Duct cleaning is an important part of mold remediation of a building but it is often overlooked as a source of problems in toxic mold exposure victims.
To make matters worse, there are countless "scam" companies out there that do not clean the duct work according to industry standards thus doing a poor job and overcharging the customer as well.
When a house has been damaged by water leaks and ends up with extensive mold growth, that mold can get dispersed into the duct work and start growing there. It can live off the dust and debris in the duct work and the humidity in the air. If the whole house has been remediated but the duct work not cleaned then it is an incomplete job. Many people exposed to toxic mold don't think of contaminated duct work and contaminated air handlers as a source of mold growth and contamination of their environment.
NADCA (The National Air Duct Cleaners Association) is a nonprofit association of companies engaged in the cleaning of HVAC systems. It's mission is to promote source removal as the only acceptable method of cleaning and to establish industry standards for the association.
If a duct cleaning company is a member of this organization than you can be fairly sure that they will be doing the job right. You can go to NADCA.com to find companies in your area that belong to this organization.
There are other ways to tell if you are dealing with a legitimate company and not a scam company. If they do not go up into your attic to clean the ducts they are scamming you. A legitimate company needs to get up in the attic to clean the duct work.
A scam company will just hook up a giant vacuum to one of your ducts and turn it on.
Also, a legitimate company will also clean the microbial growth off your air handler. Microbial growth includes mold.
And a legitimate company won't keep adding on expenses until you are giving them a small fortune just to clean your ducts.
Also, sometimes you can get a UV light put into your duct work near the air handler to prevent mold growth. A scam company will just stick it in the duct work far from the air handler. There is a certain placement for this UV light in order for it to work properly and a legitimate company will know how to do this.