Air pollution has been a global public concern. According to the World Health Organization (WHO), more than 99% of the global population lives in area with air pollution (1). In 2021, particulate matter air pollution was identified as the leading factor contributing to the global disease burden (2), accounting for 6.7 million premature deaths annually on a worldwide scale (3). Approximately 4.2 million deaths are attributed to ambient air pollution, with 25% of these fatalities linked to ischemic heart disease (4). In China, the mortality rate from CVD due to ambient particulate matter pollution has shown a consistent increase from 1990 to 2019 (5). Reports from the National Center for Cardiovascular Disease of China indicated that in 2019, more than 1.42 million excess deaths were associated with ambient particulate matter pollution (3). The high exposure risks and limited regenerative capability make the cardiovascular system become vulnerable to the hazardous effects of air pollution. Exposure to air pollution, such as particulate matters (PM₁₀ and PM_2.5), SO₂, NO₂, O₃ and CO, has been associated with a significant increase in the risk of CVD (1). A nationwide cohort study conducted in China demonstrated that each 10 μg/m³ increment in PM_2.5 exposure, corresponded to a 25.1% increase in CVD and a 16.4% rise in CVD mortality (6). Recent empirical evidence also indicates that maternal exposure to air pollution is associated with an elevated risk of congenital heart defects in newborns. A nationwide study of 1,434,998 births found a 2% increase in risk of congenital heart defects for every 10 μg/m³ rise in PM_2.5 exposure (7, 8). These results underscore the significant hazards that air pollution poses to CVD. Therefore, investigating the impact of air pollution on cardiovascular health is important. Given the vast geographical span of China, air quality exhibits considerable variation across different regions. It is crucial to analyze localized data and evaluate the detrimental effects of air pollutants in specific areas. Moreover, the relationship between air pollution and CVD has not been previously documented in Nanchong. In this study, we utilized hospital medical records from Nanchong to assess the influence of air pollutants on hospital admissions related to cardiovascular disease.

This study represents the first attempt in Nanchong to investigate the association between air pollution and cardiovascular health. Generally, this study showed that short exposure to air pollution includes PM₁₀, SO₂, NO₂ and O₃ were associated with increased risk of admission among patients with cardiovascular disease. Furthermore, the effect of PM₁₀ and SO₂ on cardiovascular disease are varied by sex.

Particle matter has been adapted as a main indicator of air pollution worldwide. Particles pollutants PM₁₀ and PM_2.5 are named according to their aerodynamic diameters. The source of particles varies among different areas. For example, compared to developed counties, Pakistan suffered a higher PM_2.5 concentration, in which also contain more heavy metal elements (13). Among the arid regions around the deserts such as Northern Africa, the Arabian Peninsula, Central Asia, the dust storms contribute significantly to local particle pollutant. Particles originating from dust storms typically contain mineral dust, whereas particulate matter emission by human activities is usually more complex and contains a higher proportion of organic and metal components (14, 15). However, due to the geographical location, air quality of Nanchong rarely affected by sandstorms. Therefore, the relationship between particles pollutant and wind speed in Nanchong is opposite with cities near sandstorms. In Nanchong, the wind can facilitate the dispersion of air pollutants, resulting in pollutant concentrations decreased in urban areas. Similar with most of the country, the concentration of particles matters peak during winter. This may be due to the dryness in winter promote the formation of particles matters, while adequate precipitation captures and remove aerosol particles from the atmosphere in summer (16). According to the update guidelines, WHO recommends a stricter criterion in which the recommendation daily concentration of PM_2.5 was 15 μg/m³ and 45 μg/m³ for PM₁₀ (17). Over the past 10 years in Nanchong, although the concentration of particles has continued to decrease, the average daily concentration of PM_2.5 is 42.39 μg/m³, which remains significantly higher than the standard. PM₁₀ and PM_2.5 shown a great effect on human hearth due to that they can penetrate respiratory system and deposited in organ and tissues following the blood follow, especially in heart. Different from the other organs, heart is characterized as high energy demand and low repair capacity. On the one hand, PM disturb the oxidative respiratory chain and oxidation–reduction system, resulting in oxidative stress in heart. On the other hand, as a foreign substance, PM arouse inflammation response, however, inflammation response does not always clear they, resulting in continuous and excessive inflammation which promote heart disease. At the same time, particle matters also disturb the coagulation system and promote the aggravation of myocardial ischemia in coronary heart disease patients. A time-series analysis conducted in New York state also revealed that a 10 μg/m³ increment in PM_2.5 concentrations contribute 1.37% increase in cardiovascular disease hospitalization (18). A prospective longitudinal cohort study conducted in Ohio found that exposure to PM_2.5 increased risk of incident myocardial infarction (19). Another research conducted by Ma et al. (20) found that 10 μg/m³ increase on the concentration of PM₁₀ was related with 14% increase in cardiovascular disease hospitalization on lag1. Furthermore, our results showed that PM_2.5 has a stronger effect on cardiovascular disease compared to PM₁₀. The greater impact of PM_2.5 may be due to its smaller particles size, which results in a higher surface area. This make PM_2.5 particles more likely to reach cardiovascular and exhibit higher reactivity ability (21, 22).

