BACKGROUND: Ambient fine particulate matter (PM2.5) exposure is associated with increased cardiovascular and cardiac arrhythmias events, but the detailed mechanism remains unclear. OBJECTIVE: We aimed to investigate the effect of PM2.5 (particulate matter鈥?鈥?.5鈥痬um in aerodynamic diameter) on the cardiac autonomics through a heart rate variability (HRV) analysis. METHOD: Among 6912 patients who had underwent 24-hour Holter ECG recordings between Oct 1st 2015 and Oct 31st 2016, 46 (25 males, 69.3鈥?/-鈥?2.1鈥痽ears old) were enrolled with confirmation of living in an environment with a reported PM2.5 level and were classified as elevated (Group 1, >36鈥痬ug/m(3), 50.73鈥?/-鈥?.50) or low (Group 2, <11鈥痬ug/m(3), 6.06鈥?/-鈥?.00) PM2.5 group. The Holter recordings and HRV parameters were evaluated. RESULT: The baseline characteristics including the comorbidities and medications were similar between the 2 groups. The Holter ECG parameters were also similar. There were no significant HRV differences between the two groups for the 24-hour interval analysis. However, the LF/HF ratio was significantly higher in Group 1 than Group 2 in the 9鈥痑m to 9鈥痯m (p鈥?鈥?.028), 8鈥痑m to 4鈥痯m (p鈥?鈥?.024), and 4鈥痯m to 12鈥痯m (p鈥?鈥?.025) periods, respectively, but not for the nocturnal HRV parameters. CONCLUSION: Our study demonstrated that an elevated PM2.5 exposure had a significant association with an increased daytime LF/HF ratio suggesting a diurnal difference in the response to PM2.5 exposure.