Patients with cardiovascular diseases who are exposed to air pollution in traffic have a greater risk of suffering from stroke or cardiac arrest

But the researchers did not find any link between reduced heart rate variability and ambient levels of air pollutants in the area where the subjects resided.
However personal exposure to traffic-related air pollutants such as elemental carbon and nitrogen dioxide led to variability in HRV as determined by the 24-hour monitors worn by the patients.
These findings may help explain why some people suffer heart attacks on days they are struck in traffic.
Source-Medindia