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(Stroke. 2003;34:2776.)
© 2003 American Heart Association, Inc.

Original Contributions

Stroke Mortality Associated With Living Near Main Roads in England and Wales

A Geographical Study

Ravi Maheswaran, MD Paul Elliott, PhD

From Public Health GIS Unit, School of Health and Related Research, University of Sheffield, Sheffield (R.M.), and Department of Epidemiology and Public Health, Imperial College Faculty of Medicine, London, UK (P.E.).

Correspondence to Dr R. Maheswaran, Public Health GIS Unit, School of Health and Related Research, University of Sheffield, Regent Court, 30 Regent St, Sheffield S1 4DA UK. E-mail r.maheswaran{at}sheffield.ac.uk

	Abstract

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Background and Purpose— Air pollution is associated with stroke, and road traffic is a major source of outdoor air pollution. Using proximity to roads as a proxy for exposure to road traffic pollution, we examined the hypothesis that living near main roads increases the risk of stroke mortality.

Methods— We used a small-area ecological study design based on 113 465 census enumeration districts in England and Wales. Stroke mortality (International Classification of Disease, 9th revision, codes 430 through 438) in England and Wales from 1990 to 1992 for people 45 years of age was examined through the use of 1991 population denominators. Exposure was calculated as distance from each enumeration district population centroid to the nearest main road. We adjusted for age, sex, socioeconomic deprivation (using Carstairs index), regional variation, urbanization, and metropolitan area using Poisson regression.

Results— The analysis was based on 189 966 stroke deaths and a population of 19 083 979. After adjustment for potential confounders, stroke mortality was 7% (95% confidence interval [CI], 4 to 9) higher in men living within 200 m of a main road compared with men living 1000 m away. The corresponding increase in risk for women was 4% (95% CI, 2 to 6) and the risk for men and women combined was 5% (95% CI, 4 to 7). These raised risks diminished with increasing distance from main roads.

Conclusions— Living near main roads is associated with excess risk of mortality from stroke, and if causality were assumed, 990 stroke deaths per year would have been attributable to road traffic pollution.

Key Words: air pollution • cerebrovascular disorders • mortality

	Introduction

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There is substantial evidence linking air pollution and stroke. Early studies found that air pollution episodes increased the risk of stroke mortality.^1–3 Several daily time series studies have also reported associations between stroke mortality and common air pollutants.^4–7 There is emerging evidence that the medium-term effects of air pollution on mortality are larger in magnitude than short-term effects, but the evidence is not specifically about stroke.^8–10 Several studies have examined the association between hospital admissions for stroke and air pollution.^3,11–15 with evidence of association in 4 studies.^3,11,12,14 Cohort studies on indoor and occupational exposures to air pollution link chronic exposure with stroke mortality.^16–18

A hypothesis that could explain the association is that fine-particulate air pollution provokes alveolar inflammation, causing the release of potentially harmful cytokines, which results in increased coagulability.¹⁹ A second potential mechanism may operate through the association between impaired ventilatory function and stroke.^20,21 Another possibility is that air pollution precipitates death among individuals who have already sustained a stroke and are disabled with compromised respiratory function.

Road traffic is a major contributor to outdoor air pollution in industrialized nations and is a major source of fine-particulate matter, carbon monoxide, and oxides of nitrogen. Concentrations of these pollutants are estimated to be high near main roads and to decrease exponentially with increasing distance from roads, reaching background concentrations by 200 m.²² However, extensive geographical data on levels of pollution near roads are scarce. We therefore examined the hypothesis that living near main roads increases the risk of mortality from stroke using proximity to main roads as a proxy for exposure to road traffic pollution.

	Methods

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We used an ecological study design with 1991 census enumeration districts (CED) as the unit of analysis based on 113 465 CEDs in England and Wales. Stroke mortality (International Classification of Disease, 9th revision, codes 430 through 438) from 1990 to 1992 for men and women 45 years of age was examined by use of 1991 population denominators corrected for underenumeration.²³ Using 1 to 200 000 resolution road network data within a geographic information system (ArcInfo, ESRI Inc), we calculated exposure as the distance from each CED population centroid to the nearest main road. We classified motorways, primary roads, and A-roads as main roads; we calculated distance from each CED centroid to the nearest road of each type and took the shortest of the 3 distances. We adjusted for socioeconomic deprivation using Carstairs index calculated at the CED level.²⁴ This index is a standardized combination of four 1991 census variables: percent of persons with no car, percent of persons in overcrowded housing, percent of persons with household head in social class IV or V, and percent of men unemployed. It is widely used in the United Kingdom, including in a national facility for small-area disease mapping,²⁵ and has been shown to predict stroke mortality at the small-area level.^24,26 The index could not be calculated for 5042 CEDs (4.4%) because of incomplete data, and these CEDs were excluded from further analysis. We adjusted for regional variation in stroke mortality using the 9 standard regions in England and Wales. We used ward-level population density as a proxy for urbanization and assigned this to all CEDs within each electoral ward. In addition, we adjusted for the possible effects of large conurbations by including metropolitan area.

