MSN.com featured this story about (non-obvious) areas of the house which might be allergen sources. Good piece. I notice however, the writer missed one: the bathroom fan.
For years and years and years (even before I came to Taiwan), I'd wake up everyday, allergy-free. By the time I'd finish shaving & showering though, the eyes were watering and nose running. Plenty of sneezing, too. And all along I just figured it was the morning air, or the shaving cream, or the shampoo, or the soap.
Wrong, Foreigner. At least once a year, unscrew the fan grille from the ceiling (don't lose the screws!), and clean it thoroughly, inside and out. There'll be plenty of dust built up on it. Next, with the grille off, clean the housing of the intake port. There may be dust and even mold in this area. And finally, stop the fan and clean it as well. There's probably dust built up on the edges of those fan blades, too.
Screw the grille back on, and you're laughin'.
(Until of course, the allergy season returns . . .)