Cleaning removes dust and other particles from your system. Performed improperly, however, duct cleaning can do more harm than good.
If the vacuum pressure isn’t applied carefully, some of the dust that settled in the ducts will be loosened by the agitation and blown into the living space after the cleaning. (This explains the results of the Canadian study in which particle levels actually rose right after a cleaning.)
Running brushes or using compressed air also risks breaking seals in the duct system, which can be especially problematic in the return air portion. Most forced-air systems are designed as closed loops, and leaks in the return-air circuit allow unfiltered air to be sucked from basements or attics, bringing with it dust and moisture.
Not every home has sheet-metal ductwork. Flexible coil-style ducts—the kind that looks like a Slinky toy—are more vulnerable to being punctured.
Ductwork fabricated from fiberglass-insulated material, which is less expensive than metal ductwork, has become more common in new homes. These ducts have fiberglass insulation on their interior surfaces. The fiberglass surface is sealed, but if a duct-cleaning company is not careful, the cleaning can damage the insulation, loosening fibers that can become airborne.