In this study, NO2 presented a more obvious effect than the other three air pollutants in coastal area. We explored outpatient visits for different diseases and in different seasons. In the cold period, there were more outpatient visits for respiratory, upper respiratory and circulatory diseases under the effects of O3, PM2.5 and PM10 than in the warm period, but in the warm period, there were more outpatient visits for lower respiratory diseases under that three air pollutants than in the cold period. In the double pollutant model, after adjusting for NO2, the effect of the other three air pollutants decreased. After adjusting for PM2.5, PM10 expressed a significant effect. After adjusting for the other three air pollutants, the ER of O3 changed only slightly. Different air pollutants presented different effects because of different conditions.
Our study showed the association between meteorological factors and the air pollutants NO2, O3, PM10 and PM2.5. We found that the action of AP, RH and T caused high concentrations of air pollutants. Other studies showed that meteorological factors had an effect on the concentrations of air pollutants, similar to our study [23, 24]. RH is known to increase haze, possibly because RH is positively correlated with NO2, which converts from the gas phase of NOx to the particulate phase in relatively low visibility conditions . We did not find a positive association between PM and RH in the Spearman correlation model, but in the contour plot, the correlation between RH and PM10 and PM2.5 first increased and then decreased between at a certain AP and T. The joint action of meteorological factors was seemingly obvious for PM. O3 had a positive association with T and a negative association with RH because sunshine might be the main promoter of O3, as O3 is enhanced by photochemical factors, and RH could affect sunshine duration . Meteorological factors could influence air pollution, thus impacting health. Therefore, meteorological factors were introduced into the GLM. The time series diagram shows that in the cold period, the air pollutants, except ozone, had higher concentrations than those in the warm period. This is due to the negative association between T and air pollutants and the positive association between AP and air pollutants, except ozone. In addition to meteorological factors, emissions also increase pollutant concentrations . In the cold period, people in Fuzhou do not keep warm with coal but usually light fires, leading to increased PM emissions. The time series diagram also shows that there were more outpatient visits for respiratory diseases, including upper and lower respiratory diseases, in the cold period than in the warm period.
The GLM showed the different aspects, including the total situation, different seasons and double air pollutant model. Because of the significant effects of the different air pollutants, we conducted a comprehensive study to evaluate the ERs of NO2, O3, PM2.5 and PM10 [28, 29]. In the overall model, we found that NO2 had a more obvious effect than the other three air pollutants on the ER of outpatient visits, especially considering the cumulative lag effect. Some studies also found that NO2 was strongly associated with hospital admissions for both respiratory and cardiovascular diseases [30, 31]. In China, the Sixth National Population Census showed that coastal area had become an old-age society, and a systematic review and meta-analysis reported that the effect of NO2 exhibited regional differences because of different proportions of elderly persons with increased susceptibility to NO2, which may be the cause of the high ER associated with NO2 . Although air quality is not bad, the elderly may be still susceptible to air pollutants.
