Eddie Hutton, Partner, answers the question this way.

You might ask if service firms can really clean ducts.  Well, because air conditioning ducts twist and turn their way through your house and because ducts have screws and edges that collect dust, duct cleaning is only 50-60% effective in removing dust and dirt. If you want a clean duct system you have to take the whole duct system out, clean it,  and put it back in. It is virtually impossible to clean a duct system while it’s in place. Most duct cleaners are a spinning polyester brush that goes through the duct.  The brush gets some of the dirt but leaves 30% or so in place. You pay a lot of money but you still have dirt and rust spots that pick up more dirt over the next few years.  Sorry, but duct cleaning is an expensive way to not get much!   The US Environmental Protection Agency (EPA) says what I just said using bureaucratese.  EPA says “no evidence suggests that such cleaning would be detrimental, provided that it is done properly.” The EPA website with lots of information is here.

Ducts twist and turn

The spinning polyester brush leaves dirt on rust, screws, etc.