There is currently no conclusive proof that air-duct cleaning is effective in reducing or preventing indoor air problems. The mere presence of dust in air ducts does not mean they need cleaning.
Duct cleaning may be appropriate if an investigation indicates any of the following problems:
Other maintenance on the ductwork may help reduce or prevent contamination problems. Water leaks into the ducts should be stopped to prevent mold and bacteria growth. Insulation of the ductwork can reduce condensation problems in ducts that result in contaminant growth.
Changing the system's filters regularly or switching to a higher efficiency filter may reduce particle contamination.
Duct cleaning can actually increase indoor contamination levels if the procedures are done improperly or carelessly. A competent trained professional should be selected using standard good consumer practices to check them out.