Association between ambient air pollution and hospitalization for ischemic and hemorrhagic stroke in China: A multicity case-crossover study☆
Graphical abstract
Percentage change (95% CI) in hospital admissions for ischemic stroke and hemorrhagic stroke associated with a difference of an interquartile range (IQR) of PM10 (85.7 μg/m3), NO2 (25.4 μg/m3), SO2 (38.3 μg/m3), CO (0.81 mg/m3), and O3 (67 μg/m3) for different lag structures in 14 Chinese cities, 2014–2015. PM10, particulate matter less than 10 μm in aerodynamic diameter; NO2, nitrogen dioxide; SO2, sulfur dioxide; CO, carbon monoxide; O3, ozone.
Introduction
Stroke is the second-leading cause of death and the third major cause of adult disability worldwide (Global, 2015, Murray et al., 2015). Approximately 10.3 million new cases of stroke were diagnosed in 2013, and 6.5 million people are estimated to have died of stroke that year; two-thirds of all strokes occurred in low- and middle-income countries (Feigin et al., 2015). In China, stroke has emerged as the most common cause of death and adult disability in recent years (Liu et al., 2011). A nationally-representative population-based survey involving 480,687 Chinese adults aged ≥20 years estimated that the age-standardized incidence and mortality rates of stroke were 246.8 and 114.8 per 100,000 person-years in 2013, respectively (Wang et al., 2017). Despite the implementation of several primary and secondary prevention strategies in various countries that have been shown as effective in randomized trials (Kernan et al., 2014, Wang et al., 2007), the incidence of stroke continues to rise, particularly in economically transitioning countries (Feigin et al., 2014). Stroke exerts considerable patient suffering and immense economic burdens (Liu et al., 2011, Tong et al., 2016). Therefore, identification of modifiable risk factors for stroke has significant public health implications.
Research has provided compelling evidence linking outdoor air pollution to hospital admissions or death from stroke (Andersen et al., 2010, Hong et al., 2002, Mateen and Brook, 2011, Wellenius et al., 2012). Unlike the established risk factors for stroke, such as smoking, alcohol consumption, and physical inactivity (O'Donnell et al., 2010), air pollution represents a potentially modifiable risk factor that is independent of individual behavioral change. Improving air quality may offer a unique advantage in enhancing prevention efforts aimed at reducing the incidence of stroke. However, most previous studies of this topic were conducted in developed countries, and only limited research data have been generated in developing countries. In view of the considerable differences in pollutant characteristics (e.g., pollution levels and components), meteorological patterns, population susceptibility, and socio-demographic status (e.g., age structure and socioeconomic characteristics) between developed and developing countries and the enormous stroke burden in the latter, an urgent need remains to evaluate the effect of outdoor air pollution on the incidence of stroke in developing countries.
China has been experiencing the worst air pollution problem in the world (Kan et al., 2012). Ambient air pollution has been a major cause of mortality and morbidity in China, accounting for an estimated 1.2 million premature deaths and 25 million disability-adjusted life years annually (Lim et al., 2012). The detrimental health effects of air pollution are of increasing concern to the public, particularly in relation to haze days. However, only a handful of studies have examined the association between air pollution and stroke in China, yielding inconsistent findings, and these studies were restricted to a single city, such as Shanghai (Kan et al., 2003), Wuhan (Xiang et al., 2013), and Beijing (Huang et al., 2016). To the best of our knowledge, only one multicity study, the China Air Pollution and Health Effects Study, has examined the acute effects of air pollution on stroke mortality in eight Chinese cities (Chen et al., 2013). However, this study did not differentiate between ischemic and hemorrhagic stroke. In addition, the study used death data rather hospitalization data. Stroke hospitalization data would yield higher numbers of patients in a given population, increasing the statistical power. Furthermore, hospitalization data can better evaluate the temporal sequence between exposure to air pollution and clinical presentation of stroke (Villeneuve et al., 2006).
The objective of this study was to examine the association between short-term exposure to air pollution and hospital admission for stroke in 14 large Chinese cities using a time-stratified case-crossover design.
Section snippets
Study population
Data on daily admissions for ischemic and hemorrhagic stroke were collected from electronic hospitalization summary reports (HSRs) of the top-ranked hospitals for care safety and quality as evaluated by the National Hospital Performance Evaluation Project of the National Healthcare Data Center of China. The hospital ranking system considers several aspects, including hospital infrastructure, medical service and management, technical level and efficiency, and quality and safety of clinical care.
Results
There were 200,958 hospital admissions for ischemic stroke and 41,746 admissions for hemorrhagic stroke that formed the basis for this study (Table 1). Demographic characteristics of stroke admissions are present in Table 1.
Table 2 shows the summary statistics of air pollutants and meteorological variables in the 14 Chinese cities during the study period. The air pollution levels in this study were much higher than those reported in developed countries.
The daily concentrations of PM10, NO2, SO2
Discussion
Our multicity analysis showed that all five major air pollutants were positively associated with risk of hospital admission for ischemic stroke in the study area. We also observed significant association of NO2 with hospitalization for hemorrhagic stroke. To the best of our knowledge, this is the first multicity study in mainland China to examine the short-term effects of various air pollutants on hospital admissions for stroke.
In the present study, a difference of an IQR of 6-day moving
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgments
This research work was funded by the National Natural Science Foundation of China (Grant No. 71402003).
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This paper has been recommended for acceptance by Dr. Chen Da.
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Hui Liu and Yaohua Tian contributed equally to this study and should be considered as co–first authors.