This article is a commentary on the following study: Schwartz J. Short-term fluctuations in air pollution and hospital admissions of the elderly for respiratory disease. Thorax. May 1995;50(5):531-8. It is one of the first studies on the health effects of air pollution to use Poisson regression models incorporating nonparametric functions. In this sense, it is an important study, even though time series analysis methods have evolved significantly since then. (R.A.)