This article is a commentary on the following study: de Hartog JJ, Hoek G, Peters A, Timonen KL, Ibald-Mulli A, Brunekreef B, Heinrich J, Tiittanen P, van Wijnen JH, Kreyling W, Kulmala M, Pekkanen J. Effects of fine and ultrafine particles on cardiorespiratory symptoms in elderly subjects with coronary heart disease: the ULTRA study. Am J Epidemiol. 2003 Apr 1;157(7):613-23. This is the multicenter Ultra study (Exposure and risk assessment for fine and ultrafine particles in ambient air) conducted during the winter of 1998–1999 in three European cities: Amsterdam in the Netherlands, Erfurt in Germany, and Helsinki in Finland. The authors’ objective is to investigate the relationship between fine and ultrafine particles and cardiorespiratory health in elderly individuals with coronary artery disease. Through this study, the authors highlight short-term associations between PM2.5 and cardiorespiratory symptoms in subjects with coronary insufficiency, but do not demonstrate a relationship with chest pain. The effect of ultrafine particles appears to be less pronounced compared to that of fine particles. Furthermore, this article highlights the difficulty of conducting multicenter panel studies and linking cardiac symptoms to urban air pollution. (Excerpt from the article).