Mold spores float in the air like pollen, and can occur indoors as well as outdoors. Outdoor mold spores begin to appear in early spring and reach their peak in July, August, September or October in the northern United States. In the South and on the West Coast, outdoor molds can be found all year long.

People increase their exposure to mold when they cut the grass, rake dead leaves, or work with hay or mulch piles. Mushrooms also have many spores under their "caps," so if you are mold-sensitive, don't kick them.

Indoor molds shed spores all year long, particularly in damp environments like basements, cellars, attics, bathrooms and laundry rooms.