This article is a critical analysis of the following study: Bateson TF, Schwartz J. Who is sensitive to the effects of particulate air pollution on mortality? A case-crossover analysis of effect modifiers. Epidemiology. 2004 Mar;15(2):143-9. This study aims to assess and quantify, in an elderly population, the modification of the effects of particulate pollution on mortality induced by various factors: history of chronic obstructive pulmonary disease (COPD), cardiac conduction disorders, congestive heart failure, diabetes, myocardial infarction, age, sex, and socioeconomic characteristics. This study is of interest because it uses a cross-sectional design to simultaneously explore changes in the short-term effects of air pollution on mortality across multiple factors. However, the lack of statistical power and the absence of certain analyses limit confidence in the interpretation of results regarding medical history. The exploration of socioeconomic characteristics is not informative. On the other hand, the study provides evidence supporting the existence of a gender-specific age effect.