The leading edge of a fan blade cuts into the air and at least some of whatever is in that air gets pushed onto the blade or fin of the fan. As the fan spins through moisture and lint dust-laden air, more and more deposits build up on the blades or fins. You don't even need that much moisture to make particulate matter stick to a fan blade. Have you ever seen the fan in a dirty-fan vacuum? I remember when Hoover would hold repair clinics at department stores. The fans in the machines that were in the stream of air before it went into the bag, the old "convertibles" or dirty-fan machines would get caked with dirt to the point where the Hoover rep would have to use the blade of a screwdriver to scrape off the dirt to get down to the metal of the fan blade to clean it. We would see the condition of the machines some people would bring in and figure that they had no shame and must have had filthy houses. Some of those vacuums should have been put in plastic garbage bags before being taken out in public, just like some people's children.