Air pollution particles are thought to kill > 500,000 people worldwide each year. The population most at risk appears to be elderly people with respiratory and cardiovascular disease. As yet, no commonly accepted mechanism has been proposed which can explain the cause of these deaths. Heart rate variability (HRV) was assessed in healthy elderly adults between the ages of 60 and 80 who were exposed twice for 2 h: once to clean air and once to concentrated ambient air pollution particles (CAPS). Changes in HRV were measured immediately before, immediately following, and 24 h after exposure. Elderly subjects experienced significant decreases in HRV in both time and frequency domains immediately following exposure. Some of these changes persisted for at least 24 h. These data were compared with HRV data collected from young healthy volunteers exposed to CAPS in a previous study, in which no CAPS-induced changes in HRV were found. These concentrated ambient air pollution particle-induced changes in heart rate variability in a controlled human exposure study extend similar findings reported in recent panel studies and suggest potential mechanisms by which particulate matter may induce adverse cardiovascular events.