A small number of products are currently registered by EPA specifically for use on the inside of bare sheet metal air ducts. A number of products are also registered for use as sanitizers on hard surfaces, which could include the interior of bare sheet metal ducts. While many such products may be used legally inside of unlined ducts if all label directions are followed, some of the directions on the label may be inappropriate for use in ducts. No products are currently registered as biocides for use on fiber glass duct board or fiber glass lined ducts, so it is important to determine if sections of your system contain these materials before permitting the application of any biocide. This is inappropriate.
Some microorganisms are always present in the air, and some growth on a settling plate is normal.
As noted earlier, only an expert can positively identify a substance as biological growth and lab analysis may be required for final confirmation. Other testing methods are not reliable. Explain why biological growth cannot be removed by physical means, such as brushing, and further growth prevented by controlling moisture. Apply the biocide only to un-insulated areas of the duct system after proper cleaning, if necessary to reduce the chances for regrowth of mold. Always use the product strictly according to its label instructions.