Dan Slepian, Dateline producer, writes:

You've probably seen the ads in the newspaper or in your mailbox: a huge, frightening-looking dust mite accompanied by warnings that your health could suffer from dirty, moldy air ducts. But have no fear: for a low price, like $49.95, you can have them cleaned -- you might even get a 10% discount if you're a senior citizen.

According to the Better Business Bureau, it's a scam called a "bait and switch", and they say it's happening all across the country. In fact, the BBB claims that dozens of connected companies are involved in the airduct cleaning scam, systematically ripping off homeowners for more than a decade to the tune of millions of dollars.

Over the years, hundreds of homeowners have logged complaints with the BBB, all sounding similar: once inside a customer's home, technicians routinely mislead them into paying hundreds or thousands of dollars for additional work. Many were told they had dangerous mold. Others had their homes unnecessarily flooded with noxious chemicals. Some said they were afraid of the workmen. All said they were duped into paying hundreds more than the promised $49.99 advertised price.

It sounded like a story for the Hansen Files, so we began our own investigation.

We found senior citizen volunteers who allowed us to wire their homes with hidden cameras to see what would happen when we responded to one of the ads. Even we were stunned by what we saw. Not only did our cameras catch the technicians scam nearly $500 from our volunteer, they actually left the house in worse condition than before they arrived.

A few weeks later, we asked our volunteer to make another appointment, this time to have her furnace cleaned. And this time, Chris Hansen was there to let the technicians know our cameras had recorded their scam, and to see what they had to say.

Early on, we learned reporting this story wouldn't be easy. We began by digging into one company, and then another, and then the histories of some of the people behind them. We found many of the businesses opened up shop only to disappear within a matter of months, but would then appear again under a different name. Sometimes, it was even hard to determine exactly who owned many of them. The paper trail left behind was limited and sometimes inaccurate. One business was registered using the name of the real owner's dead stepbrother. Another was registered in the name of an owner's bodyguard. Several to convicted felons, one a killer.

But our reporting led us to one revealing fact: dozens of the rogue businesses all across the country were run by a rotating list of the same individuals who kept popping up over and over again.

As we began to connect the dots, we saw that some Attorneys General and judges have ordered these companies shut down, forbidden their owners from doing business in their states, and ordered them to pay hundreds of thousands of dollars in damages. But most of the time, the scammers simply took off to set up shop in another state.

Bottom line: if you think you need your air ducts cleaned, check with both the Better Business Bureau and the National Air Duct Cleaners Association before answering the cheap ad in the newspaper.

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For more information on The Hansen Files, please email producer Dan Slepian: daniel.slepian@nbcuni.com