We used log-linear Poisson regression analysis to assess whether distance from the nearest main road significantly influenced stroke mortality at the CED level and to examine whether this association was independent of age, sex, socioeconomic deprivation, urbanization, metropolitan area, and regional variation. Distance to the nearest main road was entered as 4 categories, age as 5 categories, region as 9 categories, and metropolitan area as 2 categories. Population density and Carstairs index were each classified into 5 categories using quintiles. We included an age-by-Carstairs interaction term because this interaction has previously been found to be substantial.²⁶ Regression parameter estimates are presented as rate ratios with 95% confidence intervals (CIs). Statistical analysis was performed with SAS, and CIs were adjusted for overdispersion.²⁷

	Results

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There were 199 683 stroke deaths in 1990 to 1992 in England and Wales among people 45 years of age. Of these deaths, 189 966 (95.1%) could be allocated to CEDs with deprivation scores and were used in the analysis. There were 19 083 979 people 45 years of age in England and Wales in 1991 in the CEDs used in the analysis. Tables 1 and 2 show stroke deaths, population counts, and unadjusted death rates by age and proximity to main roads for men and women, respectively. We found that 30% of the population 45 years of age lived within 200 m of a main road. This percentage rose from 29% in the 45- to 54-year-old group to 33% in the group 85 years of age. Death rates increased with increasing age and tended to be higher among people living near main roads.

View this table: [in this window] [in a new window]   TABLE 1. Stroke Deaths (1990–1992), Estimated Midyear Population (1991), and Annual Stroke Mortality Rate per 100 000 Population by Age and Distance to Nearest Main Road, England and Wales: Men

View this table: [in this window] [in a new window]   TABLE 2. Stroke Deaths (1990–1992), Estimated Midyear Population (1991), and Annual Stroke Mortality Rate per 100 000 Population by Age and Distance to Nearest Main Road, England and Wales: Women

The overall mean Carstairs socioeconomic deprivation score was -0.36 (SD, 2.90) (increasing positivity indicates increasing deprivation). The mean score decreased from 0.11 (SD, 3.00) for CEDs within 200 m of a main road to -1.52 (SD, 2.25) for CEDs 1000 m from a main road, indicating that in general CEDs near main roads were more deprived than CEDs further away. However, the SDs indicate that there was substantial variation within distance categories.

Table 3 shows rate ratios for stroke mortality for CEDs in the most deprived fifth of the distribution for Carstairs index relative to the least deprived fifth. In the 45- to 54-year-old group, the adjusted rate ratio was 2.94 (95% CI, 2.59 to 3.34) for men and women combined. However, the magnitude of the association diminished and even was reversed with increasing age, with a rate ratio of 0.77 (95% CI, 0.74 to 0.80) in the group 85 years of age.

View this table: [in this window] [in a new window]   TABLE 3. Unadjusted and Adjusted Rate Ratios for Stroke Mortality (1990–1992) in the Most Socioeconomically Deprived Fifth Relative to the Least Deprived Fifth of Enumeration Districts (Based on Carstairs Index) by Age, England and Wales

Table 4 shows the rate ratios for stroke mortality with increasing proximity to main roads. After adjustment for all confounders included in this study, the risk of mortality from stroke among men living within 200 m of a main road was 7% (95% CI, 4 to 9) higher compared with men living 1000 m from a main road. The corresponding increase in risk for women was 4% (95% CI, 2 to 6). The risk for both sexes combined was 5% (95% CI, 4 to 7). The raised risks diminished with increasing distance from main roads. These fully adjusted rate ratios were broadly similar to rate ratios adjusted for age alone.

View this table: [in this window] [in a new window]   TABLE 4. Rate Ratios for Stroke Mortality (1990–1992) by Distance to the Nearest Main Road, England and Wales

In the regression model with all variables included, distance to the nearest main road, age, sex, Carstairs index, standard region, metropolitan area, and the Carstairs-by-age interaction all had highly significant associations (P<0.0001) with stroke mortality, but population density did not (P=0.14).

	Discussion

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Our results suggest that living near main roads is associated with a significant but small excess risk of mortality from stroke. The increase in risk appears to diminish with increasing distance from main roads. These results are consistent with the hypothesis that road traffic pollution is a risk factor for stroke, but this study has a number of potential limitations that need to be considered.

