Impacts Of Air Pollution On Ischemic Stroke Among Residents Of Four Cities In Gansu Province

BackgroundIschemic stroke（IS）,also known as cerebral infarction,is a cerebrovascular disease characterized by multicausal interruption of cerebral arterial blood flow,local hypoxia and ischemic necrosis of brain tissues,that correspondingly culminates with functional neurological deficits.With the accelerated population aging and changes to environmental quality and residents’lifestyles over the recent years,China has been bearing the toll of rising IS incidence,prevalence and mortality trends since the 1990s,making IS one of China’s most influential health concerns.Apart from the numerous conventionally-recognized risk factors inducing IS,such as hypertension,hyperlipidemia,diabetes,obesity/overweight,physical inactivity,and smoking,an increasing number of recent studies have revealed a significant correlation between air pollution and IS incidence.A domestic and international literature review on the correlations between air pollutants and IS incidence in recent years revealed that most past studies used mainly stroke-related hospitalization rate or mortality as exposure outcomes,and rarely,outpatient visits.In general,outpatient data are more sensitive to exposure caused by air pollution,and can more accurately reflect the actual effects of air pollution on population health than inpatient and death data.Existing Chinese research on this matter is mostly focused on economically developed areas in eastern China,such as Beijing,Shenzhen,Guangzhou and Ningbo.However,the impacts of air pollution on IS incidence in cities of different climatic zones across the northwest inland provinces have been rarely reported.Gansu Province located in the interior of northwest China,with narrow and long terrains that span thousands of kilometers from east to west,covering multiple climatic zones and affected by several climatic types.Due to the considerable regional differences in climatic conditions,the unique topography and landforms,as well as the distinctive lifestyles or heating methods of residents across its prefectures and cities.Cities situated across Gansu’s diverse climactic zones differ in air pollutant sources and pollution characteristics relative to those located along China’s eastern coasts,and thus,also significantly different harms from the various air pollutants onto its residents.Therefore,this study thoroughly considered the geographical environment,climatic conditions,socioeconomic development,and population distribution in Gansu Province,before selecting four cities representing the different climatic zones of Gansu Province,Jiuquan,Dingxi,Tianshui,and Longnan,for the case study.Quantitative analyses were carried out on the correlations between the common air pollutants and outpatient visits made by IS patients in the four cities in Gansu Province,to provide a basis for the relevant policies on air pollution and IS prevention by relevant departments.MethodsAll recorded IS outpatient data from January 1,2018 to December 31,2019 in all hospitals within the urban areas of Jiuquan,Dingxi,Tianshui,and Longnan were collected,along with the contemporaneous readings for six conventionally-monitored air pollutants(PM_2.5,PM₁₀,SO₂,NO₂,O₃8h,CO),and meteorological data（temperature and humidity）.Quantitative evaluations were carried out on the correlations between air pollutants and daily IS outpatient visits,as well as their lag effects in various cities,and the method used was the combination of the generalized additive model（GAM）based on Quasi-Poisson regression with the distributed lag non-linear model（DLNM）.Moreover,a stratified analysis was also conducted on the IS outpatients across the cities based on gender,age and season,before verifying the robustness of the models’outputs using a two-pollutant model and changing the degree of freedom of temporal variables.Results1.Descriptive analysis:From 2018 to 2019,the IS outpatient visits for residents of four cities in Gansu Province amounted to 9,490 in Jiuquan,4,278 in Dingxi,10,317 in Tianshui,and 6,213in Longnan.From the perspective of gender stratification,visits made by female IS outpatients were significantly higher than males in Jiuquan,Tianshui,and Longnan.In terms of age stratification,it can be seen that the number of visits made by IS outpatients aged less than 65 years old in the three aforementioned cities was higher than the number of visits made by those aged at least 65 years old.By contrast,little change was exhibited by the outpatient visits in Dingxi across gender and age.Seasonal variations in the concentration of the six air pollutants across the four cities were significant.Besides O₃8h,the concentration of PM_2.5,PM₁₀,NO₂,SO₂and CO was high in the winter and spring,whereas low in summer and autumn.2.Impacts of air pollutants on IS outpatient visits across the four cities:（1）Impact of PM_2.5:The harmful effects of PM_2.5 on residents of Jiuquan at lag0and lag01 to lag07 were significant,and were at their most severe at lag06,as shown by the corresponding relative risk（RR）of 1.048（95%CI:1.019,1.077）.In Dingxi,Tianshui,and Longnan,where the harmful effects of PM_2.5 on the number of IS outpatient visits demonstrated no statistical significance over various lag days.（2）Impact of PM₁₀:The harmful effects of PM₁₀ on residents of Jiuquan were significant at lag04-lag06 and were at their strongest at lag06,the RR was 1.007（95%CI:1.001,1.012）.In Dingxi,Tianshui,and Longnan,the harms of PM₁₀concentration were not statistically significant regardless of the lag days.