Beyond individual effects: joint impacts of fine particulate matter and artificial light at night on mortality in China

Nowadays, Fine particulate matter (PM 2.5 ) pollution and artificial light at night (ALAN) exposure have emerged as significant environmental health risk factors. However, their combined associations with human health remain largely unexplored. We aimed to evaluate the independent and synergistic associations of PM 2.5 and its components with ALAN on all-cause mortality. We analyzed data from four large-scale nationally representative longitudinal cohort studies conducted in China, including the China Family Panel Studies (CFPS), China Health and Retirement Longitudinal Study (CHARLS), China Health and Nutrition Survey (CHNS), and Chinese Longitudinal Healthy Longevity Survey (CLHLS). PM 2.5 and its components were from the ChinaHighAirPollutants dataset, and ALAN from China’s DMSP-OLS-like nighttime light remote sensing dataset. Time-varying covariate Cox proportional hazards regression models were employed to quantify associations between PM 2.5 , its components, and ALAN exposure with mortality. Additive interactions were assessed using relative excess risk due to interaction (RERI), attributable proportion (AP), and synergy index (S). Additionally, restricted cubic spline (RCS) analysis for dose-response relationships, mixture exposure analysis, and subgroup analyses were conducted. During 532,773 person-years of follow-up observation from 2011 to 2020, 9,897 deaths were recorded. PM 2.5 and its components were associated with increased mortality risk per IQR increase, with hazard ratios (HRs) of 1.177 (95% CI: 1.106–1.252) for PM 2.5 , 1.255 (95% CI: 1.196–1.317) for sulfate (SO 4 2− ), 1.076 (95% CI: 1.024–1.131) for nitrate, 1.096 (95% CI: 1.042–1.153) for ammonium, 1.151 (95% CI: 1.088–1.217) for organic matter, and 1.087 (95% CI: 1.004–1.177) for black carbon. ALAN showed an HR of 1.047 (95% CI: 1.023–1.071) per IQR increase. Mixture exposure analysis revealed that SO 4 2− (weight 39.9%), PM 2.5 (weight 28.2%), and ALAN (weight 27.6%) predominated in combined exposures. Significant synergistic interactions (RERI > 0, AP > 0, S > 1) were observed between PM 2.5 and ALAN on mortality: RERI = 0.570 (95% CI: 0.240–0.900), AP = 0.173 (95% CI: 0.155–0.191), and S = 1.330 (95% CI: 1.258–1.402). Individuals > 75 years, single, and with lower education were more susceptible. PM 2.5 and its components show synergistic associations with ALAN on mortality, with combined exposure associated with greater risks than individual exposures.