Distance to main roads was used as a proxy for exposure to road traffic pollution, but we did not have nationwide information on traffic flow along these roads. The accuracy of distance measurements may have been subject to error because of imprecision in CED centroid grid locations and road network vector data. We adjusted for socioeconomic confounding at a fine geographical scale, but it is possible that residual confounding remained, which could account for our findings. We did not have data on smoking and hypertension, and although socioeconomic deprivation may be a proxy for these factors to some extent, confounding as a result of these factors remains a potential explanation. We did not have any information on potential confounders early in life. This may be relevant because there is evidence for a cohort effect on stroke mortality in southeast England, indicating that early life factors influence geographical variation in stroke mortality.²⁸

The possibility of ecological bias (ie, the association found at the area level being different from that at the individual level) cannot be ruled out. We used very small geographical areas as the unit of analysis, and they are likely to have reasonable within-area homogeneity. However, a potential limitation is residual spatial autocorrelation, ie, correlation that remains between distance to roads, other covariates, and stroke mortality because these covariates were not accounted for in the statistical model. This results in overly narrow CIs, but statistical methods that take it into account tend to have little influence on the effect estimates themselves.²⁹ There may have been errors in death certification for stroke, particularly among very old people. Misdiagnosis of stroke is unlikely to be linked to distance to roads but could nevertheless dilute the magnitude of any underlying effect.

The magnitude of the association we observed is small compared with 2 cohort studies on outdoor air pollution that found relative risks for cardiopulmonary mortality ranging from 1.26 to 1.37 for the most polluted compared with the least polluted areas.^30,31 In another cohort study, the relative risk of cardiopulmonary mortality associated with living near a major road was 1.95.²⁹ These studies had data at the individual level to adjust for smoking and other confounders and still found substantial associations, so it is plausible that the association between air pollution and stroke mortality observed in our study is real, despite inherent limitations. The differences in magnitude may have resulted because the effect of air pollution on cardiac and respiratory mortality is greater than its effect on stroke mortality or because levels of exposure were higher in those studies. Another explanation may be that the imprecision in exposure measurement in our study diluted the size of the true association. We did not study stroke subtypes. The air pollution effect might be confined to ischemic stroke; further study is needed to examine this issue.

Modeled dispersion data imply that the effects of road traffic pollution on stroke should reach background levels of risk beyond 200 m of a main road.²² However, there is a suggestion from measured data that particulate levels are not especially high near main roads, implying that their dispersion pattern is different from that of other road traffic pollutants.³² We observed diminishing effects up to 500 and possibly up to 1000 m away, maybe because the combination of topographic features and prevailing winds in parts of the country results in pollutants traveling greater distances than those derived from pollution modeling. Another possibility is that this is an artifact resulting from imprecise distance measurement.

Any association between living near main roads and mortality from stroke may be due in part to noise, which has adverse effects on cardiovascular physiology.³³ Three studies have found inconsistent associations between traffic noise and cardiovascular risk.^34–36 However, there is little epidemiological evidence directly linking noise pollution and stroke, unlike the situation for air pollution and stroke. Emission of lead from automobile combustion was a significant concern before the widespread use of lead-free petroleum. However, even lead exposure in industrial settings was not strongly associated with raised blood pressure and stroke in recent reports.^37,38

Our focus was on proximity to roads and stroke mortality, but a striking observation was the substantial association between socioeconomic deprivation and stroke mortality in middle-aged people. Lower socioeconomic groups have a higher prevalence of several risk factors for stroke, including hypertension, smoking, excessive alcohol consumption, obesity, and poorer access to health care, which may explain socioeconomic inequalities in stroke mortality.³⁹ We observed a progressive diminution in the association and even a reversal in very old age, consistent with published literature.²⁶

The public health impact of the increase in stroke mortality risk is not inconsequential in absolute terms because exposure to road traffic pollution and stroke mortality are common. If we assume causality, 990 deaths from stroke per year in England and Wales among people 45 years of age would have been attributable to road traffic pollution as a result of living within 200 m of a main road. Further studies are needed to confirm our findings.

	Acknowledgments

 
This study was funded by a grant from the Stroke Association. This work uses data provided with the support of the ESRC and JISC and census and boundary material, which are copyrighted by the Crown, the post office, and the ED-LINE Consortium. The views expressed in this publication are those of the authors and not necessarily those of the funding organization.

Received January 30, 2003; revision received July 30, 2003; accepted August 11, 2003.

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This Article Free upon publication Abstract Full Text (PDF) All Versions of this Article: 34/12/2776    most recent 01.STR.0000101750.77547.11v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Maheswaran, R. Articles by Elliott, P. Search for Related Content PubMed PubMed Citation Articles by Maheswaran, R. Articles by Elliott, P. Pubmed/NCBI databases Medline Plus Health Information Stroke Related Collections Risk Factors for Stroke