Exposures to PM_2.5 components and heart rate variability in taxi drivers around the Beijing 2008 Olympic Games

Background: Carbonaceous and metallic components of particles have been shown to play a role in particles' effects on cardiac autonomic function as measured by heart rate variability (HRV). Previously we reported the association of HRV with marked changes in traffic-related particulate air pollution around the Beijing 2008 Olympic Games in a panel of taxi drivers. Objective: We further investigated the relationship between exposures to the carbonaceous and metallic components of traffic-related particles and HRV in the same population. Methods: Repeated measurements of in-car exposures to particulate matter ≤2.5 μm in aerodynamic diameter (PM_2.5), carbon monoxide and nitrogen oxides were conducted in a group of 14 taxi drivers for one work shift in four study periods around the Beijing 2008 Olympics. The quantities of organic/elemental carbons and 27 elements of the in-car PM_2.5 mass were determined laboratorially. Linear mixed-effects models were used to evaluate the impact of exposures to different PM_2.5 components on HRV while controlling for potential confounders. Results: Taxi drivers' exposures to in-car PM_2.5 and its components showed dramatic changes across the four study periods around the Beijing 2008 Olympics. Differences in associations of in-car PM_2.5 components with HRV were found. An interquartile range (IQR: 917.9 ng/m³) increase in calcium was associated with a 5.48 millisecond [ms, 95% confidence interval (CI): 0.71, 10.24] increase in standard deviations of normal-tonormal (SDNN) intervals, while an IQR (4.1 ng/m³) increase in nickel was associated with a 1.53 ms (95% CI: 0.14, 2.92) increase in SDNN index. Additionally, a decline of 8.11 ms (95% CI: -15.26, -0.97) in SDNN per IQR (481.4 ng/m³) increase in iron was also found. Conclusion: The results support associations of PM_2.5 metallic components with HRV in younger healthy individuals. Future studies are needed to clarify the interaction among different PM_2.5 components or the role of PM_2.5 mixtures.