This study investigated the association between ambient pollution levels and cardiovascular function in a repeated measures study involving 21 active Boston residents aged 53 to 87 years. The study measured particles with an aerodynamic diameter ≤2.5 μm (PM2.5) continuously using a tapered element oscillating microbalance. 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). The results showed that elevated PM2.5 levels were significantly associated with reduced HRV (SDNN and r-MSSD). During slow breathing, a reduction in r-MSSD of 6.1 ms was associated with an interquartile (14.3 μg/m³) increase in PM2.5 during the hour of and the 3 hours previous to the Holter session. A multiple pollution model, including PM2.5 and ozone, was associated with a combined effect equivalent to a 33% reduction in the mean r-MSSD. The study concluded that particle and ozone exposure may decrease vagal tone, leading to reduced HRV, which could have implications for cardiovascular health.This study investigated the association between ambient pollution levels and cardiovascular function in a repeated measures study involving 21 active Boston residents aged 53 to 87 years. The study measured particles with an aerodynamic diameter ≤2.5 μm (PM2.5) continuously using a tapered element oscillating microbalance. 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). The results showed that elevated PM2.5 levels were significantly associated with reduced HRV (SDNN and r-MSSD). During slow breathing, a reduction in r-MSSD of 6.1 ms was associated with an interquartile (14.3 μg/m³) increase in PM2.5 during the hour of and the 3 hours previous to the Holter session. A multiple pollution model, including PM2.5 and ozone, was associated with a combined effect equivalent to a 33% reduction in the mean r-MSSD. The study concluded that particle and ozone exposure may decrease vagal tone, leading to reduced HRV, which could have implications for cardiovascular health.