Industry standards used to control bacterial growth in ductwork range from the mechanical to the chemical.
The most simple is to keep it clean. Some microbes can grow on clean surfaces, but in general removing the dust buildup from ductwork significantly reduces the opportunity for microbial colonization. (Hunter, 8) But dust can often accumulate quickly. Studies show that copper remains effective even with a layer of dust between the surface and the microbes because of a phenomenon called the halo effect. Link>Read more about the halo effect in healthcare applications
On the other end of the spectrum. Biocides, either fogged into the system or applied to the duct surface before installation, can effectively eliminate or reduce microbial growth. But, as a hazardous material their use requires significant steps must be taken to ensure occupants are not exposed. For that reason the World Health Organization has explicitly discouraged their use. (Hunter, 8)
COBRE29 is a safe and natural way to control mold, mildew and fungal growth in the duct system, but its not a replacement for high quality air filtration. It is recommended to be used in conjunction with industry standards as a preventative supplement to ensure the cleanest possible air supply for building inhabitants.
* Hunter, Dr. CA, Hygenic Maintenance of Office Ventilation Ductwork, Chartered Institution of Building Services Engineers London, October 2000