Black mold is a fungus. Fungal spores are often present in the air though this is dependent on weather, time of the year, and your location. Drier areas will tend to have lower levels of fungal spores, while areas with higher humidity will often have higher fungal spore counts.
Like most fungus, black mold loves moist, humid, and dark areas – just what you find under a hot tub cover when closed.
