Your HVAC unit and air ducts deliver heated or cooled air throughout your home year-in and year-out. The air your HVAC unit produces travels through your ductwork and heats or cools your home via the air vents in each room. The dust and other particles in your home’s air are known as particulate matter, which the EPA defines as "a complex mixture of extremely small particles and liquid droplets." The EPA also notes that "particle pollution is made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles." Having clean air ducts means fewer of these particles moving through your home. If you remove your furnace filter and hold it up to the light, you can probably see several hundred of these minuscule particles floating in the air.
Duct cleaning has never been shown to actually prevent health problems. Neither do studies conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts. This is because much of the dirt in air ducts adheres to duct surfaces and does not necessarily enter the living space. It is important to keep in mind that dirty air ducts are only one of many possible sources of particles that are present in homes. Pollutants that enter the home both from outdoors and indoor activities such as cooking, cleaning, smoking, or just moving around can cause greater exposure to contaminants than dirty air ducts. Moreover, there is no evidence that a light amount of household dust or other particulate matter in air ducts poses any risk to your health.