This article is a commentary on a study published in the journal Environ. Health Perspect. 2004; 112(3):339-45. The objective of this study is to investigate statistical relationships between various measures of ambient particulate air pollution (fine particulate matter PM2.5, with or without, or partially including, the volatile fraction) and the autonomic nervous system, as measured by heart rate variability and blood markers of inflammation, in a panel of older adults. Heart rate variability appears to be a risk factor for cardiovascular events. Indeed, during exertion or "stress," the heart rate must adapt and increase. When the variability of this heart rate becomes too low, cardiac output can no longer adapt, and the risk of cardiovascular events increases. Reduced heart rate variability is a predictive sign of mortality and sudden death. In patients with heart failure, it is a recognized marker of mortality. In other words, the results of this study highlight the importance of controlling air pollution to limit inflammation, atherosclerosis, and cardiac mortality. Furthermore, these results clearly demonstrate the value of characterizing particle composition in health studies. (Authors’ objectives and conclusion).