Antimicrobial chemicals are applied by some companies to the interior surface of the air ducts to treat microbial contamination such as fungi (mold), viruses or bacteria. Before any antimicrobial chemicals are used, the system should be thoroughly cleaned. It is critical that any antimicrobial treatment used in your system be registered by the U. S. Environmental Protection Agency specifically for use in HVAC systems. The use of antimicrobial chemicals is an additional service that is not part of a typical air duct cleaning project.