['We examined whether more precise exposure measures would better detect associations between traffic-related pollution, elemental carbon (EC), nitrogen dioxide (NO2), and heart rate variability (HRV).', 'Repeated 24-hour personal and ambient PM2.5, EC, and NO2 were measured for 30 people living in Atlanta, GA. The association between HRV and either ambient concentrations or personal exposures was examined using linear mixed effects models.', 'Ambient PM2.5, EC, NO2, and personal PM2.5 were not associated with HRV. Personal EC and NO2 measured 24 hours before HRV were associated with decreased RMSSD, PNN50, and HF and with increased LF/HF. RMSSD decreased by 10.97% (95% confidence interval: -18.00 to -3.34) for an inter-quartile range change in personal EC (0.81 microg/m3).'] Does exposure error mask the relationship between traffic-related air pollution and heart rate variability?