Cleaning Air Ducts on Craigslist

I these some of these people that cleans air ducts for $99 are sort of dubious. I had one come over today and he put a big vacuum tube starting from each opening and just suck some dust inside. To me, that’s not really doing much. Is it a scam? He says there’s mold so he sprayed this mist into the air duct and charged me $80.