They contain no chlorophyll, and their spores float in the air like pollen. Mildew is caused by molds. Outdoor mold spores begin to appear after a spring thaw and reach their peak in either July, August, September, or October in the northern United States. Outside molds can be found all year long in the South and on the West coast. In the northern United States, the common molds are Alternaria, Cladosporium, Aspergillus species, and others.
Molds are present in almost every possible habitat.
Outdoors, they can be found in soil, vegetation and rotting wood. Inside, molds are found in attics, basements, bathrooms, refrigerators and other food storage areas, garbage containers, carpets, and upholstery. Pollen and mold counts measure the amount of airborne allergens present in the air. Counts are done by a variety of methods, including suctional trap devices, gravitational slides, or rotating rods prepared with an adhesive. The pollen and mold spore counts are determined daily and reported as grains per cubic meter of air.