Reduced heart rate variability (HRV) has been associated with myocardial infarction and mortality in heart failure patients, and acute decreases in HRV have been reported to precede ischemic events and paroxysmal atrial fibrillation. Associations of particulate matter (PM) and traffic-related air pollution with myocardial infarction and cardiac mortality may be related to effects on HRV, but inconsistent findings of previous studies suggest that susceptibility to effects of pollution on HRV may vary. Zanobetti et al. estimated associations between ambient air pollution [PM and black carbon (BC) measured at a central site], self-reported traffic exposure, and several markers of HRV (measured by Holter electrocardiogram monitoring over 24 hr) among 46 patients observed up to four times within the year following percutaneous intervention for myocardial infarction, acute coronary syndrome without infarction, or stable coronary artery disease.