ArticlesAssociation between mortality and indicators of traffic-related air pollution in the Netherlands: a cohort study
Introduction
Results of three prospective cohort studies1, 2, 3 have suggested that long-term exposure to paniculate matter air pollution is associated with increased mortality from respiratory and cardiovascular disease and from lung cancer. However, because these studies were done in the USA, their findings might not apply to Europe, which possibly has different air pollution mixtures. Increased mortality was identified at particle concentrations that were lower than the yearly average National Ambient Air Quality Standard in the USA (40 (μg/m3). In many European regions, including the Netherlands, the concentration of particles is presently at or above the highest concentrations recorded in the US cohort studies.4 The mortality associations are consistent with evidence that long-term exposure to air pollution causes chronic respiratory disease.5
Investigators of the cohort studies have compared several large, usually metropolitan, study regions with different ambient air pollution concentrations, on the assumption that exposure is uniform within each region. Such an assumption might have resulted in errors in estimation of exposure, especially for pollutants with important local sources. Confounding factors that vary between cities might also have distorted the relation with air pollution. The danger of such confounding is less if differences in exposure to air pollution between individuals within cities can be taken into account. In a study in four European countries,6 the concentration of nitrogen dioxide—an important traffic-related pollutant—varied between small regions within cities. Results of this study7 suggested that traffic intensity and distance to major streets are important predictors of differences in long-term nitrogen dioxide concentrations within a city. Results of several studies8, 9, 10 have shown that chronic respiratory disease in children is associated with differences in the amount of air pollution within cities, and especially with distance to or living in busy streets.
Because initiation of new cohort studies of the relation between air pollution and mortality is expensive, and many years would pass before a conclusion can be reached, we have used data from an ongoing cohort study—the Netherlands Cohort study on Diet and Cancer (NLCS).11 In this study, the cohort is spread out over the Netherlands and thus captures variation in long-term exposure to air pollution within the country. Measurements at background sites of the National Air Quality Monitoring Network have shown that the differences in particle concentrations in the Netherlands is small (30–40%).12 The long-term average concentration of ozone and secondary aerosol components (sulphate and nitrate) also does not differ by much.13 We therefore focused on traffic-related air pollutants, which are expected to show stronger spatial variation than ozone and aerosol components, and have been shown to be related to chronic morbidity.
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Participants
The Netherlands Cohort study on Diet and Cancer (NLCS)11 is a prospective cohort study that was started in 1986, with 120852 participants aged 55–69 years at enrolment (58 279 or 48% men). Participants were recruited from the 204 (29%) of 714 municipalities that had computerised population registries and were covered by cancer registries in 1986 (figure 1). All participants completed a self-administered questionnaire on diet and other risk factors in September, 1986.
The exact address of all
Results
Table 1 shows participants’ baseline characteristics. 489 participants died during the study-most from natural causes (table 2). We identified geographical coordinates for the addresses of 4466 (99%) of 4492 participants. 206 (5%) participants lived close to a major road: 132 (3%) within 100 m of a freeway, and 77 (2%) within 50 m of a major urban road.
The estimated long-term ambient concentration of black smoke and nitrogen dioxide at the 1986 home address varied greatly between participants (
Discussion
We assessed the association between long-term exposure to traffic-related air pollution and cause-specific mortality in a cohort of elderly people and identified a consistent association between cardiopulmonary mortality and living near a major road. The association between mortality and the estimated ambient background concentration of the indicator pollutants black smoke and nitrogen dioxide was less consistent.
Our results are much the same as those in two US cohort studies.1, 2 In the US
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