Global association between ambient air pollution and blood pressure: A systematic review and meta-analysis☆
Graphical abstract
Introduction
Hypertension and high blood pressure (BP) have been ranked as the leading cause of death and disability worldwide, accounting for over 10.7 million deaths per year (Forouzanfar et al., 2015). In recent years, average BP levels have decreased worldwide. However, people in developing countries have experienced an increase in BP levels and hypertension prevalence, presumably related to lifestyle changes and increasingly deteriorating environmental conditions (Danaei et al., 2011). In parallel, ambient air pollution has also become the largest global environmental health threat (World Health Organization, 2015, Giorgini et al., 2016). Mechanistic evidence indicates that the inhalation of several air pollutants could trigger autonomic nervous system imbalance, and cause systemic inflammation, endothelial dysfunction and DNA methylation disruption (Bellavia et al., 2013, Furuyama et al., 2009; Geiser and Kreyling, 2010; Miller, 2014, Perez et al., 2015, Seaton et al., 1995, Perera et al., 2011). It has therefore been speculated that air pollution may contribute to hypertension pathogenesis. During the past few decades, many human epidemiological studies have evaluated the relationships of long-term and short-term exposure to ambient air pollutants with hypertension and BP levels (Giorgini et al., 2016, Brook and Rajagopalan, 2009, Cai et al., 2016, Liang et al., 2014, Zhang et al., 2016a). However, results were inconsistent due to variations in study design, population characteristics, exposure duration, air pollutant concentrations, exposure assessments, as well as BP measurements, which make these findings difficult to interpret and prevent specific suggestions to policy makers.
Systematic review and meta-analysis are widely used methods in environmental health epidemiology, enabling researchers to quantitatively synthesize data across studies endeavoring to offset the challenge of small sample size, to solve uncertainty from single studies, and to highlight research gaps. Until now, three meta-analyses have been conducted to pool the long-term and short-term effects of ambient air pollutants on hypertension or BP (Cai et al., 2016, Liang et al., 2014, Zhang et al., 2016b). However, the meta-analyses by Liang et al. (2014) and Zhang et al. (2016b) only investigated the effects of particulate air pollutants (diameter <2.5 μm (PM2.5) and <10 μm (PM10)) on BP levels, without covering other main air pollutants such as sulfur dioxide (SO2), nitrogen dioxide (NO2), nitrogen oxides (NOx), ozone (O3), and carbon monoxide (CO). The most recent meta-analysis by Cai et al. (2016) involved seven air pollutants (O3, CO, NO2, NOx, SO2, PM10 and PM2.5), yet it only focused on hypertension (but not BP) and had apparent flaws such as missing important studies (e.g. Fuks et al., 2014) and inadvertently including two studies with different study aims (ischemic stroke (Oudin et al., 2011) and acute coronary syndrome (Qorbani et al., 2012)).
To provide researchers and healthcare professionals with more comprehensive estimates of the effect of long-term and short-term exposure to ambient air pollution on hypertension and BP levels, we systematically retrieved all relevant studies to date and conducted a meta-analysis of the literature that was specifically designed to overcome the limitations of the three aforementioned meta-analytic reviews.
Section snippets
Search strategy
The systematic review and meta-analysis was conducted according to the Preferred Reporting Items for Systematic reviews and Meta-Analysis (PRISMA) guidelines (Supplemental Table 1). We systematically searched three English databases (PubMed, Embase, ISI Web of Science) and four Chinese databases (China National Knowledge Infrastructure, Chongqing VIP Chinese Science and Technology Periodical, China Biological Medicine, and wanfang) for pertinent literature published before 25 May 2017. The
Literature retrieval and study characteristics
The initial combined search yielded 12,385 articles. After the removal of overlapping articles, the titles and abstracts of 9158 articles were assessed and 120 articles underwent full text review. Finally, 65 articles including 100 studies met inclusion criteria and were included (Fig. 1) (Ibald-Mulli et al., 2004, Zanobetti et al., 2004, de Paula et al., 2005, Ebelt et al., 2005, Mar et al., 2005, Harrabi et al., 2006, Choi et al., 2007, Liu et al., 2007, Auchincloss et al., 2008, Dvonch
Principal findings
In this systematic review and meta-analysis of up to 100 studies comprised of approximately 0.7 million participants from 16 countries, we have conducted the most comprehensive, to our knowledge, evaluation of currently available data on the associations of long-term and short-term exposure to eight ambient air pollutants with hypertension and BP levels. The overall meta-analysis showed significant associations of long-term exposure to PM2.5 with hypertension, and of PM10, PM2.5, and NO2 with
Conclusions and recommendations
Overall, the existing evidence we reviewed indicated a positive association of exposure to ambient air pollutants with increased arterial BP and established hypertension. Our results also suggest that geographical and sociodemographic factors (e.g. continent, age and sex) may modify the pro-hypertensive effects of air pollutants. However, the low quality of the evidence including high heterogeneity, small sample size for some pollutants and subgroups, and high risk of bias, preclude us from
Acknowledgements
The authors thank Emily Anne Lipton in University at Albany, State University of New York for her elaborative reviewing of this manuscript and suggestions on language use.
Sources of funding
This work was supported by grants from the National Key Research and Development Program of China (grant number 2016YFC0207000); the Fundamental Research Funds for the Central Universities (No. 16ykzd02; 17ykpy16); and the National Natural Science Foundation of China (No.91543208; No.81703179).
Conflicts of interest
None.
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This paper has been recommended for acceptance by Dr. Chen Da.