Abstract
The exposure term in the case-crossover design consists in the difference between the ambient concentration on the event day and the concentration(s) on some control day(s). So far, all air pollution case-crossover studies presented the distribution of the daily ambient pollutant concentrations but do not publish the distributional properties of the relevant exposure term--that is, the concentration difference. This article shows that this difference can be very small for a large fraction of event days, therefore, seriously limiting the statistical power to refute the null hypothesis. Publishing the distribution of the relevant differences will improve the interpretation and discussion of findings from case-crossover studies, particularly in cases with statistically non-significant associations.