Isn't natural gas supposed to be one of the cleanest fuels to burn? Last few weeks I have noticed that the filters we put in the floor vents are getting dark, and our sharper image ionic filter was full of black soot. We didn't have this problem last year when we were on propane. But, I don't want to jump to conclusions yet, because my wife was burning a large candle all day and forgot to put it out when she went to bed, so when I put it out I could see it blowing off a lot of black particles. So maybe this sudden soot problem came from that. We're changing a vent filter to see if it gets darker, and we're calling a furnance technician tomorrow. In the meantime, do we have much to worry about?