In this study, NO2 presented a more obvious effect than the other three air pollutants in Fuzhou. We explored the relationship between air pollutants and outpatient visits for different diseases and in different seasons. During the cold season, there were more outpatient visits for respiratory, upper respiratory and circulatory diseases in association with the effects of 8-h O3, PM2.5 and PM10 than during the warm season, but during the warm season, there were more outpatient visits for lower respiratory diseases in association with those three air pollutants than there were during the cold season. In the double pollutant model, after adjusting for NO2, the effects of the other three air pollutants decreased. After adjusting for PM2.5, PM10 showed a significant effect. After adjusting for the other three air pollutants, the ER of 8-h 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, 8-h 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 the findings of our study [25, 26]. 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 at a certain AP and T. The joint action of meteorological factors had seemingly obvious effects on 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 can affect sunshine duration . Meteorological factors can influence air pollution, thus impacting health. Therefore, meteorological factors were introduced into the GLM. The time series diagram shows that during the cold season, all the air pollutants except ozone had higher concentrations than during the warm season. This discrepancy is due to the negative association between T and air pollutants and the positive association between AP and air pollutants other than ozone. In addition to meteorological factors, emissions also increase pollutant concentrations . During the cold season, 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, during the cold season than during the warm season, which may be different from the findings for other regions; for example, the spring dust storm season in Lanzhou may increase emergency room visits for respiratory diseases .
The GLM reflected 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 [31, 32]. 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 [33, 34]. In China, the Sixth National Population Census showed that coastal areas had become old-age societies, and a systematic review and meta-analysis reported that the effect of NO2 exhibited regional differences because of differences in the proportions of elderly people with increased susceptibility to NO2, which may be the cause of the high ER associated with NO2 . Even when air quality is not poor, the elderly may still be susceptible to air pollutants.
However, when we examined the results according to seasons, the effect of NO2 was less significant than that of the total situation, and the ER of NO2 was lower during the warm season than during the cold season; more specifically, it lost all significance during the warm season. In addition to T, the concentrations of air pollutants differed between the cold season and the warm season. During the cold season, the concentration of NO2 was 33.11 μg/m3, while during the warm season, 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 terms of their influences on outpatient visits for upper and lower respiratory diseases. 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 effect. During the cold season, the increase in outpatient visits for upper respiratory disease was greater than that during the warm season, while the opposite results were observed for lower respiratory disease-related visits, except in the case of NO2. Nitrogen dioxide, ozone and PM2.5 caused more ERs for upper respiratory-related outpatient visits than for lower respiratory-related outpatient visits during the cold period, but nitrogen dioxide, ozone, PM2.5 and PM10 caused more ERs for lower respiratory-related outpatient visits during the warm season. T and AP were 14.66 °C and 1016.65 hPa, respectively, during the cold season and 26.40°C and 1004.66 hPa, respectively, during the warm season. Some studies reported that low AP and warm Ts increased susceptibility to respiratory-related diseases [37, 38]. A study pointed out that a greater diurnal temperature range caused more outpatient visits for the common cold . Similarly, greater temperature change affects the number of hospital admissions for chronic obstructive pulmonary disease . Fuzhou often experiences a high diurnal temperature range during cold periods. However, PM and O3 had greater effects on upper respiratory-related outpatient visits during the cold season and greater effects on lower respiratory-related outpatient visits during the warm season, possibly because the depths of the respiratory tract that pollutants are able to reach are impacted by T and AP; however, this theory needs further study. Regarding circulatory diseases, in our study, we found that during the cold season, air pollutants increased the number of outpatient visits for circulatory diseases. Some studies presented similar outcomes [34, 41]. However, a study conducted over a 17-year period in Canada reported that 1-day lagged ozone had a greater association with the three examined circulatory hospitalization causes (ischemic heart disease, other heart disease and cerebrovascular disease) during the warm season than during the cold season . A study in Hong Kong reported that PM and NO2 increased emergency hospital admissions during the warm season . During our study, increased concentrations of PM and NO2 were observed during the cold season, but an increased concentration of O3 was not. In addition to the increased concentrations of air pollutants, heat waves and other extreme high-temperature events were more likely to occur on low-temperature days, which may cause more outpatient visits for circulatory diseases . We found that during the warm season of high temperatures (> 30 ℃), pollutants cause greater damage to the cardiovascular system than when temperature are less than 30 ℃. Studies have reported that under high temperature conditions, the risk of ozone-related cardiovascular death increases, and PM has a greater impact on the cardiovascular system; thus, temperature and pollutants may have a synergistic effect on cardiovascular disease [18, 46]. However, a study in low-pollution areas found that the effects of PM2.5 were more obvious during the cool season than during the warm season.
We also conducted analyses of ozone concentrations exceeding 100 µg/m3 because ozone pollution is serious. The model with ozone exceeding 100 µg/m3 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 during the warm season, ozone exceeded 100 µg/m3 for a total of 315 days. The warm season model showed that high ozone levels had 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 the model with ozone exceeding 100 µg/m3, but the predominant diseases were different. The difference may be due to the increased concentration of O3 in the 100 µg/m3 ozone model [8-h O3 average (standard 8-h O3 concentration model): 126.36 ug/m3 vs 8-h O3 average (the warm season average): 100.48 µg/m3].
In the double model, at lag0, after adjusting for PM2.5, NO2 and 8-h O3 presented increased ERs. In contrast, after adjusting for NO2, the three other pollutants, especially PM (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 8-h O3 was independent. Previous studies also found a strong correlation between PM and gaseous air pollution, with the exception of O3, which did not change much after the other air pollutants were added to the model [6, 18]. Some mechanics studies noted that inflammation, oxidative stress, changes in systemic coagulation functioning and reduced cardiac autonomic control occurred after exposure to gaseous air pollutants and PM [44, 45], which may trigger respiratory and cardiovascular events as well as high concentrations of air pollutants, except 8-h O3, during the same period (the cold season). 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 areas, ozone pollution is more serious than PM and NO2 pollution, but in this study, NO2 increased the number of outpatient visits. Eight-hour O3 and NO2 are related to photochemical smog, and they promote one another; it is possible that 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, which may indicate that there are regional differences in the effect of PM2.5 exposure in China . 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 a comprehensive analysis of the association between air pollutants and outpatient visits. In some comprehensive studies of large cohorts in other regions, even low exposure to air pollutants can have health effects [48, 49]. 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 of long-term effects still need to be conducted in coastal areas in China.