This article is a commentary on the following study: "Timonen KL, Vanninen E, Hartog J, Ibald-Mulli A, Brunekreef B, Gold DR, Heinrich J, Hoek G, Lanki T, Peters A, Tarkiainen T, Tiittanen P, Kreyling W, and Pekkanen J. Effects of ultrafine and fine particulate and gaseous air pollution on cardiac autonomic control in subjects with coronary artery disease: The ULTRA STUDY. J Expo Sci Environ Epidemiol. 2006;16:332-41."