(My column from the June edition of the Angie's List Magazine)

There's no concrete evidence that justifies the need for homeowners to have their air ducts cleaned regularly, but a contaminated heating, ventilation and air conditioning system could necessitate a professional cleaning. Contaminated systems have either mold growth, pests, or they distribute excessive dust, debris or particles into your home. One highly rated air duct cleaner on Angie's List told us he once removed a dead raccoon from a homeowner's ducts!

But beware: there are some in the industry who are not properly trained. If you do decide to have your HVAC system cleaned, be sure the service company has the proper qualifications. The National Air Duct Cleaners Association requires its member companies to have at least one certified Air System Cleaning Specialist on staff. To get certified, contractors must receive training and pass an exam.

Tim Rogers of Dublin, Ohio, joined Angie's List in 2007 specifically to report on what he felt was an incompetent duct cleaner. Rogers hired the company based on a newspaper ad for an $85 cleaning, but ended up paying more than $400 for 90 minutes of work. "I don't want to say things were worse when they left, [but] they weren't better," Rogers says.

Allowing an unqualified contractor to clean your ducts could cause more harm than good. More dust could be released into your house - or worse - your HVAC system could be damaged. Russell Fife, who owns highly rated Destin Air Conditioning & Refrigeration in Florida, says he's often seen the aftermath of careless work. "You can end up doing more damage than anything else if you don't clean carefully," Fife says.

A qualified professional will not just clean the ducts, but all components of your HVAC system, including the coils, drain pans, fans and humidifiers. If one component of the system isn't tended to, it could result in re-contamination of the entire system and negate any potential benefit from the cleaning. A good contractor should take several hours to do the job, and the cost usually runs between $400 and $1,000. Be leery of a service company that offers a cleaning for much less than that or finishes the job too quickly.

Whether or not you think air duct cleaning is necessary, the most effective way to avert contamination in the first place is to prevent dirt and moisture from entering the system. Use high-efficiency filters and change it regularly. Vacuum your home often with a cleaner that has a good HEPA filter. If your system has any leaks or water damage, get it repaired immediately.