This article is a commentary on the following two studies: Ostro B, Feng WY, Broadwin R, Green S, Lipsett M. The effects of particulate air pollution components on mortality in California: results from the CALFINE study. Environ Health Perspect 2007;115:13-19. AND Ostro B, Feng WY, Broadwin R, Malig B, Green S, Lipsett M. The impact of particulate matter components on cardiovascular mortality in sensitive subpopulations. Occup Environ Med 2008.