WHAT TO EXPECT
Your service provider should:
Open access ports or doors to allow the entire system to be cleaned and inspected.
Use only high-efficiency particle (HEPA) air-vacuuming equipment.
Protect carpet and household furnishings during cleaning.
Use well-controlled brushing equipment with vacuums to dislodge dust and other particles.
Take care to protect duct work, including sealing and reinsulating any access holes he or she may have made.
www.epa.gov (search for "air ducts")
Source: U.S. Environmental Protection Agency