Is prehypertension more strongly associated with long-term ambient air pollution exposure than hypertension? Findings from the 33 Communities Chinese Health Study☆
Graphical abstract
Introduction
Hypertension has been consistently regarded as the leading risk factor for global disease burden, causing approximately 10.7 million deaths per year (Forouzanfar et al., 2016). In China, the estimated prevalence of hypertension increased from 18.0% in 2002 to 27.8% in 2013, which is generally comparable, yet still higher than many developed countries (Li et al., 2017). Hypertension is generally defined as blood pressure (BP) values ≥ 140/90 mmHg, as the risk of cardiovascular disease (CVD) increases sharply above this level (Petruski-Ivleva et al., 2016, Gray et al., 2011). However, even without progression to hypertension, BP is associated with an increased risk of cardiovascular morbidity and mortality (Egan and Stevens-Fabry, 2015, Grotto et al., 2006, Lewington et al., 2002, Ishikawa et al., 2010). For example, death due to CVD increases steadily when BP levels are as low as 115/75 mmHg (Lewington et al., 2002), and the risk of developing CVD is 45% greater among individuals with BP levels between 130 and 139/85–89 mmHg compared to normotensive people (Ishikawa et al., 2010). To uncover the risk associated with BP levels previously considered normal and to focus clinical and public health attention on prevention, the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC-7) introduced a new disease classification, “prehypertension”. Prehypertension is defined as systolic blood pressure (SBP) ranging from 120 to 139 mmHg and/or diastolic blood pressure (DBP) ranging from 80 to 89 mmHg (Chobanian et al., 2003).
A host of factors can cause an increase in BP, such as genetic mutations, unhealthy lifestyles and poor dietary habits (Poulter et al., 2015). Additionally, many epidemiological studies have indicated that ambient air pollution might also possess pro-hypertensive effects (Giorgini et al., 2016), but evidence is inconsistent. For example, an Asian study found that long-term exposure to air pollutants was significantly associated with high BP (Chuang et al., 2011). However, a meta-analysis of 15 European cohorts found that modeled air pollutants (PM2.5, PM2.5 absorbance, PM2.5-10, PM10, NOx (nitrogen oxides), NO2) were not clearly associated with BP or hypertension (Fuks et al., 2014). Despite this heterogeneity, one recent review summarized these published studies and concluded that the overall evidence supported a positive association of both long-term and short-term ambient air pollution with BP parameters (Giorgini et al., 2016). Notably, these studies linking air pollution and hypertension were conducted in different parts of the world with large variations in subject characteristics (healthy subjects versus patients), pollutant sources and composition, as well as the duration of exposure (long-term and short-term exposures), suggesting support for the universality of this association (Giorgini et al., 2016).
However, the extent to which ambient air pollutant exposure impacts prehypertension remains largely unknown. Given the importance of prehypertension in human health and disease prevention, the ubiquitous nature of ambient air pollution exposure, and the paucity of research on the relation between the two, this study aimed (1) to explore whether long-term air pollution exposure was related to prehypertension prevalence; and (2) to explore whether prehypertension is more strongly associated with air pollution than hypertension by analyzing data from the 33 Communities Chinese Health Study (33CCHS), a large cross-sectional study in Northeastern China. The geographic study area is comprised of wide variations in inter- and intra-city air pollution concentrations and a large population, providing an unique opportunity to address the link between ambient air pollution and prehypertension.
Section snippets
Subject recruitment
The 33CCHS study was conducted in Liaoning province, with the aim of examining the long-term effects of ambient air pollution on cardiovascular diseases and their risk factors. Liaoning is located in Northeastern China with a population of over 20 million people. It has a northerly continental monsoon climate. Winters are long, cold, and dry, while summers are short, warm and relatively wet. Annual average temperature ranges from 5 °C to 10 °C, showing a decreasing trend from coastal area to
Results
The baseline characteristics of the survey participants by BP classification and sex are summarized in Table 1 and Table S1, respectively. Overall, 51.0% were men, and the mean age of participants was 45.59 years (SD = 13.31 years). The prevalence of prehypertension and hypertension was 37.78% and 34.84%, respectively. Significant differences existed between normotension and prehypertension groups in all demographic, social-economic and behavioral variables with the only exception being
Discussion
Little is known regarding the relationship between ambient air pollution and prehypertension. The current investigation begins to fill this gap by finding a significant association between long-term exposure to ambient air pollutants and the prevalence of prehypertension, as well as arterial SBP and DBP in 24,845 adults from Northeastern China. This association was found to be stronger for females and the elderly. We also found that prehypertension may be more strongly associated with air
Conclusions
In this population-based cross-sectional study, long-term exposure to air pollutants (PM10, SO2, NO2, and O3) was associated with prehypertension and increased arterial BPs in adults. Females and the elderly were more vulnerable to ambient air pollutants. Additionally, prehypertension may be more strongly associated with ambient air pollution than hypertension. Reducing the risk of prehypertension is an added reason to reduce population-level exposure to ambient air pollutants. Further
Sources of funding
This work was supported by grants from the National Key Research and Development Program of China (2016YFC0207000); Major Program of National Natural Science Foundation of China (91543208); the Fundamental Research Funds for the Central Universities (No. 16ykzd02); and the Guangdong Province Natural Science Foundation (2014A050503027, 2016A030313342).
Conflicts of interest
None.
Acknowledgements
The authors acknowledge the cooperation of participants in this study who had been very generous with their time and assistance. We are also grateful to the anonymous reviewers (especially the reviewer #2) for their very insightful and constructive comments.
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This paper has been recommended for acceptance by Dr. Chen Da.