Although the value of routine duct cleaning remains questionable, the U.S. Environmental Protection Agency (EPA) and industrial hygienists agree that duct cleaning (or, in some cases, duct replacement) is appropriate in the following circumstances:

• Permanent or persistent water damage in ducts.
• Slime or microbial growth observed in ducts.
• Debris build-up in ducts that restricts airflow.
• Dust discharging from supply diffusers.
• Offensive odors originating in ductwork or HVAC components.