Dust mites, molds and mildews, animal dander (scales from hair, feathers, and skin), and cockroach droppings and body parts.

Older and poorly kept homes are prime residences for some of these allergens. Mold and mildew are particularly common in humid areas, and leaky roofs, windows, walls, foundations, and plumbing fixtures welcome opportunistic molds. Microscopic dust mites, which thrive on dead human skin cells and live in household textiles, are another group of troublemakers. When these textiles are disturbed, dust particles become airborne. Dust mites are especially prevalent in homes with dirty air conditioning, dehumidifiers, or ventilation systems, or with no air-circulation system at all.