In addition to being unpleasant, the dirt found in heating and air conditioning ductwork is often composed of insects, rodents, duct mites, animal hair and dander, pollen, dead skin, cigarette tar, asbestos, fungi and bacteria. These contaminants can aggravate allergies, asthma, sinus conditions and headaches in sensitive people. In its 1997 booklet, 'The Inside Story, A Guide to Indoor Air Quality,' the EPA reports that 'People with breathing problems, allergies and lung diseases are particularly susceptible to disease causing biological agents in the indoor air. Contaminated central air handling systems can become breeding grounds for mold, mildew and other sources of biological contamination and can distribute these contaminants through the building.' more... See more text