Did you know that the state of Michigan requires air duct cleaners to have a state license to clean air ducts? Any time someone accesses the duct work in a residential or commercial setting, they must have a category 3 mechanical contractors license to do this legally. In West Michigan there are several air duct cleaning companies ignoring these state requirements.
Don't let an unqualified, unlicensed air duct cleaner clean your air ducts. Every time they do this, it is illegal and if there is any kind of problem with the service, the customer has no recourse. When hiring a company to do your airduct cleaning, make sure you ask them to provide you with their state license number, then call the state to verify that it is authentic.