Background: The ‘case-crossover’ design is a strategy fitted to studying transient effects of intermittent exposure on acute-onset disease occurring shortly after exposure. It has the important advantage of eliminating most confounding variables, allowing a stronger causality inference. Objectives: The objective of the study was to evaluate the relationships between emergency room (ER) visits for asthma attacks and gaseous air pollution changes. Methods: The study included 549 individuals, 3–49 years old, visiting the ER during 1 year. Exposure to gaseous air pollutants (SO₂, NO₂, O₃) on the same day and up to 4 days before was computed according to the patient’s address. The statistical analysis included meteorological data as potential confounding variables. Results: No association could be shown between ER visits and SO₂ or NO₂ levels. In contrast, there was a statistically significant association between ER visits and mean O₃ levels, on the day of admission and also on D –2 and D –3. For an increase of 10 µg/m³, the risk of requiring an ER admission increased by 6–10%. Conclusions: Using this new strategy, we confirmed that ozone changes lead to a moderate increase in risk of requiring an ER admission in asthmatic subjects.

This paper suggests that very high relative humidity (RH) within walls is a strong risk factor for their infestation with the 'toxic mold' Stachybotrys chartarum. Besides, data from the literature demonstrate that other molds are able to produce mycotoxins when RH is very high. Thus, measurement of wall RH, which is easy to perform and very cheap, could be used as a screening tool to select those dwellings where mold identification should be performed and remediation should be promptly carried out.

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