According to the EPA, when you choose to have you air system cleaned the service provider should:
Open access ports or doors to allow the entire system to be cleaned and inspected.
Inspect the system before cleaning to be sure that there are no asbestos-containing materials (e.g., insulation, register boots, etc.) in the heating and cooling system. Asbestos-containing materials require specialized procedures and should not be disturbed or removed except by specially trained and equipped contractors.
Use vacuum equipment that exhausts particles outside of the home or use only high-efficiency particle air (HEPA) vacuuming equipment if the vacuum exhausts inside the home.
Protect carpet and household furnishings during cleaning.
Use well-controlled brushing of duct surfaces in conjunction with contact vacuum cleaning to dislodge dust and other particles.
Use only soft-bristled brushes for fiberglass duct board and sheet metal ducts internally lined with fiberglass. (Although flex duct can also be cleaned using soft-bristled brushes, it can be more economical to simply replace accessible flex duct.)
Take care to protect the duct work, including sealing and re-insulating any access holes the service provider may have made or used so they are airtight.
Follow NADCA's standards for air duct cleaning and NAIMA's recommended practice for ducts containing fiber glass lining or constructed of fiber glass duct board.