If anyone in your household has specific health concerns, such as allergies or asthma, consult your physician first. It’s important to identify the problem so your doctor can suggest alternatives to duct cleaning. Start by identifying whether your ducts are part of the problem (they probably aren’t) and whether getting them cleaned will help (it probably won’t).
Most organizations concerned with duct cleaning, including EPA, NADCA, NAIMA and the Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) do not currently recommend the routine use of sealants to encapsulate contaminants in any type of duct. Instances when the use of sealants to encapsulate the duct surfaces may be appropriate include the repair of damaged fiber glass insulation or when combating fire damage within ducts. Sealants should never be used on wet duct liner, to cover actively growing mold, or to cover debris in the ducts, and should only be applied after cleaning according to NADCA or other appropriate guidelines or standards.
Interestingly, I had this service performed several years ago, by a cheaper company. Based on the amount of dirt and junk that was removed yesterday, I now have my doubts that the previous company really performed the work. Yes, I checked this time! The owner, John, told me that there is another company working under the same name, so make sure that you are getting the Arizona Air Duct Cleaning Company, father and son team, that is being reviewed here. Not all these companies are honest and do what they claim to!
You should be aware that although a substance may look like mold, a positive determination of whether it is mold or not can be made only by an expert and may require laboratory analysis for final confirmation. For about $50, some microbiology laboratories can tell you whether a sample sent to them on a clear strip of sticky household tape is mold or simply a substance that resembles it.