Unmeasured confounders are factors that influencemortality and vary with time in amanner that is similar to air pollution. These factors produce seasonal and long-termtrends inmortality that can confound the relationship between mortality and air pollution. Influenza and respira- tory infectionsmight reasonably be considered among themost important, usually unmeasured or not readily available confounders which produce seasonal patterns in mortality. Typically, epidemic respiratory infections occur from late autumn to early spring and influenza epidemics occur in the same interval but with highly variable timing. The net effect of a respiratory virus is to increase mortality overall, explaining much of the higher mortality in winter months. Since air pollution levels.
