Background—We investigated associations between ambient pollution levels and cardiovascular function in a repeated measures study including 163 observations on twenty-one 53- to 87-year-old active Boston residents observed up to 12 times from June to September 1997. Particles with aerodynamic diameter ≤2.5 μm (PM_2.5) were measured continuously using a tapered element oscillating microbalance.

Methods and Results—The protocol involved 25 minutes per week of continuous Holter ECG monitoring, including 5 minutes of rest, 5 minutes of standing, 5 minutes of exercise outdoors, 5 minutes of recovery, and 20 cycles of slow breathing. Heart rate variability (HRV) was assessed through time domain variables: the standard deviation of normal RR intervals (SDNN) and the square root of the mean of the squared differences between adjacent normal RR intervals (r-MSSD). Mean 4-hour PM_2.5 levels ranged from 3 to 49 μg/m³; 1-hour ozone levels ranged from 1 to 77 ppb. In multivariate analyses, significantly less HRV (SDNN and r-MSSD) was associated with elevated PM_2.5. During slow breathing, a reduction in r-MSSD of 6.1 ms was associated with an interquartile (14.3 μg/m³) increase in PM_2.5 during the hour of and the 3 hours previous to the Holter session (P=0.006). During slow breathing, a multiple pollution model was associated with a reduction in r-MSSD of 5.4 ms (P=0.02) and 5.5 ms (P=0.03) for interquartile changes in PM_2.5 and ozone, respectively, resulting in a combined effect equivalent to a 33% reduction in the mean r-MSSD.

Conclusions—Particle and ozone exposure may decrease vagal tone, resulting in reduced HRV.