Estimated long-term outdoor air pollution concentrations in a cohort study
Introduction
We recently reported an association between cardiopulmonary mortality and long-term exposure to traffic-related air pollution in a random subgroup of 5000 of a cohort study of elderly people (NLCS study; N∼120 000) (Hoek et al., 2002a, Hoek et al., 2002b). Other recent studies also found associations between long-term exposure to air pollution and health effects (Nafstad et al., 2004; Filleul et al., 2005; Finkelstein et al., 2004).
Two large cohort studies in the US also showed associations between long-term exposure to air pollution and mortality. However, in the Harvard Six Cities study and the American Cancer Society (ACS) study, exposure has been estimated as the average concentration within a city/metropolitan area, ignoring small-scale variations within cities (Dockery et al., 1993; Pope et al., 1995). Exposure studies have shown large variations in concentrations of traffic-related air pollutants on a small scale within cities (Fischer et al., 2000; Jerrett et al., 2005a). A study by Jerrett et al. (2005b) in a subpopulation of the ACS study suggested that the chronic health effects associated with intraurban gradients in fine particles may be even larger than previously reported associations across metropolitan areas.
In our previous study, long-term exposure to traffic-related air pollution was therefore considered as a function of the regional background, an urban background and an indicator variable for living near a major road (Hoek et al., 2001). Following on this previous study, we conducted a study on the association between long-term exposure to traffic-related air pollution and mortality in the full NLCS cohort of ∼120 000 subjects (NLCS-AIR study). In this new study we improve the exposure assessment compared to the previous study that had no traffic intensity data and a less-accurate road network.
The purpose of this paper is to describe an improved method for the assessment of long-term outdoor air pollution concentrations in a cohort study.
Section snippets
Design
The design of the Netherlands Cohort Study on Diet and Cancer (NLCS) (van den Brandt et al., 1990) and the exposure assessment method used in the previous study have been described in detail elsewhere (Hoek et al., 2001). Briefly, the NLCS started in September 1986 when 120 852 participants from 204 municipalities, spread out over the Netherlands, were enrolled. The residential address of all participants in 1986 is known.
The NLCS was designed as a case–cohort study, i.e. mortality cases are
Regional component
We evaluated ordinary kriging and inverse distance weighed interpolation as methods to estimate regional background concentrations at the home addresses. However, there was only a limited number of BS sites (9 sites in the period 1992–1996), and therefore it was not possible to develop a suitable variogram for kriging. Further, the RIVM has used kriging procedures to decrease the density of the network during the rearrangement of the network in 1986. Therefore, it was less possible to estimate
Discussion
We described an improved method to estimate long-term outdoor air pollution concentrations in a Dutch cohort study. Using comprehensive data from existing air pollution monitoring stations, data on land use, population density, road networks and traffic density, we were able to develop exposure assessment models that explained a large part of the spatial variance in long-term averaged air pollution concentrations.
Conclusion
An improved method for assessment of long-term outdoor air pollution concentrations taking into account small-scale variations in air pollution concentrations was illustrated. Despite the limitations and methodological problems, we were able to develop an exposure assessment model using GIS-methods and traffic intensities that explained a large part of the variations in concentrations outdoor air pollutants and which can be used in epidemiologic studies to estimate air pollution levels at
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