There is no scientific perspective that ratifies the potential benefits and possible problems of air duct cleaning. The EPA does not recommend that air ducts be cleaned except when there are urgent reasons to do so. This is on account of the continuing uncertainty about the benefits of duct cleaning under most circumstances. Moreover the conditions would vary from home to home. It is not easy to specify a particular set of symptoms that would signal towards unclean ducts. However, here are some generic symptoms which can point towards unclean ducts in the house.

• If there is a family member in the house who is suffering of allergies or respiratory diseases because of no clear reasons.
• Sometimes you will find a thick layer of dust and debris near the return register. There is nothing alarming in this as dust-laden air is pulled through the grate. Here, it is filtered before sending it to the evaporator. Thus it is very likely that these will be infested with a little dirt from time to time. You do not need to place a service call for this. You can clean it yourself with a vacuum cleaner.
• You should, of course, get your ducts cleaned, if you think it is time to get a professional cleaning. A professional cleaning of ducts is quiet reasonable once in every two to three years. Although the benefits of duct cleaning is not clearly known, there is no evidence that suggests that such cleaning would be harmful, provided that it is done properly.

• If your duct cleaner say that there are visual evidence of mold growth in your ducts, ask him to show you that. You may take a black patch to be mold but it may not be so. You have to send a portion of the black patch on a sticky to the laboratory to confirm if it as mold.
• If you have insulated air ducts and the insulation gets wet or moldy it cannot be effectively cleaned and should be removed and replaced.
• However, if there is mold growth you have to opt for special treatments to terminate it. Simple cleaning will not effective as the mold will recur soon.
• A very common symptom of mold growth in the ducts is strange discoloration and bizarre smell at the vents.
• A duct covered with matted layer of debris and will send off puffs of dust in your indoor air too. In such cases you should get your ducts cleaned before long.