Estimation and characterization of children’s ambient generated exposure to PM2.5 using sulphate and elemental carbon as tracers
Introduction
Concentrations of ambient fine particles have been shown in a multitude of studies around the world to be associated with significant health effects (Pope and Dockery, 2006). An underlying assumption of these studies is that the variability in measured ambient concentration is a valid surrogate for variability in personal exposure (Mage et al., 1999, Ott et al., 2000, Wallace, 2000, Wilson et al., 2000). However, for a given ambient concentration, people may be exposed to different levels of this pollutant depending on their location and daily activities. A small number of studies have separately assessed the health impacts of the ambient and non-ambient components of PM exposure. In general, these studies have indicated more pronounced associations with the ambient generated component of personal exposure to the PM mixture (Ebelt et al., 2005, Koenig et al., 2005, Strand et al., 2006) and suggest a need to specifically determine the ambient generated contribution to exposure in areas where PM levels are problematic. Comparisons of the relationship between ambient concentrations and the ambient and non-ambient components of exposure between communities may help explain heterogeneity in epidemiological effects estimates (related to ambient concentrations) between cities. While such analyses, or studies of the contribution of ambient generated sources on indoor environments, have been conducted for a limited number of locations (Allen et al., 2003, Allen et al., 2004, Hänninen et al., 2004, Hopke et al., 2003, Meng et al., 2005a, Meng et al., 2005b, Sarnat et al., 2002, Wallace and Williams, 2005, Williams et al., 2003, Wilson and Brauer, 2006), there is a need for research that assesses the ambient generated component of exposure in colder climates where indoor PM infiltration may be reduced, especially areas where there are high levels of PM2.5 and significant health impacts are likely.
One method of determining the level of exposure due to ambient generated particles (Eag) is to use chemical components, such as sulphate or elemental carbon, that have limited or no indoor sources, as a tracer. Previous work has demonstrated that sulphate is a reliable tracer for ambient generated PM2.5 and reported higher personal-ambient correlations for sulphate than for total PM2.5 (Ebelt et al., 2000, Sarnat et al., 2000, Leaderer et al., 1999). Sulphur (predominantly in the form of sulphate) is most representative of particles in the accumulation mode (specifically particles with aerodynamic diameters of 0.06 to 0.5 μm) but indoor/outdoor sulphur ratios have been found to be significant predictors of indoor/outdoor PM2.5 ratios (Sarnat et al., 2002). In one example, however, sulphur was not useful as a tracer, perhaps due to low sulphur content in the PM mixture and/or low air exchange rates (Meng et al., 2005c). Use of a different component of the PM mixture as a tracer would be useful for areas with low sulphur content and would also provide further validation of the tracer method. Elemental or black carbon has been suggested for use as a tracer of ambient-generated PM if no combustion is allowed indoors (Ebelt et al., 2005, Wilson et al., 2000); however no studies have been published showing use of this tracer. Elemental carbon represents a different component of the PM mixture that is generally derived from more local sources compared to sulphate which is often considered to be a regional pollutant. In previous work in Prince George, British Columbia, Canada, we have shown that both sulphate and light absorbing carbon (a surrogate measure of elemental carbon) had stronger personal-ambient associations than PM2.5 mass and that each ambient component also exhibited a high correlation with the total ambient PM2.5 concentration, supporting their use as tracers of ambient PM2.5 (Noullett et al., 2006). These findings indicate that personal-ambient ratios of sulphate and elemental carbon could be used to estimate exposure to neighbourhood level ambient PM2.5 in this setting.
In this paper, we describe the use of sulphate and elemental carbon personal-ambient ratios to separate total personal exposure to PM2.5 into its ambient and non-ambient generated components. Indoor PM infiltration, air exchange rates and an exposure factor or “attenuation” of ambient PM2.5 to total personal exposure were also estimated. The main goals of this research were: 1) to determine the level of children’s exposure to ambient generated PM2.5 in a cold winter climate; 2) to assess the proportion of measured total personal exposure that originates from ambient sources; 3) to establish how these exposure levels relate to total ambient PM2.5 measured at neighbourhood and central monitoring sites; and finally 4) to determine whether elemental carbon can be used as a tracer for ambient generated exposure to PM2.5.
Section snippets
Study design
Fifteen elementary school students from non-smoking families were selected non-randomly from five schools in the city of Prince George (see Fig. 1 for locations) to participate in a study assessing personal exposure to fine particulate matter (PM2.5). Study design, methods for sampling and lab analysis, quality assurance procedures and results for the personal and ambient PM2.5 mass, sulphate, elemental carbon (via absorbance) and meteorological relationships have been described in detail by
Distribution of the measured and estimated parameters
Fig. 2 shows the distribution of personal and ambient PM2.5 mass, sulphate and elemental carbon concentrations, personal/ambient sulphate and elemental carbon ratios (exposure factors), and estimates of ambient and non-ambient generated exposure for data pooled across individuals. Table 1 provides the mean, standard deviation and percentile distribution of these measures and estimates across the fifteen individuals in the study.
Evaluation of PM2.5 ratios indicated that many of the personal
Discussion
Mean ambient generated exposures found for Prince George children were within 4.5 μg m−3 of levels estimated in other studies despite differences in the sampling period and panel characteristics (Koenig et al., 2005, Wallace and Williams, 2005, Wilson and Brauer, 2006). Exposure factors were similar when considering season with Prince George and Research Triangle Park both having a median α of 0.54; Seattle reporting means of 0.54 (using sulphur tracer method data only), 0.55 and 0.80 for the
Conclusion
On average, Eag and Enag levels for Prince George children were comparable to those found in other studies, and each contributed almost equally to total personal exposure. A strong relationship was found between Eag and ambient concentrations at both the neighbourhood school site and a central site suggesting that central-site ambient concentrations are appropriate surrogates for monitoring temporal changes in ambient-generated exposure. However, spatial differences in both ambient levels and E
Acknowledgement
This research was made possible through funding from the Natural Sciences and Engineering Research Council of Canada, Science Council of BC, the Canadian Petroleum Products Institute Clean Air Fund, UNBC Northern Land Use Institute, BC Ministry of Water, Land and Air Protection and Canadian Forest Products Ltd. In kind contributions were made by the Harvard School of Public Health, UBC School of Occupational and Environmental Hygiene and the Air Quality Research Branch of the Meteorological
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