Hay fever is caused by pollens. Certain allergens are always present. These include house dust, household pet danders, foods, wool, various chemicals used around the house, and more. Symptoms from these are frequently worse in the winter when the house is closed up. Mold spores cause at least as many allergy problems as pollens. Molds are present all year long, and grow outdoors and indoors. Dead leaves and farm areas are common sources for outdoor molds. Indoor plants, old books, bathrooms, and damp areas are common sources of indoor mold growth. Molds are also common in foods, such as cheese.