Aerosol particles generated by diesel-powered school buses at urban schools as a source of children’s exposure
Research highlights
► In this study, the indoor and outdoor aerosol at four urban elementary schools serviced by diesel-powered school buses was characterized with respect to the particle number concentrations and size distributions as well as the PM2.5 mass concentrations and elemental compositions. ► The presence of school buses significantly affected the outdoor particle size distribution, specifically in the ultrafine fraction. ► Significant associations were observed between indoor and outdoor aerosols for EC, EC/OC, and the total particle number concentration.
Introduction
Exposure to traffic-related particulate matter (PM) has been associated with various health effects, especially in children, including respiratory allergies, decreasedlung function, bronchitis, and exacerbation of existing childhood asthma (Brauer et al., 2007, Delfino et al., 2004, Gauderman et al., 2007, Gehring et al., 2010, Kim et al., 2004, McConnell et al., 2010, Morgenstern et al., 2008, Nordling et al., 2008; Ryan et al., 2005; Shima et al., 2002, Trenga et al., 2006). Diesel-engine exhaust is also listed as a potential carcinogen by the U.S. EPA, 2002a, U.S. EPA, 2002b. Particles aerosolized by diesel-powered vehicles are mainly in the ultrafine size range (<0.1 μm), which makes them capable of penetrating the lower airways (Morawska and Salthammer, 2003a) and translocating through the blood-brain barrier (Hartz et al., 2008).
A nationwide survey has shown that more than 30% of public schools in the US are located in close proximity (≤400 m) to major roadways, resulting in a potentially increased health risk (Appatova et al., 2008). In the Greater Cincinnati community 38.5% of public schools are located within 400 meters of major roads (Appatova et al., 2008). This distance also represents the area of elevated exposure to ultrafine PM (Martuzevicius et al., 2004, Morawska and Salthammer, 2003b, Reponen et al., 2003, Zhu et al., 2002a, Zhu et al., 2002b). A recent report by Richmond-Bryant et al. (2009), which established associations of PM2.5 and elemental carbon (EC) concentrations with traffic, idling, background pollution and meteorology during student dismissals at four New York City schools, singled out diesel idling and passing as significant factors contributing to EC and fine particles measured in the school vicinity.
More than 24 million American children are transported to and from school daily by over 600,000 buses most of which have diesel-powered engines (U.S. EPA, 2008, Wargo et al., 2002). Children are also exposed to freshly emitted air pollutants when boarding and riding school buses. A recent case study demonstrated that school bus traffic significantly increases the total particle number concentration as well as the concentrations of diesel-associated elements in outdoor air in the school vicinity (Li et al., 2009). The effect is especially pronounced during arrival and dismissal times when dozens of buses may arrive and depart from the school within a short time. Temperature inversions, especially during the morning hours, and cold ambient air in winter may also increase traffic-related pollutant levels (Kim et al., 2002, Zhu et al., 2004, Ning et al., 2007). In cold weather, drivers often allow engines to idle for considerable time periods thus increasing emissions. Diesel engines emit 10 times more particles per mile than conventional gasoline engines and 30–70 times more than gasoline engines equipped with catalytic converters (OECD, 1988). To reduce the exposure of children riding the school bus to diesel exhaust particles (DEP), the U.S. EPA supports programs such as the Clean School Bus USA. Exposures to DEP are also being reduced through nationwide and local efforts towards retrofitting school buses with diesel oxidation catalysts, increasing the use of low-sulfur diesel fuel, and establishing anti-idling campaigns. These initiatives, however, are far from full implementation.
Although children’s exposure to DEP generated by school buses has recently received considerable attention, there is still a lack of information regarding characteristics of aerosol in the school vicinity. Very few studies have addressed the outdoor air quality at schools whose student population is transported by “yellow buses.” Additionally, there is a lack of data regarding the association between indoor and outdoor aerosol characteristics in schools. In order to address this need, a public health partnership was formed between the University of Cincinnati and the Cincinnati Public Schools and Cincinnati Health Department to develop and implement a community-driven anti-idling campaign. The objective of this report was to establish the characteristics of indoor and outdoor aerosol at urban elementary schools, which use diesel-powered school buses for transporting students prior to the implementation of the anti-idling campaign. The aerosols were characterized with respect to their PM2.5 mass concentrations and elemental compositions as well as the particle number concentrations and size distributions. The association between indoor and outdoor aerosol concentrations was investigated at each school with respect to the particle number concentration, PM2.5, elemental carbon (EC), organic carbon (OC) and other relevant elements. Factors such as the school proximity to major highways, school building characteristics, the number of diesel-powered school buses, and the number of other vehicles during drop-off and pick-up times were also considered.
Section snippets
Materials and methods
Four urban public schools – further referred to as I, II, III, and IV – were selected for this study in four different neighborhoods of the city of Cincinnati, Ohio, USA. The schools were a priori chosen to maximize variability in exposure to buses and nearby major highways. As seen from Table 1, three of the school buildings were built in 2005 or later and one was built and renovated in 1967. Schools labeled I and IV were within <1000 m of several industrial operations and/or wastewater
Size distribution and number concentrations of aerosol particles
Fig. 1 shows representative examples (one per school) of the outdoor particle size distributions measured immediately prior to (dotted line) and during (solid line) the afternoon pick-up time period. Each curve represents a 2.5-min integrated snapshot. In spite of similarities, some differences were observed suggesting the influence of other – stationary and mobile – combustion sources in the areas. At all four sites, presence of school buses affected the outdoor particle size distribution in
Summary and conclusions
The indoor and outdoor aerosol at four urban elementary schools that use diesel-powered school buses was characterized with respect to the PM2.5 mass concentrations and elemental compositions as well as the particle number concentrations and size distributions. It was determined that presence of school buses affected the outdoor particle size distribution, specifically in the ultrafine fraction. The total number concentration of the outdoor aerosol at schools was significantly associated with
Acknowledgement
The research team would like to thank the participating Cincinnati Public Schools. This study was funded in parts by the National Institute of Environmental Health Sciences, grant number NIEHS R21ES017957, and the pilot grants from the University of Cincinnati’s Research Council and the Center for Sustainable Urban Environments.
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