This article is a commentary on the following study: "Daily variation in fine and ultrafine particulate air pollution and urinary concentrations of lung Clara cell protein CC16." Timonen KL, Hoek G, Heinrich J, Bernard A, Brunekreef B, de Hartog J, Hämeri K, Ibald-Mulli A, Mirme A, Peters A, Tiittanen P, Kreyling WG, Pekkanen J. Occup Environ Med. 2004;61;908-14.