In a recent study published in Environmental Research, researchers conducted a nationwide cross-sectional analysis of the long-term consequences of particulate matter (PM_2.5) and its five components on hypertension among participants from 13 Chinese provinces between March 1 and July 31, 2021. The PM_2.5 components analyzed in this study were organic matter (OM), black carbon (BC), sulfate (SO4₂⁻), nitrate (NO3⁻), and ammonium (NH4⁺).

In addition, the researchers studied the effects of PM_2.5 components on diastolic and systolic blood pressure, DBP, and SBP.

Background

The global burden of hypertension continues to increase worldwide, with a five-fold increase in cases in China affecting ~27.9% of their population. Among other factors, air pollution is a major etiologic factor contributing to hypertension, responsible for over 50% of cardiovascular deaths globally.

The fine particulate matter, i.e., components with an aerodynamic diameter ≤2.5 μm (PM_2.5), has been increasingly recognized as a cause of hypertension. Yet, few studies have investigated their long-term effects. Also, studies have not evaluated the magnitude of each PM_2.5 component's contribution to hypertension.

About the study

In the present study, researchers used the geocoded residential address of all the eligible participants, 113,159 in number, to assign a daily exposure concentration of PM_2.5 and its five components. They used the average exposure of the previous three years to determine the long-term exposure window of all participants. The team included only those participants in the analysis who met one or more of these criteria, had self-reported hypertension, were taking BP-lowering medication, or had SBP ≥140 and DBP ≥90 mmHg.

They used a mixed-effect regression model with covariates adjustments to assess three-year average concentrations of PM_2.5 components on the prevalence of hypertension, DBP, and SBP, and SHapley Additive exPlanation (SHAP) to compare the contribution of these components separately.

Based on the eXtreme Gradient Boosting method, SHAP helped the researchers identify the most pertinent feature for hypertension and showed how much a particular feature of hypertension altered the prediction of the study model. Finally, they compared the SHAP value of each PM_2.5 component and averaged it by the aggregate value computed from the model with all five components.

The model covariates included gender, age, body mass index (BMI), cigarette and alcohol abuse, physical activity, family history, and education. They considered the province a random variable to adjust and the clustering within provinces. The researchers stratified study data based on gender and three age subgroups, 18 to 44, 45 to 59, and ≥60 years, to examine the Chinese population's susceptibility.

Study findings

Of 113,159 eligible adult study participants, 35.22% had hypertension, with the prevalence of hypertension being higher in males than females (39.54% versus 31.88%). The mean PM_2.5 exposure of all participants was 38.81 μg/m³.

As expected, the researchers noted a significant correlation between long-term exposure to PM_2.5 and its components and the increased frequency of DBP and hypertension. Males with hypertension were more susceptible to NO3⁻, NH4⁺, and SO4₂⁻. They were also at a higher risk of DBP due to these PM_2.5 components. Likewise, the elderly were at a higher risk of all three conditions due to increased exposure to PM_2.5 and all its five components. In fact, PM_2.5 had the highest effect on hypertension, with an odds ratio of 1.10. NO3⁻ posed the most risk for hypertension, DBP, and SBP compared to other PM_2.5 components.

With the development of the economy, the number of motor vehicles in China continues to increase, reaching 298.7 million in 2020. Thus, the increased contribution of NO3⁻ to hypertension, DBP, and SBP, most likely indicated increased traffic in China. Thankfully, with the increased use of clean energy, not sulfur-containing coal, the concentration of SO4₂⁻ dropped significantly.

In the age-stratified analysis, PM_2.5 and its components had more noticeable effects on hypertension and SBP among people aged over 60 years than people in the 18 to 44 and 45 to 59-year age groups. Likewise, in the sex-stratified analysis, the effect sizes of PM_2.5 and its components on DBP were statistically greater among males than females, and males were also at significantly higher risks of hypertension due to NO3⁻. Males make more unhealthy lifestyle choices, such as smoking and alcohol abuse. Moreover, they likely work outside and drive, which increases their potential exposure to PM_2.5 and its constituents.

Similarly, deterioration of immunity makes older people more susceptible to developing hypertension, also SBP when exposed to PM_2.5 pollutants, which is a recognized risk factor for cardiovascular events. The effects of PM_2.5 components on DBP are not easily detectable in the elderly because of the reduced elasticity of their arterial walls. Thus, its effects were more pronounced in the age group 45 to 59 years.

Conclusions

The study results demonstrated a statistically significant correlation between PM_2.5  and its components and hypertension as well as blood pressure. Other notable findings were that males and the elderly were more susceptible to PM_2.5 and its five components, with NO3⁻ having the most adverse effects. These findings could inform clinical strategies to prevent hypertension exacerbated by PM_2.5 and its components.

Written by

Neha Mathur

Neha is a digital marketing professional based in Gurugram, India. She has a Master’s degree from the University of Rajasthan with a specialization in Biotechnology in 2008. She has experience in pre-clinical research as part of her research project in The Department of Toxicology at the prestigious Central Drug Research Institute (CDRI), Lucknow, India. She also holds a certification in C++ programming.