SEAL360 Magnetic Vent Covers (3-Pack), Pockets for Complete Seal, 5.5" X 12" (White) for Floor, Wall, or Ceiling Vents and Air Registers, for RV, Home HVAC and AC Vents, Vent Not Included

Humidity will make the dust more likely to stick to the fans blades. It doesn't necessarily mean more dust just more sticking. As someone else said a dehumidifier is a good idea in a basement. As for the cold air coming from the vents, it sounds like AC. You can cover the vent with pretty much anything. I have even seen magnetic vent covers.magnetic vent cover (Thia might be for US Amazon but the same items are avaliable here)