Fifty-two per cent of admissions occurred between April and September, the warmest months of the year in Canada during which people are more likely to be outside.
Therefore, air pollution must be the culprit if your correlation-causation arrow is more like a Scud missile.
Why do they reach for pollution? The dominant hypothesis of appendicitis has been obstruction of the appendix opening and cases increased dramatically in industrialized countries in the 19th and early 20th centuries but then decreased in the middle and late 20th century, coinciding with legislation to improve air quality. And the incidence of appendicitis has been growing in developing countries as they become more industrialized.
The results cannot say how air pollution might increase the risk of appendicitis but the authors suggest pollutants may trigger inflammatory responses. Of course, they recommend further studies to determine the link.
On their methodology, they used Environment Canada's air pollution data for Calgary and determined the levels of ozone, nitrogen dioxide and other air-borne pollutants along with temperature. They found correlations between high levels of ozone and nitrogen dioxide and the incidence of appendicitis between age groups and genders. More men than women were found to have the condition.
"For unexplained reasons, men are more likely than women to have appendicitis," write Dr. Gilaad Kaplan of the University of Calgary and coauthors. "Men may be more susceptible to the effects of outdoor air pollution because they are more likely to be employed in outdoor occupations," although they note that misclassifications of data could explain some of the difference.
Article: Gilaad G. Kaplan MD MPH, Elijah Dixon MD MSc, Remo Panaccione MD, Andrew Fong MSc,
Li Chen MSc, Mieczyslaw Szyszkowicz PhD, Amanda Wheeler PhD, Anthony MacLean MD,
W. Donald Buie MD MSc, Terry Leung MD, Steven J. Heitman MD MSc, Paul J. Villeneuve PhD, 'Effect of ambient air pollution on the incidence of appendicitis', CMAJ 2009. DOI:10.1503/cmaj.082068