Longitudinal Associations Of Exposure To Ambient PM_2.5 Constituents With Metabolic Syndrome In Chinese Middle-Aged And Older Adults

Objective:Limited evidence exists for chronic effects of ambient fine particulate matter(PM_2.5)constituents on metabolic syndrome（Met S）in China.We aimed to（1）assess associations between long-term exposure to PM_2.5constituents and Met S indicators[fasting blood glucose（FBG）,triglycerides（TG）,high density lipoprotein cholesterol（HDL-C）,waist circumference（WC）,systolic blood pressure（SBP）and diastolic blood pressure（DBP）]by using repeated measurements in Chinese middle-aged and older adults;（2）investigate the relationships between long-term exposure to PM_2.5constituents and the risk of Met S by designing a cohort study.Methods:Base on two waves（2011 and 2015）of the China Health and Retirement Longitudinal Study（CHARLS）,9858 participants were included for longitudinal analysis,and 3658 participants were recruited for cohort study.One-year average exposures to PM_2.5and its constituents[i.e.,black carbon（BC）,organic matter（OM）,sulfate(SO₄^2-),nitrate（NO₃^-）and ammonium（NH₄⁺）]were assessed by aggregating monthly satellite-derived estimates（1 km×1 km）at prefectural city level and then assigned to each participant.Linear mixed-effect model was used to analyze longitudinal changes of Met S indicators associated with per inter-quartile range（IQR）increase in long-term exposure to PM_2.5and its constituents.Cox proportional hazards model with age as a potential time scale was adopted to quantify associations of long-term exposure to ambient PM_2.5and its components with the risk of Met S.A restricted cubic spline function was employed to smooth the concentration-response（C-R）relationships.The overall effect of five constituents on Met S and its indicators was further evaluated using a quantile-based g-computation approach.Results:（1）Significant associations of long-term exposure to increased PM_2.5or mixture of components with the rise of FBG,TG,WC,SBP and DBP and the reduction of HDL-C were observed.The chronic effects of individual constituents were slightly different among Met S indicators,with TG significantly increased 3.68（1.57,5.79）mg/d L and HDL-C decreased 1.80（1.27,2.32）mg/d L when exposure to BC;FBG significantly raised 1.48（0.47,2.49）,2.56（1.15,3.98）,1.83（0.49,3.17）mg/d L when exposure to BC,NO₃^-and NH₄⁺,respectively;and WC significantly increased1.96-3.27 cm,SBP increased 2.05-2.67 mmg H,DBP increased 1.05-1.57 mmg H when exposure to five constituents.We observed nonlinear C-R relationships of glycolipids indicators with exposure to PM_2.5and five constituents（P<0.05）,while no evident violations of the linearity assumptions for all C-R curves of waist circumference and blood pressure（P>0.05）.（2）During 13487.15 person-years’follow-up,a total of 633 Met S cases occurred.Multivariable-adjusted Cox models estimated hazard ratios of Met S per IQR increase in exposure to PM_2.5,BC,SO₄^2-,NO₃^-and NH₄⁺were 1.38（95%confidence interval:1.20,1.358）,1.18（1.04,1.34）,1.21（1.04,1.41）1.84（1.55,2.19）and 1.66（1.37,2.01）,respectively.We did not find significant association between increased risk of Met S with exposure to OM.The risk of Met S increased by 24%（11%,39%）for each quartile rise in mixture of component exposure.We observed nearly linear relationships between exposure to PM_2.5,NO₃^-and NH₄⁺and Met S incident（P values of likelihood ratio tests being>0.05）,revealing the slopes tended to be steeper trajectories at higher concentrations.However,there were non-linear associations between the risk of Met S and BC and SO₄^2-（P<0.05）,showing risk of Met S increased rapidly at lower exposure levels and gradually decreased or leveled off at higher concentrations.Conclusions:（1）Long-term exposure to ambient PM_2.5or mixture of components,were positively associated with FBG,TG,WC,SBP,DBP,and negatively associated with HDL-C.（2）The chronic effects of individual constituents were slightly different among Met S indicators,with increased TG and decreased HDL-C associated with exposure to BC;raised FBG associated with exposure to BC,NO₃^-and NH₄⁺;and elevated WC,SBP,and DBP associated with exposure to five constituents.（3）Long-term exposure to ambient PM_2.5and specific components(BC,SO₄^2-,NO₃^-and NH₄⁺)may increase risk of Met S among Chinese middle-aged and older adults.