While it is established that appendicitis is caused by the obstruction of the appendiceal orifice, the circumstances by which the obstruction is precipitated is unclear.  Gilaad Kaplan, MD, from University of Calgary (Canada), and colleagues studied 5,191 adults who were admitted to area hospitals with appendicitis during the period from April 1999 through December 2006. Observing that slightly more than half the admissions took place in the spring and summer months, the team found that pronounced amounts of ozone, sulfur dioxide, nitrogen dioxide, carbon monoxide, and particulate matter less than 10 microns in diameter were present in July and August.  The researchers conclude that: "Our findings suggest that some cases of appendicitis may be triggered by short-term exposure to air pollution. If these findings are confirmed, measures to improve air quality may help to decrease rates of appendicitis."