “Border Commuters” had greater than a 2-fold higher concentration of 8-OHNP (0.071 ± 0.066 vs 0.032 ± 0.021 pg/mL, p=0.05 Mann-Whitney) and a 3-fold higher concentration of 8-OHNAAP (0.063 ± 0.11 vs 0.021 ± 0.013 pg/mL, p=0.11 Mann-Whitney) as compared to “Non-Border Commuters”. Home indoor concentrations of 1-NP were 30-60% of home outdoor concentrations with “Border Commuters” having higher concentrations both indoors (0.64 ± 0.81 vs 0.078 ± 0.075 pg/m3, p=0.04 Mann-Whitney) and outdoors (1.0 ± 0.93 vs 0.27 ± 0.24 pg/m3, p=0.11 Mann-Whitney) compared to “Non-Border Commuters”. Border concentrations of 1-NP weighted by the time spent at the border, total travel given season, and season were all predictors of personal exposure to 1-NP among “Border Commuters”. However, when placed in a multivariate linear regression model total travel given season was the only predictor variable to remain significant. Season was the only predictor for personal exposure to PM2.5 while total travel was the only predictor for 8-OHNP among “Border Commuters.” Median values (interquartile range; IQR) of daily averages for fixed-site measurements made at the border were as follows: 40,000 (24,000-52,000) UFP/cm3, 5 (3-6) ppm CO, 1.3 (0.5-2.6) pg/m3 1-NP, 4 (3-11) μg/m3 BC, 41 (23-57) μg/m3 real-time PM2.5, and 15 (13-22) μg/m3 gravimetric PM2.5. Wind speed was a predictor of gravimetric PM2.5 at the border explaining 22% of the variance. Relative humidity and vehicle delay were both predictors of UFP measured at the border, explaining 13% and 21% of the variance, respectively. However, when modeled together none remained significant. There were no predictors for 1-NP measurements at the border.
Conclusions: This is the first quantitative study characterizing traffic-related exposure to a vulnerable population, indicating that this vulnerable population is indeed at high risk for exposure. “Border Commuters” experience higher exposure to 1-NP and PM2.5 as compared to “Non-Border Commuters”, as determined by both personal and at-home measurements. In addition, traffic-related air pollution exposure among “Border Commuters” within 10 feet of highly concentrated traffic is of great public health concern as concentrations at the border are similar to near-roadway studies that link exposure to adverse health effects. Interventions to reduce border wait times would significantly reduce traffic pollutant exposures in this vulnerable population. However, further work needs to be done to understand the spatial heterogeneity of at-home exposures between the two study groupsURI