3.1 Descriptive statistics
A total of 33,391 participants were measured BP, and 1,256 participants were excluded due to missing information on important potential confounders and hypertension history. The basic characteristics of the 32,135 study subjects are summarized in Table 1. Participants were on average 50.0 years and 55.4% of them were women. Most participants were Han, 72.1% of them live in urban, 30.7% of them have been a smoker and 42.7% of them used to drinking. There are 40.8% and 58.5% (Table A1) of the participants diagnosed as hypertension according to Chinese and AHA Hypertension Guideline, respectively. There were significant differences in all variables between hypertension defined by Chinese or AHA/ACC guideline and non-hypertension group.
Table 2 shows the 3-year average concentrations of six air pollutants exposure for the 32,135 participants. The average concentration of SO2 was 22.9μg/m3, which were lower than the National Ambient Air Quality Standards of China (60μg/m3), however, the 3-year average concentration of PM2.5, PM10, NO2 were 73.5μg/m3, 123.0μg/m3 and 52.2μg/m3, respectively, which exceeded the National Ambient Air Quality Standards of China (35μg/m3, 70μg/m3 and 40μg/m3). The temperature of testing day was 14.7±12.2℃ and the humidity was 55.5±21.5%. The correlations of all air pollutants were moderate with each other. The mean of proximity to a major roadway was 408.5±282.3m was negatively correlated with all air pollutants.
3.2 Associations of air pollutants with blood pressure and hypertension
The associations of long-term exposures to air pollutants with blood pressure are presented in Table 3. Totally, the IQR increment in PM2.5, PM10, SO2 and NO2 were significantly associated with an elevation of 0.66mmHg (95%CI: 0.29, 1.03), 0.40mmHg (95% CI: 0.00, 0.81), 1.38mmHg (95% CI: 0.92, 1.84), 0.54mmHg (95%CI: 0.16, 0.91) in SBP, and PM2.5, SO2 and NO2 can increase PP by 0.42mmHg (95% CI: 0.09, 0.74), 1.07mmHg (95%CI: 0.71, 1.42), 0.49mmHg (95%CI: 0.20, 0.79), respectively (Table 3). Furthermore, PM2.5 and SO2 were associated with increases of 0.37 mmHg (0.09, 0.65) and 0.65 mmHg (95%CI:0.33, 0.98) in MAP, SO2 were associated with increases of 0.42mmHg (95%CI: 0.11, 0.72) in DBP.
Participants with PM2.5 and SO2 values in the fourth quartiles had much higher SBP, DBP, MAP, PP than those in the lowest quartile, and the air pollutants values in the second, third, and fourth quartiles had increased BP levels which compared with the lowest quartile (P for trend<0.01, Table 3). We found similar results between 3-y average NO2 and SBP or PP. No significant association was found between PM10 and any kinds of blood pressure.
The associations for air pollutants and Chinese/AHA hypertension guideline are shown in Table 4.
For Chinese hypertension guideline, compared with the lowest quintile (Q1) of PM2.5, SO2 and NO2, the risk of hypertension with the highest quintile (Q4) was increased by 57% (OR=1.57, 95%CI: 1.36, 1.8), 33% (OR=1.33,95CI%: 1.15, 1.53) and 13% (OR=1.13, 95CI%: 1.02, 1.26), respectively. When air pollutants were treated as continuous variables, all pollutants can significantly increase hypertension. For every interquartile range (IQR) increase in PM2.5, PM10, SO2 and NO2 exposure, the risk of hypertension increased by 14% (OR=1.14, 95%CI: 1.06, 1.23), 6% (1.06, 1.00-1.27), 9% (1.09, 1.02-1.17) and 8% (1.08, 1.00-1.16), respectively. The similar results were found in PM2.5 and SO2 with hypertension defined by AHA/ACC guideline, compared with the lowest quintile of PM2.5 and SO2, the risk of hypertension with the highest quintile was increased by 77% (OR=1.77, 95%CI: 1.54, 2.03) and 92% (OR=1.93, 95CI%: 1.21, 2.64), respectively. For every interquartile range (IQR) increase in PM2.5 and SO2 exposure, the risk of hypertension defined by AHA/ACC guideline increased by 16% (OR=1.17, 95%CI: 1.09, 1.25) and 10% (OR=1.10, 95%CI: 1.03, 1.18). However, no significantly associations were found between PM10 and NO2 exposure with the risk of AHA/ACC hypertension.
In two-pollutants model of air pollutants and blood pressure (Table A5), we found similar results as the single-pollutant model. The combined effects of SO2 with other pollutants can increase blood pressure (SBP, DBP, MAP and PP), PM2.5 combined with other pollutants can increase blood pressure except DBP, PM10 combined with SO2 can increase SBP and PP, NO2 combined with other pollutants can increase SBP. In two-pollutants model of air pollutants and hypertension (Table A6), we found PM2.5 combined with other pollutants and PM10 combined with SO2 can increase hypertension defined by both Chinses and AHA/ACC guideline. SO2 combined with PM10 can only increase hypertension defined by AHA/ACC guideline. The combined effect of NO2 and other pollutants showed non-significant results.
3.3 Associations of proximity to roadway with blood pressure and hypertension
Participants whose residential address was ≥1000 m to a roadway were excluded in this association analyses, and 23,503 (73.1%) participants were remained. The main characteristics of this study participants were similar to Tabe1 (Supplementary materials Table A2). The result for proximity to roadway and blood pressure are shown in Figure1. Participants who live within 100m from major roadway are associated with significantly higher SBP and PP, and the effect decreased with the distance of proximity to roadway (P for trend=0.004 and 0.01, respectively). Participants who live within 100m from major roadway are associated with an increasing risk of Chinese/AHA hypertension and the OR decreased with the distance of proximity to roadway (P for trend=0.03 and 0.02).
3.4 Stratified analyses for the associations of air pollutants with blood pressure and hypertension
The results of the stratified analysis of air pollution exposure and blood pressure were shown in Figure 2. The associations between PM2.5, SO2, NO2 and PP in the age group≥65 years were more significant. PM10 was associated with significantly higher DBP and MAP in the age group≥65 years. We also found higher associations of SO2 and all four blood pressure values among participants in rural area.
The results of stratified analysis for hypertension are shown in Figure 3. The associations between PM2.5 and SO2 and hypertension were significantly higher in the age group<65 years in both Chinese and AHA/ACC hypertension guideline. We also found stronger associations between SO2 and hypertension in rural participants.
3.5 Sensitivity analyses
The results of sensitivity analysis are presented in Supplementary materials. The associations of blood pressure and hypertension to long-term air pollution exposure did not substantially alter in the sensitivity analyses by using 1- to 3-year average concentrations (Table A3, A4). When we excluded the patients with antihypertensive therapy (Figure A1, A2) and cardiovascular cases (Figure A3, A4) from the data, the results remained robust.