BACKGROUND: Increasing evidence suggests that obesity may impart greater susceptibility to adverse effects of air pollution. Particulate matter, especially PM(2.5) (particulate matter with aero-dynamic diameter </=2.5 microm), is associated with increased cardiac events and reduction of heart rate variability (HRV). OBJECTIVES: Our goal was to investigate whether particle-mediated autonomic modulation is aggravated in obese individuals. METHODS: We examined PM(2.5)-mediated acute effects on HRV and heart rate (HR) using 10 24-hr and 13 48-hr ambulatory electrocardiogram recordings collected from 18 boilermakers (39.5 +/- 9.1 years of age) exposed to high levels of metal particulates. Average HR and 5-min HRV [SDNN: standard deviation of normal-to-normal intervals (NN); rMSSD: square-root of mean squared-differences of successive NN intervals; HF: high-frequency power 0.15-0.4 Hz] and personal PM(2.5) exposures were continuously monitored. Subjects with body mass index >/= 30 kg/m(2) were classified as obese. Mixed-effect models were used for statistical analyses. RESULTS: Half (50%) of the study subjects were obese. After adjustment for confounders, each 1-mg/m(3) increase in 4-hr moving average PM(2.5) was associated with HR increase of 5.9 bpm [95% confidence interval (CI), 4.2 to 7.7] and with 5-min HRV reduction by 6.5% (95% CI, 1.9 to 11.3%) for SDNN, 1.7% (95% CI, -4.9 to 8.4%) for rMSSD, and 8.8% (95% CI, -3.8 to 21.3%) for HF. Obese individuals had greater PM(2.5)-mediated HRV reductions (2- to 3-fold differences) than nonobese individuals, and had more PM(2.5)-mediated HR increases (9-bpm vs. 4-bpm increase in HR for each 1-mg/m(3) increase in PM(2.5); p < 0.001). CONCLUSIONS: Our study revealed greater autonomic cardiac responses to metal particulates in obese workers, supporting the hypothesis that obesity may impart greater susceptibility to acute cardiovascular effects of fine particles.