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Introduction
Outdoor air pollution is an important risk factor for cardiovascular disease throughout the world, with particulate air pollution alone responsible for over three million deaths each year. 1 2 Increases in concentrations of daily air pollution are associated with acute myocardial infarction 3 and admission to hospital or death from heart failure. 4 These associations could be mediated through direct and indirect effects of exposure to air pollutants on vascular tone, endothelial function, thrombosis, and myocardial ischaemia. 5 6 7 8
Stroke accounts for five million deaths each year and is a major cause of disability. 9 The incidence of stroke is increasing, particularly in low and middle income countries, where two thirds of all strokes occur. 10 The global burden of stroke related disability is therefore high and continues to rise. This has been primarily attributed to an ageing population in high income countries and the accumulation of risk factors for stroke, such as smoking, hypertension, and obesity, in low and middle income countries. 11 12 The impact of environmental factors on morbidity and mortality from stroke, however, might be important and is less certain. 13 14 15 Given similarities in the pathophysiology of acute coronary syndrome and ischaemic stroke, it is plausible that air pollution is also an important and modifiable risk factor. 3 4
To provide global policy makers with the best estimates of the effect of short term exposure to air pollution on risk of stroke, we systematically reviewed studies examining the association between air pollution and admission to hospital for stroke or mortality from stroke.
Methods
Databases, sources, and searches
We searched Medline and Embase (from 1948 to 21 January 2014), Global Health, Cumulative Index to Nursing and Allied Health Literature (CINAHL), and Web of Science with detailed search terms for: "stroke", "cerebrovascular disease", "air pollution", "carbon monoxide", "sulphur dioxide", "nitrogen dioxide", "ozone", and "particulate matter" (see appendix 1 for detailed search strategy). We also hand searched the bibliographies of all the included studies and relevant review articles to identify any remaining studies.
Selection of articles and extraction of data
Original studies were included if they evaluated short term associations (up to a lag of seven days) between carbon monoxide, sulphur dioxide, nitrogen dioxide, ozone, and particulate matter PM