PM2.5 has become the primary air pollutant affecting public health. Studies have shown that the current PM2.5 exposure in most areas of China is still at a relatively high level, and the improvement in health benefits attributed to the decline of the exposure level is less than the decline itself (Zhang et al., 2019). The daily limit of PM2.5 for Grade-I is 35 µg/m3, 70 µg/m3 for Grade-II, and over 250 µg/m3 indicating serious pollution (Zhou et al., 2016). Results of this study show that the daily average level in the past week was as high as 40.33 µg/m3, and the lowest 34.50 µg/m3 was close to Grade-I. The average PM2.5 level in the past year was 31.79 µg/m3, and the highest was in winter up to 49.33 µg/m3. Our data has similar trend with a previous study that PM2.5 concentration in winter of Jiaxing city is more serious with obvious differences in different seasons, which is greatly affected by pollutant transportation from surrounding cities (Zhao et al., 2019b). This indicates that there are still certain problems in the air quality in local. Though overall PM2.5 in long term is low, the exposure level exceeds the national standard limit within days, and season-specific especially for winter should be alerted. Therefore, the monitoring in this place should be continuously strengthened.
Hypertension is one of the risk factors for cardiovascular disease. There is 23.2% (≈ 244.5 million) of the Chinese adult population ≥ 18 years of age having hypertension, and 41.3% (≈ 435.3 million) having high-normal blood pressure (prehypertension) according to the Chinese guideline (Wang et al., 2018). Data in this study found that the average SBP was 117.78 mmHg, and the prehypertension accounted for 11.22%, and hypertension (≥ 140 mmHg) for 2.51%. The average DBP level was 75.48 mmHg with prehypertension 26.37%, and hypertension (≥ 140 mmHg) 4.53%. The prevalence of prehypertension and hypertension is lower than that in other studies according to a prior investigation in 2017 in China showing that the hypertension prevalence ≥ 18 years among 14, 220 permanent residents is up to 46.9% (Shen et al., 2017). A recent study from a cohort of 20 year-follow-up in China manifests that compared with the young and middle-aged population with no cardiovascular disease at baseline BP or lower than 120/80 mmHg, the risk of cardiovascular disease including coronary atherosclerotic heart disease (coronary heart disease), stroke, and cardiovascular death is significantly increased in the pre-hypertensive stage at 130–139/80–89 mmHg, and more than 60% of the pre-hypertensive middle-aged and young people has blood pressure progressed to ≥ 140/90 mmHg after 15 years, with their cardiovascular disease risk three times higher than that of normal BP (Qi et al., 2018). In addition, we found that the accuracy of response to the diagnostic criteria of hypertension among young individuals was no more than a half (41.41%). Among the behavioral factors, BP measurement at least once a week accounted for only 4.06%, but quite a few liked to eat oily food (25.54%) and salty food (35.08%). A small number of young students (17.18%) had mental stress in the past year, and most students stayed up late (versus 7.76% no stay-up). These results suggest that young adults are under unreasonable diet and lifestyle with little awareness of the risk of hypertension. Health promotion and education of hypertension for young people should be paid attention, and those with high-normal blood pressure should receive clinical check.
In the study, both SBP and DBP to varying degrees were positively associated with age, height, weight, BMI, smoking every day, smoking every day, excessive alcohol consumption, mental stress in the past year and stay-up. Studies have shown that the occurrence and development of hypertension is closely related to age, overweight and obesity, family history of hypertension, alcohol consumption, high uric acid, and high C-reactive protein (Flack and Adekola, 2020). This demonstrates that diet, lifestyle, and mental state are related to changes in blood pressure, and unreasonable diet, lifestyle, and mental stress are potential risk factors for hypertension. In recent years, studies have found that PM2.5 exposure levels are related to hypertension, but there are differences between the research conclusions. Ren et al. (Ren et al., 2019) used a general linear model to analyze the association between daily and fixed monitoring point PM2.5 exposure and blood pressure among young people in short-term 1 to 3 days, exhibiting that PM2.5 exposure is related to the reduction of ambulatory blood pressure, and after adjusting for age and other confounding factors, SBP and DBP was decreased by 0.54 mmHg and 0.22 mmHg for an increase of 10 µg/m3 PM2.5, respectively, while decreased by 0.95 mmHg and 0.74 mmHg at fixed monitoring stations, respectively. Baumgartner et al. (Baumgartner et al., 2011) analyzed the relationship between 24-h PM2.5 exposure and blood pressure among female adults ≥ 25 years by mixed linear models, and find that an elevation of 10 µg/m3 increase in PM2.5 is associated with an increase of 2.2 mmHg in SBP and 0.50 mmHg in DBP. Our study found that SBP was increased by 1.07, 1.25, and 1.01 mmHg for every 10 µg/m3 increase in PM2.5 at Day 1, Day 3, and Day 5, respectively, and DBP increased by 1.06, 1.28, 1.29, 0.87 and 0.56 mmHg at Day 1, 3, 5, 15, and 30, respectively. For long-term exposure, an increase of 1.17 mmHg in SBP was associated with each 10 µg/m3 increase in PM2.5 level in winter. These results show that the PM2.5 exposure level in this area is related to the increase in blood pressure. Further we adjusted the potential confounding factors, finding that for every 1 µg/m3 increase in PM2.5, the risk of SBP was increased by 6% and 4% at Day 1 and Day 5, respectively, and DBP risk increased by 3% and 16%, respectively. SBP risk was increased by 13% and 7% in spring and winter, respectively, and DBP in winter increased by 12%. Some studies have shown that by modifying blood pressure–related factors such as psychosocial stress (Hicken et al., 2014), gender and age (Baumgartner et al., 2011), the influence of short-term PM2.5 on blood pressure is changed, suggesting that potential risk factors should be considered when assessing the exposure association with PM2.5. The concentration of PM2.5 also plays a critical role as exposure in winter or spring is relatively higher than other seasons, and makes a significant change in SBP or DPB. We also tried to explore the detailed linkage between PM2.5 exposure and blood pressure by stratification of gender or age, but the model failed to distinguish due to the limited sample size of subjects when separating into different gender or age. Thus we adjusted the gender and age in the whole model. The mechanism of PM2.5 exposure for blood pressure change also has been extensively studies. PM2.5 exposure can activate the inflammatory response in the arcuate nucleus of the thalamus in the acute blood pressure response, leading to up-regulations of pro-inflammatory factors and inhibitory factors kappaB kinase (IKK)/nuclear factor kappaB (NF-κB), which is related to abnormal activation of the cerebral sympathetic nervous system (Ying et al., 2014). PM2.5 exposure can also induce inflammation and oxidative stress in the circulatory system of hyperlipidemia rats, activating the JNK/P53 pathway and promoting hypercoagulability and cardiomyocyte apoptosis (Wang et al., 2019a). Therefore, PM2.5 exposure can affect blood pressure by activating oxidative stress or inflammation.
This study also has certain limitations. First of all, the scope and time of indoor or outdoor activities of subjects are different, and the PM2.5 monitoring at a fixed station cannot accurately reflect their daily exposure level, and there is a certain bias. Secondly, the clinical recommendation of blood pressure measurement is to maintain a resting state when you wake up in the morning, and the measured blood pressure is relatively accurate. This study selected a time period in the morning which had little effect on the stable blood pressure individuals, but it might bring some changes to those with significant fluctuating people. Air pollutant exposure is quite complex posing a wide range of health effects on the population, but we did not include all such as PM10, SO2, NO, etc. We only focused on the relationship between PM2.5 exposure and blood pressure changes since the concentration of PM2.5 pollutant is highlighted and typical in this area.