（3）Impact of SO₂:The harms of SO₂ were significant in Jiuquan at lag2-lag3 and lag03-lag07,and were at their most severe at lag06,as shown by the corresponding RR of 1.348（95%CI:1.072,1.696）.In Dingxi,the harmful effects of SO₂ were significant at lag2 and lag02-lag07,culminating in severity at lag03,the RR was 1.211（95%CI:1.087,1.350）.In Tianshui,the harmful effects of SO₂ concentration on IS outpatient visits across various lag periods in days exhibited no statistical significance,presenting a maximum RR of 1.071（95%CI:0.894,1.282）at lag05.In Longnan,the harms of SO₂were significant at lag05 and lag06,most so at lag05,as evidenced by the RR of 1.114（95%CI:1.010,1.230）.（4）Impact of NO₂:The harms of NO₂became significant at lag2-lag3 and lag02-lag07 in Jiuquan,achieving its peak severities at lag05 and the RR was 1.200（95%CI:1.110,1.298）.In Dingxi,the harmful effects of NO₂ were significant at lag02-lag03and lag07,culminating in severity at lag07,the RR was 1.147（95%CI:1.027,1.282）.In Tianshui,the harms of NO₂ were significant at lag02-lag05 and were at their most severe at lag03,as shown by the corresponding RR of 1.098（95%CI:1.038,1.162）.In Longnan,the harms of NO₂ were significant at lag0,lag4-lag5 and lag01-lag07,and were at their strongest at lag06,the RR was 1.144（95%CI:1.080,1.212）.（5）Impact of O₃8h:In Jiuquan,Dingxi and Longnan,the three cities demonstrated no statistical significance in the harmful effects of O₃8h on IS outpatients at different lag days.In Tianshui,the harmful effect of O₃8h concentration was significant at lag6with a corresponding RR of 1.011（95%CI:1.002,1.020）.（6）Impact of CO:In Jiuquan,the harms of CO were significant at lag2-lag4 and lag02-lag07 and were at their strongest at lag06,the RR was 2.991（95%CI:1.895,4.722）.In Dingxi,the harmful effects of CO were significant at lag7 and lag02-lag07,culminating in severity at lag07,the RR was 2.223（95%CI:1.366,3.616）.In Tianshui,the harms of CO were significant at lag2 and lag01-lag07,and were at their most severe at lag03,as shown by the corresponding RR of 1.567（95%CI:1.280,1.919）.In Longnan,the harms of CO were significant at lag02-lag07 and were at their strongest at lag05,the RR was 1.421（95%CI:1.131,1.785）.3.Impacts of air pollutants on different IS patient groups across the four cities:Sample stratification based on gender,age and season across the four cities displayed considerable differences in the impacts of air pollutants on different subgroups.For PM_2.5,PM_2.5 affected male IS outpatient visits much more strongly than their female outpatient visits in Jiuquan,Dingxi,and Tianshui,as well as affected those aged at least 65 years old than those younger than 65 years old in Jiuquan,Dingxi,and Longnan.For PM₁₀,while PM₁₀ affected male IS outpatients much more strongly than their female outpatient visits in Jiuquan,only female IS outpatients were seemingly affected by PM₁₀in Dingxi.It can be seen that IS outpatient visits aged at least 65 years old in Jinquan and Dingxi suffered much greater consequences from the harms of PM₁₀ than their outpatients who were younger than 65 years old.For SO₂,SO₂affected female IS outpatient visits much more strongly than male IS outpatients in Jiuquan and Dingxi.IS outpatient visits younger than 65 years old were affected more seriously by SO₂than those at least 65 years old in Jiuquan,but in Dingxi and Longnan,where those at least 65 years old were being harmed much more significantly.For NO₂,NO₂impacted female IS outpatient visits more profoundly in the three cities other than Tianshui.NO₂’s harms were also much more impactful to IS outpatients younger than 65 years old in Jiuquan than those aged at least 65 years old,but the opposite was true in the other three cities.For O₃8h,O₃8h concentration fluctuations only in Tianshui more considerably impacted not only male IS outpatient visits but also IS outpatients younger than 65 years old.For CO,the harms of CO undermined female IS outpatient visits more critically in all the cities except Tianshui and impacted IS outpatients younger than 65 years old more heavily in all the cities except Longnan.During seasonally stratified analysis,it was found that cold seasons caused PM_2.5,PM₁₀,SO₂,NO₂ and CO,except for O₃8h,to be significantly detrimental to the health of IS outpatient visits across the cities.4.Exposure-response relationships between different air pollutants and IS outpatient visits across the four cities:Further analysis of the exposure-response relationships between the six air pollutants and IS outpatient visits across cities revealed approximately linear relationships between IS outpatient visits in all the cities with PM_2.5,PM₁₀,SO₂,NO₂,O₃8h and CO,showing no significant threshold.Conclusions1.Seasonal variations exist in the daily average concentration trends of the six air pollutants across the four cities.Except for O₃8h,the concentrations are all high in winter and spring,but low in summer and autumn.2.PM_2.5,PM₁₀,SO₂,NO₂,O₃8h and CO are correlated with the risk of outpatient visits due to IS among residents in the four cities.However,the pollutants exert varying impacts on IS outpatient visits across the cities,showing different single-day lag and cumulative lag effects.3.Significant differences can be observed to all the pollutants(PM_2.5,PM₁₀,SO₂,NO₂,O₃8h and CO)effects on IS outpatient visits across the cities when gender and age factors are in play.Cold seasons also cause the air pollutants except for O₃8h,to exert more harm on the IS outpatients.4.PM_2.5,PM₁₀,SO₂,NO₂,O₃8h,CO and IS outpatient visits across the four cities manifest approximately linear exposure-response curves with no threshold.