Lack of spatial variation of endotoxin in ambient particulate matter across a German metropolitan area
Introduction
Endotoxins are part of the outer membrane of the cell wall of Gram-negative bacteria. Although the term endotoxin is occasionally used to refer to any cell-associated bacterial toxin, it is properly reserved to refer to the lipopolysaccharide complex associated with the outer membrane of Gram-negative bacteria such as Escherichia coli, Salmonella and other leading pathogens. Inhalation of bacterial endotoxin or its chemically pure form called lipopolysaccharide (LPS) in doses as low as 4–15 ng m−3 has been associated with acute and chronic airways inflammation and lung function decrements (Douwes and Heederik, 1997; Milton et al., 1996). Given its toxicity and pro-inflammatory effects, endotoxin has been studied in specific occupational settings and in house dust for more than 30 yr (Douwes et al., 2000; Rylander, 2002). Ambient exposure to endotoxin, however, has not well been characterized; at present only three studies have measured systematically endotoxin concentrations in ambient particles (Carty et al., 2003; Heinrich et al., 2003b; Mueller-Anneling et al., 2004). Carty et al. (2003) found that endotoxin levels were significantly related to ambient temperature and relative humidity. Heinrich et al. (2003b) collected fine and coarse fractions of particles in the two small German towns of Hettstedt and Zerbst, which are approximately 100 km apart. The average endotoxin levels were not significantly different between the two towns, but endotoxin content in PM10 was approximately 10-fold higher than the PM2.5 content at both locations. The study of Mueller-Anneling et al. (2004) provides a characterization of endotoxin concentration across a large metropolitan area, but only in relation to PM10. They reported that the desert and mountain communities had the highest endotoxin levels, whereas samples collected in rural areas had mid-range endotoxin levels. The city of Los Angeles itself had the lowest endotoxin results.
Recent studies have focused on the physical properties, e.g. surface size or chemical compostion of ambient particles and bioaersols. However, the exact constituents of air pollution that cause disease and the precise mechanisms involved are complex. Numerous studies have been conducted to determine which components of particulate matter (PM) may contribute to airway inflammation and irritation (Soukup and Becker, 2001). Bacterial endotoxin is one such biologically active constituent of PM that has been shown to trigger a complex inflammatory response in humans. The other justifying point of this investigation is related to the characterization of ambient particle properties, which might be involved in causing the known adverse health effects.
Research on spatial patterns of airborne endotoxin levels may be useful in determining certain outdoor endotoxin sources and might improve assessment of exposure to airborne endotoxin. Several sources of indoor endotoxin in settled house dust have been described (Heinrich et al., 2001; Park et al., 2001b). Park et al. described that it is likely that outdoor endotoxin levels have an influence on indoor levels, e.g. when windows are open (Park et al., 2000). Comparisons of indoor and outdoor endotoxin concentrations in factories (Su et al., 2002), farms (Chang et al., 2001) and in urban homes (Long et al., 2001; Park et al., 2000) have also been described in the literature. In general, levels of endotoxin are higher indoors and inside factories than in the nearby outdoor environments (Long et al., 2001; Park et al., 2000; Su et al., 2002) or in the surrounding areas (includes administrative building, farm perimeters) (Chang et al., 2001). High levels of endotoxin have also been found in agriculture and related industries such as garbage handling and compost facilities (Sigsgaard et al., 1994), cotton mills (Christiani et al., 1993) and in dairy barns (Kullman et al., 1998).
As part of an international collaborative study on the impact of Traffic-Related Air Pollution on Childhood Asthma (TRAPCA), we sampled fine and inhalable PM with a 50% aerodynamic cut-off diameter of 2.5 μm (PM2.5) at 40 monitoring sites and of 10 μm (PM10) at 12 monitoring sites in ambient air across Munich, Germany for 17 months (Gehring et al., 2002; Hoek et al., 2002). In the current study, we used these sampled particles and determined endotoxin concentrations in PM2.5 and PM10.
We report endotoxin levels in PM2.5 and PM10 per milligram and per cubic meter of sampled air and to describe spatial variation of endotoxin across Munich, a German metropolitan area.
The goals of our study were to determine ambient endotoxin levels in PM2.5 and PM10 in the metropolitan area of Munich and describe the spatial varation of endotoxin in PM2.5 across the city. We also describe the correlation of endotoxin in PM2.5 and PM10 measured at the same sites and investigate whether potential sources of endotoxin are useful in predicting endotoxin ambient levels.
Section snippets
Measurement sites
In all, 40 measurement sites were located in Munich, a city of 1.26 million inhabitants located in southern Germany (Landeshauptstadt München, 2001). Munich covers a total area of 310.4 km2 (119.8 square miles) and has an average altitude of 530 m (1744 ft). Monitoring sites were within the city limits and were originally selected as part of the TRAPCA study (Gehring et al., 2002; Hoek et al., 2002). In accordance with the TRAPCA protocol and to better assess the exposures of a related birth
Statistical methods
All 158 endotoxin values, but three were above the LOD. A value of 1/2 of the LOD (0.002 EU ml−1) was used for these three measurements. Since the distribution of measured concentrations was skewed to the right, we used the geometric means (GM) to report mean concentrations for the four samples for each monitoring site. The arithmetic mean was also calculated for comparison with published data. Associations between endotoxin concentration expressed per m3 air volume and per mg of PM2.5 and PM10
Endotoxin levels
The GM endotoxin concentrations for the annual means were 1.460 EU mg−1 PM2.5 (95% CI: 1.207–1.767 EU mg−1 PM2.5) and 0.020 EU m−3 sampled air (95% CI: 0.017–0.024 EU m−3). The calculated GMs for PM2.5 for the 12 measurement sites, where both PM10 and PM2.5 were measured, were 1.300 EU mg−1 PM2.5 (95% CI: 1.011–1.671) and 0.019 EU m−3 of sampled air (95% CI: 0.014–0.024). The values for PM10 () were much higher: 3.907 EU mg−1 PM10 (95% CI: 3.034–5.033) and 0.081 EU m−3 sampled air (95% CI: 0.062–0.105).
Discussion
Most data on ambient endotoxin concentrations are derived from measurements in occupational settings and live stock buildings (Clark et al., 1983; Crook et al., 1991; Heederik et al., 1991; Seedorf et al., 1998). Several sources of indoor endotoxin have been described in literature (Heinrich et al., 2001; Park et al., 2001a), but sources of endotoxin in ambient air have not been well characterized, especially not in an urban areas. In a longitudinal study (Park et al., 2000) higher airborne
Conclusion
Athough there is little spatial variation in ambient endotoxin concentrations across Munich, there is no difference in geometric mean levels of endotoxin between the city center and the periphery. Possibly due to the low spatial variability in endotoxin or to the site selection, no clear spatial patterns were found. Potential sources of endotoxin surrounding the sites weakly explained the variation seen. The endotoxin levels in PM2.5 and in PM10 were moderately highly correlated. Future studies
Acknowledgment
The authors thank the Munich schools, churches and police stations that granted access to their property for the setup and maintenance of the PM monitors. In addition, the authors thank Andreas Schöpfer and Markus Drees for their help with measurement site selection, Martina Zeiler and Christian Harmath for filter collection, Mike Pitz and Klaus Koschine for weighing the samples, Silke Trautheim and Gabriele Brandt for completing the endotoxin extraction and laboratory analyses, and Dr. Andrea
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