A PDF from the US Environmental Protection Agency http://www.epa.gov/iaq/pubs/airduct.html .
Knowledge about Air Duct Cleaning is in its early stages, so a blanket recommendation cannot be offered as to whether you should have your air ducts in your home cleaned. The U.S. Environmental Protection Agency (EPA) urges you to read this document in it entirety as it provides important information on the subject.
The PDF file: Should-You-Have-the-Air-Ducts-in-Your-Home-Cleaned