Spatial variation in nitrogen dioxide in three European areas
Introduction
In the past decade, a large number of studies have documented effects of short-term changes in ambient air pollution on human health parameters Brunekreef et al., 1995, Linaker et al., 2000, Atkinson et al., 2001, Brunekreef and Holgate, 2002, Le Tetre et al., 2002. In contrast, only a few studies have investigated the long-term effects of exposure to ambient air pollution. However, recent studies have suggested that long-term exposure to air pollution is associated with adverse health effects, including cardiopulmonary death and lung cancer Dockery et al., 1993, Pope et al., 1995, Pope et al., 2002, Abbey et al., 1999, Nyberg et al., 2000, Hoek et al., 2002a. Most of these studies have compared large study areas (cities) with different ambient air pollution concentrations. Exposure has largely been characterised by one average concentration per city, assuming homogenous exposure within each study area.
However, some studies have attempted to relate the spatial variation in air pollution concentration within cities to health outcome, sometimes using raster measurements and interpolation (Hirsch et al., 1999). Some studies have used dispersion models Nyberg et al., 2000, Bellander et al., 2001, Zmirou et al., 2002 or statistical models for residential exposure based on geographical information systems (Brauer et al., 2003). In addition, there have been a small number of cross-sectional studies of respiratory disease employing exposure indicators, such as distance to major roads, or objectively determined or self-reported traffic intensity (see, for example, Wjst et al., 1993, Weiland et al., 1994, Pershagen et al., 1995, Brunekreef et al., 1997, Van Vliet et al., 1997, Hoek et al., 2002a).
Nitrogen dioxide has been used as an indicator of motor vehicle exhaust even if the primary emission of nitrogen oxides from vehicles is largely nitrogen monoxide (NO). Formation of NO2 from NO occurs after emission and depends on, for example, the level of ozone. The sum of NO2+NO is a better indicator of motor exhaust if the interest is in primary pollutants. If the interest is more in secondary pollutants, NO2 might be better. Further reasons to use NO2 include the ease of measurement, the large database of NO2 concentrations and the fact that environmental limit values are based on nitrogen dioxide levels.
Spatial variation in NO2 concentrations within urban areas may be substantial Lebret et al., 2000, Krämer et al., 2000. Therefore, for epidemiological studies, it is desirable to characterise urban areas with regard to small area differences in air pollution levels. In addition to dispersion modelling based on emission inventorying, regression models of NO2 spatial distribution have been found to be a promising approach Bernard et al., 1997, Briggs et al., 1997, Briggs et al., 2000, Raaschou-Nielsen et al., 2000. Spatially distributed measurements of nitrogen dioxide are necessary for further development and validation of all of these models.
The measurements described in this paper were conducted as part of a study to assess the risk of long-term exposure to traffic-related air pollution for the development of respiratory disease in children (Traffic-Related Air Pollution and Childhood Asthma [TRAPCA]). Estimation of exposure to motorised traffic emissions was added to the existing framework of three birth cohort studies on the relationship between indoor allergen exposure and development of allergy, asthma and other chronic respiratory symptoms in Munich, Germany, the Netherlands and Stockholm County, Sweden. Estimation of the long-term average exposure to NO2 and ambient particles involved a 1-year monitoring program and regression modelling using GIS data. This paper documents the spatial variation in average NO2 concentration within the three study locations. Furthermore, levels of NO2 and spatial variation were compared between the three study areas. Palmes tubes were used for the measurements and the values were compared with those from the standard chemiluminescence method. Previous studies have indicated systematic bias (over or underestimation) of nitrogen dioxide levels measured with Palmes tubes compared with chemiluminescence equipment Heal et al., 1999, Heal et al., 2000, Kirby et al., 2001, Campbell et al., 1994.
Section snippets
TRAPCA exposure assessment
The TRAPCA study involves separate cohort studies: in Munich, Germany, in three separate areas in the Netherlands and in parts of Stockholm County, Sweden. Estimation of the long-term average exposure of the children involves a 1-year monitoring program, collection of data on potential air pollution predictor variables (e.g. intensity of traffic in nearby streets) using data of geographical information systems, and regression modelling. Nitrogen dioxide monitoring involved measurement of the
Inter-laboratory comparison
There were only small differences between the samplers according to country of preparation and analysis (Table 2). The NO2 concentration obtained with Dutch tubes analysed using the Saltzman method was 15% higher than the NO2 concentration obtained with German tubes analysed by ion chromatography (the configurations used in the main study). By comparing the NO2 concentrations of the four combinations of preparation and analysis of the tube, we conclude that the difference is mostly due to
Discussion
This study shows significant spatial differences in annual average nitrogen dioxide levels within three European areas ranging in size from an urban area (Munich), a larger county area (Stockholm County) and an entire country (the Netherlands). There were substantial differences both within and between these areas. The within-area differences were related mainly to differences in proximity to city center and dense road traffic. The between-area differences were mainly in the background
Acknowledgements
The authors want to thank Martina Zeiler and Christian Harmath (in Germany), and Katinka Almrén, Stina Johnsson and Malin Kilstorp (in Sweden) for their help with the measuring program. We also want to thank Slb, the City of Stockholm Environment and Health Administration, Sweden, for giving us access to reference site and chemiluminescence values, and Niklas Berglind for statistical support. The study has been financed by the European Union (ENV4 CT97-0506).
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