"Common house dust is more dangerous than outdoor dust. House dust contains dirt, textile fibers, pollen, hair, skin flakes, residue from chemicals and household products, decaying organic matter, dust mites, bacteria, fungi, viruses and a variety of other contaminants." (Science Illustrated
3. System efficiency.
"A buildup of .042 inches of dirt on a heating or cooling coil can result in a decrease in efficiency of 21%." (Environmental Protection Agency)