O
3 was the most controversial air pollutant related to CVD. Several studies have shown that O
3 has a positive correlation with CVD [
3,
4,
8,
9,
16,
26,
30,
35]. Short-term inhalation of fine particulate matter and O
3 has caused acute conduit artery vasoconstriction [
16]. Two-pollutant models have indicated that the impact of O
3 has been significant in combination with another pollutant on warm days [
26]. The OR and 95% CI estimated from general additive models for an interquartile range increase in O
3 (20.5 ppb) was 1.010 (CI: 1.002–1.017) [
30]. However, other studies have shown that O
3 was not associated with CVD or was negatively correlated [
5,
10,
11,
13,
18,
19,
21]. All the main air pollutants, with the exception of O
3, were significantly associated with an increase in the risk of myocardial infarction; for O
3, the relative risk (RR) was 1.003 (CI: 0.997–1.010) [
8]. Hospitalization or death due to heart failure was not associated with increases in O
3 concentrations (0.46% per 10 ppb, CI: − 0.10-1.02) [
10]. The RR of admission for ischaemic heart disease and 95% CI for interquartile range increases in O
3 were 1.010 (0.990–1.032) [
5]. In some studies, the analyses for O
3 were restricted to the warm period (May to October) [
36]. In our study, O
3 levels in 365 days were negatively associated with CVD outpatient visits. However, there was no longer a negative relationship in the 183 days of the warm period, although CVD outpatient visits did not increase substantially. One study from southern China showed that the impact of O
3 on CVD mortality was stronger during high-exposure months (September to November) after adjustments were made for PM
10 [
37]; these findings differed from our results. It is well known that O
3 is the main component of photochemical smog (85%) that is the product of NO
2 and volatile organic compounds under strong sunlight. The reason for this negative correlation in our study may be that the O
3 concentration in Shijiazhuang City was significant different between the winter (for example, O
3 averaged 13.6 μg/m
3, air temperature averaged − 4.2 °C in January) and the summer (O
3 averaged 148.5 μg/m
3, air temperature averaged 24 °C in Jun), and that the amount of CVD outpatient visits was opposite to the concentration of O
3 (averaged 274 people in January, 249 people in June, without adjusting any factors). The impact of air temperature on CVD outpatient visits was significant, but that of O
3 was not significant, and there was no significant interaction between both of them (not shown). The our results after adjusting for air temperature showed an actual relationship between O
3 and CVD outpatient visits, which were similar to other studies [
1,
5,
11,
13,
18,
19,
21,
36]. The reason for the difference between the north and the south in China may be that O
3 level in winter of Guangzhou was still high (O
3 averaged 50 μg/m
3, air temperature > 10 °C in January) [
38], but O
3 level in winter of Shijiazhuang was very low (averaged 13.6 μg/m
3, air temperature averaged − 4.2 °C in January). Therefore, it should be paid an attention to the difference between the O
3 concentration and CVD outpatient visits in the warm and cold periods, especially when there is a negative correlation.
In our supplemental files, whether for the risk of increasing CVD outpatient visits per unit increase of air pollutant or for Pearson partial correlation coefficient between daily CVD outpatient visits and air pollutants, the trend of both results was consistent with the trend of the results from the main table and figures. The supplemental results also revealed that the impact of PM10, PM2.5 and CO on CVD outpatient visits was stronger than that of SO2, NO2 and O3.
Our study has three strengths. First, we explored the relationship between the different concentrations of air pollutants and the amount of CVD outpatient visits in a severe haze-fog city using GLM (LSmeans) and logistic regression (ORs). Second, our study revealed that when the concentration of air pollutants increased, an increase was seen not only in the number of CVD outpatient visits but also in the risk of the increase in CVD. Third, although the analytical methods differed from each other, the impacting trend of air pollutants in this study was consistent with those of other studies; specifically, PM10, PM2.5, and CO were the main risk factors for the increase in CVD outpatient visits. However, our results also have limitations. First, the climate in northern China (Beijing or Shijiazhuang) and southern China (Shanghai or Guangzhou) varies greatly, and our results for O3 may be different from those of other cities in southern China. Second, because Shijiazhuang is only one of many heavily polluted cities in northern China, our analytical method and results need to be validated in studies of other heavily polluted cities in the future.