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.

Methods of duct cleaning vary, although standards have been established by industry associations concerned with air duct cleaning.
Global AirPRO will use specialized tools to dislodge dirt and other debris in ducts, while the HVAC system is kept under negative air pressure.  It is important to negatively pressurize the system during cleaning so that dislodged contaminants are captured and not introduced into habitable space.