However, once we studied the different seasons, the effect of NO2 was less significant than that of the total situation, and the ER of NO2 was lower in the warm period than in the cold period; moreover, it lost all significance in the warm period. In addition to T, the concentrations of air pollutants were different between the cold period and warm period. In the cold period, the concentration of NO2 was 33.11 μg/m3, while in the warm period, it was 23.32 μg/m3. There is a dose-dependent relationship between pulmonary injuries and ambient NO2 , but for circulatory injuries, there is a lack of research. Interestingly, NO2, O3, PM2.5 and PM10 had similar results, in term of influences on upper and lower respiratory outpatient visits. Generally, PM10 has a greater impact on the upper respiratory tract than on the lower respiratory tract, while PM2.5 and O3 exhibit the opposite. In the cold period, the increase in upper respiratory outpatient visits was greater than that in the warm period, while the results for lower respiratory were in direct contradiction with the above mentioned situation, except NO2. And nitrogen dioxide, ozone and fine particle matter caused more ERs for upper respiratory outpatient visits than lower respiratory outpatient visits in cold period, but nitrogen dioxide, ozone, respiratorable and fine particle matter caused more ERs for lower respiratory outpatient visits in warm period. T and AP were 14.66℃ and 1016.65 hpa in the cold period and 26.40℃ and 1004.66 hpa in the warm period. Some studies reported that low AP and warm Ts increased susceptibility to respiratory-related diseases [34, 35]. However, PM and O3 had greater effects on upper respiratory outpatient visits in the cold period and greater effects on lower respiratory outpatient visits in the warm period, possibly because the depths of the pollutants entering the respiratory tract are impacted by T and AP; however, this theory needs further study. Regarding circulatory diseases, in our study, we found that in the cold period, air pollutants increased the number of outpatient visits for circulatory diseases. Some studies presented similar outcomes [31, 36]. However, a study over 17 years in Canada reported that warm season 1-day lagged ozone had a greater association with the three examined circulatory hospitalization causes (ischemic heart disease, other heart disease and cerebrovascular disease) than cold season ozone . A study in Hong Kong reported that PM and NO2 increased emergency hospital admissions during the warm period . During our study, increased concentrations of PM and NO2 were observed in the cold period, but an increased concentration of O3 was not. In addition to the increased concentrations of air pollutants, heat waves and the other extreme high-temperature events were more likely to occur on low-temperature days, which may cause more outpatient visits for circulatory diseases .
We also conducted exceeding 100 µg/m3 of ozone model, because ozone pollution is serious. the exceeding 100 µg/m3 of ozone model did not introduce the ns function of date and DOW because of discontinuity. Ozone exceeded 100 µg/m3 for a total of 390 days during the study period (total study period, 1096 days) and in the warm period, ozone exceeded 100 µg/m3 for a total of 315 days, and the warm period model showed a significant effect on respiratory outpatient visits at lag4 and lag5. The time at which the significant effect appeared was the same in the warm period model and exceeding 100 µg/m3 of ozone model, but the diseases were different. The difference may be due to the increased concentration of O3 in the 100 µg/m3 ozone model [O3 average (standard O3 concentration model): 126.36ug/m3 vs O3 average (the warm period average): 100.48ug/m3].
In the double model, at lag0, after adjusting for PM2.5, NO2 and O3 presented increased ERs. In contrast, after adjusting for NO2, the three other pollutants, especially particulate matter (PM2.5 and PM10), presented decreased ERs. There was a strong correlation between PM and NO2. The ER of ozone did not fluctuate considerably after adjusting for the three other pollutants. The interaction between PM and NO2 was strong, and the effect of O3 was independent. Previous studies also found that there was a strong correlation between PM and gaseous air pollution, with the exception of O3, which did not change much after adding the other air pollutants into the model [6, 17]. Some mechanics studies noted that inflammation, oxidative stress, changes in systemic coagulation functioning and reduced cardiac autonomic control appeared after exposure to gaseous air pollutants and PM [39, 40], which may trigger respiratory and cardiovascular events as well as high concentrations of air pollutants, except O3, in the same period (cold period). These factors may cause high correlations among air pollutants. Therefore, it is difficult to evaluate the independent effects of PM or NO2 because of their high correlations .
There are several limitations. In coastal area, ozone pollution is more serious than PM and NO2 pollution, but in this study, NO2 increased the number of outpatient visits. O3 and NO2 are relative to photochemical smog, and they promote one another; perhaps they exhibit joint action. However, we could not find obvious interactions in the double model; therefore, further research is required. PM2.5 increased the outpatient visit risk rate in many studies, even in areas with better air quality than Fuzhou. In our study, we did not observe a significant effect of PM2.5. If we stratify the results by different ages and diseases, we may obtain significant outcomes. Overall, our study comprised comprehensive analysis to investigate the association between air pollutants and outpatient visits. However, there is a lack of studies on the association between specific respiratory and circulatory diseases and different air pollutants. The effects observed in this study were short-term effects. Studies on long-term effects still need to be conducted in coastal areas in China.