Household cleansers, laundry detergents, air fresheners, aerosols, insecticides, insects, pet hair, dander, and litter boxes, cigarette smoke, food, dust (dust mites), people (dead skin, hair, perfume, etc.), carpet, construction materials, and anything moist can grow more. Additionally, items such as bedding/linens and furniture, even HVAC systems, can trap and collect contaminants.