Duct cleaning generally refers to the cleaning of various heating and cooling   system components of forced air systems, including the supply and return air   ducts and registers, grilles and diffusers, heat exchangers heating and cooling   coils, condensate drain pans (drip pans), fan motor and fan housing, and the air   handling unit housing.

If not properly installed, maintained, and operated, these components may   become contaminated with particles of dust, pollen or other debris. If moisture   is present, the potential for microbiological growth (e.g., mold) is increased   and spores from such growth may be released into the home's living space. Some   of these contaminants may cause allergic reactions or other symptoms in people   if they are exposed to them. If you decide to have your heating and cooling   system cleaned, it is important to make sure the service provider agrees to   clean all components of the system and is qualified to do so. Failure to   clean a component of a contaminated system can result in re-contamination of the   entire system, thus negating any potential benefits. Methods of duct cleaning   vary, although standards have been established by industry associations   concerned with air duct cleaning. Typically, a service provider will use   specialized tools to dislodge dirt and other debris in ducts, then vacuum them   out with a high-powered vacuum cleaner.