Since there are no volcanoes and heavy industry in Nanchong, the main source of SO₂ and NO₂ is the burning of fossil fuels, especially from transportation vehicles. WHO recommends 40 μg/m³ and 25 μg/m³of daily average concentrations for SO₂ and NO₂, respectively. In Nanchong, the daily average concentrations of SO₂ and NO₂ are lower than this standard. However, we also found that short exposure to SO₂ and NO₂ also can increase the number of hospitalizations count of cardiovascular disease patients. A study conducted in western China reported that cardiovascular admissions increased by 4.5%(95% CI 1–8.2%) for every 10 μg/m³ increase in SO₂ in non-dust days (20). Mechanistic investigation revealed that SO₂ induce mitochondrial and oxidation–reduction system through grapes transcriptome (23). Likewise, with pungent odor, SO₂ will induce acute lung response and inflammation, which also can affect heart function through autonomic nervous system. Furthermore, most NO₂ is quickly formed in the atmosphere from NO emissions by vehicles. A global assessment revealed that a 10 μg/m³ increase in NO₂ concentration was associated 0.37% cardiovascular mortality among 398 cities (24). A Meta-analysis enrolled 204 time-series reported that a 10 μg/m³ increase of NO₂ associated with 0.88% cardiovascular mortality (25). A prospective longitudinal observational cohort study conducted in England reported that NO₂ promotes the development of cardiac remodeling and decrease cardiac pumping function (26). Moreover, NO₂ has been implicated in promoting airway inflammation and increasing the airways’ vulnerability to viral infections (27). Among asthmatic children, NO₂ increase the risk of respiratory viral infection and resulting in severity asthmatic (28, 29). This also contributes to the development of cardiovascular disease, especially for patients with previous heart disease.

Different from other air pollutant, O₃ is not directly emitted by primary source. Nitrogen dioxide and volatile organic compounds works as a precursor in atmospheric and promote the formation of O₃, in which UV radiation or thunderstorm activity is needed (30). Therefore, we can easily be found in Figure 2 that the concentration keeps a high concentration in summer and spring. The Ozone layer in atmosphere protects human from the UV radiation; however, excessive ground-level ozone is harmful to human health. Cumulative and lag effect also have been revealed in O₃. The WHO recommended concentration of O₃ is 60 μg/m³. In Nanchong, the average concentration of O₃ is 69.63. μg/m³. A nationwide cohort conducted in China reported that per 10 μg/m³ increase in ozone concentrations related to 18.4% increase in cardiovascular disease mortality (31). As a strongly oxidative and highly reactive pollutant, O₃ may induce coronary artery spasm and trigger acute episodes of ischemic heart disease (32). Furthermore, O₃ also induce oxidative stress and inflammatory (33).

Consistent with previous studies, our results suggest that elder adults are more susceptible to air pollution, especially for 70–80 years old (34). Older adults are more likely suffer from chronic disease including CVD. Air pollution can trigger acute episode of CVD, leading to an increased need for urgent hospitalization. Furthermore, patients over the age of 80 usually have less outdoor activity and receive more care, which protect them from the hazards of air pollution. Compared to females, males show greater vulnerability to PM₁₀, possibly related to that males are more likely to work outdoor, increasing their exposure to high level of particles matter. Additionally, our results reveal that the effect estimates for moving average lags were much higher than those for single day lags. This significantly indicated that the hazardous of air pollution to CVD exhibit a cumulative effect. It’s also underscores that efforts to reduce continuous exposure to air pollution yield substantial benefits.

In this study, we employed patients’ addresses from medical records to assess air pollution exposure, which may lead to exposure misclassification. Migrant workers who provided domicile addresses rather than actual residential addresses were excluded, potentially underestimating the impact of air pollution on cardiovascular disease. Additionally, factors such as traffic volume, mask-wearing, and mode of transportation were not fully accounted for, which could introduce bias. The potential impact of participant mobility within urban areas during the study period was also not addressed. While a single-pollutant model was used for simplicity, multiple-pollutant models are needed in future research to better capture real-world exposure. Finally, since the study was limited to Nanchong, caution should be exercised when generalizing the results to other regions.

Our study assesses the impact of air pollution on CVD patients in Nanchong, a region with a subtropical humid monsoon climate and mild air pollution. Our findings reveal short exposure to high concentrations of air pollution increases the risk of hospital admission for CVD. Efforts should be devoted to avoiding exposure to high and consistent levels of air pollution, especially for elder patients.

At the same time, medical institutions should prepare enough medical sources. Furthermore, patients aged 70–80 years old are more vulnerable to high air pollutant. These results suggest that older adult patients require additional care and further precautions to avoid exposure to high pollution levels. Policymakers need to develop policies to combat air pollution and ensure sufficient medical resources are available after severe air pollution days.