Examining the causal link between long-term PM1 exposure and hypertension hospitalization risk

Hypertension is among the leading cardiovascular diseases. Despite extensive research, evidence concerning the relationship between long-term exposure to ambient particulate matter and hypertension remains limited and inconsistent, particularly with regard to submicron particulate matter (PM₁). While randomized controlled trials are considered the gold standard for causal inference, environmental epidemiological studies typically rely on observational data. Traditional approaches in observational studies are less effective than randomized controlled trials in fully controlling for confounding factors to achieve results with causal interpretability. With the advancement of causal analysis methods for observational data, an increasing array of causal inference methods that control for confounding bias are being developed and applied to mimic randomized controlled trials in obtaining effect estimates with causal interpretability.

Led by Yuan-Tao Hao from Peking University, Beijing, China, this study employed marginal structural Cox models utilizing several inverse probability weighting methods to investigate the potential causal effect of hypertension hospitalization from long-term exposure to PM₁ and PM_2.5, based on the Pearl River cohort in Guangdong, China. Exposure data were obtained from the ChinaHighAirPollutants (CHAP) dataset, provided by Professor Zhan-Qing Li and Dr. Jing Wei from the University of Maryland, which offered high-quality, high-resolution near-surface air pollution data. PM₁ and PM_2.5 concentration levels were estimated using space-time extremely randomized tree models based on high-resolution satellite data. The study focused on the endpoint of initial hospitalization for both overall hypertension and essential hypertension.

Between January 1, 2015, and December 31, 2020, a total of 36 thousand adult participants were included in the study, with males comprising 40.6%. The mean age of participants was 50.9 years, with an average of 6.0 years and approximately 210 thousand person-years of follow-up. During the study period, the annual average concentrations of PM₁ and PM_2.5 were 17.4 μg/m³ and 33.7 μg/m³, respectively. According to the current Chinese "Ambient Air Quality Standards" (GB 3095-2012), the concentration of PM_2.5 in the study was slightly lower than the annual average concentration standard of 35 μg/m³ in China However, it significantly exceeds the air quality guidelines (AQG) released by the World Health Organization in 2021, which recommend an annual average concentration of 5 μg/m³. Due to the limited research, neither international nor Chinese standards for PM₁ concentration have been established.

For every 1 μg/m³ increase in 1-year PM₁ exposure, the risks of initial hospital admissions due to overall hypertension and essential hypertension increased by 13.1% and 15.3%, respectively. For 2-year PM₁ exposure, the risks of initial hospital admissions due to overall hypertension and essential hypertension increased by 20.7% and 19.0%, respectively. These findings suggested that the harmful effects of long-term PM₁ exposure persisted for at least 2 years. The risk of hypertension hospitalization associated with long-term PM₁ exposure was 6.0%–11.0% higher compared to PM_2.5 exposure. The study revealed a linear exposure-response relationship between PM₁ exposure concentration and the outcome, suggesting that as PM₁ concentration increased, the risk of hypertension hospitalization generally increased monotonically, without a safe threshold. The study also conducted comprehensive identification of susceptible populations, revealing, for instance, that the risk of hypertension hospitalization in females was 15.3% higher than that in males. The results underscore the importance of controlling PM₁ particulate pollution and protecting vulnerable populations.

This study contributes additional evidence supporting potential causal associations between long-term exposure to PM₁ as well as PM_2.5 and the risk of hypertension hospitalization. Reducing particle pollution could be beneficial in lowering the risk of hypertension-related hospitalization within 1–2 years. These findings provide valuable scientific insights for the development of future air pollution standards and related public health policies in China.