A thorough duct cleaning done by a professional duct cleaner will remove dust and debris—pet hair, paper clips, children’s toys and whatever else might collect down there. Ideally, the inside surface will be shiny and bright after cleaning.
Duct cleaning may be justifiable to you personally for that very reason: you may not want to have your house air circulated through a duct passage that is not as clean as the rest of the house. However, duct cleaning will not usually change the quality of the air you breathe, nor will it significantly affect airflows or heating costs.
Source: © SCHL, 2007